Abstract:  Phase 1 of Guiyang rail transit line 3 involves the construction of a shield tunnel that extends from the College of Agriculture Guizhou University station to the Huaxi Park station, passing beneath the Huaxi river with complex geological conditions. The abovewater geological exploration cannot be implemented in this river because it is a water source protection area. In this study, a specialized geological exploration probe capable of launching and receiving lowfrequency of the electromagnetic wave crosshole CT scans is introduced to overcome this obstacle. This innovative probe reduces the operating frequency range from 8~64 MHz to 2~4 MHz while enhancing the power of the spark seismic source in the acoustic wave crosshole CT launching probe from 2 500 J to 5 000 J. This adjustment enables the execution of crosshole CT test at a maximum distance of 54 m and the mutual verification between the electromagnetic wave crosshole CT and the acoustic wave crosshole CT results. By adopting comprehensive exploration technology based on the use of ultralong distanc electromagnetic wave crosshole CT and acoustic wave crosshole CT technology, as well as mutual verification of exploration holes, boreholes have been successfully implemented in Huaxi river water source areas. The distribution of weak rock and soil layers involved in the shield tunnel has been identified, providing reasonable suggestions for the design and construction.

Key words: karst, shield tunnel, electromagnetic wave crosshole CT, acoustic crosshole CT, ultra longdistance crosshole CT, comprehensive exploration technology