Abstract
Measurements of ultrasonic velocity and attenuation in sandstones with a variety of saturating fluids are compared with the predictions of Biot’s theory of porous media acoustics. The data show systematic deviations from Biot theory as a function of pore fluid viscosity. Two non-Biot attenuation mechanisms are needed to explain the data: local flow in microcracks and enhanced attenuation from pore-wall roughness. A model calculation of attenuation with pore-wall roughness shows that attenuation in rocks can be many times larger than Biot predictions. This attenuation enhancement explains the inability to detect a Biot slow wave in fluid-saturated rocks and the observed correlation between ultrasonic attenuation and clay content in sandstones. The non-Biot attenuation mechanisms are negligible when extrapolated to the lower frequencies of geophysical field measurements, so ultrasonic attenuation in rocks provides no information about attenuation at the field scale.
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