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There is a need to quantify relationships among vegetation structure and soil properties in coastal areas to better understand resilience, erosion, land use impacts, eutrophication, and mobility in this terrain. Remote sensing observations have been shown to effectively capture both vegetation and soil properties. However, identifying linked vegetation-soil properties and inferring soil properties from remote sensing when vegetation obscures the pixels remains challenging. Leveraging multiscale and multisensor remote sensing data and fusion techniques, we investigated the capability to identify linked vegetation-soil properties and infer the strength and stability of a barrier island along a swath where the soil surface is bare to partially to fully obscured by vegetation. To this end, we asked (1) Which remote sensing and field measured vegetation properties relate most strongly to soil properties, especially strength and moisture? (2) Do observations of soil conditions, geotechnical descriptors, vegetation species and health, and sensible hyperspectral signatures (HSI) allow accurate characterization of the combined soil-vegetation complex? We used multilevel and multi-sensor ground-, uncrewed aerial system (UAS)-, and satellite-based observations, namely HSI, lidar, ground optical, and geotechnical measurements, to test the variability and relationships among remote sensing-based measurements and geotechnical measurements, such as soil bearing strength. Firstly, we found high capability of HSI data to discriminate soil and soil moisture when any soil was exposed (beach, foredune, back dune, and marsh) and discrimination of vegetation at UAS and satellite scale. We found strong relationships among relative vegetation structure and soil properties, namely biomass estimates and soil strength, when using combined ground-based observations and UAS-based HSI observations and relative high accuracy upscaling to high-resolution satellite level maps of soil and vegetation properties. In addition, we found that adding slope and aspect data moderately enhanced the assessment of soil strength parameters in vegetated areas, although improvement of lidar data collection protocols in subsequent data collections promise further improvements in upcoming studies. In areas with tallest vegetation or soils that were highly saturated (inundation), results were mixed, likely due to poorer inference of soil background from remote sensing and soil strength from field measurements approaching zero, respectively. However, using a combination of shortwave infrared data, full spectra for analyses (spectral unmixing, dimensionality analyses, and supervised classification techniques), water-specific indices, and vegetation type information, wetland soil delineation was improved. Differences in soil and vegetation properties detected using field optical measurements were used to test upscaling techniques, i.e. training, for UAS-based HSI. With the help from ground-based data, a framework of mapping vegetation and soil specific properties was developed which enabled finer spatial analyses to be carried out with respect to the interdependence of vegetation and soil properties from remote sensing observations on a coastal barrier island.

Improved understanding of vegetation impacts on soil strength could improve erosion and landslide assessment, stabilization and resilience efforts, and off-road vehicle mobility efforts while minimizing damage to vegetation or soil surfaces. It has been shown that belowground biomass, along with soil moisture and type, influences soil strength. Standard handheld geotechnical instruments, such as cone penetrometers, vane and torvane, typically used for measurements of soil strength were developed for civil engineering projects. These devices have been applied to studies in natural areas to determine shear strength profiles, namely wetlands, stream banks, slopes, and coastal dunes. However, in cases where soils are soft or complex, can be saturated, or where vegetation exists, such as coastal areas or wetlands, traditional geotechnical instruments provide uncertain and highly variable results, can miss vegetation contributions to soil strength, and are difficult to compare to one another. A few efforts exist to address these issues. In the case of wetlands, Sasser et al (2018) developed a Wetland Soil Strength Tester (WSST), which measures the torque required to shear the combined soil-vegetation wetland matrix using a four-pin design inserted into the wetland soil 15 cm. In our ongoing work to use remote sensing to map soil and vegetation properties, even when soil is obscured by vegetation, we found that the geotechnical measurements insufficiently capture soil strength in the presence of vegetation. We adapted the WSST design and systematically tested against traditional geotechnical measurements broadly (variety of vegetated terrain, mostly grasses and shrubs, ranging from low to high heights), not necessarily just wetlands, to investigate the utility of the measurements in applications where soil strength is a primary parameter. Soil types included sand, loam, and clay, and each were tested in wet and dry conditions. Measurements of peak torque were recorded at intervals of 90 degrees up to four revolutions with and without surface vegetation present to account for more soil properties than shear failure. Results of the analyses showed that the method can account for a greater variation in initial shear strength than the cone penetrometer-based cone index in soft to medium hard soils, such as wet and dry sands with vegetation and moist to wet clays. Analyses of torque-based measurements beyond initial shear, greater than 90 degrees, revealed a non-uniform and non-linear change in force required as sheared vegetation-soil matrix is further manipulated. Hard soils, especially dry clays were beyond the maximum limits of torque measurements and did not add valuable information beyond traditional geotechnical measurements. Based on these results, a preliminary model of the relationship between the torque-based measurement and the cone index was developed, terrain maps were enhanced, and further applications are being analyzed. The torque weighted index soil strength test techniques have potential application in stability, erosion, and mobility studies as well as our ongoing research in using remote sensing for indirect inferences of soil properties in vegetated areas.

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.

