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Davies E.J.,University of Plymouth | Nimmo-Smith W.A.M.,University of Plymouth | Agrawal Y.C.,Sequoia Scientific, Inc. | Souza A.J.,National Oceanography Center
Optics Express | Year: 2011

The use of laser diffraction is now common practice for the determination of an in situ particle size distribution in the marine environment. However, various imaging techniques have shown that particles vary greatly in shape, leading to uncertainty in the response of laser diffraction instruments when subjected to this diverse range of complex particles. Here we present a novel integrated system which combines both digital in-line holography and a LISST-100 type C, to simultaneously record in-focus images of artificial and natural particles with their small-angle forward scattering signature. The system will allow for further development of a reliable alternative to Mie Theory when using laser diffraction for the in situ measurement of complex suspended particles. A more detailed knowledge of the performance of laser diffraction when subjected to the wide variety of complex particles found in the marine environment will then be possible. © 2011 Optical Society of America. Source

Pejrup M.,Copenhagen University | Mikkelsen O.A.,Sequoia Scientific, Inc.
Estuarine, Coastal and Shelf Science | Year: 2010

It has long been recognized that the suspended sediment concentration (SSC) is one of the major determinants for the flocculation of cohesive particles into sediment flocs in estuaries. It is furthermore well known that the turbulent shear of the water significantly influences the flocculation process and the equilibrium settling velocity of flocculated sediment in a turbulent flow. A vast number of authors have reported algorithms relating the median settling velocity (W50) to suspended sediment concentration. However, only a few studies have dealt with the impact of the turbulent shear (in this paper expressed as the root mean square [rms] velocity gradient, [G]) in the water on the W50 in situ. There is a strong need to establish algorithms based on in situ measurements describing the dual impact of both SSC and G on the flocculation process, and hence, W50. The present paper addresses this topic. Field settling velocities of suspended cohesive sediment have been measured in micro-, meso-, and macro-tidal estuaries. Regression analyses between the W50, SSC and G are presented. It is shown that by including both G and SSC in the regression analyses, a significant increase in the correlation of the description of W50 and the controlling parameters from each area can be obtained. A generic algorithm describing the data from all the investigated areas is suggested. It works well within specific tidal areas but fails to give a generic description of the field settling velocity. © 2009 Elsevier Ltd. All rights reserved. Source

Mobley C.D.,Sequoia Scientific, Inc. | Boss E.S.,University of Maine, United States
Applied Optics | Year: 2012

Accurate calculation of underwater light is fundamental to predictions of upper-ocean heating, primary production, and photo-oxidation. However, most ocean models simulating these processes do not yet incorporate radiative transfer modules for their light calculations. Such models are often driven by abovesurface, broadband, daily averaged irradiance or photosynthetically available radiation (PAR) values obtained from climatology or satellite observations, sometimes without correction for sea-surface reflectance, even though surface reflectance can reduce in-water values by more than 20%. We present factors computed by a radiative transfer code that can be used to convert above-surface values in either energy or quantum units to in-water net irradiance, as needed for calculations of water heating, and to inwater PAR, as needed for calculations of photosynthesis and photo-oxidation. © 2012 Optical Society of America. Source

Lee Z.,Mississippi State University | Ahn Y.-H.,Korea Ocean Research and Development Institute | Mobley C.,Sequoia Scientific, Inc. | Arnone R.,Naval Research Laboratory Stennis Space Center
Optics Express | Year: 2010

Using hyperspectral measurements made in the field, we show that the effective sea-surface reflectance ρ (defined as the ratio of the surface-reflected radiance at the specular direction corresponding to the downwelling sky radiance from one direction) varies not only for different measurement scans, but also can differ by a factor of 8 between 400 nm and 800 nm for the same scan. This means that the derived water-leaving radiance (or remote-sensing reflectance) can be highly inaccurate if a spectrally constant ρ value is applied (although errors can be reduced by carefully filtering measured raw data). To remove surface-reflected light in field measurements of remote sensing reflectance, a spectral optimization approach was applied, with results compared with those from remote-sensing models and from direct measurements. The agreement from different determinations suggests that reasonable results for remote sensing reflectance of clear blue water to turbid brown water are obtainable from above-surface measurements, even under conditions of high waves. © 2010 Optical Society of America. Source

Slade W.H.,Sequoia Scientific, Inc. | Boss E.,University of Maine, United States
Applied Optics | Year: 2015

Measurements of the particulate beam attenuation coefficient at multiple wavelengths in the ocean typically exhibit a power law dependence on wavelength, and the slope of that power law has been related to the slope of the particle size distribution (PSD), when assumed to be a power law function of particle size. Recently, spectral backscattering coefficient measurements have been made using sensors deployed at moored observatories, on autonomous underwater vehicles, and even retrieved from space-based measurements of remote sensing reflectance. It has been suggested that these backscattering measurements may also be used to obtain information about the shape of the PSD. In this work, we directly compared field-measured PSD with multispectral beam attenuation and backscattering coefficients in a coastal bottom boundary later. The results of this comparison demonstrated that (1) the beam attenuation spectral slope correlates with the average particle size as suggested by theory for idealized particles and PSD; and (2) measurements of spectral backscattering also contain information reflective of the average particle size in spite of large deviations of the PSD from a spectral power law shape. © 2015 Optical Society of America. Source

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