Permeability and voids influence on the thermal signal, as inferred by multitemporal UAV-based infrared and visible images

This work presents for the first time associated thermal anomalies and deformations over subsurface cavities/voids located within two harbour quays in Le Havre Harbourg, Normandy, France. An U.A.V. (Unmanned Aerial Vehicle) was used to acquire visible and thermal images over a diurnal cycle (from 7 a.m. to 5 p.m.). The visible images were processed to realize an altimetric model of the platforms by mean of the photogrammetric method, while the thermal infrared (TIR) images were used to study the evolution of their surface temperatures. The obtained 3D model shows the location of five topographic depressions on both quays. The analysis of the evolution of the surface thermal field leads to the detection of cold thermal anomalies that are (1) not correlated to surface properties, and (2) spatially associated to the flanks of the five topographic depressions. Using a 2D conductive-radiative model, we show that these anomalies are not directly due to the thermally insulating effect of an air-filled area. Finally, we conclude that preferential infiltration and subsequent evaporation in the microfracturation present within the flanks of the depressions may be responsible for the strong cooling of these zones.