Mapping of swelling and shrinking clays from airborne hyperspectral data: Presentation of a coming comparison of two approaches
Résumé
Swelling soils contain clay minerals that change volume with water content and cause extensive and expensive damages on infrastructures, including houses. This study aims to combine proximal (i.e. field measurement) and remote (i.e. airborne measurement) sensing to apply and optimize hyperspectral reflectance imaging for mapping clay minerals and soil properties, so that local mapping of swelling and shrinking clays susceptibility could be assessed from an economical point of view. Results and Discussion: Based on spatial distribution of declared damages and existing geological maps, the BRGM published in 2010 a 1:50 000 swelling hazard map of France. This map was drawn using locally incomplete or imprecise geological data that does not permit mapping of smallest and isolated clays lithologies, particularly in places where house building plans are foreseen over high susceptibility areas. At local scale, characterization of soil properties and identification of clay minerals using conventional soil analysis are slow, and expensive. VNIR and SWIR spectral domains are characterized by significant spectral absorption bands that provide a largely unexploited tool for identifying swelling clays. Hyperspectral sensing, used in a proximal way, using an ASD Fieldspec Pro spectrometer, provides a rapid and less expensive field surface sensing. The AGEOTHYP Research project financed by the BRGM, General Council of Loiret (45) and Géotec, which begin early 2013; aims to provide valuable information on the presence of clays over homogeneous and continuous scales. Airborne hyperspectral images will be acquired in the Loiret at the W of Orléans (France) in February 2013. The study area (ca. 280 km²) encompasses cultivated fields, urbanized areas, and forests. The three levels of clays hazard and many infrastructure damages recorded in the national database are also covered. Airborne hyperspectral imagery will contain 380 channels ranging from 400 to 2500 nm with a 1 m spatial resolution (VNIR) and 2 m spatial resolution (SWIR). Soil sampling will be used for calibration and validation. Soil samples will be taken on different land covers and on the three levels of expansive clays hazard. The reflectance measurements will be acquired on the field with the sun as the light source and in laboratory with a quartz halogen light source. The study area will be covered by standard laboratory analyses on soil samples in order to determine their mineralogy and swelling potential. Conclusion: All the geospatial data collected during the project will be integrated in a GIS project. Spatial data analysis will be carried out on the study area and the final high resolution map will be compared with the national swelling hazard map. Novelty Statement: This project is to use different types of data (geotechnical databases, hyperspectral field, and airborne measurements) that are integrated into an operational method to detect and quantify swelling potential of soils. Summary: The AGEOTHYP Research project plans to combine hyperspectral proximal and remote sensing with geotechnical engineering to identify and map swelling and shrinking clays on a study area covering an area of approximately 280 km² located to the west of Orléans (France). Hyperspectral images will cover cultivated areas, urbanized areas, forest, grassland, and Loire River. The analysis of the Hyperspectral images currently provides a largely unexploited tool for recognize swelling minerals. Therefore, we could expect various applications on other domains like agriculture, forestry, water...