Radiation Protection Dosimetry (2014), Vol. 160, No. 1–3, pp. 70 –73 Advance Access publication 11 April 2014
doi:10.1093/rpd/ncu092
REVEALING THE HIDDEN FAULTS IN THE SE FLANK OF MT. ETNA USING RADON IN-SOIL GAS MEASUREMENT K. Johnova´1, *, L. Thinova´1 and S. Giammanco2 1 Departament of Dosimetry and Application of Ionizing Radiation, FNSPE CTU, Brˇehova´ 7, Prague 1, Czech Republic 2 Istituto Nazionale di Geofisicae Vulcanologia, Sezione di Catania, Piazza Roma, 2, 95123 Catania, Italy *Corresponding author: [email protected]
INTRODUCTION Radon, an inert gas, is one of the components of soil air. It is able to travel over a distance of tens (up to hundreds) of metres under favourable conditions for convection transport. For this reason, radon gas can be used as a tracer for obtaining information about processes in the deep parts of the earth’s crust. Fault systems may facilitate radon transport, due to the presence of more permeable and more penetrable zones along the cracks, and in these situations a significantly higher radon concentration can be measured in the fault system area. In cases where the fault system is filled with clay (or some other) material, radon anomalies occur before or after the fault system itself. Measurements of radon concentrations have previously been used to trace processes connected with degassing of the soil along tectonic faults in various parts of the world(1 – 3), including Mt. Etna(4). Ways to study the dynamics of the faults that lie hidden by recent lava cover were studied in greater detail in ref. (5). Soil radon concentration measurements may provide clear information about the presence of a fault, but in most cases it is difficult to interpret the results. It is sometimes necessary to perform other measurements in conjunction with a radon survey. Fortunately, there are several other methods that can be used for detecting the presence of faults and that can also help to clarify the results of soil radon concentration measurements. Eight methods including measurements of the radon concentration in soil gas were selected to be tested in the conditions of Mt. Etna. The main aim of this work is to evaluate the usability of these methods in the conditions of Mt. Etna, to select the best combination of methods
for identifying fault structures and to find their limitations. As a result of these radon measurements, it should also be possible to identify the best location for installing a permanent continual radon monitoring station. The measurements presented here were carried out in collaboration with the Universita` degli Studi di Catania and the Istituto Nazionale di Geofisica e Vulcanologia Sezione di Catania. LOCALITIES Mt. Etna is a strato-shield volcano located in an area where the European and African plates converge(6). Eruptions in recent times may have resulted from gravitational sliding of the unstable eastern flank towards the Ionian Sea(7). The measurements discussed in this paper were performed at four different locations along the same fault in the SE flank of Mt. Etna. The first set of measurements was collected in 2011 at two locations, Primoti and Santa Venerina, in the middle part of the fault. The Zafferana locality in the upper part of the fault was investigated in 2012. The most recent measurements were carried out in 2013 in the Dagala locality, in the lower part of the fault, where the presence of the fault was considered to be particularly obvious and the interpretation of the data to be the least complicated. Only the results of the most recent measurement (Dagala 2013) are presented in this paper. MEASUREMENT METHODS The measurements on all profiles were performed with sampling at distances between 3 and 5 m, depending
# The Author 2014. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
Downloaded from http://rpd.oxfordjournals.org/ at University of Pittsburgh on June 18, 2015
Although there are many methods for investigating tectonic structures, many faults remain hidden, and they can endanger the life and property of people living along them. The slopes of volcanoes are covered with such hidden faults, near which strong earthquakes and gas releases can appear. Revealing hidden faults can therefore contribute significantly to the protection of people living in volcanic areas. In the study, seven different techniques were used for making measurements of in-soil radon concentrations in order to search for hidden faults on the SE flank of the Mt. Etna volcano. These reported methods had previously been proved to be useful tools for investigating fault structures. The main aim of the experiment presented here was to evaluate the usability of these methods in the geological conditions of the Mt. Etna region, and to find the best place for continual radon monitoring using a permanent station in the near future.
