An Automated Electrical Resistivity Tomography System to Monitor the Freshwater/saltwater Zone on a North Sea Island

Abstract

An automated electrical resistivity tomography system was developed to monitor changes in the transition zone between the freshwater lenses and the underlying saltwater on the North Sea island Borkum. Main part of the system is a vertical electrode chain of about 20 m length comprising 78 stainless steel ring electrodes mounted on a rigid PVC pipe. The distance between adjacent electrodes is 0.25 m. On the island of Borkum two of these electrode chains were installed in 45-65 m depth in the water catchment areas Waterdelle and Ostland in September 2009. Borehole measurements ascertained the installation depths. The ongoing monitoring measurements are carried out using a modified commercial resistivity meter with active electrodes (4point light 10W). The power is supplied by solar panels and the data are transmitted to Hannover by telemetry. Within the last seven months more than 800 complete pseudosections using a Wenner-alpha array were recorded with each of the two vertical electrode chains. Significant resistivity changes were observed mainly in depths dominated by saltwater, but up to now these changes are considered to be caused by the readjustment of the disturbed conditions at the drilling locations to the normal situation.