The 3D elastic wave propagation program WPP (Petersson & Sjogreen, 2011) has been used to investigate whether the technique of Curtis et al. (2009) can be used to improve hypocentral relocations by employing virtual receivers near a cloud of microearthquakes. The virtual receiver technique can be loosely described as the "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 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, Virtual Receivers’ 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 (as described in Curtis et al., 2009). Arrival time error statistics supporting this conclusion from WPP simulations are shown. Additionally, relocations of perturbed synthetic hypocenters using virtual receiver arrivals and hypocc (Foulger and Julian, 2013) are shown. Improvements to the numerical least squares solver have been implemented, and the results indicate that the choice of solver strongly affects the quality of the relocations.

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