Description: This paper uses a Bayesian approach for inverting seismic amplitude versus offset (AVO) data to provide estimates and uncertainties of the viscoelastic physical parameters at an interface. The inversion is based on Gibbs' sampling approach to determine properties of the posterior probability distribution (PPD), such as the posterior mean, maximum a posteriori (MAP) estimate, marginal probability distributions, and covariances. The Bayesian formulation represents a fully nonlinear inversion; the results are compared to those of standard linearized inversion. The nonlinear and linearized approaches are applied to synthetic test cases which consider AVO inversion for shallow marine environments with both unconsolidated and consolidated seabeds. The result of neglecting attenuation in the seabed is investigated, and the effects of data factors such as independent and systematic errors and the range of incident angles are considered. The Bayesian approach is also applied to estimate the physical parameters and uncertainties from AVO data collected at two sites along a seismic line in the Baltic Sea with differing sediment types; it clearly identifies the distinct seabed compositions. Data uncertainties (independent and systematic) required for this analysis are estimated using a maximum-likelihood approach.

Description: Amplitude and frequency anomalies associated with lakes and drainage systems were observed in a 3D seismic data set acquired in the Mallik area, Mackenzie Delta, Northwest Territories, Canada. The site is characterized by large gas hydrate deposits inferred from well-log analyses and coring. Regional interpretation of the gas hydrate occurrences is mainly based on seismic amplitude anomalies, such as brightening or blanking of seismic energy. Thus, the scope of this research is to understand the nature of the amplitude behavior in the seismic data. We have therefore analyzed the 3D seismic data to define areas with amplitude reduction due to contamination from lakes and channels and to distinguish them from areas where amplitude blanking may be a geologic signal. We have used the spectral ratio method to define attenuation (Q) over different areas in the 3D volume and subsequently applied Q-compensation to attenuate lateral variations ofdispersive absorption. Underneath larger lakes, seismic amplitude is reduced and the frequency content is reduced to Formula, which is half the original bandwidth. Traces with source-receiver pairs located inside of lakes show an attenuation factor Q of Formula, approximately half of that obtained for source-receiver pairs situated on deep, continuous permafrost outside of lakes. Deeper reflections occasionally identified underneath lakes show low-velocity-related pull-down. The vertical extent of the washout zones is enhanced by acquisition with limited offsets and from processing parameters such as harsh mute functions to reduce noise from surface waves. The strong attenuation and seismic pull-down may indicate the presence of unfrozen water in deeper lakes and unfrozen pore water within the sediments underlying the lakes. Thus, the blanking underneath lakes is not necessarily related to gas migration or other in situ changes in physical properties potentially associated with the presence of gas hydrate.

Description: We examine the usefulness of amplitude versus offset (AVO) analysis of bottom-simulating reflections (BSRs) for estimating associated marine gas hydrate and free-gas concentrations. A nonlinear Bayesian inversion is applied to estimate marginal probability distributions (MPDs) of physical parameters at a BSR interface, which are related to overlying gas hydrate and underlying free-gas concentrations via rock physics modeling. The problem is constrained further by prior information and re-parameterization of inversion results. Inversion of BSR AVO data from offshore Vancouver Island, Canada, shows that gas hydrate and free-gas concentrations are, respectively, 0%-23% and 0%-2% of the pore volume, at a 90% credibility level. This result indicates that the data do not provide sufficient information to independently resolve gas hydrate and free-gas concentrations to useful accuracy. The Study is directed primarily at AVO for gas-hydrate-related BSRs, but may have important applicability in testing the degree of constraint of formation characteristics in other AVO studies. The inversion method is applied also to synthetic AVO data generated from Ostrander's gas-sand model. In this case, MPDs sufficiently constrain the relationship between P- and S-wave velocities in the sandstone unit to determine if it is gas-charged. The variable degree of model constraint obtained in this AVO study highlights the need to include rigorous quantitative uncertainty analysis in all AVO studies.

