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salt water interface in the coastal southeastern Sicily
ARTICLE IN PRESS
Continental Shelf Research 26 (2006) 843–851 www.elsevier.com/locate/csr
Geo-electromagnetic survey of the fresh/salt water interface in the coastal southeastern Sicily Evgeny A. Kontara,, Yuri R. Ozorovichb a
P.P. Shirshov Institute of Oceanology, Russian Academy of Sciences, Krasikova 23, Moscow 117218, Russian Federation b Space Research Institute, Russian Academy of Sciences, Moscow, Russian Federation Available online 3 April 2006
Abstract Results of geo-electromagnetic surveys and 3D mapping of the spatial distribution of fresh/salt water interface conducted during the IAEA submarine groundwater discharge (SGD) experiment in March 2002 near the boat basin in Donnalucata in the southeastern Sicily are reported. The high-resolution geo-electromagnetic profile showed the presence of several fresh/salt water horizons with various formation resistivities of geologic media. The geo-electromagnetic data confirmed the observations made by seepage metres that in the central part of the Donnalucata boat basin high seepage rates of recirculated seawater were observed. The 3D spatial distribution of formation resistivities with depth showed a saltwater intrusion at the pier, which acts as a barrier for the transport of fresh water to the sea. The geo-electromagnetic measurements showed spatial and temporal variability of the fresh/salt water interface, as measured formation resistivities were in inverse relationship with the daily tide, showing a nonlinear transformation of the boundary of the fresh/salt water interface in the process of its spreading offshore with time. r 2006 Elsevier Ltd. All rights reserved. Keywords: Geophysical survey; Geo-electric sections; Resistivity; Coastal aquifers; Submarine groundwater discharge; Fresh/salt water interface; Mediterranean Sea; Sicily
1. Introduction One of the most important contemporary challenges for the coastal zone management is the identification, measurement and monitoring of submarine groundwater discharge (SGD) and seawater intrusion, and evaluation of its influence on cumulative impacts of coastal land use decisions over distance and time (Burnett et al., 2002; Kontar Corresponding author. Tel.: +7 095 250 2515;
fax: +7 095 124 5963. E-mail addresses: [email protected] (E.A. Kontar), [email protected] (Y.R. Ozorovich). 0278-4343/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.csr.2005.12.012
et al., 2002a; Lobkovsky et al., 2003). Understanding the fact that subsurface processes are multi-variable by their nature, impose the necessity to use a new generation of non-invasive techniques for groundwater exploration (Ozorovich, 2002), which can be successfully combined with nuclear and isotopic techniques (Povinec et al., 2005a). The integrative method based on the synthesis of these approaches should improve the understanding of the SGD hydrogeological regime at a particular site. High concentration of ions dissolved in seawater (3.5 104 mg/l) enhances its ability to conduct electricity (a conductivity is 50 mS/cm) in comparison with fresh water, which has much lower
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concentration of dissolved ions (o103 mg/l) and therefore a lower conductivity (o1000 mS/cm). The large differences in the electrical properties of fresh and seawater can be therefore used for detection of saline water in an aquifer (and in a geological formation) by geo-electric measurements either in boreholes, on the surface, or in the air. A geo-electric probing of sediments and beach sands has been found as an important complementary tool to isotopic tracers for better understanding of the fresh/salt water interface in coastal areas (e.g. Fitterman and Deszcz-Pan, 1998; Stieglitz, 2005). The probing instruments have been using either direct current measurements (with electrode spacing of 1 cm) surveying surface sediment layers (e.g. Stieglitz et al., 2000), or induced electromagnetic fields which can probe geological formations down to several hundred metres (e.g. Fitterman and Deszcz-Pan, 1998; Sengpiel and Siemon, 1998). The electrical resistivity of a geological formation is described by the Archie’s law (Fitterman and Deszcz-Pan, 2001): r ¼ ar0 Fm ,
