Laboratory tests were conducted to evaluate the influence of the positions of positive and negative electrodes on the electrical response of a leak in the geomembrane. A numerical evaluation was performed to model the laboratory conditions and to investigate the effect of dipole spacing, excitation voltage, and impoundment resistivity on the leak detection sensitivity. The results of laboratory tests show that leaks cannot be identified when the positive electrode is very close to the leak and the electrical response of a leak in the geomembrane is independent of the position of the negative electrode. The results from the 3D finite element model indicate that leak detection sensitivity improves nonlinearly as the dipole spacing increases, and it tends to be stable after the dipole spacing exceeds 5% of the length of the survey line. The results also indicate that there is an almost linear increase in the leak detection sensitivity with increasing excitation voltage. Substantial improvement was obtained in the leak detection sensitivity for a greater impoundment resistivity. The numerical results indicate that 90% of leaks in a geomembrane liner can be effectively detected and accurately located for the 3D finite element model of a geomembrane lined earth-rock dam.