Effect of Varying Dielectric Permittivity in 2-D Forward and Inverse Modeling of Radiomagnetotelluric Data

Using the quasi-static approximation, which assumes that conduction currents are significantly larger than displacement currents, is common practice in applied electromagnetic geophysics. But in subsurface resistivity estimations with the Radiomagnetotelluric (RMT) method, that utilizes a frequency range of 1-300 kHz when measuring electromagnetic surface impedance, there exist several plausible scenarios where the effect from displacement currents become relevant. In this project displacement currents are accounted for by introducing a spatially variable relative permittivity (εr) in the 2-D forward and inverse modeling code emilia. For the implemented finite difference scheme, we show the sensitivity matrix when treating εr as a free model parameter for the transverse electric (TE) mode, transverse magnetic (TM) mode and the vertical magnetic transfer function (VMTF). The resulting sensitivity matrices of εr for TE and TM-mode are compared to their resistive counterparts in two different resistive environments, visualizing how current model resistivity affects the sensitivity w.r.t. εr. Through inversion of synthetic 2-D forward data, allowing εr to act as a free model parameter, final subsurface models of both resistivity and relative permittivity are created. With the same synthetic 2-D forward data set, generated from buried block anomalies, inversion is also performed using the quasi-static approximation and including displacement currents through a constant scalar a priori εr. When comparing the final models of the three different ways of treating displacement currents, it is clear that using a spatially variable εr better resolves the original subsurface resistivity structure, generates smoother models and performs better quantitatively in relation to the other two methods. This is done while, in a moderately resistive environment, also successfully recreating the buried permittivity anomaly alongside the buried resistive anomaly found in the true model. With the addition of a spatially variable εr RMT field data from Ävrö, Sweden is inverted. The final resistivity model show structures similar to those found in previous studies where a constant scalar εr is employed. The final permittivity model, which complements the qualitative interpretation of the final resistivity model, indicate the extent of a salt water intrusion at the site.  Using εr as free, spatially variable model parameter in the inversion allows for additional qualitative interpretation, does not rely on a priori knowledge of target permittivity and seem to consistently improve the data fit. We therefore recommend that this inclusion is used routinely.