How gas-hydrate saturation and morphology control seismic attenuation: A case study from the south Hydrate Ridge

Abstract

Prior studies have shown an ambiguous relationship between gas-hydrate saturation and seismic attenuation in different regions, but the effect of gas-hydrate morphology on seismic attenuation of hydrate-bearing sediments was often overlooked. We have combined seismic data with rock-physics modeling to elucidate how gas-hydrate saturation and morphology may control seismic attenuation. To extract P-wave attenuation, we process the vertical seismic profile data within a frequency range of 30–150 Hz and the sonic logging data within 10–15 kHz from three wells in the south Hydrate Ridge, offshore Oregon (USA), collected during Ocean Drilling Program Leg 204 in 2000. We calculate the P-wave attenuation using spectral matching and centroid frequency shift methods, and we use Archie’s relationship to derive gas-hydrate saturation from the resistivity data above the bottom-simulating reflection at the same wells. To interpret the observed seismic attenuation in terms of the effects of the gas-hydrate saturation and the morphology, we use the hydrate-bearing effective sediment rock-physics model. By comparing the observed and model-predicted attenuation values, we infer that (1) seismic attenuation appears to not be dominated by any single factor — instead, its variation is likely governed by the gas-hydrate saturation and the morphology; (2) the relationship between seismic attenuation and gas-hydrate saturation varies with different hydrate morphologies; (3) the squirt flow, occurring at different compliances of adjacent pores driven by pressure gradients, may be responsible for the significantly large or small attenuation over a broad frequency range.