REVEALING THE HIDDEN FAULTS IN THE SE FLANK OF MT. ETNA
and to make a deeper study of the U–Th–K ratios. The soil samples were taken at the locations where the in situ spectrometry was performed, and because of the limited measurement time available in the laboratory, the pre-selection was performed according to the radon concentration results and the electrical resistivity tomography measurements, to optimise the number of samples. A coaxial HPGe (35 %) detector was used for the laboratory gamma-spectrometry measurement of the samples in Marinelli containers with a 0.6 l volume geometry. Each sample was dried, homogenised and hermetically sealed for 1 month in a plastic container to establish equilibrium between 226Ra and 222Rn. The Automatic Resistivity System (ARES—main unit with standard accessories, multi-electrode cable sections MCS5) was used to determine the potential fault system, permeable zones (for correlation with radon, thoron and CO2 measurement) or the presence of water in underground caverns. This system can automatically apply a small electric current at two current electrodes, while at the same time it measures the drop in voltage at two other electrodes and calculates the resistivity. The position of the electrodes depends on the geometry of the measurement. The arrangements of the measurements selected for this experiment were Dipole –Dipole and Schlumberger. The second geo-electrical method is based on measuring the characteristics of long electromagnetic waves (10 –30 kHz) that are transmitted from distant transmitter sites. At a large distance from a transmitter, the electromagnetic waves travel horizontally. When they meet any conductive object, for example, a fault, the waves are disrupted along the conductor body. The largest anomaly is generated when the conductor body is elongated in the direction of the electric field and is perpendicular to the magnetic field, i.e. oriented towards the source of the waves. For the best results, the direction of the transmitter has to be in the direction of the conducting disturbing body. Very low frequencies method (VLF) is used primarily as a reconnaissance tool for identifying anomalous areas for further investigation. Unfortunately, there are only a small number of transmitters working in this frequency band at the present time. The Norwegian station JXN (Helgeland) transmitting waves with 16.4 kHz and the Italian station ICV (Tavolara), 20.27 kHz, were used for the study in the vicinity of Etna. For fault localisation, the gradient of the real component of the signal was calculated. All measurements were carried out with EM 16 and EDA instruments. Atmo-geochemical measurements may be useful for identifying various elements coming from the bedrock. Some heavy metals may occur above the surface in close relation with the presence of magma in the deeper parts of the supposed fault. Although the element transport mechanisms are still not fully understood, this method is being used successfully for 71
Downloaded from http://rpd.oxfordjournals.org/ at University of Pittsburgh on June 18, 2015
on the locations of obstacles, for example, the layout of farm buildings and garden walls, following a preliminary investigation of the area of interest. The direction of the profiles was perpendicular to the expected direction of the main fault; the midpoint of the profiles was located just above the supposed main fault line. The terrain in the selected area was typical for populated areas of Etna: usually stony ground, with a mix of cultivated and uncultivated soil, situated on series of terraces with relatively high differences in elevation. The terrace walls were assembled from basalt rocks and may have influenced the in situ gamma-spectrometry measurements. The presence of roads and some underground service networks had to be taken into account when the results were being evaluated. Measurements of the radon concentration in the soil gas were carried out using the RM-2 measurement system and the ‘lost tip method’. At each sampling point, a special metal pipe was hammered into the ground to the sampling depth of 0.8 m. Samples of soil gas (0.15 l) were taken from this depth, using a syringe equipped with a filter, and were immediately transported to small ionisation chambers (volume 0.25 l, evacuated using a pump). In a complicated rocky soil, the sampling depth was between 0.5 and 0.8 m. The radon activity was measured on site 15 min after taking the sample. The 15 min delay ensured that the thoron activity was negligible. The radon measurement was complemented by a soil permeability measurement in selected locations and by measurements using Lucas chambers 0.5 l in volume. The correlation between the results of in-soil gas radon