Such cyclic behaviors form a high-concentration (greater than hundreds of mg l –1) near-bed layer, which makes it hard to meet the optical transmittance required to acquire the proper hologram and conduct post-processing for image analysis.
For instance, flocculated cohesive sediments are readily settled and deposited on the bed during slack tides, and then resuspended into the overlying layer during tidal acceleration periods. It is because the high concentration of fine-grained suspended particles greatly reduces the optical transmittance within the water column ( Sun et al., 2002). The application of holographic techniques to cohesive sediments distributed in coastal environments has been somewhat limited.
Because holography is a proven method that enables the provision of a solution to the limitations of the focal plane, the marine science community has recently adopted it to collect information on the three-dimensional properties of particulate matter (e.g., sediment and plankton) suspended in a water column (e.g., Graham and Nimmo-Smith, 2010 Choi et al., 2018 Nayak et al., 2019 Giering et al., 2020).
Since the introduction of holography into the scientific community in the 1940s ( Gabor, 1948), it has become an indispensable technique used in various fields of fluid mechanics, metrology, and medical imaging ( Nayak et al., 2021). The reconstruction of such images is numerically performed using digitized interferograms ( Mills and Yamaguchi, 2005). The proposed method can contribute to a more accurate estimation of the transport and behavior of cohesive sediments in shallow estuarine environments.ĭigital holography is an imaging method that records holograms using a charge-coupled device (CCD) camera. When LI was lower than approximately 80, owing to the distortion of particle properties, the settling velocities derived from the contaminated holograms with mosaic patterns were underestimated compared to those from the uncontaminated holograms. Results found that as LI increased, the shape of the particle size distribution shifted from a positively skewed to a normal distribution. The outcomes from image processing support that LI normalized on a gray scale of 0 (black) to 255 (white) can be a reliable criterion for defining the contamination boundary. Using light intensity (LI), this study proposes a practical method to assess the degree of contamination and screen out contaminated images. As the number of particles in the water column increases, overlapping concentric rings (interference patterns) can contaminate the holographic images. Submergible digital holographic camera can measure the in situ size and shape of suspended particles, such as complex flocs and biological organisms, without disturbance. 2College of Harbour, Coastal and Offshore Engineering, Hohai University, Nanjing, China.1Department of Ocean Sciences, Inha University, Incheon, South Korea.Sun Min Choi 1, Jun Young Seo 1, Guan-hong Lee 1, Xiaoteng Shen 2 and Ho Kyung Ha 1*
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