Three-dimensional electromagnetic inversion of transfer function data from controlled sources

We develop a three-dimensional inversion code capable of imaging the resistivity distribution of the subsurface from frequency-domain controlled-source electromagnetic data. Controlled-source electromagnetic investigations have, besides other geophysical methods, an important role to play for geophysical prospecting purposes. To analyse controlled-source electromagnetic data collected with complex measurement setups on land, advanced three-dimensional modelling and inversion tools are required. Therefore, we adopt a preconditioned non-linear conjugate gradient algorithm to be able to invert impedance tensor and vertical magnetic transfer function data produced by multiple sets of two independent active sources. Forward simulations are performed with a finite-element solver. Increased sensitivity at source locations can optionally be counteracted with a weighting function in the regularisation term. We investigate the capabilities of the inversion code using one synthetic and one field example. The results demonstrate, that we can produce reliable subsurface models, although data sets from single pairs of independent sources remain challenging to evaluate due to a nonsymmetric illumination of the target area.