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.

位于构造板块中部（例如夏威夷）的“热点”火山的存在并不符合简单的板块构造理论，该理论只能解释此类板块边界上火山活动的存在。长期以来，人们一直认为热点是地幔柱的表面表现，是从地幔深处升起的热岩的通道。 这种管道——尽管比预期更宽——最近已通过地震层析成像技术成像，并追溯到夏威夷和冰岛主要热点地区的核幔边界。然而，这些羽流图像仍然模糊，因此很难完全理解它们在地幔动力学中的作用，而其他羽流的起源深度（例如黄石地下）仍然存在争议。一种名为“全波形断层扫描”的新方法旨在利用地震记录中包含的所有信息，越来越受到全球地震学和勘探地球物理学界的关注，因为它有望获得更清晰的结构图像，特别是在地震中。深地幔。然而，这种方法在计算上非常昂贵，尤其是在全球范围内，因为地震波传播需要在非常大的距离上进行计算，并且必须在图像的连续增量更新期间执行多次。一种有前途的解决方案是“盒式断层扫描”，其中研究的目标区域被定义在地球深部地幔内，并且从地球表面附近的地震源到遥远的记录站的完整波场在开始时仅计算一次，同时模型的连续更新目标区域内的波场计算仅在目标区域的限制内进行。我们将实施这种方法并将其应用于两个具有地球物理意义的目标：（1）黄石地幔柱和（2）靠近核幔边界的冰岛地幔柱根部，该根部已被证明包含一块极端物质特性，这可能表明存在部分熔融的岩石。 该项目的更广泛影响包括培训两名研究生以及更好地了解地球深层地幔流。地幔柱的存在长期以来一直存在争议。最近，人们通过断层扫描对明显低剪切速度的宽阔管道进行了成像，这些管道从核心-地幔边界延伸到大部分下地幔，位于冰岛、夏威夷或南太平洋超涌火山等主要热点地区附近。更好地描述这些地幔柱的细节将有助于提高我们对地球深处上升流组织的理解，以及地幔下部 2000 公里和延伸的上地幔过渡带之间对比流变学的理解。此外，可能存在不同类型的羽流，就像黄石公园的情况一样，那里没有拍摄到宽阔的下地幔羽流。为了解决这个问题，基于地震波场数值计算的全波形层析成像是一种很有前途的工具，因为它有可能通过利用整个地震图中包含的信息来提高层析成像分辨率。然而，为了达到突破图像分辨率极限所需的更高频率，一个巨大的挑战是所涉及的计算量越来越大。此外，并非地球的所有区域都被当前的地震和接收器分布充分照亮，以保证比今天更高的分辨率。一种有前途的解决方案是“盒式断层扫描”，其中将有限范围的目标体积嵌入全球 3D 地球模型中，并且成像仅限于该较小的体积，而使模型固定在其外部。在许多情况下，震源和接收器都位于该体积之外，这给连接波场以重建完整的地震图带来了挑战。我们开发了一种“盒式断层扫描”方法，允许源、接收器和目标体积的任意相对位置，并且可以应用于任何波动方程积分器。北美已经开发出了大陆尺度上地幔断层扫描的首次应用，其站位位于目标体积内。在这里，我们计划完成格林函数的计算，以推断从目标区域到外部站的波场，并将其应用于两个具有地球物理意义的目标：（1）确认中部地幔柱的存在黄石热点下方的下地幔，并提高其分辨率； （2）提高地幔底部结构的分辨率，冰岛羽流根部最近记录了一个大的轴对称超低速带，并将其与冰岛羽流底部的结构进行比较可能是黄石地幔柱的下地幔表现。 该项目将对 2 名学生进行最先进的地震学技术培训。该奖项反映了 NSF 的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

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: 2021-12
• 期刊: Geophysical Journal International
• 影响因子: 2.8
• 作者: Moulik, P;Lekic, V;Romanowicz, B;Ma, Z;Schaeffer, A;Ho, T;Beucler, E;Debayle, E;Deuss, A;Durand, S;et al
• 通讯作者: et al

Imaging deep-mantle plumbing beneath La Réunion and Comores hot spots: Vertical plume conduits and horizontal ponding zones

对留尼汪岛和科摩罗热点下方的深部地幔管道进行成像：垂直羽流管道和水平积水区

• DOI: 10.1126/sciadv.ade3723
• 发表时间: 2023-01-25
• 期刊: Science Advances
• 影响因子: 13.6
• 作者: Wamba, Mathurin Dongmo;Montagner, Jean-Paul;Romanowicz, Barbara
• 通讯作者: Romanowicz, Barbara

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: 2022-09
• 期刊: Earth and Planetary Science Letters
• 影响因子: 5.3
• 作者: Panet, Isabelle;Greff;Romanowicz, Barbara
• 通讯作者: Romanowicz, Barbara

Deflating the LLSVPs: Bundles of Mantle Thermochemical Plumes Rather Than Thick Stagnant “Piles”

缩小 LLSVP：地幔热化学羽束，而不是厚厚的静止物——堆积物——

• DOI: 10.1029/2020tc006265
• 发表时间: 2020-10
• 期刊: Tectonics
• 影响因子: 4.2
• 作者: Davaille, Anne;Romanowicz, Barbara
• 通讯作者: Romanowicz, Barbara

Accelerating full waveform inversion via source stacking and cross-correlations

通过源叠加和互相关加速全波形反演

• DOI: 10.1093/gji/ggz437
• 发表时间: 2019-10-21
• 期刊: Geophysical Journal International
• 影响因子: 2.8
• 作者: B. Romanowicz;Li;S. French
• 通讯作者: S. French