Electrical resistivity is a geophysical technique for studIyng sub-surfaces structures by electrical measurements made at the surface or by electrodes in one or more boreholes. It is closely related to the medical imaging technique called electrical impedance tomography (EIT), and mathematically it is the same inverse problem. In contrast with the medical EIT however ERT is essentially a direct current method. A related geophysical method, called induced polarization, measures the transient response. The technique evolved from techniques of electrical prospecting that predate digital computers where layers or anomalies were detected. As for other non-invasive surveys methods, the purpose of the geoelectric methods is to obtain information on the nature and structure of the subsurface through a series of measurements carried out on the surface.
The principle of the method is based on measurement of the electric field induced in the ground with suitable electrode devices, constituted by two pairs of electrodes fixed in the ground (configuration called “quadrupole”). In the quadrupole a pair of electrodes constitutes the circuit for the induction of the electric field in the ground (current dipole: conventionally electrodes A and B). The other pair (potential of dipole: M and N) represents the measurement circuit of potential difference (ddp) generated in the ground by the passage of the current itself. By Ohm‘s law (modified) the resistivity value, characteristic of these materials, can be determined (p). The value of the resistivity of the materials mainly depends on the following characteristics: degree of saturation of the pores; porosity; presence and composition of the fluids; temperature; possible presence of organic substances (hydrocarbons, solvents, etc.); clay; degree of compaction. Each type of soil or rocky body shows a wide range of variation of their resistivity values, which depend on the degree of homogeneity, the level of alteration and, in case of rocks, the degree of fracturing. In all conditions, the presence of water always plays an important role, so much that sandy-loamy or gravelly-sandy soil, if saturated, are more conductive than the same materials on dry conditions.
The basic components of the acquisition system are: transmitter or current source; receiver which measures the resulting electrode potentials; multiplexer for connecting quickly and automatically the electrodes to the transmitter and receiver and a computer for system control and data archival.
The instruments used for this method is fast multi-channel resistivity meter 10-channel system Syscal-Pro produced by IRIS Instruments). It allows the acquisition of several thousand data points in a few hours. Under typical conditions, for the Syscal-Pro operating at 4 Hz and stacking 4-5 times, it is possible to acquire data within about 1% error in the magnitude (as determined by comparing reciprocal measurements), at a rate of about 6-7000 measurements per hour.
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