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Indian Journal of Geo-Marine Sciences
Vol. 43(1), January 2014, pp. 17-21
Acoustic propagation affected by environmental parameters in
coastal waters
Sanjana M C, G Latha, A Thirunavukkarasu & G Raguraman
National Institute of Ocean Technology, Velachery-Tambaram Road, Pallikaranai,
Chennai-600 100, India.
[E-mail: [email protected]]
Received 13 July 2011; revised 26 March 2013
In coastal regions various factors affect the propagation of acoustic signals such as wind, tidal effects, off shore
currents and even river outflows. Shallow water waveguide is also characterised by site-specific source nature, bathymetry,
sediment properties and sound speed profile. Monterey-Miami Parabolic Equation (MMPE) model has been used to
study the propagation in very shallow waters of Arabian Sea with respect to environmental parameters. Sound speed
Profiles (SSP) and bottom sediment samples measured at the site characterize the water column and the ocean bottom
respectively. Bottom is clay (soft bottom) which can lead to absorption of acoustic intensity into the sediment leading
to a decrease in reflected acoustic rays. Under these conditions propagation to large distances will be associated with
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large losses in acoustic energy. Critical angle of propagation determined theoretically is found to be ±11.42 with
respect to horizontal representing low order trapped modes. SSP is downward refracting enhancing propagation along
the bottom. In this paper Transmission Loss (TL) computation have been carried out for different frequencies with
respect to range and depth. Signal loss and arrival structure and the travel time have been computed for different ranges
from 0 to 6 km. Model results have been evaluated by comparing with field measurements and are found to be suitable
for modelling extremely shallow water environments.
[Keywords: Acoustic propagation, Sound speed profile, Transmission loss, Shallow water]
Introduction
The complex environment in coastal waters is
manipulated by temporal and spatial fluctuations
which can alter the existing sound speed profile which
decides the propagation of sound. Propagation in
shallow water is also influenced by the sediment
bottom properties since multiple reflections takes
place owing to the wave guide nature of the
environment. River influx is also an entity which is
variable across the year and can influence sites at
very shallow waters. Modified Parabolic Equation
(PE) models have been used successfully in such
complex coastal environments to study propagation
effects1-3. Monterey-Miami Parabolic Equation Model
(MMPE) have been used to relate environmental
parameters to Transmission Loss (TL) in two
extremely shallow water environments4.
This work examines the propagation for a 32 m
water column with a downward refracting sound
velocity profile and a clay sea bed with sandy sub
bottom using Monterey-Miami Parabolic Equation
model(MMPE) in the frequency band up to 5 kHz.
PE models are best suited for range dependent
environments and also for broad frequency band as
is being considered here. Critical angle of seabed has
been determined from the measured bottom and water
column properties and inputs from Hamilton model.
Characteristics of near surface source propagation in
terms of travel time and arrival structure and
Transmission loss in dB with respect to range and
depth for different frequencies is investigated for a
downward refracting environment.
Materials and Methods
Study site
The environment considered is a sloping bottom
shallow water wave guide with depth ranging from
16 m to 32 m and a range extent of 6 km, with clayey
bottom (Fig.1a). Two major rivers Muvattupuzha and
Periyar empty into the site which leads to variation
in the distribution of water properties. Near to the
shore the sound velocity is less and increases as we
go offshore. Sound velocity profile is typically
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INDIAN J MAR SCI VOL. 43(1), JANUARY 2014
speed profiles have been considered starting from the
initial point and every 2 km apart in range.
We are considering 1 m layer of clay and below that
sandy layer. Sound speed in the sediment is 1596 m/s
with a density of 1.26 g/cc with a bottom attenuation
of 0.1 dB/km/Hz. A sub bottom layer with sound
speed of 1705 m/s, density of 1.5 g/cc and attenuation
of 0.1 dB/km/Hz is also considered. Source depth is
considered at just below the surface and the frequency
band considered is up to 5 kHz. Wide angle source
(approximates to a point source) is considered
initially. To perform broadband analysis centre
frequencies of 2520 Hz was used with a bandwidth
of 5000 Hz.
Results & Discussion
The critical angle at this site determined from
water column and bottom properties is
o
èc=arccos(1538/1569)=11.42 . Propagation is
characterized by normal modes corresponding to waves
striking the bottom at grazing angles lower than the
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critical angle <11.42 . So waves striking the bottom at
angles less than 11.42p will be totally internally
reflected whereas rays with grazing angles greater than
11.42p are heavily attenuated on encountering the
bottom. However in shallow water most of the energy
that propagates is along the horizontal (Fig. 2).
Fig. 1(a)–Study site and (b) Sound speed profiles at
different depths.
downward refracting enhancing propagation along the
bottom (Fig.1b). Sediment samples have been
collected at the site and sieve analysis has been carried
out. Bottom is mainly clay with admixture of sand
leading to absorption of sound into the sediment.
Shallow Water propagation modelling
The second release of the Monterey-Miami
Parabolic Equation (MMPE) propagation Model has
been used to understand the shallow water
propagation at this site. This model was developed
by5 based on Split Step Fourier (SSF) technique and
since then many improvements have been made on
this. It is a full wave underwater acoustic propagation
model that utilizes the split step Fourier marching
algorithm. Parabolic equation is popular for solving
range dependent propagation problems. Total 4 sound
Fig. 2–Reflection coefficient for the site.
