Use of hyperspectral images in the identification and mapping of expansive clay soils and the role of spatial resolution

Hyperspectral images were acquired along the Front Range Urban Corridor in Colorado to determine the feasibility of identification and mapping of expansive clay soils with two, high-SNR imaging spectrometers. Swelling soils are a major geologic hazard, and cause extensive damage world-wide every year. The cost of postconstruction mitigation and standard engineering soil tests for creation of regional maps are immense. Smectite is the clay mineral group that has the greatest swelling potential and is responsible for most of the severe swelling soil damage observed in Colorado. Data sets were acquired from 1997 to 1999 with the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) and the Hyperspectral Mapper (HyMap). Using a matched filtering algorithm, maps of exposed clay material were produced, despite a strong vegetation cover. Among those exposures, spectral discrimination and identification of variable clay mineralogy such as smectite, smectite/illite, and kaolinite, in decreasing order of swelling potential hazard, was possible. The comparison of the results from the two sensors showed that higher spatial resolution provided purer image endmembers in more heterogeneous sites, but did not exhibit more endmembers and did not identify new natural outcrops that a lower spatial resolution data set would miss in a homogeneous terrain. However, an increase in the signal-to-noise ratio (SNR) of the instrument by pixel summation made possible the identification of low reflectance exposures. This work demonstrates that, using recent instruments and well-established methodologies, imaging spectrometry can be of practical help for the detection and mapping of expansive clays.