Using acoustic Doppler velocimeter
backscatter response to determine
suspended sediment concentration
Laura Tait
Amherst College
Mentors: Carl Friedrichs and Grace Cartwright
August 3rd, 2012
Synopsis:
This study revealed both Nortek and Sontek acoustic
Doppler velocimeters (ADV) responded within the interinstrument variation for each manufacturer with a slight
downward bias in acoustic response, measured in a constant
mud concentration, to the application of anti-fouling paint on
the sensors. The accuracy in the sampling methodology used
in this study was calculated to be within 1% of the burst mean
acoustic response measured for each manufacturer. Both the
Nortek and Sontek ADVs produced the expected increase of
backscatter counts linearly proportional to the log of sand
concentration up to about 500 mg/L. The calibrations revealed
the 10 MHz Nortek was less sensitive to the 130-150 µm sand
than the 5 MHz Sontek recording less backscatter counts for
each concentration measured. Backscatter counts began to
increase less quickly with the log of sand concentration at
about 800 mg/L for both instruments and decreased for the 10
MHz case at about 1000 mg/L.
Abstract:
The first of two principle objectives of this research
was to quantify variability in the backscatter measured by the
ADVs, first between instruments, and then within the sampling
procedure itself. The inter-instrument variation in the acoustic
backscatter response for each of the ADV manufacturers,
Nortek (10 MHz) and Sontek (5 MHz) reveled burst average
variation between the instruments was on the order of the interburst variation, except for the higher response by a single
newly repaired Sontek ADV. Application of anti-fouling paint
was found to produce a slight downward bias which was well
within the inter-instrument variation determined for each
manufacturer. A method quantitation limit, a measure of the
level of accuracy of the burst mean acoustic response,
determined both ADV models are accurate to within 1% or
less. The final objective was to investigate the changes in the
regression of backscatter response versus increasing
concentrations of sand (130-150 µm) for several frequencies
(for this paper, only the 5 and 10 MHz cases from the ADVs
are considered). The ADVs responded linearly as expected to
increasing sand concentrations, recording higher backscatter
for higher concentrations up to about 500 mg/L. The 5 MHz
Sontek ADV sensor was more sensitive than the 10 MHz
Nortek, giving a higher backscatter response for each
concentration measured, as expected, due to the frequency’s
higher sensitivity to sand grainsize.
Introduction:
Sediment transport is an important process that greatly
affects the geomorphology of coastal environments such as
estuaries (Friedrichs et al. 2008). Yet the implications of this
movement of sediment extend beyond only physical changes.
Because particles can be a source of both nutrients and toxic
material like pollutants, suspended sediment transport can
distribute these, thus exerting major control on estuarine water
quality (Gartner 2004, Friedrichs et al. 2008, MacDonald et al.
2012). In limiting light transmission, suspended sediment can
also influence photosynthesis, and deposition of this sediment
can potentially render shipping channels not navigable (Gartner
2004). By their very nature, estuarine environments are
constantly changing. Aggregate sizes, suspended sediment
concentration and settling velocities can often shift on short
time scales, making it difficult to study the condition of these
highly variable systems (Fugate and Friedrichs 2003, Gartner
2004). A better understanding of sediment transport processes
is vital for better understanding nearshore sedimentary
processes (Vousdoukas et al. 2011). One crucial part of
sediment transport that needs to be better understood and
measured is the concentration of suspended solids.
Correctly estimating sediment concentration can be a
difficult feat, particularly in estuaries, where the sediment grain
size distribution changes tidally as well as seasonally
(Cartwright et al. 2011). Although originally intended to
measure velocities, the acoustic Doppler velocimeter (ADV) is
one instrument that is being used to determine suspended
sediment concentration (Fugate and Friedrichs 2002,
Cartwright et al. 2012). An estimate of the concentration is
made by relating measured acoustic backscatter “counts” to
sediment concentration. This can often be aided by laboratory
calibrations that build an empirical relationship between the
backscatter intensity and sediment concentration (Thorne et al.
1993, Voulgaris and Meyers 2004, Cartwright et al. 2009,
MacDonald et al. 2012). By emitting sound waves of a known
frequency and recording the backscatter, ADVs are able to
reasonably estimate the concentration of suspended sediment in
water. The data collected by these instruments are invaluable
in obtaining long-term concentration estimates, which are
essential for learning more about estuarine processes
(Cartwright et al. 2009).
