Meitan xuebao (May 2023)
Wave velocity model and microseismic location method of goaf overburden rock
Abstract
In order to build a small-scale regional microseismic wave velocity model in a coal mine and improve the location accuracy of microseismic source in a complex geological structure of goaf, based on the characteristics of deformation, separation, development of faults and fissures and rock expansion in goaf overburden rock “three zones”, the wave velocity structure of goaf overburden rock is described by constructing a wave velocity model of circular arc layer. The diffraction propagation path increment of microseismic wave considering the deformation characteristics of rock and the fracture rock velocity reduction based on the time-average equation are proposed to modify the velocity model. Based on the assumption of layered homogeneous media and the Snell’s law, the propagation path and travel time of microseismic wave are derived, and the anisotropic equivalent wave velocities in each direction are calculated. Based on the arrival time location theory, the difference between the measured time difference and the theoretical time difference is chosen to construct the objective function and the pattern search algorithm is used to solve the source location problem. A microseismic location method based on the anisotropic wave velocity model of goaf overburden rock is proposed. Through a 3D calculation example and the location application of coal mine ground microseismic monitoring, the feasibility and location effect of the proposed method are verified. Results show: ① Due to the wide distribution of goaf and the inevitability of microseismic wave propagating through goaf in coal mine geological environment, it is the key to establish the corresponding anisotropic wave velocity model according to the complex stratum structure characteristics of goaf in a coal mine for improving the location accuracy of microseismic source in mine environment. ② The wave velocity model of goaf overburden rock fully considers the influence of rock stratum morphology, separation, fracture, fissure development and rock expansion on microseismic wave propagation path and equivalent wave velocity. The microseismic wave propagation diffraction path increment and rock wave velocity reduction are quantitatively determined. The proposed model shows the typical wave velocity anisotropy characteristics of goaf stratum structure. ③ For the source location in stratified media, compared with the global average wave velocity model, the average error of location results is reduced from 9.7558 m to 0.4186 m by using the anisotropic equivalent wave velocity model in the 3D calculation example, which effectively reduces the location errors. ④ Taking the underground microseismic location results of large energy microseismic events as reference, the wave velocity model of goaf overburden rock is applied in the field ground microseismic monitoring. And the location results of the proposed model are closer to the influence range of coal and rock fracture determined in the field investigation results than the undetermined wave velocity. In conclusion, for the location method relying on the wave velocity model, the establishment of the corresponding anisotropy wave velocity model according to the characteristics of the geological structure is one of the breakthrough points to carry out the microseismic monitoring efficiently and improve the location accuracy of the microseismic source.
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