on VM, however, as the integrated effect of the attenuation
accumulates there is a departure from the linear curve. It can be seen that for the further
ranges from the transceiver for increasing concentrations, the backscattered pressure actually
begins to decrease. The effect of the attenuation more than offsets the signal increase due
to the linear dependence on VM and begins to dominate the backscatter response. Good
agreement is again seen between observations and predictions. This non-linear response of
upon VM with range and concentration demonstrates the difficulty in trying to
obtain an empirical calibration for an acoustic backscatter device. Comparison of the
variation with range of the ratio of the acoustic estimate of the concentration, MA, calculated
using equation (5), to the measured pumped sample value, Mp, are shown in Fig 5c. The
results were taken using a concentration of 2Kgm". Ideally the value of this ratio should be
unity and constant with range. Three scenarios are presented (ignore the dashed lines). The
centre result was computed by evaluating a, using the measured concentration, while the
upper and lower curves were obtained with no input from the measured concentration. In the
marine environment a, cannot be input to the calculation since the concentration is the value
unknown. Therefore the step-wise procedure discussed earlier was employed to calculate
M(r) and a, sequentially, and feed back these values to progressively compute the suspended
concentration profile. It can be seen that a small variation of K, or Kt, in this case an 8%,
changes the concentration from the upper line to the lower line. This results in a change of
concentration, at a range of close to Im, of over two orders of magnitude. This is a
relatively extreme result, however, it does demonstrate the possible difficulties that can be
encountered when suspended loads are high, and attenuation by the sediments dominates the
range dependence of the backscattered signal.
3.2 Marine data
The estuarine measurements were carried out during a Spring tidal period over three
consecutive floods. The bed consisted of fine sand and the area was dominated by sandwaves
which typically have a wavelength of 15-20m with a trough-to-crest height of the order of
0.8m. Measurements of the suspended load were taken above the bed at 0.1, 0.2, 0.4 and
0.8m using pumped sampling. The acoustic concentrations values were computed using
equation (5). The acoustic estimates of concentration were obtained by averaging the acoustic
data over the same period as the pumped sample data, this was 60s. The results of these
acoustic estimates of the suspended sediment concentration over three consecutive floods are
compared with the pumped sample data in Fig 6. The comparison of the concentration
estimates obtained using acoustic backscattering with the estuarine pump sample data
collected over the three consecutive floods show good agreement. The gradient of the
experimental data is close to unity and positioned about the line MA=Mp.
0 A X
10- 10- 10-1 100 ltO
6. Acoustic concentration versus pump sampled data.
The similarity of the pumped sampled data and the acoustic estimates of concentration
provide confidence in the accuracy of the acoustic technique. Th6e acoustic data can now be
employed to analyse the details of sediment transport. These results are shown below in Fig
7. Fig 7a shows the full concentration field, and Fig 7b shows values with the mean
concentration subtracted and only values above the mean plotted. The latter show the high
concentration events. These data cover a period of 80s, and illustrate the variability of the
suspended load on a time scale of the order of seconds. It is expected that by identifying the
relationship between these sediment structures, and the turbulent tidal flow, that a detailed
understanding of sediment transport process can be developed.
Fig 7. a) Measured suspended concentration over 80s. b) Fluctations from the mean over the
same period as a).
4.1 Scattering regime of the ABS
For the 1-5 MHz ABS systems currently being employed, it is useful to ascertain the
scattering regime into which the data usually fall. Most studies have been concerned with
non-cohesive sediments with particle sizes ranging from a,= 50-150/m, this covers the
ka,= 0.2-3. This regime is primarily in an intermediate region above Rayleigh scattering and
below optical scattering. Therefore in only a limited number of circumstances will the
approximations in equations (17)-(20) be valid, and typically equations (6) and (12) will need
to be employed.
