Implementation of "Box Tomography" for high resolution imaging of Target Regions in the Earth's Deep Mantle

实施“盒式断层扫描”，对地球深部地幔目标区域进行高分辨率成像

• 批准号: 1758198
• 负责人: Barbara Romanowicz
• 金额: $ 49.9万
• 依托单位: University of California-Berkeley
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2018
• 资助国家: 美国
• 起止时间: 2018-06-01 至 2023-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1758198&HistoricalAwards=false
• 关键词: Implementation; Box; Tomography; resolution; imaging

The existence of "hotspot" volcanoes located in the middle of tectonic plates, such as Hawaii, do not fit within simple plate tectonic theory, which only explains the presence of volcanism on the borders of such plates. It has long been proposed that hotspots are the surface expression of mantle plumes, conduits of hot rock rising from the deep mantle. Such conduits - albeit broader than expected - have recently been imaged by seismic tomography, and traced down to the core-mantle boundary under major hotspots under Hawaii and Iceland, for example. However, these plume images are still blurry, making it difficult to fully understand their role in the dynamics of the mantle, while the depth at which others originate, such as under Yellowstone, are still debated. A new methodology called "full waveform tomography", which aims at utilizing all the information contained in seismic records, is increasingly gaining interest in the global seismology and exploration geophysics community, as it holds the promise of obtaining sharper images of structures in particular in the deep mantle. However, such an approach is computationally very expensive, especially at the global scale, as the seismic wave propagation needs to be computed over very large distances and has to be performed many times during successive incremental updates of the image. A promising solution is "box tomography" whereby a target region of study is defined within the Earth's deep mantle and the complete wavefield from earthquake sources near the Earth's surface to distant recording stations is computed only once at the beginning, while successive updates of the model within the target region require wavefield computations only within the limits of the target region. We will implement this methodology and apply it to two targets of geophysical significance: (1) the Yellowstone plume and (2) the root of the Iceland plume near the core-mantle boundary, which has been shown to contain a patch of extreme material properties, which may indicate the presence of partially molten rock. Broader impacts of this project include training of two graduate students in addition to providing a better understanding of mantle flow in Earth's deep mantle.The existence of mantle plumes has long been debated. Broad conduits of pronounced low shear velocity have recently been imaged tomographically, extending from the core-mantle boundary through most of the lower mantle, in the vicinity of major hotspots such as Iceland, Hawaii, or the south-Pacific superswell volcanoes. Better characterizing the details of these plumes will help improve our understanding of the organization of upwelling flow in the deep earth, and the contrasted rheology between the lower 2000 km of the mantle and the extended upper mantle transition zone. Also, there may be different types of plumes, as may be the case for Yellowstone, where a broad lower mantle plume is not imaged. To address this, full waveform tomography based on numerical computations of the seismic wavefield is a promising tool, as it has the potential of improving tomographic resolution by exploiting the information contained in entire seismograms. However, to attain the higher frequencies necessary for pushing the limits of image resolution, a formidable challenge is the increasingly heavy computations involved. Also, not all regions of the earth are adequately illuminated by the present distribution of earthquakes and receivers to warrant much higher resolution than is achieved today. A promising solution is "box tomography", in which a target volume of limited extent is embedded in a global 3D Earth model, and the imaging is restricted to that smaller volume, leaving the model fixed outside of it. In many situations, both sources and receivers are located outside of this volume, presenting challenges for connecting wavefields to reconstruct complete seismograms. We have developed a "box tomography" methodology that allows arbitrary relative locations of sources, receivers and target volume, and can be applied with any wave equation integrator. A first application to continental scale upper mantle tomography with stations inside the target volume has been developed in north America. Here we plan to finish implementing the computation of the Green's functions necessary to extrapolate the wavefield from the target region to the external stations and to apply it to two targets of geophysical significance: (1) confirming the presence of a mantle plume in the mid-lower mantle beneath the Yellowstone hotspot, and improving its resolution; (2) improving the resolution of the structure at the base of the earth's mantle, within the root of the Iceland Plume, where a large axisymmetric ultra-low-velocity zone was recently documented, and comparing it to the structure at the base of what might be the lower mantle expression of the Yellowstone plume. This project will train 2 students on state-of-the art seismological techniques.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

