Towards 3D Inversion of Ground Based TEM Data

We describe the setup for inversion of ground based TEM data using a 3D modelling code and a full description of the measurement system using the system response. The response is calculated using a finite volume method where we solve for the electric field on the edges of a staggered grid and time step solutions using backward Euler steps. For the forward calculation we use an iterative solver and a preconditioner which solves the Helmholtz decomposed E-fields. In the iterative process we take advantage of the similarity between different time steps and always use the previous result as the starting point of the next iteration. To compare the calculated fields with measured data we interpolate the fields to the receiver positions and convolute the calculated fields with the system response. In this way we include all system related effects in the calculation and this is in particular relevant for the early time signal. We show that the forward code is in good agreement with the analytic response from a half space and sketch the layout of survey setup with a large, centrally positioned transmitter and many receivers located around the transmitter. We demonstrate that the code can be used to invert data from a single loop system with multiple receivers as commonly used in ground follow-up surveys in mineral exploration.