Magnetic Equivalent Circuit Model Surveys of Superparamagnetic Effect in Time-Domain Electromagnetic Method

Superparamagnetic (SPM) effect can produce a −1 ± 0.4 power-law decay of the late-stage responses in the time-domain electromagnetic (TDEM) method. The SPM effect will obscure responses from the anomalous bodies and lead to incorrect data interpretation. Therefore, accurately observing and analyzing the characteristics of SPM effect is particularly important for TDEM data interpretation. To address this problem, we propose a fractional magnetic equivalent circuit model based on the improved particle swarm optimization algorithm and the anomaly loop method, which can effectively generate the SPM effect at the late stage of the attenuation curve. By calculating the magnetic equivalent circuit model responses under the ramp-step waveform, we can analyze the influence of different transmitting and model parameters on the equivalent SPM effect. The magnetic susceptibility can be adjusted by changing the number of turns in the magnetic-induced coil, and the results show that a slower turn-off time is effective at eliminating the influence of SPM effect. Finally, the magnetic equivalent circuits of SPM effect are constructed based on the proposed design method, and the efficacy of the method is verified by field experiments. This article proposes a magnetic equivalent circuit design method to generate SPM effect in the late period of TDEM responses. The method can provide a new avenue for studying the SPM effect and verifying the design and instruments use for the SPM effect observation.