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Soil gas radon: a tool for exploring active fault zones
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Applied Radiation and Isotopes 59 (2003) 205–213
Soil gas radon: a tool for exploring active fault zones K. Ioannidesa,*, C. Papachristodouloua, K. Stamoulisa, D. Karamanisa, S. Pavlidesb, A. Chatzipetrosb, E. Karakalab a
b
Nuclear Physics Laboratory, The University of Ioannina, 451 10 Ioannina, Greece Department of Geology, Faculty of Sciences, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece Received 7 October 2002; received in revised form 5 May 2003; accepted 9 June 2003
Abstract The profile of soil gas radon was monitored in five active fault sites in northern and northwestern Greece. Measurements were carried out during summer months, using CR-39 solid state nuclear track detectors (SSNTDs). The spatial distribution of radon along lines traversing the fault zones revealed anomalies, clearly connected to the local tectonic structure. Specifically, increased radon signals evolved on the radon background level, in the vicinity of the faults’ axes and the signal-to-background ratio ranged from 2 to 13. The consistency of this pattern confirms that the radon technique is powerful in the detection and mapping of active fault zones. r 2003 Elsevier Ltd. All rights reserved. Keywords: Radon gas; Active faults; CR-39 detectors; Greece
1. Introduction Radon (222Rn) is a daughter nuclide of radium ( Ra), which in turn comes from the long-lived antecedent, uranium (238U). The short half-life of 222 Rn (t1/2=3.82 d) limits its diffusion in soil, so that radon measured at the ground surface cannot be released from a deep origin, unless there exists a driving mechanism other than mere diffusion. To explain radon migration over large distances, several models have been elaborated and it has been established that radon is transported by underground water or by carrier gases, such as CO2, CH4, He or N2 (Kristianson and Malmqvist, 1982; Rogers and Nielson, 1991; Etiope and Martinelli, 2002). These water and geo-gas discharges are strongly promoted in hydrothermal systems and seismically active zones. The distribution of radon in soil gas has therefore been employed in the exploration of geothermal energy fields (Balc!azar et al., 1991), 226
*Corresponding author. Tel.: +30-265-10-98545; fax: +30265-10-98692. E-mail address: [email protected] (K. Ioannides).
the monitoring of volcanic activities (Seidel et al., 1984; Baubron et al., 1991), the prediction of earthquakes (Hauksson and Goddard, 1981; Singh et al., 1991; Wattananikorn et al., 1998; Planini!c et al., 2001) and the mapping of fault zones (Fytikas et al., 1999; Al-Tamini and Abumurad, 2001; Guerra and Lombardi, 2001; Atallah et al., 2001; Baubron et al., 2002; Ajayi and Adepelumi, 2002). Especially in the case of assessing geological profiles and tectonic discontinuities, the radon technique seems to provide an alternative tool to geophysical methods, such as ground probing radar, electromagnetic conductivity testing, electrical resistivity or vertical magnetic gradient surveying. The present study aimed to determine a possible connection between eventual radon anomalies and active geological faults. Radon gas profiles were monitored in selected faults sites along the area of Almopia (region of Macedonia, Greek-FYROM borders), the Mygdonia basin (region of Macedonia, North Aegean border, Greece) and the Souli or Petoussi depression (region of Epirus, Northwestern Greece). A passive measuring technique based on CR-39 solid state nuclear track detectors (SSNTDs) was employed.
0969-8043/03/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0969-8043(03)00164-7
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that are located between the two main volcanic centres of the area and are comprised by debris and mudflows, fluvial and lake deposits and (b) the upper Almopia epiclastics that are sedimentary deposits, formed due to erosion of other volcanics and deposited by local streams (Vougioukalakis, 2002). Radon monitoring stations were placed at distances of few hundred meters from one another, to cover an approximately 5 km long survey line that crossed a variety of formations (Fig. 1). The northernmost stations were located on soil directly overlying the bedrock, while the rest were located on a less than 5 m thick alluvial cover, lying over volcanic material of the Almopia volcanics. The alluvia consist of coarse sand and pebbles, of both bedrock and volcanic origin, deposited due to the activity of the Almopeos River.
2. Geological overview Five sites, located in active seismic fault areas were selected on the basis of the following criteria: (a) Existence of mapped neotectonic, active, normal faults, not directly connected with any earthquake surface ruptures. Such were the cases of Almopia and Manoliassa sites (Figs. 1 and 3). (b) Existence of mapped surface ruptures generated by past seismic events. Such were the cases of Gerakarou, Nicomidino and Stivos sites (Mygdonia basin), activated after the 1978 earthquake (Fig. 2). A detailed mapping of the neotectonic faults and formations (scale 1:25 000 and 1:5000) was carried out to determine the exact positioning of the radon measuring networks (Figs. 1–3).
