Three liquid hydrocarbons of different volatilities were incrementally applied to a quartz sand substrate. These liquids were gasoline, diesel fuel, and motor oil. The reflectance spectra of the hydrocarbon-sand samples varied directly with the amount (weight) of liquid on the sand. Liquid-saturated sand samples were then left to age in ambient, outdoor, environmental conditions. At regular intervals, the samples were re-measured for the residual liquid and the associated change in sample reflectance. The results outlined temporal windows of opportunity for detecting these products on the sand substrate. The windows ranged from less than 24-hours to more than a week, depending on liquid volatility.
Each hydrocarbon darkened the sand and produced hydrocarbon absorption features near 1.70 and 2.31 μm and a hydrocarbon plateau at 2.28-2.45 μm. These features were used to differentiate the liquid-sand samples. A normalized difference index metric based on one of these features and a spectral continuum band described the reflectance-weight loss and reflectance-time relations. The normalized difference hydrocarbon index (NDHI) using the 1.60 and 2.31 μm bands best characterized the samples.
Water is a common, transient soil material that can be distributed as lattice water bound in crystalline particles, as water of adhesion on the soil particles, and as interstitial or capillary water. It can have important effects on soil reflectance spectra over the visible-near infrared-short wave infrared electromagnetic spectrum, 0.4-2.5 μm. This study's objective was to determine the changes in soil reflectance spectra relative to differences in soil water content. An initial small water application greatly reduced the soil reflectance, masked the spectral features of the air-dry soils, and enhanced water absorption features. As wavelength values increased, water absorptance increased and transmittance decreased, which created non-uniform change in the soil reflectance spectrum. These changes occurred as water filled the interstitial spaces within the soil's optical depth. Water filling the pore space below the sample's optical depth increased the substrate's moisture content but had no effect on substrate reflectance. These water absorption features were amplified over the 0.4-2.5 μm region and spectral sensitivities to water increased directly with wavelength. Soil reflectance maxima in five spectral bands, centered at 0.800, 1.080, 1.265, 1.695, and 2.220 μm, varied inversely with sample water content. The 0.800 and 1.080 μm bands varied more slowly with water content than did the 1.265, 1.695, and 2.220 μm bands. Multiple normalized difference indices (NDI) using these bands correlated strongly with sample water content.
Soil surface materials often originate from different sources and are spectrally variable. Their presence will alter soil spectral features and mask the nature of the underlying soil surface horizon. The upper-most, thin, granular layer determines a soil sample's spectra. This study's objective was to characterize the optical depth of some sandy soils and their relationship to spectral reflectance from 0.35-2.50mm. The reflectance-optical depth relationships were determined for four, air-dried, granular, sieved samples, with particle sizes of 1.0-2.0, 0.5-1.0, 0.25-0.5, 0.125-0.25, 0.075-0.125, or <0.075mm. Each particle size separate has convergent reflectance spectra associated with an optical depth that ranged from 0.2 to 8.1mm. The optical depth was greater for larger sized particles than for smaller sized particles. Normalizing the sample depth by the mean particle diameter of each sieve fraction found the optical depth-spectral feature relationships were determined by a layer of granular materials that was 5-8 particles thick. Three non-sieved, well-graded composite soils were also evaluated and their optical depths ranged from 1.4 to 3.9mm. These non-sieved composite soils include a medium fused-silica sand, a medium calcareous sand, and a medium gypsum sand.
Large size calibration panels are frequently required as reference points for in-scene calibration of remotely sensed spectral data. However, most commercially manufactured calibration panels are costly and sometimes present spectral crossover problems. The panels described in this report are made from readily available fabrics and can provide a lower cost alternative. Four 5.5 x 6.7 m fabric panels that have nominal reflectance of 85%, 38%, 18% or 3% were tested. These fabric panels cost approximately $700 to construct and the materials are available at most hardware stores, which facilitated field panel construction. When properly deployed on a uniform dark-toned surface such as asphalt, gravel, or soil, these alternative panels provide calibration for the entire 0.4 - 2.5 μm spectrum and do not exhibit spectral crossover problems. Remote sensing programs that do not have access to or resources to acquire commercial panels may find these fabric panels a suitable alternative for in-scene calibration.