measurements using small ionising chambers and Lucas cells was R 2 ¼ 0.96. In situ gamma spectrometry is a useful method for identifying differences in rock composition. Differences in the radionuclide ratios may be able to identify changes in the lava massive, due to the presence of a fault, when there is a difference in the age of the lavas or in their origin. The depth range of this method (measurement on the surface) is usually 0.35– 0.50 cm, depending on the density of the soil and the concentration of radionuclides. A portable Gamma Surveyor spectrometer was used for in situ determination of the K (%), U (ppm eU) and Th (ppm eTh) concentrations. This spectrometer consists of a probe (equipped with a 300 ` 300 NaI(Tl) detector), global positioning system (GPS) and a control panel. The spectrometer was calibrated using cylindrical standards at the calibration laboratory in Stra´zˇ pod Ralskem, Czech Republic, for measurements in 2p geometry. It was impossible to maintain the desired geometry in all measurements, and so the results need to be corrected (for example, using Monte Carlo simulations). Laboratory gamma spectrometry is widely used for detailed and precise identification of the radionuclide content of samples. The aim was to obtain a comparison between laboratory results and in situ spectrometry
´ ET AL. K. JOHNOVA (8)
several anomalies in the soil radon concentration on one side of the fault plane (95– 105 m), and there is a very low stable concentration on the other side. The second curve in Figure 1 shows the concentration of CO2 measured by the Italian group of scientists. The maximum radon concentration is complemented by the minimum CO2 flux (location 100 m), and the maximum CO2 flux is at location 105 m. The numerous radon anomalies on the right side of the fault, which were confirmed by resistivity and VLF measurements, demonstrate cracks with no direct contact with the magma and are typical for this part of the lava cover. Most of the measurements were complicated by the rugged terrain. The in situ gamma-spectrometry measurement results are shown in Figure 3, together with an outline of the terrain situation. The influences of the ‘stairs’ near the measurement point were corrected using Monte Carlo simulation. However, there is still an influence from the different composition of the ground (asphalt on the road, stones forming the walls of the stairs), which affect the results shown in Figure 3. The laboratory gamma-spectrometry measurement of samples representing terrain material such as lavas, soil and ash show similar results as in the previous localities: 96 Bq kg21 226Ra, 120 Bq kg21 232Th and 605 Bq kg21 40K in lava or 107 Bq kg21 226Ra, 110 Bq kg21 232Th and 505 Bq kg21 40K in soil. The presence of a road on profile 1 in location 85 m seriously disturbed the results of the VLF method. The road can be identified through the high maximum in the gradient in Figure 4. Figure 5 illustrates the results of the rock resistivity measurement in combination with the concentration of U from the in situ gamma-spectrometry measurement and the radon in-soil gas measurement at profile No. 2, Dagala. The fault line is characterised by the gradient in the radon concentration and the maximum in the U (ppm eU) concentrations.
RESULTS The schematic situation of the profiles at Dagala is shown in Figure 1. Typical radon concentration behaviour measured along profile No. 1 is shown in Figure 2. There are
Figure 1. Profiles across the supposed fault in the Dagala locality, projected onto Google earth maps using GPS coordinates. The white line marks the supposed fault.
Figure 2. A comparison of the radon concentration in the soil gas and the CO2 flux measured in profile No. 1 in Dagala.
Figure 3. Concentration of U ( ppm eU), Th ( ppm eTh), K (%K) found by in situ gamma spectrometry, with a scheme of the terrain situation in profile No. 1, Dagala.
72
Downloaded from http://rpd.oxfordjournals.org/ at University of Pittsburgh on June 18, 2015
exploring mineral deposits . The device consisted of a battery and a pump with a controllable air flow rate; an air circulation system with two Millipore filters connected by silicon tubes; a plastic sampling vessel 50 ml in volume with 10 ml of 0.1 molar HCl solution. The air was bubbled for 5 min at each measurement location from a height of 0.5 m above the earth’s surface. After bubbling, the sample was hermetically sealed into a small plastic sampling bottle and kept in a refrigerator. The content of Hg, Se, Cr, Fe, etc. was investigated using atomic mass spectrometry measurements in the laboratory of the Department of Nuclear Chemistry, FNSPE CTU in Prague. This type of measurement was carried out at selected locations near the supposed fault.