2.2. The Mygdonia faults 2.1. The Almopia fault The Mygdonia depression is located about 20 km NE of the city of Thessaloniki (Central Macedonia, North Aegean broader region, Greece). It is a relatively small and narrow Neogene-Quatenary basin, part of a longer seismically active belt trending NW–SE along the socalled Serbomacedonian geological zone, an old crystalline massif affected by alpine deformation, but also cut by numerous neotectonic faults (NW–SE, E–W and NE–SW trending). Five large, crustal shocks have occurred along the Serbomacedonian zone during the 20th century: 1902 in Assiros, Ms=6.6; 1905 in Athos peninsula, Ms=7.5; 1931 in Valadovo, Ms=6.7; 1932 in Ierissos Gulf, Ms=7.0; 1978 in Volvi lake, Ms=6.5
The area of Almopia is located in the north-western part of the Thessaloniki basin in northern Greece, near the Greek-FYROM borders. The study area is located in the margin between Pleiocene Almopia volcanics to the north and Quaternary basin sediments to the south. The bedrock consists of crystalline rocks that belong to the Axios geotectonic zone, mainly Upper Jurassic ophiolites and related deep-sea sediments, Upper Cretaceous transgressional rocks and older crystalline gneisses, probably associated with continental margins (Mountrakis, 1985). Volcanics of the area belong to two volcanic groups: (a) the upper Almopia volcaniclastics
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Fig. 1. Geological map of the Almopia site, showing the location of radon measuring stations.
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Fig. 2. Geological map of the Gerakarou, Nicomidino and Stivos sites (Mygdonia basin), showing the location of radon measuring stations.
(Comninakis and Papazachos, 1986; Papazachos and Papazachou, 1997). The most recent large event of 1978, caused extensive damage and serious socio-economic problems in the city of Thessaloniki and its surroundings. Older events, such as the 1902 and 1932 ones, were also extremely damaging. The neotectonics and seismotectonics of the region are discussed elsewhere (Pavlides and Kilias, 1987; Pavlides and Soulakellis, 1990; Voidomatis et al., 1990; Mountrakis et al., 1992; Martinod and Hatzfeld, 1997; Papazachos and Papazachou, 1997). It should, however, be noted that the neotectonic activity of this zone is characterized by an ongoing crustal extension (Mountrakis et al., 1983). Based on geodetic data, Martinod and Hatzfeld (1997) calculated that the extension rate during the period 1979–1994 was considerably high and amounted to 5.771.3 mm/yr. Independent calculations deduced from earthquake focal mechanism and neotectonic analysis are in agreement with the above result. An ongoing paleoseismological research, regarding the Mygdonia basin active faults, has revealed that there has been significant earthquake activity during the recent geological past, with various slip rates reaching up to 0.7 mm/yr in one of the fault branches, activated during the 1978 earthquake (Chatzipetros, 1998). The interpretation of the active faults in the Serbomacedonian zone is of crucial importance for the assessment of its seismic potential. Three fault sites belonging to the Mygdonia basin were selected for radon mapping. At the Gerakarou site, the active fault crosses the sedimentary filling of the basin while the bedrock is situated at a depth of more
than 200 m from the ground surface. Radon detectors were closely placed along a short traverse, located on soil overlying a formation of altering silt and coarse sand layers. The fault at Nicomidino site is located closer to the basin border. The nature of the soil is similar to that of Gerakarou, only slightly more coherent, extending to a depth of at least 100 m above the bedrock. As the area northern to the fault axis was inaccessible, a short survey line approaching the strike was investigated. In the case of Stivos site, faults are located both in the bedrock and the sediments. The northern fault zone is the one that was activated during the 1978 Ms=6.5 earthquake and it defines the contact between the two sedimentary formations that correspond to different faulting episodes. The geology of the ground resembles that of Gerakarou site, consisting of altering layers of silt and coarse sand. The southern fault zone defines the contact between the bedrock and the sediments. This fault is also active, although it was not directly activated during the 1978 earthquake. Bedrock in this case presents moderate schistosity and consists of mica gneiss. Radon detectors were placed at various distances crossing the northern fault and approaching the southern fault strike. 2.3. The Manoliassa fault The Manoliassa site, located 20 km SW of the city of Ioannina, is a segment of the E–W trending, left lateral strike-slip structured, Souli or Petoussi fault (Epirus, North-western Greece). Although it has been inferred that the fault has been active since the Liassic times
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Fig. 3. Geological map of the Manoliassa site (Souli depression), showing the location of radon measuring stations.
(IFP-IGRS, 1966), its major activity followed the deposition of the Oligocene–Miocene flysch. At several sites along the fault, brittle mesoscale structures were recorded in order to carry out a kinematic analysis. The morphology of many mesoscale faults affecting the underlying rocks, are characterized by fresh, polished and striated surfaces, some of which are underlain by thick cataclastic layers. These tectonic features clearly indicate several reactivations of the fault planes and thus a long tectonic history. Radon detectors were placed along a short, accessible fault traverse located on a thin alluvial cover, directly overlying the limestone bedrock of the area.