While it is true that optical instruments may also be
utilized to estimate suspended sediment concentration, the way
they detect suspended sediment differs from acoustics in
several ways. Optical methodology operates by measuring
light backscattered from the surface area of sediment while
acoustics respond to additional properties of the sediment in
suspension, such as the density contrast between the sediment
and surrounding water. This means that optical instruments are
more sensitive to fine-grain sediment, producing a greater
response for mud relative to the same concentration of sand
because mud flocs have higher surface area than sand.
Acoustics, on the other hand, are less sensitive to mud, giving a
smaller response to mud flocs for the same concentration of
sand because sand produces a stronger density contrast with
water than flocs do. One major drawback of using optical
sensors is they are much more susceptible to biological fouling,
meaning that acoustic sensors can be deployed for longer
periods of time (Gartner 2004). Previous studies suggested the
advantage of using ADVs and other acoustic instruments is that
they are relatively insensitive to aggregate size, responding to
constituent grains rather than the aggregate as a whole (Fugate
and Friedrichs 2002, Scully and Friedrichs 2007). However,
MacDonald et al. (2012) propose that acoustic sensors actually
do respond to the floc itself, not the components, thus
complicating the role of acoustics in determining suspended
sediment concentrations.
Although relatively more is
understood about how acoustics respond to sand particles in
suspension, little is known about how they respond to finegrain particles, especially when the grains bond naturally to
form flocs and/or fecal pellets or when they are part of a
natural mixed grain size distribution of suspended sediment.
The goal of this study was twofold. First, the acoustic
response of ADVs to a known concentration of fine sediment
(less than 63 microns) was tested with and without the
application of antifouling paint on the sensors. This compared
results between instruments of the same frequency and
investigated whether or not the antifouling paint affected the
acoustic backscatter recorded by the ADVs. This is necessary
to determine what the variability is between instruments used
for the long-term monitoring of suspended sediment
concentration for the National Science Foundation MUDBED
(Multi-Disciplinary Benthic Exchange Dynamics) project.
The second part of this research involved conducting an
acoustic regression for a known size distribution of sand (150180 microns) at different concentrations.
This sand
distribution was chosen because it is in region where the
acoustic attenuation relationship to grain-size and frequency is
in the much better understood area where multiple scattering
and particle-particle interactions tend to dominate the acoustic
attenuation (see Figure 1). The ADVs used for the project are
from different manufacturers and operate at different
frequencies, so this tested how each frequency responded to the
changing concentrations. Additionally, five acoustic
transducers that produce raw acoustic response were recorded
for the same sand concentrations as the ADVs. ADVs report
the acoustic backscatter as “counts,” which can then be
converted to approximate sediment concentrations. However,
the counts are possibly modified in some way to reduce error in
the velocities they were originally designed to measure. The
transducers, on the other hand, have no such modification,
which allowed a more pure acoustic response to be recorded.
This experiment was meant to verify that with a known grainsize concentration range, the instruments respond according to
accepted theory that ADV backscatter intensity increases with
increasing sediment concentration.
Methods:
Instrument Inter-comparison:
This section addressed the first objective and consisted
of two experiments (Run 1 and Run 2). In both experiments
ADVs were tested for variation within the same manufacturer
(frequency). In Run 1, acoustic response was also measured
with and without antifouling paint. Four Nortek ADVs (10
MHz) and four Sontek ADVs (5 MHz), were tested in the first
experiment. In the second experiment, one additional ADV
from each manufacturer was tested and two from each were
repeated again to provide continuity between the first and
second experiments and allow comparison of equipment that
had been deployed. Bottom sediment collected in the Clay
Bank region of the York River, VA was wet sieved using
deionized water through a 63-micron sieve and collected as a
slurry. The total solids concentration of the slurry was
determined by pipetting 20 ml into a weighed dish, dried at
103-105 oC and weighed again. An aliquot of the slurry was
then added to the 110-liter calibration chamber (Figure 2) to
bring the concentration to approximately 100 mg/L. Clay Bank
is the location of long term MUDBED tripod deployments
where suspended sediment concentrations in the range of 100
mg/L are frequently estimated using acoustic backscatter from
the ADV and in-situ pumped samples (Cartwright et al. 2009).
