FAQ / Ocean currents / Question

Can I estimate sediments, SSC or plankton from acoustic backscattering?

Before reading on we highly recommend our Technical Note on Sediments (link at the bottom of this FAQ) which contains a wealth of information specifically relating to calibrating and measuring sediment concentrations using Nortek equipment.

Estimating suspended sediment concentration acoustically has been an area of active research for many years. In some circumstances, acoustic backscatter plays a substantial role in efforts to measure and map sediment concentrations. But, the problem is that acoustic backscatter is very sensitive to the size of sediment particals. Most suspended sediment is in the "Rayleigh regime" of backscatter; that is, the particles are small compared with the wavelength of the sound. The example illustrates what happens in this regime. Imagine that you had sediment with 1 particle per unit volume. If you increase the concentration to 1000 identical particles in the same volume, backscatter would increase by 1000. So far so good. But if you increased the concentration by 1000, this time by increasing the diameter of the single particle by 10, backscatter would increase by 1,000,000. Concentration increases with the third power of particle size but backscatter increases with the sixth power of particle size. People nevertheless use acoustic backscatter to map sediment concentration. Here are some examples of how they try to handle the problem: 1) Use a wide range of acoustic frequencies to observe the sediment. 2) Assume that the partical size distribution is always the same. 3) Make independent measurements of the concentration (often including the size distribution), and use the acoustic data to interpolate between the samples.

Unless you are want to study suspended sand, the physics is against you and the size sensitivity is simply too great to handle without a lot of in-situ information. This does not mean that you can't use acoustic backscatter data for something useful - it is a great qualitative tool and it tells you where you should lower your water bottles or your optical equipment.  However, the lure of the color contour plots is mostly just that - the actual concentration can be an order of magnitude away from what you think it is.  This is especially true for "natural sediments" (i.e. dredging material, river flow, etc.) where the particle size can vary by 1-2 orders of magnitude.

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