INFRARED MEASUREMENTS OF PRISTINE AND DISTURBED SOILS 1 - SPECTRAL CONTRAST DIFFERENCES BETWEEN FIELD AND LABORATORY DATA

Comparison of emissivity spectra (8-13 mu m) of pristine soils in the field with laboratory reflectance spectra of the same soils showed tha t laboratory spectra tend to have less spectral contrast than field sp ectra (see following article). We investigated this phenomenon by meas uring emission spectra of both undisturbed (in situ) and disturbed soi ls (prepared as if for transport to the laboratory). The disturbed soi ls had much less spectral contrast than the undisturbed soils in the r esistrahlen region near 9 mu m. While the increased porosity of a dist urbed soil can decrease spectral contrast due to multiple scattering, we hypothesize that the effect is dominantly the result of a differenc e in grain-site distribution of the optically active layer (i.e., fine particle coatings). This concept was proposed by Salisbury et al. (19 94) to explain their observations that soils washed free of small part icles adhering to larger grains exhibited greater spectral contrast th an unwashed soils. Our laboratory reflectance spectra of wet- and dry- sieved soils returned from field sites also show greater spectral cont rast for wet-sieved (washed) soils. We therefore propose that undistur bed soils in the field can be characterized as ''clean'' soils (washed free of fine particles at the surface due to rain and wind action) an d that disturbed soils represent ''dirty'' soils (contaminated with fi ne particle coatings). The effect of packing soils in the field and la boratory also increases spectral contrast but not to the magnitude of that observed for undisturbed and wet-sieved soils. Since it is a comm on practice to use laboratory spectra of field samples to interpret sp ectra obtained remotely, we suggest that the influence of fine particl e coatings on disturbed soils, if unrecognized, could influence interp retations of remote sensing data. Published by Elsevier Science Inc., 1998.