Abstract

The effect of superparamagnetic minerals on the transient response of a uniform ground can be modeled by allowing the permeability of the ground mu to vary with frequency omega asEquationHere tau 1 and tau 2 are the upper and lower time constants for the superparamagnetic minerals and chi 0 is the direct current value of the susceptibility.For single-loop data it is found that the voltage will decay as 1/t, provided thatEquationHere, a is the radius of the wire loop and b is the radius of the wire, t represents time and mu 0 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 1/t 2 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 1/t (super 2.5) . 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 chi 0 of the ground. For a weakly magnetic ground the estimated conductivity sigma 1 is related to the true value of the conductivity sigma 1 = sigma [1 + 19chi 0 /14] 2 3/.

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