The scientific exploration of lost ancient cities or tombs is not merely a matter of archaeology. Useful results can also be achieved by means of technical-scientific methods such as geophysical surveys.
Tuna el-Gebel is one of the largest surviving cemeteries of antiquity: the necropolis, located in the Egyptian desert, covers an area of approximately 100 hectares. It was built by the Ptolemies and Romans as a cemetery for the ancient metropolis Hermopolis Magna (now Al-Ashm?nayn). As demonstrated by its architecture and archaeological finds, it was in use for more than 600 years, from the Hellenistic period beginning in around 300 BC to the start of Late Antiquity in around 300 AD.
The fact that we now know so much about the so-called Petosiris Necropolis is due not only to archaeologists and their excavations, but also to experts in applied geophysics. Kiel University’s research group led by Professor Wolfgang Rabbel has earned an excellent worldwide reputation with its geophysical surveys of archaeological sites. In October 2018, the Kiel team – in cooperation with the National Museum Hannover, the project coordinator – once again visited Egypt to investigate the necropolis even more extensively using the latest methods.
“We achieved very exciting results,” says Rabbel. “The ancient city street map we created by surveying ground magnetisation and using ground penetrating radar reveals the numerous buildings, both large and small, that the Romans erected for their dead. Many extend over several levels. In them, the deceased were interred in highly complex burials.” Some of the buildings are above ground and are still partially visible. However, a sizeable proportion is covered with sand, mainly because of the desert location. “Because the necropolis is so large, it cannot be excavated in its entirety,” says Rabbel. In addition, the current archaeological trend is to gain insights as far as possible using non-invasive techniques. “We now know that every major excavation also involves destruction,” says Rabbel. “By using surveying, however, the experts know exactly where to look.”
Some questions can also be answered in other ways by employing scientific and technical methods. The scientists, who are also involved in the interdisciplinary archaeological-geoscientific Collaborative Research Centre 1266, “Scales of Transformation: Human-Environmental Interaction in Prehistoric and Archaic Societies” at Kiel University, have demonstrated this in the past in Tuna el-Gebel. The first surveying campaign, in which the team magnetically mapped the area of the necropolis, that is, recorded the site as a whole, took place between 2007 and 2010.
“This is always the first step in geophysical prospecting (exploration) in archaeology,” says Rabbel, explaining the process. “The subsurface rock is magnetised by the earth's magnetic field. There is a bit of iron oxide in everything, so it is easily magnetisable, but in different ways. If we survey point for point on a surface, the results show the changes in the earth's magnetic field at a strength caused by the magnetisation of the objects in the subsurface. And that can be visualised.” In the resulting images, the outlines of the houses, individual rooms, adjoining rooms and even smaller passageways can be discovered as if on a floor plan.
The data was evaluated, and research results and images published, up to 2015. Now that the entire site has been recorded, the research team aims to dig deeper and get to the bottom of each building: new investigations, in which doctoral researcher Rebekka Mecking is involved, have been underway since 2017. With improved processing methods, she will recycle the older data and use ground penetrating radar to obtain additional point measurements of the houses. However, the work is complex. “If the subsurface permits, you can manage 50 by 50 metres per day.”
However, what we are aiming for is to record every level of the underground buildings to depth and subsequently virtually visualise them as a 3D image. “This is a real geophysical challenge, because of the multi-story buildings,” says Rabbel. “The resulting diagrams are complicated, and analysing them is extremely demanding. But that is what makes it so much fun.”
Author: Jennifer Ruske