OS41C-0483 0800h
AVO ANALYSIS OF MULTIBEAM BACKSCATTER, AN EXAMPLE FROM LITTLE BAY, NH AND SKJALFANDI BAY, ICELAND
In the seismic reflection method, it is well known that seismic amplitude varies with the offset between the seismic source and detector, and that this variation is the key to the direct determination of lithology and pore fluid content of subsurface strata. Based on this fundamental property, amplitude-versus-offset (AVO) analysis has been used successfully in the oil industry for the exploration and characterization of subsurface reservoirs. Multibeam sonars acquire acoustic backscatter over wide range of incidence angles, and the variation of the backscatter with the angle of incidence is an intrinsic property of the seafloor. With the necessary changes being made, a similar approach to seismic AVO analysis can applied to the acoustic backscatter. To illustrate this approach, AVO analysis was applied to a Simrad EM3000 (300kHz) multibeam sonar dataset from Little Bay, NH, and to a Simrad EM300 (30kHz) multibeam sonar dataset from Skjalfandi Bay, Iceland. The analysis starts with the backscatter time series stored in raw Simrad datagrams, which are then corrected for seafloor slope, insonification area, time varying and angle varying gains. Then, a series of AVO attributes (near, far, slope, gradient, fluid factor, product etc) are calculated from the stacking of a number of consecutive time series. Based on the calculated AVO attributes and the inversion of a modified Jackson et al (1986) acoustic backscatter model we estimate the acoustic impedance, the roughness, and consequently the grain size of the insonified area on the seafloor. In Little Bay, the estimated impedance and the grain size were compared to in-situ measurements of sound speed and to the direct analysis of grain size in grab samples, showing a very good correlation. In Skjalfandi Bay, the AVO attribute of fluid factor was calculated, which presented an estimate of the gas/fluid content in the sediment structure. The areas with high fluid factor anomalies correlated to regions that showed evidence of gas in seismic profiles.
http://ccom.unh.edu
OS41C-0484 0800h
Examination of Marine Seismic Reflection Data for the Occurrence and Distribution of Gas Hydrate on the Continental Margin of Fiordland, New Zealand
Evaluation of seismic data on the Fiordland margin reveals a widespread zone of Bottom Simulating Reflections (BSRs) east of the incipient subduction zone. BSRs are most commonly used to infer the presence of gas hydrates in marine sediments. We have compiled the distribution of gas hydrates on the Fiordland margin based on the occurence of BSRs in an extensive database of seismic data, acquired by academic, government and petroleum research cruises over the last 30 years. BSRs can be recognised by a negative reflection-coefficient that occurs at the boundary between shallow sediments that contain gas hydrate and deeper sediments storing free gas. Further, it is characteristic for BSR structures to follow isotherms which are nearly parallel to the morphology of the sea floor. Because methane is the prevalent enclosed gas in hydrates, it becomes an important agent of climate change in the case of fast destablisation of the hydrates. We present seismic observations that suggest that hydrate instability, due to a change in water temperature and/or of hydrostatic pressure, may cause subsea landslides on the continental slope - perhaps triggering tsunamis. Thus, evaluation of the distribution of gas hydrates can contribute to natural hazard prevention. The interest in gas hydrates as a possible resource is increasing due to the vast energy potential of methane, even though the engineering challenges of producing methane from gas hydrates are daunting. Based on the distribution of BSRs, it is therefore possible to calculate the resource potential in the gas hydrate province off Fiordland.
OS41C-0485 0800h
Erosion of the seafloor at the top of gas hydrate stability: Evidence from an uplifted ridge on the Hikurangi Margin, New Zealand.