3. Materials and methods Radon detection was carried out using CR-39 SSNTDs. Two square pieces (1.5 cm 1.5 cm) of the
detector were fixed at the upper, closed end of PVC tubes, 50 cm long by 5 cm in diameter. The tubes were placed perpendicular to the faults’ trace, in 50-cm deep holes, dug in the soil at various distances from one another. The bottom end of the PVC tubes was covered with a 50 mm thick polyethylene film to exclude 220Rn, water vapors or any contamination from the measuring arrangement. The CR-39 detectors were thus exposed to the soil gas for a period of two to three weeks, after which they were replaced by new sets of detectors. Following exposure, the detectors were etched in a 5 N NaOH solution at 80 C for 8 h. Track densities were measured through a semi-automatic measuring arrangement, employing a microscope-video camera-frame grabber-computer chain. The computer code TRACKA (Ioannides et al., 2000) was used to count the number of tracks per optical field. The detection arrangement was calibrated in the radon calibration chamber of NRPB laboratories in Didcot, UK and has a sensitivity of
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5.2 kBq m3 tracks1 cm2 d. Radon concentrations for each detector were deduced from the mean value of readings of ten optical fields. The standard deviations from the mean ranged typically between 10% and 15%. The effect of meteorological conditions, which are known to strongly influence soil radon investigations using the above detection arrangement (Ioannides et al., 1996), was minimized by conducting the survey during dry periods characterized by stable temperature and soil humidity. To verify that high radon values were related to geological anomalies rather than to increased radium content, soil samples from sites near the radon measuring stations were also analyzed by gamma spectroscopy. The samples were collected from a depth of 50 cm below the ground, dried until constant weight and measured in the standard geometry of 1 l Marineli containers, using a 1.9 keV resolution (for the 661.65 keV line of 137Cs), 22% efficiency, intrinsic Ge detector. The profile of radium concentration was thus assessed by analyzing the emission of 226Ra at 186.2 keV.
4. Results The spatial distribution of radon gas along the traverses, as well as the geological transect of the study areas are shown in Figs. 4–8. Radon data at each measuring station correspond to the mean of radon
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readings acquired during two separate monitoring periods. Standard deviations from this mean fall in the range between 2% and 30%, as shown indicatively in Fig. 4. Fig. 4 illustrates the results from the 4760 m long Almopia traverse, which crosses four, closely located fault lineaments, denoted as A1, A2, A3 and A4. The radon gas profile is related with the location of the faults. A background radon activity of approximately 2 kBq m3 can be inferred from the data, while a radon peak of at least 6 kBq m3 appears around the axis of fault A1. A second anomaly of the same magnitude is peaked in between faults A2 and A3, gradually relaxing to the background level beyond the axis of fault A4. Similarly, anomalous radon signals along the 900 m long Stivos traverse, indicate the position of the two underlying faults (Fig. 5). The study area is characterized by a relatively low radon background of 300 Bq m3, whereas the maximum concentration in the vicinity of the faults’ axes is 5–6 times higher. Along the short traverse at Gerakarou (Fig. 6), a narrow radon peak appears above the fault strike, with a signal-to-background ratio higher than 2. A similar trend is observed when approaching the fault at Nicomidino (Fig. 7), although the geomorphology of the site did not allow the measuring arrangement to cross the fault axis. However, a detector placed at around 10 m from the fault gave radon readings at least
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Radon concentration (10 x Bq m )
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Distance along the traverse (m) Fig. 4. Geological transect and radon profile across the Almopia fault. Radon concentrations measured during two different periods are shown.
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3 times higher than background. The pattern is more straightforward along the Manoliassa traverse (Fig. 8). The radon maximum recorded at the measuring station nearest the fault line, amounts to 13 kBq m3, exceeding the background level by about 13 times. This striking variation is not related to any anomalous distribution of radium along the traverse, as the radium content of soil samples is practically constant and equal to about 90 Bq kg1 (Fig. 8).
