Evaluating karst features and geological fault leakage risks based on geophysical method in Dongzhuang reservoir area, Shaanxi

Abstract

Leakage along faults, particularly in karst area faults, is a critical concern that can prevent impounding a reservoir. The Laolong Mountain fault in the Dongzhuang Reservoir Project Area in Shaanxi Province, China, is a large fault that may impede the project. To assess whether concentrated leakage can occur along this fault, we used a comprehensive approach involving footrill exploration, an electromagnetic method, optical imaging techniques, water pressure testing, and large-scale groundwater tracer tests. Footrill exploration results revealed that the karst features predominantly comprise karst cavities with some fissures and karst caves. These formations are typically clustered along fractures with a linear joint density of 0.4–1.5 joints/m. The linear karst rate was 0.39 %, and the karst cavities have small diameters of only a few centimeters and depths not exceeding 15 cm. A geophysical exploration indicated that low resistivity values (approximately 200 Ω·m) on the north side of the fault zone's hanging wall, with the lowest values being approximately 100 Ω·m. The fault's footwall was noted to have higher resistivity (approximately 2000 Ω·m). The rock properties on either side of the fault zone differ considerably. The upper plate of the fault (downstream of the river) has high-resistance carbonate rock formations with a well-preserved structure, whereas the lower plate (upstream of the river) comprises low-resistance sandstone and mudstone formations with notable fragmentation at the fault's surface compression zone. Tracer tests indicated an eastward groundwater flow from Zuantian Ridge toward the Jing River in the west. The results suggest that eastward leakage along the Laolong Mountain fault and its influence zone is unlikely upon reservoir impoundment. The geophysical exploration method in this study integrated and cross-validated the footrill exploration, hydrological test, groundwater tracing, optical imaging, and other methods; it can thus serve as a reference for future research.