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Wave attenuation in partially saturated sandstones at acoustic frequencies

Wave attenuation in partially saturated sandstones at acoustic frequencies

SEG Abstracts 1981(1): 103

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, V (sub e) and V (sub s) , and their specific attenuation, Q (super -1) (sub e) and Q (super -1) (sub s) , 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 to 400 Hz using a torsional pendulum technique. Energy loss is found to be very sensitive to partial water saturation. In Massilon sandstone, Q (super -1) (sub s) is maximum and greater than Q (super -1) (sub e) only at full saturation. Q (super -1) (sub e) rises to a strong peak at 85 percent water saturation. Energy loss drops significantly as the Massilon becomes "very dry". Q (super -1) (sub e) and Q (super -1) (sub s) in partially water saturated Massilon and Vycor are strongly frequency dependent throughout the acoustic range, exhibiting peaks between 1 to 10 kHz. Q (super -1) in dry Massilon and Vycor is independent of frequency, at least in the acoustic range. Two pore fluid mechanisms absorb energy. Bulk viscous dissipation due to fluid flow in the pores dominates in fully and partially water saturated materials. A surface capillary film mechanism operates at low moisture contents. Nonlinear frame mechanisms, such as frictional grain sliding, are not significant at normal acoustic strains, even in "dry" rocks. Compressional wave velocity and specific attenuation V (sub p) and Q (super -1) (sub p) , and bulk compressional specific attenuation Q (super -1) (sub k) , are calculated at given frequencies. While the dependent of velocities on water saturation agrees well with a simple extension of Biot's theory, there is no satisfactory theory yet available for attenuation. V (sub p) /V (sub s) and Q (super -1) (sub p) /Q (super -1) (sub s) provide sufficient information to distinguish between fully and partially water saturated Massilon sandstones, yet are insufficient to resolve the degree of partial water saturation.

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Accession: 020607646

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