Preliminary design of a RESPER probe prototype, configured in a multi dipole-dipole array

Electrical resistivity and relative dielectric permittivity are two independent physical properties which characterize the behaviour of bodies when these are excited by an electromagnetic field. The measurement of these properties provides crucial information regarding the practical use of bodies (for example, materials that conduct electricity), as well as numerous other purposes. Some studies have shown that the electrical resistivity and dielectric permittivity of a body can be obtained by measuring the transfer impedance using a system with four electrodes, although these electrodes do not require resistive contact with the investigated body [Grard, 1990a, b; Grard and Tabbagh, 1991; Tabbagh et al., 1993; Vannaroni et al., 2004; Del Vento and Vannaroni, 2005]. In this case, the current is made to circulate in the body by electric coupling, supplying the electrodes with an alternating electrical signal of low or middle frequency (LF-MF). In this type of investigation, the range of optimal frequencies for electrical resistivity values of the more common materials is between ≈10 kHz and ≈1 MHz. The lower limit is effectively imposed by two factors: a) firstly, the Maxwell-Wagner effect, which limits probe accuracy [Frolich, 1990], is the most important limitation and occurs because of interface polarization effects that are stronger at low frequencies, for example below 10 kHz depending on medium resistivity; b) secondly, the need to maintain the amplitude of the current at measurable levels because, given the capacitive coupling between electrodes and soil, the current magnitude is proportional to frequency. Conversely, the upper limit is fixed so as to permit analysis of the system under a regime of quasi static approximation, ignoring the factor of the velocity of the cables used for the electrode harness, which degrades the accuracy of the impedance phase measurements. It is therefore possible to make use of an analysis of the system in the LF and MF bands where the electrostatic term is significant. A general electromagnetic calculation produces lower values than a static one, and high resistivity reduces this differences. Consequently, above 1 MHz a general electromagnetic calculation must be preferred, while below 500 kHz a static calculation would be preferred, and between 500 kHz and 1 MHz both these methods could be applied [Tabbagh et al., 1993].