EFFECTS OF ENVIRONMENTAL VARIABILITY ON
ACOUSTIC PROPAGATION LOSS IN SHALLOW WATER
TUNCAY AKAL
TUBITAK Marmara Research Centre, Earth and Marine Science Research Institute
P.K. 21 Gebze, Kocaeli 41470, TURKEY
E-mail: [email protected]
Transmission loss as a sonar parameter describes the magnitude of acoustic energy loss
of sound propagation in the ocean. Broadband (10 Hz – 300 kHz) propagation loss
measurements conducted over the years in varying areas show the effect of the spatial
and temporal variability on acoustic propagation for bottom-limited conditions. An
example from the existing acoustic data obtained over the last three decades has been
utilized to demonstrate the effects of environmental variability on propagation loss, and
our ability to simulate the propagation conditions with existing propagation models are
presented.
1
Introduction
Acoustic propagation in the ocean is influenced by many factors: the physical and
chemical properties of the water column cause attenuation and refraction, while
variations in seafloor properties and boundary roughness complicate reflections. Thus,
any attempt to understand propagation of acoustic energy in shallow/coastal water
requires an accurate knowledge of the propagation medium, especially sound speed
structure of the water column, seafloor properties and sea surface roughness. The
shallow/coastal water environment is often characterized by wind driven flow, and/or
current flow interacting with bottom topography and increased variability in water
properties as a result of enhanced mixing. The ocean sound speed structure varies both
in time and space. There are various oceanographic processes like currents, internal
waves, fronts, eddies and thermal changes that control the sound speed structure and
thus the acoustic characteristics of this environment. In order to understand the range
and time dependence of acoustic propagation over this broad frequency range in
shallow/coastal waters, simultaneous environmental measurements are required.
As part of SACLANTCEN’s research program over the last three decades acoustic
data covering a broad frequency range (10 Hz – 100 kHz) were collected in order to
adequately understand acoustic propagation, including penetration into the seabed,
along with water column environmental characteristics. Transmission loss (TL) as a
sonar parameter describes the magnitude of acoustic energy loss of sound propagation in
the ocean. Data from a polar region are utilized to demonstrate the effects of a polar
front on acoustic propagation, and our ability to simulate the propagation conditions in
such a complex environment with existing propagation models are presented.
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N.G. Pace and F.B. Jensen (eds.), Impact of Littoral Environmental Variability on Acoustic Predictions and
Sonar Performance, 229-236.
© 2002 Kluwer Academic Publishers. Printed in the Netherlands.
230
2
T. AKAL
Experimental technique
The sound speed structure of the water column is known to be the controlling factor in
acoustic propagation. Acoustic energy propagating through a shallow water channel
interacts with the sea floor causing partitioning of waterborne energy into different types
of seismic and acoustic waves. The propagation and attenuation of these waves
observed in such an environment are strongly dependent on the physical characteristics
of the sea bottom. Transmission loss representing the amount of energy lost along an
acoustic propagation path (in range and time) carries the information relative to the
environment through which the wave is propagating.
In all the TL data reported, explosive charges and transducers were used as sound
sources and a vertical hydrophone string containing omnidirectional hydrophones was
used to receive the transmitted acoustic signals. The experimental procedure used for
the runs was the following: the receiving ship would launch a vertical hydrophone string
suspended from a spar buoy with a digital radio link; this would then be drifted away
from the ship in order to minimize the effect of ship-radiated noise and surface waves.
The number of hydrophones used varied depending on the water depth, spaced in such a
way as to cover most of the water column as shown in Fig. 1.
Radio Link
Receiving Ship
Receiving
Array
Source Ship
Sound
Source
XBT
Echo
Sounding
Figure 1. Experimental set-up for transmission loss measurements.
The source ship would then move away or towards the receiving ship at a fixed,
predetermined course launching sound sources or towing a CW/FM source transmitting
at regular intervals and set for different depths. The source ship would also launch
XBTs and obtain echo soundings along the track for the calculation of the sound speed
and bathymetric profiles.
The transmission losses reported are energy losses in dB with reference to a source
level at 1 m. For those signals in which the noise could be considered stationary, the
data have been corrected for noise by subtracting the measured noise energy of the same
time duration observed prior to the signal arrival. The data for which this was not valid,
or for which the signal-to-noise ratio was found to be less than one, have been omitted.
