A finite-volume solution to the geophysical electromagnetic forward problem using unstructured grids

The application of unstructured grids can improve the solution of total field electromagnetic problems because these grids allow efficient local refinement of the mesh at the locations of high field gradients. Unstructured grids also provide the flexibility required for representing arbitrary topography and subsurface interfaces. We investigated the generalization of the standard Yee's staggered scheme to unstructured tetrahedral-Voronoi grids using a finite-volume approach. We discretized the Helmholtz equation for the electric field in the frequency domain and solved the problem to find the projection of the total electric field along the edges of the tetrahedral elements. To compute the electric and magnetic fields at the observation points, an interpolation technique was employed, which uses the edge vector interpolation functions of the tetrahedral elements. To verify the presented scheme, two examples were evaluated, which revealed the computation of the total and secondary fields due to electric and magnetic sources in half-spaces that contain anomalous bodies. The results had good agreement with those from the literature. For the second example, accuracy studies were conducted to better understand the relative importance of different refinements in the grid and also the effect of the general quality of the grid. The results revealed the utmost importance of the quality of the tetrahedral grid and refinement at the observation locations. To validate the versatility of the approach, we synthesized helicopter-borne data for a model of the Ovoid ore body at Voisey's Bay, Labrador, Canada, which had good agreement with real data.