Development of a vertical electrode chain
Background
Geoelectric mapping and logging at the surface are frequently unable to resolve thin beds with the necessary precision. Strongly conductive intermediate beds often hide the electrical properties of the underlying beds. In these cases, and in sediment in particular, vertical electrode strings can be of major assistance.
Design
A vertical electrode string for shallow surveying needs to meet the following criteria:
- Maintaining electrode separations
- Long-term stability of the electrode connections and cable (permanent observations)
- Easy handling
The first version had stainless electrodes in the form of rings fixed to a plastic rod. The cables were fixed to the electrodes with stainless steel rivets. The advantage of this version was the easy transport of the eight-metre long unit equipped with 32 electrodes. The disadvantage is the uncased design which means that the cables and the electrode connections were exposed to the soil and the water.
This led to the development of a second version with the cables and connections lying inside a PVC pipe to protect them from the environment. Two cables are connected to each electrode and can be used to test the status of the electrode. The built-in steel cable enables the vertical electrode string to be pulled out when necessary.
Installation
The vertical electrode string can be run into holes drilled by small bore drilling equipment. The string is broken down into 2.70 m sections for transport. It is put together on site and lowered into the borehole. A plastic pipe is pushed into the borehole first to prevent the sides from caving in. This pipe is removed when logging has been completed.
Logging
Logging can be done with the usual equipment. The log here was done with a RESECS device (Geoserve). The picture shows an area at Schwerin-Parchim airport contaminated with kerosene.
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