5. Discussion The overall picture emerging from this study shows that a radon ‘‘build-up’’ phenomenon occurs in the area of active fault zones. This finding is in agreement with previously reported data, which give evidence of increased radon concentrations in the vicinity of various faults (Fleischer, 1997; Al-Tamini and Abumurad, 2001; Atallah et al., 2001; Ajayi and Adepelumi, 2002). Such anomalous signals are related to the geology and geochemistry of fractured areas. Radon can migrate through soil by diffusion, by the flow of moving soil gases or by a combination of both mechanisms. Diffusion may account for radon transport over short distances, which are typically restricted to a few meters for the case of dry soils with normal porosity and may
1.0 0
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Distancealong the traverse (m) Fig. 6. Geological transect and radon profile across the Gerakarou fault.
be much shorter for soils with high humidity and low porosity (Tanner, 1964). Long-distance transport of radon may, however, occur when radon is carried upwards by a rising flux of other soil gases, such as underground water, carbon dioxide, nitrogen or methane (Fleischer et al., 1980; Kristianson and Malmqvist, 1982). This mechanism can therefore cause significant increase of radon activities at the ground surface. Intense degassing fluxes are strongly promoted in areas of crustal discontinuities, such as fractures and faults, due to the high permeability of the bedrock and soil. To obtain some quantitative estimate of the effect of a vertical soil gas flow on radon activity, a simplified onedimensional model may be applied. The model assumes that a deep-seated radon source is overlaid by a homogeneous soil layer. Radon migrates vertically through this layer, both by molecular diffusion governed by Fick’s law and gas flow described by Darcy’s law. Radon activity will then vary following the mass balance equation: dCðzÞ D @2 CðzÞ @CðzÞ u ¼ f lCðzÞ þ ; dt e @z2 @z
ð1Þ
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Distance (m) Fig. 7. Geological transect and radon profile across the Nicomidino fault.
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ground surface (Bq l1), f the radon production rate (Bq l1 s1), l the radon decay constant (s1), e the soil porosity, D the radon diffusion coefficient in soil (cm2 s1), u the soil gas flow velocity (cm s1)—positive upward and z the depth from the ground surface (cm)— positive downward. At steady state conditions dCðzÞ=dt ¼ 0 ; the solution to Eq. (1) may be written as " rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ! # ue 2 le ue CðzÞ ¼ C1 exp z þ 2D D 2D ( " ! #) r ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi f ue 2 le ue 1 exp z ; ð2Þ þ þ þ l 2D D 2D where C1 is a constant. Applying the boundary condition that radon concentration is negligibly small at the ground surface [C(z=0)=0], Eq. (2) reduces to ( " ! #) r ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi f ue 2 le ue CðzÞ ¼ þ þ 1 exp z : ð3Þ l 2D D 2D The above expression allows a comparison between radon concentration driven by an upward flowing gas ðCu Þ and radon concentration driven by diffusion ðCu¼0 Þ: Considering that in both cases radon is measured at a
Fig. 8. Radium and radon profiles across the Manoliassa fault.
constant depth—i.e. at z=50 cm, as is the case in the present study—and assuming a soil porosity equal to 0.4, the ratio Cu =Cu¼0 may be determined as a function of gas velocity for different diffusion coefficients, D. Using typical D values reported for dry to low humidity soils (S^gaard-Hansen and Damkjær, 1987; Fleischer, 1998), the effect of a rising flow was calculated and some indicative curves are shown in Fig. 9. It is readily concluded that a small soil gas velocity—i.e. 103 cm s1—can perturb radon concentration at the measuring point by approximately a factor of 2, while higher flows increase radon concentration by more than 4–6 times, depending on the diffusion coefficient. In this context, the observed radon peaks may reasonably be attributed to a carrier gas mechanism rather than to mere diffusional migration. However, the peak intensity can not be predicted by a simple model that assumes a uniform radon production rate and diffusion coefficient and a constant gas flow velocity. It is known that even in restricted fractured areas, the structural, hydrologic and lithologic patterns of the soil and bedrock generate complex geochemical patterns in
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References
7 -1
2 7 cm s D = 0.0
6
2 -1
D = 0.05 cm s
C(=) /C (=0)
5 2
-1
D = 0.03 cm s
4
3 = 0.4 z = 50 cm
2
1 0.0
-3
5.0x10
-2
1.0x10
-2
1.5x10
(cm s-1) Fig. 9. The effect of soil gas flow velocity on the ratio of radon concentration measured at a depth of 50 cm below ground surface, for velocities u and zero. Typical values of radon diffusion coefficient in soil have been used in the calculations.
soil atmospheres (King et al., 1993; Baubron et al., 2002), which is reflected in the different radon peak intensities found in this study.
6. Conclusions The radon gas mapping, performed in active faults’ areas, demonstrated that soil radon profiles are affected by the morphology of the underlying bedrock, so that radon signatures are sensitive indicators of the existence of tectonic discontinuities. In all five sites investigated in the present study, radon levels increase as one approaches the fault line, to reach a maximum at the point above or nearest the fault axis. Concentrations at the radon peak range from 2 to 6 times the background level at the same site, while exceptionally high variations by a factor of 13 were recorded over one fault zone. Such variations are possibly related to an upward flow of geo-gas, carrying radon atoms through the fractured bedrock to the ground surface. Extending the method to a detailed radon mapping may prove a powerful and cost-effective tool for locating unknown faults.
Acknowledgements This study was partly funded by the Greek General Secretariat of Research and Technology under the project 91 ED 875, 1993–1994.
