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Acoustics of gas-bearing sediments I. Background
Abstract
Acoustical properties of water saturated and gassy sediments are observed to be significantly different. The present state of knowledge of the acoustical properties of saturated sediments, gassy water, and gassy sediments is reviewed. The dynamics of bubbles in water and in various solid materials, including sediments, are experimentally examined in a companion paper. Pulsation resonance is exhibited by the bubbles in all materials examined. Predictions of bubble resonance frequency and damping are shown to agree with the measurements. Equations for sound speed and attenuation, based on the model of resonating gas bubbles, are shown to agree with published measurements in gassy sediments. Parameters required for predicting gassy sediment acoustical properties are identified. Ranges of values of these parameters for various sediments are discussed.
... Since, it has been recognized that hydrocarbon-bearing deposits constitutes up to 30% of continental shelves (e.g., St. John 1984), which have high potential for gas accumulation (Hovland et al. 1993;Jaśniewicz et al., 2019). This allows seismo-acoustic detection to be applied (Schüler 1952;Anderson and Hampton 1980;Abegg and Anderson 1977) due to the distinct seismo-acoustic properties of gas-charged sediments (Jaśniewicz et al., 2019). In addition, seismic indicators of shallow gas deposits have been a major target for exploration geophysicists and a valuable source for the petroleum industry (Schroot and Schuttenhelm, 2003;Judd and Curzi, 2002). ...
... Seismo-acoustic pro ling methods are non-invasive, relatively low cost and are based on continuous measurement over the area of interest, which provides a fast and detailed image of shallow gas occurrences and their spatial distribution (e.g., Orange et al. 2005;Visnovitz et al., 2015;Jaśniewicz et al., 2019). This is mainly due to the very distinctive acoustic properties of shallow gas in comparison with the surrounding water and sediments (Anderson and Hampton 1980;Wilkens and Richardson 1998;Jaśniewicz et al., 2019) and also, the strong attenuation of high-frequency signals that allows the detection of very low amounts of free gas in sediments (e.g., Duarte et al. 2007;Visnovitz et al., 2015). The signal attenuation phenomenon is, in fact, an "acoustic problem" caused by shallow free gas as it has been recognized as impeding imaging of the depositional structure (Tóth et al., 2014). ...
... High acoustic impedance contrast, and also elastic contrast in relation to the surrounding medium non-gaseous sediments or water column, determines high acoustic energy scattering properties (Jaśniewicz et al., 2019). Based on high attenuation of acoustic energy and/or by reduction of acoustic signal propagation speed (Anderson and Hampton 1980;Wilkens and Richardson 1998;Jaśniewicz et al., 2019), the gas-charged deposits and/or bubbles in the water column can be simply and effectively detected. ...
The high-resolution analysis of single-channel, seismic reflection data from Lake Erçek (Eastern Anatolia) revealed a wide range of shallow gas anomalies consisting of enhanced reflections, seismic chimneys, acoustic blanking/acoustic turbidity, strong reflectors, and pockmarks, including both surface and buried pockmarks. The enhanced reflections are represented by the higher-amplitude reflection patterns resulting from high acoustic impedance variations. They are mostly clustered in the NW-corner of the lake. Seismic chimneys are represented by vertical and thinned columnar disturbances of amplitude blanking and mostly occurred in deep basinal and faulted sections in the West and East of the lake. Some seismic chimneys, occurring together with pockmarks, represent vertical vent activations. Acoustic gas-masking was represented by chaotic and diffuse seismic reflection patterns, including acoustic blanking and acoustic turbidity. As diffuse acoustic turbidity indicates gas-charged sediments, columnar disturbances showing acoustic blanking indicate degassing of the sediments. These features extend from SE to NW, coinciding with the deep basin morphology of the lake. A very local strong reflector was identified in the western section of the lake, simulating the lake floor. This reflector is due to extended enhanced reflections, suggesting shallow free gas. Pockmarks observed in the lake are structurally classified into the two distinct types; surface (active) pockmarks found in the SE-part of the lake and buried (passive) pockmarks found in the NW. The former enlarge through deeper gas reservoir feedback, as the layering is impermeable, while the latter have resulted from a cessation of the reservoir feedback mechanism and/or permeable layering. In the lake, shallow gas distribution is controlled by faults, that provide the faulting-driven depositional control and earthquakes, that provide the seismicity-driven overpressure control. The shallow gas is then both vertically and horizontally distributed and, finally, distribution is shaped by asymmetric depositional and stratigraphic factors. This study of Lake Erçek presents complementary information about a possible origin of observed shallow gas and is important in identifying the pockmark types to be targeted for oil/gas exploration and valuable geochemical proxies.
... Since, it has been recognized that hydrocarbon-bearing deposits constitutes up to 30% of continental shelves (e.g., St. John 1984), which have high potential for gas accumulation (Hovland et al. 1993;Jaśniewicz et al. 2019). This allows seismoacoustic detection to be applied (Schüler 1952;Anderson and Hampton 1980;Abegg and Anderson 1977) due to the distinct seismo-acoustic properties of gas-charged sediments (Jaśniewicz et al. 2019). In addition, seismic indicators of shallow gas deposits have been a major target for exploration geophysicists and a valuable source for the petroleum industry (Schroot and Schuttenhelm 2003;Judd and Curzi 2002). ...
... Seismo-acoustic profiling methods are noninvasive, relatively low cost and are based on continuous measurement over the area of interest, which provides a fast and detailed image of shallow gas occurrences and their spatial distribution (e.g., Orange et al. 2005;Visnovitz et al. 2015;Jaśniewicz et al. 2019). This is mainly due to the very distinctive acoustic properties of shallow gas in comparison with the surrounding water and sediments (Anderson and Hampton 1980;Wilkens and Richardson 1998;Jaśniewicz et al. 2019) and also the strong attenuation of high-frequency signals that allows the detection of very low amounts of free gas in sediments (e.g., Duarte et al. 2007;Visnovitz et al. 2015). The signal attenuation phenomenon is, in fact, an "acoustic problem" caused by shallow free gas as it has been recognized as impeding imaging of the depositional structure (Tóth et al. 2014). ...
... High acoustic impedance contrast, and also elastic contrast in relation to the surrounding medium non-gaseous sediments or water column, determines high acoustic energy scattering properties (Jaśniewicz et al. 2019). Based on high attenuation of acoustic energy and/or by reduction of acoustic signal propagation speed (Anderson and Hampton 1980;Wilkens and Richardson 1998;Jaśniewicz et al. 2019), the gas-charged deposits and/or bubbles in the water column can be simply and effectively detected. Seismo-acoustic properties of shallow gas occurrence are dependent on the concentration of gas, its distributional pattern, and the applied acoustic method. ...
The high-resolution analysis of single-channel, seismic reflection data from Lake Erçek (Eastern Anatolia) revealed a wide
range of shallow gas anomalies consisting of enhanced reflections, seismic chimneys, acoustic blanking/acoustic turbidity,
strong reflectors, and pockmarks, including both surface and buried pockmarks. The enhanced reflections are represented
by the higher amplitude reflection patterns resulting from high acoustic impedance variations. They are mostly clustered in
the NW-corner of the lake. Seismic chimneys are represented by vertical and thinned columnar disturbances of amplitude
blanking and mostly occurred in deep basinal and faulted sections in the West and East of the lake. Some seismic chimneys,
occurring together with pockmarks, represent vertical vent activations. Acoustic gas masking was represented by chaotic
and diffuse seismic reflection patterns, including acoustic blanking and acoustic turbidity. As diffuse acoustic turbidity
indicates gas-charged sediments, columnar disturbances showing acoustic blanking indicate degassing of the sediments.
These features extend from SE to NW, coinciding with the deep basin morphology of the lake. A very local strong reflector
was identified in the W-section of the lake, simulating the lake floor. This reflector is due to extended enhanced reflections,
suggesting shallow free gas. Pockmarks observed in the lake are structurally classified into the two distinct types; surface
(active) pockmarks found in the SE-part of the lake and buried (passive) pockmarks found in the NW. The former enlarge
through deeper gas reservoir feedback, as the layering is impermeable, while the latter have resulted from a cessation of the
reservoir feedback mechanism and/or permeable layering. In the lake, shallow gas distribution is controlled by faults, that
provide the faulting-driven depositional control and earthquakes, that provide the seismicity-driven overpressure control.
The shallow gas is then vertically–horizontally distributed and shaped by asymmetric depositional–stratigraphic factors. This
study of Lake Erçek presents complementary information about a possible tectono-thermal origin of observed shallow gas.
... The influence of mean grain size on sound velocity in surficial marine sediments is well known [4,5,19,83,84]. Although granulometric analyses of the cored sediments used in our study were unavailable, general grain size classes could still be determined from borehole geotechnical logs (see Section 2). ...
... The lowest sound velocity in our dataset was calculated for Borehole A-III-7/88, which penetrates a hyperbola-dense zone clearly visible on the corresponding sub-bottom sonar profile (Table 2 and Figure 3c). Since even minor gas concentrations (1-2%) dramatically reduce sound velocity in sediments [83,84,[91][92][93], we attribute this significantly lower velocity value to gas in the Quaternary sediment. In the northern Adriatic seabed, gas seeps are commonly observed and are attributed to both deep and shallow sources [94][95][96][97][98]. Since the diffraction hyperbolas in our sonar profiles are constrained only to a narrow zone in the uppermost part of the Quaternary sequence (Figures 3 and 4), we propose that the gas (probably methane) originates from a degradation of organic matter contained in the Holocene paralic and marine sediments and/or Late Pleistocene terrestrial sequences [62,97,99]. ...
... Using average sound velocities for depth conversion of a highly heterogeneous Quaternary succession can be considered a gross oversimplification as the velocity strongly varies with grain size [4,5,19,83,84]. However, when comparing the sub-bottom sonar profiles and the borehole logs used in our study ( Figure 5, Figure 3, and Figure 4), a good alignment between the reflections and the main sedimentological boundaries from the borehole logs is apparent (Figure 3b,c and Figure 4a,b). ...
Estimating sound velocity in seabed sediment of shallow near-shore areas submerged after the Last Glacial Maximum is often difficult due to the heterogeneous sedimentary composition resulting from sea-level changes affecting the sedimentary environments. The complex sedimentary architecture and heterogeneity greatly impact lateral and horizontal velocity variations. Existing sound velocity studies are mainly focused on the surficial parts of the seabed sediments, whereas the deeper and often more heterogeneous sections are usually neglected. We present an example of a submerged alluvial plain in the northern Adriatic where we were able to investigate the entire Quaternary sedimentary succession from the seafloor down to the sediment base on the bedrock. We used an extensive dataset of vintage borehole litho-sedimentological descriptions covering the entire thickness of the Quaternary sedimentary succession. We correlated the dataset with sub-bottom sonar profiles in order to determine the average sound velocities through various sediment types. The sound velocities of clay-dominated successions average around 1530 m/s, while the values of silt-dominated successions extend between 1550 and 1590 m/s. The maximum sound velocity of approximately 1730 m/s was determined at a location containing sandy sediment, while the minimum sound velocity of approximately 1250 m/s was calculated for gas-charged sediments. We show that, in shallow areas with thin Quaternary successions, the main factor influencing average sound velocity is the predominant sediment type (i.e. grain size), whereas the overburden influence is negligible. Where present in the sedimentary column, gas substantially reduces sound velocity. Our work provides a reference for sound velocities in submerged, thin (less than 20 m thick), terrestrial-marine Quaternary successions located in shallow (a few tens of meters deep) near-shore settings, which represent a large part of the present-day coastal environments.
... The high-resolution seismic analysis together with bathymetry data allowed us to identify and map different acoustic anomalies due to the occurrences of shallow gas in the Offshore Sinú Fold Belt (OSFB). Shallow gassy sediments can particularly be recognized by acoustic anomalies such as attenuation and scattering of the seismic signal [18,71,72]. Acoustic blanking (AB) zones appear as a reflection of great amplitude with extensions between 350 and 700 m. ...
... The high-resolution seismic analysis together with bathymetry data allowed us to identify and map different acoustic anomalies due to the occurrences of shallow gas in the Offshore Sinú Fold Belt (OSFB). Shallow gassy sediments can particularly be recognized by acoustic anomalies such as attenuation and scattering of the seismic signal [18,71,72]. In high-resolution seismic profiles, gas presence in the sediments produces the absorption of most of the high-frequency acoustic energy, masking the deeper horizons. ...
High-resolution seismic analysis and bathymetry data, used in the Offshore Sinú Fold Belt (OSFB), have revealed seabed and sub-surface anomalies, which were probably caused by the presence of shallow gas within the sedimentary records. Shallow gas is widely detected by the frequent presence of anomalous acoustic reflections including acoustic blanking, enhanced reflections,
acoustic plumes, pockmarks, and dome structures. More than 30 anomalies that occur within a subsurface depth of ~65 m were acoustically detected within an area of 1000 km2 on the continental
shelf and upper continental slope, in water depths ranging from −20 to −1300 m. Moreover, a map with the spatial distribution of the gas occurrences is shown. A close relationship between the locally
elevated seabed (dome structures), pockmarks, and acoustic blanking was found. Most of the active pockmarks may be closely related to the submarine path of the Uramita Fault, indicating that the gas occurrences are controlled by active faulting. The shallow gas occurrence was confirmed by the generation of authigenic carbonate and the occurrence of chemosymbiotic biological communities sampled in the area. Although there is an admixture of biogenic gas, it is believed that many of the features observed relate to thermogenic gas. The identification of these anomalies represents a useful basis for an assessment of marine geohazards and can serve as a hydrocarbon exploration tool.
... The high-resolution seismic analysis together with bathymetry data allowed us to identify and map different acoustic anomalies due to the occurrences of shallow gas in the Offshore Sinú Fold Belt (OSFB). Shallow gassy sediments can particularly be recognized by acoustic anomalies such as attenuation and scattering of the seismic signal [18,71,72]. Acoustic blanking (AB) zones appear as a reflection of great amplitude with extensions between 350 and 700 m. ...
... The high-resolution seismic analysis together with bathymetry data allowed us to identify and map different acoustic anomalies due to the occurrences of shallow gas in the Offshore Sinú Fold Belt (OSFB). Shallow gassy sediments can particularly be recognized by acoustic anomalies such as attenuation and scattering of the seismic signal [18,71,72]. In high-resolution seismic profiles, gas presence in the sediments produces the absorption of most of the high-frequency acoustic energy, masking the deeper horizons. ...