(1)
where r is the formation resistivity, r0 is the pore fluid resistivity, F is the porosity, and a and m are constants specific to a formation (a1, whereas m varies with lithology, e.g. m ¼ 1:3921:58 for marine sediments, 1.8–2.0 for sandstones). For a sand or sedimentary rock aquifer with a porosity of 30% and r0 ¼ 0:25 O m (typical of seawater), the formation resistivity would be in the range of 1.2–3.3 O m, whereas the same aquifer filled with fresh water (r0 ¼ 202200 O m) would result in a formation resistivity of 100–2000 O m. This large difference shows the advantages of the electromagnetic probing of fresh/salt water interfaces in coastal regions (Fitterman and Deszcz-Pan, 2001). If the formation factor, F ¼ r=r0 , which accounts for the influence of porosity on the electrical conductivity, can be estimated from borehole measurements, then the formation resistivity values obtained from the geo-electric surveys and laboratory measurements of sand samples can be used for estimation of conductivity of the pore fluid and of the geological formation. If, in addition, the relationship between conductivity and chloride concentration is known, then the chloride content in the formation can be estimated from the geoelectric data as well. The geo-electromagnetic probing can be carried out with two coils (a transmitter and a receiver)
placed on the land, on moving cars, helicopters or planes surveying the site (Stewart et al., 1994; Fitterman and Deszcz-Pan, 1998; Sengpiel and Siemon, 1998). A new geo-electromagnetic probing system based on the time domain electromagnetic sounding technology which uses only one coil has been recently developed by Ozorovich et al. (1999). The system can be used for fresh/salt water interface studies, water search tasks (groundwater table), waste and contamination studies, and monitoring changes in subsurface horizons (Ozorovich, 2001, 2002). The goal of this note to the SGD discussion is to contribute to a fundamental understanding of the physical processes taking place at the dynamic subsurface fresh/salt water interface by carrying out geo-electromagnetic studies on a small-scale which can provide a conceptual framework for understanding of interaction processes on a larger scale and over longer periods of time. The work has been carried out in the framework of the IAEA’s coordinated research project on ‘‘nuclear and isotopic techniques for the characterisation of SGD in coastal zones’’. In the present paper we discuss how a geo-electromagnetic survey can be used to produce resistivity sections of a geological formation. Using correlations between water characteristics (conductivity and chloride content) measured in the laboratory, and field measurements of the formation resistivity we estimated water quality in the studied area. 2. Survey area The Donnalucata study area (Fig. 1) is located in the South-eastern part of Sicily Island. The dominant geological characteristics of the territory identifying aquifers that discharge into the sea along the coastline were described by Aureli (1992). A limestone aquifer in which karstic phenomena have taken on a determining function in conditioning groundwater circulation assumes artesian characteristics. A second alluvial aquifer is partly supplied through lateral contacts and through artesianism by a deep aquifer. The links between tectonics morphology and groundwater circulation are evident. The supply of the main deep aquifer occurs on the Hyblaean Plateau, situated North of the Donnalucata area, which is primarily of carbonate in origin (Triassic–Jurassic and partly Cretaceous). The eastern part has been influenced by a volcanic activity, the western part was formed
ARTICLE IN PRESS E.A. Kontar, Y.R. Ozorovich / Continental Shelf Research 26 (2006) 843–851
Fig. 1. Physical principle of the transient electromagnetic method (TEM). A stationary current flowing in the transmitting loop creates a primary magnetic field. After this current is shut off, a secondary current is induced in the subsurface, which creates secondary magnetic field, measured in the receiving coil.
