Journal of Conference Abstracts

Progress in Finite Element Modeling of Geomagnetic Induction

Chester Weiss (guinness@beerfrdg.tamu.edu) & Mark Everett (colt45@beerfrdg.tamu.edu)

Geology & Geophysics, Texas A&M University, College Station TX, 77843, U.S.A.

Long-period geomagnetic time variations can resolve large-scale 3D mantle electrical conductivity heterogeneities which are indicators of physiochemical variations found in Earth's dynamic mantle. A prerequisite for mapping such heterogeneities is the ability to accurately model electromagnetic induction in a heterogeneous sphere.

In this contribution, a previously developed finite-element solution to the geomagnetic induction problem is improved and validated against an analytic solution for a fully 3-D geometry: an off-axis spherical inclusion embedded in a uniform sphere. The numerical problem is formulated in terms of Coulomb-gauged scalar and vector electromagnetic potentials. A new "least-squares moving averages scheme" is described for differentiating the potentials to obtain the electromagnetic response.

Geomagnetic induction is then modeled in a uniform spherical mantle overlain by a realistic distribution of oceanic and continental conductances. It is shown that the electrical constrast between oceans and continents does not cause the observed geographic variability in Earth's electromagnetic response. This argues for the existence of deeper large-scale heterogeneity, likely near the transition zones. Finally, the sensitivity of Earth's electromagnetic response to a mid-mantle spherical conductor is calculated. A semi-quantitative comparison of observations and modeling results suggests that the mid-mantle likely contains about 1 order of magnitude of lateral variability in electrical conductivity whereas the upper mantle contains at least 1.5 orders of magnitude of lateral variability in electrical conductivity.