Journal of Radioanalytical and Nuclear Chemistry, Vol. 269, No.1 (2006) 187–193
Soil gases (222Rn, 220Rn and total radon) and 214Bi measurements across El Avila fault near Caracas, Venezuela F. Urbani,¹,² J. J. LaBrecque,3* N. Flores,¹ P. R. Cordoves3 1 Universidad
Central de Venezuela, Faculty of Ingeniería, Department of Geología. Caracas, Venezuela Venezolana de Espeleología, Apartado 47334, Caracas 1041A, Venezuela 3 Instituto Venezolano de Investigaciones Científicas, Centro de Quimica, Apartado 21827A, Caracas, Venezuela 2 Sociedad
(Received January 18, 2006)
Radon measurements were performed across two sections of the Avila fault near Caracas, Venezuela. The radon concentrations clearly showed the different tectonic features and lithology at the Tacamahaca and Spanish Trail sites. 214Bi (U-cps) measurements also were related to the lithology. The passive radon method employed laboratory-made dosimeters with LR 115, type 2 celulose nitrate films as detectors. They were buried in the ground at 30 cm depth. While, the active radon method was performed with a Pylon radon measurement system with Lucas cells. The soil gas was also sampled at 30 cm depths, but for only one minute, which was sufficient to fill the 150 cm3 Lucas cells completely. The total radon counts were then separated into those corresponding to 222Rn (radon) and 220Rn (thoron) by a simple computer routine. A comparison of the active and passive methods for the Tacamahaca section over a three-month period showed that both methods could locate precisely the active fault trace.
Introduction Radon measurements in soil gas have been employed in numerous occasions to solve geological uncertainties, such as the determination of fracture zones, the location of fault traces with neotectonic activity, earthquakeseismicity related problems, geothermal activity and others.1–4 These type of studies started in Venezuela in 1995 with real-time (active) measurements of radon using a Pylon AB-5 (Ottawa, Canada) radon measurement system coupled to Lucas cells5,6 and shortly afterwards with solid state nuclear track-etch detectors (SSNTD).7 The mountainous region of northern and western Venezuela were formed by the interaction between the Caribbean and South American tectonic plates, therefore, they are seismically prone with historical evidence of earthquakes of up to 7 Magnitude (ML) of the Richter scale. Additional information on the neotectonics of the Caribbean can be found elsewhere.8 The Avila fault is situated in the northern central region near Caracas, which is the most populated area in Venezuela. Thus, this type of studies will help us to understand the degree of neotectonic activity along this fault and in turn to be able to better estimate the associated risk. A variety of techniques have been used in the past to determine radon in soil gas.9,10 Active methods were first employed with Lucas cells,11 followed by photographic films as passive detectors for counting alpha-tracks.12 This last technique cumulatively registers the alpha-tracks over long periods of time, usually from days to months, depending on the concentration of radon emanating from the ground.
As a result, the variations due to environmental parameters, such as atmospheric pressure and temperature are averaged out. In this work, we report the results of a preliminary test survey carried out using SSNTDs across two fault zones with very different geologic-tectonic settings, as well as compare the results of the radon measurements by an active and passive method for one of the section of the fault trace. Experimental Location and geological setting Tacamahaca (TA): This section crosses the El Avila Fault at the following geographical location 10°30’15”N, 66°43’10”W. By aerial photographs and geomorphologic evidence it is known that the fault trace passes somewhere within a 300 m long section of a dirt road on the Tacamahaca ridge, but knowledge of the exact location of the more active, gas permeable zone is unknown and was one of the objective of this work. The fault is normal in nature and places in contact Paleozoic quartz – plagioclase – mica gneiss of the San Julian Complex, with Cretaceous quartz – mica – calcitic – graphitic schist of Las Mercedes Schist.13 This fault is active but with a very slow movement rate. Spanish Trail (ST): Located on the highest mountain pass of the old colonial Spanish trail from Caracas to the port of La Guaira; located at 10°32’52”N 66°57’6”W. Here is a thrust fault place in sharp contact with a serpentinite body above quartz – muscovite – garnet schist and amphibolite of the La Costa Metamorphic Suite.13 This fault has probably been inactive since midTertiary time.
