MEASURING THE AZIMUTHAL VARIABILITY OF ACOUSTIC
BACKSCATTER FROM LITTORAL SEABEDS
PAUL C. HINES, JOHN C. OSLER AND DARCY J. MACDOUGALD
Defence Research Establishment Atlantic, P.O. Box 1012, Dartmouth, NS, Canada, B2Y 3Z7
E-mail: [email protected]
The acoustic backscattering strength of the seabed has been demonstrated to be one of
the key inputs required in sonar performance prediction (SPP) models. Extending range
independent SPP models to full 3-dimensional – or even the simpler Nx2-dimensional
models – requires measurements of the azimuthal variability of the backscattering
strength. DREA’s Wide Band Sonar (WBS) system, which consists of a parametric
transmitter and a superdirective receiver, is ideally suited to make these measurements.
In May 2001 the system was used as part of a joint US-SACLANTCEN-CA experiment
referred to as Boundary Characterization 2001. The system collected acoustic backscatter
data as a function of azimuth at two shallow water sites off the east coast of North
America. In addition, backscattering strength measurements were made at several subcritical grazing angles. In this paper the attributes of the WBS are outlined and some of
the results from the experiment are presented.
1
Introduction
Sonar performance prediction (SPP) models require several environmental inputs in
order to estimate detection ranges. For sonars operating in littoral waters in the low kHz
band, key inputs include bottom loss, sea surface loss, backscattering strength (BSS) of
the seabed and the sea surface, ambient noise, and the sound-speed profile for the water
column. Reasonable estimates of sea surface scatter, forward loss, and ambient noise
can be obtained using the wind speed [1–3], and bathythermographs are routinely used
to obtain the sound-speed profile. In fact, advances in satellite sensing show potential
for remotely estimating both the wind speed and sound-speed profile [2]. By contrast,
estimating those sonar parameters related to the seabed poses a greater problem. For
example, there is no easily measured correlate from which to infer the bottom
backscattering strength in the way one uses wind speed to estimate surface scatter. In
fact, the relative importance of seabed roughness, sediment type, and grain size to
bottom scattering strength is still widely debated. As a result, many sonar performance
prediction models rely on data bases to estimate seabed scatter. Often, these data bases
are sparsely sampled spatially or use broadly defined regional descriptors such as “silt”
or “rock” to estimate the scattering strength. Measurements of seabed scatter are
critical to provide ground truth validation for these data bases. Measurements should
also unravel the underlying physics of scattering which will improve ones ability to
predict scatter given geo-technical information about the seabed.
179
N.G. Pace and F.B. Jensen (eds.), Impact of Littoral Environmental Variability on Acoustic Predictions and
Sonar Performance, 179-186.
© 2002 All Rights Reserved. Printed in the Netherlands.
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PAUL C. HINES ET AL.
Scattering strength estimates for littoral seabeds have been extracted from
reverberation experiments [e.g. 3]; however, this technique requires careful
interpretation because scatter from several grazing angles having undergone different
numbers of boundary interactions arrive simultaneously.
We require direct
measurements of seabed scattering – preferably unhindered by vertically directed
sidelobes which generate unwelcome fathometer returns. Furthermore, if one hopes to
extend range independent SPP models to full 3-dimensional (or even the simpler Nx2dimensional models) measurements of the azimuthal variability of the backscattering
strength are required. In shallow water, conventional sonars are poorly suited for this
task at low kiloHertz frequencies. This is because the sonar’s size is roughly
proportional to its acoustic wavelength and to project a narrow acoustic beam, the sonar
must be many wavelengths long. This in turn places the seabed in the near-field of the
array which adds an extra layer of complexity to data interpretation. Additionally,
strong returns on array sidelobes can mask weak on-axis returns resulting in erroneous
estimates of the directional dependence of the scatter.
