Geoelectric mappings and soundings at the Earths surface are typically not able to image thin layers with appropriate resolution. In such cases, e.g. in sediment layers, permanently installed vertical electrode systems are able to monitor small changes in the conductivity of the pore fluid.
The saltwater monitoring system SAMOS has been developed for investigating and monitoring threads of aquifer by saltwater intrusions. It deploys direct-current resistivity measurements using a vertical electrode chain that is permanently installed in a borehole, along with an automatic measuring instrument at the Earth's surface Prototypes of the system have been installed in the two water work areas on the North Sea island of Borkum.
The systems on Borkum were installed in 2009 at the two boreholes CLIWAT 1 und CLIWAT 2 into the transition zone between the fresh water lense and the saltwater-bearing layers in depths between 44 m and 65 m with an electrode distance of 25 cm. Since then, several times a day the instrument (4point light 10W) measures a full Wenner pseudosection. After the measurement, the data are remotely transferred to a server at LIAG. The power supply is realized by batteries that are recharged from a photovoltaik panel.
In some depths (e.g. 55.125 m on CLIWAT 2) there are significant resistivity changes between 2011 and 2017. The seasonal changes can be attributed to varying pumping rates in the neighboring wells and to different recharge rates.