essentially from carbonate sediments. The most considerable groundwater circulation appears along the zones of cracks, fractures and karst hollows. The frequent presence of carbonate series made up of outcrops of marly or argillaceous–marly interbeds, determines impermeable levels and results in occurrence of springs in the basins of the area. Along the coast, the carbonate aquifers directly unload their waters into the sea producing numerous springs observed on beaches. The groundwater also flows through the faults directly to the sea forming submarine springs, frequently observed in southern (the Donnalucata area) and eastern Sicily (the Siracuse area). 3. Methods 3.1. Geo-electromagnetic survey A portable, geo-electromagnetic sounding instrument, Mars electromagnetic sounding (MARSES), based on the time domain electromagnetic sounding technology was used during the Donnalucata 2002 experiment. Developed within the framework of space research missions, it possesses unique meth-
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odological and technological features and advantages (a portability, PC operation, low weight and low power consumption) (Ozorovich et al., 1999; Kontar et al., 2002b; Lobkovsky et al., 2003). The system is based on the transient electromagnetic method (TEM) sounding technology which enable to conduct subsurface soundings to a depth of 300 m, depending on the used frequency (lower frequencies sense deeper into the ground) and on a geological formation (deeper for a fresh water saturated zone than for a saltwater intruded zone). The physical and mathematical principles of TEM are described by Kamenetsky (1997). The basic aspects of the theory are directly related to the TEM subsurface sounding technology, restricted to the model of the homogeneous half-space. A stationary current flowing in the transmitter loop is creating a primary magnetic field. After this current is shut off, a secondary current is induced in the subsurface, which creates secondary magnetic field, measured in the receiving coil. We shall consider an asymptotic estimation of signals for late and early stages of transience. The development of the electromagnetic signal with time, E(t), for a late stage of the transience t0 ¼ t=ðm0 R2 =rÞb1, for a transmitting coil (with radius R) and a receiving coil (with radius r), lying above the homogeneous half-space with formation resistivity r, magnetic permeability of vacuum m0 and current I (Fig. 1), is described by the formula (Kamenetsky, 1997): 5=2
EðtÞ=I ¼ 0:05ðp1=2 m0 Þ=r3=2 ðr2 R2 Þt5=2 .
(2)
The signal for t=ðm0 R2 =rÞb1 does not depend on the radius of the receiving coil (roR). The formula (2) is also valid for a height h above the surface of the half-space determined by the coil. At a late stage of transience, the signal registered in the receiving antenna is caused by currents induced in the ring inside the section with the effective radius Reff and the depth H eff Reff ¼ ðtr=m0 Þ1=2 , exceeding the radius of the transmitting coil, ReffbR. The vertical magnetic field created by the coil is homogeneous within the limits of its area at hoReff, therefore registered signals which are proportional to derivative of magnetic field over time, do not depend on the place of their receiving. This property of TEM at late stages of transience determines the maximum depth of sounding, which together with good resolution (s3=2 ) is the main advantage of the method.
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At an early stage of transient (t051) and for the identical coils (R ¼ r), the signals do not depend on the resistivity of the media EðtÞ=I ¼ m0 R=ð2tÞ.
(3)
However, for a small receiving antenna (r=R51), the signal is proportional to the resistivity of the media, but does not depend on time: EðtÞ=I ¼ 3prðr2 =R3 Þ.
(4)
Usually a geo-electromagnetic probing system is using a transmitting coil, which is continuously driven by a stationary current, and a receiving coil (antenna) to measure the electromagnetic response.
The advantage of the MARSES system is that one coil can be used both as the transmitter and the antenna. The electromagnetic response is measured at several frequencies (from 1 kHz to 1 MHz). The formation resistivity vs. depth functions, obtained for different sampling points are then integrated into a model, and 3D maps of spatial distribution of formation resistivity with depth are produced (Ozorovich, 2001). The geo-electromagnetic survey area at the Donnalucata coast (Fig. 2) was situated on the sandy beach along the pier (axis Y), the coastline (axis X) and road, which closed the study area from buildings situated on the North. The sounding
Fig. 2. Two geo-electromagnetic survey areas (with antennas 60 60 m) on the beach, where measurements were carried out in the zone between the pier, the coastal line and road. Locations of manual and automated seepage metres and drilled borehole wells are also shown.
ARTICLE IN PRESS E.A. Kontar, Y.R. Ozorovich / Continental Shelf Research 26 (2006) 843–851
antenna was arranged in the form of a square with side length of 60 m. The measurements of the geoelectromagnetic sections were carried out at two sites (Fig. 2), during various cycles of a tidal wave over a period of several days. Borehole wells 1 and 4, and 2 and 3 (drilled on the beach during the expedition) were inside the areas determined by the sounding antennas. Measured salinity of water in borehole wells 1, 2, 3 and 4 was 7.0, 17.0, 3.1 and 1.3, respectively.
