LesFab@aol.com: From Feb '96 Scientific American

[David Rosenthal <dfr@xxxxxxxxxxxxx>]

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Subject: From Feb '96 Scientific American

Seeing Underwater with Background Noise

With a technique called acoustic-daylight imaging, sounds in the sea can
"illuminate" submerged objects, thereby creating moving color pictures
without sonar

by Michael J. Buckingham, John R. Potter and Chad L. Epifanio

"Ping...ggg." The sound of a sonar transmission is familiar from classic
films on submarine warfare, such as "Das Boot" and more recently "The Hunt
for Red October." An echo provides the submariner with the clue to a target's
presence and position. Alternatively, one can passively listen for the sound
generated by the target itself. In both techniques, however, the acoustic
noise that permeates the oceans compromises the integrity of the signals.
Breaking waves, passing ships, falling rain and even sea creatures such as
snapping shrimp all contribute to this cacophony. It is only to be expected
that sonar operators have traditionally regarded background noise as a
nuisance and, accordingly, have directed great efforts to suppress the
effects of ambient noise.

Yet that approach is gradually changing, as researchers have begun to
recognize that the noise itself can be useful. Noise surrounds any object
immersed in the ocean; the object, in turn, modifies this noise field in ways
that depend on the object's shape, composition and position. Ambient noise
has a familiar optical analogue: daylight in the atmosphere. We can see and
photograph outdoor objects because they scatter, reflect and otherwise modify
the light in the air. Likewise, noise that permeates the ocean acts as a kind
of "acoustic daylight." Recent experiments have shown that we can indeed
create images of underwater objects by using ambient noise as a source of
illumination. Our results are sufficiently encouraging that we believe
acoustic-daylight imaging should prove useful for a variety of purposes, from
harbor security to underwater mine detection.

To be sure, at present the resulting pictures lack a certain aesthetic
appeal. The image resolution is no match for that achieved with optical
light. The acuity of human vision stems from the fact that the dilated pupil
is 10,000 times the size of the wavelength of visible light, enabling the eye
to "collect" a great number of light waves. Achieving a similar resolution
with sound would demand an impractically large receiver 600 meters wide. But
because seawater strongly absorbs light and all other forms of
electromagnetic radiation, sound has become the favored--and in many cases,
the only--means of acquiring information about the ocean depths.

Humanity's interest in sound in the ocean dates back to antiquity. Aristotle
and Pliny the Younger wondered if fish could hear. Fishermen in ancient China
located shoals of fish by using a bamboo stick as an underwater listening
device, placing one end in the water. Leonardo da Vinci further developed the
idea, noting in his studies of the properties of water that "if you cause
your ship to stop, and place the head of a long tube in the water and place
the outer extremity to your ear, you will hear ships at a great distance from
you."

It was not until early in the 20th century, however, that inventors fashioned
the first underwater sonic location systems, in order to counter the
submarine threat during World War I. As rudimentary as those early devices
were, they formed the basis of all subsequent sonar, the development of which
accelerated rapidly during World War II. Current sonar systems, which have
found widespread military, commercial and scientific application, have
evolved to a high degree of sophistication. Still, they operate on much the
same principles as their predecessors: they either actively transmit sounds
or passively receive sounds produced by a target.

In view of the historical emphasis on active and passive techniques, it is
not surprising that the notion that noise might provide an entirely new way
of "seeing" in the ocean evolved only recently. In the mid-1980s one of us
(Buckingham) recognized that visual imaging as performed by the eye is
neither active nor passive. That is to say, the eye functions in a manner
that differs fundamentally from the conventional ways of using acoustics in
the ocean. Once this idea had registered, it became natural to speculate on
the possibility of creating an underwater acoustic analogue of visual
imaging. On a practical level, acoustic-daylight imaging would avoid the main
drawbacks of conventional undersea detection techniques: echolocation
unavoidably reveals the presence of the operator, and passive detection,
though entirely covert, fails with quiet or silent targets.


The First Experiment


In mid-1991 we conducted the first acoustic-daylight experiments in the
Pacific Ocean off Scripps Pier at Scripps Institution of Oceanography in La
Jolla, Calif. Working for his master of science degree at Scripps was a young
navy lieutenant, Brodie Berkhout, who constructed and deployed the equipment.
The main device was an acoustic receiver in the form of a simple parabolic
reflector, 1.2 meters in diameter, with a single hydrophone (underwater
microphone) at the focus. In effect, the reflector played the role of an
acoustic lens.

