发展下地幔三维高频体波波形模拟方法及在太平洋异常体精细结构研究中的应用

The Ｄ″ region above the CMB boundary is considered to be the main place for the exchange of material and energy between the liquid outer core and the solid mantle. Studying the material state and structure of the D'' region in the lowermost mantle is the key precondition for understanding the evolution of the Earth's heat release and the convection of the mantle. Waveform modelling is one of the most effective methods for studying the structure of anomalous bodies in the lower mantle, constraining the geographical location, geometry and boundaries of anomalous bodies. At present, the calculation of the three-dimensional high-frequency body waveform of the lower mantle (≥~0.5 Hz) is challenging. This study is intended to develop a method to model the heterogeneous media of the lower mantle with both high accuracy and efficiency, and to improve the resolution of the mantle structure under dense array seismic data. The new method will use the spectral element method to accurately simulate the seismic wave propagation under heterogeneous media and irregular boundary conditions, and through the use of homogenization techniques, the model will be smoothly optimized without greatly reducing the simulation resolution，which improve the efficiency of the algorithm significantly. Finally, we use the Pacific anomalous body as a case to collect seismic observation data and apply new methods for waveform modelling to obtain the best interpretation model for the three-dimensional fine structure of the Pacific anomalous body, providing new seismic evidence for the study of the dynamic processes, mechanisms, and evolution of the lowermost mantle.

研究下地幔Ｄ″区的物质状态和结构是认识地核的热释放、地幔对流等地球动力学过程的核心问题。高精度三维全波形模拟是目前解析下地幔异常体结构，约束异常体地理位置、几何形态、边界最有效的关键技术之一。目前下地幔高频体波（≥~0.5Hz）的三维波形模拟仍存在计算瓶颈。本研究拟发展一套兼具高精度和高效率的下地幔非均匀性波形模拟方法，提高应用高密度地震台站数据探测下地幔异常体的分辨率。该方法可以兼顾谱元法模拟非均匀介质模型和不规则边界条件下地震波传播的优势，并采用均一化技术实现计算模型的平滑优化，在保证模拟精度的同时兼顾模型分辨能力和计算效率。最后以太平洋下地幔异常体的波形模拟为例，基于地震观测数据，应用该方法进行全波形拟合，获得下地幔异常体三维精细结构的最佳解释模型，为研究下地幔动力学过程、机制和演化提供高辨率的地震学证据。

研究下地幔Ｄ″区的物质状态和结构是认识地核的热释放、地幔对流等地球动力学过程的核心问题。高精度三维全波形模拟是目前解析下地幔异常体结构，约束异常体地理位置、几何形态、边界最有效的关键技术之一。目前下地幔高频体波（≥~0.5Hz）的三维波形模拟仍由于计算代价过于昂贵而很难有效开展，极大地阻碍了波形数值模拟在地球深度精细速度结构研究方面的应用。本研究发展了一套兼具高精度和高效率的下地幔非均匀性波形模拟方法——三维均一化波形模拟技术（英文简称HOMO3D），提高应用高密度地震台站数据探测下地幔异常体的分辨率。该方法通过求解与地震波动方程等效的均一化波动方程，在保证高频体波模拟精度和实际分辨能力的前提大大提高计算效率。通过设置典型三维理论异常体模型进行正演模拟，并与经典谱元法对比验证表明，新方法能够降低计算资源消耗2~8倍，这为推动高精度地震波数值模拟技术在地球深部精细速度结构中的应用，进而为研究下地幔动力学过程、机制和演化提供高辨率的地震学证据打下了坚实的基础。