Cartographie des géorisques karstiques à l'aide d'images radar application à l'Île d'Anticosti, Québec

Abstract

A significant of 10% of land surface constitute limestone, a fragile geological formation susceptible to rapid evolution. The evaluation of tools in recognising and analysing karst is therefore justified. The ability of imaging radar in measuring the intensity of karstification is of particular interest in this study.A method for mapping this georisk is developed. Interpretation of aerial photographs, representing the reality on the ground are compared with processed airborne radar images. In the latter, the karst is characterised by an important vertical hydrological flow. The correlation between the density of depressions and the length of waterways is poor. This due to the fact that onAnticosti island, several depressions are blocked after the glaciation and caused the numerous sinkholes to be pitched.A variable of the area occupied by the depressions is therefore taken into consideration. The images were analysed using a grid whose size is calculated based on the total number of depressions. While water surfaces, being good specular reflector, are easily spotted, pet bogs, acting as semi-specular reflectors, and the small sinkholes were often confounded with zones of low topographic backscatter present in the surrounding forest. The choice of the method of classification using a speckle-free image and texture analysis was therefore applied. The steep angle of the sensor create some important shadows areas witch used to get classified as water because both classes have similar numerical values. However, the results shows that a majority of the cells of the karstic intensity map of the radar image have a similar level of karstic intensity of those from the karstic intensity map made by airphoto interpretation.