The circulating pump incorporated in the design of the
chamber ensured that the concentration was well mixed for the
entire experiment to maintain a constant scattering
concentration at the sampling volume of the sensors.
In the first experiment, the sensors of each ADV were
painted with a thin coat of the antifouling paint (Trilux 33) in
the same manner as they are for deployment on the Clay Bank
tripods (Cartwright et al. 2009). The thickness of the paint was
the thinnest layer possible which did not expose the color of
the underlying transducer. The ADVs were inserted one at a
time into the chamber so that each instrument’s sampling
volume was level with the same chamber sampling tube. A
“painted” sample burst, a five-minute record with a sampling
rate of 8 Hz (Nortek) or 10 Hz (Sontek), was collected for each
ADV. All paint was then removed from the sensors and each
ADV was re-inserted one at a time into the chamber and a
“non-painted” sample burst was collected.
The second
experiment used only used unpainted ADVs so only a “nonpainted” sample burst was collected for each of the ADVs
during this experiment.
During the first experiment, in addition to the ADVs,
“non-painted” sample bursts were collected with five acoustic
transducers (0.5MHz, 1.0 MHz, 2.25 MHz, 5.0 MHz and
10MHz)
using
UTEX
Scientific
Instruments
INSPECTIONWARE program. These data will not be further
considered in this paper but will be used for future analysis.
The five transducers were strapped together and placed in the
chamber so the ADV sampling volume height was in the
middle of the acoustic profile collected one at a time by each
transducer.
A 2-liter water sample was collected at the end of each
experiment by pushing the sample tube that corresponds to the
ADV’s sample volume height into the chamber. This allows
the sample to be collected from the center of the chamber.
These suspended solids samples were filtered through a
weighed glass fiber filter (nominal pore size 0.7 𝜇𝑚), dried at
103-105o and then reweighed to determine the total suspended
solids (TSS) concentration. All recorded data from both intercomparison experiments were analyzed and graphed using
MATLAB.
Method Quantitation Limit:
To determine the repeatability of the sampling
procedure, one Nortek ADV was inserted and removed from
the calibration chamber ten times into the same concentration
as used for the first inter-comparison experiment. Care was
taken to place the instrument sample volume in the same
location of the chamber each time the instrument was inserted
and a five-minute data “burst” was recorded for each insertion.
To compare the variability between manufactures this
procedure was repeated for one Sontek ADV. The data were
analyzed and graphed with MATLAB.
Sand Calibration Study:
Quartz sand was passed through a 180-micron sieve and
captured on a 150-micron sieve to provide a relatively narrow
size distribution of sand for this portion of the study. One
Sontek ADV and one Nortek ADV, both unpainted, were
mounted in the chamber along with the same series of five
acoustic transducers used above, ensuring that the instrument
sampling volume was at the same height in the center of the
tank corresponding with a chamber sampling tube (see Figure
2b). A series of the aliquots of narrow sand distribution was
added to the chamber to bring the expected concentration to
approximately 25, 50, 100, 150, 200, 300, 400, 600, 800, 1000,
1200 and 1600 mg/L, respectively. For each concentration the
ADVs and transducers were turned on one at a time to prevent
cross-talk between the instruments and a five-minute burst was
collected. Before the addition of the next sand aliquot, a water
sample was collected from the appropriate sampling tube and
analyzed for suspended solids concentration by filtering on a
weighted glass fiber filter (~07 µm pore size), dried at 103-105
o
C. and re-weighted.
Pump sampling and filtering for the first half of the
sand calibration study was repeated, using only the
concentrations from 25 mg/L to 400 mg/L in order to obtain
more accurate TSS data. A systematic error had been found in
the pump sampling and the filtering procedure for the first set
of water samples. Because of the shape of the collection
bottles, some sand was caught in the corner of the bottles, and
it was difficult to remove enough of the sand for accurate
filtering in lowest concentration cases, thus giving erroneous
results that were much more evident at the lowest
concentrations. These concentrations were repeated in Run 2
using 2 liter graduated cylinders to collect the samples, and
they were filtered right away. As before, all data were
analyzed and graphed using MATLAB.
