Field measurements of fracture characteristics on a wave-cut platform

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 ([Formula: see text]) to tangential ([Formula: see text]) compliance ([Formula: see text]), which can be used as a proxy for fracture saturation and permeability, was observed to increase from [Formula: see text] to [Formula: see text], primarily driven by a drop in [Formula: see text]. These variations are attributed to a decrease in the water content of the main fracture set as the tide retreats.