Optimal Recovery of Rayleigh-Wave Overtones by Multi-Directional Acquisition

Rayleigh waves are ubiquitously used for subsurface characterization through dispersion curve inversion, whose quality depends on the number of useable overtones. Traditional analysis is based on vertical receivers and, for active surveys, sources. However, for layered media, eigenfunction theory shows that optimal recovery of any given dispersion mode and frequency can be achieved by either vertical source-vertical receiver or radial source-radial receiver configurations, with source directivity being more dominant. Multi-directional near-surface wave-equation modeling and field examples, including distributed acoustic sensing data, validate these predictions. Statistical analysis of compaction-type near-surface models shows that overtones are better recovered for radial-radial surveys in large portions of the useable spectrum, extending beyond 60% for the second and above modes. We conclude that incorporating radial-radial data acquisition is beneficial and should become standard procedure in active surveying, as well as analyzing the radial component in ambient noise and earthquake-induced Rayleigh waves.