Predicting the distribution of coalbed methane by ambient noise tomography with a dense seismic array

Abstract

Coalbed methane inrush is one of the commonly occurring dynamic disasters in coal mine. As the mining depth goes deeper, the coalbed methane concentration and pressure will also likely go up. As a result, the risks associated with coal and gas outburst increases. The traditional gas measurement method can only measure the gas content in local individual points, thus it is difficult to predict the gas content for the whole mining area. Therefore, it is necessary to find an effective geophysical method that can predict the concentration of coalbed methane before the mining process. The ambient noise tomography (ANT) method has begun to be widely used in near surface imaging for underground space development in the urban areas and for mineral exploration in mining areas. In this paper, we will show the first application of the ANT method to a dense array consisting of 96 stations deployed in the Sijiazhuang Coal Mine in Yangquan. The empirical Green's functions of Rayleigh waves between station pairs are obtained by the cross-correlation and stacking method. Furthermore, the dispersion curves of group velocity and phase velocity of fundamental Rayleigh surface waves for the period of 5 Hz~1.4 s are extracted. In this study, the average one-dimensional shear wave velocity model of the region is firstly obtained from the average dispersion curve as the initial model for the three-dimensional inversion, and then the three-dimensional shear wave velocity model above 1.0 km is obtained by using the surface wave direct inversion method. Finally, the coalbed methane content of No.15 coal in the Sijiazhuang mine field is predicted from the velocity variations around the coal bed based on the empirical relationship between coalbed methane concentration and seismic wave velocity obtained from the laboratory petro-physical experiment. In general, the distribution of predicted coalbed methane concentrations is consistent with the actual gas content revealed during the roadway excavation. This study shows that the ANT method provides a new technique for the study of coal mine gas distribution, and is a potential method for effectively predicting the distribution of coalbed methane.