Abstract

We have used seismic refraction surveys of a wave-cut platform from a field site in South West England to characterize the impact of natural fracture networks on seismic velocities and anisotropy. Time-lapse surveys were performed as the high tide ebbed to investigate the seismic effects of the water draining from the rock. We also deployed a drone to map the fracture sets from the air. Azimuthal variations in the P- and S-wave velocities reflect the orientation of the main east-west-oriented joint set. Seismic velocities increased as the water drained, an effect attributed to a reduction in the effective density of the medium. The ratio of fracture normal (Z(N)) to tangential (Z(T)) compliance (Omega = Z(N)/Z(T)), which can be used as a proxy for fracture saturation and permeability, was observed to increase from Omega = 0.18 to Omega = 0.48, primarily driven by a drop in Z(T). These variations are attributed to a decrease in the water content of the main fracture set as the tide retreats.