The application of geophysical methods to the study of methane hydrates is motivated by the need to characterize physical parameters or processes that may control or be affected by the distribution of gas hydrate and free gas. Although drilling provides direct samples of gas, hydrate, and sediment, geophysical methods have the capacity to estimate the distribution of free gas and gas hydrate both laterally and vertically, quickly and remotely over large areas. A particular strength of Leg 164 was the combination of direct sampling (coring) with the acquisition of in situ and laboratory geophysical data from which the elastic, thermal, electrical, magnetic, and porous media properties of the sediments and gas hydrate reservoir can be estimated.
Previous geophysical studies in the Ocean Drilling Program (ODP) Leg 164 study area have focused primarily on characterizing thermal (Ruppel et al., 1995) conditions or elastic properties that are either directly or indirectly related to the occurrence of gas hydrates. The Blake Ridge was among the earliest marine provinces recognized as a significant gas hydrate area (Markl et al., 1970), and initial single-channel seismic (SCS) and multichannel seismic (MCS) studies (Shipley et al., 1979; Paull and Dillon, 1981) focused on determining the nature and extent of the bottom-simulating reflector (BSR). Attempts to quantify the amount of gas hydrate present in this area have been made on the basis of seismic velocity analyses (Rowe and Gettrust, 1994), waveform inversion (Wood et al., 1994; Korenaga et al., 1997), and SCS reflection amplitudes (Lee et al., 1993). Other seismic data have been used to interpret connections between geologic structures (faults and slumps) and gas hydrate/free-gas deposits (Dillon et al., 1994; Dillon, Danforth, et al., 1996; Dillon, Hutchinson, et al., 1996). High-resolution seismic surveys (both surface and deep towed) have revealed finer scale structural features, including complex fault systems cutting through the BSR and overlying sediment and a BSR that appears broken and discontinuous over length scales of meters to tens of meters (Rowe and Gettrust, 1993a, 1993b; Dillon et al., 1994).
ODP Leg 164 greatly enhanced the existing geophysical data sets for gas hydrate provinces through the acquisition of zero offset and walk-away vertical seismic profiles (ZVSPs and WVSPs, respectively) and closely spaced (30-50 m) downhole temperature data at three sites located along a transect on the Blake Ridge. This paper first reviews the major results to emerge from the analysis of Leg 164 geophysical data and ancillary data sets. We then discuss the implications of the geophysical results for certain phenomena (e.g., seismic blanking); for interpretation of the base of gas hydrate stability, the base of the zone of gas hydrate occurrence, and the top of the free gas zone; and for the microscale (centimeters to meters) distribution of gas hydrate. Throughout this synthesis, we incorporate results from Leg 164 geochemical and sedimentological analyses as appropriate.
The Inner Blake Ridge, a major physiographic feature on the southeastern U.S. passive margin, is a sediment drift deposit formed by erosion of the Blake Plateau at the confluence of the late Oligocene (Tucholke and Mountain, 1986) or early Miocene Gulf Stream (Markl and Bryan, 1983; Dillon and Poponoe, 1988). The Blake Ridge has several distinct advantages for the study of the distribution of gas hydrate and free gas and of the processes related to evolution of gas hydrate provinces. First, the vertical and lateral homogeneity of Blake Ridge sediments and sedimentary processes implies that any major changes detected in seismic stratigraphy and seismic velocity structure might be directly attributable to the presence of free gas or gas hydrate. Second, the Blake Ridge lies in a tectonically quiescent setting, proximal to the passive margin. This implies that the Blake Ridge should not be significantly affected by major late Cenozoic tectonic activity or by large-scale fluid flow along major faults in the basement or sedimentary column, factors that have complicated analyses in other gas hydrate provinces (e.g., Cascadia; Westbrook, Carson, Musgrave, et al., 1994). Finally, the Blake Ridge BSR is among the best studied on Earth's continental margins and may be considered the archetypal BSR (Shipley et al., 1979), due to its conspicuous cross-cutting relationship to strata being eroded on the ridge's northeastern flank. A consistent and intriguing aspect of the Blake Ridge sediments is low reflectivity between the seafloor and the BSR (Fig. 1).
Despite the quiescent tectonic regime and uniform sedimentology of the Blake Ridge, both Leg 164 and ancillary studies have yielded results that partially challenge assumptions about the simplicity of this setting for the study of gas hydrate problems. For example, various seismic studies have revealed the presence of a large-scale collapse structure near the principal drilling transect (Dillon, Danforth, et al., 1996) and pervasive small offset normal faults rooted within the gas hydrate-bearing zone (Rowe and Gettrust 1993a, 1993b; Wood and Gettrust 1998) coincident with the transect. Just off the transect, SCS Line 16 reveals a complicated pattern of sedimentation, slumping, reflectivity, and disrupted BSRs (W.S. Holbrook, pers. comm., 1998). These observations suggest processes more sophisticated than a laterally uniform zone of high-velocity, hydrate-laden sediment immediately above a zone of gas-charged sediment.