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Abstract
The in situ observation technology of deep sea floor in China is poor now, which seriously lagging behind the security requirements of ocean engineering activities, represented by exploit of marine oil and gas resources. The in situ observation of pore water pressure can effectively react the seabed dynamic geological processes, which playing an important role in the prevention and forecast of the geological disasters. The deep sea pore water pressure observation is different from onshore and inshore observation, this paper present the types of sensors and measurement equipment to understand the pore water pressure measurement. Ordinary sensor can't satisfy the need of measurement in deep sea, while differential pressure sensors make a significant contribution to the precision and resolution of measured data. Combining with the latest research progress both at home and abroad, the in situ observation equipment about pore water pressure in deep sea are introduced in here. And this paper points out that long-term, real-time and multi parameter observation are the trend of the development of underwater observation research in future. ©, 2015, Shuili Xuebao/Journal of Hydraulic Engineering. All right reserved.
... The three countries with the most relevant literature are the United States, China, and Germany, which show large clusters. Those highly concerned countries or regions often have longer shorelines and need sufficient research to support their ocean mineral resources development, as shown in Fig. 4 (here, the data from all countries were grouped into continents, and the number of published studies is plotted with shoreline length by continent; The shoreline length data are based on Liu et al. (2015)). ...
... Figure 20 presents some tools for monitoring the internal changes in sediments, which include pore pressure probes and temperature sensors . Pore pressure can effectively reflect the external loads and geological processes of the sediments, and the in-situ observation of pore pressure plays a pivotal role in the prediction and prevention of geologic disasters (Schultheiss 1990;Liu et al. 2015). Changes in the internal temperatures of sediments can reflect the movement of fluids in pores and can be used to judge the stability of the seabed . ...
Ocean mining activities have been ongoing for nearly 70 years, making great contributions to industrialization. Given the increasing demand for energy, along with the restructuring of the energy supply catalyzed by efforts to achieve a low-carbon economy, deep seabed mining will play an important role in addressing energy- and resource-related problems in the future. However, deep seabed mining remains in the exploratory stage, with many challenges presented by the high-pressure, low-temperature, and complex geologic and hydrodynamic environments in deep-sea mining areas, which are inaccessible to human activities. Thus, considerable efforts are required to ensure sustainable, economic, reliable, and safe deep seabed mining. This study reviews the latest advances in marine engineering geology and the environment related to deep-sea mining activities, presents a bibliometric analysis of the development of ocean mineral resources since the 1950s, summarizes the development, theory, and issues related to techniques for the three stages of ocean mining (i.e., exploration, extraction, and closure), and discusses the engineering geology environment, geological disasters, in-situ monitoring techniques, environmental protection requirements, and environmental effects in detail. Finally, this paper gives some key conclusions and future perspectives to provide insights for subsequent studies and commercial mining operations.
... In addition, the light is seriously attenuated in the water, and the detection range of optical instrument is very limited (El-Fakdi & Carreras, 2013;Ishizu et al., 2012). The existing self-capacitive sensor systems (e.g., temperature probe, pressure probe, and servo-accelerometer system) can realize in-situ continuous monitoring of seabed temperature (Macdonald et al., 1994;Vardaro et al., 2006), pore water pressure (Liu et al., 2015), or topography changes (Saito & Yokoyama, 2008;Yokoyama et al., 2012), but the single monitoring node and limited monitoring range make. The submarine cabled observatory, which can achieve long-term, in-situ and real-time monitoring, has been developing rapidly and put into use in many countries and regions (Barnes & Tunnicliffe, 2008;Favali et al., 2013;Ruhl et al., 2009;Scherwath et al., 2019;Shozaburo & Walker, 2013;Taylor, 2008). ...
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.
... The probe is penetrated into the submarine sediments usually using a static penetrometer system. Liu et al. (2015c) reviewed the development status of deep-sea in situ porewater pressure probes and some typical cases of their application. They also noted that differential pressure sensors have the ability to measure directly the excess pore pressure with higher resolution. ...
