基于方位AVO正演的HTI构造煤裂隙可探测性分析

The detectability analysis on HTI tectonic coal cracks by azimuthal AVO’s forward modeling

通过建立6类HTI构造煤的理论模型,利用Hudson等效介质理论和Schoenberg所定义的传播矩阵,正演煤层顶板的方位AVO记录,最终获得了6类模型的多方位AVO记录。通过对HTI构造煤方位AVO曲线的分析可知:①不同方位AVO曲线的P值为小于零的常量,G值较大并随方位角φ的增大而减小(φ[0,90°]);②当裂隙密度增大时,P值减小,G值增大;③相对于泥岩顶板来说,砂岩顶板的P值较小,G值较大。通过对HTI构造煤GVAz曲线的分析可知:①GVAz曲线的周期为180°,并在裂隙法向方位取最小值;②随着裂隙密度的增大,GVAz曲线的波幅相应增大;③当裂隙水填充时,GVAz曲线的波幅大于裂隙气填充时的情形。因此,方位AVO的P值可以被用来识别煤层的顶板岩性,GVAz曲线的极值和波幅可分别用来获得裂隙发育法向和裂隙密度信息。就实际地震数据来说,较高信噪比(>5)是进行方位AVO分析的前提。

Established six kinds of HTI （Transverse Isotropy with a Horizontal Axis） coal models,Hudson’s effective media theory and Schoenberg’s propagation matrix were used to calculate azimuthal AVO （Amplitude Variation with Offset） records.And finally,the azimuthal AVO records according to the six kinds of models were achieved.By analyzing azimuthal AVO curves,it is easy to conclude that：① The P values are less than zero and not variation with azimuthal,the G values are positive and decrease with the increase of azimuth （φ [0,90°]）;② With the increasing of crack density,the P values decrease and the G values increase;③ Relative to the mudstone roof,P values of sandstone roof are smaller and G values of sandstone roof are lager.By the GVAz （Gradient Variation with Azimuth） curves’ analysis,the following conclusions can be made：① GVAz curves’ period is 180°,and they get their minimum at crack’s normal direction;② With the increasing of crack’s density,the amplitudes of GVAz curves increase correspondingly;③ When cracks filled with brine,the GVAz curves have larger fluctuation range than the situation of gas infilling.Therefore,P values of azimuthal AVO curves can be used to detect coal roof’s lithology and GVAz curve’s minimum and amplitude could give the information of crack’s normal direction and density respectively.As for the practical seismic data,higher signal to noise ratio （greater than five） is a precondition of azimuthal AVO analyzing.