The Coupled Effects of Topography and Substructure on Electromagnetic Data as It Applies to Impact Crater Sites

Controlled—source electromagnetic geophysical methods provide a noninvasive means of characterizing subsurface structure. In order to properly model the geologic subsurface with a controlled-source time domain electromagnetic (TDEM) system in an extreme topographic environment we must first see the effects of topography on the forward model data. When simulations are run we can see the effects on the secondary field generated by the changing topography. Simulations are run using the Texas A&M University (TAMU) finite element (FEM) code in which was included a changing 3D topography to test the limits of the code. From these models we can see how much change in topography that can be include before our forward model can no longer give us accurate data output. The simulations are based on a model of a geologic half space with no cultural noise and focus on topography changes associated with impact crater sites, such as crater rims and central uplift. Several topographical variations of the model were run but the main constant is that there is only a small conductivity change on the range of 10-1 s/m between the host medium and the geologic structure within. Asking the following questions will guide us through determining the limits of our code: What is the maximum step we can have before we see fringe effects in our data? At what location relative to the body does the topography cause the most effect? What kind of topography provides the most fringe effect? Looking at the secondary field effect can help us determine new approaches in data acquisition and after we know the limits of the code we can develop new methods to increase the limits that will allow us to better image the subsurface using TDEM in extreme topography.