Application of Audio Magnetotelluric Sounding and Acoustic Logging in Tunnel Engineering Surveys: A Case Study of a Tunnel in Eastern Fujian

As the scale and complexity of tunnel engineering continue to expand, particularly under complex geological conditions, traditional survey methods have become hard to meet modern engineering needs. The audio magnetotelluric method offers extensive coverage and significant depth but has relatively low stratigraphic resolution. In contrast, acoustic logging, while limited in depth and range, provides high-resolution acoustic data around the borehole, compensating for the shortcomings of the audio magnetotelluric method in resolution. The combination of these two methods not only enhances the accuracy of stratigraphic information but also effectively delineates key areas, guiding precise measurements in acoustic logging, thus offering an efficient and comprehensive solution for tunnel engineering surveys. This study applies audio magnetotelluric sounding and acoustic logging to a tunnel project survey in Fujian Province, accurately detecting the resistivity and velocity distribution of underground rock layers. The results indicate that sections P4 to P14 exhibit relatively low resistivity, with locally fractured surrounding rock and shallow burial depth, where relatively strong rock weathering necessitates caution is needed to prevent loose collapse upon exposure of the weathered layer. Sections P26 to P34 and P41 to P47 show two low-resistivity anomalies, possibly indicating the presence of aquifers, requiring attention to drainage and refilling to prevent water inflow. Section P34 to P41 has the greatest burial depth and highest resistivity, with tightly compressed surrounding rock, and it is suggested that the drilling of stress relief holes can release energy and facilitate construction. This research demonstrates that the combined application of audio magnetotelluric and acoustic logging methods can provide new insights for tunnel engineering surveys.