Measuring instruments, geophysical instruments

Earth resistivity

Among the first methods to be used in the history of applied geophysics, geoelectrical methods have evolved greatly over nearly a century. In earth resistivity prospecting, the physical parameter that is determined is the electrical resistivity of the formations which make up the sub-soil and which can vary according to the lithotype, the degree of homogeneity, the degree of alteration and fracturing, the content in water and salts, the particle size etc. To measure the resistivity of the sub-soil where stratification is flat and parallel, the Vertical Electrical Sounding (VES) technique is used. This includes a series of resistivity measurements carried out with a gradually increasing distance between the current electrodes (A-B) and the potential electrodes (M-N), according to a device with four electrodes (the most commonly used include the Wenner four-pin and the Schlumberger four-pin spreadings). Through the two current electrodes (A and B), a direct current (normally supplied by rechargeable accumulators or a generator), switched periodically to prevent polarisation phenomena on the electrodes, is introduced into the ground. The two measuring electrodes (M and N) measure the difference in potential generated in the sub-soil when the current passes between A and B. The measurements of the potential difference (AV) and current intensity (AI) are carried out using instruments called earth resistivity meters. To extend the resistivity measurements to gradually deeper layers, the distance between the A-B and M-N electrodes is increased. In this way, the current lines cross portions of the sub-soil that become increasingly deep. Plotting the apparent resistivity values on a bi-logarithmic diagram builds the apparent resistivity curves in which the values ?? of AB/2 on the abscissa are expressed in metres, while the resistivity values, on the ordinates, are expressed in Ohm/m. Using suitable inversion software, the apparent resistivity curves obtained in this way can be traced back to the true resistivity values and the thicknesses of the electro-coating electro-strata, thus obtaining valuable information especially when searching for groundwater.