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Published in issue No 99, January 2001 of The Hydrographic Journal
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Reflection of Sound from Submerged Plates and Bottom
Backscatter in Shallow Water
Stephen
Anthony Philpott BSc, MSc and Gwyn Jones
MInstCES, FRICS
Institute of Marine
Studies, University of Plymouth, UK
Abstract
This paper presents an
abridged version of an MSc Hydrography dissertation. The paper describes the experimental work undertaken by the
student; his experiences of setting up the experimental conditions are followed
by the results concurring with the theory expounded by Urick and others. The experimental
work enabled the student to expand his knowledge of acoustic theory and to draw
conclusions about target type and aspect and the impact of seabed relief,
giving due consideration the limitations of particular equipment.
Stephen
Philpott was awarded the RICS Prize for the ‘best overall performance’ on his
programme of study.
1.0 INTRODUCTION
The reception of echoes from reflecting or scattering targets is well
known in radar and underwater acoustic echo ranging. Prediction of the strength
and character of the echoes is however a complex task.
The sonar equation provides a useful tool for the engineer/surveyor
involved with the prediction of target strength for large, complex, extended
structures. However for the purposes of detection and classification of a
target, one must also find a means of increasing the signal-to-background noise
ratio.
Underwater targets return acoustic energy by a number of processes, which
include specular reflection, scattering by surface irregularities and resonant
effects. Resonance of a target can, in principle, lead to enhanced target
strength and ultimately a greater probability of detection. Similarly,
backscatter from sedimentary bottoms is reinforced, provided that the surface
roughness is a function of the wavelength.
Echo characteristics can also be useful to the sonar engineer since a
reflecting object imposes its own characteristics upon the echo, such that it
has a different wave shape from the incident pulse. Acoustic theory, with
particular reference to its practical applications in the detection of targets
and the function of backscattered energy, together with the sonar equation and
the application of these concepts in echo sounding are all well documented
(Urick, 1983).
Analysis of these target response factors under laboratory conditions has
been conducted at the University of Plymouth. Trials were initially conducted
via a series of simple tests in order to determine baseline equipment
specifications, and then followed by laboratory tests on scale models,
observing the variable responses from planar targets.
2.0 MODEL EXPERIMENTATION
The analysis of target strength for planar targets using scale model
measurements was conducted assuming ideal conditions, namely:
a) a homogeneous medium;
b) good acoustic contact between the transducers and the water;
c) the spreading of sound energy follows the inverse square law;
d) refraction effects caused by pressure and horizontal/vertical
temperature gradients are small and negligible;
e) the incident pulse is many
cycles long, thus can be considered as a continuous wave.
(Stephens, 1970)
2.1 The Scaling Factor
Scale model measurements made in a water tank benefit from stable
conditions for the measurement of accurate alignment/adjustment of targets and
from the ability to make repeatable measurements. However various parameters
exist which require consideration when deciding upon the scale factor. These
include:
a) dimensions of the water tank;
b) ability to manufacture targets of a tolerable accuracy;
c) beamwidth(s) of the transducer(s);
d) the bandwidth of the system;
e) attainment of an adequate
signal-to-noise ratio of the system.
It was intended to cover depths of water equivalent to approximately 15m
to 20m and to be able to deal with a wide range of frequencies. Considerations
such as these led to a scaling factor of 1:14. However, due to the lack of
suitable transducers, it was not possible to scale down the parameters that are
functions of the transducers.
It should also be stated that in the laboratory work, attenuation in the
water column has been neglected owing to the small dimensions inherent.
Observations have been primarily concerned with a study of the factors
influencing the reflected waveform of a plane target/bottom.
2.2 Initial Trials
Preliminary trials with a pair of 208kHz transducers were conducted in
the water tank facility. These were conducted using a DE-719B Fathometer, a
7245A Transducer and an Isotech ISR 420 oscilloscope.
