ACOUSTIC EFFECTS OF ENVIRONMENTAL VARIABILITY IN
THE SWARM, PRIMER AND ASIAEX EXPERIMENTS
J. LYNCH, A. FREDRICKS, J. COLOSI, G. GAWARKIEWICZ AND A. NEWHALL
Woods Hole Oceanographic Institution (WHOI), Woods Hole, MA 02543
E-mail: jlynch@whoi.edu
C.S. CHIU
Naval Postgraduate School (NPS), Monterey, CA 93943
E-mail: chiu@nps.navy.mil
M. ORR
Naval Research Laboratory (NRL), Washington, D.C. 20375
E-mail: orr@wave.nrl.navy.mil
We present an overview of how the coastal oceanographic environment affected
acoustic propagation and scattering in three recent major field experiments: SWARM
(1995), PRIMER (1996–97), and ASIAEX (2000–01). In all three of these experiments,
low frequency sound (50–600 Hz) was transmitted through strong coastal
oceanographic features to array receivers. The differences and similarities of these
experiments will be emphasized. In particular, we will focus on the effects of coastal
oceanography, i.e. fronts, eddies, and internal waves. We begin with the SWARM
experiment, which examined the effects of internal waves, particularly non-linear
internal waves, on acoustic propagation and scattering. This experiment had excellent
environmental support along a single across-shelf propagation path, and we were able to
make good progress understanding the sound scattering within the context of this
limited geometry. In the PRIMER experiment, we looked at oceanographic effects in a
fully three-dimensional configuration, being supported by numerous environmental
moorings and Sea Soar (undulating CTD) high-resolution hydrography. This experiment
observed the effects of both the shelfbreak front, eddies and filaments, and nonlinear
internal waves on the acoustic field. Finally, we examine the recent ASIAEX
experiment, which again dealt with fully three-dimensional oceanography and alongand across-shelf acoustic propagation. ASIAEX had perhaps the best environmental
support of the three experiments, including thirty environmental moorings, Sea Soar
hydrography, satellite remote sensing, and acoustic flow visualization surveys. The
acoustic monitoring included both a vertical and horizontal array, and moored and
towed sources. Oceanographically, this experimental site featured some of the strongest
non-linear internal waves we have seen to date.
1
Introduction
The Yellow Sea experiments of Zhou et al. in the 1980’s, published in the Journal of the
Acoustical Society of America in 1991 [1], created a major stir in the field of low frequency
(50–1000 Hz) shallow water acoustics. Field data showed anomalous propagation loss
3
N.G. Pace and F.B. Jensen (eds.), Impact of Littoral Environmental Variability on Acoustic Predictions and
Sonar Performance, 3-10.
© 2002 Kluwer Academic Publishers. Printed in the Netherlands.
J. LYNCH ET AL.
4
effects of up to 30 dB (depending on frequency) along with strong azimuthal dependence
of the propagation loss, with the likely cause being groups of strong nonlinear internal
waves often called “soliton trains.” Using a finite-lattice, Bragg resonance scattering
theory, Zhou et al. [1] were able to explain these data rather well, and to first order, this
looked like an effect that was found and quickly understood. However, given the
magnitude and potential importance of the effect, and the fact that not all aspects of the
scattering were measured by Zhou et al., further examination was suggested. This
motivated a number of US experiments, including the SWARM (Shallow Water Acoustic
Random Medium) experiment.
2
The SWARM experiment
The SWARM experiment [2–5] studied across-shelf propagation and scattering through
soliton trains, and had two major objectives. The first objective was to look at acoustic
intensity fluctuations as a function of time, frequency, and distance from the source, and
was spearheaded by the NRL acoustics group. This work was motivated in part by the
theoretical work of Creamer [6], who’s extension of Dozier and Tappert’s deep-water
fluctuation work [7] to shallow water predicted that the scintillation index (the
normalized variance of the intensity) should increase exponentially with range. This
intensity variability analysis of the SWARM data will be reported in another paper from
this conference [8], and so will not be pursued further here.
The WHOI group examined the time spreading of mode filtered acoustic pulses in
SWARM. It was observed that the pulse spreading measured oscillated very strongly at
the M2 (semidiurnal) tidal period. Moreover, it was seen, again in the context of mode
filtered data, that the spreading was the largest when the scatterer (the soliton train) was
in the vicinity of the acoustic receiver array. This “near receiver dominance” was
explained by Headrick et al., and we refer the reader to those papers [3,4].
The NPS group concentrated on the physical oceanography, and in some sense, this
topic contained the most surprising results [2]. To begin, the internal-wave field one
observed at the SWARM site, while being dominated by non-linear solitons, was very
different from the Yellow Sea internal-wave field. Rather than the neat, evenly spaced
“ducks-in-a-row” seen in the Yellow Sea, the SWARM wave field was temporally and
spatially complex due to multiple sources. Though the waves generated along the
shelfbreak were dominant, waves generated by local canyons also contributed
substantially to the total field. Secondly, the wave-field varied significantly in time. Thus,
the internal-wave field in this region was unpredictable without further information on the
generation processes occurring at remote sites. These space-time variations in the
internal-wave field indicated to researchers that the oceanography and the acoustic
scattering theory that applied to Zhou’s Yellow Sea experiment was not universally
applicable, and that more investigations were warranted into the coastal internal-wave
field and the acoustic scattering theory needed to describe its effects.