In response to the accelerating processes of marine natural gas hydrate exploration and gas production from hydrate-bearing sediments, their potential impacts on the environment have been attracting extensive attention from international academia as well as from industry, especially in recent years. Methane seeps related to gas hydrate degradation on the seafloor are ubiquitous on continental slopes in both active and passive continental margins. A previous series of articles suggest that hydrate dissociation (gas release or ebullition at large scale) mainly occurs as a result of faulting or decreased lithostatic pressure triggered by many external driving forces (mainly including tectonic activities, overpressure zone, earthquake and tides), and temperature change. These frequently affect methane inputs into the atmosphere, fueling seafloor oxygen consumption or ocean acidification, and even posing submarine geohazards. Gas hydrate-related areas are usually estimated from indirect biochemical indications (cold-seep communities, element and isotopic anomalies in pore water) and geophysical indications (surface morphology, gas plumes and pathways). Therefore, appropriate application of monitoring and detection methods is of crucial significance for assessing the temporal and spatial variability of related environmental indicators in gas hydrate reservoirs. Monitoring is also as an essential support for harvesting this potential alternative energy in ways that are safer, more economical, and more environmentally friendly. Following the three structural elements of a hydrocarbon seep pumping system (gas/fluid source, fluid migration pathway, and seeping structures at or near the seafloor), this paper sets forth the significance of indicators for dynamics resulting from the destabilization of reservoirs of natural gas hydrates, reviews the corresponding monitoring or detection methods and integrated monitoring systems; and especially, expatiates the in situ observation networks regarding gas hydrate reservoirs and gas hydrate production tests. In the closing section, we discuss and draw conclusions regarding the challenges for future development of hydrate environmental monitoring and significant environmental issues requiring special attention. We intend this paper to provide references and to set the scene for future research activities about gas hydrates, so as to ignite the interest of more research groups around the world.
Excess pore water pressure is an important parameter that can be used to analyze certain physical characteristics of sediment. In this paper, the excess pore water pressure of subseafloor sediment and its variation with tidal movement was measured following the installation of a wharf in Qingdao, China by using a fiber Bragg grating (FBG) piezometer. The results indicated that this FBG piezometer is effective in the field. The measured variation of excess pore water pressure after installation is largely explained by the dissipation of excess pore water pressure. The dissipation rate can be used to estimate the horizontal consolidation coefficient, which ranged from 1.3×10⁻⁶ m²/s to 8.1×10⁻⁶ m²/s. The measured values during tidal phases are associated with the variability of tidal pressure on the seafloor and can be used to estimate the compressibility and the permeability of the sediment during tidal movement. The volume compression coefficient estimated from tidal oscillation was approximately 2.0×10⁻¹¹ Pa⁻¹, which was consistent with the data from the laboratory test. The findings of this paper can provide useful information for in situ investigations of subseafloor sediment.
The swath bathymetric data acquired during the “Sumatra Aftershocks” cruise from the Sunda trench in the Indian Ocean to the
north of the Sumatra Island imaged several scars and deposits. In situ pore pressure measurements using the Ifremer piezometer
and coring demonstrate that high excess pore pressure and sediment deformation was generated by a recent event in the scar
of the slope failure zone identified by J.T. Henstock and co-authors. This excess pore pressure is localized in the upper
sedimentary layers and is not related to an interplate subduction process. Numerical simulations of the hydrological system
that take into account the hydro-mechanical properties of the upper sediment layer show that the excess pore pressure and
sediment deformations could be generated at the time of the December 26, 2004 Great Sumatra Earthquake.
The geological profile of many submerged slopes on the continental shelf consists of normally to lightly overconsolidated clays with depths ranging from a few meters to hundreds of meters. For these soils, earthquake loading can generate significant excess pore water pressures at depth, which can bring the slope to a state of instability during the event or at a later time as a result of pore pressure redistribution within the soil profile. Seismic triggering mechanisms of landslide initiation for these soils are analyzed with the use of a new simplified model for clays which predicts realistic variations of the stress–strain–strength relationships as well as pore pressure generation during dynamic loading in simple shear. The proposed model is implemented in a finite element program to analyze the seismic response of submarine slopes. These analyses provide an assessment of the critical depth and estimated displacements of the mobilized materials and thus are important components for the estimation of submarine landslide-induced tsunamis.
