• 标题：A borehole multifrequency acoustic wave system for karst detection near piles
• 关键词：Geo-investigation, Karst, Borehole, Acoustic
• 作者：Liu Liu, Zhen-Ming Shi, Ming Peng, Georgios P. Tsoflias, ChengCheng Liu, Feng-Juan Tao, Chun-Sheng Liu

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Karst cavities at or near pile locations pose a substantial danger to underground construction projects. To locate karst cavities at pile locations, advance drilling boreholes are commonly used, but a cavity can be missed due to its limited size. In this paper, we present a new borehole-based technology employing multifrequency acoustic wave (MFAW) detection of karst cavities near drilling boreholes. First, we developed an MFAW detector and signal processing software. The high- and low-frequency transmitters operate in the frequency range of 0.05 kHz to 10 kHz and the range of 1 kHz to 50 kHz, respectively. Next, we evaluated the performance of the prototype in a controlled borehole model. Lastly, the MFAW detection system was tested at a field site. A nearby borehole core was used to verify the MFAW detection results. The testing of the prototype detection system revealed the following findings. (1) The integrity of the rock surrounding the borehole can be assessed by the energy of the high-frequency zero-offset data. The zero-offset section has a  high correlation with the energy of the high-frequency detection signal. The rock is intact when the high-frequency detection signal is strong. The rock is fragmented when the test signal exhibits amplitude variance over a short-range away from the borehole. The test signal becomes weak when the borehole meets a karst cavity filled with sand, clay or water. A color bar showing the signal amplitude can represent the integrity of the borehole wall more accurately than a core sample. (2) A karst cavity near the borehole can be detected by low-frequency waves. The low-frequency acoustic detection wave field includes a reflected guided wave from the interface that the borehole meets, the wave in the borehole, and the effective reflected body wave from the karst cavity. The integrity of the rock around the borehole wall obtained by high-frequency acoustic detection can help identify the reflected guided wave. Median filtering suppresses the strong amplitude borehole wave and helps reveal the weaker reflected waves. The reflected wave from the karst cavity near the borehole can be extracted after filtering. (3) The azimuth of the cavity is identified by the different detection recordings of the azimuthal receivers. Reflected wave amplitudes obtained by the azimuthal receivers facing the karst cavity are greater than those obtained by the receivers facing away from the karst cavity. The azimuthal angle can be obtained by the three-dimensional electric compass in the downhole tool. The MFAW detection system is shown to be an effective tool for investigating the geological environments at pile locations in detailed geo-investigation periods.