Magnetic Viscosity Effect in Magnetic-Source Time-Domain Electromagnetic Surveys

The effect of magnetic viscosity (MV) has continued to be observed with the development and application of the time-domain electromagnetic method (TDEM). Field and laboratory data show that the MV effect is characterized by a −1 ± 0.4 scope power-law delay in the late stage of electromagnetic response. Research on the MV effect can improve the detection accuracy of TDEM and assist in prospecting for ferromagnetic minerals. Most of the studies are based on the Chikazumi susceptibility model and the 1-D modeling method. However, the late-stage electromagnetic response shows a −1 power-law delay, which is inconsistent with the measured data. The log-uniform distribution of relaxation time
$\tau $
in the Chikazumi model is not always appropriate. This study considers the Cole–Cole susceptibility model with a log-normal distribution of relaxation time. The 3-D modeling method of the MV effect is raised based on the rational function approximation algorithm and recursive convolution technique; the control equations and iterative process were adjusted based on finite-different time-domain (FDTD) method. The effectiveness was verified via half-space and layered models; the effects of susceptibility parameters on the response were clarified; moreover, the MV effect of the 3-D anomalous model was analyzed. Our method can model the fractional propagation process of the MV effect more efficiently and help to improve the prospecting accuracy of the TDEM method under complex magnetic geological conditions.