Side-scan sonar is a modern method of underwater imaging that can produce remarkably detailed and realistic views of shipwrecks and other bottom features using sound rather than light.
Notice all the detail in this side-scan sonar image of the Delaware.
The torpedo-like sonar tow "fish", which is towed in the water behind the boat. The speed of the boat and the length of the tow cable determine the depth of the fish.
The data recorder.
Above is a side-scan data recorder which generates a paper trace of the scan, as well as writing the data to tape. More-modern units are entirely PC-based. On the paper or the screen, the black center line is the tow fish itself, or rather the area directly below it, which it cannot see. The white area on either side of the center is area that the fish cannot see very well, although tall objects may appear. The gray area outside of that is the bottom, along with whatever objects are there ( in this case, nothing interesting. ) Note that the sonar fish scans to both sides at once.
The laptop on top of the side-scan recorder is connected to a GPS. The blip on the screen is actually the boat, displayed with its course and speed. For a good scan it is necessary to drive a very straight and precise course.
Here, the image of a tugboat emerges from the printer.
Normally, the side-scan device produces images where shadows are lighter and reflections are darker. This is confusing to the eye for most people, so I usually reverse it to get a much more natural-appearing image. Most of the side-scan images in this website ( including this one ) are actually negatives.
The final image, all cleaned-up - cropped, rotated, negative colors.
To get accurate images, the towing vessel must steer a precisely straight line at a constant speed. As anyone who has ever driven a boat can tell you, this is not always easy. Course deviations result in bent images, while speed deviations stretch out or compress parts of the image along the direction of travel ( usually the horizontal axis in the image. ) the side-scan of the Delaware at the top of this page showcases both effects - despite appearances, the wreck is actually perfectly straight, as was the vessel when it sank.
Shark River Artificial Reef - Artificial Reefs - New Jersey Scuba DivingTake a submarine trip around the Shark River Reef in this amazing animation based on side-scan sonar.
A submarine trip around the Shark River Reef via side-scan sonar
Searching for shipwrecks has always been a difficult and sometimes costly endeavor. There are several methods divers have used to conduct these searches. In many cases, divers would go out to check "hang numbers" from a local fishing boat. The spot which caused the fisherman's rig to "hang" could be anything from a single pipe to a virgin wreck. Other divers have invested time and money in machines like proton magnetometers which register anomalies when passed over any metallic object. Targets produced would then be investigated by the divers who could find the metallic object, which many times produced only 55 gallon drums. Some sophisticated wreck hunters even utilize side scan sonars which use sound waves to scan the ocean floor. In the past these machines could cost up to $100,000. Underwater searching technology has just entered the twentieth century and wreck hunters now have a new toy. Mike McMeekin and I met Doug Blaha from Marine Sonic Technology, Ltd at DEMA 95. Doug demonstrated to us some barge and wreck images he had stored in his computer. The images were so impressive that we arranged for a real life ocean demonstration. Seeing is believing in my eyes.
In the spring of 1995 Doug Blaha and Pete Wilcox of Marine Sonic came to New York to demonstrate Marine Sonics new Sea Scan PC Side Scan Sonar. This new patented technology is so advanced and yet comparably affordable that it was hard to believe Doug's claims. Doug first explained that the best way to "see" underwater is with sound. Underwater, light is quickly diffused and very ineffective. Even the brightest light yields only a few feet of visibility. A good analogy is a room filled with thick heavy smoke. Bright lights are nearly useless but sound travels well through the room. Fisherman have employed fish finding sonars for years with good success. Fish finding sonars look down through the water and display objects appearing above the bottom. Side scan sonar is engineered to look sideways through the water. Doug explained that the Sea Scan PC Side Scan Sonar was unique in at least three significant areas, cost, performance and operability. He claimed that the Sea Scan PC was an affordable, high tech system featuring performance unequaled by systems costing three times as much or more. It is portable, easy and contains features only made possible with a high performance personal computer.
Cost was, of course, the first thing that went through my mind. Marine Sonic utilizes a PC instead of a dedicated processor. PC's are manufactured in the millions thus significantly reducing one of the most expensive components of a side scan system. Added benefits from the PC are an integrated plotter for navigation, digital processing, filtering and enhancement of data and the ability to store images on disk rather than the hard thermal paper normally associated with most side scan systems.
