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10th International Symposium on Ultrasonic Doppler Methods for Fluid Mechanics and Fluid Engineering
Tokyo Japan (28-30. Sep., 2016)
Suspended Sediment characterization by Multifrequency Acoustics
Philippe Schmitt
1
, Anne Pallarès
1
,Stéphane Fischer
2
and Marcus Vinicius de Assis
3
1 Laboratoire ICube, Université de Strasbourg, 2, rue Boussingault, 67000 Strasbourg, France
2 UBERTONE, 11 rue de l’Académie, 67000 Strasbourg, France
3 Universidade Federal de Itasuba’, UNIFEI-Av.BPS, 1303 – Itasuba’-MG , Brasil
Sediment transport, either in natural environment or in sewer systems is of main interest to understand river
geomorphology or handle the wastewater regulation and treatment. Knowledge on Suspended Sediment
Concentration (SSC) and size distribution leads to a better understanding of sediment transport dynamics. In a
wide range of rivers and sewer networks, suspended solids often have a bimodal distribution composed of
mineral particles with high diameters and organic matter with smaller sizes. Acoustics methods for the
measurement of small-scale sediment processes in water have gained increasing interest over the past decades.
Ultrasonic multi-frequency profilers allowing acoustic turbidity profiles measurements at high spatial and
temporal resolution which can be linked to the particle presence. The present work will focus on the use of
acoustic signals over a wide frequency range to evaluate suspensions with monodispersed sediment distribution
and with bimodal distribution of known particle sizes and fractions. Investigations on simple models linking the
acoustic signal interpretation and the SSC will be shown, as well as the interpretation of the concentration profile
when the granulometric distribution of suspended sediment shows several modes (sand and clay). Results
obtained on laboratory test bench will be shown, as well as progress on field measurements.
Keywords: Acoustic, Backscattering, Turbidity, Sediment, Suspended Solids.
1. Introduction
The knowledge of sediment transport characteristics is an
important issue in terms of sewer and surface water
management. Indeed, the Suspended Solids (SS)
transported by the (waste)water are a vector of pollution
and they may also be physically damaging [1,2]. A
significant sedimentation in structures can lead to
progressive silting thereof. Appropriate flow
management, in order to limit these phenomena, with
high temporal frequency suspended solids data, is
needed. Suspended Solids Concentration (SSC) is usually
measured either by ad hoc analyzes on samples or
continuously by optical turbidity.
Optical turbidity is the most commonly used continuous
measurement technology for SSC as well in natural water
flows like rivers or in combined sewer systems. Optical
turbidity depends on the colour, size and shape of the SS.
As widely discussed in [3], optical turbidity can, after
adequate calibration, be linearly linked to the SSC.
However, it is a point measurement which might not be
representative of the whole flow and its sensitivity to
biofouling leads to a signal degradation.
Acoustic backscattering or acoustic turbidity is widely
used in marine environment and rivers [4]. The use of
multi-frequency instruments allows to monitor particle
size and concentration. As shown in [5,6] and the
references therein, inversion techniques exist and are
satisfying in flows with limited particle size and nature.
This is unfortunately not the case in rivers and
wastewater for which some attempts have been made [7]
but no systematic inversion technic exists.
The use of multi-frequency Acoustic Backscattering
Systems (ABS) operating at frequencies in the range 0.8
– 8MHz will fit particles in the diameter range 30μm –
300μm. In transceiver mode, the ABS measures the
backscattering and the attenuation characteristics of the
suspended sediments. The backscattered signal used to
estimate concentration depends on the size, the nature
and the quantity of particles in the flow. Thus, the
concentration estimation is difficult, because of the
intertwinement between quantity, shape and density of
particles. We will start from the hypothesis that,
considering a bimodal distribution with known particle
sizes of the fractions, it is possible to determine the
proportion of both fractions by the use of acoustic signals
over a wide frequency range.
2. Acoustic measurements basics
2.1 Pulsed Measurement Principle
ABS usually works on the pulsed Doppler principle. The
emitted signal travels along the beam axis and each
encountered particle partly backscatters a part of the
acoustic wave. This working principle allows the precise
knowledge of the position in the flow of a given
backscattered signal amplitude at a given time stamp.
In the same time, due to thermal conduction and viscosity
effects, the intensity of the ultrasonic wave propagating
in a homogeneous medium decreases. In particle laden
flows, an additional attenuation due to the scattering and
the absorption by the particles themselves contribute to
the intensity decay. This contributes to the decrease of the
backscattered signal amplitude.
