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The text below outlines some applications of using acoustics to gain
information and make measurements in the underwater environment. The section
on imaging discusses using sound to provide us with visual images of submerged
surroundings. This course is available on the internet and all text and
images herein are copyrighted. You are not permitted to copy, print, store
or distribute any of the contents including text and images for any
purpose. All rights reserved, J.P. Fish, 2002.
Early sonar systems, developed during World War I by the American, British, and French, were used to find both submarines and icebergs. They were called ASDICs (named for the AntiSubmarine Detection Investigation Committee). These early units were crude if not effective. During World War II, underwater acoustics made great leaps and enemy submarines could be detected more easily by surface ships as they sent a stronger and better formed sound pulse into the water. The pulse would bounce off the submarine's hull and give away their distance from the surface ship.
Sonar (an acronym for SOund NAvigation and Ranging), became very important for the detection of the submerged enemy. Once it was realized that sound from surface ships refracted away from submarines if they "hid" under thermoclines, or the top of layers or colder water, bathythermography (determining the temperature at depth) became important to the sub hunters.
One unique acoustic feature of the ocean is the Sofar Channel in the upper regions of the deep ocean. Within this layer of ocean water, centered at about 1250 meters below the surface in the northwest Atlantic, the temperature and pressure act in such a way as to provide a long range acoustic path or channel. The Sofar Float, successfully developed by Doug Webb and his colleague, Dr. Tom Rossby, in the late 1960's, is an instrument designed to be neutrally buoyant at a pre-determined depth (this alone is not a simple engineering feat) while transmitting a timed acoustic pulse within the sound channel. The pulse is low frequency and propagates over several thousand kilometers. Autonomous, stationary receivers moored in the sound channel over a large area receive these signals and store the time of reception. To retrieve data from Sofar Float transmissions, the moored receivers are recovered after a year or more. A variation of the Sofar Float is the Rafos (Sofar spelled backwards) Float. With this methodology, the transmitters are moored and the receivers mobile and drifting at depth. After a period of time the drifting receivers drop a ballast weight and rise to the surface. Once there, they transmit their data to satellites, which in turn transmit the data to scientists on land.
Doug Webb, now president of Webb Research Corporation, is one of the heroes of invention and ocean exploration in the 20th century. His brilliant work, accomplished after years of research and design in the 1960's and 70's, has resulted in the development of very unique but complex acoustic instrumentation. One of these is the ALACE (Autonomous Lagrangian Circulation Explorer) Float. A dedicated underwater robot of sorts, the Alace drifts for a month or more, again neutrally buoyant, measuring the ocean environment. It then rises to the surface and transmits its present position and findings to a satellite. When transmission is complete, it submerges to its pre-assigned depth for another month of measurements. The ALACE can cycle in this manner for over four years!
Another of Doug's designs is now in the test phase. It is an instrument named "Slocum." More information on Slocum and Alace can be found at Webb's homepage via a link at the end of Acoustics and Sonar.
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As technology has improved, better methods of transmitting and receiving sonar, as well as processing the received signal, have been developed. The advent of digital signal processing (DSP) has allowed great advances in the ability to view, refine and store information gathered with sonar systems.
The shape of the beam in side scan is crucial to the formation of the final image. Typically, a side scan acoustic beam is very narrow in the horizontal dimension (~ .1 degree) and much wider (40-60 degrees) in the vertical dimension.
Specifically, shipwreck location, minehunting, downed aircraft search and lost cargo search operations all require the use of side scan.
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Towing sonar in mid depths using armored cable and a winch also requires a towing block, and can benefit from the use of ancillary support equipment such as tensiometers to indicate load on the wire and meter wheels to indicate the amount wire out. Mid depth and deep towing requires constant attention by the survey team to amounts of wire out, turn radii and bathymetry.
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For timely interpretation of sonar data, it is important that the person who operates the system generating the records is the same person who is given the task of interpreting the final data. Otherwise, it would be necessary for the operator to take copious notes during record generation in order to inform the interpreter of environmental and survey related changes that might effect the data.
Shadows are not the only white areas in a sonar record. Other areas are those where little energy is being returned to the towfish by way of reflection. In an uncorrected record, the water column is an area where, assuming there are no reflectors such as fish below or around the transducers, there is little energy return. This area is often very light, if not pure white, in the classic record.
Many bottom targets, such as a gentle upward localized slope or an acoustically translucent object, cause only a slight shadow in the sonar record. Other objects, such as rocks, sand waves, shipwrecks or large fish, will cast a clear, harsh shadow. Thus the intensity of shadows tell us something about the makeup of the objects causing them. The next link shows a section of dredged seabed. Note the curved marks left from the cutter head (A) and the limits of the head sweeps (B). Although this seabed is relatively flat, darker and lighter regions in the data help with interpretation.The cause for lighter areas on a sonar record are grouped into three general categories: 1. Shadow zones that have been blocked from the sonar beam by an acoustically opaque object, 2. Areas of topography that provide less backscattering of the sonar beam such as soft or smooth sediments, 3. Areas that are oriented in such a way as to provide less backscatter, such as an area inclined away from the towfish (a sloping seabed will also provide less return on one channel of the sonar than the other).
