三维壳幔精细结构的地震体波走时层析成像技术及其应用

Physical parameters of Earth interiors are key objectives of seismic deep exploration, which involve abundant information of the structure and geologic evolution of the Earth. It is difficult to inverse fine velocity structures of crust and mantle in large scale by waveform inversion due to restrictions of calculation intesity. It is a major approach to obtain velocity images by seismic tomography using the traveltimes of P- and S- waves. Fine velocity discontinuities are often neglected by traditional seismic tomographic methods, and replaced by average effect treatment. We plan to apply the parameterizations of complex fine structure in seismic exploration to the seismic tomography of deep crust and mantle. We plan to employ block models with triangulated interfaces to describe the major and secondary velocity discontinuities. Trilinear and cubic B-spline velocity interpolations are used to describe successive velocity distributions. Partial derivatives of traveltime to interface depth and velocities in grid points are considered, as well as the nonlinear conjugate gradient method are conducted for the final seismic tomography. We will apply the method to seismic tomography of P- and S- wave in eastern Kunlun orogeny belt and adjacent areas using a combination dataset of permanent network and portable seismographic stations, in order to reveal the collision process of Indian and Asian plates, and discuss the growth mechanism of the Tibetean plateau.

地球介质参数是地震深部探测的重要目标，蕴含了丰富的地球结构和演化的信息。基于P波、S波到时的体波层析成像是壳幔精细速度结构成像的重要途径。地震体波成像的传统方法多忽略壳幔精细速度间断面，而采用平均效应处理，很难反演壳幔结构细节。本研究拟将地震勘探领域复杂精细的三维地球模型描述方法应用到深部壳幔结构成像，采用块状结构模型并结合三角形网格界面，描述壳幔中的一级和二级速度间断面，采用三线性或三维三次B-样条速度插值描述分块连续速度场，考虑射线走时对三角界面节点深度和速度网格节点的偏导数，采用非线性共轭梯度法反演壳幔精细速度结构。并尝试利用固定台网和流动台站数据，进行东昆仑造山带域及邻区三维壳幔P波和S波速度结构成像，揭示印度-亚洲大陆碰撞的深部过程，并探讨高原的生长机制。

项目围绕地震波传播与成像方法研究以及中国典型构造域壳幔精细结构研究两大目标，开展了相关研究，获得如下主要成果:.（1）开展了基于地震射线理论的正演及走时反演研究，发展了复杂模型和各向异性介质射线追踪正演和走时反演方法，包括起伏地形下椭圆各向异性介质射线追踪、三维TTI介质高效射线追踪方法、三维复杂地壳结构非线性走时反演以及起伏地形下高精度初至波与反射波联合走时反演。.（2）开展了基于地震波场理论的波形反演及偏移成像研究，发展了波场模拟方法及时间域全波形反演和最小二乘逆时偏移成像方法，包括基于优化求积点的高阶三角网格谱元法波场模拟、基于归一化零延迟互相关函数的时间域全波形反演、共轭梯度法和步长公式对时间域全波形反演的影响、基于解析优化步长的互相关目标函数最小二乘逆时偏移成像以及起伏地形下弹性波逆时偏移成像。.（3）在中国典型构造域：青藏高原、峨眉山大火成岩省及长江中下游成矿带等地区开展了深部结构研究，发现了青藏高原西缘汇聚变形、东北缘生长扩展机制和中部壳幔形变的深部结构证据；发现了古地幔柱作用的地震学证据：峨眉山大火成岩省中部（内带）地壳增厚和下地壳高速异常等特征，推断为二叠纪地幔柱活动引起的岩浆底侵作用的结果；发现了长江中下游成矿带成矿作用的深部证据：宁芜矿集区下方三维Moho 面呈上隆、但下地壳和上地幔顶部呈现低速异常特征，与燕山期地壳增厚导致下地壳榴辉岩化并发生拆沉作用有关。