Preliminary testing with
this experimental equipment highlighted a number of concerns. These included:
• Reverberation Level
The walls of the tank proved
to be very efficient reflectors of sound. A regular succession of end-to-end or
side-to-side echoes was displayed on the oscilloscope as a continual waveform
revealing high levels of reverberation.
• Amplitude Recording
Adjustment
The normal incident signal
received at the hydrophone was weak at grazing angles less than 40°, making it
increasingly difficult to measure. Amplification of the received signal was
thus required.
2.3 Acoustic Systems
Trainer for Sonar – Instrutek (UK)
Development of the project was made possible through an Acoustics System
Trainer and associated computer software. The equipment allowed for a real-time
analysis of sonar principles and techniques through appropriate scaling of
system parameters.
2.3.1 Transducer Reciprocity Calibration
The transducers used within the Acoustics System Trainer operated at
192kHz, providing a conical beam shape. Response of a transducer, functioning
as a hydrophone, can only be determined provided it has been calibrated to a
standard. Accordingly calibration of the transducers was performed by the
National Physical Laboratory using the method of three transducer
spherical-wave reciprocity.
2.3.2 Acoustic Trainer Familiarisation
Simple tests were conducted in the tank; firstly to identify any
similarities that were highlighted in previous testing, and secondly to
illustrate the predictability of the transducer response.
2.3.2.1 Transducer Resonant Frequency
It was important to determine the exact operational frequency of the
transducer(s) for reasons of accuracy and reliability of the recorded data.
From analysis, the deduced operational frequency was equal to (1/0.00515)
194.17 kHz. This value compared favourably with a voltmeter reading of natural
resonant frequency for the projector transducer equal to 192.31kHz. This
frequency variation resulted in little impact on the recorded data hence for
the purposes of these trials the frequency of 192.31 kHz was adopted.
2.3.2.2 Active Sonar Test
A single transducer was set up as a projector, operating in the
reversible (transmit and receive) mode, and mounted at one end of the tank,
positioned midway across the tank at a depth of 0.24m below the water surface.
The oscilloscope display revealed large echoes originating from the ends of the
tank, which provided for a very efficient sound-reflecting surface. It was also
possible to identify the ‘ringing’ effect (decay) of the transducer.
Clearly, in order to use the tank for continuous wave transmission it was
essential to provide a reasonably efficient non-reflecting coating to the walls
of the tank. Consequently in order to achieve a low degree of reverberation
with little or no reflection of sound, the surfaces of the tank were lined with
a selection of plastic foam sheets of varying surface textures and thickness.
It was important to choose a material with uniform absorption over a large area
at the given frequency rather than a localised area of highly absorbing material.
A number of types and thickness of foam were tried before finally opting
for the use of open cell articulated foam, consisting of an 11mm sheet of foam
covered with foam spikes about 15mm high and uniformly spaced 50mm apart in an
equilateral triangular arrangement. With strips of this foam lining the walls
of the tank, a reduction in reflected intensity of 29.93dB was obtained. The
spiked triangular arrangement prevented the formation of stationary waves
between facing surfaces and the foam preventing side-to-side and end-to-end
reflections.
It was also desirable to determine the velocity of sound. Adopting the
set-up as detailed above, it was possible to determine the velocity of sound,
which by direct measurement was 1464.828ms-1.
2.3.2.3 Transducer Beamwidth
It was important to determine the beamwidth of the transducers so that
the appropriate scaling of the target dimensions could be deduced.
The projector transducer was mounted at the centre of one end of the
acoustically damped tank and the transducer under test at the opposite
end. The separation of the projector
and hydrophone was measured at 95.6cm. The angular beamwidth of the main lobe
of the test transducer element was determined at the –3dB half power points
with the aid of a spectrum analyser.
The results of the test clearly indicated that there was variation in the
transducer beamwidths. Cross correlation of these results with the
manufacturer’s specifications suggested that there was an error of
approximately 1.6 degrees in the measured beamwidths.
