INTERPRETATION OF THERMAL PROSPECTION ON BARE SOILS

Abstract

Thermal prospection, by which we measure the surface temperature of the ground, has interesting characteristics for archaeological research, especially in surveying large areas, since a thermography corresponds to several square kilometers. Like other surveying techniques, it has its limitations and a more complex interpretation of the results than electrical or magnetic prospecting. In addition, with organisational data processing and cost problems, the difficulties encountered in interpretation explain why this method has been of little use until now. Temperature can be measured by direct contact as well as from the radiation transmitted by the ground surface: but only a radiometer can be operational in the field, because in order to compare the points of the surface it is necessary that the temperature should not vary during the time of the survey, which is limited to a few minutes. In the presence of vegetation one does not measure the ground surface temperature but that of the plants, which is controlled by plant transpiration. Archaeological features may be very well detected by plant temperature, as in the case of Villeneuve la Guyard on the 4th May 1976 (Tabbagh 1979), but it is a non-direct detection and the interpretation needs knowledge about plant metabolism and soil conditions on which it depends. This knowledge is necessary in order to determine at which stage of plant development it will be interesting to make a survey. The interpretation of results for bare soils is a direct application of the physical laws of thermal exchanges. In previous papers several rules have been established (Tabbagh 1973 and 1977a): (1) soil temperature depends on solar radiation and exchanges with the atmosphere, (2) the diurnal variation of the flux of heat in the ground has a too low penetration depth to give a measurable anomaly linked to features lying below the cultivated superficial layer, (3) slow transient variations of the flux may induce anomalies revealing archaeological features.