Real-Time Observation of Faulting Process in Rock by Seismic Computer Tomography

应用地震计算机层析成像(CT)技术，观察在单轴压缩载荷作用下裂纹扩展(微裂纹)在疲劳前的定位情况。利用这种技术，可以通过分析沿多条地震射线路径扫描的走时，来绘制岩样某一切片平面内的P波速度分布。由于地震波的传播速度受传播路径上微裂纹的密度、纵横比和体积的影响，因此这些图可能有助于了解断裂的过程。我们对大岛花岗岩样品进行了地震CT的初步应用，将其单向压缩到峰后区域，然后在不破碎的情况下恢复。地震CT被认为是观察岩石微裂纹定位的一种有利技术。该地震CT在补偿地震速度各向异性和测量系统精度方面进行了改进，最后应用地震CT观测了岩石在断层前的局部扩容情况。地震CT可以绘制P波速度的空间分布图。由于P波速度由扩容程度决定，因此绘制地震剖面图有助于非破坏性地观察扩容的渐进发展。大岛花岗岩的试件在单轴应力作用下以恒定的变形速率压缩。在不同应力水平下，在垂直于加载轴的平面内进行重建。所获得的图清楚地显示了断裂前膨胀性的变质变化。另一方面，我们应用地震CT来可视化干花岗岩中的水分渗透并表征其存在。因此，可以通过重建地震剖面来观察水在相互连通的微裂隙中的传播行为。

The seismic computer tomography (CT) was applied to observe how dialatancy (microcrack) localizes before fauting under uniaxial compression. By this technique, the distribution of P-wave velocity within a some sliced plane of the rock specimen can be mapped by analyzing the scanned travel times along a number of seismic ray paths. Because the velocities of seismic waves are influenced by the density, aspect ratio, and volume of microcracks along the transmitted path, the maps may be useful to see the process of faulting.We applied seismic CT preliminarily to the specimen of Oshima granite, once uniaxially compressed to post-peak region and then recovered without crushing. The seismic CT was found to be a favorable technique to observe the localization of microcracks in rock. This seismic CT was improved with regard to the compensation for anisotropy of seismic velocity and the accuracy of measuring systems.Finally, the seismic CT was applied to observe how dilatancy of rock localizes before faulting. The seismic CT can map the spatial distribution of P-wave velocities. Since P-wave velocity is determined by the extent of dilatancy, mapping the seismic profile is useful for observing non-destructively the progressive development of dilatancy. The specimen of Oshima granite was compressed at a constant deformation rate under uniaxial stress. The reconstructions were made within the plane perpendicular to the loading axis at several stress levels. The obtained maps clearly show the metamorphic changes of dilatancy before faulting.On the other hand, we applied the seismic CT to visualize the water infiltration into dry granite and to characterize its presence. For the result, the behavior of water spreading through the interconnecting mecrocracks could be observed by reconstructing the seismic profiles.