Using GPR Data as Constraints in RMT Data Inversion for Water Content Estimation: A Case Study in Heby, Sweden

This study uses ground penetrating radar (GPR) data as constraints in the inversion of radio-magnetotelluric (RMT) data, to provide an improved model at shallow depth. We show that modification of the model regularization matrix using all GPR common-offset (CO) reflections can mislead the constrained inversion of RMT data. To avoid such problems, common mid-point (CMP) GPR data are translated to a resistivity model by introducing a new petrophysical relationship based on a combination of Topp’s and Archie’s equations. This model is updated through a semi-iterative method and is employed as an initial and prior model in the subsequent inversion of RMT data. Finally, a water content model that fits the GPR CMP and RMT data is derived from the resistivity model computed by the constrained inversion of RMT data. To assess the proposed scheme, it is applied to a synthetic data set. Then, real RMT data collected along an 870 m-long profile across a known aquifer situated in the north of Heby, central Sweden, are inverted. By removing the smoothness constraints across GPR CO interfaces or using CMP-based inversion, thick (> 10 m) vadose and saturated zones are resolved and shown to correlate with logs from nearby boreholes. Nevertheless, the application of our CMP-based inversion was the only efficient scheme to retrieve thin (~ 3 m) saturated zones and the water table at a depth of 7–15 m in the RMT models. The estimated models of water content are in good agreement with the available hydrogeological information in the study area.