Characterization of Small-scale Heterogeneity in the Deep Earth

人们对地球深处的三维结构知之甚少，这些结构的大小不到100公里。这些小尺度的深部结构对于理解地球成分及其表面构造的历史和未来演化具有重要意义。它们影响地震波的形状和幅度，用于预测地震地面运动的大小，并预测地震和地下核试验的地震特征之间的差异。传统的地震层析成像不能对这种小尺度结构成像，它用类似于医学CAT扫描的方法处理地震波的走时。相反，该项目使用了一种技术，使用许多紧密分布的地震仪的地震记录来恢复小规模结构的尺度长度的特征谱。这种光谱限制了非均质的大小、形状和组成。该项目还培训研究生地震数据检索、处理、成像和计算机编程。这些技能在资源勘探和修复受污染的地下水方面很重要。结果将被纳入物理科学的本科课程，以加强地球物理研究在学术和工业应用方面的招聘和培训。小规模非均质性(100公里)的特征对于理解地幔化学的地表采样以及受对流和板块构造控制的化学混合的时间历史和性质非常重要。小尺度非均质对地震波的方向性散射会影响粘弹性衰减和各向异性的估计，这反过来又对温度、矿物成分和矿物相的估计很重要。由密集的地震台台阵记录的体波的振幅和相位相干性将被反演为小尺度非均质性的图像统计实现。这种随机层析成像过程将确定常规层析成像无法分辨的地球小尺度化学和热非均质的特征长度尺度和地震速度扰动。结果将检验从地球动力学模型预测的对流混合的基本假设。

Little is known about three-dimensional structures in the deep earth whose size is less than 100 km. These small-scale deep structures are important to understanding the historical and future evolution of earth's composition and its surface tectonics. They affect the shapes and amplitudes of seismic waves used to forecast the size of earthquake ground motions and to predict the differences between the seismic signatures of earthquakes and underground nuclear tests. Such small-scale structures cannot be imaged by conventiional seismic tomography, which processes travel times of seismic waves with methods similar to medical CAT scanning. Instead, this project employs a technique using seiismic recordings by many, closely spaced, seismometers to retrieve a characterisitic spectrum of scale lengths of small-scale structures. This specturm constrains the sizes, shapes, and composition of the heterogeneities. The project also trains graduate students in seismic data retrieval, processing, imaging, and computer programming. These are skills important in resource exploration and remediation of contaminated groundwater. Results will be incorporated in undergraduate courses in the physical sciences to enhance recruitment and training in both academic and industrial applications of geophysical research.The characterization of small-scale heterogeneity (100 km) is important to understand the surface sampling of Earth's mantle chemistry and the time history and nature of chemical mixing controlled by convection and plate tectonics. Directionally dependent scattering of seismic waves by small-scale heterogeneity can affect estimates of viscoelastic attenuation and anisotropy, which in turn are important for estimating temperature, mineral composition, and mineral phase. Amplitude and phase coherences of body waves recorded by dense arrays of seismic stations will be inverted to image statistical realizations of small-scale heterogeneity. This process of stochastic tomography will determine the characteristic lenth scales and seismic velocity perturbatons of the small-scale chemical and thermal heterogeneity of the earth that cannot be resolved by conventional tomography. The resullts will and test fundamental assumptions of convective miixing predicted from geodynamic models.