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Soil gas radon: a tool for exploring active fault zones K. Ioannidesa,*, C. Papachristodouloua, K. Stamoulisa, D. Karamanisa, S. Pavlidesb, A. Chatzipetrosb, E. Karakalab a
b
Nuclear Physics Laboratory, The University of Ioannina, 451 10 Ioannina, Greece Department of Geology, Faculty of Sciences, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece Received 7 October 2002; received in revised form 5 May 2003; accepted 9 June 2003
Abstract The profile of soil gas radon was monitored in five active fault sites in northern and northwestern Greece. Measurements were carried out during summer months, using CR-39 solid state nuclear track detectors (SSNTDs). The spatial distribution of radon along lines traversing the fault zones revealed anomalies, clearly connected to the local tectonic structure. Specifically, increased radon signals evolved on the radon background level, in the vicinity of the faults’ axes and the signal-to-background ratio ranged from 2 to 13. The consistency of this pattern confirms that the radon technique is powerful in the detection and mapping of active fault zones. r 2003 Elsevier Ltd. All rights reserved. Keywords: Radon gas; Active faults; CR-39 detectors; Greece
1. Introduction Radon (222Rn) is a daughter nuclide of radium ( Ra), which in turn comes from the long-lived antecedent, uranium (238U). The short half-life of 222 Rn (t1/2=3.82 d) limits its diffusion in soil, so that radon measured at the ground surface cannot be released from a deep origin, unless there exists a driving mechanism other than mere diffusion. To explain radon migration over large distances, several models have been elaborated and it has been established that radon is transported by underground water or by carrier gases, such as CO2, CH4, He or N2 (Kristianson and Malmqvist, 1982; Rogers and Nielson, 1991; Etiope and Martinelli, 2002). These water and geo-gas discharges are strongly promoted in hydrothermal systems and seismically active zones. The distribution of radon in soil gas has therefore been employed in the exploration of geothermal energy fields (Balc!azar et al., 1991), 226
*Corresponding author. Tel.: +30-265-10-98545; fax: +30265-10-98692. E-mail address: [email protected] (K. Ioannides).
the monitoring of volcanic activities (Seidel et al., 1984; Baubron et al., 1991), the prediction of earthquakes (Hauksson and Goddard, 1981; Singh et al., 1991; Wattananikorn et al., 1998; Planini!c et al., 2001) and the mapping of fault zones (Fytikas et al., 1999; Al-Tamini and Abumurad, 2001; Guerra and Lombardi, 2001; Atallah et al., 2001; Baubron et al., 2002; Ajayi and Adepelumi, 2002). Especially in the case of assessing geological profiles and tectonic discontinuities, the radon technique seems to provide an alternative tool to geophysical methods, such as ground probing radar, electromagnetic conductivity testing, electrical resistivity or vertical magnetic gradient surveying. The present study aimed to determine a possible connection between eventual radon anomalies and active geological faults. Radon gas profiles were monitored in selected faults sites along the area of Almopia (region of Macedonia, Greek-FYROM borders), the Mygdonia basin (region of Macedonia, North Aegean border, Greece) and the Souli or Petoussi depression (region of Epirus, Northwestern Greece). A passive measuring technique based on CR-39 solid state nuclear track detectors (SSNTDs) was employed.
0969-8043/03/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0969-8043(03)00164-7
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that are located between the two main volcanic centres of the area and are comprised by debris and mudflows, fluvial and lake deposits and (b) the upper Almopia epiclastics that are sedimentary deposits, formed due to erosion of other volcanics and deposited by local streams (Vougioukalakis, 2002). Radon monitoring stations were placed at distances of few hundred meters from one another, to cover an approximately 5 km long survey line that crossed a variety of formations (Fig. 1). The northernmost stations were located on soil directly overlying the bedrock, while the rest were located on a less than 5 m thick alluvial cover, lying over volcanic material of the Almopia volcanics. The alluvia consist of coarse sand and pebbles, of both bedrock and volcanic origin, deposited due to the activity of the Almopeos River.
2. Geological overview Five sites, located in active seismic fault areas were selected on the basis of the following criteria: (a) Existence of mapped neotectonic, active, normal faults, not directly connected with any earthquake surface ruptures. Such were the cases of Almopia and Manoliassa sites (Figs. 1 and 3). (b) Existence of mapped surface ruptures generated by past seismic events. Such were the cases of Gerakarou, Nicomidino and Stivos sites (Mygdonia basin), activated after the 1978 earthquake (Fig. 2). A detailed mapping of the neotectonic faults and formations (scale 1:25 000 and 1:5000) was carried out to determine the exact positioning of the radon measuring networks (Figs. 1–3).
