Gas hydrates, or clathrates, are ice-like crystalline solids composed of water molecules surrounding a gas molecule. In recent years, methane hydrates have been the focus of many studies because of their widespread occurrence in permafrost regions and in most of the world oceans (Kvenvolden, 1993). Also, immense amounts of methane may have escaped from deep-sea gas hydrates to the atmosphere as a result of dissociation of gas hydrates caused by sea-level and climatic changes, and acted as a negative feedback control on global temperature fluctuations (Dillon et al., 1991; Paul et al., 1991). Gas hydrate is also of potential importance as an energy resource and significant for seafloor stability and safety issues (Dillon et al., 1993).
Gas hydrates exhibit relatively high acoustic velocities compared to the pore-filling fluids; therefore, the velocity of gas hydrate-bearing sediments is usually elevated (Stoll, 1974; Tucholke et al., 1977). Based on the elevated velocity because of the presence of gas hydrate, a number of studies have attempted to estimate in situ hydrate amounts using seismic velocities (Mackay et al., 1995; Lee et al., 1993; Wood et al., 1994; Spence et al., 1995; Kastner et al., 1995; Yuan et al., 1996).
Kastner et al. (1995) estimated a minimum of 15% pore-space occupancy based on velocities from acoustic logs and vertical seismic profiles (VSP) on the Cascadia Margin at Ocean Drilling Program (ODP) Site 889, where the velocities of gas hydrate-bearing sediments are greater than expected by 100 m/s from a normal velocity-porosity relation. Using velocities acquired by an ocean-bottom seismometer, Spence et al. (1995) also estimated that 11%-20% of the pore space above the bottom-simulating reflector (BSR) is filled by gas hydrate at Site 889.
In this paper, the amount of gas hydrates in the pore space was estimated by applying a weighted equation proposed by Lee et al. (1996) to acoustic logs along with three distributions of porosity. This study shows that estimations using acoustic logs and core porosities are comparable to those estimated from other methods such as those using chloride anomalies or resistivity measurements.