Warren T. Wood

Methane Hydrate and Free Gas on the Blake Ridge from Vertical Seismic Profiling

Seismic velocities measured in three drill holes through a gas hydrate deposit on the Blake Ridge, offshore South Carolina, indicate that substantial free gas exists to at least 250 meters beneath the bottom-simulating reflection (BSR). Both methane hydrate and free gas exist even where a clear BSR is absent. The low reflectance, or blanking, above the BSR is caused by lithologic homogeneity of the sediments rather than by hydrate cementation. The average methane hydrate saturation above the BSR is relatively low (5 to 7 percent of porosity), which suggests that earlier global estimates of methane in hydrates may be too high by as much as a factor of 3.

Decreased stability of methane hydrates in marine sediments owing to phase-boundary roughness

Below water depths of about 300 metres, pressure and temperature conditions cause methane to form ice-like crystals of methane hydrate. Marine deposits of methane hydrate are estimated to be large, amassing about 10,000 gigatonnes of carbon, and are thought to be important to global change and seafloor stability, as well as representing a potentially exploitable energy resource. The extent of these deposits can usually be inferred from seismic imaging, in which the base of the methane hydrate stability zone is frequently identifiable as a smooth reflector that runs parallel to the sea floor. Here, using high-resolution seismic sections of seafloor sediments in the Cascadia margin off the coast of Vancouver Island, Canada, we observe lateral variations in the base of the hydrate stability zone, including gas-rich vertical intrusions into the hydrate stability zone. We suggest that these vertical intrusions are associated with upward flow of warmer fluids. Therefore, where seafloor fluid expulsion and methane hydrate deposits coincide, the base of the hydrate stability zone might exhibit significant roughness and increased surface area. Increased area implies that significantly more methane hydrate lies close to being unstable and hence closer to dissociation in the event of a lowering of pressure due to sea-level fall.

Quantitative detection of methane hydrate through high-resolution seismic velocity analysis

A laterally extensive, high-resolution travel time velocity analysis and acoustic wave form, inversion were used to quantitatively determine methane hydrate content in deep water sediments of the Blake Ridge off the southeast U.S. coast. The interval acoustic velocity (Vp) analyses were performed in the τ-p domain by interactively picking the τ-p trajectories of prominent reflections in each of 50 plane wave-decomposed common midpoint gathers. The reflections correspond to seismic stratigraphic boundaries so that lateral Vp changes due to lithology changes are mitigated, and Vp changes due to changing hydrate content are enhanced. Two separate interval Vp analyses were performed, one with thick (∼0.4 km) layers which yielded lower uncertainty but also lower resolution, and one with thinner layers (∼0.1 km), yielding higher resolution but slightly larger uncertainties. Results show no correlation between low-sediment reflectivity and Vp. However, in the areas exhibiting a bottom simulating reflector (BSR) a high Vp interval (∼2.0 km/s and 0.15 km thick) is seen immediately above the BSR. Where the BSR is strongest a 256-layer, least squares acoustic wave form inversion reveals the BSR to be caused by a Vp decrease from ∼2.0 to ∼1.5 km/s, with little or no change in density. The inversion also reveals a thin (0.025 km) layer of anomalously low Vp lying immediately below the BSR. Two models of methane hydrate distribution are tested, each indicating that the volume of methane hydrate in the intervals of elevated Vp is up to ∼25% of the total volume.

High‐resolution, deep‐towed, multichannel seismic survey of deep‐sea gas hydrates off western Canada

A multichannel seismic survey was carried out using the high‐resolution deep‐towed acoustics/geophysics system (DTAGS) to image the structure of deep‐sea gas hydrates on the continental slope off Vancouver Island and to determine the velocity profile of the hydrated sediments. The high‐frequency DTAGS data provide the means to estimate the frequency response of the bottom simulating reflector (BSR) that defines the base of the hydrate stability field in these sediments, over a broad frequency band from 15 to 650 Hz. The DTAGS sections resolved fine‐scale layering as thin as a few meters within the hydrated zone and below the BSR, and they revealed small‐scale faults and vertically oriented zones of very low acoustic reflectivity that may represent channels for upward migration of fluids or gas. Interval velocities determined from the DTAGS data indicate uniformly low values of about 1500 m/s to depths of 100 m below sea floor (mbsf), increasing to about 1850 m/s at the BSR (250 mbsf). The reflection from ...

