Stress-associated Scattering Attenuation from Ultrasonic Measurements

Acoustic attenuation has been proved to be an indicator of stress changes in solid structures. Acoustic coda, as a superposition of incoherent scattered waves, reflects small-scale random heterogeneities in solids. Acoustic coda attenuation contains information on stress changes as a result of changes in the physical state of small-scale heterogeneous structures. We measure ultrasonic properties of a cylindrical rock sample with intra-grain pores and fractures under different effective stresses to study the effect of pore-pressure induced stress changes on coda attenuation as a combination of intrinsic attenuation and scattering attenuation. We investigate the stress-associated coda attenuation quality factors Qpc and Qsc as a function of frequencies and characterize its scale dependence on stress variations in rocks by comparing with the intrinsic attenuation quality factors Qp and Qs, calculated from ultrasonic measurements. Comparisons of the P- and S-coda attenuations versus frequencies under different effective stresses demonstrate that the scattering of the S-coda is much stronger because of its shorter wavelength. The intrinsic and coda attenuations versus stress variations present quite different non-linear features, where Qp, Qs, Qpc and Qsc increase with increasing effective stress, but Qpc and Qsc show a greater sensitivity to pore pressure than Qp and Qs.