Investigating sediments and rock structures beneath a river using underwater ERT

Abstract

This article presents hydrogeophysical investigations performed in a well-developed, long-term hydrogeological gypsum karst research site where subsurface evaporite dissolution has led to the subsidence of a river dam and an adjacent highway; both constructed on gypsum-containing rock, southeast of Basel, Switzerland. An observation system was set up to improve the protection of surface and subsurface water resources during remedial construction measures of the highway and in order to understand the processes, as well as the temporal evolution, of rock water interaction (flow and dissolution). However, no detailed hydrogeological information beneath the river could be derived from the previous investigations. To supplement the basic knowledge on this area, underwater Electrical Resistivity Tomography (ERT) measurements were conducted in the river bed upstream of the dam. The ERT-data are interpreted together with drill-core information and a conceptual 3D-Model of the area behind the dam and beneath the river. Results help to delineate weathered zones, associated faults and the thickness of sediment deposits behind the dam, as well as to locate voids within the local karst system. The combination of the ERT and modeling allows the optimization of future site-specific remedial construction measures.

Determination of Lakebed Sediment Distribution Based on Underwater Electrical Resistivity Tomography Data

“…For two-dimensional ERT measurements on land, the electrodes used to inject current and measure the potential are most often hammered into the ground, while there are different methods for measurement in water. In a water environment, the electrodes can also be placed on the bottom, but more commonly, mobile systems are used where a cable with electrodes is dragged along the bottom, or floating electrodes are used on the water surface (Nyquist et al, 2009;Epting et al 2012, Kwon et al, 2005. Combined surveys with some electrodes on land and some underwater are also used (Loke and Lane, 2004).…”