Estimation of mud and sand fractions and total concentration from
coupled optical-acoustic sensors
Abstract
Optical and acoustic sensors have been widely used in laboratory
experiments and field studies to investigate suspended particulate
matter concentration and particle size over the last four decades. Both
methods face a serious challenge as laboratory and in-situ calibrations
are usually required. Furthermore, in coastal and estuarine
environments, the coexistence of mud and sand often results in
multimodal particle size distributions, amplifying erroneous
measurements. This paper proposes a new approach of combining a pair of
optical-acoustic signals to estimate the total concentration and
sediment composition of a mud/sand mixture in an efficient way without
an extensive calibration. More specifically, we first carried out a set
of 54 bimodal size regime experiments to derive empirical functions of
optical-acoustic signals, concentrations, and mud/sand fractions. The
functionalities of these relationships were then tested and validated
using more complex multimodal size regime experiments over 30
optical-acoustic pairs of 5 wavelengths (420, 532, 620, 700, 852 nm) and
6 frequencies (0.5, 1, 2, 4, 6, 8 MHz). In the range of our data,
without prior knowledge of particle size distribution, combinations
between optical wavelengths 620-700 nm and acoustic frequencies 4-6 MHz
predict mud/sand fraction and total concentration with the variation
< 10% for the former and < 15% for the later. This
approach therefore enables the robust estimation of suspended sediment
concentration and composition, which is particularly useful in cases
where calibration data is insufficient.