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 migrationbased 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.