The Response of the Oceanic Lithosphere to Electromagnetic Controlled Source Transmitters Modeled Using Local Spectral Representation

An algorithm is developed to model the frequency domain response of the oceanic lithosphere to electromagnetic transmission experiments. Within regions characterized by electrical conductivity profiles varying only in the vertical, the electromagnetic field is computed as an expansion in local normal modes. To model horizontal conductivity variation, the local spectra are then used as the basis of a high order finite element scheme. This is efficient when horizontal gradients are gentler than gradients in the vertical. The algorithm is suitable for Earth models and frequencies where the ocean and asthenosphere vertically confine electromagnetic energy. The expansions converge rapidly for relatively long range transmission, where signals penetrate into the mantle. Backscattering, multiple reflections and surface waves on all internal contrasts are included. The algorithm has been implemented for lithosphere conductivity varying in two dimensions and for sources such that the incident field can be approximated as invariant along the lithospheric direction of strike. This includes sources forming long lines parallel to strike as well as dipole sources situated at normal incidence and not too close to horizontal structure. As the analysis is largely analytical, derivatives with respect to Earth structure and conductivity could be produced at a cost of only a moderate computational overhead. Extension to oblique incidence or three dimensionally varying structure is expected to be straightforward. Views of model Poynting vectors, representing electromagnetic energy flux at depth within the Earth, are used as an aid to physical insight. Sensitivity of transmission through mid-ocean ridge type structures is studied for the polarization in which electric currents cross conductivity contrasts. Capability to discriminate between geological hypotheses is investigated. Hypotheses about temperature and fluid content in the uppermost 30 -60 km are converted to conductivity structure, and experimental response simulated. Results indicate that for likely off -axis background conductivities, crustal magma chamber presence and melt fraction may be sensed close to the ridge axis, while connected concentrations of partial melt in the root region influence cross-axis reception at longer ranges.