To address these issues the Defence Research and Development Canada (DRDC)
Atlantic (formerly DREA) has developed a wide-band active sonar (WBS) with which
to interrogate the seabed and quantify its geo-acoustic properties. The WBS employs a
parametric transmitter to make these measurements. The parametric transmitter offers
several advantages for the current experimental requirement. First and foremost, due to
the nature of signal generation in the parametric array, no sidelobes are formed. This
feature avoids extraneous boundary interactions when making measurements in shallow
water. Second, the beamwidth of the parametric array is extremely narrow relative to
the transmitter aperture. In the present case a square transducer measuring 41 cm on a
side yields horizontal and vertical beam widths of approximately 3° across the
difference frequency band. This coupled with the absence of sidelobes enables accurate
measurements of the azimuthal variability of the backscattering strength. Third, one can
obtain a wider bandwidth than that obtained using a conventional source. In the present
case a bandwidth of 1 kHz to 10 kHz is realized from a single transducer. The wide
bandwidth means that a single transducer can be used in place of a suite of transducers,
each of which may require separate power and tuning circuitry.
In May 2001 the system was used as part of a joint US-SACLANTCEN-CA
experiment referred to as Boundary Characterization 2001. The system collected
acoustic backscatter data as a function of azimuth and grazing angle at two shallow
water sites off the east coast of North America – one near New Jersey and one near
Nova Scotia. Measurements were made at grazing angles ranging from 30° down to
2.5°, at frequencies of 4 and 8 kHz. Following the introduction, the attributes of the
WBS are outlined and a sample of the measurements of backscatter variability are
presented.
2
The Wide Band Sonar (WBS)
Defence Research Establishment Atlantic has developed a wide-band sonar (WBS) for
collecting environmental acoustic data in the open ocean [4]. A schematic of the system
is shown in Fig. 1. The primary acoustic sensor suite consists of a parametric array
transmitter (PATS) and a superdirective endfire line array receiver (SIREM).
AZIMUTHAL VARIABILITY OF ACOUSTIC BACKSCATTER
181
Mechanical steering of these arrays is remotely controlled from the research ship via an
RF radio link fixed to the system’s surface float. This minimizes the risk of acoustic
interference from the ship and prevents ship motion from compromising its stability. In
addition to acoustic sensors, the system is instrumented with a range of non-acoustic
sensors to assist in the evaluation of the data. The non-acoustic sensors include depth,
tilt, roll, and heading sensors to monitor the position and direction of the parametric
array, as well as accelerometers to monitor platform vibration. The sonar head can be
panned through 360° azimuth. The system can be configured to be either bottomtethered (Fig. 1a) or bottom-mounted (Fig. 1b). In bottom-tethered mode the sonar
support arm can be rotated 180° vertically so that the sonar can be positioned above or
below the space frame. This enables measurements through 4π steradians. In this
configuration, platform stability is achieved by de-coupling the space frame from the
surface float through a weighted cable and streaming the space frame into the prevailing
shear current. In bottom-mounted mode a remote command is sent from the ship to the
surface float to flood the sub-surface floats on the space frame following deployment.
This causes the system to descend slowly to the seabed. Once the floats are fully
flooded the system weight in water is approximately 4500 N. This offers an extremely
stable platform that permits coherent averaging of multiple pings. This is used in low
SNR conditions such as measurements of backscattering strength at very shallow
grazing angles. Prior to recovery, compressed air is forced into the sub-surface floats to
evacuate the water. When the system is bottom mounted, physical constraints limit the
range of vertical angles to -30° from the horizontal up to +90°; however, the head is
still able to rotate 360° in azimuth. The experiments discussed in this document were
all performed with the system in bottom-mounted mode.
The parametric transmitter – the advantages of which have been highlighted already
– consists of nine square ceramic piston transducers arranged in a 3-by-3 grid. At full
power, the primary source level is 242 dB re 1µPa@1m at 100 kHz. Pulse duration can
range from 50 µs to 250 ms. The main disadvantage in employing a parametric array is
that the conversion of energy from the primary frequencies to the difference frequency
is a second order process, and therefore inefficient. Table 1 lists the difference
frequency source level (SLd) referred to 1 m range measured at a selection of difference
frequencies.
