THE COAST EFFECT RESPONSE IN GEOMAGNETIC-FIELD MEASUREMENTS

Two-dimensional (2D) numerical models of electromagnetic (EM) induction for anomalous conductors in continental and island coastal regions are studied to determine constraints on ocean-conductor separation distances that would permit, to within an acceptable approximation, a simple vector subtraction of the coast effect response. The anomalous conductors are in the form of an upwelling or a depression in the asthenospheric conductive substratum at depth. The coast effect response range (Y(R)) is arbitrarily taken to be the distance from the ocean at which the \B(z)/B(yn)\ coast effect response is reduced to a value of 0.2. The response ranges are determined and presented empirically as a function of period for a range of conductive substratum depths, island widths, and ocean depths. The response ranges are tested in 2D numerical models that include an anomalous conductor in the form of either an upwelling or a depression in the conductive substratum under the ocean and host Earth. The induction arrows for the anomalous conductor are shown to be in good agreement with the difference arrows obtained by subtracting the responses of a model without the anomalous conductor from the responses of the model that includes the anomalous conductor. As an application, difference arrows, obtained by subtracting three-dimensional laboratory analogue model coast effect responses from field site responses, are employed in the interpretation of induction arrow responses available for a number of sites in the Bohai Bay region of China and the Kii Peninsula region of Japan.