REVEALING THE HIDDEN FAULTS IN THE SE FLANK OF MT. ETNA
the atmo-geochemical results were present in an area of the investigated fault line.
Figure 4. VLF results from profile No. 1. Re—real and Im—imaginary components of the measured signal, and grad—gradient of the imaginary component.
ACKNOWLEDGEMENTS All measurements were made in cooperation with the National Radiation Protection Institute Prague (NRPI), Universita` degli Studi di Catania and the Istituto Nazionale di Geofisica e Vulcanologia (INGV) Sezione di Catania. The mass spectrometry measurements were carried out in the laboratory of the Department of Nuclear Chemistry FNSPE CTU in Prague. Figure 5. Concentration of radon in soil gas and uranium measured by ionisation chambers compared with the rock resistivity measurement using the ARES system on profile No. 2, Dagala.
REFERENCES 1. Israel, H. and Bjornsson, S. Radon (Rn-222) and thoron (Rn-220) in soil air over faults. Z Geophys. 33, 48–64 (1967). 2. King, C., King, B. and Evans, W. C. Spatial radon anomalies on active faults in California. Appl. Geo. 00, 11 (1996). 3. Vaupoticˇ, J. Indoor radon in Slovenia. Nucl. Technolog. Radiat. Prot. 2, 36–43 (2003). 4. Neri, M., Giammanco, S., Ferrera, E., Patane, G. and Zanon, V. Spatial distribution of soil radon as a tool to recognize active faulting on an active volcano: the example of Mt. Etna. J. Environ. Radioact. 102, 863–870 (2011). 5. Burton, M., Neri, M. and Condarelli, D. High spatial resolution radon measurements reveal hidden active faults on Mt. Etna. Geophs. Lett. 31(7), L07618 (2004). 6. Lentini, F. The geology of the Mt. Etna basement. Mem. Soc. Geol. Ital. 23, 7 –25 (1982). 7. Morelli, D., Imme´, G., La Delfa, S., Nigro, S. and Patane´, G. Soil radon monitoring in the NE flank of Mt. Etna (Sicily). App. Radiat. Isot. 64, 624–629 (2005). 8. Zhou, Z., Tao, S., Xu, F. and Dawson, R. A physical/ mathematical model for the transport of heavy metal and toxic matter from point source by geogas microbubbles. Ecol. Model. 161, 139–149 (2005).
CONCLUSION The measurements carried out on two profiles across the fault line in the Dagala locality produced the following results: † the radon-in-soil concentration at a depth of 0.8 m varied in the ranges of 5–52 kBq m23 (profile No. 1) and 5–31 kBq m23 ( profile No. 2), with the local maximum in the fault line; † this radon concentration corresponded with the CO2 maximum with a shift of 5 m; † the results for the laboratory gamma-spectrometry method showed an average concentration in lava or in soil of 96 Bq kg21 226Ra, 120 Bq kg21 232Th and 605 Bq kg21 40K or 107 Bq kg21 226Ra, 110 Bq kg21 232Th and 505 Bq kg21 40K; † a gradient in Hg, Se and Cr concentrations and a minimum in V, Mn and Fe concentrations from
73
Downloaded from http://rpd.oxfordjournals.org/ at University of Pittsburgh on June 18, 2015
Among all the methods tested here, the in-soil gas radon concentration measurements, CO2 flux, in situ gamma spectrometry, the very low frequency wave method and the rock resistivity measurements proved to be the most effective for revealing hidden faults. At least three of these methods should be applied to acquire sufficient data for comparison. Each method had limitations that had to be taken into account, above all the terrain conditions. Future measurements should be complemented by thoron in-soil gas measurements. The Dagala locality was shown to be a promising location for a continual radon and CO2 gases-in-soil measurement station. The crucial point of measurement in the area of the Etna volcano appears to be the temperature dependence of the measured values.