For the attenuation three contributions are present; the absorption by the water, the scattering
from to the sediments and the absorption due to the sediments. Values at 3MHz are chosen
to establish the relative importance. Using equation (7), and assuming a temperature of
14 o C, the attenuation is 0.28 Nepers m-' which for a range of operation of Im gives a
signal level reduction of 5 dB or close to 40%. To estimate the attenuation due to the
suspended sediment a particle size of a,=100gm is used. From data comparable to that
shown in Fig 3 it can be shown that if the sediment size is in the non-cohesive regime
scattering dominates. For a uniform concentration of 0.5 Kgm3-, aci=0.28 Nepers and
a2=0.0025 Nepers m-1. The viscous absorption due to the particles is therefore negligible,
and the sediment concentration is comparable to the water absorption. Therefore typically for
scattering by non-cohesives at Mega-Hertz frequencies, attenuation due to scattering plays
an important role in interpreting the data.
4.2 Calibration of an ABS
Ideally to evaluate equation (5) all the parameter on the RHS the equation should be known.
In general it would be useful to evaluate IK, however, this is often not practicable, since it
requires measurements of the transducer beam pattern and absolute pressure measurements.
Therefore it has to be accepted that in general K, will be treated as a scaling constant in the
system. This presents no problems as long as the system parameters remain constant. To
obtain K,, requires sediment samples comparable to the suspended sediments at the marine
site to be employed in a laboratory calibration. This will then effectively provide, IK*K, and
the value for, ,s. If it can be assumed that the particle size remains constant through the
water column in the marine environment then these parameters are constant. If a, is varying
with height above the bed the interpretation of the acoustic data becomes more difficult, and
it is then necessary to know the temporal and spatial variation of the particle size to interpret
the backscatter data. This would be difficult to obtain, however, a useful approximation
would be to measure the mean particle size profile with height above the bed and employ this
to interpret the backscatter ABS data. Therefore a basic calibration would be obtained using
in-situ samples at different heights above the bed over the measurement period at a particular
site. Half the data can then used to calibrate the acoustic data and the rest is used to test the
calibration. This calibration can then be applied to the whole acoustic data set and known
accuracies can be compute from the calibration test. Given sediment attenuations are not to
great, M(r) <0.1Kgm-3, and the variance in particle size at a given height above the bed is
reasonably low, a(a,)<20%, between in-situ samples, then the acoustic inversion should
valid. This is underpinned by the acknowledgement that
has been obtained from
about 100 backscattered profiles. This number is required to negate the variability in
backscattered signal amplitude due to the statistical distribution of the amplitude, owing to
the phasing of the echo from each of the individual particles insonified.
An examination of the application of acoustic scattering for measuring suspended sediment
concentration has been presented. A theoretical description of the interaction of sound with
suspensions has been tendered, and this compares favourably with laboratory observations.
In the marine environment time averaged acoustic concentration predictions were seen to be
in good agreement with pumped sample data taken in an estuary.
To obtain accurate results requires, significant data averaging of the amplitude of the
backscattered signal to overcome the statistical distribution of the backscattered echo,
estimates of the particle size distribution with height above the bed and careful evaluation of
Kt*K, and a,. There is also the inversion problem which is typically manifest at high
concentrations and the further ranges from the transceiver. However, the gains in the
description of the suspension, which can be attained using the acoustic approach, does
provide the impetus for resolving some of the acoustic problems, and obtaining a detailed
picture of suspension processes.
Finally the use of acoustics for measuring suspended sediments is an expanding and ongoing
process. The theory is moderately well developed, although more so for non-cohesive than
cohesive sediments. Further studies are required in the latter. Once confidence has developed
in the acoustic approach, it should provide a significant advance in a our ability to monitor
and predict sediment movement.
1 A.E. Hay. Sound scattering from a particle-laden, turbulent jet. J. Acoust. Soc. Am.
90(4) 2055-2074 1991.
2 P.D. Thorne and S.C. Campbell. Backscattering by a suspension of spheres. J.
Acoust. Soc Am. 92(2) Ptl 978-985. 1992.
3 P. D. Thorne, K. Waters, and T. J. Brudner. Scattering of sound by irregularly
shaped particles. ARL report Uni Texas at Austin. ARL-TR-92-23
3 R. J. Urick. The absorption of sound in suspensions of irregular particles. J. Acoust.
Soc Am. 20(3) 283-289. 1948.