Seismic Wave Propagation in Fractured Carbonate Rock

Laboratory experiments were performed on cubic samples of Austin Chalk to investigate the effect of fabric-induced anisotropy on the interpretation of fracture specific stiffness. The experimental results found that the fabric-induced layering complicates the interpretation of fracture specific stiffness. Seismic measurements from a sample with small layers (i.e., spacing and thickness) show that the presence of the fracture tends to interrupt the interference caused by short-path multiples and also tends to increase the high frequency components of the signal. This increase in the high frequency components is inconsistent with the prediction from the displacement discontinuity theory. For a fractured sample with only three distinct layers (i.e., large spacing and thickness), the high frequency components of the seismic waves were attenuated as expected by the displacement discontinuity theory. To investigate how the presence of a single fracture in a periodically layered medium affects seismic waves, numerical simulations were performed by combining a transfer matrix method with the displacement discontinuity theory for wave propagation across a fracture. From our current simulation results for a normally incident wave, in general, a fracture in a layered media behaved as a low pass filter and did not reproduce the behavior observed in the sample with thin layers. Future work will examine incident angles and fracture orientations that more closely match those from the experiments.