Transcript Diapositiva 1

Real time
222Rn
measurements at Stromboli Island
A. Lavagno1, M. Laiolo2, G. Gervino3, C. Cigolini2, D. Coppola2 and C. Marino3
1.- Dipartimento di Fisica, Politecnico di Torino, Italy
2.- Dipartimento di Scienze Mineralogiche e Petrologiche, Università di Torino
3.-Dipartimento di Fisica, Università di Torino, Italy
The analysis of temporal and spatial variations of the soil gases flux across the soil-air interface is a useful tool to investigate geophysical process
associated to volcanic activity. One of these gases is radon that has an unique properties: it appears in all the decay chains of the three main
primordial radionuclides of the Earth crust such as 238U, 235U and 232Th. Radon is a natural occurring noble element, chemically inert, constantly
generated in the rock matrix and in the crustal material. Being monoatomic it could easily enter the rock pores and migrate to significant distances
from the site of generation in a surprising short time. Radon is a component of the geothermal fluids, especially in the volcanic systems, measuring the
variations of the radon flux, that are only induced by physical factors since it is not a reactive species, could give valuable information on dynamical
transport process for geothermal fluid. 222Rn isotope is an a emitter (Ea=5.5 MeV) with half-life of 3.82 days, widely used as precursor of geological
risks like earthquakes and volcanic eruptions, locating buried faults, exploring for uranium ores. In recent year it has been also used as a tracer in
atmospheric research for the origin and trajectory of air masses [1]. 222Rn emanates into the air-filled soil or rock pore space from where a fraction
reaches the surface and escape to the atmosphere where it could be breathed and hence it has also been associated with lung cancer risk.. In active
volcanoes like Stromboli Island, sharp variations on 222Rn concentration may have a geological cause like magma injection, changes in the
temperature and/or depth of hydrothermal system, the effect of stress in rocks induced by seismic events. But 222Rn flux is also affected by
environmental variables, namely atmospheric pressure and temperature, soil temperature, soil moisture and humidity. Hence the environmental
modulation on the 222Rn signal could mask variation related to volcanic activity reducing its potential as a precursor of geological risk. Since 2002 up
to present days a network of field stations has been collected 222Rn concentration data in the volcanic island of Stromboli. The in-soil radon
concentration values are coupled to atmospheric pressure, soil temperature and soil humidity data. By continuously automatic measurements the role
of the previous listed environmental parameters has been studied and quantified in order to disentangle the radon variation due to geological effects
from the environmental ones.
a)Simplified structural map of
Stromboli Island with the stations
(dark grey triangles) and the
Volcano Observatory of the
National Civil Defense, where the
transmitted data from real-time
stations are stored, processed
and visualized;
b) Median 222Rn concentration on
the 21 sites of the network on the
topographic DEM map has been
obtained by means of a statistic
elaboration.
Radon measurements at the soil surface vs the temperature gradient
from 5 m depth to the surface. The same behaviour of the two curves
suggests a strong correlation
Sketch of the 222Rn real-time monitoring station placed at
Stromboli volcano. .All sensors are connected with the
radio-modem placed near the radon detector inside the
buried container, to be protected from atmospheric agents.
The radio-moden is linked by the directional antenna to
transmit the data to the control room at COA (Stromboli Obeservatory).
References
[1] J.M. Prospero and T.N. Carlson, Science 167 (1970) 974.
[2] R. Bonetti et al., Nucl. Instr. and Meth. A 18 (1991) 321.
[3] P. Kotrappa, J.C. Dempsey and L.R. Stieff, Rad. Prot. Dosim. 47 (1993) 461.
[4] C. Cigolini et al., Geophys. Res. Lett. 32 (2005) L12308.
[5] M. Grundel and J. Postendorfer, Rad. Prot. Dosim. 107 (2003) 287.
[6] G. Gervino et al., Physica A 340 (2004) 402.
a) average radon concentration in the
North side of Stromboli Volcano,
b) measurement box with inside
the radon detector and the
read-out electronics,
c) position of the two real-time station
(full red circles), the full black square
is the position of the Stromboli
Observatory COA where the it is
placed the control room.
Both real time stations are on the
same side of the volcano in order
to ease the wireless communication.
Time series for radon concentration
(Bq/m3 daily average), atmospheric
pressure, soil temperature and
cumulative rainfall. The radon
Behaviour is in anti-phase with soil
temperature curve, suggesting that
temperature gradient between surface
and in depth is the main cause of
radon emanation