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[HPL = Separation of
half power point measured from 0dB from the left,
HPR = Separation of half power point measured from 0dB from the right]
Table I:
Transducer Beamwidths
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A possible explanation for this difference was drawn from the geometry of
the transducer test conducted in the tank facility. It was appreciated that the
transducers had large diameters (6cm) in comparison with the scale being
considered. However they served their purpose by providing known directional
characteristics.
2.3.2.4 Selection of System
Parameters
The selection of system parameters would have large consequences on the
quality of the data measured. Particular attention was paid to operational
frequency, bandwidth, pulse width and pulse repetition frequency.
After much experimentation it was concluded that for the purposes of the
trials a short pulse width was required. A minimum range of half a pulse width
between the two targets was necessary to distinguish between target echoes. For
optimum detection of the target echo against the masking background noise it
was necessary for the background noise to be kept as low as possible. A short
pulse width would also provide the wide bandwidth required for good range
discrimination between targets.
To provide a continuous picture of the target, a large number of pulses
must be transmitted in a given time (Pilgrim, 1982). Therefore a high Pulse
Repetition Frequency (PRF) was used.
Following the preliminary trials, the following settings were chosen for
the investigation.
Operational Frequency
= 192.31kHz
Pulse Width
= 100msec
Pulse Repetition Frequency (PRF) = 1.53msec
Transducer Array Gain
= Maximum
To provide a steady picture of the target signature, displayed on the
oscilloscope, averaging of the received signal was conducted, ranging between
32 to 256 measurements.
2.4 Selection of Targets
The selection of targets for use in the trial was dependent upon the
following:
a) validity conditions associated
with the theoretical analysis of target strength of simple forms;
b) the range to the target as a
function of the beamwidth of the transducer in order to ensure complete
insonification of the target; and
c) manufacturing ability and
availability of materials for construction.
A summary of the theoretical maximum and minimum dimensions of the
targets that satisfy the 192.31kHz criteria can be seen in Table II.
[‘-‘ indicates no theoretical
restriction (other than the beamwidth of the transducer)].
Table II: Target
Dimensions of Simple Forms
The wavelength (l) and wave number (K) were 7.617 x 10-3m and 824.888
respectively.
From Table II, it can be seen that circular targets of dimension
<0.242cm were not practical.
Circular target dimensions at normal incidence were therefore deduced
according to the conditions described by the square target form and circular
target dimensions were thus defined as indicated in Table III.
Table III: Circular Target Dimensions
Owing to restricted manufacturing capability, it is not possible to
achieve exact target dimensions. Tolerances of 0.5mm were thus adopted in the
manufacturing of targets. Trials were carried out on the following reflectors,
specified in Table IV, for completeness including targets of greater dimensions
than specified in Table II.
Table IV: Target Dimensions
2.4.1
Reflection Coefficients of Various Materials in Water
To attain an adequate target/reverberation echo ratio, a high reflection
coefficient between the water and target was required. Analysis of a variety of
materials led to the selection of aluminium targets, because of their high
reflectivity and ductility.
2.5 Trials
Trial measurements were made for:
a) The variation in echo amplitude as a function of body shape/size of
target – averaging of rates over 32 to 256 measurements was performed to reduce
noise which may otherwise cause the received signal to vary in amplitude.
b) The aspect angle at the point of detection for circular and square
plates of equal area; 100cm²– observing angular
orientation of the plate to within 0.5°.
c) The echo amplitude for a square plate as a function of range for
various surface roughness–
using various polished, regular and random furrowed plates:
- Test Plate No. 1: 1mm
thickness; polished surface;
- Test Plate No. 2: 2cm
crest-to-trough height spaced at 1.52cm (equivalent to 2 wavelengths);
- Test Plate No. 3: 3.1cm
crest-to-trough height spaced at 1.52cm (equivalent to 2 wavelengths);
- Test Plate No. 4: Random distribution; rms height =
2.89cm.