Description: This paper presents a comprehensive, multidisciplinary study of cold vents associated with near-seafloor gas hydrate. Several cold vents characterized by seismic blank zones have been identified on the northern Cascadia margin near Ocean Drilling Program (ODP) Site 889/890. The most prominent vent site (Bullseye vent) has been the subject of intense geophysical and geochemical studies, including two- and three-dimensional (2D/3D) seismic imaging, heat flow measurements, piston coring with measurements of sediment physical properties and pore-fluid geochemistry, seafloor video observation, and sampling with the unmanned submersible ROPOS. The main seismically derived constraining observations are: (1) blanking increases with seismic frequency, (2) at low frequencies, layers can be traced through the zones, (3) blank zones widen with depth, (4) blank zones are underlain by a bottom simulating reflector (BSR), and (5) no velocity anomalies were detected across the vents. Constraints from piston core and thermal probe analyses are: (1) massive hydrate was recovered just below the seafloor at Bullseye vent, and (2) chemical alteration of sediments was observed by reduced magnetic susceptibility, increased thermal conductivity, and an elevated sulfate/methane interface. Additional constraints are: (1) no thermal anomaly was observed, (2) widespread carbonates and active chemosynthetic communities were found, and (3) elevated levels of methane were detected in the water column above Bullseye vent. We present a model for the seismic blanking at Bullseye vent that honors the constraints from all observations. The cold vents represent channels or networks of filamentous fractures containing hydrate and/or free gas. Free gas can be present within the hydrate stability field only in fractures, which may be coated with hydrate that prevents the inflow of water. The overall concentration of hydrate or gas within the vent must be small, because there was no observable velocity anomaly.

Description: The Bullseye vent, an approximately 500-m-diameter deep- sea, hydrate-related cold vent on the midslope offshore Vancou- ver Island, was imaged in a high-resolution multichannel survey by the Deep-towedAcoustics and Geophysics System DTAGS. The structure was drilled by the Integrated Ocean Drilling Pro- gram at site U1328. Towed about 300 m above the seafloor, thehigh-frequency 220–820 Hz DTAGS system provides a high vertical and lateral resolution image. The major problems in im- aging with DTAGS data are nonlinear variations of the source depths and receiver locations. The high-frequency, short-wave- length data require very accurate positioning of source and re- ceivers for stacking and velocity analyses. New routines were de- veloped for optimal processing, including receiver cable geome- try estimation from node depths, direct arrivals and sea-surface reflections using a genetic algorithm inversion method, and acoustic image stitching based on relative source positioning by crosscorrelating redundant data between two adjacent shots. Semblance seismic velocity analysis was applied to common-re- flection-point bins of the corrected data. The processed images resolve many subvertical zones of low seismic reflectivity and fine details of subseafloor sediment structure. At the Bullseye vent, where a 40-m-thick near-surface massive hydrate layer was drilled at U1328, the images resolve the upper part of the layer as a dipping high-reflectivity zone, likely corresponding to a frac- ture zone. Velocity analyses were not possible in the vent struc- ture but were obtained 180–270 m to either side. Normal veloci- ties are in the upper 50 m, but over the interval from 50 to 100 m below the seafloor at the northeast side, the velocities are higher than the average normal slope sediment velocity of approximate- ly 1590 m/s. These high velocities are probably related to the high reflectivity zone and to the bottom portion of the massive hydrate detected by resistivity measurements in the upper 40 m at U1328.

Description: We combine acoustic impedance inversion of 3D seismic data, log-to-seismic correlation, and seismic attribute analyses to delineate gas-hydrate zones at the Mallik site, Mackenzie Delta, Northwest Territories, Canada. Well-log data define three distinct hydrate zones over a depth range of 890–1100 m. Synthetic seismic modeling indicates the base of the two deeper hydrate zones are prominent reflectors. The uppermost gas-hydrate zone correlates to seismic data with a lower degree of confidence. The extent and geometry of the two lower hydrate zones suggest that local geology plays a significant role in the lateral and vertical distribution of gas hydrate at Mallik. The reliability of the hydrate concentrations calculated from the inverted impedances is qualified by the match between original and synthetic seismic data to produce confidence maps for the two lower gas-hydrate-bearing intervals. A total in-place volume estimate of solid gas hydrate for an area of 1.44 km2 around well 5L-38 yields a value of approximately 45 equivalently, 6.6 of gas. We further qualify our mapping of gas hydrates by some amount of continuous resource, defined as lateral continuity measured by seismic attribute similarity and sand-dominated rock. Using these attributes, the continuous amount of hydrate at Mallik is about half the in-place volume. Else-where within the 3D seismic cube, the seismic impedance inversion yields evidence of potential gas-hydrate deposits near wells A-06 and P-59 at levels near the predicted base of the hydrate stability zone.

Description: High-resolution multibeam bathymetry data collected with an autonomous underwater vehicle (AUV) complemented by compressed high-intensity radar pulse (Chirp) profiles and remotely operated vehicle (ROV) observations and sediment sampling reveal a distinctive rough topography associated with seafloor gas venting and/or near-subsurface gas hydrate accumulations. The surveys provide 1 m bathymetric grids of deep-water gas venting sites along the best-known gas venting areas along the Pacific margin of North America, which is an unprecedented level of resolution. Patches of conspicuously rough seafloor that are tens of meters to hundreds of meters across and occur on larger seafloor topographic highs characterize seepage areas. Some patches are composed of multiple depressions that range from 1 to 100 m in diameter and are commonly up to 10 m deeper than the adjacent seafloor. Elevated mounds with relief of 〉10 m and fractured surfaces suggest that seafloor expansion also occurs. Ground truth observations show that these areas contain broken pavements of methane-derived authigenic carbonates with intervening topographic lows. Patterns seen in Chirp profiles, ROV observations, and core data suggest that the rough topography is produced by a combination of diagenetic alteration, focused erosion, and inflation of the seafloor. This characteristic texture allows previously unknown gas venting areas to be identified within these surveys. A conceptual model for the evolution of these features suggests that these morphologies develop slowly over protracted periods of slow seepage and shows the impact of gas venting and gas hydrate development on the seafloor morphology.

