Joe Gettrust

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.

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. n nThe 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.

High-resolution MCS in deepwater

In the early 1980s, the Navy developed instrumentation to quantify the geoacoustic properties of the upper 500 m to 1 km of marine sediments in water depths to 6000 m. To obtain this information with the resolution required to support Navy systems (several meters in depth and a few tens of meters along track), a system operating from the sea surface could not be used. Thus, a new system was needed that operated below the surface.

North Cascadia Deep Sea Gas Hydrates

Abstract: The Cascadia accretionary margin off Vancouver Island is one of the best studied margins world‐wide for the determination of in situ properties of marine gas hydrates. Most quantitative information has come from cores and downhole logs of the Ocean Drilling Program (ODP) Leg 146 and from extensive seismic and other geophysical surveys. As part of the ODP site surveys, large‐offset multichannel seismic lines outlined the regional distribution of hydrate, and seismic velocity analyses coupled with full waveform inversion provided estimates of the vertical distribution of hydrate and gas. High resolution single‐channel seismic surveys indicate correlations between hydrate/gas concentrations and topographic highs, and between geothermal flux and topography; these correlations provide insight into fluid and methane flow through the sediments. A deep‐towed multichannel seismic survey (DTAGS), together with other data from 20–600 Hz, provide constraints on gradients at the base of the hydrate and gas layer. Extensive heat flow measurements, from probes and from depths of the bottom simulating reflector (BSR), have been modelled numerically, including the regional effects of sediment thickening and advective fluid flow in the accretionary prism. Other geophysical surveys include several seafloor electrical sounding experiments and seafloor compliance measurements of hydrate. ODP drilling has provided valuable downhole log data, core physical property data, and detailed pore fluid chemistry and isotopes. Several semi‐independent estimates of hydrate and gas concentrations have been obtained; all are dependent on reference sediment properties for which no hydrate and no gas is present. From a vertical seismic profile and other seismic data, the velocity increase in a hydrated region is consistent with hydrate concentrations of 20–30% of the pore space in a 100‐m interval above the BSR. Similar values are determined from log resistivity data and from core pore fluid chlorinity.

Methane Hydrate Content of Blake Outer Ridge Sedimentsa

Methane hydrate occurs within marine sediments near continental margins where temperature and pressure conditions are favorable for hydrate formation. Highresolution seismic data collected by the Deep-Towed Acoustics/Geophysics System (DTAGS) from the Blake Outer Ridge, off the east coast of the United States, resolves sediment physical properties to 10 m vertically and 100 m horizontally, enabling the extent and hydrate content of the hydrate-bearing sediments to be determined more precisely than has been possible with conventional geophysical systems. The multichannel seismic data (FIGURE 1) show a high amplitude, reversed polarity bottom simulating reflector (BSR) marking the base of the methane-hydratebearing sediment layer, 650 m below the sea floor. Normal faults extending from the seafloor through the BSR provide an avenue for migration of methane from below the BSR into overlying sediments, enabling methane hydrate to form in a thick layer above the BSR. Compressional velocities in excess of 2.0 km/s in the sediment between -200 m below the seafloor and the BSR (abnormally high for these hemipelagic silty clays), and decreased amplitude of the reflecting horizons in this region relative to overlying reflecting horizons, suggest that methane hydrate is present throughout this -450 m thick layer of sediment.’ The percent hydrate content of the Blake Outer Ridge sediments was calculated using a formula equivalent to the time-average velocity relationship.* The method assumes that methane hydrate displaces fluid in pore spaces and that the increase in compressional velocity relative to the reference velocity is due entirely to the presence of methane hydrate in the pore space. The reference compressional velocity profile used in the calculation was obtained by averaging velocity-depth profiles obtained from multichannel seismic data collected nearby in an area where expected compressional velocity, absence of a BSR, and absence of “blanking” indicated that no hydrate is present in the sediment. The subbottom material in the reference area is assumed to be composed of a sediment and pore fluid under ambient temperature and pressure conditions. Because there are no coherent reflection horizons below 400 m depth, Hamilton’s velocity-depth curve for silty clays3 was used to extrapolate the reference profile to 650 m depth. The percent hydrate content in the sediment as a function of subbottom depth and horizontal range increases from 15% at the inferred top of the hydrate-bearing sediment layer, 200 m below the seafloor, to 35%-40% at the BSR depth (FIGURE 2). The percent hydrate values represent -30%-65% of the available pore space as estimated from sediment core samples. Laboratory investigations indicate that a