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.

a b s t r a c tTo determine the impact of seeps and focused flow on the occurrence of shallow gas hydrates, several seafloor mounds in the Atwater Valley lease area of the Gulf of Mexico were surveyed with a wide range of seismic frequencies. Seismic data were acquired with a deep-towed, Helmholz resonator source (220-820 Hz); a high-resolution, Generator-Injector air-gun (30-300 Hz); and an industrial air-gun array (10-130 Hz). Each showed a significantly different response in this weakly reflective, highly faulted area. Seismic modeling and observations of reversed-polarity reflections and small scale diffractions are consistent with a model of methane transport dominated regionally by diffusion but punctuated by intense upward advection responsible for the bathymetric mounds, as well as likely advection along pervasive filamentous fractures away from the mounds.Published by Elsevier Ltd.

Data from three large‐offset seismic profiles provide information on the crustal structure beneath the Carolina trough. The profiles, obtained by the U.S. Geological Survey, the Naval Oceanographic Research Development Agency, and the Scripps Institution of Oceanography in 1985, were oriented parallel to the trough and were located (1) seaward of the East Coast Magnetic Anomaly (ECMA), which is generally thought to represent the boundary between oceanic and continental crust; (2) along the axis of the trough between the ECMA and the hinge zone, which is thought to reflect the landward limit of highly stretched and altered transitional crust; and (3) along the Carolina platform landward of the basement hinge zone on crust thought to have been thinned only slightly during rifting. These data constrain the velocity structure of the lower crust and provide evidence for a thick lens of high‐velocity (>7.1 km/s) lower crustal material that extends beneath the Carolina trough and the adjacent ocean basin. This lens reaches a maximum thickness of about 13 km beneath the deepest part of the trough, thins to about 5 km seaward of the ECMA, and is either very thin or absent landward of the hinge zone. It is interpreted to represent material that was underplated beneath and/or intruded into the crust during the late stage of continental rifting and that led to an anomalously thick plutonic layer during the early seafloor spreading phase. These data thus support the recent conclusions of White et al. (1987b) and Mutter et al. (1988) that the initiation of seafloor spreading is attended in many, if not most, cases by the generation of an anomalously large volume of melt.

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 the BSR that is normally well defined in conventional low‐frequency seismic surveys is at least twenty times weaker at the high DTAGS frequencies. The reflection coefficient‐versus‐frequency data support a new model for the velocity profile at the BSR that consists of a thin, 4–8‐m layer at the BSR in which the velocity decreases by 250 m/s. The thin transition layer at the BSR implies relatively high methane flux rates of at least 1.5 mm/year.

We review Filon's method (FM) for the quadrature o f oscillatory integrals and then introduce a generalization of Filon's method (the GFM) which enables us to treat a large class of oscillatory integrals t o which FM cannot be directly applied. One member of this class is the integral J f ( p ) exp [ s g ( p ) ] d p which occurs in the spectral WKBJ and Cagniard-de Hoop methods of seismogram synthesis. Another large class of integrals can be treated directly with FM but is better treated with the GFM since, for a given error tolerance, the GFM is simpler and faster. This class consists o f integrals of the form J f ( p ) J ( s , p ) d p in which J(s, p ) is a special function with an asymptotic expansion valid for large s. Such integrals occur in the reflectivity method. In general, every non-Filon formula for the quadrature of integrals from either class has an associated GFM formula (called the GFM analogue) which reduces to the original formula as s approaches zero but is more efficient than the original formula wher, s is large. We show how the GFM can be applied t o the computation of synthetic seismograms in the reflectivity method and the spectral WKBJ method.Although reflectivity integrals can, in theory, be computed with FM the GFM is easier to code and more economical. For reflectivity computations where: (a) the source and receiver are many wavelengths apart, or (b) the depth t o the reflectivity zone is much greater than its thickness, the GFM approach is much more efficient than any non-Filon quadrature technique. Some test calculations are presented for wavefields containing only body waves and for wavefields containing both body waves and locked modes.In the spectral WKBJ method the GFM permits the use of a much greater step size in the quadrature than would otherwise be possible. Each quadrature step contains a stationary point so no advantages accrue from deforming the contour of integration over the saddle points of the integrand.