Gas hydrates are thought to play a significant role in submarine slope failure, however, the exact mechanisms by which they affect seafloor stability are not yet understood. It is mostly assumed that overpressure caused by pore volume expansion during gas hydrate dissociation may lead to weakening and sliding along the base of gas hydrate stability (BGHS) in sediments. Sediments are predicted to be particularly vulnerable close to water depths where the BGHS pinches out (i.e., at the top of gas hydrate stability, TGHS, in the ocean) because of the lack of overburden sediments to compensate for overpressure at the BGHS. We have discovered a location on the Hikurangi margin, Southern Ritchie Ridge, where seafloor erosion appears to take place at the THGS. Late Cretaceous (?) strata are uplifted and appear to be eroded at about 600 m water depth. Pinchouts of bottom simulating reflections (BSRs) document that the ridge crest is close to the TGHS. We propose two possible mechanisms that may cause gas hydrate-related seafloor erosion. In a modification of the common model for slope failure from gas hydrate dissociation, we predict that during uplift, the BGHS moves upward with respect to the seafloor, leading to gas hydrate dissociation, overpressure, and ultimately slope failure along the BGHS. Subsequently, pressure-temperature conditions on the seafloor move back deeper into the gas hydrate stability field and the process can repeat itself during continued uplift. However, for this model, the seafloor would need to remain entirely within the stability field, which contradicts the presence of BSR pinchouts. We therefore favor a different mechanism for seafloor erosion, a process similar to frost heave. Temperature measurements at this water depth show mesoscale fluctuations by about $\pm$ 1$^o$ C suggesting that the ridge repeatedly crosses the gas hydrate phase boundary. We propose that the repeated pore volume expansion and contraction during gas hydrate dissociation and formation leads to weakening of the seafloor. The weakened material does not support the relatively steep slopes of the uplifted ridge and is predicted to slide down the ridge flanks.
OS41C-0486 0800h
Hydrate and Free Gas Concentrations in the Storegga Slide Region: Links to Mass Wasting and Climate Change
The Storegga Slide has mobilized over 55,000 cubic km of sediment during its active history. Much of this sediment was located at depths that would place it in the methane hydrate stability zone. As the northern flank of the slide has shown evidence of an active hydrate an free gas system, it has become of increasing scientific importance to understand the link between the mass wasting events at the slide and the hydrate system as it may related to regional or global climate change. To this end, we have quantified the amount of hydrate and free gas present both inside and outside the slide scar to determine the possible amount of methane that could have escaped through the mass wasting events. Concentrations were obtained through detailed P-wave velocity analysis of multi-channel seismic reflection data collected in September 2003 velocity measurements were obtained through a combination of prestack interval tomography and 1D waveform inversion. These results were then incorporated into an effective medium model for hydrate and free gas quantification. Results indicate significantly less hydrate and free gas within the slide scar but no causal link between slide movement and methane mobilization.
OS41C-0487 0800h
A New Interpretation of the 1st Storegga Slide: Evidence for an Older Event Unassociated With the Base of the Hydrate Stability Zone
Large submarine landslides on continental slopes are important geologic features because they cause 1) mass wasting, 2) tsunamis, and most importantly 3) the possible rapid release of methane, a greenhouse gas, into the atmosphere. The Storegga Slide off the coast of Norway is one of the largest underwater slides known covering an area of ~34000 km2 and displacing ~5600 km3 of sediment. We present new pre-stack depth migrated images from a multi-channel seismic data set, which comprise the most detailed profiles to date of the Storegga Slide. These images show strong evidence for multiple slide events, including a much older event, at the northern scarp of the 1st Storegga Slide that has previously been interpreted as one large recent slope failure. On-lapping features, layer thickening, and rotated fault blocks draped with undisturbed sediments support this conclusion. According to sedimentation rates on the edge of the Voring plateau, and previous dating of a layer in the Naust formation, we estimate that this older slide event occurred at a minimum of ~185 ka. We calculate thermal gradients of 54.4 and 50.0oC/km under lithostatic and hydrostatic conditions respectively at the southern edge of the Voring Plateau. Pressure/temperature modeling using these thermal gradients shows that, assuming steady state conditions, the base of the gas hydrate stability zone could not have been involved in initiation of the older event because the BSR would have been deeper than the glide plan at the time of failure.
