Abstract: This study addresses the challenge of accurately assessing surrounding rock properties during the surveying phase of mountain tunnels, which is often limited by the number of available boreholes. The authors propose a method that combines microtremor detection with magnetotelluric (MT) sounding. Using a mountain tunnel on a suburban railway in Changsha as a case study, the authors establish quantitative criteria for classifying rock mass grades, integrity, and water content based on microtremor P-wave velocity and MT resistivity, drawing on preliminary survey data and regional experience. The feasibility of these criteria is verified by comparing them with existing standards. The microtremor S-waves is utilized to identify soil layers, rock layers, and soil-rock interfaces, and microtremor Pwaves are employed to evaluate rock mass grades and integrity. In addition, MT resistivity is utilized to delineate geological features such as fault zones and jointed fracture zones, as well as to assess the water content of rock and soil layers, resulting in a comprehensive understanding of surrounding rock properties. The results are compared with those from drilling and hydrogeological tests. The key findings are as follows: (1) Microtremor S-waves accurately distinguish between soil and rock layers, with errors generally within 1 m, although greater discrepancies can occur in weathered rocks with similar lithology. (2) The rock mass integrity classification based on microtremor P-waves is consistent with drilling results. (3) The assessment of water content using MT resistivity aligns well with hydrogeological test results. These findings confirm that the proposed method is practical and effective for investigating surrounding rock properties in tunnels under complex surface and geological conditions.

Key words: suburban railway, mountain tunnel, tunnel surrounding rock, microtremor detection, magnetotelluric method, water richness