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.
The aim of this paper is to point out the advantages of multipoles for the exploration of the very near subsurface (0–3 m) by continuous profiling. We propose a new geometry with eight poles for a MUltipole Continuous Electrical Profiling (MUCEP) measuring system, where the array has a V‐shape and is thus called ‘Vol‐de‐canards’. A series of criteria including 3D numerical simulations are performed (direct and inverse modelling) to determine the optimal geometry and to compare its performance (in terms of depth of investigation and resolution of the geometry of the targets) with the other arrays (quadrupoles or rectangular‐type multipoles). This multipole was built together with a real‐time acquisition system. The multidepth maps obtained confirm the characteristics predicted by numerical simulations.
We try in this paper to characterize the magnetic behaviour of soils both in the frequency and time domain. Our aim is to understand the anomalous responses we have observed in many EM surveys.A wide range of soil samples have been analysed with two instruments specially designed for harmonic (80 Hz-10 kHz) and transient (8-100 p s after cut-off) EM low inducing fields (50 pT).The most common spectrum observed could be explained with the well-dispersed single-domain grain theory (SD): a logarithmic decrease of the in-phase susceptibility with frequency and a constant quadrature susceptibility. Moreover, measurements in the frequency domain of the quadrature susceptibility and measurements of the coefficient of viscosity in the time domain have been compared and found to be in good accordance with theory. But other types of magnetic spectrum were encountered and could not be accounted for SD theories. This results in a non-logarithmic dependence of the in-phase susceptibility, a non-constant quadrature susceptibility and in the time domain a coefficient of viscosity S which is time dependent.It is demonstrated that the response due to quadrature magnetic susceptibility in the frequency domain and the magnetic viscosity in the time domain is sufficiently strong to be responsible for the anomalous responses.It is now possible to correct in an EM survey for the soil magnetic effect for a better determination of resistivity or conductivity.
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