SEISMIC CHARACTER OF GAS HYDRATES ON THE SOUTHEASTERN UNITED-STATES CONTINENTAL-MARGIN

Gas hydrates are stable at relatively low temperature and high pressur e conditions; thus large amounts of hydrates can exist in sediments wi thin the upper several hundred meters below the sea floor. The existen ce of gas hydrates has been recognized and mapped mostly on the basis of high amplitude Bottom Simulating Reflections (BSRs) which indicate only that an acoustic contrast exists at the lower boundary of the reg ion of gas hydrate stability. Other factors such as amplitude blanking and change in reflection characteristics in sediments where a BSR wou ld be expected, which have not been investigated in detail, are also a ssociated with hydrated sediments and potentially disclose more inform ation about the nature of hydrate-cemented sediments and the amount of hydrate present. Our research effort has focused on a detailed analys is of multichannel seismic profiles in terms of reflection character, inferred distribution of free gas underneath the BSR, estimation of el astic parameters, and spatial variation of blanking. This study indica tes that continuous-looking BSRs in seismic profiles are highly segmen ted in detail and that the free gas underneath the hydrated sediment p robably occurs as patches of gas-filled sediment having variable thick ness. We also present an elastic model for various types of sediments based on seismic inversion results. The BSR from sediments of high rat io of shear to compressional velocity, estimated as about 0.52, encase d in sediments whose ratios are less than 0.35 is consistent with the interpretation of gas-filled sediments underneath hydrated sediments. This model contrasts with recent results in which the BSR is explained by increased concentrations of hydrate near the base of the hydrate s tability field and no underlying free gas is required.