Microearthquake location and uncertainty analysis using a Kirchhoff wavefront imaging method; a comparison with traveltime inversion and full wavefield imaging methods

The uncertainty and resolution of microearthquake location are an important topic for reservoir characterization and early warning of induced earthquakes. Many independent studies have either improved certain source location methods or applied a certain method to a data set without perhaps studying the full limitations compared with other methods. We have developed a P- and PS-wavefront imaging (WI) method to estimate the source origin time sequentially, which overcomes the unknown variable of source origin time in Kirchhoff-type imaging and helps to reduce the location uncertainties originating from the source origin time. We develop a 3D synthetic example by finite-difference modeling the realistic elastic 3C data using the SEG advanced modeling (SEAM) 4D earth model, and jointly compare the reservoir and basement microearthquake source location uncertainties based on traveltime inversion (TI), WI, and full wavefield imaging (FWIMG) methods. For each method, we also investigate the source location uncertainties between the P- and PS-wave results. Our results indicate that, using P waves and typical surface receiver arrays, TI has larger depth location uncertainties and cannot separately resolve the reservoir and basement events, whereas WI has larger horizontal uncertainties; FWIMG has the smallest horizontal location uncertainty, its depth uncertainty is similar to the WI method, and it is much smaller than the TI. Using PS waves can significantly reduce the location uncertainty compared with using only P waves for the three methods, and the full wavefield method has the smallest location uncertainty. The WI method has a moderate computation cost between TI and full wavefield methods. Data noise and velocity scaling errors change the source location uncertainties based on different imaging methods.