Application of in-situ digital holography in the study of particles, organisms and bubbles within their natural environment

The link between suspended particle fields, particle dynamics and bulk optical properties in natural waters is poorly known because adequate technology is lacking to fully characterize critical parameters and interactions, especially for ephemeral bubbles and aggregates. This paper highlights the capabilities of digital holography to provide non-intrusive, high-resolution 3-D imaging of particles and bubbles in their natural environment. As part of a NOPP project (HOLOCAM) to commercialize an in-situ digital holographic microscope (DHM), field data with a prototype in-situ DHM (the "Holosub") were collected in East Sound, WA. The Holosub, an in-line holography based submersible platform, was deployed in two configurations: free-drifting mode for vertical profiling, and towed mode. In free-drifting mode, vertical profiles of shear strain and dissipation rates, undisturbed size and spatial distributions of particles and organisms, and the orientation of diatom chains were recorded using the holographic images. Hydrographic and optical data, as well as discrete water samples to identify phytoplankton species were concurrently collected. In towed mode, the size and spatial distributions of bubbles just below the surface were recorded to characterize the dissipation of a wake generated by another ship, and compared to optical and acoustic scattering data recorded simultaneously. Tools to extract the size distribution and concentration of bubbles from the holographic data were developed. A preliminary data analysis indicated high concentrations of bubbles detected by all three instruments at the same locations, while comparison of the bubble size distributions indicated some similarities in trends, as well as significant differences.