Applications possibles de la télédétection à l'inventaire et à la caractérisation des ressources en eaux souterraines

Teledetection and groundwater resources

Professeur à l'Université de Montpellier II, Directeur du CERGH et de l'UER

Abstract

By its scanning field and repeatability, electromagnetic teledetection is revolutionizing methods and possibilities of listing and keeping a check on Earth's natural resources. In the case of the water cycle it already has revolutionized well established methods in use in meteorology, for current measurement and detection of pollution. The problem is more difficult in the case of ground water, where water can only be detected indirectly through its effects on the ground surface (temperature variation, effect on vegetation, etc.), and then only if such effects are sufficiently marked to be distinguishable from other electromagnetic energy emissions or absorptions related to lithology, incident sunlight, vegetation, etc. Data recording times have to be selected accordingly. Most of the factors governing or indicating the presence of water in the subsoil can be detected by direct visual observation and conventional photography from aircraft ; a major step forward has been made with multispectral recording methods whereby closer distinctions can be made between "spectral signatures", especially by establishing the intensity ratios in spectral bands of different wavelengths. The most promising technology, however, seems to be infra-red scanning radiometry (in the 3 -5 micron band, or better still, 8.5 - 14 microns - thermography) whereby one can easily distinguish between temperatures differing by as little as 0.2 °C. Owing to its high calorific value, water can appreciably affect the temperature of the rock or air with which it is in contact or of other water mixing with it. As a result of this, it is possible to detect underwater or underground springs, surface springs or losses to rivers, some types of underground drain, preferential infiltration zones, and in some cases (fissured limestone), to locate specific aquifers.