High-resolution seismic analysis and bathymetry data, used in the Offshore Sinú Fold Belt (OSFB), have revealed seabed and sub-surface anomalies, which were probably caused by the presence of shallow gas within the sedimentary records. Shallow gas is widely detected by the frequent presence of anomalous acoustic reflections including acoustic blanking, enhanced reflections, acoustic plumes, pockmarks, and dome structures. More than 30 anomalies that occur within a subsurface depth of ~65 m were acoustically detected within an area of 1000 km2 on the continental shelf and upper continental slope, in water depths ranging from −20 to −1300 m. Moreover, a map with the spatial distribution of the gas occurrences is shown. A close relationship between the locally elevated seabed (dome structures), pockmarks, and acoustic blanking was found. Most of the active pockmarks may be closely related to the submarine path of the Uramita Fault, indicating that the gas occurrences are controlled by active faulting. The shallow gas occurrence was confirmed by the generation of authigenic carbonate and the occurrence of chemosymbiotic biological communities sampled in the area. Although there is an admixture of biogenic gas, it is believed that many of the features observed relate to thermogenic gas. The identification of these anomalies represents a useful basis for an assessment of marine geohazards and can serve as a hydrocarbon exploration tool.
... Considering a relation between the Young's and Shear moduli, E = 2G(1 +ν) (where ν is Poisson's ratio), K Ic /E ratio of ~10 − 4 m 1/2 order of magnitude will be indicative for the basic values of Fracture Toughness and Young's modulus specified in Table 1 (marked by two asterisks). In addition, absolute values of E and K Ic explored in this study (Table 1) are characteristic for the shallow muddy sediments (Anderson and Hampton, 1980;Barry et al., 2013), e. g., up to 80 cm below sediment-water interface (Zamanpour et al., 2020). Importantly, sizes of mature bubbles modelled under the K Ic and E from Table 1 (up to several cm in frontal size (see Eq.(16) in Katsman, 2015) and up to 10 mm in the equivalent spherical diameter, Katsman et al., 2021), agree with those observed in the shallow in-situ sediments and in the lab measurements (Lyons et al., 1996;Wilkens and Richardson, 1998;Gardiner et al., 2003b;Best et al., 2004;Tõth et al., 2015;Liu et al., 2016Liu et al., , 2018. ...
... (Dorgan et al., 2007;L'Esperance et al., 2013) Tortuosity-corrected CH 4 effective diffusion coefficient (Boudreau, 1997), where τ 2 is tortuosity factor, τ 2 = 1 − 2ln(∅) and Dm is CH 4 molecular diffusion coefficient, Dm = 10 − 9 m 2 s − 1 ( Iversen and Jørgensen, 1993). Dissolved CH 4 (aq) concentration C0 = 6.5 mM = 0.1 kg⋅m − 3 (e.g., Martens et al., 1998) Source strength SCH 4 (aq) = 0* Fracture Toughness KIc = 30, 40, 60**, 70 Pa⋅m 1/2 (Johnson et al., 2002, 2012) Young's modulus E = 4 × 10 5 , 5.5 × 10 5 **, 7 × 10 5 Pa (Anderson and Hampton, 1980;Algar and Boudreau, 2010;Barry et al., 2012) § The effective overburden load of 5 kPa corresponds to an approximate water depth of ~0.5 m, characteristic for shallow lakes/wetlands and serving as major sources of methane fluxes to the water column and potentially to the atmosphere. * Zero source strength is used to model a bubble growth in this study. ...
Methane (CH4) bubbles in muddy aquatic sediments threaten climate sustainability and sediment mechanical stability. Mechanical response of muddy sediment to bubble growth is described by Linear Elastic Fracture Mechanics (LEFM). Minor roles of mechanical sediment characteristics in CH4 bubble solute supply and growth rates, were quantified by preceding studies, compared to biogeochemical controls. We investigate them using a coupled single-bubble mechanical/reaction-transport numerical and analytical models. We demonstrate that inner pressure of the growing bubble at fracturing, concentration at its surface, bubble size and spatial location, are uniquely defined by the Fracture Toughness of muddy sediment. However, a temporal evolution of the bubble inner pressure at expansion between the fracturing events depends on the Young's modulus. Fracture Toughness and Young's modulus thus play complementary, spatial and temporal, roles in the bubble inner pressure evolution. The proportionality suggested by LEFM manages the bubble solute exchange and growth rates. Higher Fracture Toughness controls development of longer flatter bubbles in the deeper sediments. A substantial role of the mechanical muddy sediment characteristics in the CH4 bubble growth dynamics and solute exchange is demonstrated, comparable to the role of the biogeochemical controls. Their contributions to development of “no-growth” and competitive bubble growth conditions, affecting a gas dynamics at a macro-scale, are discussed.
... Seismic signatures related to increasing CO 2 saturation in the sediment include reflectivity enhancement and energy dissipation (Anderson and Hampton 1980). Energy loss is caused by bubble fractures present in the sediment, which resonate at certain frequencies and scatter the incident sound. ...
... shows the root mean square (RMS) amplitude for a TWT time interval 1-3 ms beneath the seabed (~0.9-2.6 mbsf). As the presence of free gas greatly affects the impedance of sediment(Anderson and Hampton 1980) it is often clearly visible in RMS amplitude maps. On D+3 when gas was being injected at 6 kg/day, the RMS amplitude at the release site (12.5 m radius around the injection point) was 0.033 ±0.002 with no obvious hot spots. ...
The passage of greenhouse gases, from both natural and anthropogenic sources, through the
upper sedimentary succession and into overlying aquatic systems is a poorly understood process.
Our understanding of it however conditions our ability to detect potential leaks from Carbon
Capture and Storage sites (CCS) as well as our overall knowledge of the global carbon cycle. In this
thesis repeated high-resolution seismic reflection surveys are used to image carbon dioxide (CO2)
gas released into shallow sub-surface sediments above a potential CCS storage site in the North
Sea. Observations of temporal changes in seismic reflectivity, attenuation, unit thickness and the
bulk permeability of sediment were used to develop a four-stage model of the evolution of gas
migration in shallow marine sediments: Proto-migration, Immature Migration, Mature Migration,
and Pathway Closure. Variations in ebullition rates from natural methane (CH4) seeps in Lake
Constance (central Europe) are observed over a 9-month period using physical and acoustic
measurement techniques, demonstrating a significant negative correlation between gas flux and
in-situ pressure. The water level (hydrostatic pressure) dictates flux rates on monthly timescales,
while atmospheric pressure causes minor fluctuations on daily to weekly periods, the effect of
land-lake breeze cycles are observed for the first time. Exploiting this relationship, we find that
long-term ebullition rates are best estimated by quantifying the relationship between in-situ
pressure and gas flux, and then using this relationship to predict gas flux from more easily
measurable in-situ pressure data. Finally, the use of passive acoustic flux inversion techniques is
refined by measuring the initial amplitude of a bubble’s excitation when released from sediment,
a previously poorly constrained parameter, demonstrating a strong correlation with the bubble
equilibrium radius. We demonstrate the use of this refined acoustic inversion technique by
measuring the flux from a volcanic CO2 seep in offshore Panarea (Italy), seeing a significant
increase in precision with estimates consistent with optical and physical flux measurements.
These findings have enhanced our understanding of gas migration in the near surface and
improved our ability to measure gas emissions.
... Prior to land reclamation, acoustic detection methods exhibit good detection performance on the gas front of the bubble-bearing area (Judd and Hovland, 1992;Kim et al., 2008;Schneider von Deimling et al., 2013). The acoustic signal is strongly attenuation by the bubbles (Anderson and Hampton, 1980;Best et al., 2004;Tõth et al., 2015). Bubble-bearing areas correspond to acoustic blanking and turbidity in the acoustic profile (Fleischer et al., 2001;Judd and Hovland, 2007). ...
... There is an equilibrium bubble radius for any supersaturation; however, theoretically, small bubbles will be dissolved because the pressure at the gas-water interface increases as the bubble radius decreases. Typically, the minimum radius of bubbles is 1-10 μm (Anderson and Hampton, 1980). After that, even if the methane in pore water is no longer saturated, the bubble may be in a metastable state, which may persist for a long time or grow (Solano-Altamirano et al., 2015). ...
When land reclamation is conducted in a shallow offshore bubble-bearing sediment, methane bubbles can cause various risks, such as changes in the mechanical properties of soils and explosion during construction. The premise of assessing or addressing these risks is to understand the evolution of the bubble-bearing area during land reclamation. In this study, we selected an island-based land reclamation field in the southeastern part of Hangzhou Bay with a bubble-bearing area in shallow sediments. In this field, soft soil under the land reclamation fill was improved via prefabricated vertical drain. After the landing of the field, an electrical resistivity tomography (ERT) survey was conducted at the location of the bubble-bearing area and in an adjacent bubble-bearing area. The force and deformation of bubbles in the bubble-bearing area were analyzed at various construction stages of land reclamation. A conceptual model of the bubble-bearing area evolution in each construction stage was proposed based on linear elastic fracture mechanics (LEFM). The results of the ERT survey and theoretical analysis are in good agreement. In the process of land reclamation, although the gas pressure and size of the bubbles in the bubble-bearing area may change with the change in the environmental pressure, the bubble size is always smaller than the critical size for initiating rise; thus, the bubbles cannot rise. After land reclamation, there is no important horizontal/vertical evolution of the gas front in the bubble-bearing area.
... Highly reflective layers in subbottom data beneath pockmarks are often assumed to be the result of methane-derived authigenic carbonates (e.g., Johnson et al., 2003) or interstitial gaseous methane (bubbles) (Jaśniewicz et al., 2019). The vertically stacked reflective packages seen in the Chirp lines beneath pockmarks and throughout the region lack the distinctive diagnostic patterns (i.e., acoustic blanking, strong and locally enhanced amplitudes, and abrupt horizontal changes in reflectivity) that are considered to be indicative of the presence of gas in sub-bottom profiles (e.g., Abegg & Anderson, 1997;Anderson & Hampton, 1980;Andreassen et al., 1997;Fleischer et al., 2001;Jaśniewicz et al., 2019;Judd & Hovland, 1992;Yuan et al., 1996). In contrast, the strong reflectors that outline the lens-shaped sediment packages beneath pockmarks have similar internal geometries, change amplitudes slowly, and consistently delineate similar patterns in each successively deeper stratigraphic package. ...
Recent surface ship multibeam surveys of the Sur Pockmark Field, offshore Central California, reveal >5,000 pockmarks in an area that is slated to host a wind farm, between 500‐ and 1,500‐m water depth. Extensive fieldwork was conducted to characterize the seafloor environment and its recent geologic history, including visual observations with remotely operated vehicles, sediment core sampling, and high‐resolution, near‐bottom Chirp and multibeam surveys collected with autonomous underwater vehicles to capture the morphology and stratigraphy of the pockmarks. No evidence of high methane concentrations in sediments, chemosynthetic biological communities, or methane‐derived diagenetic byproducts was found. Chirp data and sediment cores showed alternating layers of slowly accumulating hemipelagic drapes interrupted by more reflective turbidite horizons that extend throughout the pockmark field and beyond. Chirp data showed multiple episodes of lateral migration over time in some of the pockmarks in association with erosion and infilling events. Laterally continuous turbidite horizons that overlay erosional surfaces indicated that pockmark migration occurred synchronously in multiple pockmarks separated by tens of kilometers. These shifts are presumed to be the result of asymmetrical erosion of the pockmark flanks caused by passing sediment gravity flows. While some pockmarks occur in chains, most are not clustered or randomly spaced but are regularly dispersed within the pockmark field. We hypothesize that intermittent, unconfined sediment gravity flows occurring over at least the last 280,000 years are the source of the regionally continuous turbidite deposits and the mechanism that maintained the regularly dispersed pockmarks.
... Sound speed could be evaluated as a function of gassy medium effective bulk modulus and its effective density (Mavko et al., 2003). This analysis will extend a commonly accepted dependence of the effective muddy sediment characteristics on a gas content only (e.g., Anderson and Hampton, 1980) by the additional bubble descriptors. ...
... Enhanced reflections usually occur where fine grained sediments are interbedded with layers of coarse and permeable sediments. Hence, gas voids in the finer sediments causes acoustic turbidity while the gas bubbles within the coarser sediments or gas reservoir causes scattering of seismic wavelets and brightening of the negative amplitude (Anderson and Hampton, 1980;Hovland and Judd, 1988). However, due to a lack of data, the age of the seabed from this study could not be conclusively proven. ...
... Enhanced reflections usually occur where fine grained sediments are interbedded with layers of coarse and permeable sediments. Hence, gas voids in the finer sediments causes acoustic turbidity while the gas bubbles within the coarser sediments or gas reservoir causes scattering of seismic wavelets and brightening of the negative amplitude (Anderson and Hampton, 1980;Hovland and Judd, 1988). However, due to a lack of data, the age of the seabed from this study could not be conclusively proven. ...
The influence of basin and depositional context on the formation and growth of fluid escape pipes in sedimentary basins is not fully understood. While seismic reflection data is commonly used to study these structures, direct observation and sampling of field analogues of pipes and their related structures are rare. In this study, we investigate the evolution of meter-tall fluid escape pipes (100-800 m) in the Canterbury Basin, New Zealand, using open source seismic and borehole data. Our analysis includes seismic interpretation of 19 unconformities within the Neogene-Quaternary successions, characterization of pipes and fluid-related anomalies in the subsurface, modelling of the shale volume (Vsh), lithology characterization, 3D static modelling, and seismic inversion. We identify thirty-one vertical to sub-vertical pipes with three parts consisting of top, main conduit/stem, and root zone on seismic profiles. Our results suggest that lithology plays a crucial role in the initiation and modulation of pipe structures in the Canterbury Basin, with pipes mostly formed post Tarantian (0.113 Ma) and truncated at the U19 unconformity. Furthermore, vertical fluid migration from overpressured units, such as the tops of contourite mounds, was observed in the pipes. Our findings provide new insights into the subsurface fluid flow and sedimentary architecture of the Canterbury Basin and have wider implications for other passive continental margins with similar characteristics.
... The effect of gas bubbles on sound reflection, scattering, and absorption is especially significant when bubbles are "resonant", i.e., when the eigenfrequency of their radial oscillations coincides with the soundwave frequency (Brekhovskikh & Lysanov, 1991). The resonance frequency of the gas bubble depends on bubble radius; sediment dynamic bulk and shear moduli and bulk density; gas thermal properties; and ambient hydrostatic pressure (Anderson & Hampton, 1980b, 1980aBest et al., 2004;Tõth et al., 2015). It was shown in Katsman et al. (2021) that both sediment sound speed, sediment attenuation coefficient, and reflection coefficient at the water-sediment interface show non-linear dynamics, which are dependent on the bubble size, concentration, and corresponding resonant frequency of the gas bubble. ...
Gas-rich sediments cause permanent concern due to their contribution to sediment destabilization and global warming. In this paper, an effective media theory of gassy sediments previously suggested by the authors is tested experimentally in Lake Kinneret using wideband acoustic signals. Results of experiments and the corresponding acoustic data processing is presented. Ten 5-s long wideband (0.3-15 kHz) chirp pulses were radiated by an underwater transducer mounted directly at a 30-m long seven-channel vertical line array (VLA) deployed in the central part of the lake (the seafloor depth is 35 m). Received sound field timeseries consists of a sequence of pulse arrivals comprised of specular reflections from interfaces followed by reverberation codas caused by non-specular scattering from the interface roughness and volume inhomogeneity. Having studied the frequency dependence of the signal reflected from the bottom, a dip in the reflection coefficient was found at frequencies of 4-6 kHz. This suggests the existence of bubbles with an effective spherical diameter of about 3 mm, which is consistent with previous direct measurements of bubbles in the lake sediments.
