DETERMINATION OF ELECTRICAL PROPERTIES OF THE GROUND AT SHALLOW DEPTH WITH AN ELECTROSTATIC QUADRUPOLE: FIELD TRIALS ON ARCHAEOLOGICAL SITES¹

Abstract

TABBAGH, A., HESSE, A. and GRARD R. 1993. Determination of electrical properties of the ground at shallow depth with an electrostatic quadrupole: field trials on archaeological sites. Geophysical Prospecting 41,579-597.Knowledge of both the electrical resistivity p and the dielectric permittivity E of the ground is important for the determination of characteristics such as granularity, porosity, moisture and salt content. Whereas the measurement of p is very common and can be achieved using either the d.c. resistivity method or a wide variety of EM devices, E remains practically unknown in the low-frequency domain between the IP domain and the highfrequency domain. Following the principle of the quadrupole technique used in d.c. prospecting and of the quadrupolar probe for measuring the complex permittivity of the ionosphere, we propose a new approach which does not require any galvanic contact between the poles and the ground. The transfer impedance can be evaluated using the quasi-static approximation for low frequency or the full EM theory for higher frequencies. The conditions under which both calculations apply are discussed. At low frequencies and for low resistivity ground, the electrostatic quadrupole measures p exactly as with the d.c. technique; for higher resistivities or frequencies the simultaneous measurement of both properties becomes possible. Examples in archaeological prospecting are presented and checked against independent d.c. resistivity measurements or excavation analyses. '

Fundamentals of the capacitive resistivity technique

“…For the selected frequency of 15 kHz and the range of parameter values considered in this study, we have found the variation of the complex potential with relative permittivity to be negligible in the range between ε r = 1 (free space) and ε r = 80 (water). Previous studies have shown that a measurable variation of the CR transfer impedance with ε r at audio frequencies can be obtained in highly resistive environments only (Grard and Tabbagh, 1991;Tabbagh et al, 1993).…”

“…However, as previously discussed, it is unlikely that reliable estimates of ε r can be obtained by low-frequency electromagnetic methods. Determination of ε r with the CR technique may be possible at higher frequencies and in highly resistive geological environments (Tabbagh et al, 1993).…”

“…Later studies, from the 1990s onward, include a variety of empirical, technical and theoretical information that has been incorporated into the current practical designs. Surface measurement systems include the electrostatic quadrupole as described by Grard and Tabbagh (1991), Tabbagh et al (1993) and Panissod et al (1998), the multichannel capacitive electrode system as described by Shima et al (1995;1996) and a number of commercial instruments (Møller, 2001;Lee et al, 2002;Pellerin et al, 2003;Chouteau et al, 2004). Borehole resistivity tools with capacitive sensors have been described by Bristow and Mwenifumbo (1998), Mwenifumbo and Bristow (1999) and Cottineau et al (2000).…”

Detection of abandoned mineshafts using towed-array capacitive resistivity and real-time kinematic GPS navigation

“…This "electrostatic formalism" yields a phase-sensitive expression for apparent resistivity which can be shown to reduce to the classical DC expression for the in-phase component of the transfer impedance (Tabbagh et al, 1993;Kuras, 2002):…”

Estimation of the vertical profile of the wheat crop water content with an electrostatic multipole. An experimental study

“…This article presents therefore a new instrument, an electrostatic multipole, to assess non-destructive and instantaneous estimations of plant water content. This instrument was first designed for geophysical prospecting, especially for archaeology [20]. It has recently been adapted for agronomic research and a first feasibility study has shown that this instrument is sensitive to plant water content [7].…”