HR: 1340h
AN: T43A-1080    [Abstracts]
TI: The Diurnal, Multiday and Seasonal Evolution of Rn-222 Within Rocks Along the Dead Sea Transform in Relevance to Earthquakes Related Phenomena.
AU: * Zafrir, H
EM: zafrir@gsi.gov.il
AF: Geological Survey of Israel, 30 Malkhei Israel St., Jerusalem, 95501, Israel
AB: The basic assumption underlying monitoring of Radon along the Dead Sea Transform is the capability of Radon to emanate to the porous media and be available for further transport by diffusion via the pore content (geo-gas) or to a convective migration, to a limited range (half life = 3.8 days). To this end, the association between the temporal variations of Radon at daily, multi day and seasonal scale is investigated versus other geophysical variables: pressure, temperature, precipitation, humidity, groundwater level, gravity, and geodynamic changes. Monitoring Radon in the desert-like region enables to set apart those variables that are not active in the area, like precipitation and keeping part of them stable as by installing the Radon sensors at the Amram site near Eilat. It is a deep dead-end, confined horizontal tunnel (170 meter span and 75-200 meter below surface) with well defined environmental conditions as: stable indoor humidity and temperature (28±0.3°C), low CO₂ content and very dry porous media, located high above the ground water table. Monitoring Radon by total gamma and alpha counting together with environmental parameters, since 2002, at sampling rate of several minutes, provided the following results: No correlation occurs between Radon and atmospheric pressure variations at the Amram site. Since the subsurface and external pressure is the same it does not create a local pressure gradient to produce an advection that can account for the radon signals (anomalies). In addition, there is no connection between these temporal variations of Radon and the stable subsurface temperature (28±0.3°C). Nevertheless, an apparent correlation in the time domain occurs between the Radon variation at depth and the temporal variation of the external temperature. The Radon follows sequentially the daily temperature variation with a delay of about 10 hours, consistently. It indicates that the diurnal variation of the Radon concentration at Amram tunnel is caused by the interplay of the ambient temperature gradient at the outside rock/air interface. The delay is a result of the Radon migration time towards the tunnel and back, within the fracture rock porous media. An additional external detector (at 7 meter borehole), present a clear daily correlation between the Radon and the temporal variation of the outdoor temperature, but with a delay of only 2-4 hours. Furthermore, the multi-day and the seasonal variation of the radon emanation also delineate the same interconnection with the ambient temperature. The non-linear relation between the intensity of the radon anomalies and the ambience temperature may explain the reduction of the intensities by factor of 2-5 during the winters (November-March). Similar relations occur at the shallow sites of NW Dead Sea shore, between part of the radon anomalies and the temporal variation of the environmental temperature. The rest of the radon signals could be related to other geophysical processes such as earthquakes (Geology, v. 31, 505-508). Similar phenomena have been observed in worldwide corporate research in Canada and India.
DE: 5104 Fracture and flow
DE: 5139 Transport properties
DE: 7223 Earthquake interaction, forecasting, and prediction (1217, 1242)
DE: 8177 Tectonics and climatic interactions
SC: Tectonophysics [T]
MN: 2007 Fall Meeting