The purpose of the experiment was to answer a simple question: Does the
perceived noise level at the receiver change when an object is placed in its
"beam," that is, its listening field? A rectangular plywood board, 0.9 by
0.77 meter and faced with neoprene rubber--a good reflector and scatterer of
sound--served as the target. We found that for frequencies between five and
50 kilohertz (within the range produced by breaking waves, which are often
the main source of ambient noise in the ocean), the noise intensity nominally
doubled when the target was placed in the listening field of the reflector.
This result persisted when we moved the target from seven to 12 meters from
the receiver. Moreover, the target strongly reflected some frequencies and
absorbed others, a phenomenon that can be interpreted as acoustic "color."
This development suggested that we could translate the reflected acoustic
signature into optical hues to create acoustic-daylight images in false
color.

Spurred on by this success, we began thinking about the next stage of
development. The parabolic reflector with a hydrophone at its focus "looks"
only in a single direction, corresponding to just one pixel of an image. To
create a more complete picture, more pixels are necessary, which means more
receiver "beams" are needed (rather like the compound eye of a fly). The
noise in each receiver beam could then be converted to a certain level of
brilliance in a pixel on a video monitor, with the intensity of the noise
governing the degree of the brightness. As in a newspaper photograph, the
contrast between pixels would enable the eye to interpret the result as a
more or less granular pictorial image.

With the success of the initial test, we became convinced of the feasibility
of achieving genuine acoustic-daylight images that would contain 100 or more
pixels. In mid-1992 we began designing a new acoustic lens, which came to be
known as ADONIS, for acoustic-daylight, ambient-noise imaging system. Working
in conjunction with EDO Acoustics in Salt Lake City, which produced an
elliptical array of 128 hydrophones for ADONIS, we constructed a spherical
reflector three meters in diameter and placed the hydrophones at the focus of
the dish. This system formed a total field of view of approximately six
degrees (horizontal) by five degrees (vertical), which is about one tenth the
angular view afforded by a typical camera.

We lowered ADONIS, looking rather like a satellite dish, onto the seabed for
the first time in August 1994. ADONIS was deployed from one of Scripps's
research platforms, R/P ORB, moored off Point Loma in southern California.
Square panels (one meter per side) of aluminum sheeting faced with neoprene
rubber formed the targets to be imaged. The panels were mounted in various
configurations on a square tic-tac-toe-type frame set on the seabed.
Roiled-up sediment in the busy harbor made visibility through the water
extremely poor during most of the experiment. On one occasion the turbidity
was so bad that Helene Vervoort, one of our divers, collided with the target
frame.

An electronics package housed in a sealed pressure canister rested alongside
the mast supporting the spherical dish. Among other processing tasks, the
electronic equipment, designed by our colleague Grant B. Deane, would convert
the ambient noise data acquired by ADONIS into digital form. The data would
then be transmitted to the surface and rendered into real-time, false-color
images on the screen of a Macintosh desktop computer. An immense amount of
time and effort hung in the balance as ADONIS was lowered into the sea for
its first deployment.


To See or Not To See?


The air of hushed expectancy that hung over our group as ADONIS disappeared
below the ocean surface was soon dispelled--not, however, because of an
initial, resounding success. Almost immediately, the gauges monitoring
several onboard power supplies surged--a strong indication that seawater was
flooding into the electronics canister. Sure enough, when ADONIS was hauled
up and the canister opened, saltwater gushed out. As a reflex reaction, we
removed the delicate circuit boards and soaked them in deionized water,
although nobody really believed they could be salvaged. But with help from a
number of quarters, we flushed the boards with alcohol, tested all the
electronic components of the complex 128-channel system, replaced them where
necessary and sealed the leak in the canister. Twenty-four hours later ADONIS
was again lowered into the water.

This time the tension on ORB was tangible as the divers made last-minute
checks on the equipment. When the data started to flow, the laboratory became
quiet. We had set three panels in the frame to form a simple horizontal
target, one meter high by three meters wide, at a distance of 18 meters from
ADONIS. As we gathered around the screen, we realized that a faint
rectangular shape was visible, almost filling the elliptical image space. We
were watching the first acoustic-daylight picture.

Within minutes our confidence in the imaging system had soared. Divers had
placed a sound source in the center of the target to help us align ADONIS
with the target frame. But the source proved unnecessary: we could see where
the targets were just from the ambient noise. We then extended the space
between ADONIS and the target from 18 to 38 meters, as far as we could go
without interfering with shipping traffic. At the greater range we expected
perhaps a slight degradation in performance, but astonishingly the target
became far clearer. Of course, the image was also smaller than it had been
previously, but as a result, the surrounding ocean formed a nice, contrasting
background that made the rectangular target stand out dramatically. As these
raw images continued to appear on the screen, refreshed 30 times a second, we
knew that acoustic-daylight imaging worked.