2.2 Incoherent backscattering
On the theoretical point of view, the recorded root-mean-
square voltage of the backscattered signal can be written
[8] at range r as follows (Tab.1):
r
ts
rms eM
r
kk
V
a
y
2
2
1-
= (1)
Where
( )
2
1
ss
s
a
f
k
r
=
')'(
4
3
0
drrM
ar
r
s
m
s
wsw ò
+=+=
c
r
aaaa
Table 1: variables definition
V
rms Average value of root mean square voltage
over a large number of backscattered
receptions
k
t
Acquisition system constant
ψ Near field correction
M Particle concentration
α
w
Water absorption attenuation
α
s
Particle scattering attenuation
χ
m
Normalized total scattering cross-section
k
s
Particle backscattering properties
<f> Particle averaged form function
ρ
s
Particle density
<a
s
> Mean particle radius
Thus, the backscattered signal directly includes
information about the particles encountered in the
explored medium. If the particles in the medium are well
known, in terms of shape, size and density, their acoustic
characteristics can be determined. If the content of the
flow is unknown, only a qualitative interpretation can be
made as the relative behaviour of the suspended
sediments concentration for example.
The behaviour of the form function and the normalized
scattering cross section of a particle is well-described by
the variable x = k<as>, which is the ratio between the
particle circumference and the wavelength of ultrasound
in water.
For x
≪
1, the so-called Rayleigh regime, the wavelength
of sound is much larger than the particle circumference
and thus the scattering is considered to be independent of
the particle shape. Thereby, the Rayleigh scattering
description for a sphere can be kept and this implies that
<f> varies with x2 and χm with x4.
For x
≫
1, the geometric regime, the wavelength of sound
is smaller than the particle circumference, and the
scattering cross-section is directly related to the particle’s
geometry. In this case, for a rigid sphere, <f> and χm tend
to a constant value of unity. For irregularly shaped
particles <f> and χm will tend to a constant value slightly
larger than unity.
Thus, the form function <f> and the normalized total
scattering cross-section χm have both high pass filter
behaviour, with cut frequency given by:
min
2a
c
p
n
=
(2)
The particles with smaller radius will have a lower
contribution to backscattering.
2.3 Acoustic characteristics extrapolation
Equation (1) can be rewritten under its logarithmic form:
( )
÷
÷
ø
ö
ç
ç
è
æ+-
÷
÷
ø
ö
ç
ç
è
æ
=
ss
m
w
ss
trms
a
M
r
a
M
fkrV
r
c
a
r
y
4
3
2lnln
(3)
For a homogeneous suspension (for which the
concentration won’t vary with the range), this becomes a
linear equation in ln(Vrψ) and r, and one obtains:
÷
÷
ø
ö
ç
ç
è
æ
=
ss
ta
M
fk
r
h
ln
( )
÷
÷
ø
ö
ç
ç
è
æ+=+=
ss
m
wsw a
M
r
c
aaak
4
3
22
where η and κ are respectively the intercept and the slope
obtained from the plot as expressed in equation (3). This
allows the characterization of the behaviour of an
insonified particle by specifying its form function <f>
and its normalized total scattering cross-section χm. In this
paper we will focus on the χm variable, which present the
advantage to be independent on instrument gain and
bandwidth.
3. Surrogates characteristics
The present study focuses on the acoustical
characterization of different suspensions which models
the compounds present in flows like river and
wastewater, in order to evaluate their concentration and
particle size distribution. Glass spheres (Blanpain) of
different sizes and potato starch (Sigma-Aldrich) were
used. All these components (listed in Tab.2) are
calibrated elements supplied by specialized firms.
Table 2: Particle characteristics used in laboratory experiences
particles Mean radius (µm) Density (kg/m3)
Potato starch 24 1470
Glass spheres 49 2600
Glass spheres 69 2600
4. Measurement bench
To determine the acoustical characteristics of the
particles, all measurements were performed at room
temperature in a 50 L water tank (figure 1). The
suspensions of the particles were obtained by continuous
stirring with a propeller whose frequency was adjusted to
insure homogeneous slurry.
The measurements were performed with an UB-Lab
system (Ubertone, France) and several stand-alone
transducers allowing measurements at different
frequencies growing from 2.2MHz up to 7.5MHz. Care
was taken on the pulse repetition frequency adjustment in
order to allow the sound from one emission/reception
cycle to dissipate before the following cycle. A
temperature sensor completes the test-bench in order to
compensate the temperature effects.
Figure 1: Water tank and instrumentation
For all the measurements, the following common
procedure was applied. The tank was filled with water
from the main supply, and the propeller was activated in
order to allow the air bubbles to leave the water. This
procedure was monitored and lasted until the signals
recorded by the instrument reduced to background levels.
The particles were then added at concentrations of 0.1g/l
and 0.2g/l for mineral matter and 1g/l and 1.5g/l for
organic matter. The propeller velocity was adjusted to
make sure that all the particles are in suspension. After
homogenization of the suspension, a run of six hundred
profiles was realized, each one composed of sixty-four
samples. This procedure was applied for the nine used
ultrasound frequencies.
