The influence of clay and water content in the electrical conductivity of rocks and soils has been experimentally established and is expressed by simple empirical laws: the Archie's law and the addition law between volume water conductivity and surface shale conductivity. Two independent numerical modeling techniques, the moment method and the finite difference method, are presented here and are used, first, to verify the agreement between Maxwell's equation based theoretical approaches and the empirical laws and, second, to begin to investigate for a possible effect of the microscopic geometry over macroscopic conductivity. A good agreement between simulation results and Archie's law is obtained when both randomly distributed isotropic and elongated microvolumes of conducting water are considered and a slight difference appears between these two microstructures. For low clay contents in clay‐dispersed media, the clay‐associated conductivity is shown to be proportional to a specific clay area, which is in good agreement with the addition empirical law.

La prospection géophysique en milieu urbain souffre de différentes difficultés : vibrations, bruits électrique et électromagnétiques, structure superficielle complexe et présence d'ouvrages. Pour explorer les dix premiers mètres, la méthode électrostatique peut remplacer la méthode électrique à courant continu pour fournir une image des structures du sous-sol en utilisant les mêmes outils d'interprétation.

Electrical resistivity tomography was used in Beauce (France) to assess the water extraction by corn plants (evapotranspiration). The acquired pseudosections show conductive anomalies under the plants. A 2D inversion of measurements led us to identify clear resistive features associated with the water losses under the corn‐plant rows. New models have been calculated with two different 3D algorithms (finite‐difference and moment‐method) to take into account 3D structure of the ground and to confirm that periodic resistive features may generate shifted apparent‐resistivity anomalies.

Soundings achieved with the electrostatic method cannot be interpreted correctly with 1D electrical programs if the extension of the array or the conductivity of the ground are too large. A complete calculation taking into account the induction effect due to the frequency must be performed. This paper presents the solutions for overcoming the difficulties encountered in this calculation: the iterative processes which make it possible to determine the kernel functions of the Hankel transforms and the analytic integration of the terms of the electric field. Application to practical cases first illustrates the distortion of the electrostatic curves by reference to DC sounding curves. The limitation in depth of investigation is then emphasized: in practice, the investigation is limited by the skin depth corresponding to the frequency used. The examples of the soundings obtained in the city of Alexandria (Egypt) demonstrate the importance of using the complete calculation for very conductive grounds.

Since geophysical methods are non-invasive, they can be of great help in soil studies because they disturb neither the structure nor the dynamics of the soil. Moreover, data are acquired with reliable spatial sampling. The usual ways of investigation, like augering and excavation, disturb the soil and are totally incompatible with a spatially dense sampling strategy, which would destroy the object of the study. Both approaches are complementary when excavations have a limited extent and are distributed according to the information conveyed by the geophysical investigation. A basic principle of applied geophysics is to measure different physical parameters without direct access to the studied volume. Horizontal and/or vertical variations of the parameter(s) can be recorded. Possible soil parameters should be restricted to measurements which do not alter the medium (reversible effect). To be significant, the variations of the parameter(s) should exhibit a wide dynamic range over different soil types and should be correlated in some way to soil parameters such as particle size or hydraulic conductivity. After summarising the soil properties, two examples are shown whereby electrical resistivity was used. The first example is a specific soil so-called hardpan (sandy soil in arid area) in Lagadge, North Cameroon. Using resistivity surveys the three dimensional extension of a very coherent horizon was mapped. This horizon is delineated by low resistivities <100 Omega m (conductivities > 10 mS/m) because of the disposition of clay particles around the quartz grains. In a second example, a "homogeneous" area ought to be found delimit the extent of a surface where a pesticide transfer experiment is to take place. Accurate mapping of soil horizons was not feasible by augering. Resistivity data have clearly shown the three-dimensional extension of clayey horizons in the complex delta context. (C) 2000 Elsevier Science B.V. All rights reserved.

