Delft techniques used in Leiden archaeological project reveal excluded neighbourhoods
Ostia used to be a flourishing harbour city, abandoned in the eighth century B.C., and now a Roman archaeological site some 25 kilometres to the west of Rome. Researchers from Leiden and Delft now lay bare the suburbs of Ostia and find obstructed streets.
When you walk through Ostia it is easy to imagine yourself as a citizen of this harbour city walking through its narrow streets. Roman robe, made of silk or cotton, nonchalantly draped over your shoulder and leather sandals on your feet. You would probably be on your way to the bathhouse, the marketplace or the theatre. But what if you didn’t live in the city centre? What would life have been like just outside the centre, in Ostia’s suburbs? How easy would it have been to walk from the suburbs to the centre and the public spaces situated there? Were Ostia’s suburbs connected to the city gates, to the centre and to the public buildings? And how did this connectedness influence the communities living in the suburbs? Dr Hanna Stöger, post-doctoral researcher at the Leiden University Faculty of Archaeology, is very interested in the social information we can glean from spatial data and how spatial factors can influence community building.
A road trip to Ostia
In order to find out how Ostia’s suburbs were connected to the city, a bus full of researchers from Leiden and Delft went on a road trip to Ostia to conduct archaeological-geophysical fieldwork. Dr Stöger knows that the suburbs of Ostia have not been fully excavated. Excavation is very time consuming and destructive to the site. This is why archaeologists are more and more interested in using non-invasive methods such as geophysical prospection. ‘At TU Delft, scientists constantly develop new geophysics methodologies and archaeology can benefit from those advancements,’ says Stöger. She therefore initiated a project to map Ostia’s un-excavated suburban areas using geophysics methods and other technical expertise from TU Delft. Two researchers from Leiden’s Faculty of Archaeology joined her: Dr Till Sonnemann, geophysicist and post-doctoral researcher, and Mark Locicero, a third year doctoral candidate. Together with Dr Dominique Ngan-Tillard, Assistant Professor at TU Delft’s Department of Geosciences & Engineering and affiliated to the Leiden-Delft-Erasmus Centre for Global Heritage and Development, and six students from both universities, they all headed to Ostia. ‘It was a privilege to work with Hanna. She knows so much about Ostia!’ says Ngan-Tillard.
The students were baffled. They learned a lot about working in a multidisciplinary team. Thierry van ‘t Westende, a Master’s student at TU Delft’s Faculty of Civil Engineering and Geosciences, says: ‘Archaeologists speak a different language and they have a different mind-set. Engineers want hard proof and evidence. Archaeologists interpret data on the basis of knowledge of ancient writings and artefacts. This is something that I had to get used to as an engineering student.’ Lars Schaarman, Master’s student at the Leiden Faculty of Archaeology says: ‘It was an eye-opener for me; I knew that these techniques existed but it's really different to actually work with them. I think we need more collaborations like this. The cooperation between Leiden and Delft should continue and perhaps people from TU Delft could give lectures at Leiden University.’
The actual fieldwork
To map the un-excavated neighbourhoods, the research group used three techniques: two techniques used at the Archaeology department in Leiden and one used at the Geophysics department in Delft. By combining these techniques, they were able to produce a detailed map giving a lot of spatial information.
The first technique used was photogrammetry. Dr Sonnemann took aerial pictures with a drone and from these photographs, he created a 3D map of the terrain and a reconstruction of the surface of the site.
Student with Ground Penetration Radar. Ngan-Tillard: ‘Pulling the GPR is some hard work.’
The second technique used was the Ground-penetrating Radar (GPR). GPR is one of the instruments used at the TU Delft Geophysics department to image the subsurface. Electromagnetic waves at radio frequencies are sent into the ground and part of them are reflected back and are recorded. In such a way, a reflection pattern becomes visible. Together with Dr Sonnemann and Dr Ngan-Tillard, the students formed an image of the subsurface. Through the reflected waves, the method detects differences in dielectric material properties. Dr Ngan-Tillard says: ‘It was some hard work for the students pulling the GPR device over the patches of land. But we did see some strong reflections. This could be the floor of a house, or a wall – sometimes we also had the impression of long channels and ditches.’
The third technique was the differential Global Positioning System (D-GPS). This method is commonly used in archaeology to accurately record the survey markers of the Ground Penetration Radar (GPR) along with features such as fences, walls and trees. Using this data, the students made a digital map to geo-reference the GPR survey.
‘We cannot draw any conclusions yet, but there are interesting preliminary results. These results showed us a number of obstructed streets and larger structures in the middle of passageways. This was a surprise to me and quite unexpected. I would have thought that the street space, the public space, had been respected, even over a long period of time,’ says Dr Stöger. It seems the neighbourhoods of Ostia weren’t as connected to the city centre as archaeologists thought. If you were walking along this passageway, the road to the market, bathhouse or theatre would probably have been blocked. ‘Why would they obstruct access? Why would they compromise the street network? What could this mean?’
Figure 2: GPR survey results (depth ~70-100cn), overlaying the UAS orthophotographs; highlighted in red are the structures identified so far.
So what now?
To answer this intriguing question, more research needs to be done. Another geophysics technique called seismic prospecting can be used. A combination of seismic interferometry (SI) and high-resolution reflection seismic might be a likely option. Ngan-Tillard: ‘This combination of seismic techniquesis especially suitable when they are a lot of hard archaeological remains in the shallow subsurface. The seismic waves generated by an active source are reflected and/or scattered on the remains and recorded by receivers that are placed at the ground surface. Using the recordings, after some clever signal processing, the remains can be better imaged.’
Further collaboration between Leiden University and TU Delft looks extremely promising due to the Geophysics department’s expertise with seismic interferometry (SI). Dr Ngan-Tillard explains: ‘SI has been developed over the last 15 years and is yet to be used in archaeology. We want to bring this technique to archaeology. Yet another geophysics technique that can be used is electric resistivity tomography (ERT), combined with induced polarisation (IP). An electric current is sent into the subsurface and we measure the resulting voltage and its time decay. With IS, ERT and IP, it is possible to see much deeper into the ground than with GPR. Martijn Warnaar, the TU Delft’s Applied Earth Sciences student who participated in the Ostia GPR survey last May, has secured a grant from the foundation “Stichting Molengraaff Fonds”, so first site trials can take place this summer.’
And still other techniques can be explored: techniques like remote sensing, where aerial sensor technologies are used to detect objects. Dr Stöger says: ‘I want to investigate further whether the ancient inhabitants of the neighbourhoods of Ostia were spatially privileged or spatially deprived.’ Beeld banner: