Influence of Amplitude Factor on the Attenuation of Longitudinal and Transverse Waves in Dry and Water-Saturated Sandstones

We have studied the amplitude-frequency dependence of Pand S-wave attenuation using specimens of dry and water-saturated sandstone. Measurements have been performed using reflected waves for an impulse frequency of 1 MHz in the amplitude range e ~ (0.3-2.0) 10~6 under the hydrostatic pressure 20 MPa. Attenuation decrement is found to be reciprocal to amplitude. The P-wave attenuation decrement comprises 5 % in dry sandstone while it is invariant in water-saturated one. The S-wave decrement is 8 % in dry sandstone and 4 % in water-saturated one. Relaxation spectra for the P-wave differ slightly in dry and water-saturated sandstones while those for the S-wave are radically different. The amplitude change induces the relaxation force variation, which makes P-spectra to shift along the attenuation axis. Moreover, a family of S-spectrum curves shifts to high frequencies with respect to a family of P-spectrum curves. The residual hysteresis is found in the amplitude curve of the relaxation force for water-saturated rock while it lacks for dry rock. The S-wave relaxation peak broadens (up to 40 %) in water-saturated sandstone with increasing amplitude. The unusual behavior of attenuation in response to the amplitude factor is explained by the simultaneous action of viscoelastic and microplastic mechanisms. These results can be used to improve the geological interpretation of acoustic and seismic data.