3
The New England shelfbreak PRIMER experiment
Internal waves are but one variety of oceanographic variability that can have a strong
affect on acoustics. Coastal fronts and eddies are also of importance to acoustics, and are
ACOUSTIC EFFECTS OF ENVIRONMENTAL VARIABILITY
5
common features as well. The WHOI and NPS groups had previously examined the
acoustic effects of shallow water fronts in the 1992 Barents Sea Polar Front experiment
[9–11], but this was done for a rather weak frontal system, and without large-scale
oceanographic support. A stronger frontal system, with 50 cm/s flows and frontal eddies,
was conveniently available for study in the New York Bight, and so it was decided to
examine that region.
In the summer of 1996 and the winter of 1997, two large acoustics-plusoceanography field experiments were undertaken in the vicinity of the shelfbreak front
south of Martha’s Vineyard, MA. In these efforts, three moored sources seaward of the
front transmitted signals for two weeks to two vertical line array (VLA) receivers
shoreward of the front, while the oceanography was measured concurrently by a wide
variety of means, including the Sea Soar towed CTD instrument, which provided threedimensional maps of the sound-speed field on a daily basis. We will briefly examine some
of the more interesting results from the summer 1996 experiment here, and refer the
reader wanting further detail to the literature [12–15].
An interesting study we are undertaking with the PRIMER data is that of the
amplitude fluctuations observed. One part of the work we are engaged in is trying to
explain the amplitude fluctuations observed at the receiver in terms of an “energy budget”
(i.e. energy conservation) made at that location. Looking at either of the two PRIMER
receiver sites, we note trivially that acoustic energy passing by an array has to either hit
the receiver, or go above it or below it. The total energy impinging upon the receiver is
calculated by integrating both over the temporal arrival pattern (as we sent pulses) and
over the spatial extent of the array. The energy going either above the vertical array
receiver (which goes from the bottom to 40 m water depth) or below the array (through
the bottom) is not measured, but can be estimated by computer calculations, which we
tune to match the environment of the site and the measured array data. By combining data
and model estimates, we can attempt to understand the distribution of the energy in the
water column and in time, and thus the acoustic intensity fluctuations. Moreover, the
models provide insight as to the physical causes of the fluctuations.
We now show an example of how energy conservation may be related to the
amplitude fluctuations. In Fig. 1 we display the temporally and spatially integrated
arrivals seen at the northeastern vertical line array in the summer 1996 PRIMER
experiment. Immediately evident are the fluctuations in the energy with a large M2 tidal
component and also with higher and lower frequency components. The energy changes
seen can be due to energy going above and below the receiver, as mentioned, and also
due to time dependent changes in bottom loss along the acoustic track (we disregard 3-D
effects). In order to explain the origin of these variations seen in the data, we have
simulated the acoustic propagation through a temporally and spatially evolving internal
wave field over a full M2 semidiurnal tidal cycle (12.42 hours), and then examined the
distribution of energy seen at the array.
Our initial results indicate that energy going below the array in the bottom is
minimal, not surprising since the most energetic acoustic modes have turning points
above the bottom. Somewhat more surprising is the result that the amount of bottom loss
along the track is not varying much with time, but rather that the energy is being
redistributed above the receiver in a time varying fashion. This seems to indicate that
solitons coming near the receiver at the semidiurnal tidal frequency are sending energy
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J. LYNCH ET AL.
via mode coupling to higher order modes, which puts energy higher up in the water
column. This tentative result is still being verified at the time of writing. Whatever the
final outcome, it is evident that such a computer analysis, especially when combined with
data, can be very useful in understanding the nature of the energy/intensity fluctuations
seen.
Figure 1. 1996 PRIMER experiment time series of integrated intensity at a point receiver from two
400 Hz sources, one in the southwest corner of the experimental area (a) and one in the southeast
corner (b). SI is the scintillation index, and σ is the intensity fluctuation in dB.
Another interesting intensity fluctuation study made with the PRIMER data is
relating the amount of time spreading seen in the signals to the peak acoustic intensity. As
a very simple first guess, we have first considered an energy conserving “accordion
model” that says that the area under the received pulse at a given hydrophone is
conserved in time, so that if the pulse width is expanded, its peak is correspondingly
ACOUSTIC EFFECTS OF ENVIRONMENTAL VARIABILITY
7
decreased. (This is a phase modulation effect.) In such a model, the time spreading and
the acoustic intensity at a given hydrophone are anti-correlated. We can look at this anticorrelation directly with our data and it seems to work fairly well, as shown in Fig. 2. This
would indicate that a first order effect in changing the amplitude for a point receiver is
the time spreading of the pulse. The fact that this correlation is not perfect suggests that
there are other mechanisms (amplitude modulation) also coming into play. We are
hypothesizing, based on our first example, that the vertical redistribution of the energy in
the water column is the other principal effect. This remains to be seen, and is one of the
foci of our ongoing research.
