Infrared spectroscopy (2-3-20 μm) for the geological interpretation of remotely-sensed multispectral thermal infrared data

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Abstract
Infrared spectroscopy takes advantage of the strengths and positions of absorption bands determined by composition and crystal structure of rocks and minerals to discriminate between them. The spectral features in thermal-infrared multispectral remotely-sensed data, however, may be influenced by the weathered, rough and/or varnished character of the target as well as by environmental factors such as temperature and atmospheric conditions. Hence we measured in situ the absolute spectral radiance of naturally-occurring surfaces of common sedimentary and igneous rocks to document the strengths and positions of absorption features available to the remote sensing geologist. The spectral radiance measurements (5-14 μm) were made with a portable spectral radiometer designed and built by the Jet Propulsion Laboratory and the data were used to estimate the spectral emissivity of the rocks. In addition, spectral reflectance measurements (2-3-20 μm) were made in the laboratory of samples of outcrops and soils previously measured in situ. The reflectance measurements were made with a commercial Fourier transform infrared (FTIR) spectrometer equipped with a ‘diffuse reflectance’ accessory. A modified stage facilitates the sampling of large surface areas (6 mm x 10 mm). The agreement between the two techniques is very good and either approach seems suitable for the purposes of studying the spectral character of remotely-sensed targets. The techniques differ, however, in that the field portable spectrometer has a much larger field of view and the in situ data more accurately measure the intensity of spectral features which are in part a function of temperature and atmospheric effects.