Dan's Homegrown Sidescan Sonar Page

What is Sidescan Sonar?

One of the shipwreck hunters' favorite tools is the sidescan sonar. Developed during World War II for minehunting and popularized for civilian use in the 1960's by Dr. Harold Edgerton, sidescan uses ultrasonic waves to locate objects and record bottom structure in a swath on one or both sides of its sensors. There's really no rocket science involved. A basic sidescan is nothing more than a fishfinder with the transducer turned sideways. For shallow water searching, the transducer can stay mounted to the boat. To refine the process, the transducer is removed from the hull and mounted on a towfish which can be trolled at depth. This provides several advantages: the sonar can more effectively "see" things standing off the bottom; the transducer is isolated somewhat from boat movement (roll in particular); and the transducer can be "flown" below a thermocline which might tend to reflect or attenuate the sonar signals.

References  References

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Sonar Transducers

The beam pattern from a fishfinder transducer is a cone; the angle of the cone depends on the frequency and the diameter of the transducer element: the higher the frequency, the narrower the cone angle; the larger the transducer diameter, the narrower the cone angle. Ideally, a sidescan beam pattern is fan-shaped: narrow in the horizontal plane for better detail resolution, and wide in the vertical plane so it can see up and down slopes and make the altitude above bottom less critical. Thus a transducer to produce this beam pattern would be long, for a narrow horizontal beam, and narrow, for a wide vertical pattern. While you can buy long, narrow transducers, the cost is prohibitive - I've only seen them in the 4 figure range. An array of small transducers will behave much the same as a larger transducer: a line of one inch transducers four inches long will have essentially the same beam pattern as a rectangular transducer four inches long by one inch wide.

My Sidescan Sonars

The Binford Mark I was a "proof of concept" model, intended to show that a fishfinder could be used as a sidescan. It used a single fishfinder transducer mounted in a 2 X 4 with a plywood stabilizing fin and a lead downrigger weight. It "painted" a perfectly acceptable picture of the bottom before getting snagged in a commercial fisherman's net. When I tried to pull it up, the splice between the original transducer cable and the piece of coax I had soldered on broke.
Lessons learned: Sidescan isn't rocket science. Don't introduce weak points into your tow cable. Watch out for fish nets.

The Mark II towfish was similar to the Mark I, but steamlined and with a one piece cable between the tow point on the fish and the boat.
Lessons learned: Don't use "5 minute" epoxy on anything that's going to be submerged in water - it won't hold up. Make the transducer down angle adjustable.

The Binford Mark III has four 1 inch diameter "puck" style fishfinder transducers in a row, interconnected properly to keep the impedance the same as a single transducer. Beam pattern is approximately 19° vertical and 3° horizontal. The transducers are mounted in a towfish made of 2" PVC pipe with aluminum tail fins and internal lead weights. Construction Details
Lessons learned: If the towfish bottoms out on a rocky bottom, don't try to just drag it up by brute force. (somewhere off Marquette, Michigan there is a fluorescent green pair of tail fins and tail cone stuck between some rocks on the bottom.)

For transducers, I used Lowrance / Eagle PD-W trolling motor mount transducers. These are made for Lowrance / Eagle's 192 KHz units, but would probably work with 200 KHz units also. Humminbird has some 16° 200 KHz transducers which would probably work well. All of these are available from sporting goods and marine supply stores and from mailorder houses such as Cabela's, West Marine, and Bass Pro Shops. See Lowrance / Eagle's Web Page for more information on their transducers.

At first, I thought it would be important to keep the impedance of the transducer array the same as a single transducer by connecting them in series / parallel. I now realize that the impedance of the individual elements is so high that this is really not necessary, so I'll just be hooking them up in parallel in my next array.

For a tow cable, I use RG-58 A/U coax with a flexible multiconductor center wire, such as Belden 9311. I've heard of people successfully using shielded twisted pair cable, but I've had bad luck using that on industrial sonars in my day job so I've stuck with coax. Try what you can get cheap - cable TV cable will probably work just fine, it simply may not be as flexible as would be ideal.

For all of the towfish, the electronics package has been the same: an old paper chart depth finder. I just plug in the cable from the towfish in place of the usual down-looking transducer. I use a 200 KHz model, but other people have used 50 KHz units with good results. Theoretical advantages of each: 50 KHz units tend to have more range, but 200 KHz provides narrower beam widths for a given transducer size. I tow the fish on 175 feet of coax cable (I started out with 300 feet of cable, but there seemed to be a lot of attenuation of the signal so I cut it shorter.) With only one graph recorder, I can only look to one side, but a second recorder, transducer array and cable could be added to watch both sides simultaneously.

Sample Images.

I scanned a couple of images from a test run with the Mark III towfish. The images are of the Southwest, a 137 foot wooden schooner lying in 100 feet of water. The schooner is broken apart, with its two sides, deck and bottom disarticulated.

Southwest 1 A view looking southwest at a 140 foot scale.

Southwest 2 A view looking north at a 100 foot scale.

(Both images were recorded on 4 inch thermal chart paper.)

Sidescan Sonar Links

There's a bunch of homebuilt sonar enthusiasts at the:

This guy has got to be the king of sidescan homebuilders:

Here's a page featuring some more of Sture's images:

To make homemade sidescan sonar fans green with envy, take a look at some commercial sonars...

Here's a new company developing an affordable sidescan sonar:

Transducer Sources:

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