“The city below the city – Archaeological and geophysical data to meet major heritage management challenge”

The workshop “The city below the city – Archaeological and geophysical data to meet major heritage management challenges” took place at the Saint-Romain-en-Gal Museum (Rhône, France) on April 3 and 4, 2024. It was organized by the Archéorient laboratory and Ghent University as part of PROSPECT, an international thematic network on archaeological prospection.

The city is a complex space that constantly undergoes rapid changes, a trend that has intensified since the 20th century. From an archaeological perspective, the exploration of the urban underground represents a major historical challenge as it studies long-term human occupations in a specific and limited environmental context while also having undergone numerous cultural, social, political, and economic transformations. Mainly, the city remains an extremely constrained environment for archaeological exploration, generally conducted within a preventive framework during the destruction and reconstruction of urban areas. This work is often carried out in confined spaces, without the choice of selecting excavation zones for their scientific interest. The study of the ancient topography of modern cities resembles a massive puzzle, with pieces slowly assembled by cross-referencing excavation data with other documentary sources. Although still underused, geophysical methods can provide significant help in understanding urban spaces by providing partial insights into spatial variations in between the available pieces of this puzzle. Beyond the areas that can be excavated, their non-destructive use can extend to all construction-free spaces (streets, squares, parking lots, etc.), helping to search for elements from old urban layouts over a larger area and spatially link them to archaeological surveys. Urban geophysical exploration is not new, but it is a rapidly developing field with many technical challenges still to be addressed. Therefore, the goals of this workshop were to review the various approaches developed in this area in recent years, to present archaeological feedback as well as to demonstrate the possibilities of innovative techniques and approaches.

Understanding the urban underground through geophysics requires cross-referencing with other data, from sporadic excavations that may have been conducted, old city maps, to more recent data that can help chronologically discriminate the detected urban elements. The use of geophysics in an urban context also raises the broader question of archaeological and heritage data management by local and regional archaeological resource managers and authorities and their harmonization to better understand the evolution of the city while respecting the protection and enhancement of its heritage at the same time. The second goal of this workshop was therefore to share different experiences on this issue at an international level, particularly from Western Europe, with a specific focus on the Auvergne-Rhône-Alpes region.

Finally, the workshop provided participants the opportunity to explore data processing of multichannel ground penetrating radar through open-source software.

The two day program was structured around three sessions and a practical workshop:

• Session 1: Archaeological Prospection and Data Management in Urban Environments
• Session 2: Case Studies of Archaeological Prospection using Geophysical Approaches in Urban Environments
• Session 3: Innovations in Geophysical Prospection of Urban Environments
• Session 4: Practical workshop on GPR data processing

Archaeological Prospection and Data Management in Urban Environments

This first session, led by archaeologists, facilitated a rich exchange of experiences on the management of archaeological data across several European cities, each within varied scientific, administrative, and structural contexts. All participants in this session emphasized the challenge of gathering and harmonizing very diverse data collected with different recording systems and criteria, and whose accessibility also varies greatly. The cases of several British cities presented by P. Bedford (Heritage Innovation) highlighted the difficult scientific coordination between the various actors in preventive archaeology, as well as communicating about findings to the broader public who is sometimes heavily involved in and contributing to data collection. J. Bouwmeester illustrated approaches to modelling expectations for the preservation and disturbance of urban archaeology, starting from the footprints of individual buildings and extending to the characteristics and density of buildings on an intra-city neighbourhood scale. These models consider factors such as the timing of urban development, town size, and distance from the historic center. Furthermore, he argued that the cycle of archaeological knowledge generation, part of the archaeological heritage management cycle, has evolved from being a separate entity outside of society to becoming an integral part of the public domain. This work of gathering and synthesizing data is generally the responsibility of public institutions, particularly in France. The work carried out by the archaeological service of the city of Besançon, presented by M.-L. Bassi and T. Chenal, is exemplary in this regard. It led to the publication of a historical atlas in the form of a vast three-volume synthesis, as well as the establishment of a database linked to a GIS that brings together all the archaeological data. The work of the Archaeological Service of the city of Lyon follows the same approach. The presentation by E. Hofmann and H. Tronchère also emphasized the integration of geomorphological data into databases, which gradually enriches an environmental model of the city. This model allows an estimation of the depth of the archaeological remains before any archaeological operation and enables a better assessment of the time and cost of a preventive excavation. The SITAR project, developed by the Soprintendenza Speciale Archeologia Belle Arti e Paesaggio di Roma and presented by M. Serlorenzi, allowed for the development of an information system that gathers and queries all documentary resources from underground archives, whether archaeological or environmental data. This system has enabled the management of all data on Rome with an online and open access database (Fig. 1).

