Retrieval of sand density from hyperspectral BRDF

Abstract

In past work 1 , we have shown that density effects in hyperspectral bi-directional reflectance function (BRDF) data are consistent in laboratory goniometer data, field goniometer measurements with the NRL Goniometer for Portable Hyperspectral Earth Reflectance (GOPHER), and airborne CASI-1500 hyperspectral imagery. Density effects in granular materials have been described in radiative transfer models 2,4,5 , and are known, for example, to influence both the overall level of reflectance as well as the size of specific characteristics such as the width of the opposition effect in the BRDF. However, in mineralogically complex sands, such as coastal sands, the relative change in reflectance with density depends on the composite nature of the sand 3 . This paper examines the use of laboratory and field hyperspectral goniometer data and their utility for retrieving sand density from airborne hyperspectral imagery. We focus on limitations of current models to describe density effects in BRDF data acquired in the field, laboratory setting, and from airborne systems.

Influence of density on hyperspectral BRDF signatures of granular materials

“…Instead, reflectance increases as density decreases, with no inversion at any fill factor. This supports previous observation by Bachmann et al 5,11,12 that as density increases, multiple scattering returns leaving granular substances with optically contrasting mineral content are diminished, leading to an overall reduction in reflectance. Phase Angle Figure 11.…”

“…5,11,12 Traditional radiative transfer theory, such as the model developed by Bruce Hapke 3 , suggests that the direct impact of an increase in granular material constituent density results in an increase in overall reflectance for fill factors up to approximately 0.52. 2,3 However, it has been shown that when a darker mineral fraction is present alongside larger, more translucent grains, reflectance decreases with higher density sands and increases with lower density composites.…”