Combining different 3-D global and regional seismic wave propagation solvers towards box tomography in the deep Earth
Combining different 3-D global and regional seismic wave propagation solvers towards box tomography in the deep Earth
复制标题
结合不同的 3-D 全球和区域地震波传播解算器进行地球深处的盒式断层扫描
DOI：
10.1093/gji/ggac394
复制
发表时间：
2022
影响因子：
2.8
通讯作者：
Romanowicz, B.
中科院分区：
文献类型：
--
作者：
Adourian, S.;Lyu, C.;Masson, Y.;Munch, F.;Romanowicz, B.
在之前的出版物中，我们提出了一个通用框架，我们称之为“盒式断层扫描”，它允许耦合任意两个不同的数值地震波传播求解器，分别位于目标区域或“盒”的外部和内部。这种混合波场计算的目标是当源和/或接收器距离盒子较远时，降低目标区域内结构的全波形反演背景下的计算成本。此前，我们在 2-D 声学球形地球模型中利用目标区域外部的源和接收器演示了这种方法，并在大陆尺度反演中的 3-D 球形弹性地球中演示和应用了这种方法，其中所有站都位于目标区域内。在这里，我们将该方法的实施扩展到 3-D 全球弹性地球模型的情况，其中源和站都在盒子之外。我们将全局 3-D 解算器 SPECFEM3D_GLOBE 与区域 3-D 解算器 RegSEM 结合起来，用于通过时间反转镜计算参考 3-D 模型中的波场和格林函数，用于计算框内的波场。我们简要回顾了关键的理论方面，特别展示了如何仅需要将位移存储在盒子的边界处。我们提供了实际实现的细节，包括反射镜的几何设计、如何处理两个求解器中不同大小的网格，以及如何通过使用反射镜上记录的波场的 B 样条压缩来节省内存。所提出的方法不仅在数值上高效，而且用途广泛，因为将其应用于其他求解器非常简单，并且不需要对求解器代码本身进行任何更改，只要可以在时间和空间的任何点恢复位移即可。我们提出了混合计算的基准，对照以黄石地区为中心的现实几何形状中的源和站阵列之间的波场的直接计算，在“盒子”内有或没有假设的羽流，并且具有 1-D 或 3-D 背景模型，周期低至 20 秒。这项开发的最终目标是用于对深部地幔中的远程目标区域（例如超低速区）进行成像的应用。
In previous publications, we presented a general framework, which we called ‘box tomography’, that allows the coupling of any two different numerical seismic wave propagation solvers, respectively outside and inside a target region, or ‘box’. The goal of such hybrid wavefield computations is to reduce the cost of computations in the context of full-waveform inversion for structure within the target region, when sources and/or receivers are located at large distances from the box. Previously, we had demonstrated this approach with sources and receivers outside the target region in a 2-D acoustic spherical earth model, and demonstrated and applied this methodology in the 3-D spherical elastic Earth in a continental scale inversion in which all stations were inside the target region. Here we extend the implementation of the approach to the case of a 3-D global elastic earth model in the case where both sources and stations are outside the box. We couple a global 3-D solver, SPECFEM3D_GLOBE, for the computation of the wavefield and Green’s functions in a reference 3-D model, with a regional 3-D solver, RegSEM, for the computation of the wavefield within the box, by means of time-reversal mirrors. We briefly review key theoretical aspects, showing in particular how only the displacement is needed to be stored at the boundary of the box. We provide details of the practical implementation, including the geometrical design of the mirrors, how we deal with different sizes of meshes in the two solvers, and how we address memory-saving through the use of B-spline compression of the recorded wavefield on the mirror. The proposed approach is numerically efficient but also versatile, since adapting it to other solvers is straightforward and does not require any changes in the solver codes themselves, as long as the displacement can be recovered at any point in time and space. We present benchmarks of the hybrid computations against direct computations of the wavefield between a source and an array of stations in a realistic geometry centred in the Yellowstone region, with and without a hypothetical plume within the ‘box’, and with a 1-D or a 3-D background model, down to a period of 20 s. The ultimate goal of this development is for applications in the context of imaging of remote target regions in the deep mantle, such as, for example, Ultra Low Velocity Zones.