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ENVIRONMENTAL VARIABILITY & ACOUSTIC PROPAGATION
3
Effects of an oceanographic front in space and time
Areas where oceanographic fronts are present are characterized by a strong temporal
and spatial variability due to the mixing process between two different water masses.
Acoustic and environmental data collected in an Arctic front region are used to
demonstrate effects of an oceanographic front on TL both in time and space.
The acoustic experiment was carried out in a very complicated environmental due
to the presence of the polar oceanic front where the cold Arctic water meets the warmer
Atlantic water. This boundary between two different water masses meanders with tidal
currents and creates a very complex medium that varies in both space and time.
3.1
Spatial Variability of Transmission Loss across a Front
Figure 2 shows a cross section profile of the sound speed variation perpendicular to the
front calculated from XBT recordings obtained by the source ship during TL
measurements. The front can be identified approximately at a range of 12 to 14 km
from the receiver position. As also shown in Fig. 2 is the bathymetry of the seafloor,
which is characterized by a shallow shelf with water depths varying from 100 to 130 m,
extending to 35 km distance from the receiver position, then the shelf drops from 130 to
340 m depth within 10 km distance, and a trough where the water depth remains 340 m.
At depths less than 150 m the sea floor is covered by large-grained material (i.e. sand,
gravel and boulders) with large patches of shells. As the water depth increases the
bottom composition changes from sand to silty sand, then to sand-silt-clay in the deepest
part.
0
14
1470
1460
146
6
Depth (m)
100
14
14 60
60
78
1476
58
14
200
300
14
64
14
62
Sound Speeds in m/s
Distance (km)
Figure 2. Sound speed cross section observed during acoustic measurements.
The range-dependent propagation path was perpendicular to the front with the
receiving ship situated at the Arctic water side of the front. Figure 3 show the measured
TL as a function of frequency and distance for a source at 25 m and a receiver at 50 m
depth. Within the frequency band of the measurements, an optimum propagation
frequency around 200 to 800 Hz is observed. This phenomenon is characteristic of
shallow water propagation and it is caused by a low-frequency attenuation due to
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T. AKAL
bottom interaction and attenuation of higherfrequencies due to scattering and volume
absorption [1, 2]. This optimum frequency corresponds to propagation through the
water column and the optimum frequency increases with bottom hardness and the
associated presence of high velocity shear waves and decreases with increasing water
depth due to diminishing bottom interaction.
100
90
80
400
70
800
60
50
Frequency (Hz)
2540
1600
200
100
TL in dB
50
0
10
FRONT
20
30
Distance (km)
40
50
Figure 3. Measured TL as a function of frequency and distance for a source at 25 m and a
receiver at 50 m depth.
40
50 Hz
100 Hz
200 Hz
400 Hz
800 Hz
1600 Hz
2540 Hz
TL (dB)
60
80
100
0
10
FRONT
20
30
Distance (km)
40
50
Figure 4. TL as a function of distance for selected frequencies.
Figure 4 shows the TL curves for selected frequencies. In this figure the effect of
the front is clearly seen as a 5 to 6 dB decrease within the 100 to 800 Hz intermediate
frequency band. This is due to a change in the water column sound-speed structure
within the frontal area where the distribution of acoustic energy changes, causing less
interaction with the seafloor.
Using the environmental data collected during the acoustic measurements, several
different simulations models (RAM, PAREQ and C-SNAP) [3–5] have been used to
model the TL (Fig. 5). Figure 6 is an example of the acoustic field simulated for this
particular environmental data set using the RAM model for 400 and 1600 Hz. After the
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ENVIRONMENTAL VARIABILITY & ACOUSTIC PROPAGATION
best agreement was found between simulation results and the measured data (Fig. 5), we
simulated a second case by removing the front and making sound speed profiles at the
receiver position range independent, but keeping the same bathymetric profile and
bottom properties as the range dependent case, where the effect of the front can clearly
be seen as being up to 15 to 18 dB on TL. The difference is shown in Fig. 7 where the
frontal effects are absent. Both low and high-frequency acoustic energy distributions
show completely different characteristics.
TL (dB)
40
SD: 25 m
RD: 50 m
60
80
400 Hz
100
TL (dB)
40
Measured
Simulated with Front
60
Simulated without Front
80
1600 Hz
100
Figure 5. Measured data and comparison of simulations with and without the polar front.
TL (dB re. 1 m)
50
60
70
80
90
100
0
Depth (m)
200
400 Hz
400
0
200
1600 Hz
400
0
10
20
30
Range (km)
40
50
60
Figure 6. An example for the acoustic field simulated for 400 Hz and 1600 Hz through the front.
