Enlarge /. Ground penetrating radar map of the temple in the Roman city of Falerii Novi, Italy.
Falerii Novi was once a walled city north of Rome, probably around 241 BC. It was founded as a place of resettlement for a Falisci tribe that rebelled against the Romans. Located on a volcanic plateau, archaeologists suspect that the new location was chosen because it was not so easy to defend, which discouraged further uprisings. In the third and fourth centuries BC There were probably around 2,500 inhabitants. The ruins are deep underground, but a team of archaeologists from Cambridge University and Ghent University in Belgium used ground penetrating radar (GPR) to map the entire city. They described their results in a recent article in Antiquity magazine.
From 1910, when the first patent was filed for a buried object radar system, GPR was used to measure the depth of glaciers, examine bedrock and groundwater, and to locate unexploded landmines, buried sewers, and utility lines, among other applications . The 1972 Apollo 17 mission – the last lunar landing from NASA's Apollo program – used a GPR system called the Apollo Lunar Sounder Experiment (ALSE) to record depth information on the lunar surface. The method has also proven to be a powerful tool for archaeological geophysics in recent years as it is a non-invasive means of detecting and mapping artifacts, features and key patterns beneath the surface.
GPR differs from another popular method, LIDAR, which relies on infrared light from lasers and not radio waves to map the terrain. An electromagnetic pulse is directed into the ground, and all objects or layers (stratigraphy) can be seen in the reflections picked up by a receiver as with normal radar. How long it takes for the echoes to return shows the depth, and different materials reflect the incoming waves differently. The data can then be recorded to create detailed maps of these underground features.
Falerii Novi was first excavated in the 1990s. In the decades that followed, archaeologists identified warehouses, shops, marketplaces, a theater, and a forum using various non-invasive techniques, including magnetometry – a method that measures direction, strength, or strength, relative change in a magnetic field at a particular location to detail exposed under the surface. However, the use of GPR by the British and Belgian teams has resulted in a much more detailed and complete picture of the location, allowing them to examine how the city has developed over several hundred years.
The authors attached their GPR system to the back of an ATV in order to measure the 30.5 hectares within the city walls more efficiently and measure every 12.5 centimeters. In 2017, the team used their method to find the remains of a large Roman temple, a few feet below the city, which would have been about the size of St. Paul's Cathedral.
Map showing the location of the ancient Roman city of Falerii Novi in Italy.
Co-author Lieven Verdonck and his colleagues attached a ground radar system to the back of a "quad bike" in order to better map the excavation site.
Part of Falerii Novi's GPR data reveals the outlines of the city's buildings.
The Porticos Duplex and the public monument east of the north gate of Falerii Novi.
GPR map of the theater.
This latest analysis revealed a large rectangular structure connected to a network of water pipes leading to the city's aqueduct. The authors suspect that it is the remains of an outdoor swimming pool (Natatio), which is part of a large public bathing complex. They were also surprised to see two large buildings facing each other in a covered passageway (porticus duplex), which they believe was once part of a large public monument near the city's north gate. It appears to be part of a "sacred topography" of temples on the outskirts of the city that was previously discovered by magnetometer surveys.
GPR works under certain conditions, such as B. even sandy soils, very good, but the high electrical conductivity of clays and silt can, for example, significantly dampen the signal strength. And rocky sediments scatter the signal, making it more difficult to see the patterns in the noise. "In Falerii Novi, the generally dry conditions in the summer months were well suited for the GPR examination," the authors wrote, but found that it took up to seven days in the rain before the soil was sufficiently dry to achieve optimal data quality. "
Falerii Novi was a good place to demonstrate the potential of GPR, as unlike other ancient cities, it is not buried under modern buildings. GPR, especially in combination with magnetometry, could be a useful tool to study such cities. "Neither (GPR nor magnetometry) can provide a complete picture of archeology," the authors wrote, noting that Falerii Novi's shop units appear in magnetic data, for example, but not in the GPR survey. And while the city's theater shows up in the magnetic data, the GPR survey provided a much clearer view, including at different depths, and gave insights into its structural shape, as well as indications of stone removal from walls.
The biggest challenge for the future is the sheer volume of data generated by this high-resolution mapping. According to the authors, they collected a whopping 71.7 million measurements from Falerii Novi, which corresponds to 28.68 billion data points or about 4.5 GB of raw data per hectare. Documenting a single hectare can take up to 20 hours. For this reason, the team develops automated techniques to accelerate the process with computer-aided object recognition.
"The amazing level of detail we have achieved at Falerii Novi and the surprising features that GPR has revealed suggest that this type of investigation could change the way archaeologists examine urban locations as a whole," said Co- Author Martin Millett of the University of Cambridge, adding that it should be possible to use GPR to study large ancient cities such as Miletus in Turkey or Nicopolis in Greece. "We still have a lot to learn about Roman city life, and this technology should open up unprecedented opportunities in the coming decades."
DOI: Antiquity, 2020. 10.15184 / aqy.2020.82 (About DOIs).