煤层瓦斯富集区时频域低频阴影检测

为了研究地震波在含气地层传播过程中地震波能量衰减、波形变化、横向连续性的变化等特征,实现三维地震资料瞬时谱分解,检测瓦斯富集区的低频阴影特征,在已有煤矿实际地质资料分析基础上,构建了具有代表性的煤层瓦斯富集区地质模型。通过黏弹性地震波动方程正演数值模拟获得了模拟地震记录剖面,并借助于S变换实施了地震剖面时频分析,获得了不同频率成分的处理结果。对比分析结果表明:横向上,煤层瓦斯富集区较原生煤反射振幅更强,且煤层底面反射同相轴整体下拉,瞬时频率降低,具有低Q值的瓦斯富集区地震波高频能量衰减严重,低频成分衰减慢,低频(30 Hz)瞬时剖面中对应瓦斯富集区下方表现为强能量团,当频率增加到60 Hz及以上,瓦斯富集区能量明显低于低频瞬时剖面中对应位置的能量,呈现出明显的"低频阴影"现象。选取核桃峪矿区实测三维地震资料实施时频分析处理,并重点针对8号煤层的地震反射特征和时频谱属性进行对比分析,结果表明煤层瓦斯富集区具有与正演模拟结果完全一致的"低频阴影"特征。利用"低频阴影"不仅能预测煤层瓦斯富集区,而且可以刻画富集区的边界和空间展布,减小三维地震勘探资料预测煤层瓦斯富集区的多解性;经后期实际钻采结果验证,结果吻合较好,证明了利用"低频阴影"检测瓦斯富集区的可行性。

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 exploration 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 representative 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 transform,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 reflections,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 spectral 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 reflection 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.