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T. AKAL
TL (dB re. 1 m)
50
60
70
80
90
100
0
Depth (m)
200
400 Hz
400
0
200
1600 Hz
400
0
10
20
30
Range (km)
40
50
60
Figure 7. An example for the acoustic field simulated for 400 Hz and 1600 Hz without the front.
3.2
Temporal Variability of Transmission Loss across a Front
This experiment was conducted to study the temporal variability of the TL across the
front between to positions over a 48 hour period. As can be seen in Fig. 8 the
environmental conditions did not remain constant during this period as shown by the
temperature cross section at the receiver position. On several occasions the front passed
through the receiver position and clearly affected the propagation conditions.
0
3
2
4
2
2
20
Depth (m)
4
2
2
3
40
60
Temperatures in 0C
80
0
6
12
18
24
30
Time (Hours)
36
42
Figure 8. Temperature structure as a function of time at the receiver position.
48
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ENVIRONMENTAL VARIABILITY & ACOUSTIC PROPAGATION
As shown by the TL contours in Fig. 9 the maximum TL exhibits approximately
12 h oscillations. All the losses, especially for the intermediate frequencies, show
marked oscillation with time, with variations of up to 8 to 12 dB. When the predicted
tidal curves (Fig. 9) are superimposed on the loss contours, there is very good
correlation between high tide and maximum TL. The temperature cross-section (Fig. 8)
and the current measurements made at the receiver position, supported by XBTs taken
by the source ship, strongly indicate that the best transmission was observed when the
front was furthest to the south, coinciding with the current running south and low
110
800
110
1
100
400
95
95
95
200
95
100
0
Tide Height (m)
Frequency (Hz)
2540
1600
100
110
TL in dB
50
0
6
12
18
24
30
Time (Hours)
36
42
48
Figure 9. Predicted tidal curves and TL as a function of time.
tide, while the poorest propagation was observed with the front at its northern-most
position. We are therefore dealing with two different situations as shown on Fig. 10.
When the receiving ship was in isothermal water north of the front (polar water), an
important part of the propagation in the shallow water will be under upward refracting
conditions until the position of the front, but in the deeper water part, it will be under
downward refracting conditions causing relatively small total losses. When the
receiving ship remained in the Atlantic water south of the front, the propagation was
under very strong downward refracting condition over the shallow water area causing
higher losses due to increased bottom interaction.
4
Conclusions
Acoustic and environmental data collected in an Arctic polar front region are used to
demonstrate effects of an oceanographic front on TL both in space and time. These
areas are characterized by a strong temporal and spatial variability due to the mixing
process between two different water masses. The results indicate that the polar front
can have significant effect on acoustic propagation both in space and time. Range
dependent data may have up to 15 to 18 dB different TL. The TL in the intermediate
frequencies can oscillated up to 12 dB within a 48-h period. These oscillations are well
correlated with the tide, which also control the position of the front and thereby changes
the acoustic propagation characteristics.
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T. AKAL
Receiver
1
Source
LOW TIDE
FR
ON
POLAR
WATER
ATLANTIC WATER
5
T
2
4
3
Source
Receiver
T
FRON
ATLANTIC WATER
2
5
6
4
3
HIGH TIDE
Figure 10. Simplified temperature structure during low and high tide periods.
Acknowledgements
I would like to thank G.F. Edelmann of MPL-Scripps/UCSD, M.G. Martinelli and C.M.
Ferla of SACLANTCEN for their help in running the different propagation models.
References
1. Akal, T., Sea-floor effects on shallow-water acoustic propagation. In: Bottom Interacting
Ocean Acoustics, edited by W.A. Kuperman and F.B. Jensen (Plenum Press, 1980) pp.
557–575.
2. Jensen, F.B. and Kuperman, W.A., Optimum frequency of propagation in shallow water
environments, J. Acoust. Soc. Am. 73, 813–819 (1983).
3. Collins, M.D., User’s Guide for RAM. Naval Research Laboratory, Washington, DC.
4. Jensen, F.B. and Martinelli, M.G., The SACLANTCEN parabolic equation model
(PAREQ). SACLANTCEN Undersea Research Centre, La Spezia, Italy (1985).
5. Ferla, C.M., Porter, M.B. and Jensen, F.B., C-SNAP: Coupled SACLANTCEN normal
mode propagation loss model. Rep. SM-274, SACLANT Undersea Research Centre
(1993).