OS41C-0488 0800h
Norwegian Research Strategies on gas Hydrates and Natural Seeps in the Nordic Seas Region (GANS)
Continuous leakage of methane to the oceans from hydrate reservoirs that partially are exposed towards the seafloor is an increasing international concern, as the greenhouse gas methane is significantly more (c. 20 times) aggressive than CO2. In Norway we have research groups with interest and experience on natural seeps and gas hydrates. These features, and processes related to them, are challenging research targets which demands inputs from different fields if important research breakthroughs shall be made. In February 2004 deep sea researchers from the University of Tromso, Geological Survey of Norway, Norwegian Geotechnical Institute, Statoil and University of Bergen met to obtain an overview of the research effort in the fields of natural seeps and gas hydrates in Norway and to discuss national coordination, research strategies, research infrastructure and international co-operation. The following research strategies were agreed upon: i) Strengthen multidisciplinary research on deep sea systems, ii) develop a strategy for research on natural seeps and gas hydrates, iii) contribute in national coordination of research on natural seeps and gas hydrates, iv) Coordinate the use and development of research infrastructures important for research on natural seeps and gas hydrates, and v) contribute in the international evaluations of strategies for hydrate reservoir exploitation. Proposed research tasks for GANS include: i) Gas and gas hydrate formation processes and conditions for transport, accumulation, preservation and dissociation in sediments, ii) Effect of gas hydrate on physical properties of sediment, iii) Detection and quantification of in situ gas hydrate content and distribution pattern, iv) Effect of dissociation on soil properties, v) Gas hydrates as an energy resource, vi) Rapid methane release and climate change, and vii) Geohazard and environmental impact.
OS41C-0489 0800h
Thermal Property Measurements in Tetrahydrofuran (THF) Hydrate Between -25 and +4\deg C, and Their Application to Methane Hydrate
Using a "hot wire" needle probe, we make simultaneous thermal conductivity and diffusivity measurements of pure THF hydrate between -25\deg C and the THF hydrate stability boundary near +4\deg C. Combining these measurements with published pressure and temperature dependent THF hydrate density data, we derive the sample's specific heat capacity. Over the measured temperature range, there are two distinct thermal property behavioral regimes. Between -25 and -7.5\deg C, all three thermal properties depend linearly on temperature. Thermal conductivity rises from 0.489 to 0.496 W/mK, in agreement with our previous measurements at 0.1 MPa confining pressure. Diffusivity rises from 2.50x10$^{-7}$ to 2.66x10$^{-7}$ m$^{2}$/s, and the specific heat falls from 2020 to 1930 J/kgK. Both results are in agreement with estimates calculated from published THF data. Above -7.5\deg C, the thermal conductivity rises nonlinearly, increasing to 0.58 W/mK at +3\deg C. The diffusivity also increases nonlinearly to 3.2x10$^{-7}$ m$^{2}$/s, resulting in a nearly linear specific heat decrease to 1865 J/kg K at +3\deg C. The thermal conductivity rise is less dramatic than the nearly 350% increase observed in the same system at 0.1 MPa confining pressure, but indicates a behavioral change in the thermal properties of THF hydrate for temperatures approaching the stability boundary near +4\deg C. THF hydrate has been used as a thermal property analog for methane hydrate. In contrast to our structure II THF hydrate results, our thermal conductivity measurements in porous, pure structure I methane hydrate are linear at least to +15\deg C, approximately 3\deg C below the methane hydrate stability temperature at 24.8 MPa. The nonlinear thermal properties of THF hydrate above 0\deg C do not accurately represent those of methane hydrate at the same temperature. For thermal conductivity, a closer comparison between THF and methane hydrate is achieved by extrapolating the linear THF hydrate property trends from below -7.5\deg C to positive temperatures. This procedure yields results within $\pm$0.01 W/m K of published estimates for methane hydrate up to 20\deg C.
