3D Loop‐Source EM Inversion Using Non‐Linear Conjugate Gradients

Abstract

Electromagnetic (EM) exploration on land has advanced a long way, but it still needs additional refinement. A 3D inversion would make the interpretation aspect simpler for a complicated subsurface, and make EM exploration easier to implement. We have formulated a 3D inverse solution for land‐based loop‐source EM exploration using a non‐linear conjugate gradient method. This code was developed for exploration of complex subsurface structures like those of a meteorite impact crater or of a contamination plume. A finite element forward model is used to generate predicted data. The forward model uses a finite element mesh as well as coulomb‐gauged potentials to solve the governing Maxwell equations and generate data for a single frequency. The finite element forward model code also permits discretization of the subsurface into an unstructured grid, allowing element edges to coincide with the boundaries of irregular subsurface inhomogeneities. The forward model can also take into account topography and its resultant effect on the response. A quasi adjoint Green's function approach is used to calculate the sensitivity matrix. For this presentation we will show our initial results from synthetic data as well as some preliminary results from actual field data.