Abstract A large number of the shotcrete slopes constructed during Japan's post-war economic boom are now more than 30 years old, and their structural deterioration is an ongoing problem. There-fore, it is necessary to accurately assess the stability and durability of these existing slopes. In this paper, we propose a technique that converts data from seismic velocity and electric resistivity to porosity and saturation, and we used the proposed technique to evaluate weathering and groundwater fluctuation behind the slope. Evaluation data from boring samples indicates that the distribution of porosity and saturation of rock mass around the slope as calculated using the proposed conversion system are highly similar to those of the real rock mass conditions.