各向异性薄煤层纵波AVAZ分析

通过建立HTI型薄煤层的传播矩阵,推导了水平界面情况下含各向异性薄煤层反射系数精确计算公式,并利用该公式对含有薄HTI型各向异性煤层进行了纵波AVAZ分析。经过研究发现,当HTI型煤层为薄层时(厚度小于地震波长的1/4时),其纵波反射系数与煤层为厚层时(厚度大于地震波长)顶板的纵波反射系数规律相似,但其值不仅与上下层介质物性参数有关,还与厚度有关,可见煤层为薄层时采用经典公式反演会带来误差;薄层厚度、Thomsen等效各向异性参数和裂缝密度对纵波反射系数有较大影响,裂缝纵横比对反射系数影响较小。入射波方位角为90°时,入射波在HTI介质各向同性面,此时各向异性参数对反射系数影响最小。

P-wave AVAZ analysis of anisotropic thin coal bed

By build the propagator matrix of HTI thin coal bed,we deduced the exact P-wave reflectivity equation of anisotropic thin coal bed with horizontal layers.AVAZ Analysis was made to study the reflectivity response of the HTI thin coal bed.The variations of P-wave reflectivity with incidence and azimuth angle were made for HTI thin coal bed case (layer thickness less than a quarter of seismic wavelength) and thick coal seam case (layer thickness larger than seismic wavelength).The tendency was similar but the values were different.The reflection coefficients of P wave are depending not only on physical property of the upper and lower layers,but also on the layer thickness.This indicated that applying classical equations to perform inversion for thin coal bed case was not accurate.Our study showed that layer thickness (of the thin coal bed),effective Thomsen’s anisotropic parameters for HTI and crack density had great influence on the P-wave reflectivity,while the aspect ratio of cracks changed the reflectivity tiny.When the azimuth angle was 90 degrees,the reflectivity seemed no longer affected by anisotropic parameters.This was mainly because the incidence wave was parallel to the isotropic plane of the HTI media.