CHANNEL IMPULSE RESPONSE FLUCTUATIONS AT 6 KHZ
IN SHALLOW WATER
W.S. HODGKISS, W.A. KUPERMAN AND D.E. ENSBERG
Marine Physical Laboratory, Scripps Institution of Oceanography,
University of California, San Diego, La Jolla CA 92093-0701, USA
E-mail: [wsh,wak,dave]@mpl.ucsd.edu
A shallow water (~100 m) experiment was carried out to measure the stability of
forward transmissions at 6 kHz over a 6 km propagation path. The fixed source, fixed
receiving array geometry enabled observing environmentally-induced fluctuations in the
channel impulse response. The 64-element receiving array had an aperture of 12 m and
thus a corresponding vertical angle of arrival resolution of ~1°. Source transmissions
were of duration 20 min and consisted of multiple subcomponents. Of particular interest
here are the 2 kHz bandwidth, 1 s duration FM chirps which were transmitted
continuously for 5 min and have been matched filtered to yield channel impulse
response structure. In addition to CTDs taken in the region between the source and
receiving array, a thermistor string at the receiving array site provided continuous
measurements of water column temperature fluctuations. Discussed in this paper is the
time-evolving structure of the channel impulse response observed from both individual
array elements as well as array beams which clearly show significant, environmentallyinduced fluctuations over the 5 min duration of these transmissions. Also discussed is
the relationship these fluctuations have to the characteristic ray path structure of the
channel.
1
Introduction
The shallow water environment can be quite dynamic. In addition to the background
internal wave field, rapid water column temperature fluctuations generated by the
internal tide as it progresses along the continental shelf have a substantial impact on high
frequency acoustic transmissions.
In this paper, we present results from a shallow water (~100 m) fixed source, fixed
receiving array experiment carried out to measure the stability of forward transmissions
at 6 kHz over a 6 km propagation path. First, we will overview the experiment and the
background internal wave-related temperature fluctuation characteristics. Next, we will
describe the general multipath structure between the source and receiving array. Lastly,
we will focus on a 5 min period and show the time-evolving impulse response at a single
element and how the impulse response fluctuations manifest themselves in arrival angle
vs. travel time as well as wavefronts observed across the receiving array.
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Sonar Performance, 295-302.
© 2002 Kluwer Academic Publishers. Printed in the Netherlands.
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Figure 1. Experiment overview showing the source mooring and receiving array locations (left
panel) and the hardware placement in the water column (right panel).
2
Experiment
The experiment was conducted off the coast of San Diego, CA, in October 1997
(Fig. 1a). An overview of the experiment showing the environmental and acoustic
instrumentation is shown in Fig. 1b. The fixed source, fixed receiving array geometry
enabled observing environmentally-induced fluctuations of the channel impulse response
over the 6 km propagation path. The 64-element receiving array had an aperture of 12 m
and thus a corresponding vertical angle of arrival resolution of ~1° at 6 kHz. Both the
receiving array and source mooring were deployed near the seafloor. Source
transmissions were of duration 20 min and consisted of multiple subcomponents. Of
particular interest here are the 2 kHz bandwidth, 1-s duration FM chirps which were
transmitted continuously for 5 min and have been matched filtered to yield the timeevolving channel impulse response structure. Additional results from an analysis of
acoustic communications transmissions centered at 18 kHz are given in [1].
In addition to the acoustic instrumentation, environmental measurements were made
throughout the experiment. Along with CTDs being deployed frequently in the area, a
16-element thermistor string was deployed from the R/P FLIP and measured the timeevolving temperature structure from the mixed layer through the main thermocline
(10–70 m) where internal wave activity typically is most prevalent. Figure 2 shows this
structure throughout the duration of the experiment.
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Figure 2. Time-evolving temperature structure measured by the thermistor string from the mixed
layer through the main thermocline (10–70 m).
