Abstract
Microseismic (MS) source location can help us obtain the fracture characteristics of a rock mass under a thermal-hydraulic-mechanical (THM) coupling environment. However, the commonly used ray-tracing-based location methods are easily affected by the large picking errors, multipath effects of travel time, and focusing and defocusing effects of rays in wavefield propagation, which are caused by the strong three-dimensional (3D) heterogeneity in mining areas. In this paper, we will introduce the rapidly developed waveform inversion-based location method into a mine MS field study. On this basis, the wavefields were modeled by utilizing a high-resolution 3D velocity model, a fractional-order Gaussian wavelet source-time function, and spectral element method (SEM) wavefield modeling. In order to reduce the computation cost of wavefield modeling, the 3D ray-shooting method based on a coarse grid was adopted to obtain an approximate MS event location. Based on this initial location, the multiscale waveform inversion strategy (from coarse to fine grids) and the L-BFGS iteration optimization algorithm were separately jointly selected to improve wavefield modeling speed efficiency and iterative convergence rate. Then, the IMS MS monitoring system set in the Yongshaba mine (China) and its tomographic 3D velocity model were used to conduct the synthetic test and application study. Results show that the source-time function based on the fractional-order Gaussian function wavelet can better fit complex recording waveforms compared with the conventional Ricker wavelet-based source-time function, and the waveform misfit during the L-BFGS iteration decreased rapidly. Furthermore, the multiscale waveform inversion method can obtain a similar location accuracy compared with the waveform inversion based on a single fine grid, and it can significantly decrease the iteration times and wavefield modeling computational cost. The average location error of the eight premeasured blasting events by the proposed approach is only 17.6 m, which can provide a good data research basis for analyzing MS event location and rock mass fracture characteristics in a mine.
Highlights
Microseismic (MS) source location, taking advantage of waveforms recorded by highly sensitive, broadband, and wide dynamic response sensors, plays an important role in the acquisition of fracturing location, displacement characteristics of a rock mass, inversion of a MS source focal mechanism, and magnitude along with velocity structures [1,2,3,4,5,6]
It can be seen that blasting event 6 has the largest location error of 31.6 m, while the average location error is only 17.6 m, which well satisfies the requirements for location accuracy of an engineering MS monitoring
This study successfully introduced the multiscale waveform inversion-based location method into the mine MS monitoring field with 3D complicated velocity structures
Summary
Microseismic (MS) source location, taking advantage of waveforms recorded by highly sensitive, broadband, and wide dynamic response sensors, plays an important role in the acquisition of fracturing location, displacement characteristics of a rock mass, inversion of a MS source focal mechanism, and magnitude along with velocity structures [1,2,3,4,5,6]. Researchers have conducted lots of studies on improving the precision and accuracy of MS source location methods, and the commonly used ones can be divided into ray-tracingbased location methods based on picking the travel time of a specified phase and wave equation-based location methods using waveform information directly. For the former method, the objective functions take advantage of residuals between theoretical and observed arrival time of a specified seismic phase. If the problem pertaining to computational efficiency is solved, the wave equation-based location method can have broader prospects for application
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