2.2. The Mygdonia faults 2.1. The Almopia fault The Mygdonia depression is located about 20 km NE of the city of Thessaloniki (Central Macedonia, North Aegean broader region, Greece). It is a relatively small and narrow Neogene-Quatenary basin, part of a longer seismically active belt trending NW–SE along the socalled Serbomacedonian geological zone, an old crystalline massif affected by alpine deformation, but also cut by numerous neotectonic faults (NW–SE, E–W and NE–SW trending). Five large, crustal shocks have occurred along the Serbomacedonian zone during the 20th century: 1902 in Assiros, Ms=6.6; 1905 in Athos peninsula, Ms=7.5; 1931 in Valadovo, Ms=6.7; 1932 in Ierissos Gulf, Ms=7.0; 1978 in Volvi lake, Ms=6.5
The area of Almopia is located in the north-western part of the Thessaloniki basin in northern Greece, near the Greek-FYROM borders. The study area is located in the margin between Pleiocene Almopia volcanics to the north and Quaternary basin sediments to the south. The bedrock consists of crystalline rocks that belong to the Axios geotectonic zone, mainly Upper Jurassic ophiolites and related deep-sea sediments, Upper Cretaceous transgressional rocks and older crystalline gneisses, probably associated with continental margins (Mountrakis, 1985). Volcanics of the area belong to two volcanic groups: (a) the upper Almopia volcaniclastics
N
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Fig. 1. Geological map of the Almopia site, showing the location of radon measuring stations.
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Fig. 2. Geological map of the Gerakarou, Nicomidino and Stivos sites (Mygdonia basin), showing the location of radon measuring stations.
(Comninakis and Papazachos, 1986; Papazachos and Papazachou, 1997). The most recent large event of 1978, caused extensive damage and serious socio-economic problems in the city of Thessaloniki and its surroundings. Older events, such as the 1902 and 1932 ones, were also extremely damaging. The neotectonics and seismotectonics of the region are discussed elsewhere (Pavlides and Kilias, 1987; Pavlides and Soulakellis, 1990; Voidomatis et al., 1990; Mountrakis et al., 1992; Martinod and Hatzfeld, 1997; Papazachos and Papazachou, 1997). It should, however, be noted that the neotectonic activity of this zone is characterized by an ongoing crustal extension (Mountrakis et al., 1983). Based on geodetic data, Martinod and Hatzfeld (1997) calculated that the extension rate during the period 1979–1994 was considerably high and amounted to 5.771.3 mm/yr. Independent calculations deduced from earthquake focal mechanism and neotectonic analysis are in agreement with the above result. An ongoing paleoseismological research, regarding the Mygdonia basin active faults, has revealed that there has been significant earthquake activity during the recent geological past, with various slip rates reaching up to 0.7 mm/yr in one of the fault branches, activated during the 1978 earthquake (Chatzipetros, 1998). The interpretation of the active faults in the Serbomacedonian zone is of crucial importance for the assessment of its seismic potential. Three fault sites belonging to the Mygdonia basin were selected for radon mapping. At the Gerakarou site, the active fault crosses the sedimentary filling of the basin while the bedrock is situated at a depth of more
than 200 m from the ground surface. Radon detectors were closely placed along a short traverse, located on soil overlying a formation of altering silt and coarse sand layers. The fault at Nicomidino site is located closer to the basin border. The nature of the soil is similar to that of Gerakarou, only slightly more coherent, extending to a depth of at least 100 m above the bedrock. As the area northern to the fault axis was inaccessible, a short survey line approaching the strike was investigated. In the case of Stivos site, faults are located both in the bedrock and the sediments. The northern fault zone is the one that was activated during the 1978 Ms=6.5 earthquake and it defines the contact between the two sedimentary formations that correspond to different faulting episodes. The geology of the ground resembles that of Gerakarou site, consisting of altering layers of silt and coarse sand. The southern fault zone defines the contact between the bedrock and the sediments. This fault is also active, although it was not directly activated during the 1978 earthquake. Bedrock in this case presents moderate schistosity and consists of mica gneiss. Radon detectors were placed at various distances crossing the northern fault and approaching the southern fault strike. 2.3. The Manoliassa fault The Manoliassa site, located 20 km SW of the city of Ioannina, is a segment of the E–W trending, left lateral strike-slip structured, Souli or Petoussi fault (Epirus, North-western Greece). Although it has been inferred that the fault has been active since the Liassic times
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Fig. 3. Geological map of the Manoliassa site (Souli depression), showing the location of radon measuring stations.
(IFP-IGRS, 1966), its major activity followed the deposition of the Oligocene–Miocene flysch. At several sites along the fault, brittle mesoscale structures were recorded in order to carry out a kinematic analysis. The morphology of many mesoscale faults affecting the underlying rocks, are characterized by fresh, polished and striated surfaces, some of which are underlain by thick cataclastic layers. These tectonic features clearly indicate several reactivations of the fault planes and thus a long tectonic history. Radon detectors were placed along a short, accessible fault traverse located on a thin alluvial cover, directly overlying the limestone bedrock of the area.
