Aftereffects in the Transient Electromagnetic Method: Magnetic Viscosity

One of the aftereffects inherent in geological materials is magnetic viscosity. This phenomenon consists in the time lag of changes in magnetic characteristics in relation to changes in the external magnetic field. In rocks, magnetic viscosity is mainly associated with magnetization of superparamagnetic particles. In the transient electromagnetic method, magnetic viscosity is manifested as a slowly decreasing voltage induced in a receiving loop, or, in some cases, as a nonmonotone transient voltage response. Eddy currents and viscous magnetization establish and decay independently; therefore, the induction transient response measured with a fixed-geometry TEM array gives no way of finding the vertical distribution of magnetic viscosity. In order to find it, geometric soundings are needed. At later times, the voltage induced in the receiving loop due to the magnetization decay is vastly larger than that induced by the eddy currents. Because of this, magnetic viscosity contribution to the total transient response limits the sounding depth of the transient electromagnetic method.