... Gaseous inclusions sharply differ in their properties from the solid and liquid components of soils, and their presence affects the physical [33], mechanical [16,34], and acoustic [9] properties of host soils. The largest influence on the properties of subaqueous soils is exerted by free and entrapped gases. ...
... Typically represented by seismic quality factor Q or absorption coefficient α, seismic inelastic attenuation refers to the intrinsic loss of energy that occurs during the inelastic conversion of wave energy into heat and fluid flow (Aki and Richards 2002;Müller et al. 2010;Xue et al. 2016;Tary et al. 2017). It has been verified that fluid properties are major factors of seismic attenuation in a relatively stable stratigraphic structure with little variation in horizontal and vertical lithology (Anderson & Hampton 1980;Dvorkin & Nur 1993;Castagna et al. 2003;Korneev et al. 2004;Xue et al. 2016). The overall effect of energy attenuation is that higher frequencies are attenuated more rapidly than lower frequencies for a seismic trace in gas-prone sediments, leading to attenuation anomalies in the signal (Xue et al. 2016). ...
Seismic attenuation has a considerable impact on resolution reduction and the increase in the dominant frequency period of seismic data. The absorption coefficient estimates, which measure inelastic attenuation, provide a deep understanding of the medium property changes in different geological settings. Conventional absorption coefficient estimation technologies always use time–frequency methods for seismic energy absorption analysis. However, despite continuing efforts to improve the absorption coefficient estimation, the limitation of the time–frequency methods still causes insufficient accuracy of the attenuation estimates, imposing major challenges in oil and gas hydrate exploration. In this study, a quantum mechanics-based seismic absorption coefficient estimation method was proposed for hydrocarbon detection. The seismic data were first projected on a specific basis constructed using the resolution of the Schrödinger equation. Seismic energy absorption analysis was then conducted in the potential-wave function domain. Finally, the quantum absorption coefficient estimates are given by the procedure after using a logarithmic operation and the least-squares fitting method. We examined the merits of these methods using model and field data. The gas reservoir was accurately targeted, which demonstrates that the proposed method has great potential for hydrocarbon detection.
... A more universal issue common with late-Quaternary or Holocene studies is the attenuation of the reflection signals from subsurface biogenic gas produced within pore spaces via the breakdown of Holocene organic matter (Anderson and Hampton, 1980;Anderson and Bryant, 1990;Hart and Hamilton, 1993;Figueiredo and others, 1996;Missiaen and others, 2002;Dondurur and others, 2011;Zaremba and others, 2016;Zaremba and Scholz, 2021). In the Oneida Lake seismic reflection data sets, the attenuation of the signal from biogenic gas occurs only in the deepest section of the modern lake, and where the Holocene section is thickest. ...
Ice streams are sites of ice-sheet drainage and together with other processes, such as calving, have an impact on deglaciation rates and ice-sheet mass balance. Proglacial lake deposits provide records of ice-sheet deglaciation and have the potential to supplement other paleoclimate records. Oneida Lake, northeastern USA, contains a thick proglacial lake sequence that buries evidence of ice streaming and a paleo-calving margin that developed during retreat of the Laurentide Ice Sheet. Previous high-resolution digital elevation models identified the Oneida Ice Stream from glacial landforms northwest of the lake. In this study, we utilize seismic refractions from a multichannel seismic (MCS) reflection dataset to estimate the thickness of glacial deposits using seismic tomography. With this method we constrain the depth to top of Paleozoic strata, especially in areas where the reflection data yielded poor outcomes and validate our reflection data in regions of good coverage. We demonstrate that where long offset seismic data are available, the first-arrival tomography method is useful in studies of formerly glaciated basins. Our study identifies a ~108 m thick sedimentary section and potentially long paleoclimate record in Oneida Lake, and identifies a paleotopographic low that likely encouraged formation of the Oneida Ice Stream.
... The strong amplitude anomalies at low frequencies can always be detected by employing spectral decomposition. This is mainly caused by the fact that, compared with lower frequencies, higher frequencies are attenuated more rapidly owing to the attenuation of seismic waves in gas-prone sediments (Anderson and Hampton, 1980;Xue et al., 2014). ...
The low-frequency strong reflection of coal seams always makes the gas detection of the adjacent gas reservoir difficult in a tight sandstone reservoir. An adaptive decomposition processing approach is introduced in this study to inspect the ability to target the weak seismic response of the gas reservoirs interfered by the adjacent coal seams. The variational mode decomposition (VMD)–based highlight volumes extraction approach is used to better detect the weak seismic response of the gas reservoirs concealed by the adjacent strong reflection of coal seams in a tight sandstone reservoir. The amplitudes above the average and peak frequency volumes are extracted from each intrinsic mode functions after VMD for gas detection. Field data examples show that the VMD highlights the weak seismic responses of the geological and stratigraphic information and hydrocarbon-related contents. The results obtained from the VMD-based highlight volumes extraction approach are in close agreement with previous logging interpretations and the drilling information of wells, and the lateral extent of hydrate-bearing strata is estimated with increased accuracy. When the influence of the strong reflection amplitudes caused by the coal seams is not suppressed, the characterization of the weak seismic responses in the reservoirs would prevent the detection of gas or overestimate the lateral distribution of the gas strata. The adaptive decomposition processing methodology has the ability to detect the weak seismic responses in reservoirs interfered or concealed by adjacent coal seams and yields a better hydrocarbon-related interpretation.
... Elongated discshaped bubbles with their longest axis oriented in the vertical plane are observed in the fine-grained muds (Best et al., 2004). Bubbles become more elongated as bubble size increases (Anderson & Hampton, 1980a;Robb et al., 2006). ...
Shallow gassy aquatic sediments, abundantly found in Israel and worldwide, are a source of major concern for their contribution to destabilization of coastal and marine infrastructure, ecological balance, air pollutions, and global warming. Gas bubbles within sediment change effective sediment properties, including also its geo-acoustic characteristics. Among other characteristics, sound speed is the most sensitive parameters to presence of gas. This study proposes a novel methodology for acoustic sediment characterization. Behavior of reflection coefficient is used for estimations of gas content along off-shore transect and thickness of gassy sediment layer. Study was carried our in the North-Western part of Lake Kinneret, North of Israel. Variations in the free gas content in sediments with water depth obtained using the proposed method shows an agreement with the distribution of organic matter content in and methane fluxes from sediment, both revealed by the preceding studies. Estimated in December 2016 thickness of gassy layer in the central lake Station A (water depth ≈ 37 m) is 19.5 ± 4.5 cm, whereas in the shallower Station at North-West of the lake (water depth ≈11 m) it is 32 ± 9 cm. Proposed in this study non-invasive, cost-effective methodology allows a rapid scanning over large areas of aquatic sediments. This method is especially suitable for characterizing gassy sediments and highlighting locations of potential methane emissions. This, in turn, allows a better understanding of methane gas distribution in the upper sediment layer and can be used in monitoring of ecological balance of the region.
... Seismic attenuation estimation methods are another widely used techniques which use the energy attenuation characteristics of seismic waves for reservoir characterization and hydrocarbon detection (e.g., Mitchell et al., 1996;Partyka et al., 1999;Castagna et al., 2003;Sinha et al., 2005;Xue et al., 2016a;Wang et al., 2019). Laboratory experiments and field data measurements show that seismic wave attenuated more pronouncedly for viscous fluid-saturated rocks than dry rocks in most frequency bandwidths and the high-frequency components of seismic waves are attenuated more rapidly than the low-frequency components in gas-prone sediments (Domenico, 1974;Anderson and Hampton, 1980;Dvorkin and Nur, 1993;del Valle-García and Ramírez-Cruz, 2002;Korneev et al., 2004;Xue et al., 2019). Strong amplitude anomalies of seismic waves at specific frequencies can be easily found through the seismic attenuation estimation methods such as spectrum decomposition (e.g., Castagna et al., 2003;Korneev et al., 2004;Sinha et al., 2005;Duchesne et al., 2011) and attenuation gradient estimation (e.g., Mitchell et al., 1996;Pramanik et al., 2000;del Valle-García and Ramírez-Cruz 2002;Xue et al., 2016a). ...
A direct hydrocarbon detection is performed by using multi-attributes based quantum neural networks with gas fields. The proposed multi-attributes based quantum neural networks for hydrocarbon detection use data clustering and local wave decomposition based seismic attenuation characteristics, relative wave impedance features of prestack seismic data as the selected multiple attributes for one tight sandstone gas reservoir and further employ principal component analysis combined with quantum neural networks for giving the distinguishing results of the weak responses of the gas reservoir, which is hard to detect by using the conventional technologies. For the seismic data from a tight sandstone gas reservoir in the Sichuan basin, China, we found that multi-attributes based quantum neural networks can effectively capture the weak seismic responses features associated with gas saturation in the gas reservoir. This study is hoped to be useful as an aid for hydrocarbon detections for the gas reservoir with the characteristics of the weak seismic responses by the complement of the multi-attributes based quantum neural networks.
... [33]). In specific, these data can reveal the migration of gas-rich pore fluids based on the geophysical signature of reflection disturbance because free gases can cause a decrease in P-wave velocity, severe ray bending, signal scattering, and high transmission loss [34,35]. The detected pathways favored by gas bypassing the overburden sediments include the high-angle faults and fractures; both of which largely increase the vertical permeability and hence provide the pressure communication for free gases with the surface [36]. ...
It has been two decades since the cold seeps were firstly found in the Okinawa Trough (OT). The scientific cruises and the geological surveys since then have unveiled the currently active submarine methane seeps and significantly improved the understanding of methane seeps in the back-arc basin of the OT. In this paper, we review the up-to-date progress of the research of methane seepages then put forward the promising, yet challenging, outlook by listing the unsolved questions of the cold seeps in the OT. Multiple approaches and techniques, including seismic and echo-sounder recording, dredging, gravity-piston and ROV coring, seafloor drilling, and isotopic and microarray-based genomic analysis, have been used to reveal the geological processes responsible for the seeping activities and the biogeochemical processes related to them. The geophysical signature associated with gas seeps mainly includes the acoustic turbidity in the subsurface, the anomaly of the backscattering intensity at the seabed, and the gas plumes observed in the water column. Pore water and methane-derived authigenic carbonate archive the intensification of methane seepage and the paleoenvironment changes at different time scales. The methane feeding of the seeps in the OT was generated mostly via the microbially mediated process and has an origin mixed by thermogenic hydrocarbon gas in the middle OT. Sulfate-driven and Fe-driven anaerobic oxidations of methane are suggested to be the key biogeochemical processes, which would shape the material cycling in the seeping environment. The future research on the cold seeps in the OT is worth looking forward to due to its geographic and potential geologic links with the nearby hydrothermal activities. Multidisciplinary studies are expected to concentrate on their link with the undiscovered gas hydrates, the amount of methane transferring into the oceans and its impact on the climatic change, and the evolution of the seeping activities accompanied by the biogeochemical processes.
1. Introduction
Cold seeps are seafloor expressions of the upward migration of methane-rich gases through the marine sedimentary succession [1–3]. This migration involves the varying gas fluxes with time at the seabed and the alternation between the seeping and the diffusion of gases within an area smaller than a couple of square kilometers at one seeping site [1]. It has attracted increasing scientific attention since it was detected by side-scan sonar offshore Nova Scotia, Canada [4]. Methane escaping from the submarine seeps constitutes an important part of the output of the carbon from the marine sediments into the oceans [5]. This makes the worldwide seeps a window to evaluate the impact of the methane on climatic change and study the diverse methane-dependent ecosystem [6]. The predictable consequences of considerable methane seepage from seafloor include the amplification of ocean deoxygenation and acidification and, which is still under debate, the atmospheric greenhouse gas concentrations [7–9].
The cold seeps out of the shelf of the East China Sea in the Okinawa Trough (OT) were firstly found along its western slope of the middle part (~26.2°N) during the scientific cruise in 2000 [10]. Since then, there has been some scientific cruises and geological surveys over the broad marine region (Figures 1 and 2), which provide geophysical and geochemical data (Tables 1 and 2) for continuous research and significantly contribute to the understanding of the methane seepage in the OT. Multiple ship-based approaches have been used to focus on these seeps of different spatiotemporal dimensions. The acoustic ones mainly consist of seismic and echo-sounder, and together reveal the locations of the seeps and their features from the subsurface and the seabed to the water column [10–16]. The pore water and the authigenic minerals serve as the main geological record of the seeps and have been sampled by grabbing of remotely operated vehicles (ROVs), dredging and gravity-piston coring, and seafloor drilling. Geochemical investigation of these samples reveals the involved biogeochemical processes, the origin of the methane, the interaction between the pore fluids and the seep-impacted sediments, and the evolution of the seeps [17–28]. The seeps studied most in the OT are in its northern section (30.5°N-30.8°N) and were detected by R/V Kexue Yihao from July to August 2013 [18]. These unnamed seeps become the first group of targets of multidisciplinary research.
... Co-existing gas could appear as (i) as connected inclusions in hydrate allowing local viscous flow between those inclusions and the macropores (sub-micro squirt flow), (ii) as gasfilled cavities in the pores promoting local viscous flow from different aspect ratio pores generated when hydrate grows (micro squirt flow), and (iii) as sub-spherical bubbles in the pores allowing gas bubble resonance damping effects. The peak attenuation frequency and magnitude due to gas bubble damping depends on the concentration of gas bubbles in the pore space and on gas bubble size (e.g., Anderson & Hampton, 1980a, 1980bDogan et al., 2017;Marín-Moreno et al., 2017). As gas bubbles can exhibit a distribution of bubble sizes, again a superposition of different frequency attenuation peaks can occur. ...
Gas hydrates are ice–like compounds found in marine sediments and permafrosts. A significant fraction of all known hydrocarbons in nature is in the form of hydrate. Gas hydrates are a potential energy resource, with possible roles in seafloor slope stability and climate change. As such, improved geophysical methods are needed to identify and quantify in situ natural hydrates to better study their potential impacts. Current estimates of the distribution and volume of gas hydrates vary widely, by orders of magnitude, largely because of uncertainties in geophysical inversion results. The presence of hydrate affects the geophysical properties of the host sediment, creating anomalies that can be detected by seismic or electrical methods measurements. However, the precise relationships between measured geophysical properties and hydrate content (and distribution) are not fully understood, leading to uncertainties in hydrate estimates. Previous studies have shown that both the hydrate saturation (content) and its distribution (morphology or habit) affect the geophysical properties of the host sediment, and separating these effects presents a challenge to geophysical data interpretation. As this knowledge is generally required to interpret field data, this thesis instead seeks to gain this understanding from controlled laboratory experimental studies.
