Acoustic backscatter profiling sensors (ABS)
This article is a summary of sub-section 220.127.116.11 of the Manual Sediment Transport Measurements in Rivers, Estuaries and Coastal Seas . This article describes the basic principles of acoustic backscatter (ABS) measuring instruments.
Acoustic backscatter (ABS) measurement is a non-intrusive technique for the monitoring of suspended sediment particles in the water column and changing seabed characteristics (see Figures 1 and 2). An acoustic backscatter instrumentation package comprises acoustic sensors, data acquisition, storage and control electronics, and data extraction and reduction software. An overview of the ABS-technique is given by Smerdon, Rees and Vincent Hereafter, a summary of this is given.
The basic principle of the acoustic backscatter approach is as follows. A short pulse (10 us) of acoustic energy is emitted by a sonar transducer (1 to 5 MHz). As the sound pulse spreads away from the transducer it insonifies any suspended material in the water column. This scatters the sound energy, reflecting some of it back towards the sonar transducer, which also acts as a sound receptor. With knowledge of the speed of sound in water, the scattering strength of the suspended material and the sound propagation characteristics, a relationship may be developed between the intensity of the received echoes and the characteristics of the suspended material. With typical acoustic ranges in excess of 1 metre, the acoustic head remains outside the area of study and therefore makes the instrument non-intrusive. The magnitude of the backscattered signal can be related to the sediment concentration, particle size and the time delay between transmission and reception. The acoustic backscatter intensity from a uniform field of particles of constant concentration is assumed to be an inverse function of the distance from the source with corrections for attenuation due to water and particles. Calibration in uniform suspensions is required to find this relationship. The theoretical background of the acoustics is described in detail by Thorne and Hanes (2002). Early work was done by Hay (1983).
The sensor comprises acoustic transducers that emit pulses of sound, which are incident on the seabed. They receive sound reflected by the seabed and suspended sediment in the intervening water mass. The instrument records the amplitude of reflected sound at gated intervals, thus building a reflected sound profile. With low angles of incidence, the technique may be used to monitor the formation and progress of seabed ripples. Perpendicular incidence angles will yield information on sediment suspension between the sensor head and the seabed, and on the erosion or accretion of the bed level. The vertical resolution is limited by the length of the acoustic pulse and by the speed at which the signal is digitized and recorded. A vertical resolution of about 1 cm is feasible. Temporal resolution depends on the pulse repetition rate and on the number of pulses which must be averaged to produce statistically meaningful backscatter profiles. Vincent et al. (1991) used a pulse repetition rate of 10 Hz and four profiles were averaged before storing the data on disc. On average, a profile was recorded every 0.58 s; 1250 average profiles were recorded during each burst (12 min).
Libicki et al. (1989) identified two difficulties in estimating the suspended load from the backscatter signals. First, their instrument did not measure in situ the attenuation of sound caused by the suspended load, which was introducing errors when sediment concentrations were high. Second, their instrument was unable to distinguish between changes in particle size distribution and sediment concentration, although in their experiments they showed that the assumption of a time-invariant particle size distribution did not introduce substantial errors. They felt that to decouple particle size distribution from sediment concentration would require multiple frequency devices with impractically high upper frequency limits. The acoustic method is most appropriate for particle size distributions on the order of tens to hundreds of microns (say 10 to 500 microns).
The early history of ABS use was characterised by concerns about interpretation of backscatter data. In particular, Libicki et al. (1989) were concerned both about the increased likelihood of error caused by signal attenuation by the suspended load itself, and the inability, without using many frequencies, to distinguish particle size and to separate variation in particle size from variation in suspended load. In a series of theoretical studies of backscatter characteristics from suspended glass spheres (Thorne and Hardcastle, 1991; Thorne and Campbell, 1992; Thorne et al. 1992 and Thorne, Manley et al., 1993), the general form function for a suspension of regular shaped scatterers was derived and tested against laboratory experiments. The use of glass spheres was chosen because the main constituent of both glass and marine sediments is quartz. The usefulness of these experiments was further enhanced by recent work on irregularly shaped scatterers (Thorne et al., 1995), which show the general principles applied to regularly shaped scatterers with moderate size distribution is similar to a first order approximation to that of irregularly shaped scatterers, such as would be found in marine sediment.
When sediment concentrations are high, the technique for calculating the concentration profile becomes inaccurate. This is because the attenuation term (a) includes a contribution from suspended load-related attenuation. The iterative approach required to solve for load introduces errors that can cause divergence from the solution. Recently, Thorne, Holdaway et al. (1995) examined how the attenuation due to load may be estimated independently, thereby constraining the load solution. The principle behind their method is to monitor the signal strength of the seabed echo during calm periods, when there is little or no sediment in suspension. This may be used to calculate the attenuation due to water alone, or simply to act as a reference signal strength. By monitoring the strength of the seabed return during periods of high suspended load, the attenuation due to suspended load may be derived.
A method of particle size determination using multi-frequency acoustic backscatter was described and tested by Hay and Sheng (1992). The principle is to use the frequency-dependent variation in backscatter form function for particles where ka<2 with k=wave number, f=frequency, c=speed of sound, a=mean equivalent radius. For particles that lie on this sloping region for at least one frequency and which return sufficient backscatter to be detected, a ratio between two or more frequency responses may be calculated. There must be sufficiently high concentration levels to calculate with reasonable accuracy. In the analyses described, if the measured standard error of estimates of particle size exceeded a certain level, the estimates were discounted, even though this may have resulted in some valid fluctuations in concentration being rejected. Using frequencies of 1MHz, 2.25MHz and 5MHz, they were able to estimate particle sizes in the range 50 um to 170 um to between 10% and 20%. They also note that once the relative sensitivities of the three frequencies have been established, the calibration of the system is site independent.
