Archaeologists often search for ancient buildings that are buried without a visible trace on the surface. A magnetic surveying group has found a way to address this issue, applying their expertise to help archaeologists search for buildings at the ancient Saudi Arabian city of Thaj.
Thaj is believed to have been occupied between the 3rd century BCE and the 4th century CE. An ongoing archaeology project started investigating the site in 2016. The archaeologists knew the location of the city, but the internal layout of some of the city is hidden from sight, making it difficult to determine the most productive places to excavate. Despite the lack of visual clues, the limestone buildings in the city subtly distort Earth’s natural magnetic field. Magnetic surveying techniques can use these distortions to provide a glimpse of the structures hidden underground.
Jérôme Rohmer, an archaeologist working on the Thaj project, contacted the magnetic surveying group led by Marc Munschy. This group is a part of the Géology Océans Lithosphère Sédiments team at the Institut de Physique du Globe de Strasbourg in Strasbourg, France. Although Munschy’s team does not specialize in archaeology, the techniques for detecting magnetic anomalies that they use can help with a variety of applications—detecting unexploded ordnance, for example—so they decided to take on this new challenge.
Munschy’s group was confident that they could accurately measure the magnetic fields, explained team member Paul Calou, but the type of site determines what data they obtain and how to interpret them. Calou, a Ph.D. student at the Institut de Physique du Globe de Strasbourg, helped conduct the research and presented the research at a poster session on 11 December at AGU’s Fall Meeting 2018. “So when someone calls and asks, ‘Can you come and check,’ we don’t really know what it will be,” he added. However, a member of his group had previously investigated a site with similar limestone constructions, so they were optimistic that they could get good results in Thaj.
Mapping Magnetic Fluctuations
The group uses a custom-designed system that can be worn as a backpack to collect magnetic data using four magnetometers that helps make the data collection easier to perform. The sensors they use are light and energy efficient, but they require the group to adjust for errors introduced by the sensors. Such errors include distortions from the equipment itself in addition to those introduced from the environment, such as magnetic fluctuations from the Sun.
To adjust for these errors, the group first uses models to correct for the magnetic fields that are known to be produced by Earth or nearby geological features. Then, they have to find an area of the site without large magnetic disturbances and perform a 360° sweep to collect data for determining the necessary corrections for any systematic errors from their detectors.
A member of the group then walks back and forth across a section of the area being investigated and also crosses the same area a few times from the side. This gives the group redundant data points that they can use to help ensure that they are getting consistent results.
Calou said that sometimes walking back and forth to collect the data in Thaj was boring. However, the occasional spider and scorpion sightings livened things up. “There was a spider that was looking for shade,” said Calou. The spider would follow them to stay in the shade they created. The spider “sometimes could be scary,” he added.
Once the group collected their data, they mathematically analyzed it to identify any anomalies. The deeper the source of the magnetic distortion is, the more gradual the change in the magnetic field is. Processing the data allowed the group to mathematically enhance the contrast of any magnetic anomalies they found.
Revealing the Ancient City
The survey team analyzed the processed data for straight lines or patterns that seemed likely to be signs of human creations. In Thaj, they found lines that represent limestone used in the walls and buildings of the city. One area they surveyed contained an unusual number of distortions. Excavating that area revealed the source of the distortions: a large amount of metal surrounding an ancient forge.
The archaeologists combined the data from the magnetic survey with aerial photography to produce a map of the ancient streets.
Often, geoscientists use detailed measurements of magnetic fields to learn about how Earth and rocks formed in the distant past, but using these measurements for human artifacts is less common. “It’s a very applied usage of our science,” said Laurie Brown, a paleomagnetism researcher from the University of Massachusetts Amherst who chaired and convened the section of posters but who was not involved in this research. “Most of us are doing quite different things.”