联合非纵剖面的准三维宽角地震逆时偏移成像及其应用

The wide-angle reflection seismic data play a vital role in the fine reconstruction of crustal structure. In order to acquire a three-dimensional structure of the crust, simultaneously, saving cost, we design cross-line observation profiles adjointing with the inline profiles, so as to achieve a quasi-three-dimensional detection of the crustal structure. Based on the crustal geometry structure and velocity distribution derived from the wide-angle seismic data, a wave equation pre-stack depth migration could be developed to acquire the low-frequency crustal reflection image, and this can be a valuable constraint in studying the fine structure of the crust, material composition and geodynamics. In view of the strong undulated surface where the wide-angle seismic data are acquired, we intended to modify the original irregular free surface to a plane interface through the curvilinear coordinate system, and construct the elastic wave equation and free-surface boundary condition equation in this coordinate system, and develop a three-dimensional wave field extrapolation operator with the advantage of a stable finite-difference methods. Based on the solutions of the eikonal equation under the curvilinear coordinate system, the travel-time field for the reverse-time migration condition will be derived. We can finally acquire the wave equation reverse-time migration in depth domain for the wide-angle reflected structures beneath the undulated surface. Taking advantage of the theoretical research, we'll try to apply this method in the processing and interpretation of the wide-angle seismic data collected on the Xuefengshan area in the south of China. This project can promote the application of multi-attributes of the wide-angle seimic data, and is helpful to the research of the geodynamic feature and crustal-upper mantle fine structure.

地震宽角反射资料是重建地壳精细结构的重要依据。为了获取地壳三维结构并尽可能节省成本，在二维纵剖面探测的同时实施非纵剖面观测，从而实现地壳结构的准三维探测。本项研究拟在纵剖面二维地震资料研究地壳几何结构与速度分布基础上，发展适于联合非纵剖面的准三维地震波动方程叠前深度偏移方法构建地壳宽角反射结构。其中，为适应宽角反射资料采集于复杂地表的情况，我们拟把笛卡坐标系中的非规则自由表面映射为曲线坐标系中的规则平面，进而构造曲线坐标系下的弹性波动方程和自由边界条件方程，然后采用稳定的单边修饰差分方法进行三分量地震波场外推。通过求解曲线坐标系下的程函方程获得逆时偏移成像条件下的走时场，从而实现地表起伏情况下宽角反射结构的波动方程逆时偏移成像，并尝试应用于华南地区跨雪峰山构造带的准三维宽角反射资料处理。本项研究有望提升宽角资料的信息利用以及壳幔精细结构和地球动力学特征研究。

地震宽角反射/折射是重建地壳速度等物性结构的重要手段。为了充分利用地震宽角资料的动力学信息，本项研究发展了起伏地形下的逆时偏移成像条件及成像技术。. （1）激发时间、激发振幅与互相关是偏移成像领域最重要的三个成像条件，其实际应用各有优势。本项目发展了激发振幅成像条件，使其在弹性波逆时偏移中更稳定健壮。与归一化互相关成像条件相比，稳定激发振幅成像条件具有计算量小的特点，同时水平分量和垂直分量偏移剖面的分辨率均较高，低频假象弱，硬盘需求量为零，避免了I/O吞吐开销，提高了计算效率。以上优点在三维计算中尤其明显，节省了大量的硬盘空间。. （2）针对起伏地形对地震成像精度的影响，我们将模型参数化为贴体网格，把笛卡坐标系中的非规则自由表面映射为曲线坐标系中的规则平面，进而构造曲线坐标系下的弹性波动方程和自由边界条件方程，应用互相关的成像条件，基于有限差分算法实现了起伏地表条件下弹性波场的快速偏移成像。. （3）我们将以上技术和方法应用于华南大陆陆内造山的深部结构研究，重新评估了“连县-港口”400公里剖面的地壳速度结构和地震反射结构特征。剖面的地壳和岩石圈地震波速度、反射长度以及扩展因子等系统的变化差异，解释为可能是岩石圈变形或地幔挤出导致的岩浆入侵到深地壳中而产生的岩浆活动的地震学信号。这些结论提升了对华南大陆陆内造山深部过程的理解和认识。