MMT forward modeling for a layered earth with arbitrary anisotropy

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The standard model of a layered isotropic earth is a good approximation to geophysical reality in geoelectromagnetic mapping. In regions with distinct dipping stratification, however, it is difficult to model these fine structures because of large storage and time requirements. In this case, we may approximately replace the isotropic conductors with a preferential electrical direction by macro-scale anisotropy. The marine-magnetotelluric (MMT) forward problem is formulated for a layered earth with arbitrary anisotropy. For a given layer beneath the ocean, the two horizontal components of the electric field are projected onto the principal anisotropic directions and continued from layer to layer using continuity conditions. The formulation is used to derive the impedance tensor and apparent resistivities for a uniform anisotropic half-space, which clearly distinguish the effect of electrical anisotropy on MMT responses. The principal anisotropic orientations are clearly identified in the polar plots, which show the azimuthal variation of the apparent resistivity. The resolvability analysis of a resistive intermediate layer indicates that the depth of exploration for the MMT method depends strongly on the anomaly threshold associated with system sensitivity and environmental noise level but less on the resistivity contrast between the target and surrounding media. For an anisotropic target, to obtain a larger depth of exploration the electric field should be measured in the direction of higher target-host resistivity contrast. The simulation of reservoir characterization using marine MT shows that the current MMT technique cannot be an effective tool yet for direct offshore reservoir characterization, but it can be a good complementary tool for controlled-source electromagnetic (EM) technologies emerging for offshore hydrocarbon exploration.