5. Measurements and analysis
5.1 Laboratory experiences
In a first step a run of measurements was carried out with
a single compound. Figure 2 represent a typical
recording, here for a suspension of glass spheres with a
radius of 49µm. To obtain the information about the total
scattering cross section χm, the expression in Eq. (3) was
used. The figure shows the variation of ln(rψV) as a
function of the range r from the transducer, after
suppression of the near field.
Figure 2: ln(rψV) as a function of range at different frequencies.
The slope of the curves at all frequencies gives the
attenuation due to the suspended particles. Considering
that the density, the mean size and the concentration for
the different suspended materials is well known in our
tank, we can evaluate the normalized total scattering
cross-section χm at the different ultrasound
frequencies [9].
5.2 results and discussion
Figure 3 present the theoretical curves of the normalized
total scattering cross-section χm versus frequency for the
potato starch and the glass spheres of radius 49µm [8].
Figure 3: Theoretical χm versus frequency.
The frequencies available on the UB-Lab allow only
measurements in the Rayleigh regime and in a part of the
intermediate regime. Results obtained for very small
frequencies (x<<1) might have a high degree of
uncertainty because in this case the attenuation is mainly
due to water. Nevertheless, significant measurements
were done on frequencies growing up from 2.2MHz to
7.,5MHz.
Figure 4: Xm versus frequency for potato starch and glass
spheres from two different sizes, comparison with theory
Figure 4 shows the measured normalized total scattering
cross-section χm as a function of frequency for potato
starch and two types of glass spheres. We can observe
that the ratio between the values for the mineral particles
and the organic one change in a meaningful way when
the frequencies increase: at frequencies under 3MHz, the
χm factor for potato starch is even 20 times smaller as the
χm factor for glass. At frequencies over 6MHz, this ratio
falls to 6.
Preliminary results on a combination between glass and
potato particles show a coherent behaviour. At low
frequencies the contribution of the potato starch is not
significant in the ultrasound measurements. By increasing
the frequency, the backscattered signal shows more like a
concentration combination of the two components. More
investigation has to be done on this subtraction approach,
in order to define the selection rules between mineral and
organic particles. Nevertheless, these results shows that
low frequencies allow to identify mineral particles and
higher frequencies are more sensitive to combination of
mineral and organic particles. Field measurements
presented below shows consistent results.
5.3 Field measurements
The measurements were undertaken in the entry chamber
of the wastewater treatment plant of Greater Nancy
(250 000 p.e.) from May to November 2014. Its reference
flow is 120 000 m3/day and 65% of the wastewater
comes from a combined sewer system.
An UB-Flow 315 from Ubertone was mounted on an
articulated arm. Therefore, the device was floating on the
water surface and looking down at the chamber bottom.
Measurements were taken at different frequencies to be
sensitive to different particle sizes and compositions. In
parallel to the acoustic measurements, optical turbidity
was continuously recorded by a turbidimeter (Solitax,
Hach Lange) which was mounted on the articulated arm,
next to the profiler. According to the weather conditions,
specific series of wastewater samples were collected
every hour by dry weather and every 15 minutes during a
rain event.
Figure 5: Acoustic turbidity evolution with time and frequency
for dry weather.
Figure 6: Acoustic turbidity evolution with time and frequency
for storm weather.
Figure 5 shows the change in the acoustic turbidity
(similar to amplitude but with instrument corrections)
versus time at different frequencies for dry weather and
figure 6 for a storm event. Whatever the weather, the
evolution of the acoustic turbidity is similar at the
different frequencies. During dry weather, a maximum
intensity is observed for the highest frequency, 4.167
MHz, foreshadowing the preponderance of particles less
than 60 microns radius. During storm events, the
maximum turbidity is observed for both lower
frequencies; this suggests a majority of mineral
suspended solids with radius less than 300 microns.
Furthermore, the comparison of the turbidity shows that
there is a factor of 100 between measurements in dry
weather and those in rainy weather.
6. Summary
The present study focused on the scattering properties of
suspension of potato starch and glass spheres, and
application on field measurements. It is a part of a larger
work which includes the evaluation of <f> and χm in both
Rayleigh and geometric regimes for particles which
models the compounds present in flows like river and
wastewater. The goal is to classify suspensions in particle
sizes and classes by the use of several different
ultrasound frequencies.
Measurements carried out in a wastewater flow shows
that discrimination between high mineral and smaller
organic particles can be operate. This tendency is
observed on different measurement sites, and the work
presented on this paper must be carry on in order to
create a merge between laboratory estimations and field
measurements.
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