Refocusing Surface Wave Fields Resolve Dispersion and Anisotropy

Abstract:

Converging surface wave fields create a large-amplitude feature at the origin referred to as focal spot. Its properties are governed by local medium properties and have long been used in medical imaging approaches such as passive elastography. Modern dense seismic arrays consisting of many hundreds of sensors allow now the application of noise correlation-based focal spot imaging in seismology. Here we use numerical experiments to study the resolution properties of focal spots to explore the limits of seismological data applications. Using a time-reversal approach we synthesize Rayleigh wave focal spots in a 1-D layered propagation medium from Green’s functions computed with the widely used AXITRA solver. Dispersion resolution is studied using the wavenumber dependence of the Bessel-function parameterization. Unlike the frequency-domain equivalent SPAC approach focal spots are sensitive to and can resolve azimuthal anisotropy. We demonstrate the power to resolve medium anisotropy with focal spots constructed from noise wave fields with azimuthally variable energy distributions using different filter techniques. The performance of a wavenumber filter and a covariance matrix eigenspectrum filter is similarly tested for the commonly encountered situation of impinging body waves. We demonstrate the effectiveness of the imaging method and the performance of the various data processing techniques using focal spots obtained from continuous US Transportable Array records. The results suggest that properties of Rayleigh wave propagation can be estimated from noise cross-correlation based focal spots at periods up to 300 s using the vertical-vertical and vertical-radial components of the Green’s tensor.