Abstract

Ungrounded horizontal loop transient responses of uniform conductive and magnetically viscous earth have been simulated using two different codes. One algorithm employs the relationship between viscous magnetization and the magnetic flux it induces in the receiver loop. In the other algorithm, the Helmholtz equation in a boundary-value problem is solved using the Fourier transform with frequency-dependent magnetic permeability. The two solutions are identical for noncoincident loops but differ when the transmitter and receiver loops are closely spaced (at 1 cm or less). In the latter case correct results are provided by the first code. The magnetic relaxation and eddy current responses appear to be independent at conductivities typical of the real subsurface. Therefore, TEM responses of magnetically viscous conductors can be computed using the superposition principle. Although transients change in an intricate way as a function of loop geometry and earth parameters, these changes exhibit certain patterns which may be useful at the stages of exploration and TEM data processing. In configurations where the receiver loop is laid outside the transmitter, the interaction of magnetic relaxation and eddy current decay causes sign reversal in transients. This reversal occurs at late times after an earlier sign reversal due uniquely to eddy current.

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