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Shallow gas features in incised-valley fills (Ría de Vigo, NW Spain): A case study
Abstract
Areas with gas accumulations and gas seeps, where gas escapes from the seabed to the water column have been mapped in the fill of a submarine incised valley, the Rı́a de Vigo. The various gas features have been classified into four types according to their specific seismic signatures: (1) acoustic blanket, (2) acoustic curtains, (3) acoustic columns, and (4) acoustic turbidity. At the same time, three types of gas escapes features have been distinguished: (1) acoustic plumes, (2) cloudy turbidity, and (3) pockmarks. Calculations indicate mean densities of 1.7 acoustic plumes km−2 and 1.6pockmarkskm−2. Estimations of the gas fluxes towards the atmosphere range from 144.37 to 4134.9tyr−1. It is concluded that sedimentary facies is the main factor determining whether gas accumulates or seeps, and also determines the specific type of accumulation or seep at each location.
... The most frequently occurring acoustic characters are the Acoustic Turbid Zone (ATZ) (Figure 10a,b) [16,30,[62][63][64][65] and Enhanced Reflectors (ER) (Figure 10a,b) [30,[64][65][66]. Rarely, Intrasedimentary Gas Plumes (IGP) (Figure 10b) [62,63] and Seabed Doming (D) (Figure 10b) [64,67] were observed. ...
... The study of the seismic profiles showed the existence in the area of various 'abnormal' acoustic characters, which are direct and indirect evidence for the presence of fluids in the sediments. The most frequently occurring acoustic characters are the Acoustic Turbid Zone (ATZ) (Figure 10a,b) [16,30,[62][63][64][65] and Enhanced Reflectors (ER) (Figure 10a,b) [30,[64][65][66]. Rarely, Intrasedimentary Gas Plumes (IGP) (Figure 10b) [62,63] and Seabed Doming (D) (Figure 10b) [64,67] were observed. ...
... The most frequently occurring acoustic characters are the Acoustic Turbid Zone (ATZ) (Figure 10a,b) [16,30,[62][63][64][65] and Enhanced Reflectors (ER) (Figure 10a,b) [30,[64][65][66]. Rarely, Intrasedimentary Gas Plumes (IGP) (Figure 10b) [62,63] and Seabed Doming (D) (Figure 10b) [64,67] were observed. The most extended gas-related acoustic character observed in the seismic profiles is that of ΑΤΖ. ...
The Patras Gulf Pockmark field is located in shallow waters offshore Patras City (Greece) and is considered one of the most spectacular and best-documented fluid seepage activities in the Ionian Sea. The field has been under investigation since 1996, though surveying was partially sparse and fragmentary. This paper provides a complete mapping of the field and generates new knowledge regarding the fluid escape structures, the fluid pathways, their origin and the link with seismic activity. For this, data sets were acquired utilising high-resolution marine remote sensing techniques, including multibeam echosounders, side-scan sonars, sub-bottom profilers and remotely operated vehicles, and laboratory techniques focusing on the chemical composition of the escaping fluids. The examined morphometric parameters and spatial distribution patterns of the pockmarks are directly
linked to tectonic structures. Acoustic anomalies related to the presence of gas in sediments and in the water column document the activity of the field at present and in the past. Methane is the main component of the fluids and is of microbial origin. Regional and local tectonism, together with the Holocene sedimentary deposits, appear to be the main contributors to the growth of the field. The field preserves evidence that earthquake activity prompts the activation of the field.
... The presence of gas in marine and lacustrine settings was identified about 60 years ago (Koyama 1953;Emery and Hoggan 1958;Lodolo et al. 2012), while gas-induced anomalous structures (pockmarks) were first discovered more recently in the continental shelf of Nova Scotia, Canada (King and MacLean 1970;Çifçi et al. 2003). Many more subsequent geophysical studies have focused on the seismoacoustic behaviors of gas-charged sediments (Garcia-Gil et al. 2002;Missiaen et al. 2002;Bertin and Chaumillon 2005;Gwang et al. 2005;Lodolo et al. 2012). 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). ...
... 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). They have been reported not only in marine settings (Hovland 1981a, b;Hovland and Judd 1988;Judd and Hovland 1992;Hovland 1992;Garcia-Gil et al. 2002;Gay et al. 2007;Naudts et al. 2009), but also in orogenic lacustrine basins (Toker and Şengör 2011;Toker et al. 2017a;Çukur et al. 2014;Çukur et al. 2013a, 3b;Toker and Tur, 2018). Unstable lacustrine basins in tectonically highly active orogenic settings (see unstable tectonic lakes by Hage et al. 2017;Wagner et al. 2012;Leroy et al. 2010;Jakobsson et al. 2014;Van Daele et al. 2015;Wallner, 2008;Çukur et al. 2013a, b;Solovyeva et al. 2016;Shiki et al. 2000 and many others from bibliographic research atlas of world lakes, compiled by Sammartini et al. 2020) have challenged geoscientists for decades and become a leading topic over the past 20 years (e.g., International Continental Drilling Program, ICDP-PaleoVan project by Litt et al. 2009Litt et al. , 2011Litt et al. , 2014. ...
... 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). Measurements made using high-frequency (> 1 kHz) acoustic profiling indicate "chaotic reflections" as acoustic turbidity and "absence of reflections" as acoustic blanking in muddy deposits (e.g., Garcia-Gil et al. 2002;Mathys et al. 2005;Laier and Jensen 2007;Tóth et al. 2014). The high-frequency components of seismic waves are attenuated in the gas-charged depositional layers due to scattering caused by gas bubbles. ...
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 presence of shallow gas in seismic data can totally or partially mask the stratigraphical information (Judd and Hovland 1992). Within unconsolidated sediments, gas may accumulate in extensive areas in estuaries and bays (Garcia-Gil et al. 2002;Baltzer et al. 2005;Felix and Mahiques 2013;Delavy et al. 2016a), lagoons (Baltzer et al. 2005;Klein 2005;Weschenfelder and Corrêa 2018), and shallow marine regions (Okyar and Ediger 1999;Missiaen et al. 2002;García-García et al. 2007). ...
... The biogenic processes are the primary source of gas accumulation in coastal environments (Lee et al. 2005;García-García et al. 2007;Visnovitz et al. 2015;Vardar and Alpar 2016), normally associated with a shallow basement (Garcia-Gil et al. 2002;Missiaen et al. 2002;Weschenfelder and Corrêa 2018). While thermal gas production needs high temperatures and considerable burial depths (Schoell 1988;Horsfield and Rullkotter 1994;Littke et al. 1999), methanogens microorganisms survive at temperatures between 0 and 75 °C (Zeikus 1977;Gang and Jiang 1985). ...
... Still, there is uncertainty in the estimates of its natural sources and sinks and how its variations can affect the growth rate of atmospheric CH 4 (Borges et al. 2016). Although some studies estimate the CH 4 flux from coastal gas-charged sediments to the atmosphere via immediate water plumes and pockmarks (Judd et al. 1997;Dimitrov 2002;Garcia-Gil et al. 2002), there is insufficient knowledge about methane dynamics within unconsolidated sediments. ...
High-resolution seismic surveys were carried out at the Paranaguá Estuarine Complex (Southern Brazil) to map the intrasedimentary shallow gas. The seismic signatures representing gas accumulation were separated according to the upper gas boundary characteristics in acoustic blanking with sharp top, acoustic blanking with diffuse top, turbidity pinnacles, and black shadows (gas accumulation at the water/sediment boundary). The main source of the gas has been recognized here as Pre-Holocene continental deposits. These deposits were capped by a seismic unit interpreted as a regressive mud deposited over the last 5000 years. This seismic unit is quite heterogeneous, the gas being trapped in its different internal layers. Each gas signature represents the efficiency of the sealing layer and has specific locations and burial depths. The results point to different phases of gas migration along with the sedimentary layers. Thus, we proposed a gas migration and accumulation model based on acoustic data and sedimentary inferences within the Paranaguá Estuarine Complex.
... Acoustic anomalies were first reported by Schuller (1952) in Garcia-Gil et al. (2002). Masking effects appear on echosounder records were due to free gas escape from shallow marine sediments. ...
... Presence of gas features characterized by high amplitude reflections, which appears as a strong reflector at top boundary where the gas accumulated. They usually mask the deeper event due to the presence of gas on sediments absorbs the energy of acoustic waves (Hovland & Judd, 1988;Garcia-Gil et al., 2002;Vardar & Alpar, 2016), while columnar disturbances or gas chimneys identified on sub-bottom profiler data as vertical feature across the shallow geological layers indicative of upward migration of shallow gas. ...
... These types of gas accumulations ( Figure 5) are observed on SBP profile observed at 56 locations. These zones contain gas (Judd & Hovland, 1992;Garcia-Gil et al., 2002;Vardar & Alpar, 2016). Methane gas accumulated in the study area is thought to be trapped in the upper sediment deposits of the Late Quaternary age. ...
Sub-Bottom Profiling (SBP) records and results of geochemical analysis of 12 surficial sediment cores from various water depths collected from the offshore Waropen Basin-Papua are presented. Presence of gas is clearly observed on sub-bottom profiler records. Shallow gas was identified through acoustic response due to gas accumulation and gas escape on sub-bottom profiles. Acoustic evidences of gas accumulations within near surface geology consist of high amplitude reflections and associated acoustic blanking, gas plumes and morphological features like pockmarks. Total organic carbon analysis of 12 surface sediment cores varies between 0.5% to 1.3% which indicate that the sediments have an abundance of organic matters. Gas chromatographic analysis of hydrocarbon composition detected only methane, a biogenic origin of shallow gas. Acoustic and geochemical evidence in the Waropen Basin indicates extensive shallow gas accumulations in the Late Quaternary sediments, some trapped within these deposits and some escape from seabed into the water column which then created a high distribution of pockmarks.
... Furthermore, accumulation of shallow gas can cause blow-outs during petroleum drilling, and in platform foundations (Ostanin et al., 2012;Schroot and Schüttenhelm, 2003). Gas leakage from the seafloor may also influence seawater chemistry and introduce carbon and methane into the atmosphere (Davis, 1992;Garcia-Gil et al., 2002;Hovland et al., 1993). ...
... The seismic response of shallow gas depends on the frequency used for acquisition. Thus, studies of shallow gas have used various combinations of data acquisition systems operating in different frequency ranges (e.g., Duarte et al., 2007;Garcia-Gil et al., 2002;Naudts et al., 2009;Tóth et al., 2014). In general, high-resolution shallow geophysical datasets acquired by sub-bottom profiler (SBP), single-beam echo sounder (SBES), and multi-beam echo sounder (MBES) include the images of gas bubbles, seafloor deformation, and sub-bottom sediment layers filled by shallow gas. ...
... In general, high-resolution shallow geophysical datasets acquired by sub-bottom profiler (SBP), single-beam echo sounder (SBES), and multi-beam echo sounder (MBES) include the images of gas bubbles, seafloor deformation, and sub-bottom sediment layers filled by shallow gas. These data are compared with sampling data and distribution maps of shallow gas (Donda et al., 2015;Garcia-Gil et al., 2002). In contrast, conventional 2D and high-resolution 3D seismic datasets, (e.g., P-Cable survey and autonomous underwater vehicles [AUVs]) can reveal bright spots in deeper sections of sediment and gas migration pathways (Eriksen et al., 2011;Plaza-Faverola et al., 2011). ...
This study investigates the evidence of shallow gas from a newly collected dataset comprising 2D multi-channel seismic (MCS), single-channel seismic (SCS), Chirp sub-bottom profiler (SBP), and multi-beam echo sounder (MBES) data from the southwestern continental shelf of the Ulleung Basin, East Sea, Korea. Various indicators of shallow gas were identified in this part the shelf, including seismic chimneys, acoustic flares in the water column, pockmarks, enhanced reflections, and acoustic blanking. Seismic chimneys, which are related to fluid leakage within the subsurface, are characterized weak to high amplitude, upturned or concave-upward internal reflection with polarity reversal in MCS section, whereas they are faint or transparent on SCS section with 700 Hz and Chirp SBP sections with 3.5 kHz frequency. Acoustic flares, which are diagnostics of active gas venting, were detected only on the Chirp SBP sections. There is also evidence of small-scale depressions, immediately below the acoustic flares; these are interpreted as formed by the sudden explosion of gas. Enhanced reflections (∼3 km-long) concordant with stratification are observed at different levels. Acoustic blanking anomalies, caused by absorption of acoustic energy due to the presence of gas, are seen in the range 50–200 ms TWT below seabed. Amplitude versus offset (AVO) analysis indicates the presence of free gas ca. 350 ms TWT below sea surface using the MCS dataset recorded by a 600 m-long streamer cable. The AVO cross-section shows that the lower repetitive signals in the MCS data are due to the gas plume, not lithological contrast. Our study highlights that AVO analysis is an extremely useful tool for identifying free gas, helping to discriminate water-gas contacts and bright events among the chaotic signals on the MCS data.
... Classification systems for gas-related signatures in seismic profiles have subsequently been developed (Hovland and Judd, 1988;Judd and Hovland, 1992;Taylor, 1992), providing standards against which other records may be compared. Later studies refined these early classification systems (Garcia-Gil et al., 2002;Weschenfelder et al., 2006;Iglesias and García-Gil, 2007). ...
... Gas seepage from the seabed has attracted interest due to its association with the greenhouse effect and its significance as a geohazard. Many studies have attempted to provide quantitative methane (CH 4 ) flux data, both globally and locally Hovland et al., 1993;Garcia-Gil et al., 2002;Iglesias and García-Gil, 2007;Egger et al., 2018), which help to evaluate and understand the significance of shallow gas with regard to climate change. ...
... Extensive shallow gas research has been carried out in different oceanographic settings: embayments (Wever et al., 1998(Wever et al., , 2006Rogers et al., 2006;Hu et al., 2012), rias (Garcia-Gil et al., 2002;Diez et al., 2007;Iglesias and García-Gil, 2007;Martinez-Carreno and Garcia-Gil, 2013;Benites et al., 2015), gulfs (Rogers et al., 2006;Dondurur et al., 2011), lagoons (Weschenfelder et al., 2006) and continental shelves (Figueiredo et al., 1996;Lee et al., 2005;Mazumdar et al., 2009). Generally, these are classified as either shallow semi-enclosed coastal environments or open continental shelves. ...
