SEISMIC ATTENUATION IN PARTIALLY SATURATED ROCK

Measurements of attenuation in the extensional and shear modes for a sandstone (Murphy, 1982) indicate that two separate mechanisms are operative, one at very low saturation and another at saturation levels above 50%. At saturations of less than a few percent, attenuation in extension increases to a peak and then decreases to a plateau. Attenuation in shear behaves similarly, the magnitude being 20-30% less than that in extension. At saturations greater than 50%, attenuation in shear remains at the plateau value, while attenuation in extension rises to a broad peak, finally decreasing to a low value at full saturation. We analyzed a model in which porosity of the sample is assumed to be composed of cracks and equiaxed cavities called pores. At low saturation, attenuation is assumed to be caused by viscous dissipation as fluid in cracks is squished by changes in aperture under the normal component of the wave. At higher saturation, attenuation is assumed to be caused by viscous losses as fluid is forced from completely saturated regions, composed of many pores, to dry regions. Analysis shows that the model simulates the behavior observed by Murphy for Massilon sandstone. Agreement between theory and experiment suggests that the dominant aspect ratio for cracks is of the order of 10(-3) and the size of saturated regions is about 1 mm at 90% saturation, decreasing to smaller values as the sample dries.