I studied laboratory hydrate formation and dissociation in Berea sandstone and Leighton Buzzard sand to understand their effect on P- and S-wave velocities and attenuations, and on electrical resistivity. I used high resolution synchrotron radiation X-ray tomography (SRXCT) to visualize the pore-scale evolution of hydrate morphology with saturation. These observations could be important for seismic data interpretation in terms of hydrate content and sediment strength, which are needed for natural resource and geohazard assessments (also for joint seismic and electromagnetic survey data interpretation). Hence, I was able to observe how hydrate distribution within the pores (morphology or habit) changes with hydrate formation and dissociation, and how these changes affect the P- and S-wave velocities and attenuations. I calculated hydrate saturation continuously from changes in pressure and temperature and independently from electrical resistivity during hydrate formation and dissociation. I applied a new rock physics model to relate P- and S-wave velocities and attenuations with changes in hydrate saturation and morphology.
I found that not all the gas formed hydrate, even when the system was under hydrate stability conditions with excess water. The synchrotron CT results suggest that the dominant mechanism for co-existing gas is the formation of hydrate films on gas bubbles; these bubbles either rupture, releasing trapped gas, or remain trapped within an aggregate of hydrate grains. From a geophysical remote sensing perspective, such co-existing gas could cause errors in hydrate saturation estimates from electrical resistivity as both gas and hydrate are resistive compared to saline pore fluid. I saw that hydrate starts forming in the pore-floating morphology (where hydrate grains are surrounded by brine) and evolves into the pore-bridging morphology (where hydrate connects mineral grains). Eventually, hydrate from adjacent pores joins and forms a pore hydrate framework, interlocking with the sand grain framework and separated by thin water films. I was able to relate these changes in morphology to our elastic wave measurements using the HBES (Hydrate Bearing Effective Sediment) rock physics model. For low hydrate saturations, both P and S wave velocity follows the pore-floating model curve. As hydrate formation continues, the P-wave velocity follows the pore-bridging model curve, similar to other studies. In contrast, the S-wave velocity was lower than the pore-bridging model but higher than the pore-floating model curves. I think that the presence of water films between hydrate and the rock frame inhibited the ability of pore-bridging hydrate to increase the frame shear modulus. The higher S-wave velocity than the pore-floating model predictions is likely due to interlocking rock and pore-bridging hydrate frameworks. The magnitude of relative changes in attenuation is much higher than that of velocity due to changes in hydrate content and distribution. Elastic wave attenuation frequency spectra between 448 and 782 kHz show systematic and repeatable changes during hydrate formation and dissociation. In our experiments, the dominant mechanism of attenuation and velocity changes with an increase in hydrate saturation is (i) a decrease in methane gas bubble radius and (ii) an increase in secondary porosity with hydrate formation. The accurate measurement of both velocity and attenuation at multiple frequencies in the pulse-echo system allow us to constrain the dominant attenuation mechanisms using the HBES rock physics model.
Overall, I conclude that hydrate-sediment systems are complex with interlocking solid hydrate aggregate and host grain frameworks separated by water films, with isolated pockets of gas within the hydrate. Such an interlocking pore hydrate framework and co-existing gas, if widespread in nature, should be considered in hydrate quantification from elastic wave velocities. For more reliable estimates of in situ hydrate, multiple geophysical parameter measurements are required (e.g., P and S wave velocities and attenuation, electrical resistivity, and at multiple frequencies), and hydrate estimates from seismic velocities alone could lead to significant errors at low hydrate saturations (< 40%).
... The high gas content in the sediment produces significant errors in regard to sediment thickness calculation as gas represents a barrier to sound penetration. The effects of free gas on hydroacoustics are well-investigated (Anderson and Hampton 1980;Anderson and Bryant 1990;Abegg and Anderson 1997;Lurton X. 2002). The sediment detection line derived from the 38 kHz (as shown in Fig. 2) is strongly affected by the high volumetric gas content. ...
Siltation has significant economic and social impacts as it directly reduces the useable amount of water in reservoirs. Giving a solution to the issue of sedimentation is a complicated task and maybe one of the most important engineering and environmental challenges of the 21 st century. The deposited volume and the distribution pattern of the sediment are often unknown and not easy to assess. The sedimentation process is highly dynamic, initially due to the hydrological conditions of the incoming rivers, but also due to common internal phenomena like resuspension or density currents. Sediment remediation measures such as mechanical sediment removal or flushing are planned based on the sediment thickness distribution and the overall sediment volume/ mass. Often, the sediment thickness is calculated through topographic differencing between the pre-impoundment reservoir lake bottom and the actual lake bottom. However, it is common that the previous depth distribution map is not available or in insufficient quality. In this regard, alternative measurement techniques have to be taken into consideration. In this study, we assessed the best possible approach depending on the characteristics of the sediment and of the reservoir. We combined three different acoustic systems (a mul-tibeam, a sub-bottom profiler, and a single beam dual frequency system) with sediment coring and dynamic free fall penetrometer measurements for an improved assessment of sediment stock and sediment distribution in the Passaúna Reservoir. Our results showed that topographic differencing could not be applied, as the data for the pre-impoundment lake bottom was insufficiently accurate. The parametric sub-bottom profiler could detect the sediment thickness in high accuracy, but significant limitations were recorded in areas with high gas contents. The dual-frequency echosounder derived the sediment thickness with a normalized mean absolute error of 56% due to the high volumetric gas content in the sediment. The dynamic free-fall penetrometer showed satisfying results compared to the other systems. The normalized mean absolute error was 22%, and sediment thickness could be detected in areas with up to 1.8 m of sediment. Sediment coring is also a reliable technique for sediment thickness determination. However, the results showed that if only traditional coring devices are used (gravity corer), the limited penetration depth of the equipment combined with sampling disturbances often prevent a correct assessment of the sediment thickness. The overall results of this study can help for an improved decision-making regarding reservoir management. The accurate assessment of sediment volume and distribution can reduce costs for sediment removal and assist in having a precise overview of the reservoir lifetime.
... The high gas content in the sediment produces significant errors in regard to sediment magnitude calculation as gas represents a barrier to the sound penetration. The effects of free gas on hydroacoustics are well-investigated (Anderson and Hampton 1980;Anderson and Bryant 1990;Abegg and Anderson 1997;Lurton X. 2002). The sediment detection line derived from the 38 kHz (as shown in Figure 6-10) is strongly affected by the high volumetric gas content. ...
Water and energy are the two key aspects driving economic and social development of a region. River impoundments are important structures for providing water, in case of domestic use, irrigation or mining and for providing energy as in the case of hydropower. The human interference in the riverine systems for creating these reservoirs is accompanied with several drawbacks. One and most important: ecosystem disruption. The reservoirs, which cause the disruption of a river continuum, also suffer from it by having a limited lifetime. Rivers are dynamic systems, which transport large amounts of organic and mineral material from the mountains until the sea. When they are impounded, this continuum is divided and the reservoir becomes the first sink of the particles. In order to plan remediation measures, a major scientific and engineering challenge is the assessment of sediment amounts that reach the reservoir in a certain time. The sediment volume/mass can be assessed via either the monitoring and modelling of sediment input from the hydrological catchment or, by measurements of the sediment volume in the reservoir. In the first case the spatial and temporal scale in which sediment mobilization takes place in the watershed, in addition to the episodic nature of sediment formation, make it difficult to derive reliable assessments of sediment input. On the other hand, the underwater environment and the spatial extent of the reservoir contributes also in the lack of reliable results concerning volumetric assessment of sediment.
This study aims at a better assessment of both aspects of sediment input and reservoirs’ sediment accumulation. The first part of this thesis deals with the quantification of erosion and sediment input from a watershed via modelling. The rapid population growth in many regions has dictated intense land use and landcover changes. These imply the usage of more dynamic models. Technological advancements in satellite imagery make it possible to improve the spatial and temporal resolution of the models but the overall effects of the integration of this data on the results are still not fully investigated. For assessing the improvement due to the technological advancement, the case of Passaúna catchment, located in southeast Brazil was examined. For this catchment, it was possible to quantify the sediment input and soil loss interanual dynamics in a monthly timestep, and to evaluate to what extent the inclusion of freely available satellite imagery can improve the modelling results. In other words, the integration of freely available Sentinel 2 satellite data made it possible to reduce the time and spatial resolution in comparison to the existing similar approaches.
The second part of the thesis deals with the quantification of the sediment volume in the Passaúna reservoir. In this study, five different remote sensing as well as conventional and proxy sediment sampling techniques are integrated for increasing the accuracy of sediment volume assessment. At the end, an accurate assessment of the sediment volume in the reservoir was
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achieved. In addition, a guiding diagram to choose the most suitable sediment detection method, depending on sediment characteristic (sediment magnitude and biochemical activity) was derived.
The results of both sections are closely related as the sediment input from a watershed is also the sediment amount that should be found in a reservoir like Passaúna where the trapping efficiency is ~100%. In this case, there is a difference of almost 50% between the modelled sediment input and the sediment stock in the reservoir. The most important factors contributing in this discrepancy are the non-inclusion of gully-channel erosion in the sediment input model, errors in the calculation process, internal production of the reservoir, and errors in the measuring process.
In overall based on the results of this thesis, the most important findings consist in the successful integration of freely available satellite imagery in a modelling approach to improve the sediment input assessment, and the combination of several methods for an accurate assessment of reservoir siltation. The findings of this thesis can contribute in bridging the gap between the two aspects of sediment budget by initially achieving an accurate reservoirs’ sediment stock assessment and secondly by quantifying the discrepancy of each contributing factor for a case study.
... Such kind of geophysical features has been documented along continental margins such as the Fram Strait (Plaza-Faverola et al., 2015) and well accepted as a member of the seal bypassing system (Cartwright et al., 2007). The cylindrical seismic anomalies have been interpreted as the cluster of near-vertical fractures filled with gas-rich pore fluids, which can cause a decrease of P-wave velocity, severe ray bending, signal scattering and high transmission loss (Anderson and Hampton, 1980;Cartwright and Santamarina, 2015). Comparable to the observations of the gas chimneys in offshore of Mauritania (Li et al., 2017b), the seismic features within the five gas chimneys in Fig. 6d at each reflection are similar, displayed as either positive relief or disturbed reflection (Fig. 6d). ...
Methane trapped in marine sediments surrounding continental margins can be driven by tectonic stress to escape into the oceans. What geological processes control their spatial occurrences in an extension-dominated tectonic setting is not fully understood. Here we use the multibeam echo-sounder (MBES) and the multichannel seismic (MCS) data to reveal the distribution of the gas seeps in the back-arc basin of the Mid-Okinawa Trough in the years 2013–2016. They are manifested by (a) the gas plumes in the water column, occurring either in isolation or as clusters; (b) the irregularly shaped areas of the medium- to the high-backscatter intensity at the level of the seabed and (c) some seismic gas chimneys below these areas. The spatial combination of these signature points toward the widespread upward migration of the gas-rich pore fluids. Two types of gas seeps (A and B) sustained by methane supply have been proposed. The results of the isotopic analysis (δ¹³CCH4 and δDCH4) show that methane having a mixture of thermogenic and microbial sources migrated upwards along the faults to feed type A gas seeps, while in type B ones the thermogenic methane preferentially migrated through the fracture clusters embodied as the seismic gas chimneys. The tensile faults formed during the ongoing rifting episode of the incipient back-arc basin and provided the permeable migration conduits for the gases trapped in the shallow subsurface. We propose that this faulting driven by back-arc extension determines, to a large extent, the potential locations of the temporal gas seeps in the Mid-Okinawa Trough. Some of the other free gases accumulated in the less faulted sediments and the spatial distribution of their seepage is controlled by the topographic high. Tensile faulting can disturb the overpressure build-up within the shallow sediments by releasing it prior to the seal failure in the extension-dominated regime.
... In other studies [16,22,23], dynamic light scattering (DSL) was used to analyze the size distribution of MB and the cytometer was used to determine the concentration of MB. Compare to those methods, assessment using the microscope that is utilized in the current study provides more information on the particle gathering and particle interaction. ...
... The loss of reflectivity (acoustic blanking) is another important indicator of hydrocarbons in the sedimentary layers. The presence of gas in a system can reduce the compressional wave velocity, increase its attenuation, and promote the dispersion of acoustic energy (Anderson and Hampton, 1980). Lateral reflectivity losses are observed in the seismic section shown in Fig. 14B, which were highlighted by FSE processing. ...
A special frequency spectrum expansion processing (FSE) was applied to seismic lines to identify gas chimneys (GC) in the Parnaíba Basin (NE Brazil). GC are recognized by typical vertical and low frequency chaotic disturbances in the seismic data, which are interpreted as being caused by the migration of fluids or free gas. These structures are important hydrocarbon indicators and have become a powerful exploratory tool, especially for frontier basins. In this study, an abundance of GCs was found in vast areas of the Parnaíba Basin, increasing the oil potential of this important South American Paleozoic basin. The new methodology was applied to seismic lines of the Parnaíba Basin and was used to characterize the GCs with greater security, which reduces the exploratory risks and suggest the existence of a new petroleum system associated with the pre-Silurian and Silurian deposits present in the basin grabens. The seismic expressions of the GC were successfully characterized by the FSE, and the origin of the gas chimney was associated with the regional geological structural framework, especially with the Transbrasiliano Lineament (TBL). In addition, theoretical models were developed to classify exploration plays associated with the GCs based on their probabilities of success of being economic hydrocarbon accumulations. Our results indicate a good potential for new hydrocarbon discoveries in this exploration frontier basin.
... as acoustic blanking and acoustic turbidity (Anderson and Hampton 1980;Schroot, Klaver, and Schüttenhelm 2005). ...
The West Delta Deep Marine (WDDM) concession is one of the abundant natural gas resources in the world characterized by the presence of several active gas chimneys conduit feeding pock-marks. The detection of shallow gas accumulations has been gaining importance in hazard assessments before and during offshore drilling operations, whereas there is no way to estimate the exact pressure of the gas content in sediment to expect the potential gas hazards before and during the offshore installations, operations, and drilling. Monitoring of the gas chimneys and pockmarks plays an important role as an early warning for the oil industry to more focus on this kind of activities which may represent a catastrophic event in the future for the offshore installations at the WDDM region. Moreover, the "'return period" of the pockmark activities is a region-dependence parameter according to the continuity of the gas supply and stability of the seepage pathway. However, in this study, the identification of the direct and indirect evidence of the shallow gas presence and migration pathways to the seafloor is achieved by extracting various-chosen seismic attributes, such as Root Mean Square, envelope, energy, the cosine of the instantaneous phase, variance, and chaos attributes. These attributes improve the imaging of several seismic evidence such as bright anomalies, enhanced reflectors, gas chimneys, and associated seabed features formed by the migration of the fluids from deep reservoirs through gas chimneys up to the seafloor. The presented profiles clearly show that gas chimneys and pockmarks combined with other associated features represent common features of the WDDM concession that can severely impact during and after offshore drilling operations in the aspects of safety, environment, and cost.
