GYPSUM GRAIN SIZE DETERMINATION IN THE DUNE AREA OF WHITE SANDS, N.M USING AVIRIS IMAGERY

The White Sands National Monument consists of several hundred square miles of nearly pure gypsum in the form of clean, transverse sand dunes and flat beds of gypsum. The major goal of this study is to determine gypsum grain size variations in the Dune Area of White Sands using AVIRIS imagery. Optical theory dictates a relationship between spectral reflectance/emittance and the grain size of the target material, however, very few studies have taken the opportunity to test and utilize this possibility. Gypsum samples were collected along a 5-kilometer traverse to the northwest of the loop for tourist visitors to the White Sands National Monument. Hyperspectral studies of laboratory prepared samples provide specific relationships between absorption peak depths and grain sizes of selected minerals. Three software processing steps were done to measure the grain size variations of gypsum in the Dune Area. The first step was to remove the continuum from AVIRIS pixels. The second step was to measure the band depths of the 1.76 and 2.23µm on the continuum removed spectra. The last step was to convert the continuum-removed band depths values to grain size by applying the established laboratory relationship between the band depth and grain size. The AVIRIS results showed that the grain sizes of gypsum in most of the Dune Area are mixed. However, some parts showed observable grain size variations. The results also showed that the grain size of gypsum ranges from 280 to 700 µm. A good match was noticed between ground truth data and AVIRIS results. Grain size determinations can be used to determine changing wind directions and relative magnitudes through time. These data can provide interpretations of changes in climatic conditions, and, in particular, can document the drying cycles in desert areas.