Geoelectric Method

Multi-electrodes Geoelectric, Electric Resistivity Tomography (ERT), Depth Soundings, Electromagnetics (EM)

The whole area below the earth's surface remains hidden to our eyes, since visual light is not able to penetrate the ground. But nevertheless this zone is of high economic interest.  To still get a picture of the structures and features of the subsurface it is possible to "view" the subsurface by using electric current instead of visual light. Electric current can penetrate the ground without problems. But to view the distribution of the electric current, which reflects the properties of the subsurface, it is necessary to "look on" (measure) the variations of the resulting electric potentials.  Instead of the human eyes therefore one needs a very sensitive voltmeter (we use a voltmeter from the german faktory LGM ) to "view" this electric potential distribution. But now, the resulting picture from the subsurface is different to a visible image. Instead of bright and dark areas or colors one sees here (after additional necessary processing steps) high and low electrical resistivities or polarizabilities of the subsoil which can finally be transformed to a visual image.

The higher the amount of intelligent distributed, precise measurement

data can be collected, the more detailed the analysis can be. Our well trained KNG-team, gathering the field data set, assures that the measured data set is of highest quality. The team on site supervises the automatic measurement, so it is always possible to identify problems quickly and solve on the spot. Our team works with the highest possible care to assure that the basis of every evaluation, namely the readings, are as reliable as possible.

Since the electrical resistances aren't directly visible, the measured resistance values must be changed to a picture "for the eye" again. High or low resistivities can be represented by colors.

An allocation of the resistivities to specific color ranges supplies a substantially more detailed picture, since a larger number of resistivity values can be represented. Now we have got the first result: a PSEUDOSECTION of the data.   But this pseudosection hasn't got any informations about depth or mightiness of layers. To get such informations we have to invert the data. 

The second important step is the evaluation, which enables us to get those wanted informations about depth and mightiness. Now the evaluator has to decide, which special evaluation method he will use to analyse the data to receive a subsurface picture as realistic as possible. Technical competence, experience and cross-disciplinary considerations are our strength to interpret the data despite possible ambiguities (e.g.: several formations can have similar resistivities; also disturbances in the underground or on the earth's surface, etc.), appropriately. It has been shown that the analysis of the data by the  high-quality-geoelectric-analysis-method, developed by aoProf.Dr. E. Niesner, Montan University of Leoben, in our experiences provides the best results in regard to the reflection of the subsurface structure .

Various rock formations have different electrical resistivities. Thus different resistivities (colors) in the resulting picture indicate different rock types. Though one receives a picture of the lithological structure of the subsurface. Beside different rock types also the content of water, the porosity, the tortuosity (the complexness of the pore network) affect among some other factors the electrical resistivities. Therefore additional to lithology characterisation also informations on the rock properties and the water saturation can be deduced.

The figure below shows the analysis of a geo-electrical measurement by the "Niesner method", an "image" of the distribution of the subsurface resistivities. In the subsequent interpretation of the colors in this case the lower resistivities (blue colors) correlate with water-bearing rocks and the higher resistivities (red colors) show compact rock.

In the geological specialized processing (→ by high-quality-geoelectric-analysis-evaluation) this result is then interpreted in the most realistic ground situation and yet re-adjusted by subsequent reference borehole profile.

This image shows a vertical section through the subsurface, in this case down to a maximum depth of 30 meters.

With this method it is possible to get subsurface information in a typical range from one meter or even below (e.g. for archeology) to several hundred meters depth.