The paradox of anisotropy in electromagnetic loop-loop responses over a uniaxial half-space
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The electromagnetic looploop response of a uniaxial conducting halfspace is derived. The solution is presented in terms of the Geonics EM34 apparent conductivity, which is based on an underlying isotropic halfspace model of earth conductivity. The uniaxial medium generates an apparent conductivity that depends on the angle that the line joining the transmitter and receiver loops makes with respect to the strike of the anisotropy. Furthermore, a paradox of anisotropy occurs in which apparent conductivity is higher in the resistive acrossstrike direction and lower in the conductive alongstrike direction. The paradox is resolved once it is recognized that the receiver loop response is controlled by the conductivity in the direction of the local induced electric current flow. Accurate geological interpretation of electromagnetic looploop data in fractured rock terrains depends on the ability of the geophysicist to recognize the counterintuitive effects of anisotropy.