doi:10.1093/rpd/ncu092
REVEALING THE HIDDEN FAULTS IN THE SE FLANK OF MT. ETNA USING RADON IN-SOIL GAS MEASUREMENT K. Johnova´1, *, L. Thinova´1 and S. Giammanco2 1 Departament of Dosimetry and Application of Ionizing Radiation, FNSPE CTU, Brˇehova´ 7, Prague 1, Czech Republic 2 Istituto Nazionale di Geofisicae Vulcanologia, Sezione di Catania, Piazza Roma, 2, 95123 Catania, Italy *Corresponding author: [email protected]
INTRODUCTION Radon, an inert gas, is one of the components of soil air. It is able to travel over a distance of tens (up to hundreds) of metres under favourable conditions for convection transport. For this reason, radon gas can be used as a tracer for obtaining information about processes in the deep parts of the earth’s crust. Fault systems may facilitate radon transport, due to the presence of more permeable and more penetrable zones along the cracks, and in these situations a significantly higher radon concentration can be measured in the fault system area. In cases where the fault system is filled with clay (or some other) material, radon anomalies occur before or after the fault system itself. Measurements of radon concentrations have previously been used to trace processes connected with degassing of the soil along tectonic faults in various parts of the world(1 – 3), including Mt. Etna(4). Ways to study the dynamics of the faults that lie hidden by recent lava cover were studied in greater detail in ref. (5). Soil radon concentration measurements may provide clear information about the presence of a fault, but in most cases it is difficult to interpret the results. It is sometimes necessary to perform other measurements in conjunction with a radon survey. Fortunately, there are several other methods that can be used for detecting the presence of faults and that can also help to clarify the results of soil radon concentration measurements. Eight methods including measurements of the radon concentration in soil gas were selected to be tested in the conditions of Mt. Etna. The main aim of this work is to evaluate the usability of these methods in the conditions of Mt. Etna, to select the best combination of methods
for identifying fault structures and to find their limitations. As a result of these radon measurements, it should also be possible to identify the best location for installing a permanent continual radon monitoring station. The measurements presented here were carried out in collaboration with the Universita` degli Studi di Catania and the Istituto Nazionale di Geofisica e Vulcanologia Sezione di Catania. LOCALITIES Mt. Etna is a strato-shield volcano located in an area where the European and African plates converge(6). Eruptions in recent times may have resulted from gravitational sliding of the unstable eastern flank towards the Ionian Sea(7). The measurements discussed in this paper were performed at four different locations along the same fault in the SE flank of Mt. Etna. The first set of measurements was collected in 2011 at two locations, Primoti and Santa Venerina, in the middle part of the fault. The Zafferana locality in the upper part of the fault was investigated in 2012. The most recent measurements were carried out in 2013 in the Dagala locality, in the lower part of the fault, where the presence of the fault was considered to be particularly obvious and the interpretation of the data to be the least complicated. Only the results of the most recent measurement (Dagala 2013) are presented in this paper. MEASUREMENT METHODS The measurements on all profiles were performed with sampling at distances between 3 and 5 m, depending
# The Author 2014. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
Downloaded from http://rpd.oxfordjournals.org/ at University of Pittsburgh on June 18, 2015
Although there are many methods for investigating tectonic structures, many faults remain hidden, and they can endanger the life and property of people living along them. The slopes of volcanoes are covered with such hidden faults, near which strong earthquakes and gas releases can appear. Revealing hidden faults can therefore contribute significantly to the protection of people living in volcanic areas. In the study, seven different techniques were used for making measurements of in-soil radon concentrations in order to search for hidden faults on the SE flank of the Mt. Etna volcano. These reported methods had previously been proved to be useful tools for investigating fault structures. The main aim of the experiment presented here was to evaluate the usability of these methods in the geological conditions of the Mt. Etna region, and to find the best place for continual radon monitoring using a permanent station in the near future.