We boarded the Wreck Valley joined by Captain Steve Bielenda, Captain John Lachenmayer and Mike McMeekin and started to scan shipwrecks. Marine Sonics fresh approach to side scan technology comes from the medical field. Using low power and extremely low noise electronics, the sonar emits very short, precision pulses in narrow focused beams to produce clean, crisp near photographic images. Pete demonstrated how easy it was to deploy and tow their transducer carrying fish tow. They then interfaced the computer with the Wreck Valley's loran and brought up a plotter display. We then inputted the TD's of a known shipwreck and were able to not only see a visual reference (Marker) of the wreck on the plotter, but also the position and the area we had already scanned. Doug showed how a simple key stroke changed the scanning distance or swath width. The unit is capable of scanning up to 500 meters to either side of the vessel but we would be looking for better resolution and therefore scanning at around 100 meters of ocean floor. Once we passed over the wreck the image appeared on the computer screen. Their was no interpolating the image or wondering what we were seeing. The image of the wreck, the Black Warrior, was clear and we could easily recognize details like the wreck's boiler, mast and low lying debris field. For the rest of the day we cruised around the ocean scanning wrecks, in fact, we imaged nine wrecks in one day! Doug also continued to impress us with the machine's versatility. He demonstrated how we could get larger more detailed images by scanning the same site on a smaller scale or by only scanning off one side of the tow fish. He showed how the Sea Scan PC could measure objects on the bottom and how it could triangulate to calculate the relief of the target. To say the very least everyone on the boat was very excited. We had learned more about the actual layout and surrounding areas of these nine wrecks in one day then in the last ten years of diving. We even found several targets that we want to go back and investigate on SCUBA.
Operability, on most sophisticated side scan units mandates a trained professional technician. The Sea Scan PC unit, however, is extremely user friendly. In fact, almost anyone who has ever operated a personal computer can learn the basics in just a few hours of practice. The superior performance and ease of use claimed by Doug was easy to prove. Even with yours truly behind the controls of the computer we continued to produce quality images. Experience is very important, but new operators will quickly grasp the principles and recognize the key features. Since data is stored on a hard disk in the computer, images can be reviewed later after returning from sea in the comfort of home or office.
Side scan sonar is the technology of the future that will enable us to not only produce more accurate underwater sketches of known wrecks, but to search and identify new sites to dive. For additional information on the Sea Scan PC Side Scan Sonar contact Marine Sonic Technology, Ltd., 5580 George Washington Memorial Highway, White Marsh VA 23183-0730 or call them at (800)447-4804. I will also be running a hands on basic side scan course/expedition where we will be searching for an undiscovered shipwreck. For course details you can contact me at AquaExplorers, Inc. at (516)868-2658.
About the Author: Dan Berg, of Aqua Explorers, Inc. is the author of ten shipwreck and diving related books, and the host & producer of the award-winning Dive Wreck Valley television series.
Mapping the Sea Floor Geology Offshore of the New York-New Jersey Metropolitan Area
The U.S. Geological Survey (USGS) is mapping the sea floor offshore of major metropolitan centers. The New York-New Jersey metropolitan area is one of the most populated coastal regions in the United States. The New York harbor estuary and its offshore area are used for waste disposal, transportation, recreation, and commercial and recreational fishing.
Interpretive maps of the sea-floor geology provide a fundamental framework for research and management in the coastal ocean. They show the composition of the seabed and the shape (topography) of the sea floor; they also show areas of present and past dumping of sediments and pollutants, as well as locations and impacts of other human activities. They provide information on the transport of sediment and help to define biological habitats. All this information can be used to develop predictive models to guide habitat and resource management, monitoring strategies, and other research studies.
Mapping the Sea Floor
Modern oceanographic surveys use remote-sensing techniques (sidescan sonar (fig. 1), multibeam echo sounding (fig. 2), and high-resolution seismic-reflection profiling), direct sampling, and visual observations to characterize the sea floor. The surveys provide a new, highly detailed view of the sea floor. In contrast to earlier maps that were based on widely spaced data, the new digital images are similar in detail to an aerial photograph and show the changes in seabed features over a wide range of scales. The locations and effects of human activities, such as waste disposal and bottom trawling, are often clearly observed. Sedimentary features, such as bed forms, provide information about the transport and fate of sediments and pollutants and about the importance of the underlying geologic structures and the geologic history in controlling the present distribution of surface sediments.
Upper: A sidescan sonar "fish" towed behind a vessel surveys the sea floor by sending sound to either side of the ship's path. Typical survey swaths (yellow area) are a few hundred meters wide.
Lower: the intensity and pattern of sound reflected from the ocean floor provide information on the composition of sediments and the topography. Strong reflections from boulders, gravel, and the walls of trawl grooves appear as light tones on this data record; weak reflections from finer sediments or shadows behind vertical features are dark. A composite image, pieced together from multiple survey strips, provides an image of the sea floor similar in detail to an aerial photograph.
