For about fifteen grand you could always pick up a decent little side scan sonar system. How much!!! - I hear you cry. Yeah, okay, it's a horrid amount of dosh for another black box to sit on your boat - the wife is going to love this idea, but hear me out.
When you switch on your sounder to check the depth of water or to look for a wreck, all it's doing is firing a sound pulse at the seafloor and waiting until it hears an echo. As the speed of sound traveling through water is a known quantity, the sounder knows exactly how far the sound has travelled when the echo arrives, it can therefore tell you how much water is under your boat. Likewise, when the pulse hits a shoal of fish or a wreck, it will bounce back that little bit sooner and show you on the screen that there's something down there.
Now take this further, if you could stand on the bottom and fire the sound pulse horizontally so that it travels along the seabed, it will return echoes from anything sticking up out of the sand. Keep the pulse very narrow so that it only looks straight out to the sides, not forward or backward, then move forward at constant speed and it will build an acoustic picture of what lies out to the sides of your boat.
With side scan what you end up with resembles an aerial photograph, where someone took the water away, snapped the picture and then put the water back. Well, sort of anyway, it's not quite that simple. Acoustic imaging requires that the transducers (the bits that fire the pulses) travel at constant speed, constant depth and in a straight line, difficult to achieve with the boat itself, but the transducers are mounted in a torpedo-like towfish on the end of a long cable somewhere behind the boat. Throw in a few waves, some tide and a bit of wind, together with a terrible dread of flying your ridiculously expensive towfish straight into the wreck and you'll appreciate that it's not as simple as it first seems.
Why Bother? Just think of the advantages of using side scan. If you are looking for big iron wrecks you'll probably find them quicker with a magnetometer due to the long sensing range and the fact that it's possible to tow the fish so much faster, but what if you want to locate a wooden ship? Or an aeroplane - there is so little iron in an aircraft that you almost need to crash the magnetometer fish into it before you'll find it. If it's in deep water, things can only get more difficult, same goes for wooden ships.
What if your big iron ship is in deep water, would it not be an advantage to know which way up she is, which way she is pointing, whether she's broken or intact? With a good side scan shot, the ship's orientation can be quickly established and positions calculated for the various points of interest.
On one big wreck that we dived this summer all the interest was on the stern, so we worked out a position for the stern and put the shot within ten feet of the propeller every time - it saves a lot of swimming! Here are a couple of screen shots of the same wreck, a small tug called Hercules, lying in 25m of water a couple of miles off the river Tyne. The first shot shows an intact wreck sitting on its keel with the bow facing down the screen. If this was a planned deep dive with limited bottom time, we would at least know that we weren't going to crash into an upturned hull. Notice the shadow cast on the seabed - it very clearly shows the handrail, which is intact although rotted through in places, from the tip of the bow to a point around amidships. Not too important I agree, but if the superstructure or masts were standing, we'd know about those as well.
The second shot is of the same wreck and demonstrates some of the difficulties of acoustic imaging. The wreck appears shorter, this is because the boat was travelling with the tide and the towfish went past a bit quicker making the image looked squashed. As well as this, the image is distorted - the towfish was not flying in a totally straight line at a constant speed. But, this shot shows clearly the deck beams and the boiler in the middle of the ship so we now know that the decking has gone allowing us to penetrate the wreck easily. Incidentally, if it were an upturned hull there would presumably be a hole in it somewhere or one end broken off, we could use the sonar to locate these sites and get the shot in there. Another exciting use for the sonar is to locate the debris field of a wreck. Most of our wrecks suffered a violent end and bits were blown off them at the surface by mine or torpedo. When they finally sank they had every moveable object washed off the deck during the plunge and it is possible to locate this debris on the seabed whilst establishing its direction from the main wreck site and the extent of the field.
In the third shot we can see more clearly how the sonar produces the images. Side scan, as the name suggests, looks sideways, it cannot look down or the pulses from one side would be picked up on the other, clearly not a good idea. For this reason there are two distinct channels. Up the centre of the picture is a straight line, this is the path that the boat has followed, either side of this line is black where there is no data. Then there appear two thin lines contrasting with the black. These are echoes from the water surface and tell us only that the towfish is nearer the surface than the seabed because those echoes have arrived back at the towfish first. Moving out further from the centre, the true image is next to appear and it extends outwards about 80 metres either side of the boat. Where the returns are strong, the image is green, further out as the signals come in weaker, the seabed is shown blue/black. About half way up on the left hand channel is an object that looks a bit like a wine bottle with the cork pointing towards the top left corner of the image. This is the Tyne pilot cutter Protector, sunk by a mine and now flattened to the water line. What appears to be the neck of the wine bottle is actually the tapering stern of the vessel and the opposite end is a jumbled heap of wreckage.
In this case however, it is not the wreck in which we are interested, to the left of the stern and just within the red box (shown at higher magnification upper right) is a faint object, the fourth shot shows it more clearly again. This is the wreck of a very old wooden sailing vessel complete with huge oak timbers, row upon row of copper nails and who knows what else. It turned up entirely by accident during a routine survey of the protector and then the weather closed in after we had dived it only once! These shots also show a smaller more distinct target straight off the stern of the wreck, this hasn't been investigated yet. Who knows what that one may be?
I am not suggesting that everyone remortgages their house tomorrow and buys a side scan system, but it is handy to see just what it can do. It's always possible that a club or group of dedicated individuals have a project in mind that is ideally suited to the use of side scan sonar and then it may prove worthwhile to hire one or try to involve an organisation that has access to one. It's not the ultimate piece of kit or the answer to all of our problems, it brings with it a whole new set of difficulties but it is a very powerful tool for certain applications and is worth keeping in mind.
If any readers have questions or comments on any aspect of wreck location or the equipment discussed, please feel free drop me an e-mail: Bill@Kiltech.co.uk

By Bill Smith.