REVEALING THE HIDDEN FAULTS IN THE SE FLANK OF MT. ETNA
and to make a deeper study of the U–Th–K ratios. The soil samples were taken at the locations where the in situ spectrometry was performed, and because of the limited measurement time available in the laboratory, the pre-selection was performed according to the radon concentration results and the electrical resistivity tomography measurements, to optimise the number of samples. A coaxial HPGe (35 %) detector was used for the laboratory gamma-spectrometry measurement of the samples in Marinelli containers with a 0.6 l volume geometry. Each sample was dried, homogenised and hermetically sealed for 1 month in a plastic container to establish equilibrium between 226Ra and 222Rn. The Automatic Resistivity System (ARES—main unit with standard accessories, multi-electrode cable sections MCS5) was used to determine the potential fault system, permeable zones (for correlation with radon, thoron and CO2 measurement) or the presence of water in underground caverns. This system can automatically apply a small electric current at two current electrodes, while at the same time it measures the drop in voltage at two other electrodes and calculates the resistivity. The position of the electrodes depends on the geometry of the measurement. The arrangements of the measurements selected for this experiment were Dipole –Dipole and Schlumberger. The second geo-electrical method is based on measuring the characteristics of long electromagnetic waves (10 –30 kHz) that are transmitted from distant transmitter sites. At a large distance from a transmitter, the electromagnetic waves travel horizontally. When they meet any conductive object, for example, a fault, the waves are disrupted along the conductor body. The largest anomaly is generated when the conductor body is elongated in the direction of the electric field and is perpendicular to the magnetic field, i.e. oriented towards the source of the waves. For the best results, the direction of the transmitter has to be in the direction of the conducting disturbing body. Very low frequencies method (VLF) is used primarily as a reconnaissance tool for identifying anomalous areas for further investigation. Unfortunately, there are only a small number of transmitters working in this frequency band at the present time. The Norwegian station JXN (Helgeland) transmitting waves with 16.4 kHz and the Italian station ICV (Tavolara), 20.27 kHz, were used for the study in the vicinity of Etna. For fault localisation, the gradient of the real component of the signal was calculated. All measurements were carried out with EM 16 and EDA instruments. Atmo-geochemical measurements may be useful for identifying various elements coming from the bedrock. Some heavy metals may occur above the surface in close relation with the presence of magma in the deeper parts of the supposed fault. Although the element transport mechanisms are still not fully understood, this method is being used successfully for 71
Downloaded from http://rpd.oxfordjournals.org/ at University of Pittsburgh on June 18, 2015
on the locations of obstacles, for example, the layout of farm buildings and garden walls, following a preliminary investigation of the area of interest. The direction of the profiles was perpendicular to the expected direction of the main fault; the midpoint of the profiles was located just above the supposed main fault line. The terrain in the selected area was typical for populated areas of Etna: usually stony ground, with a mix of cultivated and uncultivated soil, situated on series of terraces with relatively high differences in elevation. The terrace walls were assembled from basalt rocks and may have influenced the in situ gamma-spectrometry measurements. The presence of roads and some underground service networks had to be taken into account when the results were being evaluated. Measurements of the radon concentration in the soil gas were carried out using the RM-2 measurement system and the ‘lost tip method’. At each sampling point, a special metal pipe was hammered into the ground to the sampling depth of 0.8 m. Samples of soil gas (0.15 l) were taken from this depth, using a syringe equipped with a filter, and were immediately transported to small ionisation chambers (volume 0.25 l, evacuated using a pump). In a complicated rocky soil, the sampling depth was between 0.5 and 0.8 m. The radon activity was measured on site 15 min after taking the sample. The 15 min delay ensured that the thoron activity was negligible. The radon measurement was complemented by a soil permeability measurement in selected locations and by measurements using Lucas chambers 0.5 l in volume. The correlation between the results of in-soil gas radon