The expression for the suspended load profile relates suspended load to acoustic backscatter pressure. In a practical system it is necessary to calibrate the system response to known suspensions to take account of variations in the transducer sensitivity, amplifier gain, TVG response and perhaps transducer radiation characteristics. The latter was noted by Downing et al. (1995) to be significant in the correction for near-field results, in which the measured radiating dimensions of their test transducers varied from the physical dimensions by up to 15%. The standard method of calibration is either to use a suspended sediment jet, as described by Hay and Sheng (1992) or a sediment tank in which a homogeneous sediment suspension is circulated. The former method was used to calibrate the relative sensitivities of the three transceiver systems to jets of known particle size distribution. The latter system is commonly used to calibrate the overall system response to a uniform distribution, which may then be extrapolated using the previously described equations for suspended load profiles. Where possible, suspended or seabed sediment samples are taken from the deployment site to back up general calibration data and to analyse particle size distributions for a given site.
Field and laboratory deployments
Vincent and Green (1999) described a field arrangement on the Continental Shelf (Pacific East Coast of New Zealand) with three transducers (F1= 1, F2= 2 and F4= 4 MHz) and a pulse-repetition rate of 80 Hz; each profile recorded consisted of an average of 16 pulses (5 Hz). The vertical resolution is 1 cm. The concentration range is about 0.1 to 20 kg/m3. The system was calibrated in a laboratory recirculating suspension tank using sand from the deployment site (0.33 mm sand). The mass concentration at range r from the acoustic transducer is estimated from a function, depending on the voltage V measured at range r, the sediment density, the speed of sound in water and the attenuation of sound by water and sediment of radius a. The attenuation is a complex form function of ka, which describes the efficiency with which sediment of radius (a) backscatters sound of acoustic wave number (k). Three different acoustic frequencies are used to simultaneously determine the size and concentration of the suspended sediment involved. The strongest acoustic echoes are used to identify the position of the sand bed. Close to the bed, the bed echo dominates the backscattered signal. The concentration at 1cm above the bed is defined at the height at which the first uncontaminated echo occurs, which is identified from a break-in-slope in the concentration profile close to the maximum backscattered signal in the burst-averaged profile. The uncertainty in height is about 0.5 cm.
The concentration profiles measured by the three transducers should be identical, if the calibration conditions are perfect, which means that the suspended sand has the same size distribution at all heights in the water column at the field site and in the laboratory tank. Vincent and Green (1999) show examples of concentration profiles based on the three frequencies, which have relatively large differences (see Figure 3) in concentrations. The concentration profiles were calculated using the results of the calibration tank (based on 0.33 mm sand from the bed at the field site). The concentration profiles differ systematically with F1-concentration < F2-concentration < F4-concentration. When the backscatter data are re-processed using F1 simultaneously to obtain concentration and size, the sand concentrations are between those of F2 and F4-concentration and the suspended sand size varies between 0.25 and 0.15 mm. It is assumed that the suspended sand has a Gaussian distribution at every height and that the width of the distribution is constant. When the F1 and F4-frequency pair is used, the suspended sand sizes become smaller and the concentrations become larger; the latter show a discontinuity due to the shape of the form function yielding ambiguous results for F1-F4 pair. This latter combination of frequencies is very sensitive to small errors in backscatter intensity. These analysis results suggest that the sizes of the suspended sand at the field site differ significantly from that of the bed material used in the calibration procedure. The concentrations derived from the F1-F2 pair were found to be the most reliable. Vincent and Green concluded that the applied form function is not quite right and should be reconsidered.
Another problem is the elimination of the effects of air bubbles in the water column, if the ABS-system (highly sensitive to air bubbles) is used in the surf zone with breaking waves (Huck et al., 1999). This can be done by analysis of the time-averaged concentration profiles, which should show a decreasing concentration with increasing height above the bed. If large amounts of bubbles are present, the concentration profiles derived from the ABS will show an increase of concentration at higher levels. These data records should be excluded from the analysis. The optimum conditions for the ABS-system are: rather uniform fine sand (0.1 to 0.3 mm) in non-breaking wave conditions.
Summaries of the manual
- Manual Sediment Transport Measurements in Rivers, Estuaries and Coastal Seas
- Chapter 1: Introduction, problems and approaches in sediment transport measurements
- Chapter 2: Definitions, processes and models in morphology
- Chapter 3: Principles, statistics and errors of measuring sediment transport
- Chapter 4: Computation of sediment transport and presentation of results
- Chapter 5: Measuring instruments for sediment transport
- Chapter 6: Measuring instruments for particle size and fall velocity
- Chapter 7: Measuring instruments for bed material sampling
- Chapter 8: Laboratory and in situ analysis of samples
- Chapter 9: In situ measurement of wet bulk density
- Chapter 10: Instruments for bed level detection
- Chapter 11: Argus video
- Chapter 12: Measuring instruments for fluid velocity, pressure and wave height
- General principles of optical and acoustical instruments
- Currents and turbulence by acoustic methods
- Acoustic monitoring of marine mammals
- Acoustic point sensors (ASTM, UHCM, ADV)
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