Single-Point Acoustic Backscatter and Acousto-Optic Backscatter Sensors Mark Two Major Advances in Low-Cost Suspended Sediment Concentration Monitoring

The most widely used type of point sensor for suspended sediment concentration measurements is of turbidity type. It’s short-coming is well-known: the turbidity sensor requires re-calibration whenever particle size in the environment changes. And since PSD is constantly changing in nature, no calibration can be trusted for a long time-series. This annoying inconvenience has been tolerated, given that there was no inexpensive alternative. Now, there are two.

The first sensor is of the acoustic type, employing 8MHz frequency. The LISST-ABS (Sequoia Scientific, Inc., Bellevue, WA, USA) has the advantage of nearly flat response (Volts/[mg/L]) in the silt-sand range of grain sizes, where turbidity loses sensitivity as 1/diameter. In river applications, the LISST-ABS has been demonstrated by USGS scientists, to faithfully reproduce a Rouse-like river column concentration profile, whereas turbidity failed to do so due to its decreasing sensitivity with grain size. This new LISST-ABS [ https://www.sequoiasci.com/article/lisst-abs-river-profiles-usgs/] is increasingly being favored by US government scientists.

Even as the LISST-ABS gains in monitoring, a new concept was born that combines turbidity measurement with acoustic backscatter. The sensor, LISST-AOBS combines the measurements with turbidity sensors and LISST-ABS to produce a nearly flat response (Volts/Concentration) over a broad particle size range. The combination amounts to a weighted sum of the two outputs, and can even be applied to historical data where the two sensors were recording simultaneously by a simple procedure. The end results is a low-cost general purpose sensor for sediment monitoring that eliminates the need for sensor calibration generally, barring only extreme cases (particles <> 400 microns).

In the presentation, we shall explain the principles and show field data collected from a river in Washington state, USA.