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3.2. Laboratory measurements Laboratory geophysical measurements refer to the empirical characterization of physical rock properties such as porosity and permeability, and their relationship with other properties directly measured on the field sites (e.g. a formation resistivity, conductivity of pore water, salinity, etc.). The laboratory measurements were carried out using a special laboratory device, capable to measure resistivity of collected sand samples from the study are in the frequency interval from several kHz up to 1 MHz. The instrument (Fig. 3) is measuring the formation resistivity and the dielectric constant using a method of a variation of the inter-electrode distance at two frequencies of the electromagnetic field. Sand sample collected in borehole well 3 (Table 1) had the water content of 31.4% vol, while sand sample collected in borehole well 2 had higher water content, 38.8% vol The measured formation resistivities of samples 3 and 2 were 47.270.5 and 37.070.5 O m, respectively, showing a higher value at a lower dielectric permittivity (and a lower water content), documenting that the pore water in the sample A is fresh water (the chloride content and conductivity of pore water was 500 mg/l and 1000 mS/cm, respectively). On the basis of the laboratory measurements combined with field measurements of pore-water conductivity, chloride content and formation resistivity of borehole wells, it was possible to construct a dependence of the formation resistivity on the pore-water conductivity (or a chloride content, showing a mineralisation of the formation) for the sounding area (Fig. 4). Using this graph, a Table 1 Results of laboratory measurements of collected sand samples
Fig. 3. Condenser with varying interelectrode distance for measurement of resistivity and dielectric constant in the frequency range 30 KHz–500 MHz. 1—press nuts; 2—support for installation of a micrometre; 3—aperture for selection of a superfluous liquid; 4—glass cylinder; 5—mobile electrode; 6— sand sample; 7—non-mobile electrode; 8—protection electrode. Dimensions are given in mm.
Parameter
Well 3
Well 2
Porosity (%) Water content (% vol) Water content (% weight) Sand content (%) Air content (%) Dielectric permittivity (rel.units) (frequency 1 MHz) Dielectric permittivity (rel. units) (frequency 300 kHz) Formation resistivity (O m) (frequency 1 MHz) Formation resistivity (O m) (frequency 300 kHz)
42.570.4 31.470.3 20.670.3 57.5 11.1 19.370.5
38.870.2 38.870.2 23.970.1 61.2 0.0 24.270.7
19.771.7
26.375.3
47.270.5
36.970.5
47.270.6
37.170.7
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pore-water conductivity (a chloride content) can be estimated from the formation resistivity measured by the geo-electromagnetic survey. A similar inversion solution of the transfer of TEM geo-electro-
magnetic data to physical parameters was done by Fitterman and Deszcz-Pan (2001) for a study area in the South Florida.
4. Results and discussion
Fig. 4. Relationship between the formation resistivity, porewater conductivity and chloride content in pore-water (for a fraction 0.25odo0.5 mm, full water saturated sands), based on laboratory and field measurements of sands from the study area in the frequency range of 0.3–10 MHz.
The analysis of high-resolution geo-electromagnetic data (an example of a real field sounding curve with inversion results is presented in Fig. 5) obtained for a site located between wells 2 and 3 showed the presence of several horizons with various formation resistivities of subsurface waters. The first horizon (up to 5 m depth) represents a fresh water saturated zone with formation resistivity of 155 O m. From 5 to 23 m the formation resistivity decreased to 5.4 O m, so this horizon can be considered as a subsurface saltwater interface zone. The next horizon (between 70 and 85 m) with the formation resistivity of 3 O m represents again a saltwater horizon (a saltwater intrusion), which is, however, distinct from the top horizon. The top and bottom mineralized layers are separated with
Fig. 5. Example of a real field sounding curve with inversion results of the formation resistivity profile, measured between borehole wells 2 and 3.
ARTICLE IN PRESS E.A. Kontar, Y.R. Ozorovich / Continental Shelf Research 26 (2006) 843–851
volcanic rock with the formation resistivity of 360 O m. On the bases of the inversion solution of all geoelectromagnetic field data, a 3D model (Fig. 6) of the spatial distribution of formation resistivities was developed for the area between sites A3 and A6 (coordinates (0;90) and (0;0) in Fig. 6 are fixed at the left bottom corner of the left antenna, and at the right bottom corner of the right antenna, respectively, Fig. 2). The top 25 m represents a zone with the formation resistivity between 60 and 330 O m (expected pore water conductivity of 150–850 mS/ cm), representing a fresh water-saturated zone, with
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water flowing from the road area to the sea. However, closer to the pier a saltwater intrusion can be observed. The pier acts as a barrier for the transport of fresh water to the sea, i.e. it has blocked a superficial drain. The saltwater horizon is located at the depth between 40 and 80 m, at the right corner of the pier (Fig. 2), having a formation resistivity between 1 and 30 O m. Below a depth of 80 m a fresh water horizon is seen again, which may represent a deeper groundwater aquifer. The geo-electromagnetic data obtained for the Donnalucata boat basin confirms observations made by the seepage metres that highest SGD
Fig. 6. A 3D image of the spatial distribution of the formation resistivity with depth in the study area between sites A3, A6 and the road (coordinates (0;90) and (0;0) are fixed at the left bottom corner of the left antenna, and the right bottom corner of the right antenna, respectively). The pier is parallel along the axis Y; the coastal line is along the axis X.