Since the speed of sound is reasonably consistent and ray paths are considered straight for the purposes of side scan sonar in thermally homogeneous waters, the shape of the shadows on a sonograph is usually directly related to the shape of the objects casting them. Detailed inspection of the shape of a shadow is helpful in determining the physical condition of the objects. The shadows cast by objects are a function of the angle at which the sonar beams strike the objects. For example, an object insonified from one angle may cast a distinct shadow, while it will cast no shadow when insonified from another angle. This can be misleading and, since shadows can provide significant information about a target, objects to be identified and classified should be imaged from a number of different angles.
Since the acoustic path from the sonar is relatively straight, there is usually a right triangle formed with the three angles at the towfish, the seabed and the tip of the target shadow. The target lies along the base of this triangle with its highest point intersecting the hypotenuse. This geometry forms two similar triangles and, by using a mathematical ratio, the height of the target is calculated. The calculation used to determine the height of a target is as follows: the height (Ht) of an object is equal to the product of the acoustic shadow length (Ls) and the height of the fish (Hf) above the seabed, divided by the range to the end of the shadow.
This calculation is very accurate in normal sonar operations, however, care must be taken when operating in unusual conditions. If there are severe density differences in the water column, acoustic ray paths are not necessarily straight and if severe, the height calculation from shadow length may not be accurate. Further, if there are other targets in the area that affect the primary target's shadow, care must be taken to be certain that the proper geometry is being applied. When imaging complex targets, the exact position of the target component casting the longest shadow may be difficult to determine. In cases where target height must be calculated conservatively, the interpreter should assume the shadow causing component to be at the leading edge of the target.
Some sonar reflectors are man-made and designed to be just that, a reflector or marker. A typical example is the STARR retro-reflecting sonar target, a specially constructed, buoyant target which reflects a large percentage of the incident sonar energy back towards the sonar transducers. The sonar's side lobes help the interpreter identify the existence of a retro-reflecting target while the returns from the main lobe show precisely where the target is in the underwater environment. Targets like the STARR are used as a benchmark in marking portions of seabed for programs like seabed monitoring. They are also used in combination with a surface buoy to determine the exact position of a seabed feature that must be sampled or examined.
The towfish altitude also has a tendency to decrease when making a turn. While towing, the drag of the towcable in the water column imparts a significant upward force on the towfish. In normal operations, water mass movement relative to the towcable is balanced against the amount of cable out and the weight of the towfish which maintains fish height above the bottom. Turning reduces the water mass movement by the cable and has the same effect as slowing the towing vessel resulting in the towfish dropping. This effect of a turn increases dramatically with decreasing turning radii and with increased lengths of cable in the water. Increasing the speed of the survey vessel during turns is one method of counteracting reduction in drag. Turns executed when using a depressor will cause a less pronounced loss in towfish altitude.
Small navigational corrections are also considered turns and effect
the sonar data. This is particularly noticeable when scanning a small target.
If the survey vessel makes small course changes while countering wind or
tides, in order to traverse a reasonably straight line, these course changes
may translate to the towfish. Large turns are evident
in data by a general smeared appearance on the inboard sonar channel,
while the channel on the outboard side of the turn appears unsmeared.
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This is a portion of a 300 meters per side sonar record of flat seabed with 3 steel drums. One is positioned at about 150 meters from the towfish and the other two about 225 meters. The test was performed to determine the recognition potential of such targets during a survey of radioactive waste sites.
This record shows the results of a severe course change during a survey. With a large amount of cable out, and running sonar at short ranges, the operator may not have time to recognize a "fish drop" as when running at longer ranges. Also when the sonar display turns pure white, it may be an indication of total electrical failure to the fish (possibly from the loss of the fish). In this case, the fish dropped immediately to the bottom and rested there until rapid course correction and speed increase brought the towfish back to height. The lack of hard noise on the record indicates that the fish did not drag on the seabed to any great extent.
One of the most insonified aircraft in the world, this PB4Y rests on the bottom of Lake Washington near Seattle, WA. Low current velocities and lack of a corrosive environment has kept the basic shape of the plane intact, although structurally unsound. Many sonar manufacturers use this target in advertising because of it's image in sonar data. Note the shadow of the outboard right propeller. The aircraft is resting on its left wing as indicated by the wing's shadow.
This is a record of a sunken container ship in deep water in the northwestern Atlantic. Five of the cargo carrying containers are still in the midst of the wreck and have been the target of deep water salvors. Large plates have fallen from the vessel and litter the seabed nearby. Note that, although the target is close to the towpath, the shadow is large and indicates considerable relief from the seabed.
This corrected record was made during a cable survey. Most of the cable along its route is fairly straight. It is buried in sand in some places and exposed in others, such as shown here. The cable is on the right hand channel but has been pulled off its run by bottom fishing gear. The trawl door marks run from right to left in the middle of the record.
This record was made during a survey of a radioactive and industrial waste
site. All of the small dark targets are 30 and 55 gallon drums. Although
the areal distribution of the drums is consistent over the seabed covered
in this record, they are most notable in the outer ranges because of the
spread of the sonar beam. All the drums are old and most have been violated
by corrosion. As a result, the phenomenon of "ringing" (caused by acoustic
target wrap-around) is not evident. The large target with a shadow in the
lower left hand side of the record was optically inspected by ROV and found
to be a drum encased in concrete. These larger targets, referred to as
"vaults," were more intact than the unencased steel drums.
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Here is a link to WRC's "Webb" site for more details on remote oceanographic data collection.
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Copyright © 2002 John P. Fish, All rights reserved.
Web pages by The
Institute for Marine Acoustics