3-D Full-Waveform Modeling and Analysis of Induced Polarization and Magnetic Viscosity Effect in Time-Domain Electromagnetic Method

With the improvement in the accuracy of time-domain electromagnetic method (TDEM) observations for geophysical exploration, abnormal diffusion phenomena have become a more salient focus of research. In particular, the induced polarization (IP) effect and magnetic viscosity (MV) effect are often observed in the exploration of polymetallic ore, with IP effect leading to a negative TDEM response, and MV effect generating a power-law decay of -0.6 to 1.4 in the late stage response. Ignoring these effects can lead to incorrect data interpretation. Therefore, to model and analyze both IP effect and MV effect accurately, a 3-D numerical modeling method for IP-MV effect is proposed. The Cole-Cole conductivity and Cole-Cole susceptibility models are approximated in the time domain using the multiple-zero-pole (MZP) method. Then, the diffusion equations for the electric and magnetization intensity fields are derived as control equations, while the 3-D modeling of the IP-MV effect with full waveform is realized based on the improved recursive convolution technique and finite-difference time-domain (FDTD) method. The effectiveness is verified by comparing with the 1-D numerical integration solutions. The response characteristics of IP effect and MV effect are analyzed with full trapezoidal waveform, and a complex model including IP-MV effect is discussed. It is demonstrated that IP-MV effect can be better observed at the ON-time stage. The proposed method can effectively model the diffusion process of IP-MV effect with the full waveform, which can be helpful for improving the inversion accuracy and detection precision for complex geological formations.