Estimating fracture orientation from elastic-wave propagation: An ultrasonic experimental approach

Abstract

Elastic-wave propagation in fractured and cracked media depends on the dominant spatial orientation of the discontinuities. Consequently, compressional and shear-wave velocities can give valuable information about the orientation of the cracks. The main goal of this work is to estimate the preferential fracture orientation based on an analysis of cross-correlated S-wave seismograms and Thomsen parameters. For this purpose, we analyzed ultrasonic measurements of elastic (P and S) waves in a physical-modeling experiment with an artificially anisotropic cracked model. The solid matrix of the model consisted of epoxy-resin; small rubber strips simulate cracks with a compliant fill. The anisotropic cracked model consists of three regions, each with a different fracture orientation. We used the rotation of the S-wave polarizations for a cross-correlation analysis of the orientations, and P-and S-wave measurements to evaluate the weak anisotropic parameters and . The shear and compressional wave sources had dominant frequencies of 90kHz and 120kHz. These frequencies correspond to long wavelengths compared to the spacing between layers, indicating a nearly effective-media behavior. Integrating the results from cross-correlation with anisotropic parameter analysis, we were able to estimate the fracture orientation in our anisotropic cracked physical model. The parameter showed good agreement with the cross-correlation analysis and, beyond that, provided additional information about the crack orientation that cross-correlation alone did not fully resolve. Moreover, our results show that the shear waves are much more strongly influenced by, and can thus contain more information about, crack orientation than compressional waves. © 2012. American Geophysical Union. All Rights Reserved.