OS41C-0490 0800h
Thermal Measurements Over Hydrate Mounds in Atwater Valley, Gulf of Mexico
Heat flow measurements and cores were collected in May 2004 across two mound structures in an area designated as the Atwater Valley, a shallow trough near the distal end of the Mississippi Canyon in the northern Gulf of Mexico. The measurements were made along a NW-SE-trending U. S. Geological Survey seismic line that crossed both of the mound structures (mounds D and F). The mounds are thought to result from sediment diapirism caused by dissociation of methane hydrate at depth. A bottom-simulating reflector is visible in the seismic line near the mounds. The data show clear anomalies in sediment temperature and heat flow associated with the mounds. Measurements collected on the top of mound F show sediment temperatures elevated by 0.3$^o$C relative to the surrounding seafloor, and heat flow values of around 160 mW/m$^{2}$. Sediment temperatures decrease away from the summit of the mound, and heat flow values drop to a background level of 40 to 50 mW/m$^{2}$. Sediment temperatures at the summit of Mound D are similar to what was observed at Mound F, and heat flow values are slightly lower at around 132 mW/m$^{2}$, partly as a result of the slightly higher bottom water temperature and thus reduced thermal gradient. These thermal data are modeled to constrain the depth to the gas hydrate layer beneath the mounds and the fluid (and therefore methane) flux from the vent sites.
OS41C-0491 0800h
Characterization of Gulf of Mexico Sediment in Hydrate and Non-Hydrate Bearing Cores Using Specific Surface Area
Specific surface area (SSA) measurements made on four cores recovered in the Mississippi Canyon region of the Gulf of Mexico suggest that SSA is not a controlling factor in hydrate formation. SSA was determined using the single point Brunauer, Emmett, and Teller (B.E.T.) method on a Quantachrome Quantasorb Sorption System. This method uses thermal conductivity to measure the volume of nitrogen adsorbed and desorbed from the sample surface. Core MD02-2569, 10.35 m in length, was collected in 1032 m of water and had gas hydrate veins up to 2 cm thick. SSA measurements were made at 6.45 m below seafloor (mbsf) and 8.16 mbsf. Core MD02-2573, which also contained hydrate, was 4.2 m long, and was located 28 m from MD02-2569 in 1027 m of water. SSA measurements were made at .30, 3.25, and 4.0 mbsf. Core MD02-2569 had SSA values ranging from 26.01-27.6 m$^{2}$/g. Core MD02-2573 had higher SSA values ranging from 30.96 to 32.92 m$^{2}$/g excluding the .3 mbsf sample, which yielded a SSA value of 22.61 m$^{2}$/g. MD02-2570 and MD02-257, non-hydrate bearing cores recovered $\sim$3 km apart and $\sim$22 km southwest of MD02-2573, were collected at depths of 631 and 664 m. Each produced similar SSA values (30.94 to 32.40 m$^{2}$/g and 31.55 to 35.61 m$^{2}$/g) to core MD02-2573 and MD02-2570 produced a lower SSA value of 22.4 m$^{2}$/g at .50 m below the surface. The presence of gas hydrate in cores MD02-2569 and MD02-2573, both collected at similar water depths and locations, but with different SSA, suggests that SSA is not a dominant factor in hydrate formation. This is supported by the similarity of SSA values of core MD02-7573 as compared to cores MD02-2570 and MD02-2571, all from varying locations within the hydrate stability field. We interpret that hydrate formation may be more dependent on water depth and local environmental variations (e.g. gas saturation, gas flux) than on SSA.