There was still much to be done during this deployment, however. We wanted to
know if ADONIS could detect moving objects. A hydraulic motor mounted within
the mast supporting the dish could slowly rotate the spherical receiver in
azimuth, taking 12 minutes or so to complete a full 360-degree sweep. As the
dish panned around, we watched the target appear on one side of the screen,
creep to the center and finally drop off the far side. There was no doubt
that we could create moving images.

One more test, the most demanding of all, remained. Divers replaced the
bar-shaped target with four panels in the frame, forming a cross with
vertical and horizontal arms and a one-meter-square hole in the center.
Resolving the hole was the challenge: at a range of 38 meters, the size of
the hole would be close to the resolution limit of ADONIS.

The first raw images of the cruciform target were indistinct. We could see
the shape of the cross, but the appearance of the central hole fluctuated
from instant to instant. Since then, we have reexamined the data and applied
some computer processing. It turns out that the power spectrum of the
noise--the intensity of the sound at different frequencies--serves a
discriminatory function. It is essentially the acoustic version of color. By
using the power spectrum, the four empty corners and the hole in the
cruciform target could easily be identified and the edges of the panels
located. The panels in the target frame showed a distinctly different "color"
from the empty regions, including the central hole. It was as if the frame
looked "red," and the hole appeared "blue." Currently we are exploring this
technique as a means of enhancing acoustic-daylight images.


Imaging at Sea World


Static targets served us well in demonstrating that acoustic-daylight imaging
is a workable technique. Inspired by our results, we were anxious to try a
more difficult target: killer whales (Orcinus orca). Through the good offices
of Ann Bowles, a research biologist at Hubbs Sea World Research Institute in
San Diego, we were invited to deploy ADONIS in the outdoor killer-whale tank
at Sea World. We could try to image highly mobile marine mammals while Bowles
conducted behavioral studies on the response of the animals to a strange
object in "their" tank; the whales, it seems, feel that anything placed in
the tank, by definition, belongs to them.

In February 1995, working between the killer whales' public performances, we
set up ADONIS in one corner of the tank in rather unpleasant weather
conditions. Rain lashed down most of the time; to protect our computers and
recording equipment, we rigged up makeshift tarpaulins, but even so water
seeped everywhere.

Meanwhile, as we set up the system, the killer whales swam freely in the
tank, taking as much interest in us as we did in them. Cautious at first,
they quickly grew accustomed to the large reflecting dish. The whales became
curious on finding that because of the focusing effect of the dish, sound
reflected intensely back to them when they "pinged" in front of it. A young
male, Splash, grew more adventurous, taking one of the oil-filled electronics
cables (crunchy on the outside and chewy on the inside) into his mouth to try
some exploratory mastication. Another animal swam fast toward the dish and
breached over the top--at this point we felt that something had to be done.
The trainers moved the whales to another tank, where they could play with
their own toys until we were ready for them.

After one false start (all the electronics boards in the underwater housing
had shaken free of their connectors during transportation), we switched on
the equipment again, and data started flowing. We were not sure what to
expect. Pumps and other machinery bring the noise in the Sea World tanks to
quite high levels, comparable to those in the ocean. Despite some minor
damage that the electronics boards had sustained when they were flooded by
seawater, signals from all but two of the 128 channels were received and
displayed as real-time moving images.

As we watched the raw data (that is, with no image enhancement) on the
screen, a shadowy form suddenly appeared and remained in sight for several
seconds. At the same time, we could see (with our own eyes) one of the whales
move into the field of view of ADONIS, where it stayed as it swam directly
away from the dish. Hydrophone monitors and the trainers confirmed that the
whales themselves were not transmitting sound, indicating that the images we
saw were a direct result of acoustic daylight. We still have to examine the
images of the killer whales carefully and correlate them with the video
recordings that were made simultaneously to confirm whether we actually
imaged the whales. But the preliminary observations and the ORB experiment
off Point Loma support the analogy between conventional photography with
daylight and underwater imaging with ambient sound.

The results immediately suggest several potential applications. One is the
detection of underwater mines, some of which can be rigged to detonate on
receiving a sonar signal. An acoustic-daylight system might be able to locate
these devices without triggering them. Imaging with ambient noise could
provide vision for autonomous underwater vehicles, enabling them to steer
around obstacles without help from a human operator on a surface ship and to
monitor the structural integrity of oil rigs and other large maritime
platforms. The inherently covert nature of acoustic-daylight imaging also
makes it suitable for monitoring harbors--just as video cameras keep
vigilance in shopping malls--and for counting marine mammals, because there
would be no sonic interference with the animals themselves. (That, in turn,
raises the question of whether marine mammals themselves use acoustic
daylight to acquire information.)

