Sea floor imaging with side-scan transducers.

Sea floor imaging: how to get the best from your side-scan transducer.

I don’t know about you, but I find “seeing” beneath the waves, and wrecks in particular, fascinating. I’m no diver, so this is probably the closest I’ll get to exploring the wreck itself, imagining her last hours as she sank and came to rest on the sea floor. Thanks to the capabilities of today’s recreational sonar equipment, high quality images of wrecks are easily within reach of the recreational boater and boat angler.

Configuring your transducer can be a minefield of options and settings. Generally, I find that less is more – most fish-finders are pretty well configured out of the box and I don’t find I need to tweak or change settings that often. There is however one caveat to that statement, and that’s transducer frequency.

Transducer Frequency: I use 455khz for general side-scan imaging – this tends to give the strongest returns, and therefore the brightest images, but I'll switch to 800khz when conditions are perfect and I want a really detailed view of the wreck structure. Here’s a side by side comparison of the same wreck (the Camswan in Sandown Bay, Isle of Wight, UK) at 455 and 800 khz. These images were taken using a Lowrance HDS9 Live chartplotter/fishfinder unit, coupled with a Lowrance Active Imaging 3-in-1 transducer.

As you can see, the 800khz image has better resolution, and is therefore more detailed, but is a fainter image than at 455khz.

You’ll notice the banana shape of the wreck in the first image. This is what happens when you change course during your pass over the wreck.

Scanning range: I've found that a side-scanning range of 60 metres gives a good compromise between being able to see far enough to either side in order to pick up objects and locate wrecks, yet still gives a bright and detailed enough image. The farther the range, the weaker the sonar returns will be, and therefore the poorer the image quality. You could tighten up the range to 30m or even less, but then it gets hard to pinpoint the wreck and you'll need to pass very close to the wreck to get a decent image.

Wave action can also affect the quality of the image you get. In the 800khz image above the wavy lines on either side of the sonar track are caused by the changing height of the transducer above the wreck, which is caused by waves making the boat rise and fall. In rough conditions the wave action can often cause fragmentation of the image, such as on this image of the Kurland below.

Weather conditions: Generally I find flat calm conditions at slack water best for side-scan imaging, and good conditions can bring out the best in your equipment. The image below was taken at slack water with very light winds – not quite flat calm, but not far off, and you can see the  resolution of the 800khz image really pops out, with very little fragmentation of the image due to wave action.

Boat speed: With fish-finders, the size of the objects they show on the sea floor is a function of time; the slower you pass over the object, the larger it will appear on the screen. The inverse is also true, as this image of the Highland Brigade below illustrates.

I buzzed over the Highland Brigade at 18kts. For those that don't know, she's quite a large wreck but see that that she's relegated to a relatively small blob of structure at speed - she'd be easy to miss, so it's always worth scouting an area at slow speed to pick up the wreck/reef/feature more clearly on the finder. I find a speed of 3-5kts about right for side-scan images.

Below is a side-scan pass of the same wreck, showing her true size. The image contains about 80m of wreckage and was taken at a speed of 4kts.

As you can see, the slower the pass, the bigger the wreck image - the transducer has more time to ping the wreck and collate the ping returns into useful images for us to view.

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In summary then, for best sonar image quality:

• 455khz transducer setting for brighter images
• 800khz for greater detail
• Side-scan range set to 60m for the best compromise between having the range to find objects and imaging them in sufficient detail
• calm weather conditions
• at slack water
• speeds of 3-5kts

I’ll leave you with this image, again of the Camswan. The image is skewed due to a change of course during the pass, but what stands out is the ribs that are clearly visible sticking up from the wreck. Divers have confirmed their presence, and what this tells me is that she will likely be tackle hungry. For the angler doing a side-scan pass before drifting with lures, this will give you an insight into how you might choose to fish the wreck – with caution!