Acoustic properties of sediments saturated with gas hydrate, free gas and water

Abstract

ABSTRACT We obtain the wave velocities and quality factors of gas‐hydrate‐bearing sediments as a function of pore pressure, temperature, frequency and partial saturation. The model is based on a Biot‐type three‐phase theory that considers the existence of two solids (grains and gas hydrate) and a fluid mixture. Attenuation is described with the constant‐ Q model and viscodynamic functions to model the high‐frequency behaviour. We apply a uniform gas/water mixing law that satisfies Wood's and Voigt's averages at low and high frequencies, respectively. The acoustic model is calibrated to agree with the patchy‐saturation theory at high frequencies (White's model). Pressure effects are accounted by using an effective stress law for the dry‐rock moduli and permeabilities. The dry‐rock moduli of the sediment are calibrated with data from the Cascadia margin. Moreover, we calculate the depth of the bottom simulating reflector (BSR) below the sea floor as a function of sea‐floor depth, geothermal gradient below the sea floor, and temperature at the sea floor.