# Multi-Parametric Imaging of Etruscan Chamber Toms:
## 3 Results
### Radon and Thoron Results  
Rn concentration anomalies must be analyzed site by site because of their dependence on local geological-structural settings.  
According to [21] and [27], the presence of voids facilitates gas accumulation, which may propagate outward through fractures, enabling higher Rn mobility.  
The gas concentration anomaly of a buried structure can be substantially observed in two different ways: (1) as a spot with a high concentration, in cases of an empty volume where gas accumulates due to the lack of air exchange with the atmosphere or (2) as a low concentration spot if the same volume is partially filled by soil, with no radioactive elements, thus revealing only the structure perimeter.  
In addition, the determination of the Th/Rn ratio is crucial as it provides information on the shallow gas circulation and on the effective permeability of the volcanic gas source. Due to the significant difference of the half-decay between the two Rn isotopes (3.8 days for Rn and 55 s for Th), a high Th concentration indicates that this gas is confined close to the surface, and thus, a high Th/Rn ratio is directly linked to a shallow circulation and diffusive gas migration mechanism. Conversely, a low Th/Rn ratio suggests a deeper gas source, which involves a longer transport and the concomitant Th decay, given its low half-life compared to the Rn. Moreover, the Rn longer transport needs a gas carrier (i.e., CO\({}_{2}\)) to bring it up to the surface and which we found to be close to zero, after a preliminary CO\({}_{2}\) flux survey that we conducted in the study area (not reported in this paper). This supports the hypothesis that the presence of Rn and Th was only due to the local volcanic rocks with no input from deeper gas sources.  
Figure 7 reports the measured Rn and Th concentrations and their ratio. The acquired dataset shows min, max, and mean values of 213, 10,154, and 4872 Bq*m-3 for the Rn, while Th are 668, 61,989, and 22,225 Bq*m-3, respectively. In general, the measured concentrations mostly range between average soil and atmosphere values, with the exception of some sectors in which anomalous emissions were detected.  
In particular, along the northwest-southeast oriented trail, some samples showed very high (Rn > 7000 Bq*m-3 for samples SGC1 and SGC7) and high (4000 < Rn < 7000 Bq*m-3 for samples SGC3, SGC4, SGC8, and SGC9) Rn concentrations. In the same line, the measured Th concentration had very high values (Th > 40,000 Bq*m-3) for SGC1, SGC2, and SGC7. Meanwhile, on the flat area, corresponding to the CCR2, GPR2, and ERT profiles, we observed high to middle-high Rn and Th concentrations (GC2, GC6, GC8, and GC10 in Figure 7a) with Rn values in the range of 4000-7000 Bq*m-3 and Th values well above 30,000 Bq*m-3.  
Figure 7b shows the calculated Th/Rn ratio. The highest ratio values (Th/Rn > 9, Figure 7b) were obtained in four samples (GC1, GC9, GC11, and GC13) that corresponded to anomalies pointed out by the geophysical methods, while the other three (GC3, GC7, and GC10) showed moderately to high Th/Rn activity ratios (6 < Th/Rn < 9, Figure 7b). The distribution of these points suggests and delineates areas characterized by the accumulation and circulation of gases probably facilitated by the presence of shallow voids.