The industry's recognized authority for design, engineering and application of equipment and services in the global ocean community
Issue link: http://sea-technology.epubxp.com/i/639963
www.sea-technology.com February 2016 / st 15 M any widely studied aspects of the ocean are controlled by particles less than a few millimeters in size, including water clarity and remotely sensed ocean color, sediment transport, particulate carbon and fux, photosynthetic pro- duction, and harmful algal blooms. Techniques to characterize properties of particles often in- volve passing a sample through an orifce into some kind of fow cell. The associated shear can break up sensitive aggregates, cell chains, and perhaps even cells themselves, modifying size distributions and skewing results. For example, simple in-water light transmission measurements serving as a proxy for particle concentration are signifcantly different when a water sample is undisturbed in the path of the light beam as op- posed to actively pumped through a fow cell. Ef- fects of any preferred particle orientation within a natural fow regime, as well as any fne-scale particle distributions, are also randomized when the sample is pumped through a fow cell. These combined effects have the potential for measurements not to be representative, i.e., contain inherent bias, particularly for the core optical pa- rameters and techniques that are directly related to the areal cross-section of a particle feld. Exploring natural, undisturbed states of particle felds is an emerging research focus that has received limited atten- tion previously because of associated technical challenges. Techniques with the ability to image individual microscopic particles typically must do so in very small, constrained sample volumes where avoiding disruption of the natural state of the free particle or aggregate may be impossible. Open-path optical methods such as attenuation and an- gular scattering can be made in bulk, undisturbed particle felds, but these may only be relevant for the specifc ge- ometry/orientation that the devices were deployed in if the particle feld exhibits any preferential orientation. One can readily imagine that a sensor attempting to make replicate open-path measurements in several orientations in the same sample volume simultaneously will interfere with natural particles from localized turbulence associated with the bulk of the sensor itself. Technologies and methods were devel- oped as a frst step toward addressing characteristics of un- disturbed particle felds, including scanning lidar, an in-situ holographic imaging microscope, and a technique to sam- ple optical properties of water simultaneously with different sensor orientations. The entire suite of instrumentation was co-deployed recently in East Sound, a partial fjord of Orcas Island within the San Juan Island chain in Washington. Optical Property Sensing We developed a free-falling profling package to mea- sure optical and acoustical parameters. A key aspect was the alignment of multiple open-path sensors resolving the same optical measurements, at the same horizontal plane Studying Undisturbed Particle Fields in the Ocean Novel Instrument Suite for More Representative Particle Measurements By Dr. Michael Twardowski • Dr. Jim Sullivan • Dr. Fraser Dalgleish Deployment of a 4-by-4-by-3.5-ft. profling package in East Sound, Washington, with all open-path sensors mounted with sample volumes in the same horizontal plane. Sensors include two custom multi-angle scattering devices (MASCOTs) in or- thogonal relative orientation (blue arrows), a custom holo- graphic video microscope, fve WET Labs C-Stars (i.e., trans- missometers; yellow arrows), two WET Labs ac9 devices, SBE CTD, Nortek Vector ADV, and Nortek Aquadopp.