Microseismic source location using a 3D velocity model: From the ray tracing method to waveform inversion

Abstract

Microseismic (MS) source location is important in MS monitoring, providing the basis for determining fracture zones and calculating seismic source parameters (e.g., event magnitude and focal mechanisms). To date, homogenous, 1D, and simple 3D velocity models have been adopted in MS source location. However, a mine is usually characterized by a strong velocity heterogeneity due to engineering geology, 3D geostress, and excavation. In this work, we adopted a travel-time tomography-based high-resolution 3D velocity model and then applied 3D ray tracing based on the shooting method, 3D Gaussian beam-based reverse-time method, and waveform inversion method to MS source location. A semiautomatic waveform-cut method based on the cross-correlation (WCC) technique was developed for a quick, robust, and precise determination of the direct P-phase relative delay times. Additionally, the spectral element method for wavefield modeling, multiscale grid (coarse grid + fine grid) 3D waveform inversion, and L-BGFS iterative method were applied in the waveform inversion. Our results show that the commonly used ray tracing method may be affected by multiray path effects and waveform focusing/defocusing during wavefield propagation, whereas the Gaussian beam method has a frequency-dependent width, and the waveform inversion method has a broader frequency width, which can effectively overcome the issues of ray tracing. The average location errors of eight blasting events obtained using the 3D ray tracing, Gaussian beam, and waveform inversion methods are 26.2, 17.0, and 17.6 m, respectively, which are smaller than those of previous researches obtained using the homogenous-velocity model (average location error > 40 m). In conclusion, the high resolution of the location methods based on 3D velocity models can provide an effective approach to improve the MS source location accuracy and exhibit broad application potential.