3. Materials and methods Radon detection was carried out using CR-39 SSNTDs. Two square pieces (1.5 cm 1.5 cm) of the
detector were fixed at the upper, closed end of PVC tubes, 50 cm long by 5 cm in diameter. The tubes were placed perpendicular to the faults’ trace, in 50-cm deep holes, dug in the soil at various distances from one another. The bottom end of the PVC tubes was covered with a 50 mm thick polyethylene film to exclude 220Rn, water vapors or any contamination from the measuring arrangement. The CR-39 detectors were thus exposed to the soil gas for a period of two to three weeks, after which they were replaced by new sets of detectors. Following exposure, the detectors were etched in a 5 N NaOH solution at 80 C for 8 h. Track densities were measured through a semi-automatic measuring arrangement, employing a microscope-video camera-frame grabber-computer chain. The computer code TRACKA (Ioannides et al., 2000) was used to count the number of tracks per optical field. The detection arrangement was calibrated in the radon calibration chamber of NRPB laboratories in Didcot, UK and has a sensitivity of
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5.2 kBq m3 tracks1 cm2 d. Radon concentrations for each detector were deduced from the mean value of readings of ten optical fields. The standard deviations from the mean ranged typically between 10% and 15%. The effect of meteorological conditions, which are known to strongly influence soil radon investigations using the above detection arrangement (Ioannides et al., 1996), was minimized by conducting the survey during dry periods characterized by stable temperature and soil humidity. To verify that high radon values were related to geological anomalies rather than to increased radium content, soil samples from sites near the radon measuring stations were also analyzed by gamma spectroscopy. The samples were collected from a depth of 50 cm below the ground, dried until constant weight and measured in the standard geometry of 1 l Marineli containers, using a 1.9 keV resolution (for the 661.65 keV line of 137Cs), 22% efficiency, intrinsic Ge detector. The profile of radium concentration was thus assessed by analyzing the emission of 226Ra at 186.2 keV.
4. Results The spatial distribution of radon gas along the traverses, as well as the geological transect of the study areas are shown in Figs. 4–8. Radon data at each measuring station correspond to the mean of radon
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readings acquired during two separate monitoring periods. Standard deviations from this mean fall in the range between 2% and 30%, as shown indicatively in Fig. 4. Fig. 4 illustrates the results from the 4760 m long Almopia traverse, which crosses four, closely located fault lineaments, denoted as A1, A2, A3 and A4. The radon gas profile is related with the location of the faults. A background radon activity of approximately 2 kBq m3 can be inferred from the data, while a radon peak of at least 6 kBq m3 appears around the axis of fault A1. A second anomaly of the same magnitude is peaked in between faults A2 and A3, gradually relaxing to the background level beyond the axis of fault A4. Similarly, anomalous radon signals along the 900 m long Stivos traverse, indicate the position of the two underlying faults (Fig. 5). The study area is characterized by a relatively low radon background of 300 Bq m3, whereas the maximum concentration in the vicinity of the faults’ axes is 5–6 times higher. Along the short traverse at Gerakarou (Fig. 6), a narrow radon peak appears above the fault strike, with a signal-to-background ratio higher than 2. A similar trend is observed when approaching the fault at Nicomidino (Fig. 7), although the geomorphology of the site did not allow the measuring arrangement to cross the fault axis. However, a detector placed at around 10 m from the fault gave radon readings at least
NNW
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Radon concentration (10 x Bq m )
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6
4
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Distance along the traverse (m) Fig. 4. Geological transect and radon profile across the Almopia fault. Radon concentrations measured during two different periods are shown.
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Distance along the traverse (m) Fig. 5. Geological transect and radon profile across the Stivos fault.
3 times higher than background. The pattern is more straightforward along the Manoliassa traverse (Fig. 8). The radon maximum recorded at the measuring station nearest the fault line, amounts to 13 kBq m3, exceeding the background level by about 13 times. This striking variation is not related to any anomalous distribution of radium along the traverse, as the radium content of soil samples is practically constant and equal to about 90 Bq kg1 (Fig. 8).
