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 particles. 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 particle 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 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.
Question from AquaPro HR user:
I have been looking into using the signal amplitudes (acquired with an HR profiler) as a proxy for SSC. In Nortek Technical Note I found the following equation for range-normalized echo level:
EL = Amp*.43 + 20log(R) + 2alpha_w*R + 20R integral(alpha_p)*dr
I used only the first two terms, i.e. EL = Amp*.43 + 20log(R), and find that the EL values are too low in the first few range bins. Is there a way to correct this? The range bins are from 0.2 to 1m at 5-cm intervals. The profiler was up-looking with tidal depths that ranged from 0 - 2m.
The equation in the technical note is valid for the acoustic far field and it is cannot really be used close to the transducer. To get a good expression, you have to integrate the nearfield equation (can at least be done numerically). Another way to do this is to establish a base profile when the water is fairly clear and assume this represents of a low but vertically constant scattering level. You can then subtract this base profile from all the other profiler and think of it as an "anomaly profile". Just be sure to compensate for large variations in temperature (which can change the base profile) and changes in the supply voltage over time.