**3.2. Underground measurements**

The use of absorption muon tomography is of course not only limited to the study of large mountain structures, but proves much more useful for the investigation of smaller geological locations, underground cavities, caverns, mines and tunnels, due to the reduced thickness to be explored, hence to the large flux being measured. There are several examples of the use of this technique for these applications [21–25]. As a recent example, in one of these investigations [22], carried out to explore underground cavities in the Naples area, a muon detector similar to that employed for volcano muography was employed, with size 1 m x 1 m, and segmented into 32 scintillator strips.

The vertical rock thickness above the detector was in the previous case about 40 m, which did not reduce too much the muon count rate, allowing for a significant result to be obtained in less than one month of data taking. Actually, for many of these applications, the range of rock thickness usually amounts to a few tens metres, which is a value much less than the values of interest for large volcanic structures. The possibility to install the detector in places which are not so prohibitive as for volcanic explorations gives larger opportunities to use this technique, which will likely be employed more and more in the near future to investigate underground environments.

### **3.3. Archaeology**

muons originating from the front side and the corresponding map produced by the muons

**Figure 5.** A simple geometrical sketch showing the principle of muon tomography applied to the study of mountain structures. A tracking telescope is placed downstream of the structure being explored, reconstructing muon tracks which ideally have traversed a thickness of solid rock. A comparison with the tracks coming from the open sky or from the backside is used to provide a 2D density map of the structure. A nonnegligible background however may originate from

It must be remembered that other Projects [20, 21] are exploiting the possibility to employ the Cerenkov light produced by the muons in the air after traversing the large thickness of the rock by a Cerenkov detector prototype (ASTRI), originally devised for astrophysical investi-

The interest in this field is twofold: from one side methods based on muon tomography may complement and sometimes even surpass the potential offered by traditional methods in the understanding the inner part of these structures, revealing empty spaces, or different density profiles inside the mountain. On the other side there is the hope to reach the resolution and capability to monitor in real time the time evolution of the subsurface structures, in order to control potential activities giving rise to explosions and lava eruptions. This last possibility at the moment is still far from being fully reached, while static investigations have offered beautiful pictures of the interior of mountains and volcanoes in several parts

An important aspect of the technique, which in some cases offers a better figure of merit in comparison to geological and geophysics methods, is the spatial resolution, which can be

∆*x* = *L* ∆*θ* (6)

where *L* is the distance between the detector and the structure being probed, and *∆θ* is the angular resolution of the tracking device. As an example, for a distance *L* = 500 m, and an angular resolution of *∆θ* = 1°, a spatial resolution of the order of 10 m is obtained, which is

gation in view of the large Cerenkov Telescope Array (CTA) Project.

muons which are scattered either from the solid rock or from the air.

better than typical values of other geophysical methods.

coming from the back side.

48 Cosmic Rays

of the world.

expressed as

As recalled at the beginning of this Chapter, one of the first examples of muon absorption tomography is represented by the well-known work by Alvarez and collaborators [2], who employed a muon detector inside an Egyptian pyramid to search for possible hidden chambers. The interest in the study of these very old structures is still very large, and in 2017 a recent study [26] was reported by the ScanPyramid Project, supported by many Institutions, who succeeded to find a very large (~ 30 m) unknown chamber in the Great or Khufu's Pyramid. Such void was first explored by nuclear emulsions and then confirmed by measurements carried out with scintillation hodoscopes and gas detectors; hence, it represents a beautiful example of interrelations between different observation techniques pointing to the same body of evidence. Additional examples of the use of the muon absorption technique for archaeological studies have been reported over the last years [27–29], among which is a study of the cavities in the Teotihuacan Pyramid of the Sun [27].
