Prediction of Subsurface Crack Propagation by the Acoustic Emission Moment Tensor Analysis

研究了矩张量分析在声发射中的应用，以发展一种裂纹扩展预测技术。利用基于P波振幅的矩张量反演法确定了6个独立的张量分量。开发了一种用于声发射反演分析的计算机程序SIGMA(SIMPLIZED Green‘s Functions for the Moment Tensor Analyses)。为了对裂纹类型进行分类，并根据弯矩张量分量确定裂纹方向，提出了特征值的统一分解。分解的目的是确定剪切和拉伸对声发射源的贡献比例，并将裂纹分类为主导运动的裂纹类型。将Sigma反演和统一分解应用于现场水压致裂试验中检测到的声发射波形。对水压致裂试验中部署的8个加速度计探测到的声发射波形进行了分析。所确定的裂纹类型和方向与钻孔电视观测预测的破坏面相吻合。结果表明，在弱接缝处首先产生拉伸裂纹，然后在张开的接缝处产生剪切裂纹。将Sigma方法改进为求解二维问题，并应用于带内缝的二维模型的实验。研究了单轴压缩下缝隙中复合型裂纹的扩展，并用四通道声发射系统记录了声发射波形。基于力矩张量分析，根据声发射波形确定裂纹位置、裂纹类型和裂纹方向。将计算结果与模型中观察到的裂纹轨迹进行了比较。结果表明，Sigma分析方法适用于地下材料的失效过程预测。

An application of a moment tensor analysis to acoustic emission (AE) is studied to develop a prediction technique of crack propagation. A moment tensor inversion based on P wave amplitude is employed to determine six independent tensor components. A computer code named SiGMA (simplified Green's functions for the moment tensor analysis) is developed for the AE inversion analysis. To classify crack type and to determine crack orientation from moment tensor components, a unified decomposition of eigenvalues is proposed. The aim of the decomposition is to determine the proportion of shear and tensile contribution on AE sources, and to classify cracks into a crack type of the dominant motion. The SiGMA inversion and the unified decomposition are applied to AE waveforms detected during an in-situ hydrofracturing test. AE waveforms detected by eight accelerometers deployed during the hydrofracturing test are analyzed. Crack types and orientations determined are in reasonable agreement with a predicted failure plane from bore-hole TV observation. The results suggest that tensile cracks are generated first at weak seams, and then shear cracks follow on the opened joints.The SiGMA procedure is modified to solve the 2-D problem and is applied to experiments of a 2-D model with an internal slit. Mixed-mode crack propagation from the slit under uniaxial-compression is investigated and AE waveforms are recorded by a four-channel AE system. Crack locations, crack types, and crack orientations are determined from AE waveforms, based on the moment tensor analysis. The results are compared with crack trajectory observed in the model. The results confirm that the SiGMA analysis is applicable to predicting failure process of subsurface materials.