Sequoia’s LISST-Holo2 captures holograms of suspended particles. Upon processing, particle size, distribution, concentration and images of all particles in the beam are recovered. The LISST-Holo2 has traditionally been used for suspended particles in the aquatic environment. However, Cohn et al. (2022) used their LISST-Holo2 on a beach to measure the grain size variation and concentration within the lower 50 mm of the aeolian saltation layer.
The LISST-Holo2 was dug into the sand, with the optics end sticking out:
It then sampled at 10 Hz for an hour, collecting 30,000 holograms, which were subsequently processed. The LISST-Holo2 records not only the x,y position of each particle in the laser beam, but also the z position. This enabled Cohn et al. to compute the particle concentration at 5 mm bins (height intervals) over the 50 mm path length and study the vertical concentration profile of the saltating grains. The figure below shows a five-minute period:
The individual profiles generally show a concentration profile where concentration decreases with height, but occasionally large concentrations are observed higher ups, related to saltating grains. Also, periods with hardly any suspended grains are seen. Averaging all grain-size distributions in each 5 mm bin supports this: The mode at 7.5 mm above the beach is approximately 460 um, while the mode at 42.5 mm above the beach is approximately 340 um:
Overall, Cohn et al.’s paper concludes that the LISST-Holo2 measures properties of aeolian saltation close to the bed that are qualitatively similar to existing sensors, but at much higher spatial and temporal resolution than these. The paper is FREE to download and published in ESPL – download it here!