... Die Reflektionen am Gas sind durch einen hohen Impedanzunterschied (rot) charakterisiert, was ein weiteres Eindringen der Schallwellen in das Sediment verhindert. Informationen unterhalb dieses Reflektors werden dadurch maskiert (Anderson & Hampton, 1980). ...
Der 52 km² große, mesohaline Wasserkörper der Schlei ist das größte Brackgewässer in Schleswig-Holstein. Neben natürlich vorhandenen Stressfaktoren sind es die in der jüngeren Vergangenheit hinzugekommenen anthropogenen Einflüsse, die die Schlei zu einem äußerst empfindlichen Gewässer gemacht haben. Ihr derzeitiger ökologischer Zustand wird als schlecht eingestuft. Als Grundlage für eventuelle Maßnahmen zur Verbesserung dieses Zustandes wurde die Schlei von März bis April 2017 umfassend mit hydroakustischen und sedimentologischen Methoden kartiert. Mit dem Seitensichtsonar und einem parametrischen Sedimentecholot konnten auf 400 km Profillänge entsprechend 30 km² das Sedimentverteilungsmuster und der vertikale Aufbau des Meeresbodens erfasst werden; die übrigen Bereiche waren aufgrund zu geringer Wassertiefe oder Hindernissen (z.B. Stellnetze) nicht zugänglich. Um diese Daten verifizieren und sedimentologisch interpretieren zu können, wurden 166 Greiferproben und 24 Sedimentkerne entnommen. Neben der Aufbereitung der digitalen Daten und der Erstellung von Karten und Profilen erfolgten Korngrößenanalysen und die Bestimmung von Kohlenstoffgehalten. An den Sedimentkernen erfolgten Nährstoffanalysen des Porenwassers, Röntgenfluoreszenzanalysen, Röntgenaufnahmen sowie Altersdatierungen zur stratigraphischen Einstufung.
Von der kartierten Sedimentoberfläche bestehen 22 km² (74%) aus mehr als 50% Silt und Ton (Korndurchmesser <63 µm). Dieses Sediment tritt überwiegend in der inneren und mittleren Schlei auf, wobei die mittlere Schlei generell etwas feinere Sedimente aufweist. Der übrige Anteil der kartierten Fläche liegt im Korngrößenbereich des Sandes. Das Sediment in den Engen besteht überwiegend aus Sand und einem Feinsedimentanteil (<63 µm) zwischen 10% bis 50%. Dieses Sediment tritt auch im Mündungsgebiet der Füsinger Au, in der Mittleren Schlei und in Gebieten auf, wo spätglaziale Beckenablagerungen (Sande und Silte) den Meeresboden leicht erhöhen. Weiterhin kommt Sand in der Äußeren Schlei, östlich der Enge von Rabelsund, bis Maasholm vor. Sande mit weniger als 10% Silt und Ton kommen besonders in den Ufergebieten der Inneren Schlei vor. Vom Meeresboden aufragende Sandinseln gibt es in den zentralen Bereichen der Kleinen und Großen Breite, sowie der Büstorfer Breite, hier auch abseits der Ufergebiete. In der Äußeren Schlei bilden Sande östlich von Maasholm im Bereich der Fahrrinne die Sedimentoberfläche. Südlich von Maasholm treten im Untergrund geschichtete Sandkörper mit über 3 m Mächtigkeit auf. Weiter östlich, in Richtung der Schleimündung, bilden Sande Rippelfelder aus. In den Engen von Missunde und Kappeln liegt teilweise sedimentfreier Schill an der Oberfläche. Geschiebemergel bzw. aus diesem herausgewaschene Blöcke und Steine befinden sich nördlich von Missunde, südlich von Kappeln und nahe der Schleimündung. In der Inneren Schlei konnten im Untergrund liegende Gyttjen, Schilllagen und Torfe, Letztere mit einem Alter von ca. 6.000 Jahren vor heute, nachgewiesen werden. Unterhalb dieser organogenen Sedimente befinden sich glazifluviale Schmelzwassersande. In der Mittleren Schlei können quer zu ihrer SW – NO - Ausrichtung immer wieder aufragende, geschichtete kalkreiche Beckensilte oder Sande beobachtet werden.
Der organische Kohlenstoffgehalt des Oberflächensedimentes reicht bis zu 12,8% mit auffällig hohen Gehalten vor Schleswig, in der westlichen Großen Breite und in der Büstorfer Breite. In
der innersten Schlei sind die Werte gegenüber früheren Untersuchungen etwas geringer. Östlich der Brücke von Lindaunis nehmen die Organikgehalte ab.
Das organikreiche, siltige Sediment setzt sich auch im Vertikalprofil fort und erreicht durchschnittliche Mächtigkeiten in der inneren Schlei von 29,6 cm, in der mittleren Schlei sind es 28,5 cm. In der südlichen Großen Breite und der Karschauer Breite kommen auch Mächtigkeiten über 50 cm vor. Es wird als Halbfaulschlamm klassifiziert, zeigt in den tieferen Lagen des Vertikalprofils aber auch einen sapropelartigen Charakter. Zu den Ufern hin nehmen die Mächtigkeiten dieses Sedimentpaketes ab. Nährstoffmessungen am Porenwasser der Sedimentkerne zeigen für den Beprobungszeitraum oxische Verhältnisse in den oberen Zentimetern. Im Sedimentprofil zeigen die Nährstoffkonzentrationen im Porenwasser für die innere Schlei etwas höhere Werte als für die mittleren Schlei.
In früheren Untersuchungen wurden für die innerste Schlei Sedimentationsraten bis zu 8 mm/Jahr festgestellt. Eine Fortsetzung dieser hohen Sedimentationsraten konnte im Rahmen dieser Untersuchungen nicht festgestellt werden. Weder die gemessenen Mächtigkeiten des siltigen Oberflächensedimentes noch die Messungen von Sedimentationsraten mit den Radionukliden 210Pb und 157Cs können derart hohe Sedimentationsraten bestätigen. Ein Kern aus der mittleren Schlei zeigt, dass bis ca. 1980 bestehende Sedimentationsraten in der Größenordnung von 4,2 mm/Jahr auf 2,2 mm/Jahr abgesunken sind.
Die Menge des siltigen Oberflächensedimentes, das in der Vergangenheit nur für die innerste Schlei bestimmt wurde, ist für diesen Bereich im Zeitraum von 35 Jahren (1981 – 2017) mit 1,36 ± 0,12 x 106 m³ nur leicht angestiegen. Es sollte jedoch berücksichtigt werden, dass dieses Sediment auch in der Großen Breite und in der mittleren Schlei vorkommt. Die Gesamtmenge dieses Sedimentes beträgt für die innere und mittlere Schlei ca. 6,3 ±0,33 x 106 m³.
Neben Schill, Blöcken und Steinen, die als Hartsubstrate ein hohes Besiedlungspotenzial innerhalb des vorwiegend siltig bis sandigen Sediments der Schlei darstellen, konnten Seegraswiesen vor Maasholm und Miesmuschelbänke vor Kappeln und Schleimünde mit Hilfe der Sedimentkartierung ausgewiesen werden.
... A velocity of 1514 m/s (average value of the speed of sound in the water column obtained with the SVP 15 Reson Sound velocity profiler before each survey) was used for the conversion. This value is similar to the previously used values for sound velocity of the first few meters of seabed sediments in the northern Adriatic (Trobec et al., , 2018Ronchi et al., 2018bRonchi et al., , 2019 and corresponds to average values for water saturated mixed sandy and fine-grained sediments (Anderson and Hampton, 1980;Yuan et al., 1992;Innomar, 2009;Qiu et al., 2019). The bathymetry of the studied area is based on multibeam and singlebeam data acquired with the Reson SeaBat 8125 sonar (at 455 Hz) and Elac Hydrostar 4300 sonar (at 50 and 200 Hz) which were elaborated in the Reson PSD 2000 software in order to create a 10 m × 10 m cell size digital bathymetric model (Fig. 2). ...
We use geophysical and sedimentological data to study the high-resolution acoustic stratigraphy and sedimentology of a Late Quaternary alluvial plain (located in the Gulf of Trieste, northern Adriatic) which was transgressed during the post-LGM sea-level rise. With sub-bottom sonar profiling we determined six acoustic facies, of which the top five acoustic facies (A-E) were sampled with a gravity corer. We performed core descriptions, radiocarbon dating and granulometric analysis with a laser particle analyser. The acoustic facies and their corresponding sediments are associated with three general sedimentary environments: Last Glacial toYounger Dryas alluvial (E-B), Early Holocene transgressive (Ab) and Holocene shallow marine (Aa). Acoustic facies E with low-amplitude chaotic reflection geometries is represented by cross-stratified sandy mud deposited by braided or wandering rivers. Acoustic facies D with high-amplitude and high- frequency sub-horizontal reflection geometries is represented by graded deposits (sandy mud and sandy clay grading into clay) of braided or wandering rivers. Acoustic facies C and B with individual low to middle amplitude reflections in an otherwise transparent facies are represented by overbank fine-grained sediments deposited by a meandering river systems.The top surface of AF B presents the Younger Dryas paleosurface in the geophysical record. Acoustic facies Ab with onlapping and concordant middle to low amplitude reflection geometries is represented by bioclastic transgressive sandy mud containing brackish mollusc assemblages. Acoustic facies Aa is acoustically transparent and contains bioclastic sandy mud with shallow marine mollusc assemblages. Our work provides a reference for future studies of the Late Pleistocene-Holocene transition on well-preserved low-gradient mid-to-low latitude continental shelves.
... Frequency-dependent amplitude anomaly information is widely used for hydrocarbon detection since seismic wave energies exhibit obvious high-frequency attenuation in or beneath gas-bearing layers [2][3]. Spectral decomposition is commonly used for detecting the frequency-dependent amplitude anomaly information through a series of common frequency volumes [4]. ...
We apply the variational mode decomposition (VMD)-based instantaneous centroid estimation method for interpreting a 11-meters main gas-producing interval of the Longmenshan foreland basin, China. Five VMD-based instantaneous centroid slices are generated for fine interpretation. The results are evaluated by the measured well data. The application results show that the VMD-based instantaneous centroid estimation method targets the 11m main gas-producing interval very well. Field data application shows that the VMD-based instantaneous centroid estimation method can give a fine interpretation of hydrocarbon-related information with high-precision.
... πR γP ρ f 1/2 3 / (Anderson and Hampton, 1980) f = resonant frequency. R = radius of the cavities. ...
The disinfection potential of ultrasound was evaluated for fish production in aquaculture systems by investigating the sensitivity of a wide range of different model organisms representing different taxa of common fish pathogens such as bacteria or parasites and algae at a size range of a few micrometers to 1 mm. Therefore, dose-dependent inactivation rates depending on consumed energy (kJ/L) at low (20 kHz, LFUS) and high (850 kHz, HFUS) frequency ultrasound were determined in in-vitro tests using a laboratory-scaled set-up for three zooplankton and five algae species as well as for heterotrophic marine and fresh water bacteria separated from recirculating aquaculture system water. LFUS and HFUS were effective against most tested eukaryotic organisms even at low energy input and the dose-dependent inactivation could be well described by functions of an exponential decay. HFUS seemed to be more effective compared to LFUS regarding the inactivation of bigger-sized algae species (≥14 μm). Within the tested zooplankton species, no systematic correlation between the size of the organisms and the effect of ultrasound frequency on the inactivation efficiency became apparent, suggesting a strong species-dependency likely caused by additional relevant morphological and physiological factors. Our data also suggest ultrasound treatment alone to be insufficient for an efficient bacterial reduction as LFUS and HFUS did not significantly reduced total viable counts even at high energy inputs up to 144 kJ/L. Measurements of total residual oxidants (TRO) revealed a higher formation potential of sonochemically-produced oxidants for HFUS compared to LFUS as well as an increased TRO accumulation at saline waters most likely due to the formation of more persistent halogenated secondary oxidants. Hence, particularly when HFUS is applied in a marine process chain total residual oxidants should be monitored in order to avoid deleterious impacts on animal health.
This study proposed an operation mode that actively releases shallow gas from an ultra-deep water shallow gas reservoir by drilling pilot holes. This operation mode can help discharge underground shallow gas in advance and better guarantee the security of the subsequent drilling operation. The flow law of shallow gas in the release process was simulate according to the field data of the Shenhu area in the South China Sea to study the mechanism and feasibility of actively releasing shallow gas in ultra-deep water area. The findings of the study revealed that shallow gas and seawater driving each other behavior (DEO behavior) in the shallow gas reservoir and borehole zones (underground zone: refers to the zone comprising the borehole and shallow gas zones) existed in the early release process, and the DEO behavior exhibited an obvious cyclic phenomenon. Simultaneously, the morphological development of the shallow gas spewing into the static seawater and real seawater zones was studied to obtain a clearer understanding of the gas morphological development in seawater. Based on the above findings, further investigations revealed that the operation that released shallow gas with 8-3/8″, 9-5/8″, and 12-1/4″ pilot holes in the target field did not affect the buoyancy of the seawater or sink the platform; there was almost no fluctuation to the sea surface. This study examined and compared the shallow gas flow law and remaining shallow gas volume in the reservoir with time under the application of 8-3/8″, 9-5/8″ and 12-1/4″ pilot holes. Consequently, it found that under the condition of target field larger hole sizes exhibited lower collapsing and dispersing heights (the distance between the last shaped shallow gas bubble about to burst and the mud line) and faster shallow gas release velocity. Furthermore, a numerical simulation study on the operation mode of releasing shallow gas from an ultra-deep-water shallow gas reservoir by drilling pilot holes can provide theoretical support for subsequent shallow gas treatment decisions in ultra-deep-water drilling construction.
Reflection and transmission of acoustic waves at the interface between water and a porous medium saturated with bubbly liquid and the porous medium saturated with a bubbly liquid–water interface at normal incidence of these waves on the indicated interfaces have been studied theoretically. It is shown that there exists a range of frequencies, for which reflection of acoustic waves from the interface between water and a porous medium saturated with bubbly liquid is similar to reflection of these waves from a free surface, and for reflection of acoustic waves from the medium saturated with bubbly liquid/water interface it is similar to reflection from a rigid wall.
Superheated nanodroplets (NDs) were recently proposed for in vivo proton range verification, owing to their ability to vaporize into echogenic microbubbles (MBs) upon exposure to ionizing radiation. In a previous publication, vaporization events were detected with 2D Ultrasound Localization Microscopy (ULM) based on a pulse-echo method. Here, we introduce P-ULM, a passive version of ULM, based on the detection of acoustic signatures emitted by vaporizing NDs, without actively transmitting ultrasound. Due to the lack of a time reference for the trigger of the ND vaporization, the time differences of arrival to each transducer element are used to retrieve the position of vaporizing NDs. P-ULM, compared to ULM, can continuously detect and super-localize sparse radiation-induced vaporization events with inherent specificity against already existing microbubbles, which otherwise would hinder range verification in the presence of vascular flow. We evaluated the localization performances of both methods theoretically and experimentally, by interleaving active and passive acquisitions on ND-phantoms irradiated with protons. P-ULM offered a higher sensitivity to vaporizations, as it detected twice as many events as ULM. Both methods retrieved, in the acoustic lateral direction, the proton range and range dispersion with sub-millimeter accuracy. In the acoustic axial direction, despite a degraded theoretical resolution limit, P-ULM retrieved the proton spot size with an accuracy similar to ULM. Importantly, P-ULM detected vaporization events with high specificity in the presence of flowing MBs, which makes the technique a candidate for in vivo proton range verification in the presence of flow.
