ADJUSTMENT OF THE ELECTROMAGNETIC-FIELD DISTORTED BY 3-D HETEROGENEITIES

The 3-D adjustment distance of electromagnetic fields distorted by lateral contrasts of conductivity was studied, with applications to conductivity anomalies occurring near coasts or islands. Investigations of the electrical conductivity structure of the mantle depend critically on whether the distortion of the electromagnetic data is caused by local or by remote contrasts of conductivity. With the help of simple models it is shown that the actual adjustment distance of the electric field along the off-strike direction to the conductivity contrast is a balance between an adjustment distance related to the galvanic coupling of the upper layer to the substratum and a distance describing the horizontal scale of the electric currents deviated by the contrast in conductivity. It is shown that this adjustment distance cannot exceed the effective dimension of the conductivity contrast. Consequently, published adjustment distances of several thousand kilometres are unlikely to exist. As a result, sea-floor electromagnetic soundings carried out at large distances from coastlines are useful for probing the local conductivity structure of the oceanic mantle. The 2-D interpretation of the electric field in the offshore direction at stations perturbed by the coastal effect has a high probability of bias because the field is affected by the combination of the 2-D galvanic coupling and the effect of the horizontal short-cut by the closest conductive channels of the electric current flowing in the ocean.