Anisotropic attenuation in rocks: Theory, modelling and lab measurements

Abstract

Anisotropic attenuation affects seismic observations and complicates their interpretations. Its accurate determination is, however, difficult and needs extensive measurements of wavefields in many directions. So far, the traveltime and amplitude decay of waves are usually measured along a sparse grid of propagation directions, and methods for inverting for anisotropic attenuation are not fully developed. In this paper, we present theory allowing a description and parametrization of general triclinic anisotropic attenuation. We focus on a correct recalculation of ray quantities usually measured in lab to phase quantities needed in the inversion. We develop and numerically test an iterative inversion scheme for determining the parameters of anisotropic attenuation. We present a lab facility that allows for measuring anisotropic attenuation using the P-wave ultrasonic sounding of spherical samples in 132 directions distributed regularly over the sphere. The applicability of the proposed inversion method and the performance of the experimental setup are exemplified by determining triclinic anisotropic attenuation of the serpentinite rock from Val Malenco, Northern Italy. The ray velocity and ray attenuation were measured on a spherical sample of the rock with diameter of 45.5 mm at the room temperature and under two pressure levels: 0.1 and 20 MPa. The measurements confirmed that anisotropic attenuation is remarkably sensitive to confining pressure. Since cracks are closing with increasing pressure, attenuation decreases. However, changes in pressure can also induce changes in the directional variation of attenuation and rotation of anisotropy axes. The obtained results for the serpentinite rock sample are unique because they represent the first accurately determined triclinic anisotropic attenuation from lab measurements.