Offshore Le Croisic (West coast of France), the test site SEM-REV, offers opportunity to study the impact of a marine renewable energy structure on the benthic environment. A buried electrical cable that links the sea site to the shore, crosses over a specific benthic ecosystem built by small bio-engineer organisms: the tube-dwelling amphipods Haploops. To define the effect of the cable installation on this particular ecosystem, three different approaches are used. First, acoustics surveys were conducted, once a year, over three years (2016-2018) to monitor the physical impact after the cable burial in 2012. The surveys allowed to analyse and map the space-time evolution of the Haploops settlement. Our results confirm the systematic overlap between Haploops and active pockmarks field, already observed in other sites of South-Britanny.
Then, the study of sedimentary geochemistry allows to clarify the possible link between methane rises and Haploops, and explores the influence of the thick layer of tubes on sediments. The flow of water through the tube system modifies the distribution and flow of chemical elements to and from the sediment. The latter appears to be an important source of nutrients (P and N) that could support the local primary production required to maintain dense Haploops population
Finally, benthic foraminifera faunas were used to assess the environmental change brought by the electric cable installation and to describe the environmental quality status, inside and outside the cable trench.
Remote characterization and parameterization of gassy sediments have significant environmental importance for quantifying the global methane budget and assessing its impact on climate change. Acoustic techniques that have been developed hold advantages over direct sediment sampling (e.g., using pressurized and frozen cores), as they permit comparatively quick and cost-effective assessments over the large bottom areas. This paper proposes a non-invasive acoustic method that allows simultaneous assessments of the free gas content (Θ) and the thickness (d) of a gassy layer in the aquatic surface sediments. The method is based on amplitude measurements and frequency analysis of the bottom reflection coefficient in the wide frequency band (300-3500 Hz). The spatial variability of Θ and d in freshwater Lake Kinneret (Israel) is studied, where the upper sedimentary layer is characterized by a high organic matter content, high methane production rates, and a large Θ. The assessed values of Θ and d varied from 0.1% to 0.6%, and from 20 to 40 cm, respectively, depending on the location of measurements. These results are in reasonable agreement with gas void fractions measured directly in frozen sediment cores, where the depth-averaged Θ varied from 0.4% to 1.3%. The suggested methodology should have considerable practical implementation for remote spatiotemporal monitoring of shallow gassy sediments in aquatic ecosystems. (free paper link https://aslopubs.onlinelibrary.wiley.com/doi/10.1002/lom3.10506)
Seismic investigation in marine gas‐bearing sediments fails to get information below the acoustic mask created by free gas. To circumvent this problem, we combined collocated multichannel ultra‐high resolution seismic imaging, marine electrical resistivity tomography (MERT) and core sampling to study the physical properties of gas‐bearing sediments in the Bay of Concarneau (France). We obtained sections of compression (P‐) wave velocitvalues where free gas was identified in seismic data. We tested a joint processing workflow combining the 1D inversion of the MERT data with the 2D P‐wave velocity through a structural coupling between resistivity and velocity. We obtained a series of 2D resistivity models fitting the data whilst in agreement with. The resulting models showed the continuity of the geological units below the acoustic gas fronts which is associated with paleo‐valley sediment infilling. We were able to demonstrate relationships between resistivity and velocity differing from superficial to deeper sediments. We established these relationships at the geophysical scale then compared the results to data from core sampliand porosity). We inferred the porosity distribution from the MERT data. At the core locations, we observed a good agreement between this geophysical scale porosity and the core data both within and outside the gas‐bearing sediments. This agreement demonstrated that resistivity could be used as a proxy for porosity where no was available below gas caps. In these regions, the observed low resistivity showed a high porosity (60‐70%) down to about 10–20 m in depth in contrast with the surrounding medium with porosity less than 55%. These results support the hypothesis that failures inside the paleo‐valley sediment could control the gas migration. This article is protected by copyright. All rights reserved
Recent studies have confirmed the coexistence of gas hydrate (GH) and free gas (FG) in gas hydrate stability zone (GHSZ). However, the acoustic properties of the coexisting sediment, GH-bearing sediment, and FG-bearing sediment are needed to be specified, for discriminating these three coexisting phases using sonic logs. In this study, the compressional- (P-) and shear (S-) wave attenuations of the sediment with coexisting GH and FG are calculated from the sonic logs in the South China Sea (SCS). The obtained results reveal that the P-wave and S-wave velocities of the sediment with coexisting GH and FG are decreased by 14% and 11%, P-wave attenuation enhanced by 104%, and S-wave attenuation decreased by 44%, respectively, as compared with the sediment only containing GH at well SHSC-4J1. A similar result can be obtained for well GMGS6-SH02. The highest P-wave attenuation appears in the sediment with coexisting GH and FG, ∼0.66 at well SHSC-4J1 and ∼0.60 at well GMGS6-SH02, respectively. Moreover, cross plots of the measured P- and S-wave velocities and attenuations suggest that attenuation is more sensitive to the coexistence of GH and FG than velocity. The saturation of coexisting GH and FG in sediment can be calculated from the sonic velocities using rock physics modeling, a maximum value of ∼26% (av. 13%) at well SHSC-4J1 and a maximum value of ∼23% (av. 7%) at well GMGS6-SH02.
The available measurements of the acoustic attenuation coefficient, α, in aqueous suspensions of glass beads and sand are investigated for 10−3<𝑘𝑎<30 (where k is the acoustic wavenumber and a the grain radius) and volume concentrations, 𝜙, up to 0.65. The data are found to collapse substantially when dividing by volume concentration, consistent with the expected first-order linear dependence. Equations of the form 𝛼𝑎=𝐵1𝜙+𝐵2𝜙2, with ka-dependent coefficients, provide a prediction that is within a factor of 2 for low and intermediate values of ka.
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The focus of this chapter is P- and S-wave propagation in porous rocks. Starting from the wave equation for isotropic solids, we extend the description to poroelastic and anisotropic rocks, including dispersion and attenuation mechanisms. We emphasise the important link between static rock mechanics and dynamic rock acoustics. We outline the basics of seismic and sonic borehole measurements, and give a brief background of acoustic emission mechanisms with relevance to microseismicity.
Gassy sediments of shallow lakes are a source of permanent concern due to their contributions to sediment destabilization and global warming. In this study, a “microscopic” bubble model is suggested that describes shapes and sizes of methane bubbles in muddy aquatic sediments predefined by sediment mechanical properties. An effective media model specifying a bubble size distribution is designed based on the microscopic model. An acoustic approach for verification of this model is proposed. It is based on the estimation of the frequency dependence of sound reflection coefficient from gassy sediments, which exhibits the resonant behavior. An experimental implementation of the method is discussed.
The need to predict acoustic propagation through marine sediments that contain gas bubbles has become increasingly important for civil engineering and climate studies. There are relatively few in situ acoustic wave propagation studies of muddy intertidal sediments, in which bubbles of biogenic gas (generally methane, a potent greenhouse gas) are commonly found. We used a single experimental rig to conduct two in situ intertidal acoustical experiments to improve understanding of acoustic remote sensing of gassy sediments, eventually including gas bubble size distributions. In the first experiment, we measured sediment sound speed and attenuation between four aligned hydrophones for a quasi-plane wave propagating along the array. The second experiment involved a focused insonified sediment volume created by two transducers emitting coincident sound beams at different frequencies that generated bubble-mediated acoustic signals at combination frequencies. The results from sediment core analyses, and comparison of in situ acoustic velocity and attenuation values with those of water-saturated sediments, together provide ample evidence for the presence of in situ gas bubbles in the insonified volumes of sediments. These datasets are suitable for linear and non-linear inversion studies that estimate in situ greenhouse gas bubble populations, needed for future acoustical remote sensing applications.
The ability to detect and monitor any escape of carbon dioxide (CO2) from sub-seafloor CO2 storage reservoirs is essential for public acceptance of carbon capture and storage (CCS) as a climate change mitigation strategy. Here, we use repeated high-resolution seismic reflection surveys acquired using a chirp profiler mounted on an autonomous underwater vehicle (AUV), to image CO2 gas released into shallow sub-surface sediments above a potential CCS storage site at 120 m water depth in the North Sea. Observations of temporal changes in seismic reflectivity, attenuation, unit thickness and the bulk permeability of sediment were used to develop a four-stage model of the evolution of gas migration in shallow marine sediments: Proto-migration, Immature Migration, Mature Migration, and Pathway Closure. Bubble flow was initially enabled through the propagation of stable fractures but, over time, transitioned to dynamic fractures with an associated step change in permeability. Once the gas injection rate exceeded the rate at which gas could escape the coarser sediments overlying the injection point, gas began to pool along a grain size boundary. This enhanced understanding of the migration of free gas in near-surface sediments will help improve methods of detection and quantification of gas in subsurface marine sediments.
Owing to the decomposition of organic material and other reasons, the actual marine sediment contains gas bubbles, and the existence of gas bubbles will significantly affect the low-frequency acoustic characteristics of sediment. Therefore, it is significant to investigate the effect of gas bubbles on the low-frequency sound velocity in the sediment. Considering the uncontrollable environmental factors of field experiment, an experiment platform for obtaining acoustic characteristics of a large-scale gas-bearing unsaturated sandy sediment is constructed in the indoor water tank. Considering the long wavelength of low-frequency acoustic wave and the multipath interference in water tank, the transmitted acoustic signals are received by hydrophones which are buried in the unsaturated sediment. The sound velocity data (79-142 m/s) in the gas-bearing unsaturated sediment are acquired by using a multi-hydrophone inversion method in the bounded space for the first time in a 300-3000 Hz range, and the sound velocity data (112-121 m/s) are also acquired by using a double-hydrophone method in the same frequency range. The refraction experiments at different horizontal distances between the source and the hydrophones are conducted, which verifies the reliability of sound velocity data acquired by using the multi-hydrophone inversion method and the double-hydrophone method. At the acoustic frequency well below the resonance frequency of the largest bubble in the sediment, the pore water and the gas bubbles are regarded as an effective uniform fluid based on effective medium theory. On this basis, the density and the bulk elastic modulus of pore water in the effective density fluid model are replaced by the effective density and the effective bulk modulus of the effective uniform fluid, then a corrected effective density fluid model is proposed in gas-bearing unsaturated sediment. The numerical analysis indicates that when the gas bubble volume fraction is small (1%), a small increase in the gas bubble will cause a significant decrease in the effective bulk elastic modulus of sediment, but the density of pore water is much greater than the density of gas bubbles, the presence of a small number of gas bubbles hardly changes the density of pore fluid and certainly does not change the density of sediment, which results in a significant decrease at a low-frequency sound velocity in the gas-bearing unsaturated sediment. Furthermore, with the increase of gas bubble volume fraction, the sound velocity predicted by the corrected model gradually decreases, and the decreasing trend gradually becomes gentle. The corrected model reveals the effect of gas bubbles on the low-frequency acoustic characteristic of sediment. By analyzing the sensitivity of the predicted sound velocity to parameters of the model, the gas bubble volume fractions (1.07%-2.81%) of different areas are acquired by inversion according to the measured sound velocity distribution and the corrected model. In the future, it will provide a new method of obtaining the volume fraction and the distribution of gas bubbles in the sediment.
We study the effect of using different rheological schemes in models of Frenkel-Biot elastic waves of P-type in porous media. Two rheological schemes are considered-one with the bubbles and the other without. The bubble-including scheme consists of segments representing the solid continuum and bubbles inside the fluid, while the bubble-free scheme is represented by the standard solid-fluid rheological model. We derived the dispersion relations for the wave equations in their linear forms and analyzed the decay rate, λ, versus the wave number, k. We compared the λ(k)-dependence for the two rheologies under consideration using typical values of the mechanical parameters of the model. We observed, in particular, that an increase of the radius and the number of the bubbles leads to an increase in the decay rate.
The Seabed Characterization Experiment was carried out from March 5 to April 10, 2017 (SBCEX17) on the New England Mud Patch, approximately 90 km south of Martha's Vineyard. The SBCEX17 experimental site covers an area of 11 km × 30 km with water depth in the range of 75–80 m. The Sediment Acoustic-speed Measurement System (SAMS) is designed to measure sediment sound speed and attenuation simultaneously over the surficial 3 m of sediments. During SBCEX17, SAMS was successfully deployed at 18 sites, which were chosen to coincide with coring locations, with the goal of developing a geoacoustic model for the study area. In this article, a summary of SAMS operation during SBCEX17 is presented, as well as preliminary results for sediment sound speed and its spatial variation in the frequency band of 2–10 kHz. It is found that in mud, the sound-speed ratio is in the range of 0.98–1. Little dispersion was observed in this frequency band. Using the preliminary SAMS sound-speed results measured at different depths, the sound-speed gradient in mud within the surficial 3 m favors an exponential rather than a linear dependence at SBCEX17 site. Large gradients are observed for depth shallower than 1.5 m. For the sandy basement beneath the mud layer, the sound-speed ratio is as high as 1.105.
Modern and ancient delta plains in Louisiana that border the northern Gulf of Mexico are undergoing unprecedented rates of land loss. Subsidence due to compaction and loss of pore water is believed to be one of the primary land loss factors, whereas subsidence due to gas emission is generally believed to play a significant role only in the upper one or two meters of a wetland. Evidence to the contrary is presented here that suggests that degasification subsidence is a more important mechanism contributing to land loss than previously thought. In addition, potential degasification subsidence can be quantified if the depression from normal compressional and shear wave velocities is known. Velocity reductions of 30 to 75% from compressional wave velocity in water and extending to depths of tens of meters have been observed in a saltwater marsh in Louisiana. An order of magnitude reduction in velocity due to the presence of free gas has been observed previously in gas-rich sediments on the adjacent continental shelf, where biogenic gas is observed to depths of 1 km or more. The ratio of normal to depressed compressional wave velocities is used to calculate potential degasification subsidence.
Seismic reflection measurements in Suva Harbour have shown the existence of a mud layer between 50 and 140 m thick covering the bottom of the harbour entrance and the inner harbour. Average compressional wave velocities in the mud increase with increasing thickness of the mud layer, and vary from 0 · 54 km/sec to 1 · 10 km/sec, indicating a steep velocity gradient in the mud.
The low mud velocities can be explained by the presence of free carbon dioxide resulting from decaying organic matter; the existence of CO2 in the mud has been proved by chromatographic analysis. Taking into account a decrease of porosity and gas Concentration with increasing thickness of the mud layer (and water depth) using data from Lake Mead, the observed average velocities can be explained by a CO2 fraction which reaches a maximum value of about 0·2% at a hydrostatic pressure ot 3·8 atm and which decreases by about two orders of magnitude at 80 m below the top of the mud layer. The uppermost layer of the mud contains no gas.
