Abstract: The development of karst features in offshore areas has a significant impact on the construction of underwater tunnels. Therefore, it is critical to determine the distribution limits and morphological characteristics of the karst features before the construction. This article presents the preliminary survey data from a shielded tunneling section of the Dalian Metro Line No. 5 located underwater. By employing the ground penetrating radar (GPR) and the borehole sonar technology for an actual surveying in the well-developed karst area, the study investigates the applications of the borehole radar and the borehole sonar technology in the offshore karst surveys. The results demonstrate the effective application, successfully clarifying the limits of the karst development and the spatial morphological characteristics within the work area, thus obtaining the accurate data related to the degrees of the karst development. The application results from the engineering case history indicate that these two technologies possess high accuracy and reliability in the practical application of the offshore karst engineering surveys. The GPR can determine the depth and undulations of the bedrock surface, the distribution of karst rock cavities, and the development range of the dissolution fractures, while the borehole sonar can analyze the development limits and the spatial morphological characteristics of the karst cavities. The rational and the integrated use of borehole and these two down-the-hole detection methods can effectively clarify the limits and the spatial morphological characteristics of the offshore karst terrain, providing accurate data on the karst development within the impact range of the underwater tunnel design. This information guides the construction of the shielded tunneling machine, ensuring a smooth crossing of the underwater karst area for the metro tunnel project.

Key words: Offshore Karst Investigation, Borehole Radar, Borehole Sonar, Underwater Tunnel