Through theoretical calculation of ρ_s and η_s, anomalies and observational signal on the spheroid, this paper expounds anomalous characteristics of the "bipole-dipole"device, and the main advantages of this device in enhancing obser vational signal and saving electric energy as compared with the "dipole-dipole"device. Besides, the calculation leads to formulating the concept of "criticalAB/MN value", which might serve as theoretical basis for the choice of bipolar distance AB in the practical work.