Abstract: There are many geophysical methods for advanced geological prediction. Due to the complexity of spatial distribution of Karst development and diversity of fillings of Karst waterbearing media, single advanced geological prediction method has limitations. It is necessary to select suitable geophysical methods or combinations of geological prediction to improve the accuracy of prediction results according to the geophysical characteristics of Karst. The theoretical basis and method selection of advanced geological prediction method for Karst tunnels are discussed by analyzing the working principle, effective detection distance and geophysical characteristics of main geophysical prediction methods such as TSP method, geological radar method and transient electromagnetic method. Several main advanced geological prediction methods and combinations used in Hejia Tunnel of Jiujingqu Railway are discussed in detail during construction. Through comparative analysis, the applicability and accuracy of prediction results of several main geophysical prospecting methods are verified and summarized. The applicable results show that: (1) Single advanced geological prediction method has limitations and is difficult to cope with complex geological conditions and high construction risk of Karst tunnels; it is necessary to adopt the measures of "three combinations" and select different advanced prediction methods as combinations to verify each other. (2) The TSP method fails to distinguish between Karst and fault fracture zones, so it must be combined with other methods to carry out comparative verification in order to distinguish them; the combination of TSP method and geological radar method is a common combination for simple Karst tunnels. (3) The transient electromagnetic method has obvious effect in Karst tunnel detection with high accuracy of interpretation, and is suitable for large stronglydeveloped Karst areas. The combination of advanced horizontal drilling + transient electromagnetic method + geological radar method + hydrological monitoring geological prediction is an effective combination for advanced geological prediction of complex Karst tunnels. The analysis of superimposed advanced horizontal drilling and transient electromagnetic profiles can provide a detailed analysis of the spatial distribution and morphological characteristics of Karst.

Key words: Karst tunnel, advanced geological prediction, geophysical characteristics, transient electromagnetic method