New seismic study of deep sea gas hydrates results in greatly improved resolution

A multichannel seismic survey has resulted in greatly improved resolution of structural details of deep sea gas hydrates off the west coast of Canada, revealing numerous geological features not before evident. The survey using the Naval Research Laboratory deep-towed acoustic/geophysics system (DTAGS), provided high-resolution images and layer velocities more than 10 times better than those obtained in the past using conventional systems. Vertical resolution within 2 m and horizontal resolution within 20 m were achieved. The work demonstrates that high-resolution seismic surveys with deep towed multichannel systems can provide important new information about pathways of fluid and gas migration that control the formation of gas hydrates. Conventional surface-towed seismic systems are unable to do this.

SITE SELECTION FOR DOE/JIP GAS HYDRATE DRILLING IN THE NORTHERN GULF OF MEXICO

Studies of geologic and geophysical data from the offshore of India have revealed two geologically distinct areas with inferred gas hydrate occurrences: the passive continental margins of the Indian Peninsula and along the Andaman convergent margin. The Indian National Gas Hydrate Program (NGHP) Expedition 01 was designed to study the occurrence of gas hydrate off the Indian Peninsula and along the Andaman convergent margin with special emphasis on understanding the geologic and geochemical controls on the occurrence of gas hydrate in these two diverse settings. NGHP Expedition 01 established the presence of gas hydrates in Krishna- Godavari, Mahanadi and Andaman basins. The expedition discovered one of the richest gas hydrate accumulations yet documented (Site 10 in the Krishna-Godavari Basin), documented the thickest and deepest gas hydrate stability zone yet known (Site 17 in Andaman Sea), and established the existence of a fully-developed gas hydrate system in the Mahanadi Basin (Site 19).

Imaging a hydrate-related cold vent offshore Vancouver Island from deep-towed multichannel seismic data

The Bullseye vent, an approximately 500-m -diameter deep-sea, hydrate-related cold vent on the midslope offshore Vancouver Island, was imaged in a high-resolution multichannel survey by the Deep-towed Acoustics and Geophysics System (DTAGS) The structure was drilled by the Integrated Ocean Drilling Program at site U1328. Towed about 300 m above the seafloor, the high-frequency (220–820 Hz) DTAGS system provides a high vertical and lateral resolution image. The major problems in imaging with DTAGS data are nonlinear variations of the source depths and receiver locations. The high-frequency, short-wavelength data require very accurate positioning of source and receivers for stacking and velocity analyses. New routines were developed for optimal processing, including receiver cable geometry 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 bycrosscorrelating redundant data betw...

A combined genetic and linear inversion algorithm for seismic waveform inversion

We combine a genetic algorithm (GA) with a linearized inversion (LI) scheme to develop a new approach to seismic waveform inversion. By incorporating the LI method into GA we intend (i) to overcome the limitations of the knowledge of a good starting model in LI and (ii) to reduce the computational cost of GA. The new method takes advantage of the convergence properties and local search approach of the linear method while the global search is carried out using GA. The two methods working together improve the directivity of the model ensemble increasing the fitness and accelerating the convergence to near the global optimum. To illustrate the procedure, we derive estimates of vp, and density for a 1-D elastic earth structure by modeling plane wave decomposed seismic data.

The low-frequency sound speed of fluid-like gas-bearing sediments

The low-frequency sound speed in a fluid-like kaolinite sediment containing air bubbles was measured using an acoustic resonator technique and found to be 114 ms with negligible dispersion between 100 and 400 Hz. The sediments void fraction and bubble size distribution was determined from volumetric images obtained from x-ray computed tomography scans. A simplified version of Woods effective medium model, which is dependent only upon the ambient pressure, the void fraction, the sediments bulk mass density, and the assumption that all the bubbles are smaller than resonance size at the highest frequency of interest, described the measured sound speed.

Simultaneous deconvolution and wavelet inversion as a global optimization

Estimates of the source wavelet and band-limited earth reflectivity are obtained simultaneously from an optimization of deconvolution outputs, similar to minimum-entropy deconvolution (MED). The only inputs required beyond the observed seismogram are wavelet length and an inversion parameter (cooling rate). The objective function to be minimized is a measure of the spikiness of the deconvolved seismogram. I assume that the wavelet whose deconvolution from the data results in the most spike-like trace is the best wavelet estimate. Because this is a highly nonlinear problem, simulated annealing is used to solve it. The procedure yields excellent results on synthetic data and disparate field data sets, is robust in the presence of noise, and is fast enough to operate in a desktop computer environment.