... (acoustic blankets) [2,16,17] (acoustic curtains) [16~18] (acoustic columns) [2] (acoustic turbidity) [17,19] . ...
... (acoustic blankets) [2,16,17] (acoustic curtains) [16~18] (acoustic columns) [2] (acoustic turbidity) [17,19] . ...
... , [2,16,17] . ...
... In these environments, the high flux of organic matter to the sediment favours gas generation, which can accumulate in clayey environments (Zeikus, 1977;Whitman et al., 2006;Judd and Hovland, 2007). The Galician rias represent a good example of these shallow muddy environments, high productive and acting as sediment traps, in which gas production is favoured García-Gil et al., 2002), providing an ideal context for the study of shallow gas accumulations and the stratigraphic context in which they are found. ...
... Thanks to the obtaining of sedimentary material with datable organic remains from some of the oldest seismic units in the Ría de Vigo, the existence of deposits more than 40,000 years old was demonstrated. Thus, a new chronostratigraphic model was elaborated for the Ría de Vigo in which the most probable hypothesis is the recording of up to five cycles of sea-level variation (from 450 kyr BP to the present) Introduction basins, which act as sediment traps, are ideal environments for the generation and accumulation of shallow gas, mainly methane ( García-García et al., 1999;García-Gil et al., 2002; García-Gil, 2003). ...
... The Galician rias represent highly productive shallow environments that act as sediment traps, favouring depositions of muddy sediment rich in organic matter where biogenic methane is produced ( García-Gil et al., 2002, 2015Kitidis et al., 2007). Evidences of shallow gas accumulations of varied dimensions have been identified in most of the rias , which have been studied for more than 30 years (Acosta, 1984;García-García et al., 1999;García-Gil et al., 2002;Ferrín et al., 2003;García-Gil, 2003;Diez et al., 2007aDiez et al., , 2007bDurán et al., 2007;Iglesias and García-Gil, 2007;Kitidis et al., 2007;García-Gil et al., 2015;Martínez-Carreño and García-Gil, 2013;de Carlos et al., 2017;Martínez-Carreño et al., 2017a). ...
Estuaries, rias and incised valleys in general, as well as their deposits, are important sedimentary environments of wide relevance, both economic and environmental. Large human populations and ports are located in their coasts with intense economic activity (fishing, aquaculture, trade...). Their sedimentary deposits constitute excellent oil traps and represent archives, safeguarding much environmental information associated with changes in sea level, from lowstand to highstand. Estuaries, rias and incised valleys are a complex and possibly unique group of sedimentary environments, since their formation and development involves the creation of accommodation space normally by a single process (river erosion) and their infill is the consequence of a wide range of processes (river, tides and waves) in the presence of variable salinity water. The complexity of these sedimentary environments, with a mixture of facies influenced by waves, tides and rivers in a restricted area, has led to the late development of facies models of their infill with respect to other environments, such as rivers or beaches.
The Galician rias have been intensely studied by geologists and oceanographers to understand their origin and evolution, as well as the processes taking place within them and the adjacent continental shelf. All the research work focused on their sedimentary infill up to 2015 was based fundamentally on seismic records and interpreted that most of the sedimentation within the rias corresponded to the transgressive period after the Last Glacial Maximum (about 20 kyr ago). Therefore, the rias were interpreted as simple incised valleys. However, in the light of the most recent results of the Ría de Vigo, which demonstrated the existence of sediments more than 40000 years old, the chronostratigraphic context for the rias was completely changed.
The fundamental purpose of this research work is (1) to establish the chronostratigraphic framework of the Quaternary sedimentary fill of the rias of Ferrol and Arousa in order to (2) carry out a detailed reconstruction of the evolution of sedimentary environments in both basins and (3) to contextualize the methane reservoirs found within them. In order to carry out these objectives, the stratigraphic architecture of the rias of Ferrol and Arousa is characterised, as well as its sequential analysis. The seismic and sedimentary facies, as well as their associations, are analysed in detail to interpret the different sedimentary environments preserved within these incised valleys in order to elaborate a conceptual facies model of their sedimentary infill and evolutionary models that allow to compare them with known examples and to evaluate the main controlling factors.
Both rias are interpreted as incised valleys with a compound sedimentary infill, recording multiple cycles of sea level variation. In the Ría de Ferrol, multiple sequences from the Quaternary period have been identified, and in the Ría de Arousa, one of Neogene age and several of Quaternary age. Sea level and physiography are the main factors controlling the sedimentary architecture of both ria. While sea level controls the spatial distribution of sedimentary environments along each incised valley, physiography modulates the relative importance of fluvial, tidal and wave processes, i.e., energy. Tectonic activity, with the differential movement of blocks and the deformation of deposits, has changed the accommodation space during the Neogene and the Quaternary, being able at the same time to favour the preservation of multiple sequences within the rias. High-frequency climatic oscillations (at higher frequency than glacial/interglacial cycles) cause sedimentary responses in both systems, particularly in the Holocene record. Several phases of aggradation have been differentiated in tidal flats that coincide with high-frequency climatic cycles described on the European Atlantic coast. In the storm fans identified in the Ría de Arousa, five sets of prograding clinoforms were differentiated, whose ages coincide with the periods of high incidence of Holocene storms described on the French coast. The morphology of the basement exerts a key control on the hydrodynamic patterns of both rias. The Strait of Ferrol in the Ría de Ferrol and the strait partially submerged between the Rúa Island and the Xidoiros Islands in the Ría de Arousa restrict the action of the waves in both rias and favour the development of tidal environments. This control takes place until the present in the Ría de Ferrol, but has lost relevance as the transgression advanced in the Ría de Arousa. Sediment input and source have varied throughout the most recent postglacial transgressive period. The fluvial contribution has decreased from the lowstand in favour of local and marine sources. Human activity affects the most recent deposits of the HST and its impact is much more evident in the smaller basin, the Ría de Ferrol. In this ria, dredging activities, dams and buildings reduce the sedimentary contribution, limiting the progradation of coastal sedimentary bodies. The dredging also affects the depth of the acoustic front of the gas fields and causes their escape, with the formation of pockmarks. Aquaculture activities in the Ría de Arousa favour anoxic conditions on the sediment surface beneath mussel raft polygons, generating gas in very shallow horizons.
... Gas in sediments can be found in various types of coastal environments, where normally the ideal conditions for biogenic gas formation due to high biological productivity occur. Gas-charged sediments is a common phenomenon worldwide and so have been related in the most coastal environments as shallow or enclosed seas and shelves (Emeis et al., 2004;Terra et al., 2014), lakes (Lafferty et al., 2006), bays (Baptista Neto et al., 1996;Quaresma et al., 2000;Catanzaro et al., 2004;Jensen & Bennike, 2009;Marino et al., 2013;Aliotta et al., 2014;Klein et al., 2016), rias (Garcia-Gil et al., 2002;Diez et al., 2007;Duarte et al., 2007;Iglesias & Garcia-Gil, 2007), deltas , lagoons (Baptista Neto et al., 2011;Weschenfelder et al., 2006Weschenfelder et al., , 2016 and estuaries (Frazão & Vital, 2007;Pinet et al., 2008). The gas sources and accumulation modes are closely related to the sedimentary and evolutionary processes occurring in the depositional environment (Garcia-Gil et al., 2002). ...
... Gas-charged sediments is a common phenomenon worldwide and so have been related in the most coastal environments as shallow or enclosed seas and shelves (Emeis et al., 2004;Terra et al., 2014), lakes (Lafferty et al., 2006), bays (Baptista Neto et al., 1996;Quaresma et al., 2000;Catanzaro et al., 2004;Jensen & Bennike, 2009;Marino et al., 2013;Aliotta et al., 2014;Klein et al., 2016), rias (Garcia-Gil et al., 2002;Diez et al., 2007;Duarte et al., 2007;Iglesias & Garcia-Gil, 2007), deltas , lagoons (Baptista Neto et al., 2011;Weschenfelder et al., 2006Weschenfelder et al., , 2016 and estuaries (Frazão & Vital, 2007;Pinet et al., 2008). The gas sources and accumulation modes are closely related to the sedimentary and evolutionary processes occurring in the depositional environment (Garcia-Gil et al., 2002). Hence, the various gas reservoir and trapping sites, together with their associated types of gas accumulation and environmental settings provide valuable clues to the evolution of coastal and shallow marine environments (Garcia-Gil et al., 2002). ...
... The gas sources and accumulation modes are closely related to the sedimentary and evolutionary processes occurring in the depositional environment (Garcia-Gil et al., 2002). Hence, the various gas reservoir and trapping sites, together with their associated types of gas accumulation and environmental settings provide valuable clues to the evolution of coastal and shallow marine environments (Garcia-Gil et al., 2002). ...
Gas-charged sediments are very common in coastal environments worldwide, and their occurrence is usually clearly revealed as acoustic anomalies by a number of different seismic tools at differing scales and resolutions. This paper presents examples of gas-related acoustic anomalies in high resolution sub-bottom profiles (SBP) from the Patos Lagoon, Southern Brazil. The echograms show acoustic gas-related anomalies, which can present distinctive morphology for sediment-trapped gas, leaking or free gas in the water column. The paleo-topographic depressions filled with gas-charged sediments are related to former drainage systems developed in the coast in response to Quaternary sea level up and down events. Hence, such shallow gas occurrences seem to be controlled by the previous environmental configuration, the transgressive infilling arrangement of the basin,
and the hydrodynamic conditions. Since the establishment of the present configuration, following sea level highstand of the Holocene, the Patos Lagoon interior seems to behave as a coastal trap for fine-grained, organic-rich gas-generator sediments, bordered by coarser gas-free sediments.
... A sub-bottom profiler (SBP) is designed to image underwater sub-bottom sediment for waterway dredging, marine scientific research, etc. [1][2][3][4][5][6][7]. Nowadays, SBP imaging is widely used as a tool with which to map and interpret changes in sub-bottom reservoirs. ...
... Nowadays, SBP imaging is widely used as a tool with which to map and interpret changes in sub-bottom reservoirs. However, the existing use of sub-bottom profilers has remained limited mostly to the delineation of sub-bottom layers, though remote acoustic classification of the physical properties of sub-bottom sediments is of paramount interest to the geotechnical and geological communities [1][2][3][4][5]. Sediment types are still difficult to discern because of uncertainties in rock-physics models [7]. ...
Sub-bottom profile data have the potential to characterize sediment properties but are seldom used for offshore site investigations because of uncertainties in rock-physics models. Deep-learning techniques appear to be poised to play very important roles in our processing flows for the interpretation of geophysical data. In this paper, a novel deep learning-based method for this task is proposed in which a nonlinear mapping between the observed data and sediment types is learned using a multi-attribute temporal convolution network (MATCN). Firstly, empirical mode decomposition (EMD) is employed for the original data, and intrinsic mode functions (IMFs) with multiple time scales are generated. Based on different IMFs, instantaneous frequency (IF) data under different IMFs can be obtained, while instantaneous phase (IP) and instantaneous amplitude (IA) data are obtained based on the original data. IF, IA and IP data are called attribute data, and are highly related to the attenuation, reflection, and interior structure of the sediment. Thus, IA, IF, and IP are used as the inputs, and a 1D convolutional neural network (CNN) and a time convolution network (TCN) are used to extract sequential features. Different feature representations are then fused. Combining cross-entropy loss function and class-edge loss function, the network is encouraged to produce classified results with more continuous sediment distributions compared with the traditional loss function. The real-data experiments demonstrate that the proposed MATCN has achieved good performance with an F measure greater than 70% in all cases, and greater than 80% in most cases.
... For this purpose, we investigated Fe speciation and mineral phases of sediments and pore water composition in two pockmarks and two reference stations in the Baltic Sea. Similar structures were found in multiple estuaries globally, e.g. in NW Spain (García-Gil et al. 2002), NE USA (Kelley et al. 1994), Kongo (Gay et al. 2003), Taiwan (Chuang et al. 2010) and Black Sea (Knab et al. 2009). These coastal methane reservoirs may be formed e.g. ...
... Higher CH 4 concentrations at MET1-BH compared to MET1-MP may also be a result of higher organic matter content of the sediment (the average LOI at MET1-BH is 19.1%, while at MET1-MP is 14.4%) combined with the reduced availability of sulfate for its decomposition as a result of groundwater outflow. Similar correlations between sulfate depletion, high organic matter content, and methanogenesis were found in the Black Sea (Knab et al. 2009) and NW Spain (García-Gil et al. 2002). In addition, in active pockmarks, as in the case of MET1-BH and MET1-MP, as a result of ebullition, methane from deeper layers is transported upward, and the gas concentration at the surface tends to be high (Idczak et al. 2020). ...
Pockmarks are formed as a result of gas (methane) or/and groundwater outflow from the sea bottom. Methane, the second most important (after CO2) greenhouse gas, has a significant impact on biogeochemical processes in the bottom sediments by affecting the cycling of some elements, e.g. C, Fe, and S. Active pockmarks may also lead to changes in water column conditions by causing nutrients release from sediments. In the present study, we have focused on the impact of biogeochemical processes in pockmarks (methanogenesis, anaerobic methane oxidation, and groundwater seepage) on the transformation of iron (Fe) and the mineral composition of the sediment. In pore water, concentrations of hydrogen sulfide, phosphate, ammonia, sulfate, chloride, dissolved inorganic carbon, iron, and methane were analyzed. In the sediment, Fe speciation was performed using sequential extraction. The mineral composition was determined using powder X-Ray diffraction and scanning electron microscopy. The results from two pockmarks (with active gas seepage and groundwater infiltration) and two reference stations in the southern Baltic Sea show that geochemical conditions in pockmark sediments are significantly different from those in the typical muddy sea bottom. Pore water in pockmarks is characterized by lower sulfate and higher dissolved carbon concentrations as compared to areas of the seafloor where such structures are absent. This is due to the outflow of groundwater, which was confirmed by lower chloride concentration. In addition, sulfate is used to oxidize methane diffusing from deeper layers. Sediments in pockmarks are enriched in Fe(II) carbonates and depleted in Fe(III) (oxy)hydroxides, resulting from the anaerobic oxidation of methane with Fe(III) (Fe-AOM). Ferrous iron produced in large quantities during Fe-AOM is precipitated with carbonates.
