Based on a study of the regularity governing the variation of exploration sensitivities of different power-supply polar distances with the depth under the horizontal layering terrestrial condition, the authors have discovered that, when the power-supply polar distance AB/2 is equal to 4h, the exploration sensitivity is the highest at the depth of h. On such a basis, this paper puts forward the simple relationship of the exploration depth to the power-supply polar distance as well as to the optimum power-supply polar distance.