Sea Technology

APR 2013

The industry's recognized authority for design, engineering and application of equipment and services in the global ocean community

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Choosing Side Scan Sonar Frequencies Dual-Frequency Options Allow for Flexibility in Capturing Images By Doug McGowen ��� Rob Morris F or decades, side scan sonars have been imaging the deep The target images that were captured are those of an sea, ports, harbors, rivers, lakes, fjords, inlets, lagoons, old plane that went down into Buzzards Bay off the coast locks, ponds, estuaries, coves, creeks, basins, bays, bayous of Massachusetts in the late 1940s while making practice and ��� the list goes on. Like everything else in the realm bombing runs on the Weepecket Islands. The U.S. Navy SBof technology-based solutions, the systems have improved 2C aircraft crashed and settled in about 45 feet of water after greatly since the days of single-frequency, continuous wave the pilot escaped safely. The wreck was discovered in 1999, (CW) towfsh. With the availability of high-speed digital and the frst diver to see the ruin in its shallow-water grave signal processors, frequency-modulated (FM) pulses, somewas Rob Morris of EdgeTech, who also captured the four times called chirp or swept-frequency pulses, have become side scan sonar images of the wreck in this article. prevalent. In addition, frequency choices have increased. When studying the images closely, one can begin to Instead of trying to operate with one very sharp acoustic recognize the subtle but important differences in the fne peak pulse, like conventional details. For example, honing in on the cockpit area CW systems, the chirp sonars presents discernible imaging differences among the spread out the transmission. Image from 400-kilohertz frequency various frequencies. In the 400-kilohertz image, the One of the benefts of chirp sig- used to survey Buzzards Bay plane area is grainy. As the side scan sonar frequency innal processing is the use of a wreck. creases to 600 and 900 kilohertz, the area starts to long-duration, broad-bandwidth become more defned. transmitting pulse that sweeps The 1,600-kilohertz out over a range of frequencies, image has remarkable which can be compressed into detail showing the a short duration pulse on recepmetal tubing and frame tion for higher resolution. This outlining the cockpit generates a great deal of acousstructure that is now Image from 600-kilohertz fretic energy in the water, which exposed to water. quency used to survey Buzzards results in more defned images. Across-track resoluBay plane wreck. Now, systems with FM pulstion for the EdgeTech es and a myriad of frequency 4125 used in the surchoices are available in the marvey is as follows���400 ketplace. With this evolution in kilohertz: 2.3 centimetechnology, users are faced with ters, 600 kilohertz: 1.5 a vast array of options, choices and trade-offs. centimeters, 900 kilohertz: 1.0 centimeters New England Plane-Wreck Survey and 1,600 kilohertz: Frequencies, once limited to a standard single-frequency 0.6 centimeters. The selection, such as 100 or 400 kilohertz (in CW), now inalong-track resolution clude a myriad of single- and dual-frequency operational at a 25-meter range is���400 kilohertz: 20 centimeters, 600 pairs (in chirp). A survey was done in January off the coast kilohertz: 14 centimeters, 900 kilohertz: 12 centimeters and of New England that provides an informative side-by-side 1,600 kilohertz: 8 centimeters. look at what frequency choices can mean for the operator. The images show that as the frequency increases, the The survey was conducted by EdgeTech (West Wareham, images become sharper, and noticeable along- and acrossMassachusetts) as part of routine testing using a 4125 side track resolutions between the frequencies are evident. The scan sonar. A single object on the seafoor was scanned on resolution of a sonar is its ability to determine the presence the same day, with the same system, by the same operator at of two small objects separated by a distance, X. If the resolufour different frequencies. tion of the sonar is greater than X, the two targets will appear 44 st / April 2013

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