Vertical electric sounding (VES): Schlumberger and Wenner method

Hi Steemians. There are several specific geophysical techniques to perform these measurements by means of a technique called Vertical Electrical Probes (SEV) by means of suitable instruments. At the end of the measurement, the results have to be interpreted to produce an equivalent model for the electrical performance of the earth, which allows adequate land modeling (according to the thickness of the layers), by means of one of the methods of scientific interpretation known as Interpretation by means of pattern curve.

Vertical electrical sounding (VES) is a geophysical method for investigation of a geological medium. The method is based on the estimation of the electrical conductivity or resistivity of the medium. The estimation is performed based on the measurement of voltage of electrical field induced by the distant grounded electrodes (current electrodes).

Soil resistivity measurement
Current Geoelectric methods use four electrodes which are buried in horizontal terrain, where a couple of them inject electric current into the ground and the other pair measure the potential difference between them as a result of the passage of the electric current they determine the depths of the subsoil layers, with the objective of knowing the geological characteristics of the subsoil or the Measurement of Resistance of Grounding.

There are two methods that allow the measurement of resistivity in a flat terrain, named respectively Wenner and Schlumberger.

Source

In the Wenner method, four electrodes are buried in a flat terrain in a straight line, each one separated at the same distance and these must move at the same distance until reaching the desired measurements of terrain. The electrodes are evenly spaced in a line. The spacing will be where a = L1 = L2 = L3. By Ohm's law, the ratio of V/I of the result of the apparent resistivity remains as:

ρ = 2 * π * a * R (1)

For the Schlumberger method, four electrodes are buried in a flat ground in a straight line and the central electrodes are kept fixed in the same distance and the electrodes A and B of Figure 1 move at the same distance, but at different distances from the electrodes. Central electrodes M and N. It is obtained from L1 = L3 = by L2 = a. For this configuration the apparent resistivity graphs are generated as a function of the distance L1, L2, and L3 between the measurement center and the current electrodes, then L - a / 2 and L + a / 2. Therefore, the Equation for the Schlumberger configuration, is as follows:

ρ = π* R*a {(L / a)2 - 1/ 4)} (2)

Source

References

Orellana y Mooney (1966). Master Tables and Curves for Vertical Electrical Sounding over layered structures. Madrid, España: Interciencia.

A. A. R. Zohdy (1989). A new method for the automatic interpretation of Schlumberger and Wenner sounding curves.

Jorge Carranza (1994). Tendencias tecnológicas en mallas de tierra. V Seminario de Electrónica de Potencia. Universidad de Concepción. Departamento de Ingeniería Eléctrica.