Strain amplitude-dependent attenuation of P and S waves in dry and water-saturated sandstone under confining pressure

The dependence of P- and S-wave attenuation on strain amplitude and frequency has been studied experimentally in dry and water-saturated sandstone samples under a confining pressure of 20 MPa. Attenuation of P and S reflections was measured at a frequency of 1 MHz in a strain range of ε ∼(0.3−2.0) × 106. The measured P-wave attenuation (QP−1) in dry sandstone and S-wave attenuation (QS−1) in dry and saturated samples turned out to be inversely proportional to strain amplitude while QP−1 in saturated sandstone showed no strain dependence. The frequency-dependent attenuation spectra in dry and saturated sandstone differed considerably in S waves but were generally similar for P reflections. Strain amplitude variations were found out to influence the frequency dependence of attenuation and to shift the relaxation spectra of S waves toward high frequencies relative to those of P waves. As strain increased, the S-wave attenuation peak in saturated sandstone became notably (to 40%) narrower. The unusual strain amplitude-dependent behavior of attenuation may be due to joint action of viscoelastic and microplastic mechanisms. The reported results may be useful for improving geological interpretation of acoustic and seismic data.