Identification of contaminated soil using CO₂ laser reflectance ratios

A mid-infrared laser reflectance sensor operating within the 9-11 μm wavelength range was used to study the reflectance characteristics of soil contaminated with various commercially available chemical liquid materials. Measurements were made at ten wavelengths and three incidence angles under both co-polarized and cross-polarized scatter conditions. Data were gathered from bare soil, as well as soil saturated with anti-freeze/coolant, unused motor oil, and used motor oil. Calibration was performed using a Labsphere Diffuse Reflectance Standard of 94% nominal reflectivity. The measurements were used to compute reflectance ratios, i.e., ratios of reflectivities at various two-wavelength combinations. Our study indicates that it is possible to detect the presence of as well as to identify the type of contamination present in the soil by computing reflectance ratios at judiciously selected wavelengths. Use of wavelength ratios obviates the need for absolute calibration if the atmospheric transmission characteristics are the same at the two probing wavelengths. This technique is well-suited for standoff detection of contaminated soil.