Periglacial morphogenesis in the Paris Basin: insight from geophysical survey and consequences for the fate of soil pollution.

International audienceGeophysical survey by of the Pierrelaye-Bessancourt area revealed conductive polygon patterns of 20-30 m diameter detected between 0.5 and 1.7 m depth. The patterns are formed by greenish glauconite and carbonated sand hollows where clay-rich pedological horizons bend downward, forming narrow tongs extending up to 2-3 m depth. Such structures were interpreted as a buried polygonal ice-wedge network. Geometrical relationships between the lithological units allowed the identification of successive landscape events and a landscape chronology. The sequence started during the Saalian glaciation with (1) development of patterned grounds by thermokarstic cryoturbation; (2) consecutive deflation/erosion during post-permafrost aridity; (3) loess and eolian sand deposits; (4) weathering of the former deposits with development of pedogenic horizons during the Eemian interglacial; (5) recurrent cryoturbation and thermal cracking leading to infolding of the pedogenic horizons during the Pleniglacial optimum (Weichselian); (5) finally erosion that levelled the periglacial microreliefs, leading to the modern landscape. In this agricultural area, urban waste water has been spread and has led to high levels of metal pollution in the surface horizons of the soils. The polygonal cryogenic structures have major impacts on soil hydrology and dispersion/distribution of heavy metals toward the geological substrate

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

Magnetic signal prospecting using multi parameter measurements

The magnetic signal of soils The magnetic signal of soils is divided between remanent (Jr) and induced magnetization (Ji) The former has diverse origins (heating, magnetic viscosity, slow deposition of magnetic bulk in a magnetic field) and is proof of the undisturbed state of features. The other, Ji, is acquired in the terrestrial magnetic field and is governed by magnetic susceptibility (κ). But the behavior of this property can be complex when the gain or loss of induced magnetization is d..

Etude d'évaluation du risque de mobilité des métaux dans l'agrosystème contaminé de Pierrelaye-Bessancourt : prospections du site, fonctionnement des sols et tests de lixiviation.

Ce sont les mécanismes de fixation et de libération des métaux dans ces sols pollués qui ont été approchés. Dans ce sens, l'étude se veut un outil d'aide à la décision pour les aménagements futurs du territoire. 1- Les de structures polygonales reconnues par géophysique conditionnent les modes et lieux d'infiltration des eaux. 2- L'irrigation a conduit à la déstabilisation des argiles des sols qui de ce fait ne jouent plus le rôle de rétention. Toutes les phases porteuses de métaux sont plus ou moins directement liées aux anciennes conditions d'hydromorphie (irrigation). L'arrêt de l'irrigation va déstabiliser progressivement ces complexes porteurs et libérer métaux retenus. 3- Des antibiotiques ont été mis en évidence dans les sols. Ils sont susceptibles d'agir sur les microorganismes des sols, et de ce fait présenter des risques environnementaux. 4- Les tests de lixiviation (lessivage/mobilisation) montrent la sensibilité des métaux à l'acidification du milieu et à l'oxydation de la matière organique. En raison de l'arrêt de l'irrigation il y aura accélération de la migration des métaux. 5- Les options de réaménagement sont discutées sous le regard sols/paysages : (1) en cas de boisement, l'acidification entraînera leur évacuation ; (2) les sols en place génèrent des poussières (dispersion dans l'environnement et inhalation humaine) ; (3) La dispersion incontrôlée des terres lors de terrassements est certainement un risque majeur

Magnetic signal prospecting using multi parameter measurements

The magnetic signal of soils The magnetic signal of soils is divided between remanent (Jr) and induced magnetization (Ji) The former has diverse origins (heating, magnetic viscosity, slow deposition of magnetic bulk in a magnetic field) and is proof of the undisturbed state of features. The other, Ji, is acquired in the terrestrial magnetic field and is governed by magnetic susceptibility (κ). But the behavior of this property can be complex when the gain or loss of induced magnetization is d..

Strategies for using magnetic susceptibility to reconstruct ancient landscapes on a large scale: A case study from Persepolis (Central Fars, Iran)

International audienc

Strategies for using magnetic susceptibility to reconstruct ancient landscapes on a large scale: A case study from Persepolis (Central Fars, Iran)

International audienc

Strategies for using magnetic susceptibility to reconstruct ancient landscapes on a large scale: A case study from Persepolis (Central Fars, Iran)

International audienc

Investigating Magnetic Ghosts on an Early Middle Age Settlement: Comparison of Data from Stripped and Non-stripped Areas

International audienceStatute laws concerning preventative archaeology adopted in France in 2001 for threatened sites required development of evaluation techniques. The common methodology used during the past 10?years has been extensive excavation. This method delivers a direct record of the discovered archaeological features and of their distribution. However, the success of visual observation can be variable, depending on geological context and the feasibility of envisaging the entire archaeological settlement from the part that is observed. We report here a case study from an early Middle Age settlement located on a floodplain. It was partially excavated and the alluvial context makes the features difficult to recognize for archaeologists. We show how the geophysical survey complemented the archaeological survey and allowed the detection of previously unrecognized features. In a second step, we compare the results of surveys with magnetic and electromagnetic methods on a topsoil stripped area and the neighbouring surface. From this analysis we conclude that a significant part of the unrecognized features discovered were present above the level of stripping. This example underlines that the practice of topsoil stripping has to be reconsidered, and should be integrated with geophysical survey to limit the potential loss of archaeological information. Copyright (C) 2012 John Wiley & Sons, Ltd

Which EM Properties with Multi-frequency Low Induction Number Slingram Apparatus?

International audienceIn the 3 kHz-300 kHz frequency range, electromagnetic measurements achieved with limited size Slingram instruments are not only sensitive to the electrical resistivity but also to the complex magnetic susceptibility and to the electrical polarization. A process allowing the determination of all the properties is proposed and applied to case studies with the use of commercial instruments