Multisensor piezometer probes were deployed at four different sites in the Mississippi Delta in water depths ranging from 13.5 to 43.6 m with sensor penetration depths of up to 15.6 meters. Absolute and differential pressure sensors were used to measure pore water pressure and excess pressures, respectively. The free water column pressure was measured with absolute pressure sensors. Pore pressures induced by probe insertion were determined as well as ambient excess pore pressures following the time-dependent decay of induced pressures. Significant differences in the pore pressures and related geotechnical properties were found between East Bay and Main Pass sediments. Generally higher probe insertion pressures and lower ambient excess pore pressures were characteristic of Main Pass compared to East Bay. Probe insertion pressures (Ui) were found to correlate well with the undrained shear strength (Su) of the sediments, indicating reasonably good agreement with the predicted relation: Ui = 6Su as suggested by an earlier study42. Using this relationship undrained shear strengths were calculated and compared with measured values.
In this paper, we present the current practice of investigations of seafloor instabilities and deformation processes, based on extensive research conducted over the last years, which sets the scene for future research activities in this field. The mapping of the continental margins and coastal areas with ever increasing resolution systematically reveals evidence of instabilities and deformation processes, both active and palaeo-features. In order to properly assess the hazards and risks related to these features, an integrated and multi-disciplinary approach is essential, but challenging. Such an approach consists of combining field data (geophysics, geology, sedimentology, geochemistry and geotechnical data) with numerical simulations constrained by results from laboratory data. As such, it is of paramount importance to build a common knowledge base and understanding that unify these disciplines into more complete and conceptual models constrained by all the data.
This field study was designed to examine the effect of storm waves on a portion of weak under-consolidated sediments in the South Pass area of the Mississippi Delta . The study centered within a long linear bottom deformational feature. Detailed bathymetric, high resolution seismic and side scan sonar profiles show this feature to be a rough blocky zone of hard sediment. Pi ston cores taken on either side of the feature show low shear strength sediment whereas the sediment from within the feature has higher shear strength and contains gas bubble structures. A wave rider and bottom pressure transducer were used to determine wave characteristics while accelerometers and piezometers were used to determine bottom sediment motion and pore water pres sures. These data were collected on a continuous basis for a year. Although the 1976-77 winter season did not yield any storms of major proportion, waves of 8-10 feet and bottom displacements of 4-6 inches were observed with passage of storm fronts. Significant bottoe pressures and accelerations were measured with the passage of Hurricances Anita and Babe . (A)
One successful in situ test in the Wilkinson Basin, in a water depth of 274 m, yielded a maximum excess pore-pressure of 59 kPa after the probe was driven about 3. 2 m into the silty-clay bottom. An excess pore-pressure of 9. 8 kPa was measured 5-10 h after emplacement of the probe. Implications of excess pore-pressures cyclically generated by storm and internal wave loading of sea-floor soils is discussed. It is concluded that a better understanding of submarine slope stability through the use of the effective stress principle should now be possible by measuring pore-pressure in situ.
SEASWAB is one element of the Delta Project of the U.S. Geological Survey, a cooperative effort with several universities and other governmental agencies to investigate the processes that cause marine‐sediment instability. The basic purpose of the SEASWAB experiment was to obtain field measurements of sediment motion and pore‐pressure variations in soft sediment affected by wave‐pressure perturbations. This article serves as an introduction to the six papers that follow and that together make up a report on the results of SEASWAB.
