"Proceedings, Federal Interagency Workshop,
"Sediment Technology for the 21'st Century,"
St. Petersburg, FL, February 17-19, 1998"

Acoustically Measuring Suspended Sediment Concentration and Size Distribution

By R.W. Derrow II, R.A. Kuhnle, and
I.P. Jones,

Abstract

Use of acoustic techniques to measure suspended sediment concentration have been ongoing for several years (Urick, 1948). In most of these systems a short burst of high frequency (>1 MHz) sound is emitted from a transducer(s) and is scattered off the sediment in suspension. The magnitude of the backscattered energy is related to the concentration and size of the sediment. Current technology can measure suspended sediment concentration with limited size distributions with high accuracy (Schat, 1997), but success has been limited at determining sediment size distribution to a reasonable accuracy. Very high concentrations where multiple scattering effects are more dominant are especially problematic (Crawford and Hay, 1993; Thorne et al., 1994).

Work has been underway by the National Center for Physical Acoustics in collaboration with the National Sedimentation Laboratory to develop a triple frequency acoustic system to measure volumetric suspended sediment concentration and size distribution in lab and field environments that extend the current sampling and concentration/size ranges. Work to this point has focused on (1) developing a water/sediment re-circulation calibration system, (2) analyzing existing backscatter models and assembling a PC based acoustic backscatter measurement system (ACMS), and (3) taking preliminary acoustic backscatter measurements with simulated sediment (i.e. glass beads).

The water/sediment re-circulation system is used to maintain various sizes and densities of sediment in suspension. This system functions as a calibration and development device to aid in the testing of backscatter models and algorithms. The system is called the Suspended Sediment Calibration System (SSCS). The impeller driven pump system has built-in sample ports for using a pump sampler or other devices (i.e. OBS, etc.) for comparison with the acoustic backscatter measurements. Existing backscatter models and algorithms have been compared and parameters identified that need modification for the NCPA-NSL system. The ACMS is a multiplexed, triple frequency system controlled by a PC computer. It has a time-varied-gain (TVG) receiver to compensate for the acoustic transducer geometrical beam spreading and absorption. Preliminary acoustic backscatter measurements have been made with 600-700(m glass beads at various concentration levels that show the expected correlation between concentration and increasing backscatter signal level. It was also shown that for a given concentration, the backscatter level is unaffected by flow velocity in the SSCS. Work in the future will concentrate on using the ACMS with the modified models and algorithms to first develop a laboratory system and then build a field system.

References

Crawford, A.M. and Hay, A.E., 1993, Determining suspended sand size and concentration from multifrequency acoustic backscatter, J. Acoust. Soc. Am., 94 (6), 3312-3324.

Schat, J., 1997, Multifrequency acoustic measurement of concentration and grain size of suspended sand in water, J. Acoust. Soc. Am., 101 (1), 209-217.

Thorne, P.D., Hardcastle, P.J., Flatt, D, and Humphery, J.D., 1994, On the Use of Acoustics for Measuring Shallow Water Suspended Sediment Processes, IEEE J. Oceanic Engineering, 19 (1), 48-57.

Urick, R.J. 1948, The absorption of sound in suspensions of irregular particles, J. Acoust. Soc. Am., 20, 283-289.

Workshop Contributions

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