2012 Geophysical Survey

Fig. 1 ‘Unsinkable II’ towing the yellow side-scan sonar ‘fish’ in very shallow water

December 8, 2012: In November 2012, the team of Dr. G. Papatheodorou from the University of Patras, Laboratory of Marine Geology and Physical Oceanography of the Department of Geology, returned to the Piraeus. Their aim was to place the finishing touches on the already detailed mapping of the seafloor that was done last year. With them they brought a van full of scientific gadgets.

The first few days were devoted to filling in the gaps of the mapping outside the harbours of Zea and Mounichia, and the Piraean Akte. The setup on the boat ‘Τριας’ included a sub-bottom profiler, a side-scan sonar and an echo-sounder. This configuration allowed for the creation of a detailed three dimensional map of the sea floor, a bathymetric map, as well as a profile of everything that potentially lies beneath the muddy bottom. The sub-bottom profiler in particular has the capacity to penetrate sediments down to depths of 10 meters, enabling the geologists to identify among other things ancient coastlines, as well as potential targets of interest for the archaeological team.

On top of the usual approach described above, this year the geophysical team also had a “never been tried before” trick in mind. The idea was to use the side scan sonar on depths of less than a meter. If successful, this would create a detailed map of all the submerged shipsheds, fortifications and quarries that exist in both naval harbours. In order to achieve this improvisation was the name of the game. Using the small inflatable, ‘Unsinkable II’ of the Zea Harbour Project the team managed to place the side scan sonar, echo sounder, differential GPS, transceiver, two laptops, a small generator and three people on less than 4 meters of floating air (Fig. 1). The benefit was the almost non-existing draft of the particular boat. As the boat drifted at around 2 knots per hour on top of the submerged shipsheds the team held their breaths while looking at the computer screen and wondering if there would be too much “noise” to pick up the structures. Very soon some very well known features popped up in front of their eyes. It could only be described as a great success!

Taking advantage of last year’s geophysical data provided by the University of Patras crew, the Zea Harbour Project decided to put it into use in the field. Working with the geological team of Dr. K. Pavlopoulos of the Harokopio University and in collaboration with the École Française d’Athènes (represented by Professor Dr. Eric Fouache, Sorbonne University) a series of core drillings took place in both Zea and Mounichia Harbours. The aim was to remove three cores in each harbour in order to gather information about the depositional processes in the areas. The sub-bottom profiler maps created in 2011 were spread out and the points were picked (Fig. 2). The ancient entrances of both harbours were the primary candidates since that were where the thicker sediments of up to 9 meters were identified. Beyond that, a set of core drills would be attempted along the lines of the walls of the submerged shipsheds, in order to determine whether these structures were constructed on land or in the sea.

A 5 by 5 meter platform weighing in the excess of 4 tons, that supported the drill, was towed into place (Fig. 3). Each time the ZHP divers would carefully evaluate the position underwater. The drillings on the harbour mouths proved successful and relatively easy. It was then that the ZHP team had to prove how precise they are. The challenge was to position the “little” platform on top of one of the shipsheds in a target area that measured 0.80 by 0.60 meters in almost 1 meter depth of water. Once again, success was the name of the game.

  • Fig. 2 Isopach map showing the seabed sediment thickness in the Zea Harbour

  • Fig. 3 ‘Unsinkable II’ towing the drilling platform to the target area