Efficiently identifying coalbed methane enrichment areas by detecting and locating low-frequency signals in the coal mine

Low-frequency signals have been widely found in the conventional oil/gas field and volcanic region as well as during hydraulic fracturing of unconventional oil/gas reservoirs. Their generation mechanism has been ascribed to the flow of gas/fluid in the fractures, which can induce the Krauklis wave around fractures and can further excite low-frequency seismic body wave signals at diffraction points. Thus, it is theoretically feasible to determine the gas/fluid enrichment areas and migration pathways by locating the low-frequency signals. Here we have utilized a surface dense seismic array deployed above the Sijiazhuang coal mine in Shanxi province to detect and locate such low-frequency signals that are dominant in the frequency range of 1.5–4.0 ​Hz. Waveform migration-based location method is employed to locate these signals that have low signal to noise ratios. We further compare the distribution of low-frequency signals and coalbed methane concentrations that are estimated based on ambient noise tomography result with the same seismic array. The spatial consistency between low-frequency signals and coalbed methane enrichment areas suggests that detecting and locating low-frequency signals with a surface seismic array is an efficient way to identify gas enrichment areas and potential gas migration pathways.