Strength characteristics of the flocculated aggregates (flocs) of clay minerals in an aqueous marine environment are important in many modeling applications, such as penetration of heavy objects in the cohesive seafloor, hydrodynamic transport of coastal and seafloor sediments, dredging, and remote sensing among many. Measuring floc strength accurately has been a difficult task due to the fragile, amorphous, and transient nature of the flocs. Here, we describe a micromechanical technique that was used to measure the tensile strength of soft flocculated aggregates produced from simulated marine clay sediments. In this study, artificial flocs similar to the ones that are found on and near the seabed, were prepared using Na-montmorillonite, guar gum, and sea salt. Using the micromechanical technique the average tensile strength of the flocs was found to be 667 ± 189 Pa. The range of tensile strength values is consistent with the nature of the flocs and the measurement technique.

This paper describes measurements of the phase angle dependence of sand density observable in hyperspectral reflectance. Bi-directional reflectance distribution measurements in the principal scattering plane were recorded for sand samples prepared near minimum and maximum relative densities. Radiative transfer theory for granular media of a single constituent would predict increased reflectance with increased density. However, sands are usually composite materials, and we observed that reflectance can actually decrease with increasing density when the dominant constituents of the sand are semi-translucent, and a darker fraction with typically smaller grains is also present. We postulate that under these circumstances, as density increases, multiple scattering modes are being suppressed as the darker absorbing fraction occupies more optimally the available pore space.

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.

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.

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.

In June 2011, a multi-sensor airborne remote sensing campaign was flown at the Virginia Coast Reserve Long Term Ecological Research 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 and shallow water bathymetry. 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.

[1] Sediment characteristics largely govern tidal flat morphologic evolution; however, conventional methods of investigating spatial variability in lithology on tidal flats are difficult to employ in these highly dynamic regions. In response, a series of laboratory experiments was designed to investigate the use of temperature diffusion toward sediment characterization. A vertical thermistor array was used to quantify temperature gradients in simulated tidal flat sediments of varying compositions. Thermal conductivity estimates derived from these arrays were similar to measurements from a standard heated needle probe, which substantiates the thermistor methodology. While the thermal diffusivities of dry homogeneous sediments were similar, diffusivities for saturated homogeneous sediments ranged approximately one order of magnitude. The thermal diffusivity of saturated sand was five times the thermal diffusivity of saturated kaolin and more than eight times the thermal diffusivity of saturated bentonite. This suggests that vertical temperature gradients can be used for distinguishing homogeneous saturated sands from homogeneous saturated clays and perhaps even between homogeneous saturated clay types. However, experiments with more realistic tidal flat mixtures were less discriminating. Relationships between thermal diffusivity and percent fines for saturated mixtures varied depending upon clay composition, indicating that clay hydration and/or water content controls thermal gradients. Furthermore, existing models for the bulk conductivity of sediment mixtures were improved only through the use of calibrated estimates of homogeneous end-member conductivity and water content values. Our findings suggest that remotely sensed observations of water content and thermal diffusivity could only be used to qualitatively estimate tidal flat sediment characteristics.

Our multi-sensor experiments are focused on coastal characterization from hyperspectral, thermal, LiDAR, and SAR. Cal/val activities include new instrumentation such as a field portable hyperspectral goniometer.

Various branches of offshore civil and petroleum engineering, as well as the Navy, have an expressed need for tools that allow for a rapid assessment of the surficial (top 2–3m) marine sediments. Undrained shear strength of soft cohesive (clayey) deposits is one of the central parameters required in these investigations. This information is critically important in evaluations of bearing capacity of sediment under the load of objects and structures placed on the sea sediment surface or embedded in it. It is also a primary parameter guiding the response of these types of sediments in resistance to dynamic penetration events, including anchors, probes, and various other bodies deployed in free fall through the water column. Dynamic free-fall penetrometers offer a potential for such rapid strength assessment and are used widely, but have typically been employed in conjunction with mostly empirical relationships to retrieve the undrained shear strength of sediments [1–5]. In this work, we analyze the behavior of a typical representative of these types of penetrometers, the STING (Sea Terminal Impact Naval Gauge). One of four available foot geometries is examined herein (50 mm), with probe configured to achieve up to 2m soil penetration. We develop an approach for modeling the velocity decay profile of the probe, as it's decelerating in the sediment after free-fall through the water column. We utilize a commercial finite-element code (ABAQUSTM) to develop a full dynamic solution of the penetrometer impact and burial into marine sediment, with undrained shear strength varying linearly with depth. We conduct a comprehensive study of convergence and solution accuracy, utilizing Arbitrary Lagrangian-Eulerian dynamic remeshing schemes in a fully dynamic elasto-plastic model. We explore the conditions of fully-developed plastic flow around the foot of the penetrometer and compare our conclusions with those made utilizing other numerical approaches [6,7], primarily, due to the unavailability of laboratory controlled experimental data on the details of the plastic flow of cohesive materials around objects penetrating soft cohesive marine soils in the velocity regime of interest (O[10 m/s]). The numerical solution and prediction of the probe velocity decay, as it's penetrating the soil, is compared with the field-measured velocity time series from a STING deployment into soft marine clay.

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. In order to investigate these effects, a series of variable rotational rate vane shear tests were performed using a precision rheometer and spanning the rotational rate range of 0.25 to 1000 [1/min]. A wide range of water contents from 65 to 95% was examined as a primary influence on the response of a particular saturated silty clay material. Non-linearities in the behavior of the soft mud were analyzed and applicability and precision of various models examined. A modified rate equation is suggested yielding good correlation with experimental data at all water contents and all rotational rates explored.

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 (, Oceans '08 MTS/IEEE Quebec, (Oceans, Poles and Climate: Technological Challenges) 15-18 September, 2008