... In these cases, the seismic signal is strongly attenuated, reflections are discontinuous and no clear image of the subsurface can be obtained (Judd and Hovland, 1992). Such locally confined acoustic blanking or the presence of chaotic reflections described as acoustic turbidity is a known phenomenon in hydroacoustic and seismic data, for examplein the Black Sea, Baltic Sea or modern rivers, and is attributed to shallow gascharged sediment (Okyar and Ediger, 1999;Garcia-Gil et al., 2002;Mathys et al., 2005;Laier and Jensen, 2007;Tóth et al., 2014). In the German North Sea, investigations of shallow gas as a potential energy resource detected multiple acoustic anomalies (Trampe et al., 2014), but the exact origin and differentiation between shallow anomalies and fluid conduits in the North Sea is not well understood. ...
... Possibly, biogenic gas generated within the fluvial or tidal deposits simply escaped in most cases while only at selected locations, some form of barrier trapped gas within the sediments leading to sufficient accumulation to produce the blanking effect on seismic data. Fine-grained muds from meadow or floodplain deposits at the river flanks could form this barrier if the porosity contrast between them and underlying gas-bearing sediments is sufficiently high (Garcia-Gil et al., 2002). In Figure 4a, the horizontal subunit U3 at the flanks of the channel, which we interpreted as floodplain deposits, could form such a barrier. ...
Seismic data from the North Sea commonly show vertical acoustic blanking (VAB) often interpreted as fluid conduits with implications for Quaternary development. The robustness of this interpretation has long been controversial as the infill of tunnel valleys can also cause vertical blanking. Using 2D and 3D seismic data and sediment echosounder data from the German North Sea, we investigate VAB to determine a geological or imaging origin of these anomalies. We detected multiple VAB occurrences throughout the North Sea. 3D data from the Ducks Beak ('Entenschnabel') reveal a correlation of VAB with bright spots in incised channels directly below the seafloor. Large source-receiver distances allow imaging the subsurface below the channel without signal penetrating through it (undershooting). This method removes the blanking. Energy absorption by shallow biogenic gas trapped within the channels explains the observed VAB. Hence, the blanking represents an imaging artifact, highlighting the need for careful seismic processing with sufficient offset before interpreting such anomalies as fluid pathways. The channels belong to a postglacial channel system related to the now submerged lowlands of Doggerland. This work demonstrates the usability of mapping VAB to detect shallow features for paleo-landscape reconstruction and identification of shallow gas for hazard assessments, for example.
... Globally, coastal sediments have experienced dramatic sedimentological changes (Garcia-Gil et al., 2002;Lin et al., 2004;Okay & Aydemir, 2016;Rogers et al., 2006;Schroot & Schüttenhelm, 2003a;Weschenfelder et al., 2016). Since the Late Pleistocene, the global sea level has changed many times, which have all been recorded in offshore sediments. ...
... At the same time, shallow sea sedimentary facies arose with the rise in sea level during interglacial periods. Previous studies have focused on the geophysical characteristics of shallow biogenic gas (Benites et al., 2015;Fleischer et al., 2001;Garcia-Gil et al., 2002;Rogers et al., 2006;Schroot & Schüttenhelm, 2003b), and the methane-related microbial communities in marine surface sediments (Y. Chen et al., 2022;Giovannelli et al., 2016;Niu et al., 2018;Zhuang et al., 2018). ...
To better understand the depositional constraints on the fingerprint characteristics of methane isotopes, we present a set of carbon/hydrogen isotopic data for CH4, CO2, pore water, carbonate, and total organic carbon along a 70‐m sedimentary core. The sedimentary facies (Units I, II, and III from upper to bottom) suggested depositional environments of the present estuary, shallow marine, and floodplain‐estuary. Calculations suggested that around 86% of methane was produced through the CO2 reduction pathway. In this pathway, the hydrogen in CH4 is from ambient water, while the carbon is from dissolved inorganic carbon. Correspondingly, our study showed that the values of δDCH4 displayed similar trends with those of δDH2O and Cl⁻ concentrations along the depth profiles. The low δDCH4 below 44.5 m corresponded to low δDH2O and low salinity during the cold and low‐sea‐level period. The values of δ¹³CCH4 generally synchronously changed with those of δ¹³CCO2. The variation trends of δ¹³CCH4 and δ¹³CCO2 were the same with δ¹³Ccarbonate from 10 to 70 m depth but decoupled above 10 m. The values of δ¹³CCH4 in Units II and III were correlated with the δ¹³Ccarbonates, which is related to the sedimentary processes. But decoupling of low values of δ¹³CCH4 and δ¹³CCO2 from δ¹³Ccarbonates in Unit I may be related to preferential microbial consumption of labile compounds with light carbon isotopic compositions, such as lipids. In short, the variations of δ¹³CCH4 and δDCH4 were largely related to the sedimentary process, which controlled the isotopic compositions of both water and carbonate.
... In one ping measurement, the SBP transducer transmits acoustic waves and then receives reflected echoes. The recorded sequential echoes reflect the structural information of the measured sediment layers, and the reflection strength and IF of SBP data may aid in the lithology identification of sub-bottom layers [1][2][3][4][5][6][7]. Remote sediment classification based on SBP data enables the sub-bottom to be mapped more quantitatively, thus improving the degree of confidence of geological predictions, and this in turn has the potential to decrease the reliance on costly seabed sampling. ...
... Remote sediment classification based on SBP data enables the sub-bottom to be mapped more quantitatively, thus improving the degree of confidence of geological predictions, and this in turn has the potential to decrease the reliance on costly seabed sampling. However, the existing use of SBP is still mostly limited to the delineation of stratigraphic structures, although the remote acoustic classification of the physical properties (reflection coefficient, attenuation coefficient, etc.) of shallow sediments is of paramount interest to the geotechnical and geological communities in the field of mining and exploration for placer deposits and other marine mineral resources [4][5][6][7][8][9]. For example, SBP sediment classification based on the data acquired by De Beers Marine helped its offshore mining and exploration programs for diamondiferous placer deposits, situated on the continental shelf off the west coast of South Africa [9]. ...
The shift in IF (instantaneous frequency) series and the corresponding relaxation time have the potential to characterize sediment properties. However, these attributes derived from SBP (sub-bottom profiler) data are seldom used for offshore site investigations because of the unsound-ness in attribute calculation. To overcome this problem, a new reliable method combining VMD (variational mode decomposition) and WVD (Wigner-Ville distribution), as well as relaxation time, is presented. Since the number of modes in classical VMD should be provided in advance, a modified VMD algorithm, MVMD (modified variational mode decomposition), is proposed here, where the distribution of the frequency domain of modes is taken into account to automatically determine the number of modes. Through the relaxation time model, the IF data of a series of pings calculated through MVMD-WVD are transformed into a relaxation time map. A robust estimation algorithm is applied to the relaxation time map to reduce the effects of interferences and obtain robust relaxation times. The final relaxation time data are used to determine the sediment types. Real data from SBP experiments, as well as borehole sampling and geotechnical analysis results, verified the good performance of the proposed method.
... Furthermore, the accumulation of shallow gas can cause blowouts during petroleum drilling and platform foundations (Ostanin et al., 2012;Schroot and Schüttenhelm, 2003). Gas leakage from the seafloor may also influence seawater chemistry and introduce carbon dioxide (CO 2 ) and methane (CH 4 ) into the atmosphere (Davis, 1992;Garcia-Gil et al., 2002;Hovland et al., 1993), which are greenhouse gasses that increase the Earth's surface temperatures (e.g., Kiehl and Trenberth, 1997) while methane's initial impact is about 100 times greater, and its warming impact over the first 20 years is 72 times greater than that of CO 2 after it reaches the atmosphere (IPCC Fourth Assessment Report, 2007). ...
... Vertical gas migration is represented by the presence of seismic chimneys (Figs. 4 and 11) (e.g., Garcia-Gil et al., 2002;Hustoft et al., 2007Hustoft et al., , 2009Løseth et al., 2009). Seal-weakening, formation-wide overpressure, and free gas are required to initiate the formation of vertical fluid conduits. ...
High resolution multi-channel seismic reflection data (∼1000 km) and multibeam echosounder bathymetry from the southeastern Korean continental shelf of the East Sea (Japan Sea) reveal numerous shallow gas indicators and seepage-related features, such as bright spots, enhanced reflections, seismic chimneys, acoustic blanking, pockmarks, and bathymetric mounds. Bright spots, indicating gas-charged layers, appear as local negative-polarity reflection anomalies (up to 5 km wide) and occur at various stratigraphic levels within a subsurface depth of ∼320 m. Bright spots covering an area of ∼60 km² are clustered at the tip of NE-SW–trending reverse faults in the northeastern and southeastern part of the investigated region, suggesting gas entrapment. Enhanced reflections (ca. 20-km-long) are developed along erosional unconformities and tilted sedimentary layers below them. This suggests that unconformities formed during sea-level low stands in the study area are potential reservoirs and may have acted as potential conduits for lateral migration of gas-rich fluids due to their permeable nature. Some enhanced reflections are formed along interfluves of channels where channel walls cut them, and thus they may potentially act as fluid reservoirs. Seismic chimneys, expressed as vertical disturbances in seismic data, are interpreted as the upward movement of fluids (i.e., either in liquid or gaseous form). Lack of faulting in some seismic chimneys suggests higher permeability in the sedimentary interval, which would allow the migration of deeper-sourced fluids. Pockmarks (up to 500 m in diameter) are typically associated with seismic chimneys in the sub-seabed, suggesting that they were formed by the explosive emission of gas or gas fluids. Some exhibit mound-like features near their crests that are interpreted as carbonate mounds. The locations of mounds above uplifted fault blocks in the central part suggest a structural control on the formation of these seabed features.
... The profiles marked with blue rectangles in Figure 13 show acoustic blanks and acoustic turbidities. In some areas in the SBP image, there are large acoustic impedance differences because of gas seeps or carbonate rock, which cause the acoustic blank phenomenon [22]. Gas accumulation can also induce acoustic turbidities, which may be caused by the reflection and scattering of the acoustic energy by myriads of in situ gas bubbles, causing dark smearing on the SBP image. ...
... These phenomena disturb the interface structure and weaken the line-like characteristics of interfaces [22]. In these disturbed areas, only the main interfaces are picked. ...
Traditional manual horizon picking is time-consuming and laborious, while automatic picking methods often suffer from the limited scope of their applications and the discontinuity of picked results. In this paper, we propose a novel method for automatic horizon picking from sub-bottom profiles (SBP) by an improved filtering algorithm. First, a clear and fine SBP image is formed using an intensity transformation method. On this basis, a novel filtering method is proposed by improving the multi-scale enhancement filtering algorithm to obtain clear horizons from an SBP image. The improvement is performed by applying a vertical suppression weighting term based on the form of logistic function, which is constructed by using the eigenvectors from the Hessian matrix. Then, the filtered image is segmented using a threshold method, and the horizon points in the SBP image are picked. After that, a horizon linking method is applied, which uses the horizon directions to refine the picked horizon points. The proposed method has been verified experimentally, and accurate and continuous horizons were obtained. Finally, the proposed method is discussed and some conclusions are drawn.
... Stratigraphic studies of the sub-bottom below the sea floor have been undertaken using high resolution seismic reflection. Various studies have been carried out on passive marginal coasts around the world (Somoza, 1989;Duncan et al., 2000;Fleischer et al., 2001;García-Gil et al., 2002;Iturralde-Vinent, 2003;Weber et al., 2004;García-García et al., 2004;Simó et al., 2005;Nordfjord et al., 2005;Lavoie et al., 2008;Tang et al., 2010;Donnici et al., 2015;Meireles et al., 2016;Xu et al., 2016;Alves and de Mahiques, 2019;among others). The continental shelf, coastal and transitional zones in Argentina have also been recorded (Parker and Violante, 1982;Isla, 1989;Parker et al., 1999Parker et al., , 2008Cavallotto et al., 2004;Schnack et al., 2005;Violante et al., 2010Violante et al., , 2014Bozzano et al., 2017;Ercilla et al., 2019;Isola et al., 2020). ...
... These provoke the acoustic masking (or anomalies) in seismic records. These anomalies can induce patterns of reflection that have been observed and described in various environments by several authors: in marine sectors (Judd and Hovland, 1992;Hovland et al., 1992aHovland et al., , 1992bKarisiddaiah et al., 1993;Karisiddaiah and Veerayya, 1994;Fleischer et al., 2001;Taylor, 1992), coastal environments (Taylor, 1992;Weschenfelder et al., 2006Weschenfelder et al., , 2014Baltzer et al., 2005), deltaic environments (Hart and Hamilton, 1993;Figueiredo et al., 1996), in the continental shelf (García-Gil et al., 2002;Missiaen et al., 2002), estuaries (García-Gil, 2003;Iglesias and García-Gil, 2007;Aliotta et al., 2002Aliotta et al., , 2006Aliotta et al., y 2011Martínez-Carreño and García-Gil, 2013;Benites et al., 2015), in bays (Wever and Fiedler, 1995;Karnaukh et al., 2016) and lagoons (Hovland et al., 2012), among others. In BBE the presence of this kind of gas was revealed by Aliotta et al. (2002Aliotta et al. ( , 2006Aliotta et al. ( and 2011. ...
The Quaternary sedimentary deposit of the inner sector of Bahía Blanca estuary (BBE), Argentina, was studied using high-resolution seismic profiles, core samples and correlation with continental – marine units outcropping in the continent. We interpreted four prominent seismic reflection surfaces and the acoustic basement which together defined five seismic stratigraphic units (S0, S1, S2, S3, S4) and four related systems tracts. Studying those units, we proposed a fourth Stage deposition evolutionary model that took place in a relative sea-level fluctuation context after the Miocene in the BBE area. The first stage contained the deposition of the regressive seismic units S0 and S1. The S0 was determined as an acoustic basement. The top of the S0 erosional surface (SU) was determined and it was formed by subaerial processes during the Late Pleistocene fall in sea-level and the following marine reworking during transgression. The unit was correlated with the oldest continental Miocene loess deposit and was associated with an Early Lowstand Systems Tract (ELST). Unit S1 was associated with a Late Lowstand Systems Tract (LLST). An irregular stratigraphic surface was observed above it. It was inferred as a Maximum Regressive Surface and the Ravinement Surface (RS/MRS) which are related to a maximum regression and subsequent transgression. The S1 unit was correlated with Pliocene-Pleistocene continental deposits. The Miocene - Pleistocene coastline movement in this sector would be associated with both Neogene Andean dynamics and the Last Maximum Glacial. After that, during the second Stage in the Late Pleistocene - middle Holocene the sea-level on the Bahia Blanca coast rose and unit S2 was formed. We interpreted unit S2 as a Transgressive System Tract which developed until the maximum sea-level of the Mid-Holocene. The coastline extension on the continent conformed the Maximum Flooding Surface. After a stabilization period to the mid Holocene, the sea-level started falling, offering conditions for the deposition of the High Stand System Tract (S3 – S4). Following stabilization, the third Stage took place and a progradation unit S3 was deposited overlying the MFS and a minor stratigraphic surface Bedset Boundary (BsB) separated this unit from S4. During the fourth phase, the sea-level continued to fall until the present level and the last HST (S4) was interpreted, which is interpreted as an aggradational – progradational deposit formed in the Middle Holocene to the present day by a change in the rate of sediment supply or energy flux.