Conceivably, we can take acoustic-daylight imaging further, for it is still a
nascent concept. In recent tests, ADONIS successfully imaged plastic floats,
titanium spheres and polyvinyl chloride oil drums containing wet sand and
foam. Preliminary analysis indicates that the barrels can be seen even when
they are on the seafloor. We have reached a stage rather like the earliest
days of television: what is important is not the quality of the images but
the fact that there are images at all. In the months ahead, we plan to
replace the spherical reflector with a phased array containing as many as
1,000 hydrophones. At the same time, we shall be developing dedicated
algorithms to provide image enhancement and automatic image recognition.
These efforts will, we hope, improve the quality of acoustic-daylight images
significantly and perhaps make the successors to ADONIS the underwater video
cameras of the future.


BOX: Sounding Out New Uses for Noise

Acoustic-daylight imaging is just one form of remote-sensing technology that
relies on the background noise in the seas. Oceanographers have recently
demonstrated other examples of similar techniques. One is to use ambient
noise to determine the acoustic properties of the seabed and hence to
determine its composition to some extent. In the shallow waters over the
continental shelves, where the depth is less than about 200 meters, the noise
reflects off the seafloor. The manner in which the sound bounces off
indicates the speed with which vibrations move in the floor. That, in turn,
reveals the composition of the bottom: sound travels at different speeds
through bedrock than it does through sand, for instance.

To carry out such measurements, one can deploy a fleet of hydrophone-dangling
buoys to map the seabed using ambient noise. The hope is that this technique
will offer a cost-effective alternative to conventional methods, such as the
often slow and laborious practice of bouncing sonar signals off the sea
bottom.

Background sounds may also prove beneficial in the study of processes
occurring at the sea surface. In particular, they can reveal the amount of
atmospheric gas the oceans are absorbing. Crucial for models of global
warming and the greenhouse effect, the extent of gas exchange has been
difficult to quantify. Ambient noise may help, because the phenomenon mostly
responsible for the sound also happens to govern the transfer of gas from the
air to water--namely, wave breaking. In driving air into the water, the
process creates a layer of bubbles immediately below the surface. These
bubbles modify the sound of the breaking waves in a characteristic way,
leaving an acoustic signature for hydrophones below the bubbles to detect.

>From such a simple acoustic measurement, it may be possible to infer the
amount of air in the bubble layer and the depth to which the bubbles extend.
Both quantities are related to the amount of gas entering the ocean. Some
preliminary testing suggests the idea is feasible; major experiments are
currently under way.--M.J.B.


The Authors

MICHAEL J. BUCKINGHAM, JOHN R. POTTER and CHAD L. EPIFANIO developed
acoustic-daylight imaging at the Scripps Institution of Oceanography in La
Jolla, Calif. Buckingham is professor of ocean acoustics there and holds a
visiting professorship at the University of Southampton in England. He
received his Ph.D. in physics from the University of Reading and has written
and edited numerous articles and books on acoustics. An itinerant yachtsman,
Potter sailed across the Pacific Ocean last fall to direct the Acoustic
Research Laboratory of the National University of Singapore. After spending
four summers on the Antarctic peninsula, he received his Ph.D. from the
Council for National Academic Awards and the University of Cambridge.
Epifanio is close to completing his Ph.D. on acoustic-daylight research at
Scripps. He received his B.S. in electrical engineering from Bucknell
University in 1991. The authors are grateful to Sea World in San Diego, to
Hubbs Sea World Research Institute and to the U.S. Office of Naval Research
for their research support.


Further Reading

IMAGING THE OCEAN WITH AMBIENT NOISE. Michael J. Buckingham, Broderick V.
Berkhout and Stewart A. L. Glegg in "Nature," Vol. 356, pages 327-329; March
26, 1992.

THEORY OF ACOUSTIC IMAGING IN THE OCEAN WITH AMBIENT NOISE. Michael J.
Buckingham in "Journal of Computational Acoustics," Vol. 1, No. 1, pages
117-140; March 1993.

ACOUSTIC IMAGING USING AMBIENT NOISE: SOME THEORY AND SIMULATION RESULTS.
John R. Potter in "Journal of the Acoustical Society of America," Vol. 95,
No. 1, pages 21-33; January 1994.

ACOUSTIC DAYLIGHT IMAGING: VISION IN THE OCEAN. Michael J. Buckingham and
John R. Potter in "GSA Today," Vol. 4, No. 4, pages 97-102; April 1994.


SCIENTIFIC AMERICAN  February 1996  Volume 274  Number 2  Pages 86-90



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