Abstract
Two dimensional numerical models of the California coast anomaly have been constructed to compare their predictions with observations, with a view to infer the electrical conductivity structure of crust and upper mantle within this area. The third axis of these models is directed along the coast. Only induction processes with magnetic fluctuations normal to it are considered (Transverse Electric Mode). The ocean is represented by a this sheet with a conductivity equal to the vertically integrated oceanic conductivity. Crust and upper mantle are simulated by free and finite conductivity distributions extending downward several skin depths before termination by an infinitely conductting horizontal plane. The upper boundary, which contains the source field, is transparent to the upward reflected energy, thus simulating the high impedance of the ionosphere. The horizontal wave length of the source fields is long, though arbitrary, and these fields can simulate horizontal perturbations, stationary or progressive. The predicted fields are obtained by solving for the stream function φ of the magnetic fluctuation vector B over a rectangular area centered around the coast. φ has to satisfy the diffusion equation Δ2φ+iμμ0ωσφ=0 subject to the aforementioned boundary and interface conditions. Application of these techniques to interpret observations of electromagnetic fluctuations in the range 2-2 to 21 cph across the central California coast suggests a substantial increase of conductivity of the basement, seaward of the continental slope.