d) The variation of echo amplitude as a function of target aspect for
Test Plate No. 2.
e) The echo amplitude as a function of range for normal and grazing
incidence (to determine beamwidth effects) – rotating the projector
through 156° allowing grazing incidence adjustments to 0.5°.
f) The bottom backscattering strength for a flat bottom as a
function of grazing angle – using various fine sands to simulate scaled coarse
sands through to boulders.
g) The bottom backscattering strength for an undulating sand bottom as
a function of grazing angle – using an irregular undulating bottom surface to
simulate sand waves.
h) The scattering strength as a function of grazing angle for a
circular target and flat sand bottom – allowing for the angular
orientation of the plate through 90°, situated on the sand bottom.
3.0 ANALYSIS
Digitised printouts were obtained for the individual trials performed in
the tank. The echo amplitude of each reflector was extracted and tabulated with
cross-referencing, for each individual plot along with graphs to show scatter
and the regression lines derived.
3.1 Target Strength of
Simple Forms
It was confirmed that at high frequency (192.31kHz) and short wavelength
the body shape of a target can affect its target strength. The response of the
targets is detailed in Figure 1. It would appear that a linear
relationship exists between surface area and received echo amplitude. A
doubling of circular area provided an echo amplitude increase in the ratio
2:3. Similarly a doubling of square
area provided an increase in the ratio 2:2.7.
Fig. 1: Average Curves for Echo Amplitude as a
Function of Target Area.
The experiment confirmed the findings of Meixner and Andrejewski (Urick,
1983) for circular and square plates with surface areas greater than
65cm². This behaviour however was not
evident for areas <65cm². It is hypothesised that this inverse behaviour
corresponds to different modes of oscillation or vibration of the target.
3.2 Variation of Target
Strength with Aspect
Trials demonstrated that circular plates exhibit the strongest return
intensity at all aspect angles relative to square plates. The relationship
between echo amplitude and direction of incidence was observed to be
exponential as seen in Figure 2. At a 90° beam aspect, the maximum echo
amplitude of the circular and square plates was 0.0230V and 0.011V
respectively.
It appeared
that detection of a circular plate was possible at shallower aspect angles. The
oblique angle at which reflection was just detectable for circular and square
plates approached 70° and 77.5° respectively.
Fig. 2: Average Curves for Echo Amplitude as a
Function of Direction of Incidence (Aspect)
3.3 Variation of Target
Strength with range and Surface Roughness
It was evidenced that target roughness, determined as a function of
wavelength, produced a higher scattering intensity than simple/smooth plates.
These results displayed a similar behaviour to the backscattering theory of
Marsh (1963), for both sea-surface and sea-bottom backscattering reinforcement.
Marsh’s theory states that backscattering reinforcement depends upon ‘the ratio of the average path difference
between the peaks and troughs of the rough surface to a wavelength’.
Figure 3: Average Curves for Echo Amplitude as a
Function of Range
3.4 Variation of Target
Strength with Aspect for a Square Target, Surface Roughness = 2cm
An angular
relationship was evident for square plates.
At normal incidence (90°) the plate exhibited the strongest return
intensity, 0.020V, with a steady decay at shallower aspect angles. At shallower
aspect angles, in the range 80° to 40° inclusive, no detection of the target
was possible. Aspect angles of 30° and less, however, display a steady increase
in echo intensity (amplitude). Detection is made possible at these shallow
angles by the geometry of insonification: ie at angles, £30° the slope heights of the
peaks and troughs approach normal incidence resulting in weaker detection.
This confirmed the observations of Urick (1983), who stated that ‘it is apparent that a rough surface will
become effectively smooth when the wavelength with which the surface is
insonified becomes large enough, or when the grazing angle is small enough’.
