A damped forward EMI model for a horizontally stratified earth

TL;DR

This paper introduces a new analytical EMI forward model for horizontally stratified earth that accounts for eddy current interactions, improving accuracy and applicability over existing models for geophysical surveys.

Contribution

The paper develops a damped forward EMI model that includes eddy current interactions, extending the validity range and reducing errors compared to previous models.

Findings

Model accurately predicts magnetic fields in broader conductivity ranges

Error compared to exact solutions is significantly reduced

Vertical sensitivity aligns well with exact model predictions

Abstract

If a magnetic dipole is placed above the surface of the earth, the Electromagnetic Induction (EMI) effect, encoded in Maxwell's equations, causes eddy currents in the soil which, on their turn, induce response electromagnetic fields. The magnetic field can be measured in geophysical surveys to determine the conductivity profile of the ground in a non-destructive manner. The forward model used in the inversion of experimental data usually consists of a set of horizontal homogeneous layers. A frequently used analytical model, proposed by McNeill, does not include the interaction between the eddy currents, and therefore fails for larger conductivities. In this paper we construct a new forward, analytical, model to estimate the magnetic field caused by a horizontally stratified earth but which approximates the interaction between eddy currents. This makes it valid for a broader range of parameters than the current state of the art. Furthermore, the error with the (numerically obtainable) exact result is substantially decreased. We also calculate the vertical sensitivity ("depth of exploration") of the model and observe that it is in good agreement with the values obtained from the exact model.