Low-frequency shadow detection of coalbed methane in time-frequency domain

Investigating the coal seam methane concentration area is one of vital research subjects for coalfield explora- tion and development,and the prediction of reservoirs fluid properties using seismic attenuation has become a research hotspot in recent years. In order to study the characteristics of the seismic wave energy attenuated and the waveform change as seismic wave propagates through the gas-bearing reservoirs,implement instantaneous spectral decomposition of 3-D seismic data,and detect low-frequency shadow related to the coal seam methane concentration area,the repre- sentative geologic model of coal seam methane concentration area was established based on the actual geological data of coal mine. Then,through the forward modeling by visco-elastic wave equation based on the Kelvin-Voigt model,the seismic record section was obtained. Also,with the time-frequency analysis of seismic profile with the help of S trans- form,ultimately,the processing results of different frequency components were achieved. Analytical results show that seismic wave presents obvious low-frequency shadow phenomenon. Comparing with the primary structure coal,the coal seam methane concen-tration area with lower Q value has the seismic characteristics including strong amplitude reflec- tions,the coal seam floor reflection of wave axis moves down transversely,instantaneous frequency attribute reduction, the high frequency energy of seismic waves in coal seam methane concentration area attenuates seriously,contrarily, low frequency components decay slowly. On low-frequency (30 Hz) instantaneous spectral analysis sections,the spec- tral energy of the area under coal seam methane concentration area is much stronger than that of the same area on high-frequency instantaneous spectral analysis sections when the frequency increased to more than 60 Hz. After further time-frequency processing of 3D seismic data for Hetaoyu mining area and comparative analysis of the seismic reflec- tion characteristic and time spectrum attribute for main mining coal seam ( No. 8 coal seam),the authors found that coal seam methane concentration area possesses the coincident low-frequency shadow feature with forward simulation results. Consequently,using low-frequency shadow effect not only predicts the coal seam methane concentration area, but also depicts its boundary and space distribution,which reduces the multiple solvability of methane concentration area prediction by 3D seismic data. The subsequent drilling results has vali-dated the validity of this method,which further prove that detecting methane concentration area by low-frequency shadow effect is feasible.