Geoelectric

      Geophysical methods

Electrical prospecting uses a large variety of techniques, each based on some different electrical property or characteristic of materials in the earth. There for this techniques are divided into three major categories; for resistivities, being a method with a large applicability and complex, there are subcategories retaliated.

     Electrical rezistivity

The 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. Most recently it has been applied in the search for geothermal power because subterranean steam affects the resistivity of formations in a way that can often be diagnostic.

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. These apparent resistivities are usually functions of a variable that is related to the depth of penetration. Consequently, the apparent resistivities are of important intuitive value and in practical quality control and interpretation procedures.

Depending on the type and complexity of the array used for determining apparent rezistivity, this method is classified in three major categories, also adding a special section with the theory for the different arrays.

Vertical Electrical Sounding (VES)

This method is by far the most used method for geo-electric 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 the 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.

This method is most useful in surveying at shallow depths (the first 300 m), but there were cases when the investigation depth was of thousands of meters. In cases like these powerful electric energy sources must be provided.

The principle of this method is to insert a 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 resistvity. 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.