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F. URBANI et al.: SOIL GASES (222Rn, 220Rn AND TOTAL RADON) AND 214Bi MEASUREMENTS
Measuring methods Determination of radon by solid state nuclear tracketch detectors (passive method): The soil gas radon sampling was performed using Kodak Pathè (France) LR-115 Type 2 film which detects alpha-particles with energies of 1.7–4.2 MeV.14 The radon concentration was calculated as: C = [30.7 (Ts–Tb)/t] in which C is the radon concentration in kBq.m–³, Ts is track.cm–² for the total time of exposure of the film strip, Tb is tracks.cm–² for a blank, t is the exposure time in days and 30.7 is a conversion factor experimentally determined. Track counting was operator-based using an optical microscope equipped with a grided eyepiece. The dosimeters were exposed for periods between 1 and 3 months giving a time integrated radon concentration of the soil gas. The dosimeters were laboratory-made with a regular aluminum beverage can (250 ml in volume) with top completely removed, on the inside of the bottom a styrofoam piece was glued having a small incision in which the tip of the film was inserted in a vertical hanging position.15 The top of the can was sealed with kitchen-quality thin polyethylene film. The dosimeters were placed inverted, with the top facing down so that the soil gas could pass through the thin polyethylene film at depth about 30 cm and covered with the same excavated material. Determination of radon with a radon measurement system (active method): Soil gas was sampled through steel sampling tubes which were placed permanently in the soil at a depth of 30 cm with another 10 cm above the surface. The tubes were sealed with rubber corks between measurements. Before the measurements, the tubes were checked to ensure that they were free from water or other materials by inserting the same rod into them that was used to insert them into the ground. Further information on these sampling tubes can be found in Reference 16. Total radon measurements were performed with radiation monitors (AB-5 Pylon, Ottawa, Canada) coupled with 150-cm3 Lucas cells. The monitors were adapted with a simple external filtering system to eliminate moisture and small soil particles. Flow meters were also employed between the filtering system and the Lucas cells to ensure the cells were completely full with soil gas at the end of the pumping cycle. The pumping cycle was for one minute followed with five one-minute counting periods. Finally, total radon, 222Rn (radon) and 220Rn (thoron) were calculated by a simple computer routine.17 Determination of 214Bi (U-cps): A portable differential gamma-ray spectrometer (Scintrex GRS500, Concord, Canada) with four windows, total counts, 188
“potassium”, “uranium” and “thorium” was used to obtain profiles across the same sections in which radon concentrations were measured. The measuring time was 100 seconds for the “uranium” window (actually the 1.746 MeV gamma-rays from 214Bi) at each of measuring points. Since 222Rn is a decay product of the 238U disintegration series, the uranium-window gammaray profile [indicated in the figures as “U (cps)”] was helpful to obtain complimentary information of the lithology variation and relative availability of 238U across the sections. Results and discussion Relation of the radon and 214Bi measurements to the lithology Tacamahaca: The measurements of radon by the passive method and 214Bi (U-cps) along this 265-m section are shown in Fig. 1 with its’ geological context. The south side of the fault trace was characterized by Cretaceous schist, while the north side of the fault trace was characterized by Precambrian gneiss. The radon measurements clearly define the active fault trace between the 175- and 205-m sampling sites, about 30 m wide with the maximum radon concentration at the 190m sampling point. The radon concentrations over the fault trace ranged from 15.9–26.5 kBq.m–3, well above the 4.8 and 6.6 kBq.m–3 averages on the southern and northern sides, respectively. The total radon peak is due to the higher emanating of radon in the fractured soil near the fault trace, rather than higher concentrations of uranium in the soils. Thus, the location of the active fault trace by this method was much more precise than the 300-m range estimated by aerial photographs and geomorphologic evidence. The 214Bi (U-cps) activity was uniform over the complete section with 105±21 counts for the 100 second measurements. This is compatible with the fact that the rocks on both sides of the fault trace contain similar amounts of mica, which is the main component for the uranium content in the soils and rocks. Spanish Trail: The measurements of radon by the passive method and 214Bi (U-cps) were only performed along 155-m section, since the location of the fault trace is clearly seen with an uncertainty of about ±1 m at the 80-m sampling point. The results of these measurements and the geological parameters are shown in Fig. 2. The west side of the fault trace was characterized by serpentinite, while the east side of the fault trace was characterized by schist and amphibolite. The radon concentrations were in the range of 0.4 and 0.6 kBq.m–3 over the serpentinite on the western side of the fault and between 1.6 and 7.2 kBq.m–3 over the schist and amphibolite on the eastern side of the fault. The radon concentrations near the region of the visual fault trace
F. URBANI et al.: SOIL GASES (222Rn, 220Rn AND TOTAL RADON) AND 214Bi MEASUREMENTS
(between the 60- and 95-m sampling points) ranged from 2.7 and 3.3 kBq.m–3, well above average on the western side but below the 4.5 kBq.m–3 average on the eastern side. If one considered that the fault trace is at the 80-m sampling point, it is sealed and dips with low angle to the west. The average radon concentration was around 4.0 kBq.m–3 over the fault trace due to the fact that just below a couple of meters below the serpentinite there was the schist. Similar radon concentration patterns were reported18 before and explained by the fact that the fault trace was sealed. The 214Bi (U-cps) activity over the serpentinite on the western side of the fault trace was only between 4 and 8 counts for the 100-second measurements with respect to between 16 and 24 counts over the schist and amphibolite on the eastern side. Both these 214Bi measurements and the radon measurements are consistent with the mineralogy of the involved rocks whose uranium content diminishes from the mica schistamphibolite to serpentinite. Thus, the activities of these radionuclides, e.g., 214Bi and 222Rn have shown the effect of the lithology. The 214Bi (U-cps) activity from the fault trace at 80-m sampling point to the east rises very significantly with an average 214Bi activity (U-cps) of 56 counts. 