Abstract: The Opposing Coils Transient Electromagnetic Method (OCTEM) is widely utilized in engineering and geological surveys for its high shallow resolution and rapid,non-destructive detection capabilities.This study applied OCTEM to investigate a river-type significant leaky zone in the southern mountainous area of Jinan.The primary objective is to evaluate karst development and infer seepage directions based on differences in electrical properties between rock and groundwater.Based on the SimPEG open-source framework,we conducted three-dimensional forward simulations for karst detection.The results show that dual opposing coils concentrates energy more effectively in the center-coupled earth field,thus improving lateral resolution.However,opposing excitation reduces the amplitude of the induced secondary field and shortens the decay time,thereby decreasing depth sounding capability.Based on detection results,we interpreted the electrical distribution in the 0~150 meter range of the leakage zone.The Quanlu River channel formations show distinct low-resistivity characteristics at elevations below 130m,with two low-resistivity zones extending from the surface to deeper layers and spreading to surrounding areas.It is inferred that atmospheric precipitation and surface water flow into the river channel and low-lying areas,becoming surface runoff.Some of this water infiltrates and replenishes groundwater in karst development sections through stratigraphic joints and fractures.In the middle reaches of the river channel,a low-resistivity zone extends northwestward,as confirmed by tracing experiments.The remaining electrical data align with the hydrogeological characteristics of the study area.The OCTEM method,with its high resolution in shallow layers,is suitable for conducting shallow water-richness detection in large areas and complex terrain.