Seismic characterization of fracture compliance in the field using P- and S-wave sources

Near-surface seismic field experiments using both P- and S-wave sources were carried out to image fractured limestones at two sites in southwest England. We measured P- and S-wave seismic velocities at multiple azimuths to aligned fracture sets, allowing us to determine the seismic anisotropy generated by these fractures. The effect of aligned fractures on seismic anisotropy is commonly modelled in terms of the additional compliance introduced by the fractures. Therefore, an understanding of fracture compliance is crucial both in terms of interpreting observations of anisotropy in the field and in forward modelling the effects of fractures on seismic wave propagation. Of particular concern is (1) the scaling of fracture compliance with fracture length scale, and (2) the controls on the ratio of normal to tangential compliance of the fractures (Ω = Z_N/Z_T). Our experimental design allows us to image both, and we find thatΩ = 0.37 ± 0.06 and Ω = 0.75 ± 0.10 for our two study sites, while the absolute values of the tangential compliance range from 0.66 × 10⁻¹¹ to 5.0 × 10⁻¹¹ Pa⁻¹ m.