Geoelectrics
Geoelectric measurement in the field
Source: BGR
Geoelectrics is one of the classical methods of exploration geophysics. Main applications are groundwater investigation and soil-scientific studies, but the method also plays an important role in the search for cavities and faults and is applied for archaeological studies and in borehole measurements.
Principle of a Schlumberger Sounding
In Geoelectrics, a direct current of strength I is injected into the subsurface at two points A and B and the electrical potential U generated thereby is measured between two other points M and N. From the two measured values I and U and a factor K calculated from the distances of the four electrodes A, B, M and N the so-called apparent electrical resistivity rho (a), the reciprocal of the electrical conductivity, is calculated.
During a measurement, the distances between the current electrodes are systematically increased and the apparent electrical resistivity is determined as a function of the electrode distances AB. The result of the measurement is the apparent electrical resistivity Rho (a) as a function of the electrode spacing. Its color-coded graphical representation is referred to as pseudosection.
In case of a homogeneous subsurface the apparent electrical resistivity rho (a) is identical to the electrical resistivity. However, such ideal conditions can hardly be found in nature. The model that geophysicists mostly use is therefore the layered subsurface.
Pseudosection of an Wenner-Alpha configuration and representation of the apparent influence of resistance. The contrast between the copper slate flute and the bedrock can be clearly seen here (Röhrig mine, Wettelrode)
Source: BGR
For the interpretation of the measurement the rho (a) data are “inverted” , often also called inverse modeling. The aim of the inversion is to determine a model of the resistivity of the subsurface, that fits the measured data as best as possible (accounting for measurement errors). The method is inherently ambiguous. There are many (uncountable) models that fit the data "equally well". Additional information about the subsurface is important, through drilling, plausibility assumptions or experiences, in order to select the most suitable one from the multitude of models. Interdisciplinary and transdisciplinary cooperation with the task-specific disciplines such as geology, mineralogy, hydrogeology, soil science, archaeology, forestry etc. is particularly important for this decision-making.
Geoelectric measurement
Source: BGR
In practical use, linear or 3D- electrode arrays are often used, i.e. many steel electrode are hammered into the subsurface along a profile or in a defined area, which can be used both as current and voltage measurement electrodes. It is therefore possible to measure various electrode configurations without changing the position of the electrodes. Usually, soundings are carried out, that means primarily the changes in the resistivity with depth is studied. However, there are also electrode arrangements that provide detailed information about the lateral resistivity changes
Thanks to technical developments, it is now possible to measure a few hundred electrodes and a few thousand electrode configurations in less than one hour, which also means that subsurface flow processes can be observed.
A very descriptive and helpful tutorial on how to perform resistivity soundings was written by H. Flathe and W. Leibold already a few decades ago; it can be downloaded here:
Projects:
Groundwater Geophysics in Burundi to support the Technical Cooperation Project
Additional information on groundwater geophysics can be found here.
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