Table 1. Parametric array source levels measured at several difference frequencies.
fd (kHz)
1.0
2.0
4.0
8.0
2
SLd (dB//1µPa )
166
172
182
188
To complement the compactness of the parametric transmitter, a six-channel
superdirective hydrophone line array (SIREM) was developed as the principal receiver.
SIREM is mounted on the parametric array head and turns with the array. (Recall
Fig. 1). A superdirective array relies on computing pressure gradients and as such
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PAUL C. HINES ET AL.
requires inter-element spacings that are a small fraction of an acoustic wavelength.
Thus, by its very nature, a superdirective array is much more compact than a
conventional array. The principal disadvantage with a superdirective array is its
susceptibility to incoherent noise such as sensor self-noise and inter-channel phase
errors. That is to say, the very process of taking the difference between the acoustic
signals at two sensors means that the signal to noise ratio (SNR) must degrade. In
practice the array yields gains of up to 15 dB across the sonar's frequency band from an
array aperture only 0.8 m long.
The system also has a tri-axial intensity array known as SIRA (Sound Intensity
Receiver Array) fixed to the spaceframe. This secondary receiver is used to measure
ambient noise directionality. The superdirective array weights can then be optimized for
the specific ambient noise field. (SIRA can also be used to localize the platform during
bistatic experiments. It’s bearing accuracy is better than ±0.5°.) Note that all data
reported in this paper were collected using a single hydrophone from SIREM.
In the following section, an experiment to measure the scattering strength of the
seabed as a function of azimuth and grazing angle is described. Particular emphasis will
be placed on the variability of the scattered energy as a function of azimuth.
Figure 1. Schematic of Wide Band Sonar bottom-tethered (left), bottom-mounted (right). Note
that the pivot point for the sonar support arm is different for the two configurations.
3
The scattering experiments
A series of experiments to measure the dependence of seabed backscatter on azimuth
and grazing angle was performed at two shallow water sites on North America’s Eastern
Seaboard – one site known as Strataform is located off the New Jersey coast and the
second site is about 100 nmi. southeast of Halifax, Nova Scotia on Canada’s Scotian
AZIMUTHAL VARIABILITY OF ACOUSTIC BACKSCATTER
183
Shelf. The sites were chosen primarily because of the availability of supporting geotechnical measurements to assist in data interpretation and modeling [5, 6].
Figure 2 contains a sketch of the geometry for the measurements of azimuthal
scattering. The array was pointed at a grazing angle in the range of 5° to 15°. At each
azimuthal angle, a series of 50 pulses was transmitted by the WBS at 4 and 8 kHz and
Figure 2. Geometry for measurements of the azimuthal variability of acoustic scattering.
acoustic backscatter from the seabed was recorded on SIREM. A pulse duration of 2 ms
was used with a pulse repetition frequency (PRF) of 4 per second. The parametric array
transmitter was rotated approximately 1.5° in azimuth and the sequence was repeated.
This sector scan covered approximately ∆θ = 25°. Time constraints did not permit a
360° sector scan; as a compromise, the parametric array was rotated 90° in azimuth
relative to the center of the sector scan and the experiment was repeated for 2 azimuthal
angles separated by 3°. This procedure was adopted to allow an examination of the
small scale transition of backscatter across azimuth, while at the same time providing an
opportunity to observe any drastic variations that might only show up with a substantial
change in azimuth. Backscattering strength was measured at grazing angles ranging
from 20° down to 2.5° at the midpoint and the extrema of the sector scan. In the
following section, samples of the azimuthal variability and the grazing dependence of
the backscatter are presented.
PAUL C. HINES ET AL.