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fluxes were observed at site A3, up to 35 cm/day (Taniguchi, 2005). In situ measurements of salinity and 222Rn concentration at this site showed that the discharged water is salty water (salinity 38.7, the highest one in the boat basin) with low 222Rn concentration (0.2 kBq/m3) (Povinec et al., 2005b). This would indicate that the SGD observed at this site is represented by a re-circulated seawater, in agreement with the formation resistivity profile shown in Fig. 5. The geo-electromagnetic measurements were carried out during several days at various tide levels. They revealed a spatial variability of the fresh/salt water interface with time, showing an inverse relationship with the daily tide. A negative correlation between SGD and sea level changes, as observed by measurements of seepage rates in the Donnalucata boat basin have been attributed to the tidal effect, when a greater hydraulic gradient is observed at low tide, and a smaller one at high tide, causing increasing SGD fluxes during decreasing sea level, and opposite (Taniguchi, 2005). The tidal effect observed in the spatial distribution of formation resistivities (Fig. 5) showed a nonlinear transformation of the boundary of the fresh/salt water interface with time in the process of its spreading offshore. 5. Conclusions The geo-electromagnetic sounding in combination with nuclear and isotopic techniques is a useful tool for investigation of SGD, as it can provide additional information on the spatial and temporal fluctuations of the fresh/salt water interface in the coastal zone. Several observations were made on the basis of the inspection of the geo-electromagnetic data obtained for the Donnalucata coast:
The high-resolution geo-electromagnetic profile showed the presence of several horizons with various formation resistivities of the geologic media. The geo-electromagnetic data confirms the observations made by the seepage metres that highest SGD fluxes were observed in the central part of the basin, as well as in situ measurements of salinity and 222Rn concentration, which showed that the discharged water is recirculated seawater. The 3D spatial distribution of formation resistivities with depth helped to identify in the area close to the pier a saltwater intrusion, as the pier
acts as a barrier for the transport of fresh water to the sea. Below a depth of 80 m a fresh water horizon is seen again, which may represent a deeper groundwater aquifer. The geo-electromagnetic measurements have revealed spatial and temporal variability of the fresh/salt water interface in the coastal zone. The measured formation resistivities were in inverse relationship with the daily tide, showing a nonlinear transformation of the boundary of the fresh/salt water interface in the process of its spreading offshore with time.
The geo-electromagnetic surveys helped to expand information on spatial and temporal variations in the fresh/salt water interface, its structural and geological properties, which are not possible to obtain using other techniques. Thus we have an opportunity to construct complex measures in the salt/fresh water interface, as a synergy of in situ isotope and geophysical measurements. Further developments in the geo-electromagnetic techniques are in progress which include a new software for interpretation of 3D data and modelling of the spreading of the fresh/salt water interface with time, as well as development of a new survey technique based on simultaneous measurements with four sounding loops for high-resolution spatial and temporal studies of geo-electromagnetic sections. References Aureli, A., 1992. The submarine springs in Sicily. Report of the Instituto di Geologia e Geofisica Universita di Catania, Sicily, 21pp. Burnett, W.C., Chanton, J., Christoff, J., Kontar, E.A., Krupa, S., Lambert, M., Moore, W., O’Rourke, D., Paulsen, R., Smith, C., Smith, L., Taniguchi, M., 2002. Assessing methodologies for measuring groundwater discharge to the ocean. EOS 83, 122–123. Fitterman, D., Deszcz-Pan, M., 1998. Helicopter EM mapping of saltwater intrusion in Everglades National Park, Florida. Exploration Geophysics 29, 240–243. Fitterman, D., Deszcz-Pan, M., 2001. Saltwater intrusion in Everglades National Park, Florida, measured by airborne electromagnetic surveys. First International conference on Saltwater Intrusion and Coastal Aquifers Monitoring, Modeling and Management. 23–25 April, Essaouira, Morocco, pp. X1–X7. Kamenetsky, F.M., 1997. Transient geo-electromagnetics. GEOS, Moscow. In: Burnett, W.C., Chanton, J.P., Kontar, E.A. (Eds.), Submarine Groundwater Discharge, Biogeochemistry, vol. 66, 2003. Kluwer Academic Publishers, The Netherlands, 202pp.
