Linking goniometer measurements to hyperspectral and multisensor imagery for retrieval of beach properties and coastal characterization

Abstract

In June 2011, a multi-sensor airborne remote sensing campaign was flown at the Virginia Coast Reserve Long Term Ecological Research 10 site with coordinated ground and water calibration and validation (cal/val) measurements. Remote sensing imagery acquired during the ten day exercise included hyperspectral imagery (CASI-1500), topographic LiDAR, and thermal infra-red imagery, all simultaneously from the same aircraft. Airborne synthetic aperture radar (SAR) data acquisition for a smaller subset of sites occurred in September 2011 (VCR'11). Focus areas for VCR'11 were properties of beaches and tidal flats and barrier island vegetation and, in the water column, shallow water bathymetry. On land, cal/val emphasized tidal flat and beach grain size distributions, density, moisture content, and other geotechnical properties such as shear and bearing strength (dynamic deflection modulus), which were related to hyperspectral BRDF measurements taken with the new NRL Goniometer for Outdoor Portable Hyperspectral Earth Reflectance (GOPHER). This builds on our earlier work at this site in 2007 related to beach properties 3 and shallow water bathymetry 2. A priority for VCR'11 was to collect and model relationships between hyperspectral imagery, acquired from the aircraft at a variety of different phase angles, and geotechnical properties of beaches and tidal flats. One aspect of this effort was a demonstration that sand density differences are observable and consistent in reflectance spectra from GOPHER data, in CASI hyperspectral imagery, as well as in hyperspectral goniometer measurements conducted in our laboratory after VCR'11.

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.…”

“…12 Results for the 65°illumination zenith angle show the expected behavior predicted by the Hapke radiative transfer model where reflectance increases with increase in density until a fill factor of 0.52 is reached and then proceeds to decrease with further increases in fill factor. For the 20° illumination zenith angle, this trend does not hold.…”

“…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.…”

Retrieval of sand density from hyperspectral BRDF

“…In these sands, increased density led to a decrease in reflectance. 1,3 An apparent explanation may be a combination of factors 3 : the onset of coherent effects at sufficiently high fill factor and the fact that the dark absorbing mineral fraction should more optimally fill the available pore space as the density increases; this darker absorbing mineral fraction would then tend to dampen the multiple scattering pathways. Figures 2 -4 show examples that illustrate this point using observations in the laboratory, ground observations in the field, and airborne hyperspectral imagery.…”

A next generation field-portable goniometer system

“…5 Many types of goniometer systems have been created to conduct measurements in both field and laboratory settings. 1,3,4 These devices are purpose-built to measure specific target sets, such as vegetation, man-made surfaces, or sediments, and their overall design can vary widely based on the type of target being measured. In the creation of such devices, the designer must consider the standard operating environment that the devices are expected to perform in, the level of accuracy they are expected to obtain in their measurements, their size, and any other required constraints.…”