... Na Baía Norte (Fig.3) dos 142 perfis, 45 apresentaram presença de gás. Sendo que na porção Norte, próximo ao estreitamento da Baía os gases apresentam profundidades entre 0,8 e 3m e estão na forma de Coberturas Acústicas, onde alguns pontos possuem associações de Pináculos de Turbidez, que de acordo com Garcia Gil et al. (2002), são uma variação da frente da Cobertura Acústica que aparece como em forma de tenda, obscurecendo completamente os refletores subjacentes que sobem até 1m acima da média dos gases, às vezes atingindo o fundo do mar. A parte central da Baía apresenta gases com profundidade de 0,7m até a superfície, com predomínio de Coberturas Acústicas com presença de Pináculos de Turbidez e Plumas intrasedimentares, as quais são feições que consistem de anomalias parabólicas atravessando refletores reais (Garcia Gil et al., 2002), que na Baía se tornam cada vez mais próximo da superfície. ...
... Sendo que na porção Norte, próximo ao estreitamento da Baía os gases apresentam profundidades entre 0,8 e 3m e estão na forma de Coberturas Acústicas, onde alguns pontos possuem associações de Pináculos de Turbidez, que de acordo com Garcia Gil et al. (2002), são uma variação da frente da Cobertura Acústica que aparece como em forma de tenda, obscurecendo completamente os refletores subjacentes que sobem até 1m acima da média dos gases, às vezes atingindo o fundo do mar. A parte central da Baía apresenta gases com profundidade de 0,7m até a superfície, com predomínio de Coberturas Acústicas com presença de Pináculos de Turbidez e Plumas intrasedimentares, as quais são feições que consistem de anomalias parabólicas atravessando refletores reais (Garcia Gil et al., 2002), que na Baía se tornam cada vez mais próximo da superfície. ...
... Most of the above-named gas escape structures have been documented worldwide (e.g. Hovland & Judd, 1988;Hovland, 1992;Premchitt et al., 1992;Soderberg & Floden, 1992;Judd, 2001;Garcia-Gil et al., 2002) and, as is shown by the present study, also occur extensively in the western Irish Sea region. ...
... Decrease in seismic velocity and lower density in gas-charged sediments can also cause a polarity reversal of the corresponding reflections (Garcia-Gil et al., 2002;Kim et al., 2020). ...
New 2D high-resolution seismic and hydro-acoustic data demonstrate the presence of methane in the shallow
sediments and its origin in the Pomeranian Bight, south of the Baltic Sea area. Various shallow gas features
including chimneys, bright spots, acoustic blanking, pockmarks, and polarity reversal were identified in the
Gryfice block, along the inverted Trzebiatow ´ fault zone. Structural and stratigraphic interpretation with support
of seismic attributes shows the potential of fluid migration pathways from the Upper Triassic formation reser�voirs to shallow sediments below the seafloor. It also helps in the explanation of how this natural gas escapes to
the sea bottom. Amplitude-vs-offset (AVO) analysis proved free gas existence in the potential Upper Triassic
reservoir, and helped locate free gas deposits within sediments. Hydro-acoustic data illustrated the gas chimneys’
anomalies and the corresponding free gas accumulation in Pleistocene to Quaternary successions. Leaking of gas
to the seafloor was also proved by the exposure of pockmarks on multibeam (bathymetry) data at the seafloor.
We combine seismic, hydro-acoustic data, and information on petroleum system from previous studies to explain
the signatures of free gas and its migration from lower reservoirs to shallow sediments.
... This sedimentary structure (source-reservoir-cap assemblage), the fine particles with high organic matter content at the bottom, coarse grains with high porosity in the middle, and low permeability fine particles at the top, is conducive to the formation of gas pockets (García- García et al., 2013). The sedimentary facies are the main factor determining whether gas accumulates or seeps, and this has also been demonstrated in another incised valley fill (Garcia-Gil et al., 2002). In some places, the upper layers were not well sealed, leading to the release of biogenic gas from deeper layers along the fissures (Dalla Valle and Gamberi, 2011;. ...
Marine sediments are the world's largest known reservoir of methane. In many coastal regions, methane is trapped in sediments buried at depths ranging from centimeters to hundreds of meters below the seafloor, in the forms of gas pockets, dispersed gas bubbles and dissolved gas, also known as shallow gas (methane-dominated gas mixture). The existence of shallow gas affects the engineering geological environment and threatens the safety of artificial facilities. The escape of shallow gas from sediments into the atmosphere can even threaten ecosystem security and affect global climate change. However, until now, shallow gas has remained a mystery to the scientific community. For example, how it is generated, how it distributes and migrates in sediments, and what are the factors that influence these processes that are still unclear. In the context of increasingly intense offshore development and global warming, there is a huge gap between existing scientific understanding of shallow gas and the need to develop scientific solutions for related problems. Based on this, this paper systematically collects the information on all aspects of shallow gas mentioned above, comprehensively summarizes the current scientific understanding, and analyzes the existing shortcomings, which will provide systematic references for the research on environmental disaster prevention, engineering technology, climate change, and other fields.
... These features are frequently observed together with acoustic turbidity zones (Judd and Hovland, 1992). Decrease of seismic velocity and lower density in gas-charged sediments can also cause polarity reversal of the corresponding reflections (Garcia-Gil et al., 2002;Kim, 2020). ...
New 2D high resolution seismic and hydro-acoustic data demonstrate the presence of methane in the shallow sediments and its origin in the Pomeranian Bight, southern of the Baltic Sea area. Various shallow gas features were identified in the Gryfice block, along the inverted Trzebiatów fault zone, including chimneys, bright spots, acoustic blanking, pockmarks, and polarity reversal. Structural and stratigraphic interpretation with support of seismic attributes was carried out to show the potential of fluid migration pathways from the Upper Triassic formation reservoirs to shallow sediments below seabed and helps in explanation of how this natural gas escapes to the sea bottom. Amplitude-vs-offset (AVO) analysis verified remnants of free gas existence in the Upper Triassic potential reservoir and helped locating free gas deposits within sediments. Hydro-acoustic data illustrated the gas chimneys’ anomalies and corresponding free gas accumulation in Pleistocene to Quaternary successions. Leaking of gas to sea surface was also proved by exposure of pockmarks on multibeam (bathymetry) data. We combine seismic, hydro-acoustic data and information on petroleum system from previous studies to explain signatures of free gas and its migration from lower reservoirs to shallow sediments.
... Faults may also influence shallow gas migration, as observed in other coastal regions (Dimitrov, 2002;Garcia-Gil et al., 2002;Schroot and Schuttenhelm, 2003). Faults have been observed in the Pearl River Estuary, as well as in the southern part of the estuary near the entrance (Fig. 1c) (Li et al., 2006;Shi et al., 2013). ...
High-resolution seismic and side-scan surveys were conducted to investigate shallow gas accumulation and seepage in the Holocene sediment of the Pearl River Estuary. Extensive shallow gas accumulation and active seepage were detected in the Holocene sediments of the estuary and adjacent shelf regions. Laterally continuous and extremely shallow gas fronts were observed in areas with relatively higher sedimentation rates, whereas shallow gas was absent or discontinuous in regions with the sediment with lower sedimentation rates. Gas accumulation was present at extremely shallow depths in the West Shoal, which is the recent depocentre location. The co-occurrence of shallow gas distributions and variations in the sedimentation rate was related to the role of sediment accumulation in organic carbon burial and preservation, both of which are required for gas generation and accumulation in sediment. In addition, the consistency between the distribution of highwater column methane concentrations in the West Shoal, the super shallow gas accumulation pattern in the sediment profile, and active seepage indicate that shallow gas in the Holocene sediment is a methane source in the Pearl River Estuary. Further, considerable evidence of recent or relic seepage is present in areas that have experienced active anthropogenic disturbances, including sand mining, dredging, and bridge building. The disrupted and discontinuous gas fronts at these sites suggest that active shallow gas releases were influenced by anthropogenic activity.
... At a depth of approximately 3.4 s TWT, the fault reaches an acoustic turbidity region most likely corresponding to free gas accumulations and possibly acting as a suitable pathway for deeper gas to be transported to shallow sedimentary units. Acoustic turbidity zones mask out deeper reflections due to the high and diffuse reflectivity of gas present at their top (e.g., García Gil et al. 2002;Ergün et al. 2002;Dondurur et al. 2011). When reaching the gas hydrate stability zone in ~ 200-250 ms TWT below seafloor, gas migrating along the fault will form gas hydrates, which act as a cap rock as indicated by shallow gas anomalies underneath the BSR under the ridges of the study area (Figs. ...
Widespread indications of free and hydrate gas accumulations and mud volcanoes were imaged using multichannel seismic reflection, chirp sub-bottom profiler, multibeam bathymetry, and deep-towed side-scan sonar data collected along the Southwestern Black Sea margin, offshore Akçakoca. These indications are typically associated with sedimentary ridges along the continental slope and rise, particularly with the “Ereğli Plateau” (850–1350 m water depth), where 20 mud volcanoes were found. Two types of bottom-simulating reflections (BSR) were identified, both mimicking the seafloor relief: Type-1 crosscuts the sedimentary reflections with amplitudes similar to the surrounding strata, while Type-2 shows higher amplitudes that terminate against the base of the gas hydrate stability zone. These types were observed over large portions of the continental rise, indicating the base of gas hydrate accumulations. Analyses of the BSR depth indicate that thermogenic gas is possibly present in the gas hydrates. A fault-driven hypothetical model was then developed to describe the formation of gas hydrate and mud volcanoes and the effect of submarine fluid flow in the area. According to this model, Type-1 BSRs form through biogenic gas accumulations. The presence of fault regions with Type-2 BSRs suggests active fluid transfer between permeable and impermeable units, where the fault surfaces act as possible conduits for thermogenic gases produced in the deeper sediments and transported into the shallower subsurface where then thermogenic biogenic gases coexist.
... The lowermost valley surface, horizon H3, is a varying amplitude reflector that commonly separates low-amplitude laminated seismic facies above from highamplitude laterally accreting and chaotic reflectors or complete acoustic blanking below (Figures 8,10,12). For the majority of the study area, H3 forms a sharp acoustic contact, and the observed masking is characteristic of biogenic gas accumulation along a lithologic contact as observed previously within the Trinity valley and elsewhere (e.g., Rodriguez et al., 2005;Thomas and Anderson, 1994;Garcia-Gil et al., 2002). Where this acoustic blanking is not present lateral accretion surfaces and potential channel forms are observed. ...
Modern lowland river systems show transitions in flow characteristics near coastlines that lead to systematic changes in sediment deposition and stratigraphic architecture. Sensitivity of fluvial morphodynamics to base-level has important implications for the prediction and interpretation of fluvial stratigraphy, particularly in deposits formed during periods of relative sea-level rise such as the early Holocene. Improving our understanding of how fluvial stratigraphy is created and preserved in such environments is crucial to paleoenvironmental reconstruction, sand resource estimation, and mapping subsurface facies distributions. A significant challenge has been capturing the spatiotemporal evolution of such systems on long timescales (centuries to millennia), often due to poor data coverage and resolution. Here we investigate the offshore stratigraphic architecture of the Trinity coastal river system, Texas. The paleo-valley is interpreted to have formed during the last lowstand and filled by backstepping fluvial-deltaic and estuarine sediments over the Holocene. However, the nature of this transition and resulting stratigraphy is unclear. We present unprecedented imaging of the incised valley and fluvial stratigraphy using a combination of over 500km2 of 3D seismic and nearly 700km of full waveform chirp data with 250m line spacing, combined with sediment cores and geotechnical borings. Our chirp processing technique creates images of strata on the decimeter scale, allowing for near outcrop scale mapping and interpretation. Archival industry 3D seismic data show that the basal valley fill is comprised of a highly-amalgamated fluvial channel belt with numerous avulsions and loop cutoffs. The chirp data show the transition of individual fluvial channels from sand rich, laterally migrating systems to muddy channels that depict high rates of vertical aggradation and little lateral movement. We observe the interaction of these channels with the paleo-floodplain and show that aggradation and infilling of the incised valley is dominated by fluvial processes before a transition to bay and estuarine conditions. Our work illustrates that traditional models of incised valley filling and fluvial response to transgression fail to fully capture the morphodynamics of coastal river systems.
... While the plumes' effect is just an hypothesis (García-Gil et al., 2002), the presence of different water layers is clearly proven by Fig. 11, displaying the seafloor and the shallowest reflections along the line G05-C02, acquired closer to the coast. We vertically exaggerated the time span from 4 to 20 ms, i.e. between the sea surface and seafloor, and used a large global scaling factor. ...
Recent studies revealed the emission of fluids from the seafloor at various locations in the Gulf of Trieste (northern Adriatic Sea). To image the shallow sediments where this happens, several Chirp surveys were carried out. The shallow water reverberations limited the interpretation of this data, so we set up a workflow that attenuated the multiple reflections significantly. The new images of the shallow formations allowed improving our understanding of the most recent geological phenomena. Also, faint reflections from water layers with different acoustic impedance, probably due to temperature differences, were revealed, and several plumes were imaged. These images allow linking the shallow stratigraphy of the seafloor to ongoing fluids' emissions.
... The detailed horizons play an important role in the discovery of marine mineral resources. For example, the migration of gas hydrate under the seabed can produce small reflector changes in high resolution seismic profiles, namely the acoustic blank and acoustic turbidity, etc. [8][9][10]. The rare earth marine mineral resources in the sub-bottom can produce reflections of low intensity and can generate detailed reflectors on SBP images [11,12]. ...