ARTICLE IN PRESS E.A. Kontar, Y.R. Ozorovich / Continental Shelf Research 26 (2006) 843–851 Kontar, E.A., Burnett, W.C., Povinec, P.P., 2002a. Submarine groundwater discharge and its influence on hydrological trends in the Mediterranean Sea. Proceedings of the CIESM Workshop: Tracking Long Term Hydrological Change in the Mediterranean Sea, 22–24 April 2002, Monaco. pp. 109–114. Kontar, E.A., Ozorovich, Yu.R., Salokhiddinov, A., Azhigaliyev, Ye.B., 2002b. Study of groundwater–seawater interactions in the Aral Sea Basin. Proceedings of the International Conference on Low-lying Coastal Areas—Hydrology and Integrated Coastal Zone Management, 9–12 September 2002, Bremerhaven, Germany. pp. 225–230. Lobkovsky, L.I., Kontar, E.A., Garagash, I., Ozorovich, Yu.R., 2003. Monitors and methods for investigation of submarine landslides, seawater intrusion and contaminated groundwater discharge as coastal hazards. In: Beer, T., Ismail-Zadeh, A. (Eds.), Risk Science and Sustainability: Science for Reduction of Risk and Sustainable Development of Society. Kluwer Publishers, NATO, pp. 191–207. Ozorovich, Yu., 2001. Operational system for groundwater and pollution determination and monitoring. In: A New Hydrology For the A Thirsty Planet. Proceedings of IAHS Scientific Assembly, 18–27 July 2001, Maastricht, The Netherlands. Ozorovich, Yu.R., 2002. Operational system for groundwater and pollution determination and monitoring into the coastal zones. Proceedings of the SWIM17—17th Salt Water Intrusion Meeting, May 2002, Delft. Ozorovich, Y.R., Linkin, V.M., Smythe, W., 1999. Mars electromagnetic sounding experiment—MARSES. Proceedings of LPI Conference, Houston. pp. 97–101.
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Povinec, P.P., Aggarwal, P.K., Aureli, A., Burnett, W.C., Kontar, E.A., Kulkarni, K.M., Moore, W.S., Rajar, R., Taniguchi, M., Comanducci, J.-F., Cusimano, G., Dulaiova, H., Gatto, L., Groening, M., Hauser, S., Levy-Palomo, I., Oregioni, B., Ozorovich, Y.R., Privitera, A.M.G., Schiavo, M.A., 2005a. Characterisation of submarine groundwater discharge (SGD) offshore south-eastern Sicily—IAEA-UNESCO SGD collaboration. Journal of Environmental Radioactivity (submitted for publication). Povinec, P.P., Comanducci, J.-F., Levy-Palomo, I. Oregioni, B., 2005b. Monitoring of submarine groundwater discharge along the Donnalucata coast in the south-eastern Sicily using underwater gamma-ray spectrometry. Continental Shelf Research, this issue, doi:10.1016/j.csr.2005.12.009. Sengpiel, K.P., Siemon, B., 1998. Examples of 1-D inversion of multifrequency HEM data from 3-D resistivity distributions. Exploration Geophysics 29, 133–141. Stewart, D.C., Anderson, W.L., Grover, T.P., Labson, V.F., 1994. Shallow subsurface mapping by electromagnetic sounding in the 300 kHz to 30 MHz range: model studies and prototype system assessment. Geophysics 25, 1201–1210. Stieglitz, T., 2005. Submarine groundwater discharge into the near-shore zone of the Great Barrier reef, Australia. Marine Pollution Bulletin 51, 51–59. Stieglitz, T., Ridd, P.V., Hollins, S., 2000. A small sensor for detecting animal burrows and monitoring burrow water conductivity. Wetlands Ecology and Management 8, 1–7. Taniguchi, M., 2005. Submarine groundwater discharge measured by seepage meters in Sicilian coastal waters. Continental Shelf Research, this issue, doi:10.1016/j.csr.2005.12.002.
