La prospection : évolution de la sous-discipline, évolution du métier

Les méthodes géophysiques utilisées en archéologie s’insèrent dans le cadre général d’une démarche, celle de la prospection archéologique, où la mise en relation des différentes informations passe aujourd’hui par l’utilisation d’un système d’information géographique. Les premiers chercheurs spécialisés à plein temps dans cette voie s’y sont engagés à la fin des années cinquante. Les méthodes mises en œuvre au sol sont principalement basées sur les propriétés électromagnétiques : conductivité électrique, permittivité diélectrique et susceptibilité magnétique complexe. La mécanisation des mesures permet aujourd’hui la couverture de grandes surfaces à maille fine. Les développements actuels et futurs portent sur des acquisitions et une interprétation multi-propriétés en 3D.The application of geophysical survey methods fits into the global archeological prospection approach with the present use of Geographic Information Systems. The first full time researchers began their work at the end of the fifties. Ground methods are based on the measurement of electromagnetic properties: electrical conductivity, permittivity, complex magnetic susceptibility. The improvement of the method through mechanization now allows the coverage of great surfaces with a fine meshing. Recent and future progresses address multi-property acquisitions and 3D interpretation

Albert Hesse: life and work (1938-2022)

Albert Hesse was one of the pioneers in archaeometry in France. He has developed the electrical methods, and more specifically the continuous electrical surveying methods and focused very early on urban archaeology and geophysics. This paper focusses on his work, showing specific interactive maps to highlight the position of his different surveys world-wide

Perrot, J., (dir.), 2010. Le palais de Darius à Suse. Une résidence royale sur la route de Persépolis à Babylone

Ce livre magnifique tant par ses illustrations qui en font un ouvrage d’art que par son propos historique et documentaire, présente les résultats acquis sur le site de Suse par la Délégation Archéologique Française en Iran entre 1969 et 1979 sous la direction de Jean Perrot, avant que l’évolution de la situation politique n’entraîne l’arrêt des missions. Sous un titre pertinent mais plutôt limitatif il replace l’ensemble des découvertes réalisées à Suse dans le cadre de la civilisation achémé..

Perrot, J., (dir.), 2010. Le palais de Darius à Suse. Une résidence royale sur la route de Persépolis à Babylone

Ce livre magnifique tant par ses illustrations qui en font un ouvrage d’art que par son propos historique et documentaire, présente les résultats acquis sur le site de Suse par la Délégation Archéologique Française en Iran entre 1969 et 1979 sous la direction de Jean Perrot, avant que l’évolution de la situation politique n’entraîne l’arrêt des missions. Sous un titre pertinent mais plutôt limitatif il replace l’ensemble des découvertes réalisées à Suse dans le cadre de la civilisation achémé..

Can electrical resistivity tomography describe soil structure evolution?

Variations of soil structure is significant for the understanding of water and gas transfer in soil profiles. In the context of arable land, soil structure can be compacted due to either agriculture operation (wheel tracks), or hardsetting and crusting processes. As a consequence, soil porosity is reduced which may lead to decrease water infiltration and to anoxic conditions. Porosity can be increased by cracks formation due to swelling and shrinking phenomenon. We present here a laboratory experiment based on soil electrical characteristics. Electrical resistivity allows a non destructive three dimensional and dynamical analysis of the soil structure. Our main objective is to detect cracks in the soil. Cracks form an electrical resistant object and the contrast of resistivity between air and soil is large enough to be detected. Our sample is an undisturbed soil block 240mm*170mm*160mm with an initial structure compacted by wheel traffic. Successive artificial cracks are generated. Electrodes built with 2 mm ceramic cups permit a good electrical contact at the soil surface whatever its water content. They are installed 15 mm apart and the electrical resistivity is monitored using a dipole-dipole and wenner multi-electrodes 2D imaging method which gives a picture of the subsurface resistivity. The interpreted resistivity sections show the major soil structure. The electrical response changes with the cracks formation. The structure information extracted from the electrical map is in good agreement with the artificially man-made cracks. These first results demonstrate the relevance of high resolution electrical imaging of the soil profile. Further experiments need to be carried out in order to monitor natural soil structure evolution during wetting-drying cycles

Soil cracks detection by 3D electrical resistivity.

Soil cracks, whose formation are associated to natural climate phenomena such as swelling and shrinking, play an important role in water and gas transfers. Up to now, their 3D structure was characterised either by serial sections (Cousin, 1996) which is a destructive technique or X-ray tomography (Macedo et al., 1998) which is applicable on limited size sample. Three-dimensional electrical resistivity prospecting enables now to monitor crack development and to characterise their geometry without any destruction of the medium under study. Three-dimensional electrical resistivity surveys are commonly gathered by a network of in-line survey arrays, such as Wenner, Schlummberg, or dipole-dipole (Xu and Noel, 1993; Zhou et al., 2002). As emphasized by Meheni et al. (1996) the resulting apparent resistivity maps are often different depending on the array orientation related to an electrical discontinuity. Chambers et al. (2002) underline that in heterogeneous medium 3D electrical resistivity model resolution was sensitive to electrode configuration orientation. Indeed asymmetric bodies or anisotropic material exhibit different behaviours depending on whether the current passes through them in one direction or in another (Scollar et al., 1990). It would be all the more true for medium having very contrasted resistivities like cracking soil. In that case the electrical current does not encounter the same resistance when it passes perpendicular or parallel to the resistant bodies. Measurements of apparent resistivity depend then on the location and orientation of the current source relative to the body under study (Bibby, 1986). Studies conducted by Habberjam and Watkins (1967) emphasized that the square array provide a measurement of resistivity less orientationally dependent than that given by a in-line array investigation. Intending to lead a more 3D accurate inversion, we have chosen to focus our attention on a 3D electrical resistivity data acquisition. We present here a three-dimensional electrical survey carried out by a square array quadripole for characterising the soil cracks network developing during a desiccation period