Results:
Instrument Inter-comparison:
The measured results of the suspended solids
concentration in the calibration chamber was 130 ± 12 and 80
± 6 mg/L for Runs 1 and 2, respectively. The Nortek burst
mean responses for Run 1, with paint, range between 159.2 and
151.7 counts with burst standard deviations about the mean of
1.8 to 1.9 counts. (Figure 3 and Table 1). The mean standard
deviations between the three beam responses for each record
range between 2.3 and 6.1. Nortek Run 1 responses, without
paint range between 160.7 and 153.2 counts. The Run 2
responses range between 151 and 146 counts (slightly lower
because of the lower suspended solids concentration).
The Sontek burst mean responses for Run 1, with paint,
range between 157.7 and 124.2 counts with burst standard
deviations about the mean of 3.5 to 4.0 counts (Figure 3 and
Table 1). The mean standard deviations between the three
beam responses for each record range between 2.9 and 3.9.
Sontek Run 1 responses, without paint range between 127.5
and 160.3 counts. The Run 2 responses range between 122.4
and 145 counts (slightly lower because of the lower suspended
solids concentration).
Method Quantitation Limit (MQL):
The Nortek and Sontek ADVs both demonstrated a
similar response for the method detection limit and were fairly
consistent during the ten runs. The Nortek range of the burst
means was 143.1 to 146.1 with a mean of the 10 burst means of
144 counts and a standard deviation about that mean of 0.067
counts (Table 2 and Figure 4). The MQL (3 times standard
deviation) was calculated to be 0.199 counts. The Sontek ADV
range of burst means was 123.6 to 126.0 with a mean of the
burst means of 124 counts, a standard deviation of 0.47 counts
and a MQL of 1.41 counts (Table 2 and Figure 5).
Sand Calibration Study:
Overall, the Nortek and Sontek instruments recorded an
increase in backscatter counts corresponding to increased sand
concentration. There are no visual upward or downward trends
observed in the burst data in time, although it is evident some
of the sand was trapped during the circulation process. Figures
6 and 7 show strong linear relationships between the amount of
sand added to the chamber and that measured in the water
sample collected. The relationship between the log10 expected
concentration and the log10 measured concentration was linear
with an R2 value of 0.989 (Figure 6). Figure 7 shows only 63%
of the expected concentration, based on the amount of sand
added to the calibration tank, was actually kept in suspension.
Figures 8 and 9 show a steady upward trend between
the log10 concentration and the acoustic response until about
500 mg/L. At that concentration the relationship appears to
taper off and by 1000 mg/L the Nortek is actually recording a
lower response for an increase in concentration (Figure 8).
almost the same to painting. In addition, these results
demonstrate the potential importance of manufacturing date
and repair history in acoustic response. It is possible that,
because the five Nortek ADVs were built around the same
time, they show a more consistent response across instruments.
The outlying Sontek ADV, V20358, is the only Sontek recently
repaired, perhaps accounting for its anomalously high
response. Further, the other four Sontek instruments are of
varying ages and come from different manufacturing batches.
This may explain in part why the Sontek ADV responses are
more scattered than the Nortek ADVs.
The acoustic response of both the Nortek and Sontek
ADVs was highly correlated with the log10 concentration of
sand in suspension within the linear range of less than 500
mg/L (R2 = 0.969 and 0.982, respectively). The Nortek ADV
exhibited a lower sensitivity by recording a lower number of
counts for the same concentration as the Sontek. The Nortek
ADV also had a smaller range of responses with only 28 counts
separating the highest and lowest concentrations in the linear
range, while the Sontek ADV had a greater range of 47 counts.
The similar results for both Nortek and Sontek ADVs
for the MQL signify that both instrument types are generally
consistent even when removed and re-inserted into the
calibration tank. Therefore any error that may occur from this
process is likely small and will not greatly affect the ADV
response. However, the fact that the backscatter was generally
higher for the 10 MHz Nortek ADVs in both the intercomparison and the method quantitation limit suggests an
important role of frequency in acoustic backscatter.
