A multidisciplinary team at Tel Aviv University has developed the technology that will enable "X-ray imaging" of the underground for archaeological excavations.
A technological breakthrough at Tel Aviv University offers a revolution in the world of archaeology: the first demonstration of the feasibility of locating underground spaces using cosmic ray detectors – muons, which are created when cosmic rays hit the Earth’s atmosphere. Muons penetrate the ground before losing their energy and stopping, so their detection can be used by archaeologists to map hidden spaces such as tunnels and canals. As part of the study, the research team demonstrated the effectiveness of the technology at the City of David archaeological site in Jerusalem and showed how the system was able to map underground spaces through changes in the permeability of the ground to cosmic ray particles.
The research was conducted under the leadership of Prof. Erez Etzion. from the School of Physics and Astronomy at Tel Aviv University, and Prof. Oded Lipshitz From the Department of Archeology and Ancient Near Eastern Cultures at Tel Aviv University. Also participating in the study were Prof. Yuval Gadot from the Department of Archaeology and Ancient Near Eastern Cultures at Tel Aviv University, Prof. Yan Ben Hamo, Dr. Igor Zolkin and doctoral student Gilad Mizrahi, from the School of Physics and Astronomy, Dr. Yiftach Silver and Dr. Amir Weisbein from Rafael and Dr. Yiftach Shalio from the Israel Antiquities Authority. The results of the study were published in the Journal of Applied Physics.
Swiss cheese under the rock
"From the pyramids in Egypt, through the Mayan cities in South America to the ancient sites in Israel, archaeologists have difficulty finding underground spaces," says Prof. Lipshitz. "Archaeology excavates the structures above ground relatively easily, and there are also various methods for locating walls and structures below the surface. But there are no good methods for conducting comprehensive surveys of underground spaces – which are located beneath the bedrock on which the ancient sites are located. In the lowlands, for example, beneath the layer of hard limestone (the Nari rock), lies the soft limestone (the chalk), so that someone who carves and penetrates the Nari rock from above or enters underneath it from the side can easily create very large spaces for storing water, for various agricultural uses, for storage and even for living. It is clear to us that most of the archaeological sites on the surface are nothing more than Swiss cheese beneath the bedrock. But we have no way of knowing that. If by chance we dug above ground, reached the bedrock and identified an entrance to the space, we can dig it out. But we have no way of locating the subsoil in advance. In the current study, we are proposing for the first time an innovative method that has proven to be very effective in locating underground spaces using cosmic ray detectors – "Muons".
The researchers explain that a muon is an elementary particle similar to an electron, but 207 times more massive. Muons are created in the atmosphere when energetic particles, mainly protons, collide with the nuclei of molecules in the air. This collision creates unstable particles called paeons, which decay very quickly into muons. Muons also have a very short lifespan, and they decay after 2.2 microseconds, but they travel at a speed close to the speed of light – and in that time they have enough time to reach the ground.
Small, portable detectors for identifying voids in the ground
"The muon meter that hits the ground does so at a constant and known rate," explains Prof. Etzion. "Unlike electrons that stop in the ground after a few centimeters, muons lose energy at a slow rate as they pass through the ground, and therefore some of them penetrate deep into the ground. The most energetic of them can penetrate even to a depth of one hundred meters. Therefore, if we place a muon detector underground and measure the environment, we can identify empty spaces where the energy loss is negligible. Why is this similar? It's like reflecting an X-ray: we place an X-ray beam on one side and a camera on the other, to illuminate the body we want to photograph - the bones and joints, etc., since these stop the beam better than fat and flesh, for example. Thus, the muons are the X-ray beam, our detector is the camera, and the underground systems are the human body."
As mentioned, the researchers conducted an impressive demonstration in a rock-cut structure, known as the "Jeremiah's Pit" at the City of David archaeological site. As part of the demonstration, the researchers combined high-resolution LiDAR scanning of the interior with muon flux simulations, thereby successfully mapping structural anomalies. The system successfully detected changes in the permeability of the soil to muons, thereby demonstrating the feasibility of using muon tomography for archaeological imaging.
The field is resurgent
"This article is a first milestone," says Prof. Lipshitz. "We want the archaeological need to push physicists to produce smaller, simpler, cheaper, more durable and more accurate detectors, which also consume less electricity. The next step will be to combine physics and archaeology with artificial intelligence, which will know how to take the replica data that the detectors will produce to produce a 3D image of the subsoil. Our test site will be Tel Azka in the central Shephelah, above the Elah Valley."
"This is not our invention," adds Prof. Etzion. "Back in the 1960s, muons were used to search for hidden rooms in the pyramids in Egypt, and recently the field has been reawakened. Our innovation is that we have developed small, portable detectors and learned to operate them at archaeological sites. After all, there is a difference between a detector in laboratory conditions and one that needs to be placed in a cave or excavation to measure its environment - and suddenly practical problems of electricity, temperature, and humidity arise. The detection ranges are a function of the measurement time; the further the detector is, the fewer particles reach it, but realistically, it is possible to analyze images from a distance of up to 30 meters in a reasonable time. Therefore, the goal is to place several detectors, or move one detector from place to place, in order to simulate the underground of entire sites in 3D. Therefore, we are only at the beginning. The next stage is a stage of sophisticated analyses, which will allow us to map everything that is underfoot - even before the first backhoe reaches the site."
More of the topic in Hayadan:
