Optical Signatures of Buried Mines

Soils can be disturbed by the burial of mines. When disturbed, the soils have a smaller effective size distribution than when undisturbed and this difference can be detected optically. Apparent emissivity differences in the thermal infrared is an effective passive optical technique developed by others for discriminating disturbed silicate soils from undisturbed soils and appears particularly sensitive to the presence of grains less than about 60 μm in diameter. It is not, however, an effective means for detecting grain size variations in carbonate sands. Optical techniques that may prove less sensitive to composition include 1) Changes in the reflectance in the shortwave infrared outside of absorption features nominally near 2.4 μm, and 2) Reflectance changes due to photometric backscattering by particulate surfaces. The reflectance of particulate surfaces increases with decreasing phase angle, peaking sharply at opposition when the illumination and detector are collocated (0o phase angle). For a given high-albedo material, finer-grains are brighter than larger-grained powders when observed at