The purpose of electrical surveys is to determine the subsurface resistivity distribution by making measurements on the ground surface. From these measurements, the true resistivity of the subsurface can be estimated. The ground resistivity is related to various geological parameters such as the mineral and fluid content, porosity and degree of water saturation in the rock. Electrical resistivity surveys have been used for many decades in hydrogeological, mining and geotechnical investigations. More recently, it has been used for
environmental surveys.
The resistivity measurements are normally made by injecting current into the ground through two current electrodes (C1 and C2 in Figure 1), and measuring the resulting voltage difference at two potential electrodes (P1 and P2). From the current (I) and voltage (V) values, an apparent resistivity (pa) value is calculated.
Pa = K V/I
where k is the geometric factor which depends on the arrangement of the four electrodes. Figure 2 shows the common arrays used in resistivity surveys together with their geometric factors. In a later section, we will examine the advantages and disadvantages of some of these arrays.
Resistivity meters normally give a resistance value, R = V/I, so in practice the apparent resistivity value is calculated by, Klik This for the complete book or klik above image....
