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

位于构造板块中间的“热点”火山的存在，如夏威夷，不符合简单的板块构造理论，这只能解释火山活动在这些板块的边界上的存在。长期以来，人们一直认为热点是地幔柱的表面表现，是热岩从地幔深处上升的通道。 这种管道--尽管比预期的要宽--最近已经通过地震层析成像技术成像，并追踪到了夏威夷和冰岛等主要热点下的核幔边界。然而，这些羽流图像仍然模糊，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，如黄石公园，仍然存在争议。一种称为“全波形层析成像”的新方法旨在利用地震记录中所含的所有信息，它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，特别是在地幔深处。然而，这种方法在计算上非常昂贵，特别是在全球范围内，因为地震波传播需要在非常大的距离上计算，并且必须在图像的连续增量更新期间执行多次。一个有前途的解决方案是“盒层析成像”，即研究的目标区域被定义在地球的深部地幔和完整的波场从地震源附近的地球表面到遥远的记录站计算只有一次在开始时，而连续更新的目标区域内的模型只需要波场计算的限制内的目标区域。我们将实施这种方法，并将其应用于两个目标的地球物理意义：（1）黄石羽和（2）冰岛羽的根附近的核幔边界，已被证明包含一个补丁的极端材料属性，这可能表明部分熔融岩石的存在。 该项目的更广泛影响包括培训了两名研究生，此外还使人们更好地了解了地球深部地幔的地幔流。最近，在冰岛、夏威夷或南太平洋超井火山等主要热点附近，用断层摄影术对具有明显低剪切速度的宽管道进行了成像，这些管道从核幔边界延伸到大部分下地幔。更好地描述这些羽流的细节将有助于提高我们对地球深部上涌流动组织的理解，以及地幔下部2000 km和延伸的上地幔过渡带之间的对比流变学。此外，可能有不同类型的羽流，就像黄石公园的情况一样，那里没有成像广泛的下地幔羽流。为了解决这一问题，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，因为它具有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的潜力。然而，为了获得推动图像分辨率极限所需的更高频率，一个艰巨的挑战是所涉及的越来越繁重的计算。此外，并不是地球上的所有地区都能被目前的地震分布和接收器充分照亮，以保证比今天更高的分辨率。一个很有前途的解决方案是“箱式层析成像”，其中有限范围的目标体积嵌入全球3D地球模型中，成像仅限于较小的体积，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，这对连接波场以重建完整的地震图提出了挑战。我们已经开发出一种“盒层析成像”的方法，允许任意相对位置的源，接收器和目标体积，并可以应用于任何波动方程积分。在北美首次开发了利用目标体积内的台站进行大陆尺度上地幔层析成像的应用。我们计划完成绿色函数的计算，将波场从目标区外推到外部台站，并将其应用于两个具有地球物理意义的目标：（1）确认黄石热点下中下地幔中地幔柱的存在，并提高其分辨率;（2）提高地幔底部结构的分辨率，在冰岛地幔柱的根部，最近记录了一个大的轴对称超低速区，并将其与可能是黄石火山柱下地幔表现的底部结构进行比较。 该项目将培训2名学生掌握最先进的地震学技术。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.