OS41C-0492 0800h
Estimation of Free Gas Saturation Using AVO Analysis on 3D Seismic Data at South Hydrate Ridge, Cascadia Accretionary Complex
In 2000 we conducted a high-resolution 3D seismic survey of a 4$\times$10 km$^{2}$ region on south Hydrate Ridge on the Oregon continental margin. The objective of the survey is to characterize the regional pattern of fluid and gas migration and its relationship to hydrate accumulations on Hydrate Ridge. These data were acquired with a high-resolution seismic source with source-receiver offsets of up to 644 m, which results in incidence angles of up to 20 degrees. In 2002 new 2D seismic data with source-receiver offsets of up to 1500 m, producing incidence angles of up to 40 degrees, were collected during R/V Ewing Cruise EW0208, which was coordinated with Ocean Drilling Program (ODP) Leg 204. Prior to AVO analysis, we conducted true amplitude recovery by using the seismic range equation, and true amplitude processing through prestack time migration. We also calibrated the 3D data by using the 2D data to remove unfavorable acquisition effects in the 3D survey. On prestack-migrated gathers, we measured the seismic amplitude from the three surfaces associated with free gas accumulations: 1) the bottom simulating reflection (BSR), 2) Horizon A, and 3) Horizon B'. Horizon A is a $\sim$4-m-thick turbidite interval where it was drilled during ODP Leg 204 and it is identified as a primary conduit along which free gas migrates from deep sources to the summit of Hydrate Ridge. Horizon A focuses gas hydrate formation and feeds gas vents near the southern summit of Hydrate Ridge. Horizon B' is a volcanic glass-rich horizon that is also a stratigraphically defined gas conduit. Intercept and slope are obtained from AVO fitting, and the Poisson's ratio is obtained by using intercept and slope with the constraints of well-log data. Gas saturation for the three surfaces is estimated by fluid substitution technique. The distribution of free gas inferred from AVO analysis shows that the highest free gas concentrations lie directly beneath the highest gas hydrate concentrations estimated from Leg 204 core data. For Horizon A, the gas saturation increases toward the summit as Horizon A shallows. Free gas within Horizon A is distributed in isolated pockets rather than in continuously connected veins.
OS41C-0493 0800h
3D Seismic Imaging of the Blake Ridge Gas Hydrate Province: Evidence for a Dynamic Gas System
New 3D multi-channel seismic reflection images of the Blake Ridge gas hydrate province, located 400 km east of Savannah Georgia, suggest that a dynamic gas-hydrate/free-gas boundary exists. Complex ocean bottom currents above the Blake Ridge Depression create both high rates of sediment deposition (~250m/k.y.) and erosion, resulting in changes in sediment overburden and thermal variability at the bottom simulating reflector (BSR). 3D images combined with heatflow analysis show that the BSR below these sediment waves is not in equilibrium, and that fluctuating ocean currents and sedimentary processes may cause gas overpressure and escape. Amplitude analysis of the 3D volume reveals significant amplitude variability at the BSR, suggesting highly variable free-gas concentrations beneath the Blake Ridge. Specifically, compared with the crest of the Blake Ridge, little gas exists along the eastern erosional flank and within the Blake Ridge Depression where faults extend continuously from the BSR to the seafloor. The results imply faults that outcrop at the seafloor act as primary conduits for gas migration into the water column.
OS41C-0494 0800h
Analysis of Sonic Velocity in an Active Gas Hydrate System, Hydrate Ridge, Offshore Oregon
One of the best recognized and most intuitive influence of gas hydrate on its host sediment is the change in its mechanical and elastic properties. This is identified through an increase in acoustic velocity, which is partially responsible for one of the most distinct signatures of gas hydrate presence, the Bottom Simulating Reflector (BSR). The unstable nature of gas hydrate makes the in situ recording of their properties by downhole logging the best way to identify and quantify its distribution. During ODP Leg 204 on Hydrate Ridge, offshore Oregon, acoustic logs were recorded in seven holes and vertical seismic profiles (VSP) were acquired successfully in four holes. These data, recorded within a wide range of frequency and scales provide a unique and extensive survey of the acoustic properties of a dynamic gas hydrate system. Because of the poorly consolidated nature of the Hydrate Ridge sediments, automatic picking of velocity was only partially successful and a complete post cruise reprocessing of the sonic waveforms was necessary to draw accurate compressional (Vp) and shear velocity (Vs) logs. Synthetic seismograms generated with the Vp and density logs allow to confirm the nature of the main reflectors identified in a 3-D seismic survey of Hydrate Ridge, such as the BSR and various faults underlying the southern Hydrate Ridge system. Despite the highly heterogeneous distribution of gas hydrate, the Vp logs and interval velocities calculated from the VSP clearly identify the presence of gas hydrate and the eventual presence of free gas directly underneath the hydrate stability zone or within the faults feeding the ridge system. We use various elastic models to try to estimate gas hydrate and free gas saturations from sonic velocity and from bulk moduli. The best agreement with independent estimates derived from resistivity logs and other methods indicate that gas hydrate interact with the host sediment through cementation, which contributes also to a significant energy loss in the recorded waveforms.