A sub-bottom profiler (SBP) can capture the sediment interfaces and properties of different types of sediment. Horizon picking from SBP images is one of the most crucial steps in marine sub-bottom sediment interpretation. However, traditional horizon picking methods are good at obtaining the main horizons representing the main reflectors while ignoring the detailed horizons. While detailed horizons are the prime objective, many tiny structures caused by interference echoes will also be picked. To overcome this limitation, an integrated horizon picking method for obtaining the main and detailed horizons simultaneously is proposed in this paper. A total of three main process steps: the diffusion filtering method, the enhancement filtering method as well as the local phase calculation method, are used to help obtain the main and detailed horizons. The diffusion filtering method smooths the SBP images and preserves reflectors. Enhancement filtering can eliminate outliers and enhance reflectors. The local phase can be used to highlight all of the reflections and help in the choosing of detailed horizons. A series of experiments were then performed to validate the effectiveness of the proposed method, and good performances were achieved.
... The first type of seismic anomaly is polarity reverse. It marks a localized decrease in acoustic impedance (the product of velocity times density) [36,37], which indicates the presence of free gas in the strata [38]. The sediment interface in the shallow strata is normally displayed in the seismic section as a positive polarity reflection; however, polarity reverses break the continuous reflection event and exhibit a negative seismic phase (Figures 3 and 4). ...
The 3D seismic data acquired in the central Qiongdongnan Basin, northwestern South China Sea, reveal the presence of shallow gas hydrate, free gas, and focused fluid flow in the study area, which are indicated by multiple seismic anomalies, including bottom simulating reflectors, polarity reverses, pulldowns, minor faults, and gas chimneys intensively emplaced within the shallow strata. A new cold seep is also discovered at approximately 1520 m water depths with an ~40 m wide crater in the west part of the study area. Water column imaging, seafloor observation, and sampling using the remotely operated vehicle “Haima” demonstrate ongoing gas seepages and shallow gas hydrates at this site. Thermogenic gas in the study area migrates from the deep reservoir through the gas hydrate stability zone along deep faults and gas chimneys, forms shallow gas hydrate and free gas, and sustains localized gas seepage within this cold seep. The results provide insight into the relationship between shallow gas hydrate accumulation and deep hydrocarbon generation and migration and simultaneously have important implications for hydrocarbon explorations in the Qiongdongnan Basin, northwestern South China Sea.
... The presence of all these fluid flow features usually denotes subsurface hydraulic activity, over-pressurization, fluidization, and degassing processes, as well as sudden fluid (gas and/or liquid) release that may produce gas explosions, slope sedimentary instabilities, and an uplifting/subsiding seafloor [122,[147][148][149] (Figure 6c). These processes can have major impacts on seabed infrastructures and on those requiring piles that are driven into the seafloor. ...
Offshore geological hazards can occur in any marine domain or environment and represent a serious threat to society, the economy, and the environment. Seismicity, slope sedimentary instabilities, submarine volcanism, fluid flow processes, and bottom currents are considered here because they are the most common hazardous processes; tsunamis are also examined because they are a secondary hazard generated mostly by earthquakes, slope instabilities, or volcanic eruptions. The hazards can co-occur and interact, inducing a cascading sequence of events, especially in certain contexts, such as tectonic indentations, volcanic islands, and canyon heads close to the coast. We analyze the key characteristics and main shortcomings of offshore geological hazards to identify their present and future directions for marine geoscience investigations of their identification and characterization. This review establishes that future research will rely on studies including a high level of multidisciplinarity. This approach, which also involves scientific and technological challenges, will require effective integration and interplay between multiscale analysis, mapping, direct deep-sea observations and testing, modelling, and linking offshore observations with onshore observations.
... Pockmarks are circular or sub-circular seabed depressions, which may reach diameters of hundreds of metres and depths of tens of metres (Judd and Hovland, 2007;King and MacLean, 1970). It is now understood that pockmarks are globally widespread, occurring in the abyssal plains and continental margins (Nelson et al., 1979;Paull et al., 2002;Picard et al., 2018;Pilcher and Argent, 2007;Skarke et al., 2014), but also in shallow coastal settings such as estuaries and bays (Brothers et al., 2011;Garcia-Gil et al., 2002;Jordan et al., 2019;Szpak et al., 2015;Wildish et al., 2008) and freshwater lakes (Pickrill, 2006;Wirth et al., 2020). Recent surveys highlight very high densities of pockmarks, or 'pockmark fields' in shallow coastal settings: for example, densities of up to 1200 km 2 in the German Bight (Krämer et al., 2017) and up to 5500 km 2 in the Bay of Concarneau, France (Baltzer et al., 2014). ...
Marine pockmarks are globally widespread seabed depressions, conventionally thought to be formed by the accumulation and expulsion of microbial and thermogenic gas. However, other putative fluids and processes have been implicated in pockmark formation and gas escape to the atmosphere may be underestimated. Given the complex spectrum of aquatic settings, morphologies and sizes, there may also exist a spectrum of physical, chemical and biological processes that form pockmarks. Pockmarks in shallow coastal waters are now understood to be widespread, but the influence of physical dynamics (e.g. tides, storms, etc.), terrestrial processes and anthropogenic activities add considerable spatiotemporal complexity and uncertainty to our understanding of these features. Here, we revisit a field of small (ca. 2 m diameter), shallow (<1 m depth) pockmarks in Dunmanus Bay, Ireland. The presence of muddy surface sediments overlying sand in the pockmarked area indicates that gas accumulation within fine-grained surface sediments contributes to formation of the features. Previous work indicates that CH4 is an important seepage fluid in Dunmanus and neighbouring bays. However, based on evidence from multiple surveys, we observe considerable spatiotemporal complexity, and the transient nature of the gas within sediments points to the potential for fluids other than traditional microbial or thermogenic CH4, migrating from sources tens to hundreds of metres below the seafloor. We observed atypical porewater profiles where millimolar concentrations of H2S concentrations are observed in surface sediments in the absence of SO42− depletion, together with NH4+ build-up from ammonification of sedimentary organic matter. Archaeal methanogens, anaerobic methanotrophic archaea and SO42--reducing deltaproteobacteria co-occur in surface sediments in the pockmark field and NMR revealed the presence of non-competitive substrates for methanogens. We hypothesize that in-situ methanogenesis and production of other volatile metabolites besides CH4 (CO2, dimethyl disulfide) from microbial degradation of organic matter are potential gaseous fluids and could contribute to the formation of small pockmarks.
... In SU3 shallow gas cutting, the plan-parallel depositional style was evident. The presence of gas in shallow seismic studies is commonly noted in incised valley fill (Garcia-Gil et al. 2002;Gomes et al. 2016), estuarine environments (Frazão and Vital 2007), lagoon and lake environments Weschenfelder et al. 2016), and continental shelves (Vardar and Alpar 2016). Gas accumulation occurs in Quaternary fine sedimentary deposits, which possess a strong relation with the paleotopography-incised valley systems (Weschenfelder et al. 2016). ...
This study aimed to understand the key surfaces and the sedimentary filling pattern of the Coreaú incised valley (CIV) system (NE Brazil) using high-resolution seismic stratigraphy. Six key surfaces (Ss) and five seismic units (SUs) were identified. The difference in the basal reflector (S1) and the paleointerfluve (modern sea floor) was verified to be up to 40 m of incision, of which only 20 m are filled. S1 (sequence boundary—SB) was associated with the acoustic basement (Tibau Formation/Barreiras Group) and presented strong control (antecedent topography) in the incision (re-incisions in the Pleistocene lowstands) and sedimentary stacking pattern. The CIV fill showed lowstand system tract, LST (chaotic facies)–transgressive system tract, TST (horizontal layers to chaotic facies)–highstand system tract, HST (chaotic facies) from the base to top. Thin fluvial deposits above SB correspond to LST. Overlapping this system tract is the first marine flooding surface—transgressive surface. Afterwards, the largest seismic unit (lateral and vertical) with sub-parallel and parallel (onlap) seismic facies, interpreted as related to a low energy system, with shallow gas, and vertical accretion (central basin of estuary) occurred. This environment is related to the drowning of the shelf from the shelf break (a decrease in gradient). A tide ravinement surface truncated the top of SU3 and favored coastal to shallow marine deposition. The transgressive system tract (TST) is thicker in the middle (mainly) and outer sectors in the fluvial troughs (depocenter). The maximum flooding surface appears near the sea floor and favored the development above of carbonate sedimentation in the HST. These seimostratigraphic stacking patterns showed a strong influence of semi-arid climate and antecedent topography/structural inheritance. Thus, it is evident that the filling of the CIV differs from other valleys on semi-arid systems and/or influenced by paleotopography (e.g., South Africa and Texas Shelf), which demonstrates the importance of local settings. The great influence of estuarine to coastal sediments in the valley infilling is also observed in the modern Coreaú coastal plain incised valley.
... In high-resolution acoustic profiles or seismic profiles, the gas-bearing sediments appear as acoustic chaos, acoustic blanking, velocity pull-down, "bright spot", acoustic curtain, acoustic column, acoustic blanket, and enhanced reflections (Judd AG and Hovland M, 1992;Baraza J and Ercilla G, 1996;Garcia-Gil S et al., 2002;Wang JL et al., 2018a). Among the above, the acoustic blanking zone is the most common feature (Lee G et al., 2005;Jensen J and Bennike O, 2009;Mazumdar A et al., 2009). ...
Seabed fluid escape is active in the Makran subduction zone, Arabian Sea. Based on the new high-resolution 2D seismic data, acoustic blanking zones and seafloor mounds are identified. Acoustic blanking zones include three kinds of geometries: Bell-shaped, vertically columnar and tilted zones. The bell-shaped blanking zone is characterized by weak and discontinuous reflections in the interior and up-bending reflections on the top, interpreted as gas chimneys. Vertically columnar blanking zone is interpreted as side-imaged gas chimneys associated with focused fluid flow and topped by a seafloor anomaly expressed as a localized reflection discontinuity, which may together serve as a vent structure. Tilted acoustic blanking zone could be induced by accretionary thrust activity and rapid sedimentation surrounding slope. Seafloor mounds occur at the sites of bell-shaped acoustic blanking zone and may be associated with the material intrusion. Bottom simulating refectors (BSRs) are widely distributed and exhibit a series of characteristics including diminished amplitude, low continuity as well as local shoaling overlapping with these acoustic blanking zones. The large amount of gases dissociated from the gas hydrates migrated upwards and then arrived at the near-seafloor sediments, followed by the formation of the gas hydrates and hence the seafloor mound.
... In high-resolution acoustic profiles or seismic profiles, the gas-bearing sediments appear as acoustic chaos, acoustic blanking, velocity pull-down, "bright spot", acoustic curtain, acoustic column, acoustic blanket, and enhanced reflections (Judd AG and Hovland M, 1992;Baraza J and Ercilla G, 1996;Garcia-Gil S et al., 2002;Wang JL et al., 2018a). Among the above, the acoustic blanking zone is the most common feature (Lee G et al., 2005;Jensen J and Bennike O, 2009;Mazumdar A et al., 2009). ...
... In high-resolution acoustic profiles or seismic profiles, the gas-bearing sediments appear as acoustic chaos, acoustic blanking, velocity pull-down, "bright spot", acoustic curtain, acoustic column, acoustic blanket, and enhanced reflections (Judd AG and Hovland M, 1992;Baraza J and Ercilla G, 1996;Garcia-Gil S et al., 2002;Wang JL et al., 2018a). Among the above, the acoustic blanking zone is the most common feature (Lee G et al., 2005;Jensen J and Bennike O, 2009;Mazumdar A et al., 2009). ...
... In high-resolution acoustic profiles or seismic profiles, the gas-bearing sediments appear as acoustic chaos, acoustic blanking, velocity pull-down, "bright spot", acoustic curtain, acoustic column, acoustic blanket, and enhanced reflections (Judd AG and Hovland M, 1992;Baraza J and Ercilla G, 1996;Garcia-Gil S et al., 2002;Wang JL et al., 2018a). Among the above, the acoustic blanking zone is the most common feature (Lee G et al., 2005;Jensen J and Bennike O, 2009;Mazumdar A et al., 2009). ...
Seabed fluid escape is active in the Makran subduction zone, Arabian Sea. Based on the new high-resolution 2D seismic data, acoustic blanking zones and seafloor mounds are identified. Acoustic blanking zones include three kinds of geometries: bell-shaped, vertically columnar and tilted zones. The bell-shaped blanking zone is characterized by weak and discontinuous reflections in the interior and up-bending reflections on the top, interpreted as gas chimneys. Vertically columnar blanking zone is interpreted as side-imaged gas chimneys associated with focused fluid flow and topped by a seafloor anomaly expressed as a localized reflection discontinuity, which may together serve as a vent structure. Tilted acoustic blanking zone could be induced by accretionary thrust activity and rapid sedimentation surrounding slope. Seafloor mounds occur at the sites of bell-shaped acoustic blanking zone and may be associated with the material intrusion. BSRs are widely distributed and exhibit a series of characteristics including diminished amplitude, low continuity as well as local shoaling overlapping with these acoustic blanking zones. The large amount of gases dissociated from the gas hydrates migrated upwards and then arrived at the nearseafloor sediments, followed by the formation of the gas hydrates and hence the seafloor mound.
... In addition, intrinsic attenuation, which is related to the properties of the materials, also affects the envelope. The envelope variations are also associated with the measuring situation, sediment types, and distributions of the mediums, for example, a strong envelope exists in sand or gas accumulation areas (Garcia-Gil et al., 2002). All of these bring difficulties to horizon picking just based on the envelope image, and can cause discontinuous picked horizons. ...
We propose a new method that considers the envelope, phase attributes, and texture analysis of the subbottom profile to automatically obtain continuous and accurate horizon picking. This method overcomes the shortcomings of the traditional methods in envelope and phase automatic horizon picking in terms of accuracy and continuity, and in efficiency when it comes to manual picking. Under the constraint of the envelope threshold from the envelope image and the texture segmentation from the phase image, the accurate envelope horizons are picked from the envelope image. Through mean filtering and template enhancement, the fine phase horizons are picked from the phase image. Finally, by combining the two kinds of picked horizons, more continuous, accurate, and finer horizons are obtained. The proposed method was tested on a real data set.
