The Dunwich Bank wreck site is notorious for its ‘black water’. This is caused by silt and other particulates in the water which are so dense that it is almost always impossible for a diver to see anything underwater. That means that everything has to be done by touch, which can disorientate the divers and greatly complicates the work.
To make things easier and much quicker we are using a hand-held high frequency sonar, using sound to replace the divers’ eyes. This can look forward several metres and is accurate enough to measure the dimensions of archaeological objects on the seabed, as well as providing a very clear picture of the environment of the site. As the diver would not be able to see a screen on the seabed because of the poor visibility, we see the sonar image on a monitor on the dive boat for the diver. We can also navigate the diver, scanning from side to side until we locate the object they are looking for and telling him or her how far in front the object they are trying to find is.
Although the sonar we are using is much more advanced and is often used during the construction of offshore wind farms, his type of technology was used several years ago to survey some of the remains of the medieval churches of Dunwich. This important medieval town was gradually lost because of coastal erosion, with the buildings falling off the cliffs as the sea advanced. Although the wreckage of the wooden houses is long gone, the rubble of some of the large stone buildings is still on the seabed a short distance from the beach.
The pictures below show what the diver could never see at the time – left is the top of the mysterious central mound of archaeological material which we now know contains iron bars. This is the only part of the site that is exposed at the moment and you can see that the sonar is even able to see the tiny ripples in the surface of the sandy seabed – these are like the ripples you see on a sandy beach when the tide goes out. On the right is a sonar image of the ‘shot weight’, a heavy weight that we lower on a rope to the seabed from the dive boat and which allows the diver to descend and ascend safely. The sonar can measure the size of this object to millimetric accuracy.