Time-resolved tomography using acoustic emissions in the laboratory, and application to sandstone compaction
Time-resolved tomography using acoustic emissions in the laboratory, and application to sandstone compaction
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DOI：
10.1093/gji/ggy068
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发表时间：
2018-06-01
影响因子：
2.8
通讯作者：
Brantut, Nicolas
中科院分区：
文献类型：
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作者：
Brantut, Nicolas
声发射(AE)和主动超声波速度监测通常是在实验室岩石变形实验期间进行的，但通常是分开处理的，以产生均匀的波速测量和近似的震源位置。在这里，我介绍了一种数值方法及其在自由软件中的实现，以实现对实验室样品的声发射位置和三维各向异性P波结构的联合反演。所使用的数据是从声发射和主动超声波测量中获得的P波初至。模型参数是材料中离散点的震源位置、纵波速度和各向异性参数(假设横向各向同性)。正问题用快速推进法求解，反问题用拟牛顿法求解。这些算法是在一个名为FaATSO(使用标准优化的快速行进声发射断层扫描)的集成自由软件包中实现的。该程序被用于研究多孔砂岩中压实带的形成。在变形过程中，AEs的前锋从样品的一端前进，与一系列水平压实带的形成有关。在活动前锋后面，只观察到稀疏的声速，但层析成像显示，P波速度下降了15%，各向异性增加了20%。因此，砂岩中的压实带会导致地震性质的急剧变化。这一结果突出了该方法在成像复杂岩土材料中弹性特性的时间变化方面的潜力，包括与微破裂和损伤产生相关的戏剧性、局部性变化。
Acoustic emission (AE) and active ultrasonic wave velocity monitoring are often performed during laboratory rock deformation experiments, but are typically processed separately to yield homogenized wave velocity measurements and approximate source locations. Here, I present a numerical method and its implementation in a free software to perform a joint inversion of AE locations together with the 3-D, anisotropic P-wave structure of laboratory samples. The data used are the P-wave first arrivals obtained from AEs and active ultrasonic measurements. The model parameters are the source locations and the P-wave velocity and anisotropy parameter (assuming transverse isotropy) at discrete points in the material. The forward problem is solved using the fast marching method, and the inverse problem is solved by the quasi-Newton method. The algorithms are implemented within an integrated free software package called FaATSO (Fast Marching Acoustic Emission Tomography using Standard Optimisation). The code is employed to study the formation of compaction bands in a porous sandstone. During deformation, a front of AEs progresses from one end of the sample, associated with the formation of a sequence of horizontal compaction bands. Behind the active front, only sparse AEs are observed, but the tomography reveals that the P-wave velocity has dropped by up to 15 per cent, with an increase in anisotropy of up to 20 per cent. Compaction bands in sandstones are therefore shown to produce sharp changes in seismic properties. This result highlights the potential of the methodology to image temporal variations of elastic properties in complex geomaterials, including the dramatic, localized changes associated with microcracking and damage generation.