5. Discussion The overall picture emerging from this study shows that a radon ‘‘build-up’’ phenomenon occurs in the area of active fault zones. This finding is in agreement with previously reported data, which give evidence of increased radon concentrations in the vicinity of various faults (Fleischer, 1997; Al-Tamini and Abumurad, 2001; Atallah et al., 2001; Ajayi and Adepelumi, 2002). Such anomalous signals are related to the geology and geochemistry of fractured areas. Radon can migrate through soil by diffusion, by the flow of moving soil gases or by a combination of both mechanisms. Diffusion may account for radon transport over short distances, which are typically restricted to a few meters for the case of dry soils with normal porosity and may
1.0 0
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Distancealong the traverse (m) Fig. 6. Geological transect and radon profile across the Gerakarou fault.
be much shorter for soils with high humidity and low porosity (Tanner, 1964). Long-distance transport of radon may, however, occur when radon is carried upwards by a rising flux of other soil gases, such as underground water, carbon dioxide, nitrogen or methane (Fleischer et al., 1980; Kristianson and Malmqvist, 1982). This mechanism can therefore cause significant increase of radon activities at the ground surface. Intense degassing fluxes are strongly promoted in areas of crustal discontinuities, such as fractures and faults, due to the high permeability of the bedrock and soil. To obtain some quantitative estimate of the effect of a vertical soil gas flow on radon activity, a simplified onedimensional model may be applied. The model assumes that a deep-seated radon source is overlaid by a homogeneous soil layer. Radon migrates vertically through this layer, both by molecular diffusion governed by Fick’s law and gas flow described by Darcy’s law. Radon activity will then vary following the mass balance equation: dCðzÞ D @2 CðzÞ @CðzÞ u ¼ f lCðzÞ þ ; dt e @z2 @z
ð1Þ
where, CðzÞ is the radon concentration at depth z below
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ground surface (Bq l1), f the radon production rate (Bq l1 s1), l the radon decay constant (s1), e the soil porosity, D the radon diffusion coefficient in soil (cm2 s1), u the soil gas flow velocity (cm s1)—positive upward and z the depth from the ground surface (cm)— positive downward. At steady state conditions dCðzÞ=dt ¼ 0 ; the solution to Eq. (1) may be written as " rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ! # ue 2 le ue CðzÞ ¼ C1 exp z þ 2D D 2D ( " ! #) r ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi f ue 2 le ue 1 exp z ; ð2Þ þ þ þ l 2D D 2D where C1 is a constant. Applying the boundary condition that radon concentration is negligibly small at the ground surface [C(z=0)=0], Eq. (2) reduces to ( " ! #) r ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi f ue 2 le ue CðzÞ ¼ þ þ 1 exp z : ð3Þ l 2D D 2D The above expression allows a comparison between radon concentration driven by an upward flowing gas ðCu Þ and radon concentration driven by diffusion ðCu¼0 Þ: Considering that in both cases radon is measured at a
Fig. 8. Radium and radon profiles across the Manoliassa fault.
constant depth—i.e. at z=50 cm, as is the case in the present study—and assuming a soil porosity equal to 0.4, the ratio Cu =Cu¼0 may be determined as a function of gas velocity for different diffusion coefficients, D. Using typical D values reported for dry to low humidity soils (S^gaard-Hansen and Damkjær, 1987; Fleischer, 1998), the effect of a rising flow was calculated and some indicative curves are shown in Fig. 9. It is readily concluded that a small soil gas velocity—i.e. 103 cm s1—can perturb radon concentration at the measuring point by approximately a factor of 2, while higher flows increase radon concentration by more than 4–6 times, depending on the diffusion coefficient. In this context, the observed radon peaks may reasonably be attributed to a carrier gas mechanism rather than to mere diffusional migration. However, the peak intensity can not be predicted by a simple model that assumes a uniform radon production rate and diffusion coefficient and a constant gas flow velocity. It is known that even in restricted fractured areas, the structural, hydrologic and lithologic patterns of the soil and bedrock generate complex geochemical patterns in
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References
7 -1
2 7 cm s D = 0.0
6
2 -1
D = 0.05 cm s
C(=) /C (=0)
5 2
-1
D = 0.03 cm s
4
3 = 0.4 z = 50 cm
2
1 0.0
-3
5.0x10
-2
1.0x10
-2
1.5x10
(cm s-1) Fig. 9. The effect of soil gas flow velocity on the ratio of radon concentration measured at a depth of 50 cm below ground surface, for velocities u and zero. Typical values of radon diffusion coefficient in soil have been used in the calculations.
soil atmospheres (King et al., 1993; Baubron et al., 2002), which is reflected in the different radon peak intensities found in this study.
6. Conclusions The radon gas mapping, performed in active faults’ areas, demonstrated that soil radon profiles are affected by the morphology of the underlying bedrock, so that radon signatures are sensitive indicators of the existence of tectonic discontinuities. In all five sites investigated in the present study, radon levels increase as one approaches the fault line, to reach a maximum at the point above or nearest the fault axis. Concentrations at the radon peak range from 2 to 6 times the background level at the same site, while exceptionally high variations by a factor of 13 were recorded over one fault zone. Such variations are possibly related to an upward flow of geo-gas, carrying radon atoms through the fractured bedrock to the ground surface. Extending the method to a detailed radon mapping may prove a powerful and cost-effective tool for locating unknown faults.
Acknowledgements This study was partly funded by the Greek General Secretariat of Research and Technology under the project 91 ED 875, 1993–1994.
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