Résumé. La reconnaissance de la structure des sols urbains est aujourd'hui une préoccupation majeure pour des raisons environnementales, historiques et techniques. Ces sols sont caractérisés par leur grande complexité structurelle qui rend nécessaire une approche pluridisciplinaire. Les méthodes géophysiques permettent d'acquérir des informations de façon non destructive. Cependant, le contexte urbain (structures 3D superposées, espaces accessibles limités, perturbations électromagnétiques) limite considérablement le champ des méthodes adaptables à ce type de reconnaissance. Un protocole de mesure associant le radar-sol et la méthode électrostatique a été défini pour la reconnaissance et l'évaluation du patrimoine présent sur les sites historiques urbains. Nous présentons quelques exemples des résultats obtenus : étude de bâtis anciens sous les édifices actuels (la cathédrale de Gérone, Espagne), étude de fortifications (La Rochelle, France) et recherche de voies anciennes (l'Heptastade d'Alexandrie, Egypte).

As concerns the links between ancient Alexandria and its famous light house on the island of Pharos, Strabo describes the so-caIled Heptastadium as "an embankment (which) forms a bridge extending from the mainland to the western portion of the island and leaves open only two passages ... which are bridged over; ...this work formed ... also an acqueduct...". Alluvium deposits on which now lies a -large part of the present city of Alexandria, have completely masked the traces of the Heptastadium: its position in the large existing isthmus was, till now, entirely conjectural and, from the middle nineteenth century, its orientation was believed to be largely different from that one of the antique streets network.

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).

The latest generation of mobile electric and electrostatic arrays allows the measurement of the apparent resistivity over large areas with a high spatial sampling and for several depths of investigation. This paper presents three experiments undertaken over three archaeological sites of three different historical periods (Iron Age, Roman, medieval) and three different subsoil covers (meadow, ploughed field, asphalt). The archaeological structures (ditches, enclosures or walls) are described with a good accuracy. They are integrated in their environmental and geological context. Copyright © 1998 John Wiley & Sons, Ltd.

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.

The electrostatic niethod constitutes a generalization of the well known electrical method. As the eleetrostatic poles are located in the air, not far from the ground, the method can be used whatever the type of ground surface (tarmac, slabs, concrete,.,.). The developments during the last ten years of this method increased considerably the application field of the electrical prospecti.ng, especially for civil engineering and archaeological applications (Tabbagh et al., 1993, Panissod et aL, 1996). In urban areas, where classical electroznagnetic methods cannot be used because of the metallic feature disturbances, the electrostatic quadripole is a good device for the subsurface prospecting and gives complementary informations to those which can be obtamed with a ground penetrating radar. This paper presents complete resuits about electrostatic mapping and soundings on the geophysical test site of the LCPC (Laboratoire Central des Ponts et Chaussées, Centre de Nantes, France).

Many geophysical surveys performed for archaeology, pedology and environmental studies use electrical methods. Indeed, they are easy to handle, not to expensive, and give informative results. Among the different objectives that can be chosen when using multielectrodes systems, we adopt that which consists in combining measurements over a series of electrodes in order to obtain the best depth of investigation, the most isotropic result and to reject the superficial geophysical noise. This is achieved by transposing to surface measurement the so called ‘focusing’ concept developed in resistivity logging. We develop thus a new ‘potential focusing’ concept, the advantages of which are studied using both synthetic models and field trials over a test site and two archaeological sites. As a result of this study, we conclude that a very simple system, based on the pole-pole array and comprising a central electrode surrounded by four potential ones, is a good compromise to achieve this objective. By reference to the work performed forty years ago in resistivity logging, we conserve the word of focusing and the new array was named I.F.M.P.P. for Isotropic Focusing MultiPole-Pole.

Multimethod prospection at shallow depth emphasizes capabilities of detection through the different sensitivity of each method. A comparison between four 'integrative' methods (electrical/ electromagnetic) and one 'quasi-punctual' method (GPR) is performed to analyse individual efficiency with various physical or geometrical parameters. The chosen site is located at 'Le Gué Goujard' (Nièvre), known to exhibit shallow archeological features at depths rather equivalent to limestone geological settings. The different methods involved allow to distinction between them.