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在方位各向异性介质中,方位各向异性的主方向就是似P波和似S波以纯P波和纯S波方式传播的方向。如果方位各向异性是由作用于一个方位各向同性背景中的定向垂直裂缝引起的,则这些主方向中的其中两个对应平行和垂直于裂缝的方向。横波穿过一个方位各向异性介质时,对与主方向有关的传播方向很敏感。因此,可以用多分量的一次S波或转换S波资料采获得主方向除了采集费用高以外,地震工业界目前还没有完全开发出多分量资料处理和解释所需要的技术另一方面,利用常规P波资料检验各向异性还只限于一些不同方向的AVO定性研究。为了要定量研究方位AVO,我们研究了P波资料在固定炮检距处振幅随方位的变化。研究结果显示振幅随方位角的变化是以2δ为周期的,其中δ是激发方向与某个主方向的夹角。对于裂缝引起的各向异性,主方向对应平行或垂直于裂缝的方向。用P波反射振幅的随方位角呈周期性变化的关系来识别两种不同的各向异性检测情况。第一种情况是恰好正定的,我们已经用三个方位的测线对每个共中点(CMP)位置进行了观测,并推导出这种情况下计算主方向的方程。第二种情况是超定的,我们用多于三个方位的测线进行了观测。在这种超定情况下,主方向的取向能采用在各个方位方向反射振幅的最小平方拟合得到,或通过求解多个恰好完全确定的问题来得到。在上述两种情况下,除了取向角度外,还能获得方位各向异性程度的定性度量。如果方位各向异性是由定向垂直裂缝引起的,则各向异性的定性度量与裂缝密度成正比。使用合成地震记录,显示了用常规P波地震资料计算主方向方法的稳健性。我们还将该技术应用到一个宽炮检方位分布上采集的P波资料中。用该方法计算出的主方向取向与该地区用其它地质和地球物理证据推断的总的裂缝方向一致。
In the azimuthally anisotropic medium, the main direction of azimuthal anisotropy is the direction in which P wave and S wave resemble pure P wave and pure S wave. If azimuthal anisotropy is caused by directional vertical cracks acting in an azimuthally isotropic background, two of these main directions correspond to directions parallel and perpendicular to the fracture. When a transverse wave passes through an azimuthally anisotropic medium, it is very sensitive to the propagation direction associated with the main direction. Therefore, multi-component primary S-wave or S-wave data can be acquired in the main direction. In addition to the high acquisition cost, the seismic industry has not yet fully developed the technology required for multi-component data processing and interpretation. On the other hand, P wave data to test anisotropy is only limited to a number of different directions of AVO qualitative research. In order to quantitatively study the azimuth AVO, we studied the variation of amplitude of P-wave data with azimuth at a fixed offset. The results show that the variation of amplitude with azimuth angle is based on 2δ period, where δ is the angle between the excitation direction and a main direction. For cracks caused by anisotropy, the main direction corresponds to the direction parallel or perpendicular to the crack. Two different anisotropy detection scenarios were identified by the periodically varying azimuth of the P wave reflection amplitude. The first case is exactly positive. We have observed each co-ordinate (CMP) position with three azimuth lines and derived the equation for calculating the main direction in this case. The second case is overdetermined, and we have observed with more than three directions of the line. In this overdetermined case, the orientation of the main direction can be obtained by fitting the least square of the amplitude of the reflection in each azimuth, or by solving a number of exactly determined problems. In both cases, in addition to the orientation angle, a qualitative measure of the degree of azimuthal anisotropy can also be obtained. If the azimuthal anisotropy is caused by directional vertical cracks, the qualitative measure of anisotropy is proportional to the fracture density. Synthetic seismograms are used to show the robustness of the principal direction method using conventional P-wave seismic data. We also apply this technique to P-wave data acquired over a wide range of shotguns. The principal direction calculated using this method is consistent with the general fracture direction inferred from other geologic and geophysical evidence in the area.