Errors in microseismic events locations introduced by neglecting anisotropy during velocity model calibration in downhole monitoring

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Microseismic monitoring is the only tool to provide locations of microseismic events induced e.g. during hydraulic fracturing of unconventional reservoirs. Obtained locations become a baseline for the evaluation of stimulation effectiveness. However, due to several factors, they can be different from the true ones and lead to misinterpretation. Most important is the inaccuracy of the velocity model used in the location procedure, which is often amplified by seismic anisotropy. Even though the anisotropy is a common feature of organic-rich mudrocks, it is often neglected in the current industry practice. The usual approach is to produce a set of isotropic models, valid for the limited subsurface area only, which compensate for anisotropy effects. Here we explore how the accuracy of microseismic events' locations is affected when neglecting the anisotropy during velocity model building using traveltimes of calibration shots. We analyse events' mislocations using the synthetic modelling that simulates an ongoing multi-stage treatment in a vertically-transverse isotropic (VTI) media, monitored with a single vertical borehole. Finally, we provide a quantitative interpretation of location errors expected to be present when VTI anisotropy is being neglected. The most important result is the demonstration of how spatially limited are single-stage isotropic models in terms of correct event locations, despite performing an accurate calibration. Also, we show the limited gain in terms of location accuracy when multi-stage isotropic velocity models are constructed. As the ultimate solution to reduce location errors, we propose to account for anisotropy by inverting VTI anisotropy model parameters even when only part of calibration data is available.