Spectra were taken that describes free water, ice, and snow, and vegetation and inorganic backgrounds. The reflectance of water films, ranging from 0.008 to 5.35 mm, on a spectralon background varied with water depth and the water transmittance and absorbtance properties. Thin water films, > 3.5 mm, quenched the short wave infrared (SWIR) reflectance, even though moderate visible-near infrared reflectance occurred from the water-spectralon surfaces. Ice and snow have a similar number of absorption bands as water but their absorption maxima varied from those of water. River float-ice and glacial ice have diagnostic absorption features at 1.02 and 1.25 μm and negligible reflectance in the > 1.33 μm region. New powder snow, new wet snow, and older deep snow packs have similar shaped reflectance spectra. Thin snow accumulations readily masked the underlying surfaces. These snow pack surfaces have a small asymmetric absorption features at 0.90 μm and strong asymmetric absorption features at 1.02, 1.25, and 1.50 μm. These snow packs have measurable SWIR reflectance. An avalanche snow pack had low SWIR reflectance, which was similar to ice spectra. Water, ice and snow and ice surfaces have spectrally distinct features, which differentiates them and the background surfaces.
Spectral reflectance measurements were acquired at various viewing angles at three sites along the James Riber representing both tidal and non-tidal waters. The upper James River reaches were characterized by optically clear waters and resulted in spectral measurements that represented bottom substrates. In contrast, the lower James River sites were characterized by turbid waters having high suspended sediment and algal chlorophyll. Concurrent pyranometer measurements showed the maximum downwelling radiation occurring from 1030 to 1330 local sun time. During this period, two strategies emerged for consideration when collecting water column reflectance data. In optically clear waters, statistical analysis using the variance of the 575 nm waveband (as a reference) showed a nadir viewing angle (90 degree(s)) and upsun (+30 degree(s)) off-axis viewing angle were the most effective at characterizing the bottom substrates. The conclusion drawn for optically clear waters was that (independent of sun angle), nadir position of the sensor optics is critical, but confident measurements can still be acquired up to +30 degree(s) off axis. In contrast, lower James River sites (turbid reaches) showed no correlation between the nadir and off-axis measurements using the variance of the 680 nm chlorophyll absorption line. Furthermore, the conclusions drawn from these reaches demonstrated that reflectance data are best acquired at nadir viewing angles for highly turbid waters. These measurements could have implications for both non-imaging and imaging remote sensor data.
The objective of this study was to describe the excitation- emission spectra of seed pubescence, pollen and spores, and senesced plant materials that could be carried in the air column. Reference samples were a mature green-colored corn leaf, green-, yellow- and brown-colored soybean leaves, cellulose, commercial grade cotton batting and a soil. Spectral luminescence signatures were collected over the 300 to 800 nanometer region using a scanning spectrofluorometer. The excitation-emission spectra were broadband emission centroids in the 400-nm to 600-nm spectrum. Emission maxima were associated with the 440-nm, 470-nm and 370-nm excitation bands and the 455-nm to 590-nm emission bands. The coma of milkweed, silkvine, cotton (raw), cottonwood seeds and yellow- colored pollen and spores were highly fluorescent. The pappus of thistles, dandelion and goat's beard seeds and newly senesced grass leaves and glumes had moderate to high fluorescence. Dark brown-colored mushroom spores and weathered, senesced plant materials had low fluorescence. The emission spectra resembled that of regent, microcrystalline cellulose although impurities incorporated within the plant materials altered their emission intensities from that of cellulose. Moderate to low emissions were from tan- to dark brown-colored materials, whereas the white-colored or light, tan-colored materials had high emissions.