measurements using small ionising chambers and Lucas cells was R 2 ¼ 0.96. In situ gamma spectrometry is a useful method for identifying differences in rock composition. Differences in the radionuclide ratios may be able to identify changes in the lava massive, due to the presence of a fault, when there is a difference in the age of the lavas or in their origin. The depth range of this method (measurement on the surface) is usually 0.35– 0.50 cm, depending on the density of the soil and the concentration of radionuclides. A portable Gamma Surveyor spectrometer was used for in situ determination of the K (%), U (ppm eU) and Th (ppm eTh) concentrations. This spectrometer consists of a probe (equipped with a 300 ` 300 NaI(Tl) detector), global positioning system (GPS) and a control panel. The spectrometer was calibrated using cylindrical standards at the calibration laboratory in Stra´zˇ pod Ralskem, Czech Republic, for measurements in 2p geometry. It was impossible to maintain the desired geometry in all measurements, and so the results need to be corrected (for example, using Monte Carlo simulations). Laboratory gamma spectrometry is widely used for detailed and precise identification of the radionuclide content of samples. The aim was to obtain a comparison between laboratory results and in situ spectrometry
´ ET AL. K. JOHNOVA (8)
several anomalies in the soil radon concentration on one side of the fault plane (95– 105 m), and there is a very low stable concentration on the other side. The second curve in Figure 1 shows the concentration of CO2 measured by the Italian group of scientists. The maximum radon concentration is complemented by the minimum CO2 flux (location 100 m), and the maximum CO2 flux is at location 105 m. The numerous radon anomalies on the right side of the fault, which were confirmed by resistivity and VLF measurements, demonstrate cracks with no direct contact with the magma and are typical for this part of the lava cover. Most of the measurements were complicated by the rugged terrain. The in situ gamma-spectrometry measurement results are shown in Figure 3, together with an outline of the terrain situation. The influences of the ‘stairs’ near the measurement point were corrected using Monte Carlo simulation. However, there is still an influence from the different composition of the ground (asphalt on the road, stones forming the walls of the stairs), which affect the results shown in Figure 3. The laboratory gamma-spectrometry measurement of samples representing terrain material such as lavas, soil and ash show similar results as in the previous localities: 96 Bq kg21 226Ra, 120 Bq kg21 232Th and 605 Bq kg21 40K in lava or 107 Bq kg21 226Ra, 110 Bq kg21 232Th and 505 Bq kg21 40K in soil. The presence of a road on profile 1 in location 85 m seriously disturbed the results of the VLF method. The road can be identified through the high maximum in the gradient in Figure 4. Figure 5 illustrates the results of the rock resistivity measurement in combination with the concentration of U from the in situ gamma-spectrometry measurement and the radon in-soil gas measurement at profile No. 2, Dagala. The fault line is characterised by the gradient in the radon concentration and the maximum in the U (ppm eU) concentrations.
RESULTS The schematic situation of the profiles at Dagala is shown in Figure 1. Typical radon concentration behaviour measured along profile No. 1 is shown in Figure 2. There are
Figure 1. Profiles across the supposed fault in the Dagala locality, projected onto Google earth maps using GPS coordinates. The white line marks the supposed fault.
Figure 2. A comparison of the radon concentration in the soil gas and the CO2 flux measured in profile No. 1 in Dagala.
Figure 3. Concentration of U ( ppm eU), Th ( ppm eTh), K (%K) found by in situ gamma spectrometry, with a scheme of the terrain situation in profile No. 1, Dagala.
72
Downloaded from http://rpd.oxfordjournals.org/ at University of Pittsburgh on June 18, 2015
exploring mineral deposits . The device consisted of a battery and a pump with a controllable air flow rate; an air circulation system with two Millipore filters connected by silicon tubes; a plastic sampling vessel 50 ml in volume with 10 ml of 0.1 molar HCl solution. The air was bubbled for 5 min at each measurement location from a height of 0.5 m above the earth’s surface. After bubbling, the sample was hermetically sealed into a small plastic sampling bottle and kept in a refrigerator. The content of Hg, Se, Cr, Fe, etc. was investigated using atomic mass spectrometry measurements in the laboratory of the Department of Nuclear Chemistry, FNSPE CTU in Prague. This type of measurement was carried out at selected locations near the supposed fault.