OS41C-0495 0800h
Failure of Marine Sediments due to Gas Hydrate Dissociation
Methane gas hydrate (MGH) dissociation in the pore space of marine sediments may be caused by various natural and human-induced processes including sea level decrease, tectonic uplift of continental margins, global warming, and petroleum operations. While these processes generally have different spatial and temporal scales, they result in MGH dissociation, and the released gas and water tend to expand. This may change the pore pressure in the sediments, affecting their mechanical state and failure processes. If the pressure does not change, the hydrate dissociation may still affect the sediment properties by perturbing particle cementation and by introducing phase interfaces (e.g., capillary menisci). In this work, the pressure change has been calculated by coupling the dissociation rate with fluid flow in the sediments based on thermodynamic considerations. The common seafloor failure, submarine landslides, can reach a length of $\sim$100 km, with a length-to-thickness ratio as large as $\sim$1000. It is often assumed that the Storegga Slides were caused by earthquakes that instantaneously created a shallow discontinuity ($\sim$100 m below the seafloor) along the entire slide length of $\sim$100 km. Instead, {\it Puzrin and Germanovich} [2004] reasoned that the MGH dissociation may have resulted in an initial flaw at the scale of only $\sim$1 km. They explained the landslide evolution in submarine slopes by the mechanism of catastrophic shear band propagation of this flaw. Our modeling suggests that the sediment de-cementation and the excess pore pressure due to MGH dissociation may indeed have determined the scale of $\sim$1 km of this initial defect. Our calculations also suggest that dissociation-affected submarine landslides may be common for shallow sea water depths of $<$ 1 km and involve thin sediment layers (usually $\sim$100 m or less). However, the MGH dissociation may also occur underneath a massive and horizontally extended MGH layer, which could serve as a seal or cap-rock. In this case, the excess pressure can be as high as tens of MPa if the sediment permeability is much lower than $\sim$10$^{-16}$ m$^{2}$. Therefore, an excess pore pressure, sufficient for sediment fracturing, may occur at water depths of $>$1 km. In particular, we argue that the seafloor collapse structure at the Blake Ridge site [{\it Dillon et al.}, 2001] may be explained by this mechanism. Alternatively, the excess pore pressure may be sufficient to initiate vertical hydraulic fractures above the dissociation area [{\it Zuhlsdorff and Spieb}, 2004]. We hypothesize that the MGH dissociation occurring in the pore space and MGH lenses [{\it Suess et al.}, 1999] supplies the growing fissures with the fracturing fluid. In this scenario, the phase transition in the pore space is due to the pressure decrease (rather than increase), which further enhances the dissociation and fluid supply. Our calculations suggest that once the dissociation-driven fractures reach a size of up to tens of meters, they separate from the originating MGH region and propagate towards the seafloor. Similar to the models of deep magma transport, this propagation is essentially due to buoyancy. We argue that such periodic episodes of hydraulic fracturing may explain methane ejections, mud volcanoes, and pockmarks observed on the seafloor. Furthermore, the fractures are likely to migrate along closely spaced trajectories. Eventually, quasi-vertical elongated regions of disturbed sediment form. Their horizontal dimensions are unlikely to exceed $\sim$100 m, which is consistent with the quasi-vertical channels discovered in seismic studies.
