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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. '

Application of mobile electrical and electrostatic quadripoles during the past ten years has allowed a considerable increase in the size of the surveyed areas, together with keeping a high spatial resolution and a reduction of the total cost of a survey. Two new developments of towed arrays are illustrated here: (1) a pole‐pole array pulled by the operator provides a lightweight solution for mapping large surfaces at a unique given depth of investigation, as shown by the prospection of the Roman‐British city of Wroxeter; and (2) a multipole, multidepth system allows a 3-D investigation of the ground resistivity, as illustrated by the experiments undertaken on the test site of Garchy and on the archaeological site of Montbaron (Indre, France).

Three different improvements in the galvanic method of shallow‐depth electrical prospecting need to be made. (1) A better definition of the electrode array(s) is needed in order to reduce the “apparent anisotropy” effect and increase the depth of investigation; the square array is one solution. (2) A resistivity meter that can make measurements very rapidly needs to be built. The rapid decrease of the induced electric field for small electrode distances makes such measurements possible. We propose a resistivity meter with an 8 ms measurement time. (3) The electrode‐ground contact problem needs to be solved by using a resistivity meter that can endure high contact resistances. As an alternative solution, we also propose a new type of electrode: a liquid jet one. These improvements could make the resistivity method very attractive and competitive, and open large future developments for the method.

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