Electric dipole fields over an anisotropic seafloor: theory and application to the structure of 40 ma pacific ocean lithosphere

SUMMARY Seismic anisotropy has been detected in the oceanic crust and upper mantle, and likewise it is geologically reasonable to expect that a certain amount of lateral anisotropy exists in sea£oor electrical properties. Anisotropy in Earth properties can often lead to surprising eiects on geophysical responses that are not anticipated from simple isotropic theories. Here, we investigate the eiects of lateral anisotropy on the frequency-domain, controlled-source electromagnetic (CSEM) response of a uniaxially conducting, non-magnetic sea£oor excited by a horizontal electric dipole whose moment is oriented obliquely with respect to the electrical strike direction. A ‘paradox of anisotropy’ is observed, in which the sea£oor electric ¢eld strength is enhanced in the most conductive direction of the sea£oor. This enhancement is opposite to what one would expect based on naive isotropic theory. We also show that it is possible in certain circumstances to extract the along-strike electrical conductivity from marine controlled-source electromagnetic data using only isotropic modelling. The extraction of across-strike conductivity, however, requires full anisotropic modelling. The physical insight into electromagnetic induction in uniaxial media that is presented here should greatly assist the geological interpretation of marine CSEM experimental data. Applying our algorithm to the PEGASUS data set (CSEM data collected over 40 Ma Paci¢c Ocean lithosphere) produces a model with conductivity in the fossil spreading direction that is seven times greater than the conductivity perpendicular to spreading. Strain-aligned mineralogical fabric, as predicted by tectonic modelling, would explain our result, with enhanced conductivities caused by hydrogen conduction along the olivine a-axis or connected accumulations of trace conductors such as graphite or magnetite.

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