OS41C-0496 0800h
Methane Hydrates and Fluid Flow Along the Chilean Margin
An international collaboration between the Naval Research Laboratory and Pontificia Universidad Catolica de Valparaiso (Chile) was developed to investigate methane hydrate distribution along sections of the Chilean margin. Preliminary data collected along the Chilean margin in 2003 by researchers from Chile and the Universities of Bremen and Kiel (GEOMAR) found a clear discrepancy between estimated heat flow inferred from the depth of the bottom simulating reflector (BSR) and direct measurement using a heatflow probe. The data indicated that fluid migration enhanced heat flow in the upper section of the sedimentary column. We conducted a more extensive and higher resolution survey in October 2004 to evaluate this discrepancy and determine if the phenomenon is a local or regional phenomenon. Multichannel seismic data collected in the region, suggest the BSR is shallower than expected. It is possible that tectonic movements present that shifted the BSR upward but did not immediately destabilize the hydrates. This investigation will address possible "in situ" destabilization kinematics. Complimentary pore water geochemical profiles from piston cores and thermal data collected with a heatflow probe instrument, will help reconcile the discrepancies observed between the seismic and heat flow observations. Preliminary sulfate profiles collected in April 2003 indicate downward fluid advection but no heatflow or thermal information was collected to support this observation. Data collected in October 2004 will provide a more thorough analysis of this relationship. Previous DTAGS data is coupled with the heat flow data to interpret the variation observed in the geochemical profiles Also, heatflow data collected during Fall 2003 and Spring 2004 will be compared to determine if seasonal variations in bottom water temperature affect near surface sediment heat flow.
OS41C-0497 0800h
3D-Seismic and Acoustic Imaging of Gas Migration and Gas Hydrate Accumulations Beneath Pockmarks in Hemipelagic Sediments off Congo, SW Africa
Seismic and acoustic imaging is a major tool to study basic geological parameters controlling the occurrence of gas hydrate as well as of free gas within the gas hydrate stability field. For sufficient upward fluxes of hydrocarbon gases, gas hydrates grow within pore spaces or gases are trapped beneath efficient seals, thereby revealing information about the nature and efficiency of fluid flow pathways and about the time scale on which fluid transport occurs. In the hemipelagic sedimentary sequences off the Congo, where layering and uniform properties exist at the time of deposition, modification of sediment physical properties due to mixing between water, gas and hydrates within pore spaces affects amplitude and phase properties of seismic reflections. Furthermore, fluid flow and gas or hydrate accumulations are often associated with sediment deformation or faulting on different scales. Thus, an integrated interpretation of seismic, acoustic, and surface mapping data sets was used to optimize lateral and vertical resolution at each depth level and to connect deeper processes to their surface expressions. 3D seismic data across seafloor pockmarks indicate that the typical low-amplitude signature of opal-rich and water-rich sediments is superimposed by high amplitude zones near faults and potential fluid pathways. A high amplitude patch observed in 40-50 m depth is interpreted as a gas hydrate cap that plugs the feeder channel of a pockmark and initiates hydrate growth parallel to the bedding. The upflow zone at greater depth is characterized by amplitude blanking, indicating free gas bubbles that scatter seismic energy. A package of high amplitude reflector elements at 150-200 m sub-bottom depth suggests the presence of trapped gas beneath a low permeable layer. This package is bent upwards at the vicinity of the pockmark, probably indicating a deeper salt diapir, that is associated with faulting and probably higher permeability above the diapir. However, the creation of pathways beneath the pockmarks is not yet completely understood. Preliminary results based on 3D mapping of fault plane orientations suggest that faulting due to diapirism may be superimposed by zones of weakness within a regional fault pattern that probably is of polygonal structure.