In this experimental study evidence was obtained on the relationship between the velocity of sound and the moisture content of the soil. The velocity of sound in un- saturated soils at shallow depths was shown to be proportional to the 1/6 power of the effec- tive pressure. Thus the effective stress concept, used in conjunction with the Hertz theory, was proved valid.
When sound of high amplitude is transmitted into a liquid by means of a mechanical driving device, the ultimate limitation to the power that can be transferred is cavitation or breakdown of the liquid under high internal stresses. A study of cavitation has resulted in establishing the following results. Under steady-state conditions, light liquids filled with air cavitate when the negative acoustic pressure reaches the atmospheric pressure. When liquids are degassed, their natural cohesive pressure becomes effective and they will withstand a negative acoustic pressure. It is found that the total negative pressure required to cause cavitation is equal to the sum of the cohesive pressure—tensile strength—and the ambient pressure. Viscous liquids have a higher cohesive pressure and a proportionality has been established between the logarithm of the viscosity and the cohesive pressure. The amount of power that a liquid can withstand increases markedly as the pulse length is shortened.
An explanation of these phenomena is attempted on the basis of Eyring's theory of viscosity, plasticity and diffusion. On this theory natural holes exist in the liquid into which molecules can jump, leaving holes behind them. A jump occurs when the molecule has accumulated enough heat energy to surmount an activation potential barrier of energy value E0. Cavitation appears to be the result of coalescing of the natural holes in the negative pressure phase of the cycle. Since a molecule has to jump from a hole in order that this can coalesce with another hole, the cavitation pressure is proportional to the activation energy which in turn is proportional to the logarithm of the viscosity. The increased power-transmitting capacity for short pulse lengths is a result of the finite time taken for the small holes to grow in size to a large enough hole to cause rupture of the liquid.
It is generally accepted that acoustic echoes received from marine sediments are relatable to a number of sediment geotechnical descriptions. To investigate these relationships, mathematical modeling, preliminary system design concepts, specific instrumentation, and a computer aided analytical approach have been developed. Techniques have been implemented and applied to at-sea data acquisition, employing a towed array, which allow simultaneous remote estimates of compressional wave velocity, attenuation rates, and the reflection coefficient as a function of incident angle. Initial correlations between measured acoustic indices and sediment physical parameters are presented and the potential for remote classification discussed.
The acoustical properties of liquid-saturated and gas-bearing sediments are discussed and related to other sediment properties. A system is described which has been developed for attachment to sediment corers in order to obtain an in situ sound-speed profile during a coring operation. The system uses two electroacoustic transducers mounted in the cutting head of the corer and associated electronic circuitry to measure the travel time of an acoustic pulse traversing the diameter of the sediment core. Results of laboratory and field tests are presented. There is an ongoing study of the feasibility of expanding the sound speed measurement system capabilities to include a measure of sediment acoustic attenuation and internal volume scattering. Each of these measured acoustical parameters is useful for sediment description and gas assessment.
Measurements of acoustic and engineering properties of seafloor sediments show that while grain size parameters, along with porosity, dominate in any study of variations of the former, reasonably good prediction equations exist to be able to define the one knowing the other. Thus, while the static elastic moduli are several orders of magnitude less than the equivalent dynamic parameters, it is still possible to make an assessment of their magnitude from acoustic information.
Many of the measured variations can be predicted by the use of normal elastic theory, but such theory cannot account fully for the observed manner in which acoustic energy is lost in marine sediments. While inter-granular friction seems to be the main dissipating mechanism (giving a linear frequency dependence), viscous losses must also exist in any sediment.
Many shallow water, fine-grained sediments apparently contain appreciable quantities of interstitial gas bubbles produced by the biochemical degradation of organic matter. There have been relatively few direct observations of entrapped bubbles, but the presence of extensive gaseous sedimentary zones has frequently been inferred from high resolution sub-bottom profiling records. Frequently, large segments of many records made in estuaries and bays with high-frequency, low-energy profilers are confused and characterized by bands of strong diffuse reflection that mask the underlying features. Investigators have repeatedly attributed this anomalous acoustic behavior to the presence of interstitial gas bubbles, which produce excessive reverberation of sound within the sediment. Until recently, however, this hypothesis for the explanation of the acoustically impenetrable or ‘turbid’ character of sediments had apparently not been tested.
The accuracy of predictions of sediment properties is dependent on relationships between various physical properties and/or their expectable statistical variability. In shallow water, sediment types, and consequently their properties, may vary greatly over short distances. In the deep sea, sediment types are fewer, and the main types occupy vast areas. Within the term “deep-sea sediments” must be included the turbidites which form large abyssal plains, rises, and fans in the deep sea.
Prediction of sediment surface properties in deep-sea sediments falls into two categories: (1) a sediment sample is available, or (2) location, only, is provided. Given a sediment surface sample in which water content, grain size, and grain density can be measured, porosity and density can be computed or measured. Given only a dried sample, grain size data provide predictive relations with the other properties. Having derived one or more of these properties, velocity of compressional waves can be predicted within about 1 to 2% because of its relations with grain size and porosity. Given certain restrictive conditions, reflection coefficients and bottom losses at normal incidence can be computed. All of these properties can be corrected to in situ conditions. With less confidence, values of compressional wave attenuation can be approximated because of relations with grain size and porosity, and values of elastic properties (e.g., shear-wave velocity) can be computed. The values of attenuation and the elastic properties require additional confirmation by future in situ measurements. There is apparently no usable relationship between soil mechanics shear strength and velocity, but there may be a relation between dynamic rigidity and attenuation of compressional waves.
If sediment samples are not available and location, only, is provided, the geologist or geophysicist must use available information to predict, in sequence, the physiographic province, sedimentary environment, and sediment type. After the sediment type is predicted, available tables can be entered to determine averaged laboratory properties which can then be corrected to in situ values.
Although surface properties may be reasonably predicted, there is a lack of data to predict many property gradients with depth in the sediment body. At present, compressional velocity vs. depth in sediments can be reasonably predicted because of sonobuoy measurements, but more measurements are needed. The attenuation of compressional waves with depth is not known, but is speculated to decrease with overburden pressures. A small amount of data is available on consolidation, which, if supplemented, will allow prediction of the variations of density and porosity with depth. The least known properties in deep-sea sediments are the velocity and attenuation of shear waves.
Tables of sediment properties previously published by the writer are supplemented by additional measurements from 12 different sources, and new regression equations and diagrams illustrate some interrelationships.
Predictions of sound transmission in the deep ocean ordinarily rely on ray diagrams and ray trace calculations, wherein the ocean floor is assumed to be an impenetrable boundary that does nothing more than reflect sound back up into the sea. Yet, there is a growing body of experimental evidence to indicate that refracted sound traveling through the ocean floor makes a significant contribution to the sound in the sea. This evidence includes recent work with a geophone as a sensor of underwater sound, a measurement of sound transmission up the continental slope, and some recent findings on low-frequency, low-angle bottom loss. In this paper, this experimental data will be reviewed in terms of the role that the sea bed may play in long-range sound transmission in the sea.
Experimental measurements of sound velocity and attenuation constant in a mixture of air bubbles in fresh water, using a standing wave tube, are described. Bubble sizes in the mixtures were controlled between about 0.08- and 0.26-in. diameter, concentrations ranged from 0.03 to 1%, and applied frequencies from 60 to 20 000 cps. Bubbles in each mixture were of a single uniform size except for one series of experiments in which mixtures of bubbles of two discrete sizes were used. Attenuation constants were obtained for each mixture through a range of frequencies, including natural frequencies of the bubbles in the mixture. Velocity measurements were not obtained near the natural frequencies of the bubbles because high attenuation prevented the establishment of standing waves.
Data obtained in the tubes were reduced to mean, infinite conditions and compared with available theory. The measurements show that theory gives at least a good estimate of both velocity and attenuation constant in the region investigated.
The natural frequency of a spherical gas-filled bubble oscillating in a viscous compressible liquid is derived. Surface tension is also included in this solution, but gravity, gas diffusion and thermal conduction are neglected. The solutions of Minneart Richardson, Neppiras and Noltingk, Hirose and Okuyama, and Houghton are shown to be special cases of this solution. The effects of liquid compressibility, surface tension, and viscosity on the natural frequency of a bubble in water is determined as a function of the static radius. The effect of ratio of specific heats of gas in a bubble is also examined.
Recent in situ and laboratory determinations of density and velocity of sound in fine‐grained, high‐porosity sediments of the sea floor off San Diego, California, reveal several stations at which the velocity of sound in the sediment was 2% to 3% less than the velocity of sound in the water just above the bottom. Comparison of the compressibility of the sediment computed in two ways and of the ratio: velocity in sediment/velocity in water computed and actually measured indicates that these high porosity sediments are approximately described acoustically by the velocity formula which applies to a suspension. The theoretical explanation for this phenomena was apparently made by R. J. Urick, [J. Appl. Phys. 18, 983 (1947); J. Acoust. Soc. Am. 20, 283 (1948)] and R. J. Urick and W. S. Ament [J. Acoust. Soc. Am. 21, 115 (1949)].
Preliminary research confirms that microbubbles become stabilized in a body of water and persist for long periods of time in contradiction of classical theory. The evidence was obtained by generating bubbles at the bottom of a water tank and then measuring the decay of ultrasonic attenuation as bubbles of various sizes rose to the surface. Comparison of experimental and classical decay curves disclosed persistence that appeared to be a function of the solid particle content of the water and indicated that bubbles may stabilize at sizes as large as 30 μ radius in water of high particulate content. In waters of low particulate content the decay of attenuation lasted over 100 hr.
Substantial persistent abnormal attenuation was detected in all fresh tap water measured, amounting to a minimum 44% over that of distilled water (at 5.125 Mc) and is believed to be caused by stabilized bubble nuclei somewhat less than 0.8 μ in size. An interesting discovery was that attenuation rose in an uncovered test basin but fell in a covered basin, indicating that microbubbles may enter water on dust particles.
A hypothesis presented to explain the persistence effect suggests that solid particles collect on the bubble surface, forming a compressed wall capable of supporting the excess pressure necessary to halt gas diffusion. Other hypotheses explore the mechanism by which dust particles may carry air globules below the water surface, and the dynamics of microbubble populations.
1. The experiments showed that the damping of pulsating air bubbles in water for bubbles with resonant frequencies up to 100 kc/s can be explained theoretically by the energy losses due to sound radiation and heat conduction.2. From the result of the first part of the measurements, the validity of Minnaert's relation between radius and resonant frequency of the bubbles for frequencies higher than 50 kc/s became doubtful. Further investigations including direct measurements of the radii showed that the resonant frequencies of bubbles are considerably influenced by dust particles at higher frequencies. By association of dust particles at the surface of a bubble in the frequency range of 100 kc/s, the resonant frequency may rise up to 1.5 times the value given by Minnaert's formula.3. The damping of ayo bubbles is not influenced by association of dust particles.4. In liquids with small surface tension, where no dust particles remain at the surface, all bubbles showed normal properties.5. The damping due to viscosity seems to be considerably smaller than would be expected from Lamb's theory.
This report includes discussions of elastic and viscoelastic models for
water-saturated porous media, and measurements and computations of
elastic constants including compressibility, incompressibility (bulk
modulus), rigidity (shear modulus), Lamé's constant, Poisson's
ratio, density, and compressional- and shear-wave velocity. The
sediments involved are from three major physiographic provinces in the
North Pacific and adjacent areas: continental terrace (shelf and slope),
abyssal plain (turbidite), and abyssal hill (pelagic). It is concluded
that for small stresses (such as from a sound wave), water-saturated
sediments respond elastically, and that the elastic equations of the
Hookean model can be used to compute unmeasured elastic constants.
However, to account for wave attenuation, the favored model is `nearly
elastic,' or linear viscoelastic. In this model the rigidity modulus
μ and Lamé's constant λ in the equations of elasticity,
are replaced by complex Lamé constants (μ + iμ') and
(λ + iλ'), which are independent of frequency; μ and
λ represent elastic response (as in the Hookean model), and
iμ' and iλ' represent damping of wave energy. This model
implies that wave velocities and the specific dissipation function 1/Q
are independent of frequency, and attenuation in decibels per unit
length varies linearly with frequency in the range from a few hertz to
the megahertz range. The components of the water-mineral system bulk
modulus are porosity, the bulk modulus of pore water, an aggregate bulk
modulus of mineral grains, and a bulk modulus of the structure, or
frame, formed by the mineral grains. Good values of these components are
available in the literature, except for the frame bulk modulus. A
relationship between porosity and dynamic frame bulk modulus was
established that allowed computation of a system bulk modulus that was
used with measured values of density and compressional-wave velocity to
compute other elastic constants. Some average laboratory values for
common sediment types are given. The underlying methods of computation
should apply to any water-saturated sediment. If this is so, values
given in this paper predict elastic constants for the major sediment
types.
The sensitivity of bottom reflection loss to a density gradient is examined within the context of a physically meaningful model consisting of a fluid sediment layer having depth-dependent density and sound speed overlying a substrate having solid properties. The bottom grazing-angle range is divided into two regions, the low-angle and the high-angle regions. In the low-angle region sediment sound-speed gradients refract incident energy upward before it encounters the substrate. In the high-grazing-angle region, energy impinges on the substrate. Through the use of a numerical plane-wave reflection coefficient model it is shown that the effect of a density gradient on low-angle bottom loss is small. An expression indicating this effect is derived and is shown to agree with numerical bottom-loss calculations. The high-angle bottom loss is more influenced by a density gradient because of the impedance changes at the sediment-substrate interface which accompany a density gradient. It is shown that introduction of density gradient of 0. 002 g/cm**3/m can cause the high-angle bottom loss to change on the average by 12. 5%, 7%, and 3% in 100-m clay, silt, and sand layers, respectively.
In‐situ measurements of compressional (sound) velocity and attenuation were made in the sea floor off San Diego in water depths between 4 and 1100 m; frequencies were between 3.5 and 100 khz. Sediment types ranged from coarse sand to clayey silt. These measurements, and others from the literature, allowed analyses of the relationships between attenuation and frequency and other physical properties. This permitted the study of appropriate viscoelastic models which can be applied to saturated sediments. Some conclusions are: (1) attenuation in db/unit length is approximately dependent on the first power of frequency, (2) velocity dispersion is negligible, or absent, in water‐saturated sediments, (3) intergrain friction appears to be, by far, the dominant cause of wave‐energy damping in marine sediments; viscous losses due to relative movement of pore water and mineral structure are probably negligible, (4) a particular viscoelastic model (and concomitant equations) is recommended; the model appears to apply to both water‐saturated rocks and sediments, and (5) a method is derived which allows prediction of compressional‐wave attenuation, given sediment‐mean‐grain size or porosity.