REVEALING THE HIDDEN FAULTS IN THE SE FLANK OF MT. ETNA
the atmo-geochemical results were present in an area of the investigated fault line.
Figure 4. VLF results from profile No. 1. Re—real and Im—imaginary components of the measured signal, and grad—gradient of the imaginary component.
ACKNOWLEDGEMENTS All measurements were made in cooperation with the National Radiation Protection Institute Prague (NRPI), Universita` degli Studi di Catania and the Istituto Nazionale di Geofisica e Vulcanologia (INGV) Sezione di Catania. The mass spectrometry measurements were carried out in the laboratory of the Department of Nuclear Chemistry FNSPE CTU in Prague. Figure 5. Concentration of radon in soil gas and uranium measured by ionisation chambers compared with the rock resistivity measurement using the ARES system on profile No. 2, Dagala.
REFERENCES 1. Israel, H. and Bjornsson, S. Radon (Rn-222) and thoron (Rn-220) in soil air over faults. Z Geophys. 33, 48–64 (1967). 2. King, C., King, B. and Evans, W. C. Spatial radon anomalies on active faults in California. Appl. Geo. 00, 11 (1996). 3. Vaupoticˇ, J. Indoor radon in Slovenia. Nucl. Technolog. Radiat. Prot. 2, 36–43 (2003). 4. Neri, M., Giammanco, S., Ferrera, E., Patane, G. and Zanon, V. Spatial distribution of soil radon as a tool to recognize active faulting on an active volcano: the example of Mt. Etna. J. Environ. Radioact. 102, 863–870 (2011). 5. Burton, M., Neri, M. and Condarelli, D. High spatial resolution radon measurements reveal hidden active faults on Mt. Etna. Geophs. Lett. 31(7), L07618 (2004). 6. Lentini, F. The geology of the Mt. Etna basement. Mem. Soc. Geol. Ital. 23, 7 –25 (1982). 7. Morelli, D., Imme´, G., La Delfa, S., Nigro, S. and Patane´, G. Soil radon monitoring in the NE flank of Mt. Etna (Sicily). App. Radiat. Isot. 64, 624–629 (2005). 8. Zhou, Z., Tao, S., Xu, F. and Dawson, R. A physical/ mathematical model for the transport of heavy metal and toxic matter from point source by geogas microbubbles. Ecol. Model. 161, 139–149 (2005).
CONCLUSION The measurements carried out on two profiles across the fault line in the Dagala locality produced the following results: † the radon-in-soil concentration at a depth of 0.8 m varied in the ranges of 5–52 kBq m23 (profile No. 1) and 5–31 kBq m23 ( profile No. 2), with the local maximum in the fault line; † this radon concentration corresponded with the CO2 maximum with a shift of 5 m; † the results for the laboratory gamma-spectrometry method showed an average concentration in lava or in soil of 96 Bq kg21 226Ra, 120 Bq kg21 232Th and 605 Bq kg21 40K or 107 Bq kg21 226Ra, 110 Bq kg21 232Th and 505 Bq kg21 40K; † a gradient in Hg, Se and Cr concentrations and a minimum in V, Mn and Fe concentrations from
73
Downloaded from http://rpd.oxfordjournals.org/ at University of Pittsburgh on June 18, 2015
Among all the methods tested here, the in-soil gas radon concentration measurements, CO2 flux, in situ gamma spectrometry, the very low frequency wave method and the rock resistivity measurements proved to be the most effective for revealing hidden faults. At least three of these methods should be applied to acquire sufficient data for comparison. Each method had limitations that had to be taken into account, above all the terrain conditions. Future measurements should be complemented by thoron in-soil gas measurements. The Dagala locality was shown to be a promising location for a continual radon and CO2 gases-in-soil measurement station. The crucial point of measurement in the area of the Etna volcano appears to be the temperature dependence of the measured values.
