Abstract:
The elastic properties of hydrated sediments are not well-known, which leads to inaccuracy in the evaluation of the amount of gas hydrate worldwide. Elastic impedance inversion is useful in estimating the elastic properties of sediments containing gas hydrate, or free gas trapped beneath the gas hydrate, from angle-dependent P-wave reflections. We reprocess the multichannel U.S. Geological Survey seismic line BT-1 from the Blake Ridge off the east coast of North America to obtain migrated common-angle aperture data sets, which are then inverted for elastic impedance. Two new algorithms to estimate P-impedance and S-impedance from the elastic impedance are developed and evaluated using well-log data from Ocean Drilling Program (ODP) Leg 164; these new algorithms are stable, even in the presence of modest noise in the data. The Vs/Vp ratio, Poisson's ratio, and Lame parameter terms lambda rho and lambda mu are estimated from the P-impedance and S-impedance. The hydrated sediments have high elastic impedance, high P-impedance, high S-impedance, high lambda rho , slightly higher Vs/Vp ratio, slightly lower Poisson's ratio, and slightly lower lambda mu values compared to those of the surrounding unhydrated sediments. The sediments containing free gas have low elastic impedance, low P-impedance, nonanomalous background S-impedance, high Vs/Vp ratio, low Poisson's ratio, low lambda mu and low lambda mu values. We conclude that some parameters such as Vs/Vp ratio, Poisson's ratio, and lambda mu although they help identify the free-gas charged layers, cannot differentiate between the hydrated sediments and nonhydrated sediments when gas hydrate concentration is low, and cannot differentiate between the hydrated sediments and free-gas charged sediments when the gas hydrate concentration is high. Three distinct layers of gas hydrate are interpreted as being caused by gas hydrates with gas of different molecular weights, with correspondingly different stability zones in depth. Free gas appears to be present below the two deeper gas-hydrate layers, but not below the shallowest one because the lack of a trapping structure. The gas hydrate has an average concentration of approximately 3-5.5% by volume, and is highest (9%) at the base of the lower gas hydrate stability zone. The free-gas concentration ranges from 1 to 8% by volume, and is most developed beneath the local topographic high of the ocean bottom.