... In addition, intrinsic attenuation, which is related to the properties of the materials, also affects the envelope. The envelope variations are also associated with the measuring situation, sediment types, and distributions of the mediums, for example, a strong envelope exists in sand or gas accumulation areas (Garcia-Gil et al., 2002). All of these bring difficulties to horizon picking just based on the envelope image, and can cause discontinuous picked horizons. ...
Abstract We propose a new method that considers the envelope, phase attributes, and texture analysis of the subbottom profile to automatically obtain continuous and accurate horizon picking. This method overcomes the shortcomings of the traditional methods in envelope and phase automatic horizon picking in terms of accuracy and continuity, and in efficiency when it comes to manual picking. Under the constraint of the envelope threshold from the envelope image and the texture segmentation from the phase image, the accurate envelope horizons are picked from the envelope image. Through mean filtering and template enhancement, the fine phase horizons are picked from the phase image. Finally, by combining the two kinds of picked horizons, more continuous, accurate, and finer horizons are obtained. The proposed method was tested on a real data set.
... Acoustic masking due to the presence of gas in the sediment (Garcia-Gil et al. 2002) blocked the visualization of subsurface features in the central portion of the clinoform (Fig. 1). Thus, the data presented and discussed herein regards mainly the distal portions of the modern clinoform not affected by gas blanketing. ...
The present study used shallow high-resolution seismic surveys to characterize the architecture and stratigraphic evolution of the
last depositional sequence of the São Francisco Delta (SFDS). The sequence was accumulated within a bathymetric low (BL) on
the shelf located in front of the São Francisco river. This antecedent topography provided additional accommodation of tens of
meters in comparison with the rest of the shelf. The BL and its sediment-trapping effect allowed the accumulation of a complete
stratigraphic record of the eustatic sea-level rise since the Last Glacial Maximum (LGM). The SFDS comprises six stratigraphic
units. Because cores and radiocarbon dates are not available for this study area, the depth of occurrence of the top and base of each
unit and their lateral relationships with wave-cut terraces carved into the BL walls were used in an effort to tentatively tie the
mapped units to known eustatic sea-level curves available in the literature. Results suggest that major episodes of accumulation
occurred during intervals of reduced rates in sea-level rise. The lower five units mapped accumulated in a large bay-estuarine
protected environment, which resulted from the inundation of the BL. Only the topmost unit (the present clinoform) accumulated
in an open ocean environment when sea level reached approximately its present position after 8 ka and the whole shelf was
flooded. The most important delta backstepping event was probably associated with Meltwater Pulse 1B, which followed the
Younger Dryas and produced one of themost significant stratal surfaces of the SFDS.Among the existing wave-dominated deltas
in the Brazilian coast only, the São Francisco was able to construct a well-developed sigmoidal muddy clinoform in an otherwise
very shallow shelf subjected to high wave-energy levels. This was only possible because of the existence of a bathymetric low in
front of the river mouth, which illustrates the role that antecedent topography might play in providing larger amount of available
accommodation space called in this paper of additional accommodation.
... Comparison of stations A, B, and C revealed that differences in the activity and flow of cold seeps lead to different reflections of their biochemical characteristics, which are often closely related to their regional geology. In recent years, various geophysical techniques such as three-dimensional seismic data have been widely used in the study of cold seeps and their submarine plumbing systems as a powerful tool for examining deep structures [65][66][67]. In addition, the permeability of migration pathways is believed to be a vital factor in the distribution and temporal and spatial variability of seeps [68]. ...
Submarine cold seeps have recently attracted significant attention and are among the most effective indicators of gas hydrate in the oceans. In this study, remotely operated vehicle (ROV) observations, seismic profiles, core sediments, bottom seawater, and fluid vented from cold seeps in the deep-water Qiongdongnan Basin were used to investigate the origin and evolution of cold seeps and their relationships with gas hydrate. At stations A, B, and C, inactive cold seeps with dead clams, cold seep leakage with live clams, and active cold seeps with a rich mussel presence, respectively, were observed. The salinity and Na ⁺ and Cl ⁻ concentrations of the cold seeps were different from those of typical seawater owing to gas hydrate formation and decomposition and fluid originating from various depths. The main ion concentrations of the bottom seawater at stations B and C were higher than those at station A, indicating the substantial effects of low-salinity cold seep fluids from gas hydrate decomposition. The Na ⁺ -Cl ⁻ , K ⁺ -Cl ⁻ , Mg ²⁺ -Cl ⁻ , and Ca ²⁺ -Cl ⁻ diagrams and rare earth element distribution curves of the water samples were strongly affected by seawater. The concentrations of trace elements and their ratios to Cl ⁻ in the bottom seawater were high at the stations with cold seeps, suggesting the mixing of other fluids rich in those elements. Biochemical reactions may also have caused the chemical anomalies. Samples of HM-ROV-1 indicated a greater effect of upward cold seep fluids with higher B/Cl ⁻ , Sr/Cl ⁻ , and Ba/Cl ⁻ values. Moreover, the Re/Cl ⁻ value varied between fluid vents, possibly due to differences in Re precipitation strength. Differences in cold seep intensity are also believed to occur between areas. The cold seep fluxes changed from large to small before finally disappearing, showing a close connection with gas hydrate formation and decomposition, and influenced the local topography and ecosystems.
... Acoustic masking due to the presence of gas in the sediment (Garcia-Gil et al. 2002) blocked the visualization of subsurface features in the central portion of the clinoform (Fig. 1). Thus, the data presented and discussed herein regards mainly the distal portions of the modern clinoform not affected by gas blanketing. ...
The present study used shallow high-resolution seismic surveys to characterize the architecture and stratigraphic evolution of the last depositional sequence of the São Francisco Delta (SFDS). The sequence was accumulated within a bathymetric low (BL) on the shelf located in front of the São Francisco river. This antecedent topography provided additional accommodation of tens of meters in comparison with the rest of the shelf. The BL and its sediment-trapping effect allowed the accumulation of a complete stratigraphic record of the eustatic sea-level rise since the Last Glacial Maximum (LGM). The SFDS comprises six stratigraphic units. Because cores and radiocarbon dates are not available for this study area, the depth of occurrence of the top and base of each unit and their lateral relationships with wave-cut terraces carved into the BL walls were used in an effort to tentatively tie the mapped units to known eustatic sea-level curves available in the literature. Results suggest that major episodes of accumulation occurred during intervals of reduced rates in sea-level rise. The lower five units mapped accumulated in a large bay-estuarine protected environment, which resulted from the inundation of the BL. Only the topmost unit (the present clinoform) accumulated in an open ocean environment when sea level reached approximately its present position after 8 ka and the whole shelf was flooded. The most important delta backstepping event was probably associated with Meltwater Pulse 1B, which followed the Younger Dryas and produced one of the most significant stratal surfaces of the SFDS. Among the existing wave-dominated deltas in the Brazilian coast only, the São Francisco was able to construct a well-developed sigmoidal muddy clinoform in an otherwise very shallow shelf subjected to high wave-energy levels. This was only possible because of the existence of a bathymetric low in front of the river mouth, which illustrates the role that antecedent topography might play in providing larger amount of available accommodation space called in this paper of additional accommodation.
... 以上识别羽状流大都通过摄影及声纳声学技术, 而地震方法也可以识别出浅层气溢到海水中的气泡 及水合物甲烷羽状流. 海水中气泡在地震剖面上已 有响应, 如 Garcia-Gil 等 [25] 在海底浅层气逸出区(水 面可见到气泡逸出形成的"开锅沸腾"区)发现了海水 中气泡形成的声学羽流、云状扰动等地震反射特征, 在里海沿岸的海底浅层气逸出区记录到了串珠状的 反射信号 [26] ; 鄂霍次克海水合物调查中探测到了水 合物分解产生的气泡形成的"火焰"状反射 [27] . 顾兆 峰等 [28] 在南海地质调查中发现, 浅层地震剖面记录 到海水中大量分布的斑点状反射, 南黄海西部地区 浅层气在海水中的地震特征为声学羽流、云状扰动、 点划线反射 [29] . ...
... So far, there are over 900 cold seepage activity areas including the Arctic shelf NW-Svalbard (Veloso, 2015), Okhotsk Sea (Luan et al., 2010), Hydrate Ridge Cascadia (Heeschen, 2003), Northern Okinawa Trough of South China Sea (Feng et al., 2015) and Black Sea (McGinnis, 2006) around the world. At present, the side scan sonar, single-beam sonar and multibeam sounding are three main ways to detect cold seepage, which can clearly identify the bubble plume through sonar acoustic impedance profiles of the strong wave impedance interface (Garcia-Gil et al., 2002). Unlike those small-scale detection methods, the seismic method is a more large-scale and economical choice in marine exploration to regard plume bubbles as scattering points since the seismic acoustic source wavelength is much larger than the scale of a plume bubble. ...
Understanding past and present hydro-system feedbacks to global ocean-atmospheric interactions represents one of the main challenges to preventing droughts, extreme events and related human catastrophes in the face of global warming, especially in arid and semiarid environments. In eastern Africa, the El Niño-Southern Oscillation (ENSO) was identified as one of the primary drivers of precipitation variability affecting water availability. However, the northern East African Rift System (EARS) still suffers from ENSO climate teleconnection and the underrepresentation of predictive models because of the scarcity of local-to-regional historical or palaeo-data.
In this paper, we provide a 50-year seasonal flood/drought chronicle of the Awash River catchment from the study of laminated sediment from Gemeri and Afambo lakes (Central Afar region, Ethiopia), with the aim of reconstructing the magnitude of regional hydro-climatic events. Pluri-centimetric micro-laminated lithogenic facies alternating with pluri-millimetric carbonate-enriched facies are investigated in both lakes. We couple dating methods including radiocarbon, short-lived radionuclides, palaeomagnetic field variations and varve counting on both lake deposits to build a high-resolution age model and to discuss the regional hydro-sedimentary dynamics of the Awash River over the last ~700 years, with a focus on the last fifty years.
Using a multiproxy approach, we observe that following a multi-centennial enhanced hydrological period, the two lakes experienced a gradual decrease in river load inflow since 1979 CE, attaining extreme drought and high evaporative conditions between 1991 and 1997 CE. In 2014, the construction of a dam and the improvement of agricultural hydraulic management in the lower Awash River plain impacted the erodibility of local soils and the hydro-sedimentary balance of the lake basins, as evidenced by a disproportionate sediment accumulation rate.
Comparison of our quantitative reconstruction with i) lake water surface evolution expressed in Km2, ii) the interannual Awash River flow rates expressed in mm/yr, and iii) the El Niño 3.4 model highlights the intermittent connections between ENSO SST anomalies, regional droughts and hydrological conditions in the northern EARS.
Submarine seep plumes are a natural phenomenon in which different types of gases migrate through deep or shallow subsurface sediments and leak into seawater in pressure gradient. When detected using acoustic data, the leaked gases frequently exhibit a flame-like structure. We numerically modelled the relationship between the seismic response characteristic and bubble volume fraction to establish the bubble volume fraction in the submarine seep plume. Results show that our models are able to invert and predict the bubble volume fraction from field seismic oceanography data, by which synthetic seismic sections in different dominant frequencies could be numerically simulated, seismic attribute sections (e.g., instantaneous amplitude, instantaneous frequency, and instantaneous phase) extracted, and the correlation between the seismic attributes and bubble volume fraction be quantitatively determined with functional equations. The instantaneous amplitude is positively correlated with bubble volume fraction, while the instantaneous frequency and bubble volume fraction are negatively correlated. In addition, information entropy is introduced as a proxy to quantify the relationship between the instantaneous phase and bubble volume fraction. As the bubble volume fraction increases, the information entropy of the instantaneous phase increases rapidly at the beginning, followed by a slight upward trend, and finally stabilizes. Therefore, under optimal noise conditions, the bubble volume fraction of submarine seep plumes can be inverted and predicted based on seismic response characteristics in terms of seismic attributes.
As a new and unconventional reservoir, Quaternary mudstone biogas reservoirs in the Qaidam Basin are widely distributed and thick. They have great potential for biogas exploration. However, the mudstone biogas reservoirs in this area characterized by weak cementation, high clay content, and easy hydrolysis. Using existing methods, it is difficult to obtain the rock electrical parameters and evaluate the saturation. This greatly affects the evaluation of oil and gas reserves. This study selected core samples from the Quaternary mudstone biogas reservoir in Qaidam Basin, prepared the samples through core conformation pretreatment, and conducted rock electrical experiments using the self-absorption water augmentation method to obtain the rock electrical parameters of the mudstone biogas reservoirs for the first time. Then, the factors influencing the rock electrical parameters of the mudstone biogas reservoir were analyzed. The results showed that: (1) the porosity of the mudstone core samples was generally greater than 20%, and the permeability was in the range of 0.05–5 mD. The rock mineral composition was dominated by clay minerals, followed by carbonate minerals, quartz, and feldspar. The clay mineral composition was mainly illite, followed by montmorillonite and chlorite, with a small amount of kaolinite. (2) The use of wire-electrode cutting and high-temperature, heat-shrinkable tube to wrap the core sample played a supporting and protecting role in the core. Using the self-absorption water augmentation method, information was obtained showing that the distribution of cementation index m of the reservoir core samples ranged between 1.89–2.08, with an average value of 1.99, and the distribution of saturation index n ranged between 1.872–2.270, with an average value of 2.09. (3) Organic matter content had no obvious effect on rock electrical parameters. With the decrease of clay mineral content, the quartz content increased, the permeability increased, and the cementation index increased gradually. The saturation index increased with the increase in clay mineral content and the decrease of quartz content and permeability. The above results laid a petrophysical basis for the evaluation of the saturation of mudstone biogas reservoirs and could provide technical support for the comprehensive evaluation of the reservoir and the calculation of reserves.
