Computed seismic speeds and attenuation in rocks with partial gas saturation

Abstract

Abstract Calculations for an unconsolidated sand with partial gas saturation show a 20 percent increase in compressional wave velocity between 1 and 100 hz and attenuation of 27 db/1000 ft at 31 hz and 82 db/1000 ft at 123 hz.Shear velocity and attenuation are not affected. Fluid-flow waves are shown to be responsible for the dispersion and attenuation at low frequencies; relations are derived by extending Gassmann's viewpoint to include coupling between fluid-flow waves and seismic body waves. This appears to be an important loss mechanism for heterogeneous porous rocks.