Mapping faults in the laboratory with seismic scattering 1: the laboratory perspective

Seismic waves produced by stressed and deforming rocks lose coherence when they cross regions of high heterogeneity. The delay in the arrival of maximum seismic energy amplitude (peak delay), an essential attribute to model earthquake source characteristics, is increasingly used to map complex crustal geology, heterogeneous reservoirs and fault networks. However, no laboratory calibration for the sensitivity of this parameter to fractures is currently available due to both experimental challenges and the difficulty in modelling wavefields in the near field. In this study, peak delays have been measured and mapped in space in the frequency range 50 kHz to 1 MHz using acoustic emission data recorded during a triaxial deformation experiment of Darley Dale Sandstone. Peak delays can increase dramatically throughout the experiment, but their behaviour depends on frequency and, especially, anomalous azimuth-dependent scattering. The changes in frequency depend on strain. At low frequencies, peak delays are sensitive to surface waves generated at the sample boundaries, but they also mark the zones of shadow and intense/intermediate strains expected for an heterogeneous sample. At high frequencies, peak delays detect the zone of intense strain corresponding to the post-deformation shear zone. Temporal variations of peak delays show a frequency-dependent sensitivity to fracture nucleation, fault coalescence and sample failure. Scattering from these heterogeneities produces waves reverberating through seismic coda if the source-station path is close to an acoustic boundary, such as the fault zone or the sample boundaries. Our results confirm that peak delay has notable sensitivity to heterogeneity and can map and monitor structural-and deformation-induced changes in the near-field. The companion modelling paper tests this sensitivity and the corresponding imaging potential.