Submarine hydrate mounds are important indicators of submarine methane seepages, hydrocarbon reservoirs, and seabed instability. In order to fully understand the formation of hydrate mounds, here, we review the study of hydrate mounds, in which the morphology, the formation mechanism, as well as the research techniques are introduced. The formation mechanism of hydrate mounds can be classified into: (1) The sediment volume expands due to the formation and accumulation of shallow hydrates; (2) unconsolidated shallow sediment layers respond mechanically to increasing pore pressure caused by shallow gas accumulation; (3) materials extrude from submarine layers driven by the over-pressure caused by shallow gas accumulation; and (4) the interaction of multiple factors. Most hydrate mounds occur in submarine gas hydrate occurrence areas. Active hydrate mounds are circular or ellipse well-rounded shaped, with gas seepages and abundant organisms, whereas inactive hydrate mounds are rough or uneven irregular shaped, with low flux of fluid in the migration channel. Due to the limitation of long-term in-situ observation technology, the existing observation method makes it possible to provide basic morphology features, stratigraphic structures, and fluid migration channels of the hydrate mound. Future research should be focused on the long-term in-situ monitoring technology, the formation mechanism of the hydrate mounds, and the role of gas hydrates in the seafloor evolution. In addition, the features of hydrate mounds (e.g., gas chimneys and fluid migration conduits) and the relationship between hydrate mounds and pockmarks could be further studied to clarify the influence of methane release from hydrate mounds on biogeochemical processes and the atmospheric carbon contents.
•L’estuaire de la vilaine est localisé sur la façade atlantique en bretagne méridionale. Depuis 1970, le barrage d’arzal protège des inondations les marais de redon constitue une réserve importante d’eau douce et un axe routier. La présence de cet ouvrage a perturbé dans le temps et l’espace la dynamique hydro sédimentaire de l’estuaire ; ceci se traduit principalement par un envasement prononcé de la portion estuarienne à l’aval du barrage. Il a fallu une vingtaine d’années environ pour que cet ouvrage soit intégré au paysage estuarien et que l’action des paramètres hydrodynamiques naturels reprenne ses droits. Les impacts anthropiques doivent donc être pris en compte dans le fonctionnement morpho sédimentaire pour prévoir leur évolution et tester les solutions curatives et préventives. L’héritage géomorphologique par l’intermédiaire des paramètres hydrodynamiques (vent houle et dynamique tidale) a régi les dépôts sédimentaires et la vallée estuarienne inondée il y a environ 10000 à 9000 ans BP. La puissance de ces dépôts est importante : localement elle atteint une quarantaine de mètres. Les cinq séquences de dépôts marins transgressifs reconnus dans la bais de la vilaine sont également présentes au sein de la vallée de l’estuaire et perçues jusqu’à 40 km environ à l’intérieur des terres. Le contrôle morphologique ainsi que l’architecture des dépôts sont significatifs d’un environnement sédimentaire de type ria. L’espace estuarien de la vilaine a donc enregistré la somme des facteurs naturels et anthropiques et connaîtra de nouvelles modifications en lien avec la remontée du niveau marin au cours de ces prochaines années.
L’objet de cette thèse a été la réalisation d’un état des lieux de la sédimentologie du Golfe du Morbihan à partir de différents approches : une approche géomorphologique (bathymétrie, sismique), une approche biosédimentaire (échantillons de surfaces, foraminifères actuels) et une approche biostratigraphique (carottes, foraminifères fossiles, datations). Ainsi, on a pu d’abord caractériser l’état actuel du Golfe en tant qu’environnement sédimentaire macrotidal en différenciant chaque subenvironnement et ensuite reconstituer l’histoire environnementale du Golfe, depuis son envahissement par la mer jusqu’à la situation actuelle, afin d’apporter des informations pour mieux comprendre l’évolution actuelle et mieux estimer les évolutions futures. Le rapport de thèse est donc construit sur le schéma suivant : Un premier chapitre réunit les informations bibliographiques dont nous disposons.Cela nous permet de faire le bilan de ce que nous savons déjà sur le Golfe et de ce que nous ignorons, en ouvrant l’esprit à de nouvelles questions à poser. Un deuxième chapitre conceptuel aborde le contexte climatique de notre fenêtre de temps à étudier (en plus du contexte actuel) pour faire un rappel et un résumé de la variabilité climatique et eustatique de l’Holocène, des forçages externes et internes aux niveaux global, régional et local.Le troisième chapitre est centré sur le contexte géomorphologique et sédimentologique du Golfe, il aborde les différents sous-environnements tidaux du système macrotidal du Golfe du Morbihan à différents niveaux : (1) au niveau des objets de dépôt (catalogue de formes de fond), (2) au niveau de chaque palier bathymétrique (l’intertidal, le subtidal…), (3) au niveau du système total et de sa dynamique sédimentaire par rapport aux courants tidaux et à la répartition des environnements sédimentaires en fonction du gradient hydrodynamique. De même dans le chapitre quatre, la répartition de biofaciès est étudiée et présentée à partir des assemblages de foraminifères benthiques, en fonction de plusieurs variables environnementales (pas seulement le gradient hydrodynamique). Il s’agit d’une étude de la distribution de foraminifères benthiques qui nous aidera à répondre à différentes questions : comment la diversité et la dominance d’espèces varient dans l’espace ? quelle est la provenance des espèces allochtones ? comment se distribuent les espèces de foraminifères dans le Golfe ? quels facteurs environnementaux contrôlent leur absence, leur présence et leur dominance ? Enfin, un scénario de comportement biosédimentaire du Golfe est proposé. Le dernier chapitre concerne l’histoire sédimentaire et environnementale du Golfe à partir de l’étude lithologique et micropaléontologique des carottes sédimentaires prélevées dans la partie orientale du Golfe. Pour l’interprétation des carottes, nous nous sommes appuyée sur les chapitres précédents. Ainsi le contextegéomorphologique nous a servi pour mettre en relation les carottes en voyant la continuité des corps sédimentaires dans l’espace. De même la connaissance des assemblages actuels de foraminifères a été utilisée pour calibrer les assemblages fossiles afin d’avoir une interprétation plus précise. Parallèlement, cette interprétation est discutée dans le cadre du contexte paléoclimatique résumé précédemment. Avec l’ensemble de ces données nous avons pu décortiquer l’histoire sédimentaire enregistrée dans les sédiments du bassin interne du Golfe du Morbihan. Une synthèse et des conclusions générales rappellent les points les plus importants de cette thèse. Un commentaire autocritique analysant les points faibles desapproches méthodologiques mises en oeuvre dans les études des environnements littoraux clôt ce manuscrit.
Mounds and depressions containing dense fossil accumulations are similar to modern features containing dense accumulations of live species in the North Sea. The fossil accumulations relied on a similar local upward flux of minerals and nourishment. -from Author
Circular depressions, or pockmarks, cover the sea floor in many
estuarine regions of the western Gulf of Maine. In Belfast Bay, Maine,
they are found in densities up to 160/km2, are up to 350 m in
diameter and 35 m in relief, and are among the largest and deepest
known. The pockmarks appear to form from the escape of biogenic natural
gas and pore water and are far larger than features associated with
thermogenic gas elsewhere. These pockmarks are thought to have formed
(1) catastrophically during an earthquake, tsunami, or storm, or (2)
slowly over thousands of years. Recent observations of bubble releases
suggest continuing activity and a potential geologic hazard. The
pockmarks involve a poorly documented coastal process of sediment
redistribution and methane release, largely unrecognized in the rock
record but widespread in middle- to high-latitude embayments.
Two occurrences of active gas seepages are described from the North Sea. The southernmost one, situated above a salt diapir in Norwegian block , has been studied and sampled by use of a remotely operated vehicle (ROV). This seepage consists of about 120 single seeps located within a diameter of ∼ 100 m. It is estimated to produce ∼ 24 m3 of methane gas per day (at ambient pressure, 75 m water depth). Isotope values of the methane gas and higher hydrocarbon gases in the surrounding seafloor sediments, show that their origin is from a deep seated, thermogenic source. No typical gas-induced erosion features are found on the seafloor at this location, probably due to the lack of very fine grained material.The second occurrence is located in U.K. block (Geoteam, 1984), where the seepage is associated with a very large pockmark depression, measuring 17 m in depth and 700×450 m in width. This depression represents an eroded fine grained sediment volume of ∼ 7.105 cubic metres. No detailed inspection or sampling of the gas has been performed here. However seismic reflection anomalies are seen on airgun seismic records at various levels down to a depth of at least 1100 m below seafloor. The seeping gas, possibly mixed with liquids, at this location is therefore also expected to be of a thermogenic origin.
It has been inferred that the rias of Galicia on the passive margin of northwest Spain which trend nearly at right angles to the Paleozoic structural grain appear to be the result of erosion along a weathered fault zone. New data from the Ria de Pontevedra appears to verify such an origin. The sediment fill in Pontevedra indicates that deposition along the eroded fault zone probably began during the emplacement of the late Pleistocene S1 sequence. Deposition of the Holocene S2 sequence, resting unconformably on S1, reflects not only the Younger Dryas regression, but also the transgressions prior and subsequent to the Younger Dryas regression. Both the Pleistocene and Holocene sequences are disrupted by mud diapirs at the mouth of the Ria de Pontevedra; the Holocene sequence at the mouth on its southeast side also is disrupted by faults which may be due to slope failure. Mobilization of the mud intruding the strata is probably due to sediment loading.
The effects of methane gas seepage were studied at an intertidal/shallow subtidal site on the
Kattegat coast of Denmark. A 30 m wide zone, containing approximately 65 gas seeps, extended over 70 m seawards from the shore. This was characterised by the presence of slabs, pavement and small pillars of carbonate-cemented sandstone which formed a partially buried reef. The escaping gas contained 91-100% methane with some carbon dioxide, 0.6-0.9%, and hydrogen sulphide. The hydrogen sulphide concentration varied over time and between individual seeps. Gas flow rates of individual seeps ranged up to 21 litre h-1 and the estimated total flow was 110 litre h-1. Seeps were often stopped by sand movement, but the overall gas flow from the site appeared to be constant. The escaping gas generated an interstitial water circulation and drew overlying seawater into the sediment. Water pumped out by the seeps was enriched in phosphate and ammonia. Sulphate reduction rates in the seep area were 1.1-17.1 m moles sulphate reduced and aerobic methane oxidation rates were 0.2 - 5.5 m moles methane consumed m-2 day-1. The composition of the flora and fauna surrounding the seeps was affected by the presence of hard substrate (the cemented sandstone). Epifauna was more abundant in the seep zone than elsewhere, whereas the macrobenthic infauna was reduced in the seep zone, possibly due to the cementation. The sediment was almost devoid of meiobenthic organisms, except nematodes. Nematode species numbers, abundance and biomass were lower at the seeps than 5-20 cm away.
The nematode fauna penetrated deeper into the sediment close to the seeps than at the seeps themselves. This is explained by the interstitial water circulation at and close to the seeps. I4C measurements showed that little methane carbon was entering the food web surrounding the seeps.
Zones of acoustic masking in sediments in the continental shelf of Cádiz, and in the Ria de Muros have been mapped utilizing 3.5 kHz and Uniboom seismic reflection records. In Cádiz the masking may be generated by shell beds, source of the shells is a Pliocene conglomerate that corresponds to the acoustic basement in the area. Acoustic masking in the sediments of Ria de Muros was generated by the presence of gas within the sediments. The gas is due to the decay of organic matter within the sediments.That area of acoustic masking in the Ria de Muros roughly coincides with the zone of mud with high (15.5%) organic content, and that gas bubbles were noted in the water column in the same region, support this hypothesis.
This paper demonstrates the importance of integrated studies of marine geology and geochemistry in the environmental management of an urbanized coastal inlet, using Halifax Harbour, Nova Scotia, as an example. The harbour receives 170 million litres of raw sewage per day; other sources of contamination include landfills, industrial activity, surface runoff and dredging. Geological and oceanographic conditions strongly influence the present environmental quality of the harbour.
Shallow interstitial gas within marine and lacustrine sediments restricts seismic reflection imaging of sub-gas events. The effect is sometimes termed “gas blanking, acoustic turbidity or acoustic masking” [Hovland and Judd, 1988]. The gas scatters or attentuates the acoustic energy, preventing penetration. Coherent reflection events (sediment bedding horizons) that border the areas with high gas content are often deflected downward (pull downs): the result of reductions in the speed of acoustic propogation through the gassy sediments. Examples are presented from Atlantic Canada (Figure. 1).
Natural gas seepages occur on the United Kingdom's continental shelf and although published reports suggest that they are very rare, the petroleum industry has identified, but not publicly reported, many more. There is also very little data on the flux of gas from seabed seepages, and even less on the contribution of seepages to atmospheric concentrations of gases such as methane. Potential gas source rocks include Quaternary peats as well as petroliferous source rocks such as the Carboniferous Coal Measures and the Upper Jurassic Kimmeridge Clays. There are also other organic-rich sediments which are potential source rocks. Together these cover a considerable part of the U.K. continental shelf. Analogue seismic reflection (pinger) profiles acquired during the British Geological Survey's regional mapping programme have been reviewed to identify water column targets including fish and plumes of gas bubbles. The ability to distinguish targets is critical to an assessment of the distribution of gas seepages. Both theoretical predictions of target identity and the habits of shoaling fish have been investigated in order to identify a method of distinction. Data from seabed seepages and measurements of seepage rates have been used to establish likely ranges of gas flux rates and the sizes of gas bubbles. The likelihood that a rising bubble will survive and escape into the atmosphere is determined primarily by bubble size and water depth; methane, the principal constituent of seepage gas, is relatively unreactive and sparingly soluble. The studies have enabled a new estimate of the distribution of gas seepages on the U.K. continental shelf, and of the contribution to atmospheric methane levels. The results suggest that natural gas seepages are significantly more important as a source of methane than had hitherto been established. It is estimated that between 120,000 and 3.5 mtonnes of methane per year come from a continental shelf area of about 600,000 km(2). This represents between 2% and 40% of the total United Kingdom methane emission. It is suggested that similar contributions arise from other continental shelf areas worldwide, and that geological sources of atmospheric methane are more significant than is generally acknowledged.
A combined high resolution seismic, sub-bottom profiling, and multi-beam echo-sounding survey in the Skagerrak (Danish sector of the North Sea) together with gas analyses at a station along the profile exhibit the expulsion of gas (mainly methane) and the presence of gas-charged sediments at shallow depth. The echo-soundings yield detailed insight into the distribution and shape of typical sea-floor features associated with gas seepage, such as pockmarks. The pockmarks reach dimensions of 800 m in length, 300 m in width, and 15 m in depth, with the long axis running parallel to the slope of the Norwegian Trench. Processing of the multi-channel high resolution seismic data and the digitally recorded sub-bottom profiler signals indicate an internal compressional velocity of about 1050 m s-1 within the gas-charged sediments reaching from the sea-floor to a sub-bottom depth of about 23 m. Using the lateral distribution and thickness of the gas-charged sediments in conjunction with a mean concentration of gas of 3000 ppb, the present amount of trapped gas is estimated to be 6·45 × 1011 g CH4. The flux of methane through the sea-bed into the water column appears to be 7·2 × 1010 g CH4 per year. To explain the small difference in size between the methane pool in near-surface sediments and the annual flux through the sea-bed, a constantly high supply of methane from leaking hydrocarbon reservoirs at greater depths has to be active.