Attenuation is frequency-dependent, so for the fine sediment
used in these tests, the higher-frequency Nortek ADVs
produced a higher response than the Sontek ADVs.
Discussion:
The instrument inter-comparison tests suggest several
key facts about the effect of antifouling paint on acoustic
response as well as variation within the same frequency ADVs.
Overlapping error bars between painted and non-painted
instruments for both Nortek and Sontek indicate that while
there is a slightly negative backscatter bias for painted ADVs,
in general the difference is not significant enough to strongly
skew data. Overall, both instrument types seem to respond
Interestingly, for the sand calibration study, on average,
the Nortek ADV recorded lower backscatter counts than the
Sontek, reinforcing the fact that attenuation and the transition
between absorption (mud) and scattering (sand) are dependent
on frequency. The general trend of increasing backscatter
counts for both instrument types with increasing sand
concentration adheres to accepted theory and verifies the
acoustic response of both the Nortek and Sontek ADVs. As
expected, with increasing sand concentration, both instruments
eventually begin to respond less due to the attenuation of
acoustic energy by the high sediment concentration. Higher
frequencies tend to exhibit effects of attenuation at lower
concentrations, so because the Nortek ADV operates at 10
MHz and the Sontek only at 5 MHz, the Nortek response
weakens first. This produces the slight decrease in Nortek
backscatter intensity at the highest concentration. On the other
hand, the Sontek response creates more of a plateau consistent
with its lower frequency. The backscatter count range
difference between the Nortek and Sontek ADV may also be a
result of their frequencies, though it could possibly be merely
due to instrument differences.
well as for a method quantitation limit. Further, the sand study
has allowed for the calibration of Nortek and Sontek ADVs,
relating recorded backscatter counts to suspended sediment
concentration. This work substantiates the theory that acoustic
backscatter can be used to determine sediment concentrations.
However, previous studies have shown that acoustic response
acts as expected only for sand concentrations while other
sediment types, particularly mixed sediments, are less clear.
The studies carried out in this experiment set the stage for
future work on this subject to better understand how acoustic
backscatter from ADVs responds to more complex types of
sediment concentrations.
The variation between expected and measured sand
concentration also points to a significant issue about sampling
from the calibration tank. The TSS calculations for mud were
not problematic because mud is so easily suspended in water.
Heavier sand, on the other hand, is more likely to settle in the
chamber and pump, making it more difficult to obtain accurate
samples through the sampling tubes. This additional noise
added by the sampling combined with error during filtering
may account for the discrepancy between expected and
measured concentrations. Nevertheless, despite the fact that
concentration added and concentration measured did not have a
perfect 1:1 ratio, their relationship was still very linear,
meaning that although the sand may not be evenly distributed
throughout the chamber, the area of acoustic sampling is fairly
well-mixed and consistent for each concentration.
The
resulting calibrations for both Nortek and Sontek produced
good linear regressions with a fairly small margin of error in a
95% confidence interval.
Acknowledgements:
I thank everyone who contributed ideas, technical help,
or equipment; particularly my incredible mentors Carl
Friedrichs and Grace Cartwright. I also thank Kelsey Fall,
Carissa Wilkerson and the rest of the CHSD lab for all their
help. Thanks to Drs. Rochelle Seitz and Linda Schaffner for
coordinating the Research Experience for Undergraduates
program at the Virginia Institute for Marine Science and the
National Science Foundation grant OCE-0552612. This work
was also made possible by additional funding from NSF,
Division of Ocean Sciences, grant OCE-1061781.
At the conclusion of this research, all ADVs of both
types have been tested for variability between instruments as
Literature Cited:
Cartwright, G.M., C.T. Friedrichs, P.J. Dickhudt, T. Gass, and
F.H. Farmer. 2009. Using the acoustic Doppler
velocimeter (ADV) in the MUDBED real-time
observing system. Proceedings, OCEANS 2009
MTS/IEEE, CD ISBN: 978-0-933957-38-1, 9 p.
Cartwright, G.M., C.T. Friedrichs, and L.P. Sanford. 2011 (in
press). In situ characterization of estuarine suspended
sediment in the presence of muddy flocs and pellets.
N.C. Kraus and J.D. Rosati (eds.), Coastal Sediments
2011, American Society of Civil Engineers, 14 p.
