Interpretation of an airborne geophysical survey in southern Paris Basin: towards a lithological cartography, key tool for the management of shrinking/swelling clay problems
Résumé
Regolith formations support a full spectrum of human activities. Among others, they provide a source of extractable materials and form the substratum of soils. As such, they should be considered as a capital to be managed and protected. Moreover, one of the main challenges for present and future land settlement is to prevent house building programs from being planned inside shrink-swell risky areas which is only possible thanks to an complete lithological mapping of the french regolith. We illustrate here the results of the geological interpretation of an airborne geophysical survey carried out in "Région Centre" administrative region in the southern part of the Paris Basin, in France. Among other techniques, airborne geophysics is appropriate to quickly provide information on near surface, because of i) its high spatial coverage ii) the rapidity of acquisition and iii) the variety of available sensors (magnetic, spectral radiometry, electromagnetic...). Spectral radiometry data were collected with a line spacing of 1 km. This method provides maps of potassium (K), uranium (U) and thorium (Th) which are the only naturally occurring elements with direct or indirect radioisotopes that produce gamma rays of sufficient intensity to be measured at airborne survey heights. On the radiometric data we applied the HAC (Hierarchical Ascendant Classification) computation procedure: taking into account several variables, the statistical HAC method groups individuals based on their resemblance. Also in this study, calibrated Total Count channel (TCm) is compared to an estimated dose rate (TCe) computed from the measured radioelement abundances: TCe = 13.078 * K + 5.675 * U + 2.494 * Th. Our results show that the ratio TCe/TCm came out to be a good indicator of ground property changes within Sologne mixed sandy-clay environment. Processed geophysical data are cross-checked with geological data (from field observations) and field or laboratory measurements of mineralogical data from X-Ray diffractometry and hyperspectral method. Our study demonstrates that in the silico-clastic basin of Région Centre, aerial gamma-ray data mostly reflect concentrations of radiogenic elements in clay minerals. Joint analysis of above mentioned data allow the discrimination in clay mineral nature. A detailed interpretation of gamma-ray data allows broad lithological changes to be readily mapped at 1/100 000 scale. Despite the former 1/50 000 scale geological mapping, the geological interpretation of airborne data led to the discovery of several unknown geological units. Our study demonstrates the consistence of airborne geophysical methods for geological prospection, especially in forested and hardily accessible area. The lithological mapping of the sedimentary units is the angular stone for the management of the shrinking/swelling clay problem. We demonstrate here that 90% of cracked houses of "Région Centre" can be located thanks to the TCe/TCm ratio algorithm.