‘Combing the seabed’

October 27, 2011: This past week we got a perfect weather window, which happened to coincide with the long awaited geophysical survey. Since early on in 2011 we have planned together with the Department of Geophysics and Oceanography from the University of Patras to run a geophysical survey that would cover the ancient harbours of Zea and Mounichia, as well as the areas between them and all the way to Kantharos. Given the logistics of such a survey and a relatively constricted time frame available, everything had to be prepared perfectly in order to be as efficient as possible.

The team led by Dr. George Papatheodorou of Patras University arrived early in the morning in a van filled to the ceiling with all kinds of technical equipment. Three different methods of surveying were to be used. The first was the side-scan sonar, a torpedo-like device (aka the ‘fish’) that is towed behind the boat (Fig. 2). The side-scan sonar provides very realistic imagery of what is lying on the sea bottom, and if it is run on high frequency it can pick up details as small as a chain from a moored boat. For the side-scan sonar we used a 6.80 meter RIB (rigid inflatable boat), due to better maneuverability in the constricted areas of the two harbours. On the RIB we had an extremely accurate GPS receiver that was connected to a navigating program. The skipper had a computer screen attached on the cockpit allowing him at all times to monitor the route of the boat. High accuracy is required on each line that is followed, with a margin of error only at plus or minus 5 meters.

Fig. 1 Stavroula Kordella recording and keeping track of the geophysical survey data

The second method of surveying is the sub-bottom profiler, a device attached to the side of the boat which is capable of penetrating the sea floor to a depth of up to 10 meters and providing information about what might be lying beneath the surface. The sub-bottom profiler is useful both for archaeological purposes, as it can pick up buried harbour structures or remains of shipwrecks, as well as for providing information about the paleogeography of a particular area (ancient coastlines). Due to the size and weight of the device, a special method of attachment to the boat is required, which features a large swinging arm. This was not possible to place on the RIB and therefore midway through the survey we switched to a larger boat in order to accommodate this bulky equipment. A 9 meter long 5 ton center console plastic boat was used that provided ample space and stability (Fig. 3). On this boat we were able to attach both the sub-bottom profiler and the side-scan sonar, as well as our third surveying system, the echo-sounder. The echo-sounder is basically a depth measuring device. With all three systems attached we were able to map the interiors of both Zea and Mounichia in a way that has never been done before! Moreover we surveyed to an extent of approximately 2 nautical miles off shore, thus creating a record of all the ancient coastlines that date back to tens of thousands of years ago (Fig. 1).

The processing of all acquired data that will take about four months, and will be carried out by the geophysicists at the University of Patras. We are waiting with anticipation to see the results, as some of the mysteries that surround the harbours of Zea and Mikrolimano might finally become unraveled.

  • Fig. 2 The ‘Fish’

  • Fig. 3 The boat used during the final phase of the geophysical survey