3
Multipath structure
The general nature of the sound speed profile observed during the experiment is shown
in Fig. 3a which was derived from one of the CTD casts. The mixed layer is relatively
deep followed by a sharp thermocline. Of note is a second thermocline in the 70–90 m
depth range. A range-independent ray trace of all significant eigenrays from the source
to the receiving array is shown in Fig. 3b. As is evident, one group of rays turns over in
the lower thermocline, another group turns over in the upper thermocline, and lastly a
group of rays interacts with the sea surface.
Another useful display of the eigenrays is to plot their transmission loss (TL) vs.
arrival time at the array and their vertical angle of arrival vs. arrival time. These are
shown in Fig. 4
4
Channel impulse response
The matched filtered FM chirp transmissions enable observing the time-evolving
channel impulse response with a 1-s update rate. A 5 min period of relatively high
dynamics was selected for detailed analysis (JD 301: 2207-2212 UTC). The water
column temperature structure over the 16 hour interval bracketing this period is shown in
Fig. 5.
4.1
Single-Element Impulse Response
The time-evolving channel impulse response from the source to array element #1
(highest in the water column) is shown in Fig. 6. The general structure is consistent with
the eigenray predictions in Fig. 4a. Although the single-element impulse response is
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Figure 3. Typical sound speed profile (left panel) and corresponding ray trace of all significant
eigenrays from the source to the receiving array (right panel).
Figure 4. Eigenray transmission loss vs. arrival time (left panel) and vertical angle of arrival vs.
arrival time (right panel).
Figure 5. Water column temperature structure over the 16 hour period bracketing the 5 min period
selected for detailed analysis (JD 301: 2207-2212 UTC).
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relatively stable for a few seconds to a few tens of seconds at a time, there are substantial
fluctuations over the total 5 min observation period.
4.2
Arrival Angle vs. Travel Time
The transmissions were observed on a 64-element array. Thus, we can decompose the
channel impulse response structure in both space and time. The average arrival angle vs.
travel time structure over the entire 5 min observation period is shown in Fig. 7.
Although fluctuations are evident in the average, the underlying structure is consistent
with the eigenray predictions in Fig. 4b. Individual examples of the spatial structure of
the channel impulse response are shown in Fig. 8 where the results represent
transmissions 4 min apart.
4.3
Wavefront Arrivals
In addition to the spatial decomposition provided by the beamforming results, displays
of the wavefront arrivals also are informative. For the same transmissions displayed in
Fig. 8, the wavefront arrivals are shown in Fig. 9.
5
Summary
The fixed source, fixed receiving array transmissions at 6 kHz discussed here provide a
detailed look at the time-evolving dynamic structure of the shallow water channel
impulse response. In this case, the water column was characterized by having a relatively
deep mixed layer followed by a sharp upper thermocline. Of note was a second
thermocline in the 70–90 m depth range. Thus, there are ray groups which turn over in
the lower thermocline, turn over in the upper thermocline, and interact with the sea
surface. Fluctuations of the channel impulse response relate directly to the dynamics of
each of these regions of the water column. For the data analyzed, the impulse response
structure is somewhat stable for a few seconds to a few tens of seconds at a time but over
the total observation period of 5 min there are substantial fluctuations.
Acknowledgements
This research was supported by the U.S. Office of Naval Research.
References
1. Carbone, N.M. and Hodgkiss, W.S., Effects of tidally driven temperature fluctuations on
shallow–water acoustic communications at 18 kHz, IEEE J. Oceanic Eng. 25, 84–94
(2000).
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Figure 6. Time-evolving channel impulse response from the source to array element #1.
Figure 7. Arrival angle vs. travel time structure averaged over the 5 min period.
CHANNEL IMPULSE RESPONSE FLUCTUATIONS
Figure 8a. Arrival angle vs. travel time (JD 301: 2207 UTC).
Figure 8b. Arrival angle vs. travel time (JD 301: 2211 UTC).
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Figure 9a. Wavefront arrival structure across the receiving array (JD 301: 2207 UTC).
Figure 9b. Wavefront arrival structure across the receiving array (JD 301: 2211 UTC).