Geotechnical Site Characterization with 3-D Seismic Waveform Tomography

意想不到的场地条件，如高度变化的土壤和岩层，嵌入低速异常（软土或空洞），在基础施工期间和之后会造成重大问题。对异常的了解是至关重要的，因为异常可能导致结构损坏或倒塌，从而导致重大财产损失和偶尔的生命损失。该奖项支持基础研究，以提供量化嵌入异常和高分辨率表征可变土壤/岩石层所需的知识。这项研究的结果将加速经济和实用的创新无损检测（NDT）方法的实施，以保护和改善我们国家的基础设施。开发的分析可用于从毫米到数百米的各种尺度的材料成像和表征，用于几种工程应用，例如岩土工程地下场地条件的评估，桥梁基础冲刷，混凝土结构部件和沥青路面。本研究的目的是在地表地震波场中表征30米深度的三维地下结构在米尺度上的横波（s波）和压缩波（p波）速度剖面。地下结构由土壤、岩石和充满空气或水的空隙组成。为了实现这一目标，研究将包括(i)开发地震波场的三维全波形反演（3-D FWI）分析，以获得地下场地条件的高分辨率（米像素）特征，以及（ii）通过全尺寸现场实验验证3-D FWI分析。由于地震波的传播特性受到异常和地层界面的调制，完整的地震表面波场将用于量化嵌入异常并表征可变的土壤/岩层。利用地面均匀二维网格上的传感器和震源，通过地球物理测试获取地震波场，然后进行反演，提取三维次地表波速结构。研究将解决小尺度层析成像固有的问题，包括瑞利波分量占主导地位、波激发不一致、衰减强、先验信息差、近地表土壤/岩石高度可变等。利用测量波形数据中包含的全部信息，FWI分析有望提供比目前实践中使用的方法更详细、更准确的材料表征，而目前的方法仅使用部分测量数据。这将带来工程地球物理领域的新知识，并且是使用有效的地球物理方法进行现场调查以改进基础和其他岩土结构设计的关键一步。

Unanticipated site conditions such as highly variable soil and rock layers with embedded low-velocity anomalies (soft soils or voids) cause significant problems during and after construction of foundations. Knowledge of the anomalies is crucial, as the anomalies can cause structural damage or collapse that can result in significant property damage and occasional loss of life. This award supports fundamental research to provide the knowledge needed to quantify embedded anomalies and characterize variable soil/rock layers at high resolutions. Results from this research will accelerate economical and practical implementation of innovative non-destructive testing (NDT) methods for protecting and improving our nation's infrastructure. The developed analysis can be used for material imaging and characterization at various scales from millimeters to hundreds of meters for several engineering applications such as evaluation of geotechnical subsurface site conditions, bridge foundation scour, concrete structural components, and asphalt pavements. The goal of this research is to characterize both shear wave (S-wave) and compression wave (P-wave) velocity profiles of 3-D subsurface structures at meter scales down to 30 meter depth from surface-based seismic wave fields. The subsurface structures consist of soil, rock, and voids filled by air or water. To achieve the goal, the research will include (i) to develop a three-dimensional full waveform inversion (3-D FWI) analysis of seismic wave fields for high-resolution (meter pixel) characterization of subsurface site conditions, and (ii) to verify the 3-D FWI analysis by full scale field experiments. Full seismic surface wave fields will be used to quantify embedded anomalies and characterize variable soil/rock layers, as the propagation properties of seismic waves are modulated by the anomalies and layer interfaces. The seismic wave fields are acquired from geophysical testing using sensors and sources located in uniform 2-D grids on the ground surface, and then inverted for the extraction of 3-D subsurface wave velocity structures. The research will address inherent issues of small scale tomography including dominant Rayleigh wave components, inconsistent wave excitation, strong attenuation, poor a priori information, and highly variable of near surface soil/rock. Using full information contained in the measured waveform data, the FWI analysis is expected to provide more detailed and accurate characterization of materials than methods currently used in practice, which use only portions of the measured data. This will result in new knowledge in the area of engineering geophysics, and it is a critical step toward using an effective geophysical method for site investigations to improve design of foundations and other geotechnical structures.