3D electrical resistivity tomography of karstified formations using cross-line measurements

The acquisition of a full 3D survey on a large area of investigation is difficult, and from a practitioner's point of view, very costly. In high-resolution 3D surveys, the number of electrodes increases rapidly and the total number of electrode combinations becomes very large. In this paper, we propose an innovative 3D acquisition procedure based on the roll-along technique. It makes use of 2D parallel lines with additional cross-line measurements. However, in order to increase the number of directions represented in the data, we propose to use cross-line measurements in several directions. Those cross-line measurements are based on dipole-dipole configurations as commonly used in cross-borehole surveys. We illustrate the method by investigating the subsurface geometry in a karstic environment for a future wind turbine project. We first test our methodology with a numerical benchmark using a synthetic model. Then, we validate it through a field case application to image the 3D geometry of karst features and the top of unaltered bedrock in limestone formations. We analyze the importance of cross-line measuring and analyze their capability for accurate subsurface imaging. The comparison with standard parallel 2D surveys clearly highlighted the added value of the cross-lines measurements to detect those structures. It provides crucial insight in subsurface geometry for the positioning of the future wind turbine foundation. The developed method can provide a useful tool in the design of 3D ERT survey to optimize the amount of information collected within a limited time frame. (C) 2017 Elsevier B.V. All rights reserved.