位于构成板块中间的“热点”火山的存在，例如夏威夷，不适合简单的板块构造理论，这仅解释了在此类板块边界上存在火山的存在。长期以来，一直有人提出，热点是地幔羽的表面表达，即从深幔上升的热岩石的导管。 例如，这种导管（尽管比预期的要广泛）最近是通过地震层析成像成像的，并将其追溯到夏威夷和冰岛下的主要热点下的核心掩体边界。但是，这些羽状图像仍然模糊，因此很难完全理解它们在地幔动力学中的作用，而其他人（例如黄石之下）的深度仍在辩论。一种称为“完全波形层析成像”的新方法，旨在利用地震记录中包含的所有信息，它越来越对全球地震学和勘探地球物理学群落产生兴趣，因为它具有在深层大衣中特别获得尤其是结构的更清晰的结构图像。但是，这种方法在计算上非常昂贵，尤其是在全球尺度上，因为需要在非常大的距离上计算地震波传播，并且必须在图像的连续增量更新中进行多次执行。一个有希望的解决方案是“盒子层析成像”，从而在地球的深幔中定义了目标区域，并且在开始时仅计算一次地球表面到遥远记录站的地震来源的完整波场，而目标区域内仅在目标区域内的连续更新需要在目标区域范围内进行波场计算。我们将实施这种方法，并将其应用于地球物理意义的两个靶标：（1）黄石羽流和（2）冰岛羽流的根部附近核心壳边界附近的根部，已显示出包含一堆极端材料特性，这可能表明存在部分熔融岩石。 该项目的更广泛的影响包括对两名研究生的培训，除了对地球深层地幔中的地幔流提供更好的了解。最近在层压仪上成像了明显的低剪切速度的广泛导管，从核心掩体边界通过大多数下地幔延伸，在主要热点（例如冰岛，夏威夷或南部太平洋的Superswell火山）附近。更好地表征这些羽流的细节将有助于我们对深层地球上上流流的组织的理解，以及地幔下2000公里和扩展的上层地幔过渡区之间的对比。同样，可能存在不同类型的羽流，例如黄石可能的情况下，没有成像宽阔的较低地幔羽。为了解决这个问题，基于地震波场的数值计算是一个有前途的工具，因为它有可能通过利用整个地震图中包含的信息来改善层析成像分辨率。但是，为了达到推动图像分辨率限制所需的较高频率，涉及越来越重的计算。同样，并非地球上的所有区域都被当前地震和接收者的分布充分照亮，以便比今天所获得的要高得多的分辨率。一个有前途的解决方案是“盒子层析成像”，其中将有限范围的目标体积嵌入到整体的3D地球模型中，并且成像仅限于较小的体积，使该模型固定在其外部。在许多情况下，来源和接收器都位于此卷之外，这给连接波场重建完整的地震图带来了挑战。我们已经开发了一种“盒子层析成像”方法，该方法允许任意的相对位置的源，接收器和目标量的相对位置，并且可以与任何波方程集成器一起应用。北美已经开发了针对目标体积内部的大陆尺度上地幔层析成像的第一个应用。在这里，我们计划完成对绿色功能的计算，以将波场从目标区域推断到外部站点，并将其应用于两个地球物理意义的目标：（1）确认在黄石热点以下的中较低的地幔中存在地幔羽流，并改善其分辨率； （2）最近记录了冰岛羽流的根部，在冰岛羽流的根部改善结构的分辨率，最近记录了一个大轴对称超低速度区域，并将其与黄石羽流的下层表达的基础上的结构进行了比较。 该项目将培训2名学生的最先进的地震学技术。该奖项反映了NSF的法定任务，并被认为是值得通过基金会的知识分子优点和更广泛的影响审查标准来评估值得支持的。

项目成果

Global reference seismological data sets: multimode surface wave dispersion

全球参考地震数据集：多模面波频散

• DOI: 10.1093/gji/ggab418
• 发表时间: 2021
• 期刊: Geophysical Journal International
• 影响因子: 2.8
• 作者: Moulik, P;Lekic, V;Romanowicz, B;Schaeffer, A;Beucler, E;Debayle, E;Deuss, A;Durand, S
• 通讯作者: Durand, S

Combining different 3-D global and regional seismic wave propagation solvers towards box tomography in the deep Earth

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
• 发表时间: 2022
• 期刊: Geophysical Journal International
• 影响因子: 2.8
• 作者: Adourian, S.;Lyu, C.;Masson, Y.;Munch, F.;Romanowicz, B.
• 通讯作者: Romanowicz, B.

Dense mantle flows periodically spaced below ocean basins

洋盆下方周期性分布的致密地幔流

• DOI: 10.1016/j.epsl.2022.117745
• 发表时间: 2022
• 期刊: Earth and Planetary Science Letters
• 影响因子: 5.3
• 作者: Panet, Isabelle;Greff-Lefftz, Marianne;Romanowicz, Barbara
• 通讯作者: Romanowicz, Barbara

Accelerating full waveform inversion via source stacking and cross-correlations

• DOI: 10.1093/gji/ggz437
• 发表时间: 2019-10
• 期刊: Geophysical Journal International
• 影响因子: 2.8
• 作者: B. Romanowicz;Li-Wei Chen;S. French