High-resolution multibeam mapping systems use sound from arrays of 60 to more than 150 electronically separated transducers to measure water depth as well as sediment characteristics of the sea floor. The transducers form beams a few degrees wide that produce a footprint of a few square meters on the sea floor in water depths of 50 meters. Because the system is fixed to the ship's hull, the data can be easily georeferenced and surveys can be run at speeds of 15 knots. The multibeam surveys provide a new, highly detailed view of the sea floor. Used with the permission of the University of New Brunswick.
Mapping the Area Offshore of New York and New Jersey
Sidescan sonar and multibeam systems emit pulses of sound that reflect off the sea floor. Features of the sea floor are identified by the pattern and varying levels of sound reflected (backscattered) from the sea floor. One of the most striking characteristics of the backscatter maps of the New York Bight area (fig. 3) is the variability in sediment characteristics over scales of tens to hundreds of meters. Sampling, bottom photography, and the high-resolution seismic observations show that the bottom sedimentary environments in this area range from outcropping rock to muds. The maps show a complex pattern of sediment properties that is a result of the underlying geology, modern processes, and anthropogenic activities.
The USGS has mapped the sea floor in the New York Bight apex and along the southern shore of Long Island by using sidescan sonar, and the Hudson Shelf valley and adjacent shelf by using multibeam. This combined backscatter image (red is high backscatter and blue is low backscatter) shows a complex pattern of sediment properties that is a result of the underlying geology, modern processes, and anthropogenic activity. The backscatter intensity is related to sediment texture, as well as other properties. In general, high backscatter indicates coarse-grained sediment or outcropping rock, and low backscatter indicates fine sands, silt, or clays. The Hudson Shelf valley is floored with fine-grained sediments. The box outlines the location of the multibeam image shown at right; the black arrow indicates the look direction -- to the southwest. From Schwab and others (1997); Butman and others (1998).
Top left: Along the southern shore of Long Island, Cretaceous age (>65 million years old) rocks outcrop approximately 6 kilometers off central Fire Island (bright high-backscatter region) and exert a primary control on observed patterns of coastal change. This area acted as a headland during times of lower sea level about 10,000 years ago. Erosion of this headland during subsequent sea-level rise furnished sediments to the inner shelf downdrift to the west. These sediments, in turn, were reworked by oceanographic processes into a series of sand ridges. Note the halo of coarser sediment to the west of the outcropping rock. A westward and onshore sediment flux from these ridges may supply sediment to the beaches of western Fire Island and may influence the pattern of erosion of the barrier-island system. From W.C. Schwab and others, written commun. (1998).
Lower right: Sun-illuminated perspective view (looking to the southwest) of the topography and sediment characteristics of the shelf at the head of the Hudson Shelf valley (area in box, fig. 3). The image was constructed by draping color-coded backscatter intensity over the bathymetry, as measured by a multibeam system. Water depth at the northern portion of the region is approximately 30 meters; water depth in the Hudson Shelf valley at the left of the image is about 50 meters. Features observed include relatively smooth mounds composed of material dumped since the 1800's; mounds (as high as 10 meters) of dredged material from more recent disposal; a smooth, roughly circular region that resulted from disposal of contaminated sediments and is capped with coarse sand; outcrops of southwestward-dipping Cretaceous age coastal plain strata at the head of the Hudson Shelf valley; low-relief (amplitude <1 meter) sand waves suggesting sediment transport to the southwest; and individual dumps of material, some arranged in lines, in some cases probably large rocks from construction activity in New York (referred to as "derrick stones"). From Butman and others (1998).
The surveys off the New York-New Jersey area are being carried out by the USGS in cooperation with the U.S. Army Corps of Engineers, Texas A&M University, the State University of New York at Stony Brook, the University of New Brunswick, the Canadian Hydrographic Service, Wesleyan University (Connecticut), and Coastal Carolina University.
Butman, Bradford, Danforth, W.W., Schwab, W.C., and Bucholtz ten Brink, M.R., 1998, Seafloor topographic and backscatter maps of the Upper Hudson Shelf valley and adjacent shelf, offshore of New York: U.S. Geological Survey Open-File Report 98-616.
Schwab, W.C., Allison, M.A., Corso, W. Lotto, L.L., Butman, B., Bucholtz ten Brink, M., Denny, J., Danforth, W.W., and Foster, D.S., 1997, Initial results of high-resolution sea-floor mapping offshore of the New York - New Jersey metropolitan area using sidescan sonar: Northeastern Geology and Environmental Sciences, v. 19, no. 4, p. 243-262.
For more information, please contact:
U.S. Geological Survey
Woods Hole Field Center
384 Woods Hole Road
Woods Hole, MA 02543-1598
I make no claim as to the accuracy, validity, or appropriateness of any information found in this website. I will not be responsible for the consequences of any action that is based upon information found here. Scuba diving is an adventure sport, and as always, you alone are responsible for your own safety and well being.