We present a theoretical study of the thermodynamic chemical equilibrium of gas hydrate in soil by taking into account the influence of temperature, pressure, pore water chemistry, and the mean pore size distribution. The model uses a new formulation based on the enthalpy form of the law of conservation of energy. The developed model shows that due to a temperature and pressure increase, hydrates may dissociate at the top of the hydrate occurrence zone to ensure a chemical equilibrium with the surrounding bulk water. This original result confirms what has been already shown through experiments.The second part of the paper presents an application of the model through a back-analysis of the giant Storegga Slide on the Norwegian margin. Two of the most important changes during and since the last deglaciation (hydrostatic pressure due to the change of the sea level and the increase of the sea water temperature) were considered in the calculation. Simulation results show that melting of gas hydrate due to the change of the gas solubility can be at the origin of a retrogressive failure initiated at the lower part of the Storegga slope. Once again, the developed model leads to predictions, which are supported by laboratory experiment results, but contradictory to previous interpretations and beliefs considering that hydrate dissociation occurs only at the bottom of the gas hydrate stability zone.
The study area, offshore Nigeria, is located in one of the compressional zones within the Niger Delta, which is characterized by imbricate thrust structures. Although the low mean slope angle (around 2°), bathymetry data from the study area have shown the existence of several submarine landslides which coincide with known subsurface faulted compressive features.In this paper, we have focused on a submarine slide occurring in water depths ranging between 1690 and 1750 m. Headwall scars, internal architecture and associated deposits have been characterized using a combination of 3D seismic data, near-bottom echosounder seismic profiles, Kullenberg cores and in-situ geotechnical measurements. The slide shows horseshoe shaped headwall scars and depositional lobes with positive relief. Monitoring of excess pore pressure for 12 months indicates the presence of negative hydraulic gradients, which is either an indication of a local present-day mechanical activity of subsurface faults or related to the regional extension of the Niger Delta and the possible creation of a regional depression in the hydraulic regime.In order to identify the triggering mechanism of the observed landslide, a three-dimensional slope stability model (SAMU-3D) based on the upper bound theorem of plasticity was used. Calculation results have shown that the gravity loading generated by the sediment weight alone is not sufficient to explain the observed submarine slide. Cylindrical cavity expansion theory was used to locally simulate the compressional structure movements and to evaluate the strength generated within the upper sediment layers. Slope stability assessment carried out by considering this additional structural strength has shown that regional compressional gravity driven deformation can explain the observed submarine failures.
Based on acquired geophysical, geological and geotechnical data and modeling, we suggest hydrate dissolution to cause sediment collapse and pockmark formation in the Niger delta. Very high-resolution bathymetry data acquired from the Niger delta reveal the morphology of pockmarks with different shapes and sizes going from a small ring depression surrounding an irregular floor to more typical pockmarks with uniform depression. Geophysical data, in situ piezocone measurements, piezometer measurements and sediment cores demonstrate the presence of a common internal architecture of the studied pockmarks: inner sediments rich in gas hydrates surrounded by overpressured sediments. The temperature, pressure and salinity conditions of the studied area have allowed us to exclude the process of gas-hydrate dissociation (gas hydrate turns into free gas/water mixture) as a trigger of the observed pockmarks. Based on numerical modeling, we demonstrate that gas-hydrate dissolution (gas hydrate becomes mixture of water and dissolved gas) under a local decrease of the gas concentration at the base of the gas-hydrate occurrence zone (GHOZ) can explain the excess pore pressure and fluid flow surrounding the central hydrated area and the sediment collapse at the border of the GHOZ. The different deformation (or development) stages of the detected pockmarks confirm that a local process such as the amount of gas flow through faults rather than a regional one is at the origin of those depressions.
The difference in pressure between the sea floor and a point in the underlying sediment is an indicator of the velocity of pore-water flow. This differential pore pressure has now been measured at five sites using a free-fall piezometer (pop-up pore pressure instrument, PUPPI) in a distal turbidite province of the Madeira Abyssal Plain. We found that at one of the five sites the pressure was slightly positive, at two others it was effectively zero, but at the remaining two sites the pore pressure was significantly negative (-450 and -120 Pa at 4 m depth). The implication is that at these last two sites water is moving downwards at rates of 3.1 and 0.8 mm yr-1 (calculated using laboratory-measured hydraulic conductivities from core samples). If this downward movement is part of a hydrogeological system which includes upward movement of pore water elsewhere, there are obvious implications for schemes to dispose of highly radioactive waste by burial in deep-sea sediments.