The seepage issue of dam foundation is a critical factor determining whether the dam can op_ erate stably in long term. The rock mass fracture zone in dam foundation is often the direct cause of seep_ age. Taking the geophysical investigation work of the dam in Guazhongling reservoir as an example

the frequency electromagnetic sounding method is adopted. By comparing with subsequent drilling and sam_ pling results

the result shows that the frequency electromagnetic sounding method has high accuracy in detecting rock fracture zones in dam foundation

which can provide reliable evidence for the design of seepage prevention treatment plans for dam foundation.