Description: A 3D seismic survey (Mallik 3D), covering 126  km2 in the Mackenzie Delta area of Canada’s north, was conducted by industry in 2002. Numerous lakes and marine inundation create a complex near-surface structure in the permafrost terrain. Much of the near subsurface remains frozen but significant melt zones exist particularly from perennially unfrozen water bodies. This results in an irregular distribution of permafrost ice creating a complex pattern of low and high frequency near-surface velocity variations which induce significant traveltime distortions in surface seismic data. A high resolution 3D traveltime tomography study was employed to map the permafrost velocity structure utilizing first-arrival traveltimes picked from 3D seismic shot records. Approximately 900,000 traveltime picks from 3167 shots were used in the inversion. Tomographic inversion of the first-arrival traveltimes resulted in a smooth velocity model for the upper 200 m of the subsurface. Ray coverage in the model is excellent down to 200 m providing effective control for estimating velocities through tomographic inversion. Resolution tests conducted through horizontal and vertical checkerboard tests confirm the robustness of the velocity model in detailing small scale velocity variations. Well velocities were used to validate tomographic velocities. The tomographic velocities do not show systematic correlation with well velocities. The velocity model clearly images the permafrost velocity structure in lateral and vertical directions. It is inferred from the velocity model that the permafrost structure in the near subsurface is discontinuous. Extensions of surface water bodies in depth, characterized by low P-wave velocities, are well imaged by the velocity model. Deep lakes with unfrozen water, inferred from the tomographic velocity model, correlate with areas of strong amplitude blanking and frequency attenuation observed in processed reflection seismic stack sections.

Description: This paper examines the usefulness of amplitude versus offset (AVO) analysis for marine and permafrost gas hydrate settings. In marine environments, AVO analyses have traditionally focused on bottom-simulating reflections (BSRs) for estimating associated marine gas hydrate and free gas concentrations. A nonlinear Bayesian inversion is applied to estimate marginal probability distributions (MPDs) of physical parameters at a BSR interface, which are related to overlying gas hydrate and underlying free-gas concentrations via rock physics modeling. The problem is further constrained by prior information and re-parameterization of inversion results. Inversion of BSR AVO data from offshore Vancouver Island, Canada, shows that gas hydrate and free-gas concentrations are, respectively, 0-23% and 0-2% of the pore volume, at a 90% credibility level. However, it should be noted that these two parameters are not independent in the analysis, but instead inversely related. Thus, 0% in gas hydrate concentrations requires some percent of free gas within the range stated (and vice versa). This result indicates that the data do not provide sufficient information to independently resolve gas hydrate and free-gas concentrations to useful accuracy. The same Bayesian inversion method is applied to synthetic AVO data generated from well-log data obtained at the Mallik 5L-38 well in the Mckenzie Delta, NWT. The model cases representing typical permafrost gas hydrate occurrences are investigated and include shale over gas hydrate-bearing sand, gas hydrate-bearing sand over shale, and gas hydrate-bearing sand over water-saturated sand scenarios. The AVO inversion sufficiently constrains the shear-wave velocity for reliable quantitative analysis, only if the gas hydrate concentration exceeds ~40%. The variable degree of model constraint obtained in this AVO study highlights the need to include rigorous quantitative uncertainty analysis in all AVO studies.

Description: This paper reviews various seismic inversion techniques (AVO, acoustic-, and elastic-impedance, pre-stack waveform inversion) for assessing elastic parameters of sediments and more specifically hydrate-bearing sediments. Several theoretical approaches are described and examples of the application of the inversion schemes to assess gas hydrate deposits in three different geological environments are compared. The first example is from a permafrost-related gas hydrate deposit at Mallik, the second example is from the Blake Ridge offshore Carolina (location of Ocean Drilling Program Leg 164), and the third example is from the Golf of Mexico (Atwater Valley and Keithley Canyon). The techniques used in these areas are band-limited acoustic impedance inversion (Mallik), post-stack elastic impedance inversion (Blake Ridge), and a hybrid inversion scheme utilizing pre-stack waveform inversion with post-stack AVO-inversion (Golf of Mexico).