Ultrasound scattering and absorption in a cracked rock

Ultrasonic experiments were performed on room-dry and wet shale samples—composites consisting of sand grains and clay with extremely low porosity and permeability, and exhibiting a layered texture. Wetting caused significant shear wave scattering and a reduction in shear wave velocity, likely due to moisture-induced microcracks from the swelling of clay minerals. To analyze these effects, we used the Monte Carlo method to solve the radiative transfer equation for finite-size samples, extracting the scattering mean-free path and absorption length from ballistic and coda waves. The frequency-dependent scattering mean-free path indicated that wet shale could be modeled as a medium with randomly oriented and located cracks, characterized by a low crack density and power-law crack length distribution. This was supported by 2-D finite-difference simulations of shear wave scattering in a similar cracked medium. As confining pressure increased, microcracks in the shale closed, leading to higher shear wave velocity and scattering mean-free path. Interestingly, the absorption length was found to decrease under pressure, which contrasts with the behavior typically observed in sandstones (e.g., squirt flow dissipation). This result aligns with observations in weakly wet granular media, where water films trapped by asperities at grain surfaces play a key role.