Determining suspended sand size and concentration from multifrequency acoustic backscatter

Abstract

An inversion algorithm for extracting suspended sand size and concentration from simultaneous backscattered acoustic pressure amplitude at three operating frequencies is presented. The algorithm is based on the differences in signal amplitude between different frequency pairs, and is tested using laboratory measurements of multifrequency backscatter from a turbulent sediment-carrying jet. Concentration and size profiles inverted from field and laboratory data are compared with results from a previously developed algorithm based on signal ratios. The difference inversion scheme is less sensitive to errors arising from low signal levels, allowing the size/concentration measurement range to be extended to regions of lower concentration. The concentrations from the field data agree well with independent optically determined estimates. The results demonstrate sensitivity to the backscatter form factor. PACS numbers: 43.30. Gv, 43.30.Ft signal levels (particularly in the denominator of a ratio) contribute to increased noise level in the inverted results. An alternative method is presented here which, by using the differences between signal levels, is less prone to noise in regions of low scatterer concentration. This paper contains a description of the new inversion method and results from laboratory and field experiments.The inverted sizes and concentrations from the laboratory experiments are compared to physical measurements of these quantities and concentrations determined from the field data are compared to optical measurements. Comparison to results obtained using the ratio algorithm are also presented.Proper extraction of the size information from the multifrequency data by any method requires prior knowledge of the size/frequency dependence of the backscatter signal. Factors such as the material composition of the scatterers can introduce variation in the scattering cross section, even locally. The sensitivity of inverted results to small differences in the backscatter form factor is investigated. In addition, it is necessary to know the effect of attenuation of the signal by scatterers. Measurements and discussion of scattering attenuation are presented in the Appendix. I. MEASUREMENT TECHNIQUEThe multifrequency acoustic backscatter system, RAS-TRAN system 1, has been described in detail elsewhere. •ø'•2 The transceivers operate at three frequencies ( 1, 2.25, and 5 MHz), and over a range of about 1 m. Data from each of the units is logged on a CAMAC-crate and PC-based data acquisition system. •2'•3In the laboratory studies, •3 an axisymmetric region of statistically steady scatterer concentration is maintained by a recirculating jet carrying sand of known size. Independent concentration measurements are obtained by syphon between acoustic runs. The beams intersect at the center of the jet 28 cm downstream from the nozzle, 55 cm away from the three transceivers. Backscatter profiles across the jet were acquired at a rate of 6.6 Hz, with four-ping ensemble averaging and block averaging of three adjacent sample ...

Measuring suspended sand transport using a pulse-coherent acoustic Doppler profiler

“…Methods to compute c directly from backscattered pressure, accounting for signal loss due to water absorption and sediment scattering (e.g. Betteridge et al [2]; Thorne and Hanes [4]; Crawford and Hay [5]) are difficult to apply for single-frequency sensors when the sediment is inhomogeneous, consisting of a mixture of sand and wash load, and they usually further require an independent estimate of c at some point within the water column. The latter could not be achieved for the deployments at Vejers because the tripod settled some 15 cm into the bed upon deployment and an OBS-3 optical backscatter sensor mounted at a nominal elevation of 0.05 m above the bed was below bed level, or interfering with the bed during the deployment periods.…”

Sediment Size Effects in Acoustic Doppler Velocimeter-Derived Estimates of Suspended Sediment Concentration

“…Here, α s was calculated for each experiment using Equation (5). This approach was proposed by Crawford and Hay [10] as a modification of Urick's (1948) Equation [52]:…”

“…In some cases, the spatial variation in concentration might also be quantifiable. Methods employing acoustic signals tend to be less problematic logistically, as optical sensors are typically much more sensitive to biofouling in natural environments [10][11][12][13]. Multi-frequency acoustic sensors also have the potential to provide information on the size distribution of the suspended sediment.…”

Cross-shore distribution of sediment texture under breaking waves along low-wave-energy coasts

“…The optical and acoustical techniques developed by Hay and Sheng (1992) and Crawford and Hay (1993) are capable of estimating vertical profiles of sediment concentration and mean grain size simultaneously with much higher temporal and spatial resolutions than the pumping, suction, and trapping techniques. Application of the promising acoustic (Hay and Sheng 1992) and laser (Agrawal et al 1996) techniques inside the surf zone is currently limited by several factors, including: (1) harsh environmental conditions caused by high wave energy; (2) significant amount of air bubbles in the water column; (3) large sediment particles in suspension; (4) difficulty in obtaining near-bed (e.g., Ͻ 5 cm) measurements; and (5) high cost.…”

“…Information on the cross-shore and vertical distributions of grain size also enters engineering applications, such as determining compatibility of beach nourishment material and designing of coastal structures to hold sand. Although there have been a large number of studies on the concentration of suspended sediment in the nearshore zone (e.g., Fairchild 1977;Kana 1979;Nielsen 1984;Zampol and Inman 1989;Zampol and Waldorf 1989;Greenwood et al 1991;Barkaszi and Dally 1992;Hay and Sheng 1992;Crawford and Hay 1993), only a few studies have addressed the vertical grain-size distribution of the suspended sediment (Kana 1979;Kennedy et al 1981;Kraus et al 1988;Hay and Sheng 1992), and these studies were conducted on beaches composed of fairly well-sorted fine sands with little cross-shore variation in either mean grain size or grain-size distribution.…”