Combined vertical‐incidence and wide‐angle seismic study of a gas hydrate zone, Blake Ridge

Abstract

Wide‐angle ocean bottom seismic data and single‐channel seismic reflection data were collected in June 1992 over an area where gas hydrates are thought to be extensive on Blake Ridge, offshore South Carolina. Wide‐angle reflections were observed on four Woods Hole Oceanographic Institution ocean bottom hydrophones at offsets up to 15 km. Results from traveltime inversion show that the bottom simulating reflector (BSR) that marks the base of the hydrate stability field lies 400–500 m below the seafloor and is overlain by a 200‐ to 300‐m‐thick layer of average velocity 1.9 km s−1. There is no evidence for significant lateral velocity variation associated with lateral changes in BSR character. Ray‐tracing calculations show that the observation of the BSR out to large offsets (7 km) constrains the maximum vertical velocity gradient to about 0.5 s−1. Amplitude‐versus‐offset (AVO) analysis was performed for ranges up to about 6 km and incidence angles of more than 70°. The high amplitude of the BSR in the normal incidence data can be explained by two models, both of which include a strong negative impedance contrast: the “hydrate wedge” model and the free gas layer model. Synthetic seismogram calculation for the wedge model, using the reflectivity method, shows a strong increase in the BSR amplitudes at offsets between 3 and 5 km because of postcritical reflections and diving waves within the wedge. This amplitude increase is clearly not observed in the data, which shows only a relatively modest increase in amplitude with offset. This observation agrees better with the calculated AVO for the gas layer model. The lack of a distinct reflection from the base of the gas layer implies that the gas layer is not thicker than 25 m and/or that the base of the gas layer is a gradational boundary.

A laboratory investigation into the seismic velocities of methane gas hydrate‐bearing sand

“…It was also assumed that low-amplitude reflections observed above some BSRs [Shipley et al, 1979] could be used to assess the concentration of gas hydrate within the stability zone [Lee et al, 1992]. However, the use of differing seismoacoustic acquisition techniques, such as multichannel seismics, ocean bottom seismometers [Katzman et al, 1994;Vanneste et al, 2002], deep tow seismics [Gettrust et al, 1999], vertical seismic profiling [Bangs et al, 1993;Holbrook et al, 1996], and down hole logging [Guerin et al, 1999;Lee and Collett, 2001], has led to a greater understanding of the nature of the BSR. The BSR is now considered to result from free gas in sedimentary layers beneath the hydrate stability zone (HSZ) [Holbrook et al, 1996;Korenaga et al, 1997;MacKay et al, 1994;Minshull et al, 1994;Singh et al, 1993].…”

Seismic and seafloor evidence for free gas, gas hydrates, and fluid seeps on the transform margin offshore Cape Mendocino

“…[16] The seafloor reflection coefficient of 0.27 ± 0.03 near the Gorda Escarpment is higher than ''typical'' seafloor reflection coefficients, which are generally 0.1 -0.2 [e.g., Katzman et al, 1994;Yuan et al, 1999], but is similar to values obtained along the crest of accretionary ridges of the Cascadia subduction zone, where local carbonate crusts lead to reflection coefficients that are typically $0.3 [e.g., Fink and Spence, 1999]. We attribute the high seafloor reflectivity south of the Gorda Escarpment primarily to the erosional nature of the seafloor, although diagenesis related to fluid flow may also be a factor.…”

Leg 164 Introduction

“…These low velocities have been attributed to gas-charged sediments (Dillon et al, 1980), such as those that have been found beneath BSRs elsewhere (Bangs et al, 1993;MacKay et al, 1994). Velocity estimates for the sediment section directly above the BSR are variable, with some relatively high (>2200 m/s) estimates (Dillon and Paull, 1983;Rowe and Gettrust, 1993a) and other more modest (>2000 m/s) estimates (Wood et al, 1994;Katzman et al, 1994). Moreover, unusual V s velocity structures have been inferred (Ecker and Lumley, 1993).…”