Electrical resistivity survey in soil science: a review .

Electrical resistivity of the soil can be considered as a proxy for the spatial and temporal variability of many other soil physical properties (i.e. structure, water content, or fluid composition). Because the method is non-destructive and very sensitive, it offers a very attractive tool for describing the subsurface properties without digging. It has been already applied in various contexts like: groundwater exploration, landfill and solute transfer delineation, agronomical management by identifying areas of excessive compaction or soil horizon thickness and bedrock depth, and at least assessing the soil hydrological properties. The surveys, depending on the areas heterogeneities can be performed in one-, two- or three-dimensions and also at different scales resolution from the centimetric scale to the regional scale. In this review, based on many electrical resistivity surveys, we expose the theory and the basic principles of the method, we overview the variation of electrical resistivity as a function of soil properties, we listed the main electrical device to performed one-, two- or three-dimensional surveys, and explain the basic principles of the data interpretation. At least, we discuss the main advantages and limits of the method

Interpretation of shallow electromagnetic instruments resistivity and magnetic susceptibility measurements using rapid 1D/3D inversion

International audienceWe have developed an inversion process of electromagnetic induction (EMI) data based on a two-step approach with 1D inversion of the entire studied surface and a fast 3D inversion applied over limited areas. This process is similar to that formerly used in resistivity prospection. For the study of soil (environmental, engineering, or archaeological explorations), low-frequency electromagnetic instruments (referred to as Slingram EMI) have highly useful specificities. They are light, are easy to move in the field, and can simultaneously measure the ground’s electric conductivity and magnetic susceptibility; they have thus been used to map these properties over large surface areas, within relatively short periods of time, and at reasonable expense. The possibility of combining several coil geometries has opened up the potential for multidepth techniques and systematic 1D inversion, which are found to be sufficiently revealing to allow larger portions of surveyed areas to be analyzed. In the “targeted areas” selected for 3D inversion, the geometries of the 3D features and the resistivity and/or susceptibility contrasts are determined. This step is based on the method of moments, where only 3D heterogeneities are meshed, and only a small number of major characteristics, such as contrast, thickness, width, etc., are sought. We first applied this process to synthetic data, then to data acquired at an experimental test site, and finally to field cases. The rapid 3D inversion complements the 1D inversion by solving a series of issues: correction for the apparent anisotropy generated by the instrument configuration, multiarched anomalies, precise location of lateral changes, and determination of the properties contrasts. Our inversion results highlighted the importance of the instrument geometry. We also have determined that apparent magnetic susceptibility data can be more appropriate for the determination of the volume of man-made features and can be highly complementary to conductivity data

Stratégies d’approvisionnement en eau dans l’agglomération gallo-romaine de Cassinomagus (Chassenon, Charente)

Le but de cette étude était de retrouver les ressources en eau en contexte de métamorphisme de choc (astroblème de Rochechouart-Chassenon), de reconstituer l’organisation spatiale des structures gallo-romaines de Cassinomagus, implantées à Chassenon, et de tenter de comprendre les relations entre les constructions gallo-romaines et les ressources en eau. Nous avons répondu à cette problématique par la réalisation de prospections géophysiques. La réalisation de prospections électriques et magnétiques à maille fine, couplées à des mesures de susceptibilité magnétique, ont permis d’entrevoir l’extension du bâtiment thermal, l’agencement des quartiers privés à proximité du complexe monumental ainsi que l’extension du réseau hydraulique superficiel. Le traitement des sondages électromagnétiques à maille large a permis de produire des cartes de résistivité électrique pour différentes profondeurs de terrains ; ces cartes ont révélé la présence d’un aquifère dans les impactites et ont montré d’étroites relations entre la position du captage, des galeries drainantes et l’extension des formations aquifères.The aim of this study was to determine water resources in a shock-metamorphism context (Rochechouart-Chassenon meteorite crater), to investigate the spatial extent of Gallo-roman buried structures of Cassinomagus and to try to understand relationships between various Gallo-roman buildings and water resources. We have conformed to this problematic by the realisation of geophysical surveys. The magnetic and electric surveys and measures of magnetic susceptibility highlight archaeological buried structures as extension of thermae, the organization of domestic dwellings near the monumental complex and the extent of hydraulic structures. Electromagnetic soundings indicate the presence of a subsurface aquifer and reveal connexions between its geometry and the water catchment area