Case Studies of Archaeological Prospection using Geophysical Approaches in Urban Environments

During this second session, several case studies on the intensive use of geophysics in urban environments were presented. From a methodological perspective, ground-penetrating radar (GPR) is the primary tool used in urban contexts. The case of the city of York, presented by J. Creighton (University of Reading), is highly representative in this regard, as the entire urban area covering the Roman city was surveyed using GPR, including streets, squares, and green spaces. While the quality of the results naturally varies, this systematic survey has greatly helped to refine the ancient topography of the city. A similar approach was adopted in the Rome Transformed Project, presented by S. Kay (British School at Rome), which explored several neighbourhoods, such as the areas around Saint John Lateran and the Sessorium. In France, Inrap has gained solid experience in this field, with nearly 80 geophysical surveys conducted in urban contexts since 2001, a synthesis of which was presented by G. Hulin. Most of these operations, conducted in preventive contexts, have led to the establishment of an optimized intervention protocol. The opening of archaeological trenches following the surveys also allowed for a direct comparison between GPR data and the excavated remains. The electrostatic method, developed in the early 1990s by Alain Tabbagh (UMR Metis, Sorbonne University), is now considered as the most promising alternative to GPR in urban contexts. The synthesis of the evolution of this method, presented by M. Dabas (UMR AOROC, ENS Paris), demonstrated how the combined use of GPR and electrostatics provides highly complementary results in urban settings (Fig. 2).

Innovations in Geophysical Prospection of Urban Environments

During this final session, C. Schamper (UMR Metis, Sorbonne University) reviewed recent technical developments in the electrostatic device. Today, two complementary devices can be used: the MP3 device, which allows for detailed surveys up to about 2 meters deep, and the “sliding carpet,” which can reach depths of 4 meters with a lower resolution. Through the example of surveys conducted in Vienne (as discussed in a previous post), C. Schamper also highlighted the importance of measuring electrical permittivity, in addition to electrical resistivity, particularly in conductive environments. L. Verdonck (University of Cambridge) discussed the evolution of ground-penetrating radar (GPR) devices (multi-antenna, autonomous driving) and data processing, where the use of machine learning and deep learning is opening up particularly interesting possibilities for radar data characterization and classification. D. Boddice (University of Birmingham) simulated the potential of quantum technology gravity sensors, particularly in a noise cancelling gradiometer configuration, for improving detection of smaller and deeper subsurface features in comparison to conventional methods, expecting significant detectability improvements. N. Papadopoulos (Institute of Mediterranean Studies) demonstrated the various applications of electrical resistivity tomography in urban contexts. By using bentonite to hold electrodes on hard surfaces (asphalt, cobblestones, masonries), he and his team achieved very interesting results in different contexts, both on streets and vertically to assess the stability of certain buildings, as in the case of the Rethymnon fortress in Greece (Fig. 3). The second part of this session also presented the first results obtained from the Archémuons project (introduced in a previous post) at the Palais du Miroir in Saint-Romain-en-Gal. J. Marteau (IP2I, University of Lyon 1) showcased the results obtained through muography; seismic approaches were discussed by L. Bodet (UMR Metis, Sorbonne University) for surface wave surveys (Fig. 4) and by B. Tauzin (LGL-TPE, University of Paris 1) for fiber optic surveys. These approaches still need further development, but initial tests suggest that new methods may soon be more widely used for urban reconnaissance.

Practical workshop on GPR data processing

The second day concluded with a practical workshop where T. Wunderlich (Kiel University) presented the radar data processing software she developed in Matlab. Participants were then able to use the software themselves and test it on datasets provided to them. As we discussed over the course of these two days, ground-penetrating radar (GPR) remains the most commonly used method in urban contexts for now, but data processing can be particularly time-consuming. The availability of optimized software is therefore a crucial issue, especially as new types of processing are being developed, as L. Verdonck had demonstrated earlier that morning.

The workshop, also followed online, brought together an average of 70 participants from various countries over the two days. This success highlights the scientific and heritage interest in the use of geophysical methods in urban contexts as well as the importance of bringing not only archaeological and geophysical researchers together but also hearing of challenges from the heritage management perspective and evaluating if and how these can be met by geophysical methods. As recent developments have shown, technological advancements are numerous and quick regarding a better understanding of the urban environment, which nevertheless remains one of the most complex contexts for geophysics.

Les auteurs

Christophe Benech est Chargé de Recherche au CNRS. Spécialiste en géophysique appliquée à l’archéologie. UMR 5133-Archéorient, Maison de l’Orient et de la Méditerranée, Lyon.

Jeroen Verhegge is a postdoctoral researcher in the Department of Archaeology and the Department of Environment at Ghent University (Belgium), specializing in archaeological prospection using non-invasive and minimally invasive methods. He coordinates the activities of the international thematic network PROSPECT.

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