Anomaly effect-driven optimization of direct-current geoelectric mapping surveys in large areas

In most hydrogeological, geotechnical, archaeological, and other geoscientific applications, we need to understand the lateral extent and connectivity of system-relevant subsurface features. Towards this end, direct-current electric resistivity tomography (ERT) with several 2-D profiles or 3-D grids provides a powerful tool for non-invasive resolution of electrical resistivity anomalies. On the downside, many hours of fieldwork to set up and break down long electrode profiles limit this method to study areas of few thousand square meters, as the workload multiplies with the number of profiles. In many projects, however, determining the extent and connectivity of subsurface anomalies and therefore their potential relevance to the system, may only require the target to be spatially traced instead of fully resolved. We therefore propose geoelectric mapping with a target-specific fixed electrode spacing as an efficient way to trace a resolved resistivity anomaly away from an initial ERT profile, which should be particularly valuable for large study areas. The target-specific electrode spacing is hereby determined by evaluating the effects of the targeted anomaly in the raw data of the preliminary ERT profile. We therefore introduce an anomaly effect applicable to measurements in environments with spatial trends in resistivity distribution. In synthetic simulations, we demonstrate that our approach can efficiently delineate lateral boundaries of resistivity anomalies in ERT data space and we visualize this in pseudosections of anomaly effects. We then apply this method to tracing a gravel-filled paleo-channel in the 8 km(2) Ammer flood-plain near Tubingen, Germany and determine a suitable electrode spacing for a subsequent mapping campaign from the ranges of anomaly effects. We traced the paleo-channel over several hundreds of meters away from an initial 550 m long ERT profile within 19 h, the same time needed to set up, measure, and dismantle the single initial ERT profile. The evaluation of anomaly effects proves to be an efficient tool to detect resistivity anomalies in geoelectric data and determine suitable electrode spacings for large-scale mapping campaigns. Once identified, anomalies and project-relevant subareas can be the target of more detailed investigations. (C) 2020 Elsevier B.V. All rights reserved.