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| Titel |
Soil radon and electromagnetic anomalies before the Ileia(Greece) M6.8 earthquake |
| VerfasserIn |
D. Nikolopoulos, E. Vogiannis, A. Louizi, A. Zisos |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250022302
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| Zusammenfassung |
| Radon (222Rn) is a radioactive gas generated by the decay of the naturally occurring 238U
series. It is considered very important from radiological point of view, since it accounts for
more than half of the natural exposure of the general public. Radon has been used as trace gas
in several studies of Earth, hydrogeology and atmosphere, due to its 3.82-day half-life
(which allows migration at long distances) and its alpha decay (which enables low
level of detection). It has been accounted in the search of earthquake precursors,
volcanic processes, fluid circulation in karstic sources and in the study of natural
ventilation of underground cavities. Radon anomalies impending great earthquakes
have been observed in groundwater, thermal waters soil gas and in underground
tunnels.
Ileia is a very active tectonic site located in SW Greece, dominated by extensional active
seismicity structures (e.g. Alfeios, Neda, Melpeia, Kiparissia-Aetos). Its instrumental and felt
seismicity is very high, with more than 600 earthquakes of magnitude greater than 4.0 R in
the last 100 years two of which occurred during the last 15 years and were very
destructive (5.8 R on 26/3/93 and 6.8 R on 8/6/08 respectively). Hence, it is an area
benefiting from the installation of a geophysical monitoring station, where radon
exhalation associated with the accumulation or release of tectonic strain can be
studied.
In the aforementioned consensus, a station for the surveillance of soil radon has been
installed in Kardamas Ileias, 3 km south from Amaliada which is the second highly populated
city. The station consists of a high precision (calibration certified) active instrument (Alpha
Guard-AG, Genitron Ltd.), equipped with an appropriate unit designed for pumping and
measurement of radon in soil gas (Soil gas Unit, Genitron Ltd.). Soil radon is driven into AG
via a 1-m probe (to minimize meteorological influences) and a 25-m radon proof 25-mm
tube (to avoid simultaneous measurement of soil 220Rn-Thoron) with the aid of a
mechanical pump working continuously at a rate of 1 L/min. Radon is continuously
measured every ten minutes. Additionally, calibrated passive radon dosimeters based on
CR-39 Solid State Nuclear Track Detectors (SSNTD’s) are periodically (every two
weeks) installed in 50 cm holes dug near the 1-m probe and exposed (passively)
to soil radon. After the end of each exposure period, the dosimeters are collected
and the SSNTD’s are measured via standard methods (optical microscopy track
counting).
Both methods (active with AG and passive with SSNTD’s) provide similar estimates of
mean soil radon concentration. Active techniques are much more precise and quick, however,
they indicated the necessity of periodical checks for the pumping and measurement status,
especially after strong rainfalls. The mean soil radon concentration was found fairly constant
(to within +/- 10%) and approximately equal to 25-27 kBq m-3. Numerous soil radon
concentration anomalies (sudden statistically significant (p |
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