Measuring sediments: Backscatter optics, laser diffraction, acoustics, and combined optics and acoustics

Abstract

Over the last 3-plus decades, point measurements of sediment concentrations in water have mostly been made using optical backscatter. Physics tells us that this signal correlates with particle area concentration so that while measuring volume concentrations, suspended load (sand) can be swamped by wash load (fines) and remain unseen. Over 2 decades ago, we introduced laser diffraction to the marine-aquatic environment, delivering concentration, size distribution (and settling velocity spectrum when equipped with a setline tube), with the characteristic of uniform sensitivity to a 200:1 range of grain sizes, excluding flocs which have variable fractal dimensions. Most recently, we introduced a high-frequency 8 MHz acoustic backscatter point-measurement system with the attraction of enhanced sensitivity to suspended load, i.e., opposite of backscatter optics, a higher limit to sediment concentrations, and greater tolerance to fouling. Very recently, a small conceptual jump was made to combine backscatter optics and 8MHz backscatter acoustics into a sensor with nearly uniform sensitivity to sizes over a 1—500 micron size range. I will review each briefly and illustrate how well these methods work in nature. Finally, I will offer thoughts on the information content of multi-frequency acoustics.