Fig. 4: Echo Amplitude as a
Function of Direction of Incidence (Aspect) for a Square Plate; Surface
Roughness = 2cm
3.5 Transducer Beamwidth
Effect (Side-Lobe Energy)
Fig. 5: Average Curves for
Echo Amplitude as a Function of Range at Normal and Grazing Incidence
At normal
incidence the acoustic axis of the transducer array is in the direction of
maximum sensitivity and maximum in-phase condition. This is displayed, in
Figure 5, as maximum echo amplitude of 0.05V at short range (45.5cm) and normal
incidence, slowly decaying at greater ranges. Grazing angles of 60°, 40°, 30°,
25° and 24° associated with ranges of 45.5cm, 60.5cm, 75.5cm, 90.5cm and
95.6cm, respectively, resulted in weaker echo amplitudes. Thus it was
determined that the response of the transducer array must vary with direction
(grazing angle) in a manner specified by the beam pattern of the array. This
again corresponds with the theory stated by Urick (1983): ‘the response of the transducer array varies with direction relative to
the array’.
Side-lobe energy from the transducer orientated at grazing incidence was
also noted to have an effect on the results, such that it is possible for
reflections from the water surface to arrive at the hydrophone out of phase
thus resulting in poorer detection.
3.6 Measurements of Backscatter of Sound from
Various Bottom Types
Fig. 6: Average Curves for
Bottom Backscattering Strength as a Function of Grazing Angle for Flat
Sedimentary Bottoms
The experimental plot (Figure 6) details backscattering of fine sand,
limestone chippings and a random soil type distribution displaying good
agreement with the theory, outlined by McKinney and Anderson (1964), namely
that backscattering strength increases with a respective increase in grazing
angle.
However the backscatter of sound for the randomly distributed soil type
displayed no angular dependence, thus resulting in scattering of the data
points.
3.7 Measurements of Backscatter of Sound from an
Undulating Bottom
It can be seen from the above that particle size serves as a means of
classifying bottoms in terms of acoustic backscattering. In addition, one might
also expect the magnitude and nature of the scattering to be a function of both
the particle size and the surface bottom relief.
Fig. 7: Average Curves for
Bottom Backscattering Strength as a Function of Grazing Angle for an Undulating
Sand Bottom
The data given in Figure 7 shows enhanced returns at grazing angles less
than 50° and at normal incidence. The reduced backscattering strengths in the
range 60° to 80° and the scatter of the data suggests that roughness of the
bottom surface is important. A possible
reason for the reduction of backscatter at these mid-angles may be the method
of insonifying the bottom and the geometry of the undulating bottom relative to
the beam aspect of the transducer.
Thus a complete description of particle size distribution and bottom
relief must be accurately known if attempts are made to compute the
backscattering strength of various soil types.
3.8 Scattering Strength
Measurements from a Circular Target Positioned on a Sand Bottom
Fig. 8: Average Curves for
Scattering Strength as a Function of Grazing Angle for a Circular Target (f = 11cm) and Sand Bottom
The results obtained for a circular plate (f = 11cm) insonified on a fine sand, flat bottom
displayed weaker scattering strengths than for an identical target insonified
at equal range at mid depth. The introduction of a sand bottom illuminated at
60° and 30° resulted in a reduction in echo amplitude of 86.9% and 65%
respectively. Thus the sand bottom could be seen to scatter the incident wave
in a direction that is not normal to the hydrophone face. Comparing the data
for the 11cm circular target at mid depth and the same target positioned on a
sand bottom, it could be seen that at shallow grazing angles equal to 25°, the
echo amplitudes differ by 0.002V, thus representing a reduction of echo
amplitude of only 11.1% when insonifying on a sand bottom. In conclusion
therefore, the probability of detection of a circular plate is less with a sand
bottom and grazing angles >30°. At grazing angles <25°, however, the
probability of detection is not a function of the bottom type.
The behaviour of a circular plate, f = 4cm was also observed, revealing greater
backscattering strengths than for a larger circular target; f = 11cm (Figure 9).
