The transient electromagnetic response of a magnetic or superparamagnetic ground

Abstract

The effect of superparamagnetic minerals on the transient response of a uniform ground can be modeled by allowing the permeability of the ground μ to vary with frequency ω as [Formula: see text] Here [Formula: see text] and [Formula: see text] are the upper and lower time constants for the superparamagnetic minerals and [Formula: see text] is the direct current value of the susceptibility. For single‐loop data it is found that the voltage will decay as 1/t, provided that [Formula: see text] and [Formula: see text] Here, a is the radius of the wire loop and b is the radius of the wire, t represents time and [Formula: see text] is the permeability of free space. Even if a separate transmitter and receiver are used, the transient will still be anomalous. For this case the 1/t term in the equations is less important, and more prevalent now is the [Formula: see text] term. These results show that a uniform ground behaves in a similar way to a ground which only has a thin superparamagnetic layer. A difference is that whereas the amplitude of the 1/t term could be drastically reduced by using a separate receiver, this is not the case for a uniform ground. A magnetic ground for late times will decay as [Formula: see text]. However, if the conductivity of the ground is estimated from apparent conductivities it will be found that the value of the conductivity will be incorrect by a factor that is related to the susceptibility [Formula: see text] of the ground. For a weakly magnetic ground the estimated conductivity [Formula: see text] is related to the true value of the conductivity [Formula: see text].