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

The RESPER probe has been assembled, except the ferrules that must ensure the contact, by insulating materials and more specifically in Tufnol, as regards the support plates, and Teflon, as regards the standoffs. A series of holes has been drilled on the surface of support plates in order to allow approaching each other of the two central electrodes to external ones, from a minimum of 4.29 cm to a maximum of 10 cm. The dipole-dipole array defined by the integer parameter n = 6 could not be implemented, as the positioning of suitable “spring” shafts requires 6 mm holes and an adequate space could not be available to carry out the drilling. The presence of these “spring” shafts allows reaching a right prominence of the tip from the base and, at the same time, a certain amount of pressure which ensures the proper adherence to the artifact that must be tested. There is a brass screw within each shaft, which edge has been turned to the measurement of 1.4 mm. Four metal spacers are replaced of time in time depending on the configuration that is to be adopted. The achievement of height l(n) from the plates is ensured by the presence of metallic spacers. To complete the description, a copper cable, 1 m long and with a 4.0 mm2 section area, has been welded to the head of each electrode. The two transmitting or reading cables are kept at a fixed distance L(n) between them by means of Teflon rods in which has been applied the same series of holes existing on the Tufnol plates.

💡 Research Summary

The paper presents a comprehensive account of the preliminary design, construction, and theoretical validation of a RESPER (RESistivity‑PERmittivity) probe prototype intended for simultaneous measurement of electrical resistivity and relative dielectric permittivity in terrestrial and concrete media. The authors begin by reviewing the fundamentals of electrical spectroscopy, emphasizing that resistivity (ρ) and permittivity (ε_r) are independent parameters that can be extracted from the transfer impedance measured with a four‑electrode configuration, even when the electrodes are coupled capacitively rather than galvanically. They discuss the practical frequency window (approximately 10 kHz to 1 MHz) within which the probe can operate, noting the limiting effects of the Maxwell‑Wagner polarization at low frequencies and the need for sufficient current amplitude at higher frequencies due to the capacitive coupling between electrodes and the investigated medium.

A detailed theoretical model is developed for both galvanic and capacitive contact scenarios. For galvanic contact, the probe’s complex impedance Z_N is expressed as a parallel combination of a resistance R_N(L,σ) that depends on electrode spacing L and medium conductivity σ, and a capacitance C_N(L,ε_r) that depends on L and the medium’s permittivity. The authors derive the cutoff frequency f_T = σ /