214Bi is a decay product in the uranium series after radon (222Rn), thus it can also be used in some cases to located fault traces. The employment of 214Bi to locate fault trace has been recently published.19 Comparison of the active and passive radon methods across the Tacamahaca section The parameters of the radon measurements by both the active and passive methods are listed in Table 1. It should be noted that the three series of active measurements were taken in the beginning of and at the end of the two series of passive measurements. All the active measurements only sampled soil gas for one minute, leaving only 150 cm3 of soil gas in the Lucas cell to be counted in respect to the soil gas which penetrated the ethylene film of the passive detectors during an one or two-month period of time. The zero measuring point was assigned a position near (but not exactly) the active fault trace determined from the previous measurements in 1996 (the 190-m sampling point of Fig. 1), the minus sampling points are
on the south side and the plus ones on the north side. With respect to the original measurements by the passive method in 1996, the second series of measurements by the passive method during April-May 1999 (Fig. 3), the radon concentration pattern and the intensity at the fault trace were very similar, even though the measurements (Fig. 4) were performed for only 33 days instead of 63 days. But, the third series of radon measurements by the passive method for 29 days showed a much less intense peak at the fault trace and with only one sampling point significantly above the background with respect to three sampling points in the previous measurements. These less intense radon measurements can possibly be explained by the fact that during these radon measurements less soil gas would have been transported to the detector as a result of the much higher soil moisture due to the amount of rainfall. With respect to the first series of radon measurements on the 15 April 1999 by the active method, it was clearly shown in Fig. 5, that there was a wide intense total radon concentration peak at the fault trace, a 222Rn (radon) peak slightly to the north side and a 220Rn (thoron) peak slightly to the southern side. The 222Rn pattern was very similar to the radon pattern of the first and second series of measurements by the passive method. With respect to the first active measurements on 15 April 1999 and the second series of measurements about one month later, on 18 May 1999 as shown in Fig. 6, it can be seen that the total radon, 222Rn (radon) and 220Rn (thoron) concentration patterns were similar, but were about 25% less intense near the fault trace. Similarly for the third series of radon measurements by the active method on 16 June 1999 (Fig. 7). It can be seen in Figs 5 to 7 that the 222Rn (radon) concentration patterns more clearly define the location of the fault trace than those of the total radon. Finally, it can be seen that the active fault trace can be easily located by both the active and passive methods in all the radon measurement series except the third passive series when only one sampling point was very significantly above the background measurements. When this happens, further measurements are needed around the anomalous sampling point. In the case of the active method, more measurements at different sampling points can be performed the same day, since the results are seen immediately in the field.
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F. URBANI et al.: SOIL GASES (222Rn, 220Rn AND TOTAL RADON) AND 214Bi MEASUREMENTS
Fig. 1. Radon (kBq.m–3) and 214Bi (U-cps) measurements across the Tacamahaca section of the Avila fault between May and August, 1996 employing the passive radon method
Fig. 2. Radon (kBq.m–3) and 214Bi (U-cps) measurements across the Spanish Trail section of the Avila fault between July and October, 1996 employing the passive radon method
Table 1. Parameters of the radon measurements Figure 1 2 3 4 5 6 7
Site TA ST TA TA TA TA TA
TA: Tacamahaca. ST: Spanish Trail.
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Method SSNTD/LR 115 SSNTD/LR 115 Lucas cell SSNTD/LR 115 Lucas cell SSNTD/LR 115 Lucas cell
Exposure time intervals 31/05/96-02/08/96 04/07/96-11/10/96 15/04/99 15/04/99-18/05/99 18/05/99 18/05/99-16/06/99 16/06/99
Sampling time 63-days 69-days 1-min 33-days 1-min 29-days 1-min
F. URBANI et al.: SOIL GASES (222Rn, 220Rn AND TOTAL RADON) AND 214Bi MEASUREMENTS
Fig. 3. Radon measurements across the Tacamahaca section of the Avila fault between April and May, 1999 employing the passive radon method
Fig. 4. Radon measurements across the Tacamahaca section of the Avila fault between May and June, 1999 employing the passive radon method
Fig. 5. Total radon (counts) measurements across the Tacamahaca section of the Avila fault on 15 April 1999 employing the active radon method showing the total radon, 222Rn (radon) and 220Rn (thoron) counts
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F. URBANI et al.: SOIL GASES (222Rn, 220Rn AND TOTAL RADON) AND 214Bi MEASUREMENTS
Fig. 6. Total radon (counts) measurements across the Tacamahaca section of the Avila fault on 18 May 1999 employing the active radon method showing the total radon, 222Rn (radon) and 220Rn (thoron) counts
Fig. 7. Total radon (counts) measurements across the Tacamahaca section of the Avila fault on 16 June 1999 employing the active radon method showing the total radon, 222Rn (radon) and 220Rn (thoron) counts
Conclusions
*
It was shown that in different geological settings how the radon concentrations in soil gas are influenced by either lithology and/or tectonics. Both, the active and passive methods employed in this work were precise to locate the fault traces and reproducible. These methods were very economical and employed very common technology, thus these types of studies could be carried out by low skilled personnel and still produce significant results for the assessment and risks for fault zones.
The authors would like to thank the students of the School of Geology of the Universidad Central de Venezuela who assisted in the field work in 1996.
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