184
4
Results and discussion
Figure 3 shows waterfall displays of the azimuthal dependence of the backscattered
energy vs. time as measured on a single hydrophone in SIREM. The data are from the
Strataform site at 4 kHz (left) and 8 kHz (right). The top (dashed) trace is the average of
the 17 angles used in the sector scan. The 17 solid lines below it correspond to the data
at each azimuthal angle. Each trace in the waterfall represents the coherent average of
50 pulses at a single azimuth. These data were taken with the parametric array pointed
at 10° grazing. This corresponds to a two-way travel time of approximately 20 ms at the
center of the beam. The two dashed lines at the bottom correspond to measurements at
azimuths of 90°and 93° from the center of the sector scan. Unfortunately, experimental
constraints required that for these measurements the sonar be pointed at 13° grazing
(rather than 10° as was used during the sector scan). This corresponds to a two-way
travel time of 15 ms to the center of the beam.
The waterfall display shows rich structure in the azimuthal dependence of the data.
For example the arrow in the 8 kHz data points to a single peak splitting in two and then
consolidating back into a single peak. This occurs a couple of times as one passes
through azimuth. Similar features occur in the 4 kHz data. Additionally at 4 kHz, a
sharp ridge (denoted by the arrow) can be seen in the upper traces that disappears near
the bottom of the figure. At this time it is unclear whether these features result from
interface structure or possibly shallow sub-bottom layering.
4 kHz
8 kHz
50
50
0
0
-50
-50
-100
-100
-150
-150
0
20
40
time (ms)
60
80
0
20
40
60
time (ms)
80
Figure 3. Waterfall display of azimuthal dependence of backscattered energy vs. time for the
Strataform site taken at 4 kHz (left) and 8 kHz (right). The topmost (dashed) trace is the average
for the entire sector scan. The two dashed lines at the bottom are measurements taken at azimuths
90° and 93° relative to the center of the sector scan (see text).
185
AZIMUTHAL VARIABILITY OF ACOUSTIC BACKSCATTER
The backscattered energy was measured with the acoustic axis centered at slant
angles of 5°, 7.5°, 10°, 15°, and 20°. To prevent discrete features from biasing the
results, measurements were made at several azimuthal angles and the rms average was
computed. The difference frequency source level at the seabed was estimated using a
combination of calibration data and model evaluation [7]. Correcting the source level
for the parametric array beam pattern and then converting time of flight to grazing angle
allowed a range of grazing angles to be spanned for each slant angle. This procedure
allowed for some overlap in the curves thereby increasing confidence in the results.
-10
-20
BSS (dB)
8 kHz
4 kHz
-30
-40
-50
-60
20
15
10
5
0
grazing angle (degrees)
Figure 4. Backscattering strength vs. grazing angle at 4 and 8 kHz for the Strataform site.
-10
-20
BSS (dB)
8 kHz
4 kHz
-30
-40
-50
-60
20
15
10
5
0
grazing angle (degrees)
Figure 5. Backscattering strength vs. grazing angle at 4 and 8 kHz for the Scotian Shelf site.
PAUL C. HINES ET AL.
186
Figure 4 shows the BSS at 4 and 8 kHz for the Strataform site plotted as a function of
grazing angle. There appears to be little, if any, frequency dependence in the scattering
strength. The slope of the 8 kHz data appears to be slightly steeper than for the 4 kHz
data; however, this may result from interference from the noise floor at 4 kHz at the
shallow angles. Figure 5 shows the BSS at 4 and 8 kHz for the Scotian Shelf site plotted
as a function of grazing angle. As for Strataform, the BSS appears to be independent of
frequency within the statistical accuracy of the measurements.
5
Concluding remarks
The Wide Band Sonar is an effective tool for examining seabed acoustics. It permits a
wide range of experimental geometries, minimizes the risk of acoustic interference from
the ship, and prevents ship motion from compromising array stability. It’s narrow beam
width makes the system particularly well suited to measure the variability of acoustic
scattering from the seabed in addition to making direct (rather than inferred)
measurements of the backscattering strength of the seabed.
In this paper, the azimuthal variability of the scattered intensity and the grazing
angle dependence of the seabed backscattering strength are reported for frequencies of 4
and 8 kHz. The measurements were made at two shallow water sites referred to as
Strataform and Scotian Shelf. Initial results show substantial azimuthal variability of
seabed backscatter. At both sites, the backscattering strength measurements appear to
be independent of frequency within the statistical accuracy of the data.
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