Laboratory measurements of the sound speed and attenuation in natural sea‐floor sediments have shown that the speed ranges from 0.997 to 1.19 that in sea water, and that the attenuation is directly proportional to frequency over the range of the measurements (15–1500 kHz. Values of the attenuation coefficient range from 0.7 dB/ft at 15 kHz for silty‐clay sediment to 75 dB/ft at 1500 kHz for sand.
This is a summary of an experimental study to measure the acoustic properties of water‐saturated sediments. The sediments used were laboratory prepared to allow control of physical parameters (such as grain size, volume concentration, compressibility, etc.) and to approximate natural sediments. Acoustic velocity and attenuation in the sediments were measured over the frequency range 4–600 kHz. Acoustic measurements were made at high frequencies by means of two probes inserted in the sediments, and at low frequencies by means of a specially constructed rigid‐wall standing‐wave tube. The data presented show the frequency dependence of attenuation and velocity in the laboratory‐prepared sediments and the change in this frequency dependence with changes in physical parameters of the sediments. Sediments composed of pure kaolinite, or kaolinite and sand up to 15% (by weight), show an f 1.37 frequency dependence of attenuation. Sediments with greater than 30% sand (by weight), including pure sand, exhibit an f 0.5 frequency dependence of attenuation. The measuredvelocity dispersion approximately 2% over the frequency range 4–200 kHz. Velocity increases with frequency. All measurements reported are for sediments free of entrapped gas.
Measurements of the attenuation of sound in water containing air bubbles were undertaken in order to confirm some aspects of a theory due to L. L. Foldy. Bubbles were produced by forcing compressed air through the cloth covering of several trays lying on the bottom of the Black Moshannon Lake. The distribution in size of the bubbles and the number of bubbles per unit volume were determined by photographing the bubbles and by collecting the volume of air which rose over a given area in a given time, the terminal velocity of rise being a known function of bubble diameter. The attenuation of sound was found to be very large at frequencies for which resonant bubbles were present, and much smaller at other frequencies, indicating that resonanceabsorption was the principal phenomenon observed. For the particular bubble distribution investigated, which contained 0.045 percent air by volume, the attenuation was found to be in satisfactory agreement with the theory and a maximum attenuation of approximately 20 db per inch was measured. Amplitude and phase fluctuations made it impossible to measure the phase velocity, but this quantity was computed from the bubble distribution.
At low frequencies consideration of the quasi‐static elastic moduli and densities of the gas and liquid parts of the mixture yields a value of sound velocity which is much lower than the velocity in either constituent. The computation has been confirmed experimentally in the concentration regions of 1 to 60% air by volume. The sound velocity was found from the change of phase in a plane progressive wave as a function of distance. Concentrations were determined from a measure of the hydrostatic pressure of the mixture. The “froth” was made by pumping the water rapidly past a porous glass filter through which the air was forced. A trace of detergent retarded the coalescence of bubbles. Minimum sound velocity in a water air mixture occurs at 50% concentration where the velocity is approximately 22 mps.
This is the first of a series of reports on sound velocity, elasticity,
and related properties of marine sediments from sedimentary environments
associated with three major physiographic provinces in the North Pacific
and adjacent areas: the continental terrace (shelf and slope), the
deep-water abyssal plain (turbidite), and the abyssal hill (pelagic).
The following properties are listed in tables and illustrated in
diagrams that interrelate various properties: grain size (mean diameter,
percentages of sand, silt, and clay), bulk density, density of mineral
grains, porosity, sound velocity, velocity ratio (velocity in
sediment/velocity in sea water), impedance, and density ×
(velocity)2. Values are given for each sediment type within
each environment. Significant differences in the density and porosity of
the environments studied are caused by mineralogy, size and shape of
grains, and sediment structure; presence of diatoms and clay mineralogy
are particularly important. General equations and diagrams relating
density and porosity to velocity should be abandoned in favor of entry
into diagrams or equations for a single environment; velocity is then
predictable within 1 to 2 per cent in most environments. Mean grain size
has one of the best empirical relationships with velocity, which permits
derivation of useful data from size analyses of dried cores. Porosity
and density are excellent indices by which to determine values of
impedance and density × (velocity)2. There is no
usable, empirical relationship between sound velocity and shear strength
(cohesion) as measured in soil mechanics tests. No anisotropic velocity
relationships were measured in surficial sediments, and none is
predicted for the upper few hundred meters in sea-floor sediments.
The scattering strength of air bubbles in water has been determined experimentally at frequencies of 1 and 5 MHz. The bubble size and sound frequencies represent the short wavelength limit, where ka > 1. By scanning the sound fields with a PZTtransducer of 1 mm radius, angular distributions have been obtained out to an angle of 35° with and without the scatterers present. The results agree well with recent scattering measurements in suspensions and also with scattering theory.
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This paper considers the acoustical effects of variations of sediment absorption with depth for several sediment types. A simple measure of absorptioneffects is given by the total absorption along the sub‐bottom ray paths. This total absorption is considered for a variety of absorption and sound‐speed gradients for both semi‐infinite and finite layers. The plane wavereflection loss (bottom loss) is then considered for several treatments of attenuation, layering, and sediment type. It is found that, for clay sediments, plausible variations in absorption gradients can result in variations in bottom loss of several decibels. These conclusions are reinforced by consideration of the absorption gradient effects on normal modeattenuation coefficients.
Laboratory measurements have been made of the compressional wave velocities in samples of deep-sea sediments using ultrasonic methods. The variation of velocity with compaction pressure has been determined. From the results it has been concluded that a vertical gradient of velocity exists in the sedimentary layer of the ocean.
The hypothesis discussed that the cavitation nuclei consist in gas bubbles. Due to surface tension, small bubbles would dissolve in a very short time. If the bubbles are larger than 5×10−3 cm, or if the liquid is supersaturated, they may last longer or even be stable, but then no cavitation threshold exists.
The hypothesis expressed that the nuclei are very small bubbles, stabilized by an organic skin, which mechanically prevents loss of gas by diffusion. The cavitation occurs when the skin breaks and the threshold is determined by the breaking strength of the film and the size of the bubble.
A theory is developed for the soundvelocity and attenuation in a medium composed of closely packed solid particles immersed in a fluid. The absorption mechanism considered is that of viscous motion of fluid between the particles, and the macroscopic point of view is taken. For high enough frequencies the attenuation is proportional to the square root of the product of frequency and static flow resistance of the medium. Comparisons are made with data reported previously by others, and the agreement with the theoretical results is good for cases involving particles of nearly uniform size. Still another frequency effect, dependent on the size distribution, is evident for unsorted granular substance.
A theory is outlined for the propagation constant in media containing numerous small spherical particles. Using expressions derived by Lamb for the zeroth- and first-order scattering coefficients of a particle free to move in a sound field, an expression for the complex propagation constant is derived whose real part yields a velocity which reduces to the homogeneous case for extremely small particles, and whose imaginary part yields an absorption coefficient identical with that derivable from the viscous-drag theory outlined in a previous paper. Using both an interferometer and a pulse-reflection method, measurements of sound velocity and absorption at megacycle frequencies have been made on mercury-in-water and bromoform-in-water emulsions of non-uniform particle size up to a volume concentration of about 50 percent of emulsified liquid. These materials, though showing considerable deviation from a homogeneous behavior, are found to have a velocity and absorption in good agreement with the theory up to a concentration of about 25 percent by volume. Similar measurements on suspensions of quartz sand in water exhibit small deviations of velocity from the theory that may possibly be attributed to the non-spherical shape of the quartz particles.
The phase velocity and absorption of a continuous train of sound waves in water containing air bubbles were measured as functions of frequency from 10 kc/sec to 1 mc/sec. The data are in fairly good agreement with theories by Meyer and Skudrzyk and Carstensen and Foldy. For the bubbles investigated which constituted 0.02 percent of the volume, phase velocity varied from 500 m/sec to 2500 m/sec and peak absorption was over 30 db/cm.
This paper considers a mathematical model to describe the propagation of low‐amplitude waves in saturated sediments. Losses due to inelasticity of the skeletal frame and to motion of the pore fluid relative to the frame are both accounted for, and each is found to be significant in a different frequency range. The theory shows favorable agreement with experimental results where available for both sands and finer‐grained sediments over a wide range of frequencies.
The substitution of a logarithmic variable in the Gaussian function yields a logarithmic probability law (the normal phi curve) which has all the geometrical properties of the conventional normal law. That is, it is bell-shaped and symmetrical about its mean value, and it is completely described by two parameters, the phi mean and the phi standard deviation. Simple analytic and graphic methods are described for testing data for normalcy. Finally, some of the theoretical implications of normal phi curves are given, in terms of environmental factors which may control them.
A number of mechanisms have been proposed to account for the attenuation of compressional waves in water‐saturated sediments. These include viscous losses between the particles and the fluid, and “solid friction” losses between the particles. The mechanisms are discussed and it is shown how the low values of attenuation observed in pure clays arise from the electrical interaction forces between the surface‐active particles. It is proposed that the attenuation in clay‐ and silt‐size sediments (up to 6 phi mean diameter) arises from viscous interaction between the clay‐water “fluid” and the non‐surface‐active particles. Both new and published experimental measurements indicate that the proposed mechanism is valid, at least in a frequency range 30 to 370 kHz. For sediments of mean particle diameter greater than 6 phi, both new and published experimental results are presented to show that, although under the circumstances of a very well sorted sediment under zero overburden pressure a viscous dissipation mechanism may be dominant, the “solid friction” mechanism is dominant for poorly sorted sediments buried to a depth of 2 m.
Ocean‐bottom sediment porosity and acoustic reflectivity are related by a simple linear equation. The equation was derived empirically from data published by several investigators and is valid for a wide variety of sediment types and ocean conditions. The degree of validity of the relationship appears to be excellent as the correlation coefficient between the porosity and acoustic reflectivity is 0.97.
Laboratory measurements of compressional sound speed, and absorption, have been made on 111 unconsolidated marine sediment samples, ranging from shallow water sands to deep‐sea clays. In addition, determinations were made of porosity, wet density, and grain size distributions. Frequencies between 20 kc/sec and 37 kc/sec were used for the acoustic studies. Sound speed values at room temperature range from 1.474 km/sec for a red medium clay to 1.785 km/sec for a medium sand. More than one‐third of the values are lower than the value for sea water alone. Variations in the speed of sound in unconsolidated sediments as found in nature are caused by the following factors, in order of importance: (1) porosity, because of the great difference in compressibility of water and mineral grains; (2) the factor which produces rigidity, which appears to be related to the abundance of coarse grains; (3) pressure; (4) temperature; (5) compressibility of the grain aggregate, determined from compressibilities of individual minerals. Sound absorption measurements ranged from 0.5 db/m for a medium clay (28.4 kc/sec) to about 20 db/m for silts and fine sands (between 30 and 37 kc/sec). An absorption maximum occurs for sediments of intermediate porosity (0.45–0.6) and intermediate grain size (0.031 mm–0.25 mm). The expression α = MA s , where α is the linear absorption coefficient, M is a frequency‐dependent factor related to the sediment volume fraction of grains in mutual contact, and A s is a computable total acoustically effective grain surface area, predicts the absorption values and the absorption maximum. Absorption measurements at more than one frequency between 20 kc/sec and 37 kc/sec were obtained for 65 samples. Assuming that absorption is directly proportional to frequency raised to a power n, the data yield an average value of n equal to 1.79, with a standard deviation of 0.98.
A portable pulse-echo, pulse-scatter acoustical system has been used to
make in situ measurements of excess attenuation and scatter over the
frequency range 20 to 200 kHz, and thereby to infer numbers of bubbles
of radius approximately 180 to 18 microns at sea. The study was made at
various depths to 50 feet in isothermal coastal waters, at sea states
one and two, over 24-hour periods. At sea state one densities of bubbles
of the order of 1000/m3 are identified in 1-micron bands of
incremental radius; these microbubbles, with decreasing numbers as
radius is increased (proportional to R-4), are postulated to
be entrained by aerosols as they fall into the ocean. Densities of the
order of 100/m3 in a 1-micron band are found for bubbles of
radii greater than 40 microns; for the largest bubbles the numbers vary
approximately as R-2. Numbers of bubbles of radius 60 microns
or larger have a depth dependence Z-1/2, whereas numbers of
smaller bubbles follow the exponential law e-Z/L with L
approximately 7 meters. Analysis in the Lagrangian frame suggests that
the bubbles originate at the surface, with the smaller bubbles,
particularly, losing mass as they are carried downward by convection and
diffusion.
Marine sediments that emit considerable quantities of gas when exposed to surface conditions are found in many parts of the sea. The discharge of gas is often violent enough to disrupt core samples, and has been observed in conjunction with ice formation in some eases where the cores are exposed to the atmosphere immediately after sampling. In recent drilling operations by the Glomar Challenger on the Blake-Bahama ridge (Deep Sea Drilling Projeet, leg 11), such gassy materials were encountered to depths of over 600 meters in the sediment at water depths averaging 3600 meters. Variable amounts of gas, predominantly methane, were released with what appeared to be a general increase in quantity with depth. On the basis of the most obvious correlation between seismic reflections and lithologic changes at the drilling sites, the interval velocity in the gassy layer is about 2.2 km/see, an anomalously high value when compared to velocities in non-gassy sediments with similar porosity and grain size. This apparent anomaly in velocity is the motivation for the work described in this paper wherein a tentative physical explanation is explored and tested. The key to an explanation of the higher velocity appears to lie in the fact that water and many gases, particularly those of small molecular size, combine to form gas hydrates
Objects, believed to be gas bubbles, rising from the bottom (197 m) of Saanich Inlet British Columbia, have been recorded by stationary echo-sounders of 12, 50 and 200 kc/s. If the ship carrying the transducers is under way, the traces of the objects can readily be confused with fish echoes. The upward velocities of the bubbles near the bottom (16 to 30 cm/sec) and inferred size distributions (0·9 to 1·6 mm diam.) have been determined from the 200 kc/s records. These data, together with records obtained at 12 kc/s, indicate the bubbles become resonant at 12 kc/s at depths between about 100 and 30 m. The size of the bubbles is such that at 50 kc/s they are rarely detected and do not become resonant. The bubbles are visible for at least 15 min and increase in size as they rise. The gases in the bubbles appear to originate in the highly organic, anaerobic sediment of the bottom, but their exact composition remains an interesting problem.
A system of grain-size nomenclature of terrigenous sediments and sedimentary rocks is introduced wherein fifteen major textural groups are defined on the ratios of gravel, sand, silt, and clay. Further subdivision of each class is based on the median diameter of each size fraction present. Next, the mineral composition of terrigenous sedimentary rocks is considered. A triangular diagram is used to define eight rock types (orthoquartzite, arkose, graywacke, and five transitional types) based on the mineralogy of the silt-sand-gravel fraction and ignoring clay content. The writer contends that the current practice of calling all clayey sandstones "graywackes" is not valid, inasmuch as it represents a confusion of texture with composition. It is suggested that sedimentary rocks may be best defined by the use of a tripartite name, based on the following pattern-(grain size): (textural maturity) (mineral composition).