Geoelectrical resistivity method is designed to yield information on formations or bodies having anomalous electric conductivity and it has been used for a long time to map boundaries between layers having different conductivities. It is employed in engineering geophysics to map bedrock, for determining the homogeneity of the terrain and possible sinkholes, in groundwater studies to determine salinity and the depth to the water table.
It is convenient and customary for most resistivity techniques to define a response function called apparent resistivity (Roa), which can be evaluated or estimated from surface measurements and correlated with geological information, resulting maps and sections of the electrical resistivity variation correlated geologically.
Direct current (DC) geoelectrical methods are frequently used with good results to investigate the geological structure of water covered land (rivers, lakes).
GEOELECTRICAL RESISTIVITY METHODS
* APPLICATIONS OF GEOPHYSICAL INVESTIGATIONS:
- Archeological Surveying
- Borehole Investigation
- Determining Geological and Hydrological Conditions
- Lake and River Prospecting
- Mapping Unexploded Bombs (UXO)
- Minerals Exploration
- Research on Inorganic Contaminants
- Utility and Buried Objects Detection
- Wind Farms and Photovoltaic Sites
Vertical Electrical Sounding (VES)
Vertical Electrical Sounding (VES) is by far the most used method for geoelectric surveying, because it is one of the cheapest geophysical method and it gives very good results in many area of interest.
The field measurements technique is adjustable for the different topographic conditions and the interpretation of the data can be done with specialized software, with a primary interpretation immediately after the measurements. The results of VES measurements can be interpreted qualitatively as well as quantitatively. The quantitative interpretation, in the case of mineral exploration, can give the chance to evaluate the reserves in a certain area of interest.
The principle of this method is to insert an electric current, of known intensity, through the ground with the help of two electrodes (power electrodes – AB) and measuring the electric potential difference with another two electrodes (measuring electrodes – MN). The investigation depth is proportional with the distance between the power electrodes.
Vertical electric sounding method, as all geophysical methods, needs for a good interpretation the presence of a contrast in resistivity. As the contrast is greater, the results are better.
The technology is applicable on rivers and lakes also, making it possible to find out the geology of under water terrain. This method has proven efficient in designing vertical guided boreholes and other engineer projects.
The electric resistivity profiling method uses the same type of arrays as in the vertical electric sounding (VES), just that in this case the interest is concentrated on several levels of depth. There for, there are arrays conceived with fixed distances between the electrodes so the measuring technology becomes simple and fast, the number of operators is smaller, the costs are smaller and the results can be visible immediately.
Geoelectrical profiling is ideal when we need to survey for shallow depth ground properties but on long distances. It is successfully used in determining soil contamination (for pipe protection, in agriculture, etc.), determining depth to bedrock, discontinuities in mechanic properties of rocks (useful in designing pipe and cable diggings), and tracking of pollutants. The method has new applicability in engineering geophysics.
Geoelectrical tomography can be considered the lead technique in geoelectrical measurements. This method combines modern techniques of data acquisition with performing interpretation solutions.
The data acquisition is made uniform along the profile with a density established by the distance between the electrodes and the type of array used. With one length of the multielectrod cable we can obtain hundreds of resistivity measurements thus creating a 2D image of the underground resembling a tomography. This data sets are afterwards filtered and processed with a specialized software that does a 2D inversion, unlike the other methods, thus driving to the best results possible.
The high density of measuring points that are achieved with just one layout of the array make this method very useful in engineering projects. The resolution of the geoelectric tomography is very good thus making possible pointing out diverse objects in the ground.
Using special configurations of the measuring arrays, 3D images of the ground resistivity can be obtained thus making it very easy to visualize underground features (archaeological, construction or natural features).