Sonar illusion symmetry

The RTE TV News story of Friday 27th April 2012 about the Lough Foyle sonar illusion struck me as being rather strange. There seemed to be far too much of the symmetry of some sort of ship in the sonar image for it to be an ‘illusion.’ Symmetry in an image of the sea bed implies the same symmetry on the sea bed. I would find it difficult to believe that this symmetry is just an illusion or that it is not in some way related to the shape of a ship-shaped object, or collection of objects, of sleek proportions. My curiosity is aroused and I would be very interested in a plausible explanation. The diagram above is a quick trace I made of the main symmetry lines I could see in the image on the RTE News web site.