Fig. 9: Average Curves for Scattering Strength as a Function of
Grazing Angle for various Circular Targets and Sand Bottom
It is surmised that this enhanced scattering strength for a smaller
target area is a function of the incident frequency. The similar behaviour of
the circular plates at angles less than 45° would suggest that resonance is
dependent upon aspect. This performance supports the theory outlined by
Urick (1983).
3.9 Trial Comparisons
with Theory
In concluding this section, it is appropriate to make comparisons with
theoretical analysis conducted by Urick. This can be achieved by comparing
trends and highlighting similarities.
The theoretical results reveal that the strongest reflector is a perfect
circular disc, yet there is a strong logarithmic relationship linking target
area and target strength for all reflector types.
The dependence of target strength on plate area is evident in the
empirical analysis for identical circular and square plates. The trial response
of rectangular plates was at odds with this theory. It is therefore supposed
that the structure used to support the target for measurement purposes in the
trials had influenced the data.
The ability to compare data involving grazing incidence was limited. The
transducer beamwidth and its directional properties limited the reception of
the incoming acoustic waves, yet theoretical analysis highlighted similar
characteristics to those demonstrated in the laboratory. As the grazing angle
approaches normal incidence the increase in target strength is rapid for all
target forms, as would be expected because of the specular reflection from a
flat plate.
It is speculated that the scatter in the data points for square plates,
within the trials, was a result of the numerical accuracy adopted in the
determination of the theoretical target strength. Theoretical target strengths
of rectangular plates display a greater scatter than those of square plates at
angles £10° and 50°. The steady
increase in theoretical target strength for rectangular plates at shallow
grazing angles £15° accordingly suggested
that the expression used to derive target strength does not function well at
small grazing angles.
4.0 CONCLUSION
The objective of the experiment was satisfied. The extent to which
comparison could be made between empirical results and theoretical analysis was
however limited to identifying trends in the two data sets. Further field
trials would be necessary in order qualify the laboratory results, given
realistic and real-world variables, such as temperature, salinity and velocity
gradients. Measurements from the laboratory tank did however allow for a more
accurate assessment of target/bottom scattering strengths.
The trials were limited to aluminium plates of thickness 2mm and to a
frequency of 192.31kHz. The validity of the conclusions drawn from the results
is thus limited to this range of plate thickness and frequency.
Reflected intensity was found to decrease with range for all surface
roughness types. Plate surfaces that were defined as a function of wavelength
proved to be greater reflectors of sound displaying a similar relationship to
the work conducted by Marsh (1963).
Backscatter data from a fine sand (500 ® 355mm) and limestone (4 ® 1mm) bottom displayed harmony with the work conducted
by McKinney and Anderson (1964), indicating that particle size is a significant
factor in the backscattered return. Tests conducted on an undulating sand
bottom revealed marginally higher backscattering strengths than for a flat
bottom at grazing angles £50° and 90°, and lower backscattering strengths for
grazing incidence in the range 60° to 80°. Thus bottom relief was also shown to
be an important factor in the backscattering of sound.
It was found that the reflected intensity was greatest for circular
plates, featuring a surface roughness defined by a wavelength, at normal
incidence. At grazing incidence >30° the echo amplitude of a circular plate
situated on a sand bottom is a function of the bottom. At grazing incidence
<25° detection of a circular plate is not dependent upon the bottom type.
It was thus concluded that a knowledge of the particle size distribution
and bottom relief must be accurately known along with a complete description of
the target in terms of shape, aspect, grazing incidence and surface roughness
if attempts are to be made to compute the target strength of objects lying on
the bottom.
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McKinney, C..M. and C.D. Anderson. (1964). Measurements of Backscattering
of Sound from the Ocean Bottom. J. Acoust. Soc. Am, 36: 158
Marsh, H. W. (1963). Sound
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Pilgrim, D.A. (1982). Attraction
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