Understanding how sound is propagating at a site
is important in assessing the ambient noise field and
the contribution due to different sources. For a broad
band point source at 0.1 m and 3 m depth from the
water surface, at a water column depth of ~16 m, the
output starting field data is given in Fig. 3. As the
frequency increases the noise is restricted more and
more towards the surface and maximum noise seen
at the source depth for all frequencies. For frequencies
above 2 kHz, sound waves are mostly restricted to
the upper 4 m and 6 m of the water column at the
start of the simulation for source depths of 0.1 and 3
m respectively.
SANJANA et al.: ACOUSTIC PROPAGATION IN COASTAL WATERS...
19
Fig. 3–Output starting field data for a source beneath the
surface at 3m and 0.1 m, in ~ 16 m water column.
Transmission loss arrival structure: At 16 m water
depth, for a near surface source, the arrival is crisp
with no signal distortion with a sound speed
fluctuation of only 1 m/s (Fig. 4). The transmission
of the high frequency signal produced more multipath
arrival structure and a weaker head wave due to more
attenuation in the floor sediment. At 25 m depth, the
sound speed gradient is 4.5 m/s and we can observe
slight distortion in the reflected rays. In order to test
the validity of the model, the 2.5 kHz source responses
Fig. 4–Transmission loss arrival structure for the source signal at 16 m, 25 m & different ranges.
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INDIAN J MAR SCI VOL. 43(1), JANUARY 2014
were recomputed out to a range of 6 km. It is clear
that the perturbation does impact the propagation at
larger ranges. Travel time for the signal at different
ranges for centre frequency of 2.5 kHz is given in
Fig. 4. At 0.5 km range ( 20 m depth) sound is seen to
decay fast due to intense surface and bottom
reflections compared to 6 km range (32 m depth).
Since the waves are striking at much greater angles
than the critical angle, energy is lost fast into the
bottom.
Transmission loss: The term transmission loss
describes the drop in sound energy level as it
propagates from one point in the ocean to another. It
can be considered as the signal which is lost due to
the sum of geometric spreading, refraction,
interference and the loss due to attenuation in the
ocean. Since the environment is shallow water,
cylindrical spreading is assumed to dominate the
propagation. In cylindrical spreading the intensity
decreases linearly with increasing range.
Transmission loss outputs from MMPE are in three
different forms, TL at a single frequency versus range
and depth, TL at a single range versus frequency and
depth and TL at a single depth versus frequency and
range. Transmission loss for different frequencies
with respect to tange and depth are given in Fig.5.
For 0.1 & 0.25 kHz, there is more penetration into
the sediment both surface and subsurface layers
whereas for higher frequencies the penetration is only
into the surface clay layer.
Fig. 5–Transmission loss for different frequencies (a) with range at 15 m depth (b) with depth at
250 m range (c) with depth at 6 km range.
Transmission loss for different ranges in the
frequency band up to 5 kHz is given in Fig. 6. At 0.1 km
range the sound speed profile is almost is o-velocity
with a gradient of 1 m/s between surface and bottom
and the corresponding Fig. shows noise spread out
along the depth with penetration into the bottom
especially at low frequencies.
Model Evaluation
Difference techniques measure the distance
between the model prediction and a standard (which
can comprise field measurements or outputs from
other models) in terms of dB differences at a given
range, or over a set range interval. These techniques
are best suited to comparative model evaluations
conducted in research environments2. Model accuracy
was evaluated by examining field measurements with
respect to model output as a function of range and
frequency (Fig.7).
Conclusions
Fig. 6–Transmission loss for 0.1 km range.
The sound propagation cone for the shallow water
channel is defined in terms of critical angle. The
critical angle of propagation at the site is estimated
SANJANA et al.: ACOUSTIC PROPAGATION IN COASTAL WATERS...
21
Transmission loss at short ranges clearly shows the
influence of lossy bottom on acoustic propagation for
low frequencies. From the different model runs, it is
found that the model is able to incorporate water
column variability, sediment properties, broad band
sources etc and is suitable for modelling extremely
shallow water environments. Model results have been
compared with field measurements at the site and are
found to be satisfactory.
Acknowledgements
Authors thank Director, NIOT for his support in
carrying out this work. Prof. Gopu R Potty, University
of Rhode Island, USA is gratefully acknowledged for
his valuable guidance in running the model.
Fig. 7–Comparison of model with field results.
o
to be 11.42 based on measurements and the source
in such a waveguide propagates at an angle
o
confined to a cone of 2*èc =22.8 . The sound speed
profile is downward refracting enhancing propagation
along the bottom with a clay sediment layer which
leads to loss in acoustic energy. Transmission loss
analysis was done to illustrate the effects of shallow
water variability on sound propagation. The signal
arrival structure has been determined and signal
distortion in terms of sound speed fluctuation
examined. Signal distortion is observed for sound
speed gradients at long ranges. On examining the TL
for different frequencies over range and depth it shows
that the loss with respect to range is more for low
frequencies and decreases with increase in frequency.
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