Andrei Abelev

Flexible field goniometer system: the Goniometer for Outdoor Portable Hyperspectral Earth Reflectance

Abstract. This paper describes a portable hyperspectral goniometer system for measurement of hemispherical conical reflectance factor (HCRF) data for terrestrial applications, especially in the coastal zone. This system, the Goniometer for Portable Hyperspectral Earth Reflectance (GOPHER), consists of a computer-controlled Spectra Vista Corporation HR-1024 full-range spectrometer mounted on a rotating arc and track assembly, allowing complete coverage in zenith and azimuth of a full hemisphere for recording HCRF. The control software allows customized scan patterns to be quickly modified in the field, providing for flexibility in recording HCRF and the opposition effect with varying grid sizes and scan ranges in both azimuth and zenith directions. The spectrometer track can be raised and lowered on a mast to accommodate variations in terrain and land cover. To minimize the effect of variations in illumination during GOPHER scan cycles, a dual-spectrometer approach has been adapted to link records of irradiance recorded by a second spectrometer during the GOPHER HCRF scan cycle. Examples of field data illustrate the utility of the instrument for coastal studies.

Sensing shallow seafloor and sediment properties, recent history

Near surface seafloor properties are needed for recreational, commercial, and military applications. Construction projects on the ocean seafloors often require extensive knowledge about strength, deformability, hydraulic, thermal, acoustic, and seismic characteristics for locating stable environments and ensuring proper functioning of structures, pipelines, and other installations on the surface of and buried into the marine sediments. The military is also interested in a variety of seafloor properties as they impact sound propagation, mine impact burial, trafficability, bearing capacity, time-dependent settlement, and stability of objects on the seafloor. Point measurements of sediment properties are done using core samplers and sediment grab devices (with subsequent lab analysis) and in-situ probes. These techniques are expensive in terms of ship time and provide limited area coverage. Sub-bottom acoustic and electromagnetic sensors can provide profiles of near surface sediment information with improved coverage rates. Fusion techniques are being developed to provide areal extent of sediment information from multiple sensors. This paper examines the recent history of techniques used to measure sediment properties in the upper portions of the seafloor and in shallow (< 100 m) water.

Strain-Rate Dependence of Strength of the Gulf of Mexico Soft Sediments

Many marine civil engineering applications require knowledge and understanding of the behavior and strength properties of soft cohesive marine sediments under high strain-rate conditions, typically encountered during impact penetration events of sediment probes and other objects as a result of free fall through the water column. To investigate these effects, a series of variable rotational rate vane shear tests was performed on a sample of Gulf of Mexico marine mud, using a precision rheometer and spanning the rotational rate range of 0.25-1000 r/min. A wide range of water contents from 55% to 95% (liquidity index: 1.5-3.3) was examined as a primary influence on the response of a particular saturated silty clay material. Nonlinearities in the behavior of this soil were analyzed, and applicability and precision of various models were examined. A modified rate equation is suggested yielding good correlation with experimental data at all water contents and all rotational rates explored. A testing schedule for a complete material constant derivation procedure is outlined.

Rheological Behavior of Artificial Flocculated Clay/Guar-Gum Suspensions

In this paper, artificial flocs, similar to the ones that are found in a recently deposited cohesive seabed, were prepared using Ca-montmorillonite and kaolinite (two of the most abundant clay minerals), guar gum (a polysaccharide closely resembling natural marine organic matter), and artificial sea water, in order to investigate the effect of the composition on their rheological response. Shear rate effects on viscosity of these suspensions were also systematically addressed. These effects typically have a strong influence on the overall rheological response of materials incorporating any polymeric substance. It was found that the addition of guar gum, even as little as 5% relative to the clay content, increases the viscosity of the floc suspensions significantly. The higher the guar-gum and clay content, the greater is the viscosity of the floc suspensions, within the guar-gum loading range investigated in this study. The floc suspensions with higher solid content were found to exhibit a slight shear thinning behavior, apparently due to the breakdown of the floc structure under high shear stress. This behavior is more pronounced in montmorillonite floc suspensions than in the kaolinite suspensions due to the differences in the clay mineral surface charge properties and surface-to-mass ratios. We explore different rheological models in their applicability to describing the observed viscous response and conclude that a cross-type formulation may be most appropriate.

Retrieval of sand density from hyperspectral BRDF

In past work, 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 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. 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.