Figure 2. Time series of pulse spreading (sec) versus inverse measured intensity (arbitrary linear
units) for the two sources discussed in Fig. 1.
J. LYNCH ET AL.
8
The other topic of interest from PRIMER is the physical oceanography and its
variability, which we are endeavoring to correlate to the acoustic field and its time and
space variability. The PRIMER experimental site featured extremely variable and
complicated coastal oceanography, with the prime features being the shelfbreak front,
Gulf Stream eddy forcing from the continental slope, and the nonlinear internal tide (i.e.
the soliton trains and tidal bore). The summer Shelfbreak front oceanography has been
examined by Gawarkiewicz et al. [15], and we will just mention some of his more
interesting results which are of importance to the low-frequency acoustic field. To begin
with, the sound-speed field was found to be very inhomogeneous, both in the across-front
and the along-front directions, somewhat at variance with popular wisdom. The correlation
lengths and times of the oceanography near the front were estimated to be of order 7–15
km and 1–2 days, as measured by the Sea Soar hydrography and moorings. The warm
bottom layer beneath the front was seen to be a consistent and acoustically important
oceanographic entity. The eddy field spawned by the Gulf Stream was seen to interact
with the front in a very complicated fashion, modulating its shape and thus its acoustic
effects, and this is the subject of intense further study. Regarding the internal-wave field,
Colosi et al. [16] have studied it from moored records and have again found it to be
extremely variable, both in time and space. Significant interaction between the internal
tides and the larger scale oceanography is seen, leading to a picture of a field that most
likely has to be described by a combination of deterministic and random contributions.
There is much more that can be said regarding the PRIMER oceanography, but due to the
limits of space, we will just refer the reader to some of the literature for now [15,16].
4
The ASIAEX experiment
The final experiment we will touch on here is the recent ASIAEX experiment [17], which was
conducted in the South China Sea. Since this experiment is rather recent, we have fewer
detailed results to show, but we can at least talk about what the data set includes and the
analyses it will support.
The biggest improvements in ASIAEX over the previous shallow water experiments, as
regards acoustics, were: 1) the inclusion of a 400 m length, 32 element horizontal array, in
addition to a 79 m long, 16 element vertical array — previous experiments only had 16
element vertical arrays, 2) more acoustic frequencies, filling the 50–600 Hz band — previous
experiments had only 224 Hz and 400 Hz transmissions, 3) a longer overall time series (three
weeks), so that we could examine a full spring-neap tidal cycle, 4) longer duration moored
transmissions, so that we could unambiguously look at temporal decorrelation times for the
acoustic field, and 5) a variety of towed source tracks, allowing us to look at the range
variability of the acoustic field, which cannot be done with purely moored transmissions.
Turning to the oceanography, we had far more oceanographic environmental data than any of
the previous efforts. In addition to having thirty oceanographic moorings deployed in the
experimental region, along with intense satellite imagery support, we also had three ships
performing measurements from onboard, including Sea Soar, ADCP current measurements,
and high frequency acoustic flow visualization. Geologically, measurements included a highresolution bathymetry survey, chirp sonar imagery along the fixed acoustic paths, and
numerous cores. Thus, our ASIAEX data set is the most complete coastal acoustics-plusenvironment measurement set in our possession to date.
ACOUSTIC EFFECTS OF ENVIRONMENTAL VARIABILITY
9
Given the new dimension of the horizontal array, one of the first things we are
looking at in ASIAEX is the coherence of the acoustic signal across the array. In Fig. 3,
we show a typical 224 Hz pulse arrival structure across the horizontal array. Due to the
array having a somewhat irregular shape, and to the sources being off-broadside, the
acoustic pulses arrive skewed in time. This is being corrected by time delay beamforming,
or more prosaically, lining up the initial peaks in this figure. Looking at the arrivals in
Fig. 3, one is struck at their general regularity, with some difference in the later arrivals.
This general regularity indicates a rather high degree of coherence of the signal across the
array (with exact numerical estimates of that number forthcoming), a result which is
somewhat surprising, given that the ASIAEX was located in a very active oceanographic
area. It is our conjecture that the near-bottom position of both source and receiver is
mitigating against seeing strong scattering effects, and we propose to test this by looking
at the variability in the arrivals for receiver elements higher in the water column in the
vertical array.
Figure 3. Typical arrival structure seen across the bottom-lying horizontal array in ASIAEX.
There are numerous other topics in ASIAEX that we will be looking at, but rather
than enumerate them, we will just mention that the intercomparison of various acoustic
effects between the SWARM, PRIMER, and ASIAEX sites is one topic that has been
enabled by finally having high-quality data from multiple sites. One of the key issues in
shallow water is the generality and transportability of results from site to site, and we
hope that these three data sets will allow us to begin answering that question.
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Acknowledgements
We would like to thank our numerous colleagues and shipmates from the SWARM, PRIMER
and ASIAEX projects for their hard work and collegiality. The three projects described in this
report (SWARM, PRIMER, and ASIAEX) all were supported by the Office of Naval Research.
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