Frequency dependent anisotropy in saturated porous rocks with aligned fractures

Porous rocks with aligned fractures exhibit frequency dependent seismic anisotropy due to wave induced<br>fluid flow between pores and fractures. To model this frequency dependent anisotropy, we combine the low<br>frequency anisotropic Gassmann model with the frequency dependency from a penny shaped crack model.<br>Predictions of the anisotropic Gassmann model, and the combined frequency dependent model, are compared<br>to angular dependent wave velocities measured on a synthetic porous sample with aligned cracks. For the<br>saturated sample, the anisotropic Gassmann model predicts qualitatively accurate velocities, but P-wave<br>anisotropy is overestimated by approximately 25%. This discrepancy can be explained by the low frequency<br>assumption of the anisotropic Gassmann model, which does not account for fluid diffusion effects occurring<br>at the relatively high frequencies used in the experiment. The combined frequency dependent model accounts<br>for fluid diffusion effects, resulting in excellent agreement for predicted P- and S-wave velocities.