Effects of partial water saturation on attenuation in Massilon sandstone and Vycor porous glass

The advent of new high resolution seismic reflection and borehole sonic techniques has stimulated renewed interest in what information stress wave propagation may carry about rock properties and pore fluids in situ. We have measured extensional and shear wave velocities. Ve and Vs, and their specific attenuation, Q−1e and Q−1s, in Massilon sandstone and Vycor porous glass as a function of continuously varying partial water saturation and relative humidity. Measurements were made at frequencies from 300 Hz to 14 kHz using a resonant bar technique and from 25–400 Hz using a torsional pendulum technique. Energy loss is very sensitive to partial water saturation. In Massilon sandstone, Q−1s is maximum and greater than Q−1e only at full saturation. Q−1e rises to a strong peak at 85% water saturation. Energy loss drops significantly as the Massilon becomes ’’very dry.’’ Q−1e and Q−1s in partially saturated Massilon and Vycor are strongly frequency dependent throughout the acoustic range, exhibiting peaks between 1–10 kHz. Q−1 in dry Massilon and Vycor is independent of frequency, at least in the acoustic range. Two pore fluid mechanisms absorb energy. Viscous dissipation due to fluid flow in pores dominates in fully and partially water saturated materials. A surface capillary film mechanism dominates at low moisture contents. Nonlinear frame mechanisms such as frictional grain sliding are not signficant at normal acoustic strains, even in ’’dry’’ rocks. Compressional wave velocity and specific attenuation, Vp and Q−1p, and bulk compressional specific attenuation, Q−1k, were calculated at given frequencies. While the dependence of velocities on water saturation agrees well with a very simple explanation, there is no satisfactory theory yet available for attenuation. Vp/Vs and Q−1p/Q−1s provide sufficient information to distinguish between fully and partially water saturated Massilon sandstones, yet are insufficient to resolve the degree of partial water saturation.