Karst cavities beneath bored cast in situ piles are hazardous to the stability of infrastructure projects. Therefore, it is important to detect karst cavities during the construction of piles. Downward-looking sonar deployed at the bottom of a pile hole can be used to detect cavities; however, interference of multiple reflected surface waves from the walls of the pile hole masks the weak cavity reflections. We have developed a sonar method that exploits the instantaneous phase difference between signals recorded at two receivers to detect karst cavities beneath piles. The receiver separation is set to half the dominant wavelength of surface waves propagating along the pile hole. We define the instantaneous phase difference intensity (IPDI) as an index that measures the similarity of instantaneous phase between the signals at the two receivers. Higher IPDI values signify that the two signals have a similar instantaneous phase at that time, which implies the arrival of a reflection from a cavity. Reflected surface-wave arrivals exhibit low IPDI by design of the receiver geometry. Thus, the first break of reflected P-waves from the roof and floor of a cavity can be identified. We evaluate the effectiveness of the IPDI-based analysis method using numerical tests simulating the varying depth, azimuth, and size of karst cavities. A prototype using the IPDI analysis demonstrates the application of the new pile hole sonar method at two field investigations. Advance drilling, borehole optical image logs, and crosshole tomography verify the IPDI detection results. We conclude that the two-receiver sonar instrumentation along with the IPDI analysis are effective for detecting cavities beneath piles.

中文翻译：

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利用两个接收器记录的瞬时相位差检测现浇桩下的岩溶腔

钻孔灌注桩下方的岩溶洞对基础设施项目的稳定性有害。因此，重要的是在桩的施工过程中检测岩溶腔。向下放置在桩孔底部的声纳可用于检测空腔。但是，来自桩孔壁的多个反射面波的干扰掩盖了微弱的腔反射。我们已经开发出一种声纳方法，该方法利用两个接收器记录的信号之间的瞬时相位差来检测桩下的岩溶腔。接收器间隔设置为沿桩孔传播的表面波主波长的一半。我们将瞬时相位差强度（IPDI）定义为衡量两个接收器信号之间瞬时相位相似度的指标。较高的IPDI值表示这两个信号当时具有相似的瞬时相位，这意味着从腔体反射的到达。通过设计接收器的几何形状，反射的表面波到达显示出较低的IPDI。因此，可以识别出从腔体的顶部和底部反射的P波的第一次破裂。我们使用数值测试来评估基于岩溶腔的深度，方位角和大小的数值测试，从而评估基于IPDI的分析方法的有效性。使用IPDI分析的原型演示了新的桩孔声纳方法在两个现场调查中的应用。预先钻探，钻孔光学图像记录和井间断层扫描可以验证IPDI检测结果。