Natural gas seepages occur on the United Kingdom's continental shelf and although published reports suggest that they are very rare, the petroleum industry has identified, but not publicly reported, many more. There is also very little data on the flux of gas from seabed seepages, and even less on the contribution of seepages to atmospheric concentrations of gases such as methane.Potential gas source rocks include Quaternary and Tertiary peats as well as petroliferous source rocks such as the Carboniferous Coal Measures and the Upper Jurassic Kimmeridge Clays. There are also other organic-rich sediments which are potential source rocks. Together these cover a considerable part of the U.K. continental shelf.Analogue seismic reflection (pinger) profiles acquired during the British Geological Survey's regional mapping programme have been reviewed to identify water column targets including fish and plumes of gas bubbles. The ability to distinguish targets is critical to an assessment of the distribution of gas seepages. Both theoretical predictions of target identity and the habits of shoaling fish have been investigated in order to identify a method of distinction.Data from seabed seepages and measurements of seepage rates have been used to establish likely ranges of gas flux rates and the sizes of gas bubbles. The likelihood that a rising bubble will survive and escape into the atmosphere is determined primarily by bubble size and water depth; methane, the principal constituent of seepage gas, is relatively unreactive and sparingly soluble.The studies have enabled a new estimate of the distribution of gas seepages on the U.K. continental shelf, and of the contribution to atmospheric methane levels. The results suggest that natural gas seepages are significantly more important as a source of methane than had hitherto been established. It is estimated that between 120,000 and 3.5 mtonnes of methane per year come from a continental shelf area of about 600,000 km2. This represents between 2% and 40% of the total United Kingdom methane emission. It is suggested that similar contributions arise from other continental shelf areas worldwide, and that geological sources of atmospheric methane are more significant than is generally acknowledged.
Zusammenfassung Es wird über eine Methode zur Feststellung von Schlickmächtigkeiten, von Bodenschichtungen in weichem Boden und zur Ermittlung des Tiefenprofils des harten Untergrundes berichtet. An Hand von Echogrammen und im Vergleich mit Bodenproben wird die Methode diskutiert.
A regional estimation of the contribution to atmospheric methane by natural gas seepages on the UK continental shelf was undertaken by Judd et al. (Mar. Geol. 137(1/2) (1997) 165). This paper is the second in the series, and provides an estimation of the atmospheric methane flux from Bulgarian Black Sea continental shelf.Potential gas source rocks include Holocene gas-charged sediments, Quaternary peats and sapropels, and deep-lying Palaeocene and Neogene clays, Cretaceous coals, and other sediments of late Jurassic to early Cretaceous age. These cover almost the whole continental shelf and slope and, together with irregularly developed seal rocks and widespread active and conducting faults, provide good conditions for upward gas migration.A total of 5100 line kilometers of shallow seismic (boomer) and echo-sounder records acquired during the Institute of Oceanology's regional surveys, and several detailed side-scan sonar lines, have been reviewed for water column targets. Four hundred and eighty-two targets were assigned as gas seepage plumes. It is estimated that a total of 19,735 individual seeps exists on the open shelf. The number of seeps in coastal waters was estimated to be 6020; this is based on available public-domain data, specific research, and results of a specially made questionnaire which was distributed to a range of “seamen”.More than 150 measurements of the seabed flux rates were made in the “Golden sands” and “Zelenka” seepage areas between 1976 and 1991. Indirect estimations of flux rates from video and photo materials, and a review of published data have also been undertaken. Based on these data, three types of seepages were identified as the most representative of Bulgarian coastal waters. These have flux rates of 0.4, 1.8, and 3.5l/min.The contribution to atmospheric methane is calculated by multiplying the flux rates with the number of seepages, and entering corrections for methane concentration and the survival of gas bubbles as they ascend through seawater of the corresponding water depth. The estimation indicates that between 45,100,000 (0.03Tg) and 210,650,000m3 (0.15Tg)methaneyr−1 come from an area of 12,100km2.
Single-channel seismic recording, water chemistry determination, and sediment sampling were carried out off the northern Kuril Islands and the southern tip of Kamchatka. A bottom simulating reflector (BSR) was mapped in detail. The shipboard analyses of bottom water samples yielded methane concentrations up to 1000 nl/1. West off Paramushir Island, venting of methane-rich fluids from the seafloor into the water column is related to unusual trajectories of the BSR which emerges towards the seafloor. The destabilisation of gas hydrates and the origin of the vent site may correspond to anomalous high heatflow caused by recent volcanic intrusions into the sediment. No venting was found in the areas off the southern tip of Kamchatka and off Onekotan Island.
Gassy sediments and large seafloor pockmarks are identified from seismic profiles in the Gulf of Cadiz slope. Gas-charged sediments are represented by zones of acoustic turbidity and other acoustic anomalies on seismic profiles. Most of the gas is believed to be biogenic in origin, resulting from the decay of organic matter contained within rapidly developed regressive shelf-margin deltas. A group of large asymmetric seafloor pockmarks also occurs downslope of the gassy area, both on the seafloor surface and deeply buried in the sediment column. Sediment sorting by the Mediterranean Undercurrent seems to act as a controlling factor for the development of each type of gas-originated feature. The area where the strong Mediterranean Undercurrent outflow impinges the seafloor is covered by sandy sediments of high porosity which allow the interstitial gas to escape through the sediment pores, whereas the areas affected by weaker currents are covered by clays, and the gas tends to accumulate in the sediment beneath impermeable horizons.
Lake Rotoiti formed 11 850–20 000 years ago when lava flows dammed the drainage system through the Okataina caldera, one of the major rhyolitic centres of the intracontinental, back-arc, Taupo Volcanic Zone of North Island, New Zealand. The lake morphology reflects this complex origin, with remnants of the old caldera rim, the lava dam and relict river system being preserved. High resolution seismic reflection profiles (7 kHz) show the drowned river system channelled inflowing water into the lake basin, scouring and depositing sediments and maintaining an active sublacustrine channel in the former river valley. Airfall tephras are preserved in lake sediments. The tephrachronology, established from the surrounding catchments, can be correlated with the siesmic stratigraphy for eight major eruptions over the last 11 850 years. Hydrothermal fields underlying the lake generate gaseous sediments which mask seismic penetration. This is indistinguishable from seismic masking caused by biogenic gas accumulation within the sediments. Venting of hydrothermal gases at the surface creates pockmarks. From the seismic stratigraphy beneath pockmarks seven stages of pockmark genesis have been identified which show pockmarks grow, decay, migrate and persist over several thousand years.
Gas seepage rates were measured over a two year period at an intertidal/shallow subtidal methane-derived carbonate-cemented sandstone reef off north-east Denmark. The positions of some of the gas outlets at the intertidal site changed with time but the overall gas flux of the reef area remained approximately constant, with a mean flow-rate of 59.4 ml h−1 m−2 or over 100 l h−1 for the seepage area. In the shallows the flow-rates of seeps varied with the depth of the overlying water. The methane from the seeps had a radiocarbon age of > 39 000 years bp and was depleted in heavy isotopes, δ13CCH4 varied from −68.6 to −75.3‰ and δD from −240 to −249‰, suggesting a bacterial origin. The most 13C-depleted methane came from the most active of the seeps tested.
The first discovery of pockmarks (gas-escape craters) was in muds of the basins of the central Scotian Shelf off eastern Canada. Since then, pockmarks have been found in many continental shelf environments of the world. They may be used as a hydrocarbon exploration tool and recently their role as foci of intense biological chemosynthetic activity has attracted considerable attention.Pockmarks occur in the Gulf of Maine, Passamaquoddy Bay, eastern Scotian Shelf, Scotian Slope, Laurentian Channel, Gulf of St Lawrence, Halibut Channel and on Labrador Shelf. Their formation in Passamaquoddy Bay may be enhanced through earthquake activity as their greatest density occurs in association with a large fault zone. Pockmarks on Labrador Shelf and in Laurentian Channel may have been formed preferentially as a consequence of disturbance of the seabed by grounded moving icebergs. Pockmarks in Placentia Bay, Newfoundland occur in an area of large megaflutes at the seabed and the pockmarks may have provided an initial seabed roughness for subsequent formation of the megaflutes by a tsunami-generated turbidity current.Subsurface zones of gas-charged Quaternary sediments are often associated with areas of pockmarks. Gas-charged sediments in the nearshore frequently appear as thick Holocene deposits occurring in areas overlying buried channel systems. In the nearshore, biogenic methane is the probable source. In other areas, such as Downing Basin on the Grand Banks of Newfoundland, the gas-charged sediments contain ethane, butane and methane, suggesting a deeper hydrocarbon source. Active gas venting has been observed at the seabed in Downing Basin without the formation of pockmarks.Seabed observations from research submersibles and bottom photography have identified areas of white filamentous bacteria on sandy sediments sometimes associated wtih circular patches of dense benthic communities. Seismic reflection profiles from these areas also indicate the presence of shallow gas (bright spots) within Tertiary age sediments. Taken in their entirely, the broad distribution of gas-related features suggests that the major basins of the eastern Canadian continental shelf vent gas, but pockmarks or other morphologic venting features are only produced where soft cohesive sediments are present at the seabed. The chronic release of gas from the seabed may contribute to the biological productivity of the offshore areas as well as provide a significant, but as yet unquantified, volume of methane and other gases to the ocean and atmosphere.
In this paper the authors explain the biogenic and petrogenic origin of gas in marine sediments in non-technical terms. In such sediments methane arises from bacteriological reduction of organic matter and by geochemical transformation of buried biota. The microbiological mechanisms are described along with the geochemistry of thermogenic gas formation. The process of hydrocarbon migration is briefly outlined together with the means to determine whether a gas sample is biogenic or thermogenic.
The occurrence of shallow gas in seafloor sediments within 50 km of the coast of the British Isles is generally attributed to the degradation of organic matter within post glacial deposits. However there is evidence that buried rockhead topography may also be a factor governing the existence of shallow gas in some localities. In such cases gas may be escaping from the rock through fissures. Where channels have been cut into the rockhead and are now filled with unconsolidated sediments, gas often accumulates in the central portion of the channels. This may be due to the greater thickness of sediment in the centre of the channels generating sufficient gas to be detectable in seismic profiling data. The process of compaction causing gas to be squeezed out of the pore spaces may also be a contributory factor. In other localities no such relationship appears to exist and the gas is present at very shallow levels (within 5 m of the seabed) in the superficial sediment column. Detailed analyses of high resolution continuous seismic reflection profiles from Cardigan Bay, the northeast Irish Sea and the Firth of Forth, would appear to permit the categorization of nearshore shallow gas zones around these parts of the U.K. continental shelf and probably elsewhere, in order to assist with the explanation of their occurrence and the likely hazard that the zones present to seabed and sub-seabed structures.
The sediments of Eckernförde Bay (southwest Baltic Sea) are known for their high contents of gas. It causes acoustic turbidity, i.e., the masking of sedimentary horizons by absorption and scattering of seismic energy due to free gas bubbles, also known as “basin effect”. Repeated surveys with side-scan sonar and subbottom profiler along coincident profiles gave new insight into the variability of the acoustic turbidity. For the first time it is evidenced that its depth varies by several decimeters with an annual cycle. We explain this by the temperature-dependent solubility of methane. The depth of the acoustic turbidity, however, does not vary in phase with the annual surface temperature cycle. The delay is an effect of the poor heat conductivity of the overlying water and sediment.
High resolution seismic reflection profiles on the inner continental shelf (less than 60 m water depth) off India from 10 degrees N to 22 degrees N show 5-35 m of weakly stratified acoustically transparent clays. At water depths of 15-50 m, the subbottom profiles are characterizEd. by anomalous seismic signatures in the form of acoustic maskings. These extend from the underlying Late Pleistocene unconformity, oftern stretching along the bedding planes, and occasionally surfacing on the sea bed. These acoustic maskings could be due to shallow hydrocarbon gases (mainly methane) which might have been derived from biogenic sources or geothermal processes, or both
High-resolution shallow seismic reflection data obtained in the continental shelf sediments off western India from 10°N to 22°N reveal the characteristic anomalous seismic signatures in the form of acoustic maskings of methane-rich horizons. A conservative estimate of the potential subsurface methane in these gas-charged sediments is of the order of 2.6 Tg, and its contribution to the atmosphere with an annual flux of 0.039 Tg CH4 appears to be quite significant. From our limited results coupled with worldwide reports, it is concluded that the seepage of methane (8 to 65 Tg) through the seabed of the continental shelves worldwide cannot be ignored for the estimation of global oceanic flux of methane and its budget.
High-resolution seismic reflection and sub-bottom profiling on the continental margin off western India has revealed the presence of characteristic acoustic maskings in the form of wipe-outs, reflector terminations and seep-associated features in the inner shelf. These maskings suggest the presence of gas-charged sediments. Further seaward on the outer shelf-middle slope, pockmarks and prominent plumes in the overlying water column indicate a significant seepage of gas from the slope sediments, and it is the seepage which may demonstrate the existence of source rocks. Seismic profiles also revealed the presence of bottom simulating reflectors (BSRs) in the mid-lower slope-rise regions, presumably suggesting the presence of gas hydrates. The BSRs occur at roughly 300-600 ms (TWTT) beneath the sea floor at water depths between 525 and 2200 m; they occasionally show discontinuities. Distinct blanking zones as well as acoustic voids have also been observed above the BSR. In contrast, chaotic and/or scattered hyperbolic reflctors occur in places below the BSR, which may suggest the presence of gas-charged sediments. Folds, diapiric features and faults present in the slope-rise areas may probably werve as traps and conduits for upward migration of fluids and methane gas from the deep
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Quantitative Study of the Rı́a de Vigo Gas Fields
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Giant sea-bed pockmarks
- Kelley