Hypocentral Relocations Aided by Virtual Receivers Constructed Via Seismic Interferometry

Abstract

A 3D elastic wave propagation program was used to investigate whether the Double Difference technique can improve hypocentric relocations by employing virtual receivers (VRs) near a cloud of microearthquakes. The VR technique can be loosely described as a dual of the ambient noise technique from seismic interferometry - replacing noise sources on the boundary of a region of interest with physical receivers. Seismograms from events in the interior of the region of interest can be cross-correlated and stacked/integrated over all boundary receivers to estimate a seismogram from one of the interior events as if it were recorded at the location of another interior event. Unlike ambient noise interferometry, where raypaths from all directions impinge on the region of interest, VR raypath directions are constrained by the location of the physical receiver array. Hence, approximating the surface integral plays a large role in the practical success of the technique. Fortunately, stationary-phase arguments suggest that only a few physical receivers nearby the interior-source to virtual-receiver ray direction suffice to reconstruct the seismogram. Arrival time error statistics and relocations of perturbed synthetic hypocenters using virtual receiver arrivals will be shown at the meeting. Improved performance in hypocc - by replacing the embedded LSQR solver with LSMR or a sparse SVD - will also be discussed.