Parametric estimators for current excitation on a thin plate

Abstract

Parametric analysis is used to predict whether induction or current channeling dominates the current excitation in a conductive body in the earth. Knowing that one mode dominates the current response permits the use of relatively simple models that account for only a single mode of excitation in place of more complicated general ones that account for both modes of the current response. This is useful both in forward modeling and in inversion. In interpretation, predicting the current excitation is useful for verifying that the assumed mode of excitation is consistent with the interpreted body. Prediction is done with a set of “current excitation ratios” that we demonstrate for a thin conductive plate in a conductive half‐space. To derive the excitation ratios, parametric theory is used to estimate the strength of the inductive and galvanic modes of the current response of the plate. The ratios then follow by dividing the inductive estimate into the galvanic one. When this ratio is less than one, induction will dominate the current response. When it is greater than one, current channeling will dominate. Current excitation ratios are simple to calculate, and consist of two components. One component is a product of model parameters such as conductivity, dimension, and permeability, and can be calculated by hand. The second component consists of what we term the “local impedance” of the source field at the conductor. This component can be calculated with a simple half‐space or layered earth electromagnetic modeling algorithm and then contoured for later reference. The predictive capability of the current excitation ratios is tested by calculating the current response on a vertical plate in a half‐space with a full electromagnetic scattering solution. We find the correspondence between the two to be very good, and that it is possible to successfully predict the dominant mode of the current response through parametric theory where assumptions used in the parametric analysis are valid.