Cartwright, G.M., C.T. Friedrichs, and P.D. Panetta. 2012.
Dual use of a sediment mixing tank for calibrating
acoustic backscatter and direct Doppler measurement of
settling velocity. Submitted to: Oceans 2012,
MTS/IEEE, Virginia Beach, VA, 14-19 Oct.
Friedrichs, C.T., G.M. Cartwright, and P.J. Dickhudt. 2008.
Quantifying benthic exchange of fine sediment via
continuous, non-invasive measurements of settling
velocity and bed erodibility. Oceanography, 21(4): 168172.
Fugate, D.C., and C.T. Friedrichs. 2002. Determining
concentration and fall velocity of estuarine particle
populations using ADV, OBS and LISST. Continental
Shelf Research, 22: 1867-1886.
Fugate, D.C., and C.T. Friedrichs. 2003. Versatility of the
Sontek ADV: measurements of sediment fall velocity,
sediment concentration, and TKE production from
wave contaminated velocity data. In: R.A. Davis et al.
(eds.), Coastal Sediments 2003, ASCE, CD ISBN: 978981-238-422-5, 14 p.
Gartner, J.W. 2004. Estimating suspended solids
concentrations from backscatter intensity measured by
acoustic Doppler current profiler in San Francisco Bay.
Marine Geology, 211 (2004): 169–187.
MacDonald, I.T., C.E. Vincent, P.D. Thorne, and B.D. Moate.
2012. Acoustic scattering from a suspension of
flocculated sediments. Submitted to: Journal of
Geophysical Research.
Scully, M.E., and C.T. Friedrichs. 2007. Sediment pumping by
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Thorne, P.D., P.J. Hardcastle, and R.L. Soulsby. 1993.
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Figures & Tables
Figure 2: (a)110-liter calibration chamber used for the ADV
inter-comparison and calibration studies (b) sensors mounted in
chamber.
170
Painted Record Mean
Painted Burst means
Non-Painted Record Mean
Non-Painted Burst means
Run 2 Record Mean
Run 2 Burst means
160
Acoustic Backscatter Signal (counts)
Figure 1: Acoustic Attenuation Relationship to grain-size and
frequency.The black box represents the sand distribution used
in this study.
150
140
130
120
110
V20359
V20358
V20146
Sontek Instruments
V20361
V20366
Figure 3: Nortek ADV instrument inter-comparison acoustic
backscatter counts
Method Detection Limit
152
Record Mean
Burst means
Acoustic Backscatter Signal (counts)
150
148
146
144
142
140
138
Run 1
Run 2
Run 3
Run 4
Run 5
Run 6
Nortek
Run 7
Run 8
Run 9
Run 10
Figure 4: Nortek ADV method quantitation limit
Method Detection Limit
132
Record Mean
Burst means
Acoustic Backscatter Signal (counts)
130
128
126
124
122
120
118
Run 1
Run 2
Run 3
Run 4
Run 5
Run 6
Sontek
Run 7
Run 8
Run 9
Figure 5: Sontek ADV method quantitation limit
Run 10
3.5
1:1
Y = 1.0666X + -0.40815
R2 = 0.98938
log10 Concentration Measured (mg/L)
3
2.5
2
1.5
July 20
July 11
All
1
1
1.5
2
2.5
log10 Concentration Added (mg/L)
3
3.5
Figure 6: log10 measured sand concentrations versus log10
expected sand concentrations
1600
1:1
Y = 0.63711X + -9.7819
1400
R2 = 0.98167
Concentration Measured (mg/L)
1200
1000
800
600
400
July 20
July 11
All
200
0
0
200
400
600
800
1000
Concentration Added (mg/L)
1200
1400
1600
Figure 7: Measured sand concentrations versus expected sand
concentrations
Figure 8: Nortek ADV backscatter counts versus log10
measured TSS concentrations for sand
Figure 10: Nortek calibration relating acoustic backscatter
counts to suspended solids concentration for sand
Figure 9: Sontek ADV backscatter counts versus log10
measured TSS concentrations for sand
Figure 11: Sontek calibration relating acoustic backscatter
counts to suspended solid concentrations for sand