Three Step Depth Focussed Inversion as a Tool to Resolve Small Resistivity Contrasts by ERT

In more recent times Electrical Resistivity Tomography (ERT) has become a valuable tool for many environmental research themes. Within these topics the resistivity contrasts of the different layers and zones are of importance, i.e. sand, clay, water salinity and preferential infiltration pathways are quite small. Inclusions with such small resistivity contrasts to the surrounding are difficult to resolve by ERT inversion and typically the following problems occur: 1.) the true resistivity contrasts are underestimated 2.) the size of the anomalous zone is overestimated 4.) “fake” anomalies arise beside and between the true anomalous zones. 4.) surface heterogeneities infer “fake” anomalies into the subsurface. A two step inversion method is suggested to overcome these problems at least partly. Firstly the near surface heterogeneities are reconstructed by inversion. A forward model comprising the near surface structures only is extracted and used to remove their effect from the data by the application of a reference inversion method. Forward modelling is used to estimate the true sizes and the true resistivity contrasts of the inclusions. These methods are applied to array electrical resistivity tomography measurements at a sandy site with distinct heterogeneities at a depth of 30 – 60cm and show improved resolution capabilities.