Field Validation of Hyperspectral Imaging for Ballast Fouling Assessment

This study evaluates the performance of hyperspectral imaging (HSI) as a non-contact method for assessing railroad ballast fouling. A severely degraded ballast sample was collected from a derailment site. Conventional fouling indices were measured, indicating extreme ballast deterioration and fouling. To establish a quantitative baseline for degradation severity, hyperspectral reflectance data in the Visible–Near Infrared (VNIR) and Near Infrared (NIR) ranges were acquired for field samples under fouled-wet (as-received), fouled-dry (oven-dried), and clean-dry (oven-dried and sieved) conditions. Field spectra were compared with laboratory-fabricated ballast mixtures containing clay and coal fouling agents to ensure the results were not skewed due to the sampling procedure. Spectral similarity analysis using the Spectral Angle Mapper (SAM) was employed to quantify differences across ballast conditions. The maximum SAM angle reached approximately 0.45 radians between the as-received and clean-dry states in the NIR range, reflecting the combined effects of fouling and moisture. Comparisons between field and laboratory-fabricated samples showed moderate similarity, with SAM angles below 0.30 radians, indicating general agreement between field and laboratory spectra while capturing differences related to fouling agents, moisture retention, and compositional variability.