Microseismic hydraulic fracture mapping is a very valuable tool for monitoring hydraulic fracture growth and behavior in unconventional reservoirs. During the development of these unconventional reservoirs, there are many engineering decisions that need to be made based on the microseismic event locations. In addition to recording the microseismic data with adequate signal-to-noise ratio, measuring, calibrating, or otherwise building an accurate velocity structure through the formation layers is another key element necessary to obtain acccurate event locations for making sound engineering decisions. The velocity building process starts with an initial velocity model usually derived from a dipole sonic log. After the receivers have all been placed, source shots in known positions, such as perforations or string shots, then are used to calibrate the velocity model, which is used to solve a nonlinear and mostly underdetermined system of equations, depending on the acquisition geometry and the number of layers in the model. In this paper, a modified Newton-Raphson method coupled with singular value decomposition (SVD) is presented to optimize the velocities and anisotropic parameters for individual layers, and the effectiveness of this method is demonstrated with examples.

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