Effects of a filled discontinuity in a rock mass on transmission losses of compressional and shear wave of full-waveform sonic log data

Abstract

The presence of discontinuities in a rock mass affects the propagation of compressional (P) and shear (S) waves recorded by an acoustic borehole logger. The purpose of this study is to develop a new approach for characterizing such discontinuities using the full-waveform (FW) sonic tool. The approach focuses on evaluating the sensitivity of P-wave and S-wave transmission losses to the mechanical and geometrical properties of the discontinuities intersecting the borehole. Numerical simulations enabled us to quantify the impact of a discontinuity on full-waveform sonic log data using two transmission loss factors—velocity variation and amplitude attenuation—associated with P- and S-waves. These factors were determined by comparing the arrival times and amplitudes of waves propagating across discontinuous and intact media. We performed a sensitivity analysis for a wide range of discontinuity widths, lengths, P- and S-wave velocity, and density of the infilling material, and for different mechanical properties of the embedded rock. S-waves were found to be generally more sensitive to the presence of a discontinuity than P-waves. The discontinuity length influences both the compressional and shear wave responses depending on their relation to the prevailing wavelength. We demonstrate that velocity variation is more indicative of the discontinuity width than amplitude attenuation. Overall, the analysis of our dataset indicates that the P- and S-wave transmission loss factors provide complementary information, as they tend to be more sensitive to a different discontinuity property. This finding, in turn, may lead to the development of integrated algorithms for robustly characterizing discontinuities.