Near-Field Imaging Based on High Resolution Focal Spot Properties

Abstract:

The dense spatial wavefield sampling provided by modern seismological acquisitions allows the resolution of the near-field focal spot. The large-amplitude focal spot emerges from the superposition of a collapsing, time reversed wavefront. We discuss an imaging method that is based on high resolution reconstructions of the focal spot that is obtained from cross correlation. This approach exploits the dependence of the spot's shape on local properties of the propagation medium, i.e., the distance of the first zero crossing is proportional to the wave length. We construct noise correlation functions from data collected by a highly-dense Nodal array centered on the San Jacinto fault zone south of Anza. The focal spot can be obtained from the amplitude distributions at zero lag time. We repeat this analysis using each geophone location as the collapsing point to which the wave length estimate is related. The anisotropic intensity of the converging wavefields leads to distorted focal spots. In addition, strong body and fault zone waves that are associated with the complex fault zone structure prohibit the straightforward analysis of the spatially variable zero-lag time distributions. We discuss strategies to mute the influence of the wavefield anisotropy and the removal of the body and fault zone wave components for improved reconstructions of the symmetric surface wave focal spot. The overall consistency of the local wave speed estimates from the zero crossing and images obtained with more traditional far-field travel time inversions validates the near-field approach. We discuss causes for the remaining inconsistencies and compare limits of the resolution in both cases. At higher frequencies (>3 Hz) both methods reveal a heterogeneous velocity structure that exhibits pronounced low-velocity zones. In addition, near-field based images obtained at wave lengths that are too large for the application of far-field methods suggest a strong velocity contrast across the fault.