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Feature Article

Choosing Side Scan Sonar Frequencies

By Doug McGowen • Rob Morris

Image from 400-kilohertz frequency used to survey Buzzards Bay plane wreck.
For decades, side scan sonars have been imaging the deep sea, ports, harbors, rivers, lakes, fjords, inlets, lagoons, locks, ponds, estuaries, coves, creeks, basins, bays, bayous and ' the list goes on. Like everything else in the realm of technology-based solutions, the systems have improved greatly since the days of single-frequency, continuous wave (CW) towfish. With the availability of high-speed digital signal processors, frequency-modulated (FM) pulses, sometimes called chirp or swept-frequency pulses, have become prevalent. In addition, frequency choices have increased.

Instead of trying to operate with one very sharp acoustic peak pulse, like conventional CW systems, the chirp sonars spread out the transmission. One of the benefits of chirp signal processing is the use of a long-duration, broad-bandwidth transmitting pulse that sweeps out over a range of frequencies, which can be compressed into a short duration pulse on reception for higher resolution. This generates a great deal of acoustic energy in the water, which results in more defined images.

Now, systems with FM pulses and a myriad of frequency choices are available in the marketplace. With this evolution in technology, users are faced with a vast array of options, choices and trade-offs.

New England Plane-Wreck Survey
Frequencies, once limited to a standard single-frequency selection, such as 100 or 400 kilohertz (in CW), now include a myriad of single- and dual-frequency operational pairs (in chirp). A survey was done in January off the coast of New England that provides an informative side-by-side look at what frequency choices can mean for the operator. The survey was conducted by EdgeTech (West Wareham, Massachusetts) as part of routine testing using a 4125 side scan sonar. A single object on the seafloor was scanned on the same day, with the same system, by the same operator at four different frequencies.

The target images that were captured are those of an old plane that went down into Buzzards Bay off the coast of Massachusetts in the late 1940s while making practice bombing runs on the Weepecket Islands. The U.S. Navy SB-2C aircraft crashed and settled in about 45 feet of water after the pilot escaped safely. The wreck was discovered in 1999, and the first diver to see the ruin in its shallow-water grave was Rob Morris of EdgeTech, who also captured the four side scan sonar images of the wreck in this article.

When studying the images closely, one can begin to recognize the subtle but important differences in the fine details. For example, honing in on the cockpit area presents discernible imaging differences among the various frequencies. In the 400-kilohertz image, the area is grainy. As the side scan sonar frequency increases to 600 and 900 kilohertz, the area starts to become more defined. The 1,600-kilohertz image has remarkable detail showing the metal tubing and frame outlining the cockpit structure that is now exposed to water. To continue this article please click here.

Doug McGowen is the director of new business development and marketing at EdgeTech. He has 20 years of experience in product development strategy and the promotion of technology-based products and solutions, including side scan sonars and sub-bottom profilers. He has a bachelor's in business and economics, and an M.B.A.

Rob Morris is the EdgeTech customer service manager. He has field experience performing underwater surveys, and training field personnel on side scan sonars and sub-bottom profilers. With more than 20 years of experience, he has completed underwater surveys worldwide. He is also a certified scuba-diving instructor and skilled ROV operator.

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