Passive Tomographic Study on Velocity Changes in Underground Mines Using Tabular Mesh Grids

Abstract

Double difference tomographic inversion on measurements of travelling time and location are performed to analyze the velocity structure within rock mass in underground mining. Residuals of each iteration are estimated to evaluate the conversion of computation. Average wave propagation velocities in tabular areas are assessed to compare the velocity change affected by mainshocks. It is summarized that velocity increases before mainshocks and then reduces with temporal evolution after them. Possible explanations include static stress buildup that enhances the wave propagation before mainshocks and static stress reduction that weakens the waveform propagation. Additionally, wave propagation is attenuated by the dynamic-shaking induced fractures and ruptures within rock masses. Velocity change is shown to be of importance in assessing the stress redistribution and stability of rock masses. It is found that P-wave velocity increased by 1% ~5% before the occurrence of mainshocks. After the mainshocks, the velocity turned into decreasing and eventually dropped to a level that was even lower than the average level before the mainshocks. It can be inferred that stress increased and formed a stress concentration region before the mainshocks. The occurrence of mainshocks caused damage and stress relaxation in the rock mass, leading to a significant velocity decrease.