Continental Shelf Research 26 (2006) 843–851 www.elsevier.com/locate/csr
Geo-electromagnetic survey of the fresh/salt water interface in the coastal southeastern Sicily Evgeny A. Kontara,, Yuri R. Ozorovichb a
P.P. Shirshov Institute of Oceanology, Russian Academy of Sciences, Krasikova 23, Moscow 117218, Russian Federation b Space Research Institute, Russian Academy of Sciences, Moscow, Russian Federation Available online 3 April 2006
Abstract Results of geo-electromagnetic surveys and 3D mapping of the spatial distribution of fresh/salt water interface conducted during the IAEA submarine groundwater discharge (SGD) experiment in March 2002 near the boat basin in Donnalucata in the southeastern Sicily are reported. The high-resolution geo-electromagnetic profile showed the presence of several fresh/salt water horizons with various formation resistivities of geologic media. The geo-electromagnetic data confirmed the observations made by seepage metres that in the central part of the Donnalucata boat basin high seepage rates of recirculated seawater were observed. The 3D spatial distribution of formation resistivities with depth showed a saltwater intrusion at the pier, which acts as a barrier for the transport of fresh water to the sea. The geo-electromagnetic measurements showed spatial and temporal variability of the fresh/salt water interface, as measured formation resistivities were in inverse relationship with the daily tide, showing a nonlinear transformation of the boundary of the fresh/salt water interface in the process of its spreading offshore with time. r 2006 Elsevier Ltd. All rights reserved. Keywords: Geophysical survey; Geo-electric sections; Resistivity; Coastal aquifers; Submarine groundwater discharge; Fresh/salt water interface; Mediterranean Sea; Sicily
1. Introduction One of the most important contemporary challenges for the coastal zone management is the identification, measurement and monitoring of submarine groundwater discharge (SGD) and seawater intrusion, and evaluation of its influence on cumulative impacts of coastal land use decisions over distance and time (Burnett et al., 2002; Kontar Corresponding author. Tel.: +7 095 250 2515;
fax: +7 095 124 5963. E-mail addresses: [email protected] (E.A. Kontar), [email protected] (Y.R. Ozorovich). 0278-4343/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.csr.2005.12.012
et al., 2002a; Lobkovsky et al., 2003). Understanding the fact that subsurface processes are multi-variable by their nature, impose the necessity to use a new generation of non-invasive techniques for groundwater exploration (Ozorovich, 2002), which can be successfully combined with nuclear and isotopic techniques (Povinec et al., 2005a). The integrative method based on the synthesis of these approaches should improve the understanding of the SGD hydrogeological regime at a particular site. High concentration of ions dissolved in seawater (3.5 104 mg/l) enhances its ability to conduct electricity (a conductivity is 50 mS/cm) in comparison with fresh water, which has much lower
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concentration of dissolved ions (o103 mg/l) and therefore a lower conductivity (o1000 mS/cm). The large differences in the electrical properties of fresh and seawater can be therefore used for detection of saline water in an aquifer (and in a geological formation) by geo-electric measurements either in boreholes, on the surface, or in the air. A geo-electric probing of sediments and beach sands has been found as an important complementary tool to isotopic tracers for better understanding of the fresh/salt water interface in coastal areas (e.g. Fitterman and Deszcz-Pan, 1998; Stieglitz, 2005). The probing instruments have been using either direct current measurements (with electrode spacing of 1 cm) surveying surface sediment layers (e.g. Stieglitz et al., 2000), or induced electromagnetic fields which can probe geological formations down to several hundred metres (e.g. Fitterman and Deszcz-Pan, 1998; Sengpiel and Siemon, 1998). The electrical resistivity of a geological formation is described by the Archie’s law (Fitterman and Deszcz-Pan, 2001): r ¼ ar0 Fm ,
(1)
where r is the formation resistivity, r0 is the pore fluid resistivity, F is the porosity, and a and m are constants specific to a formation (a1, whereas m varies with lithology, e.g. m ¼ 1:3921:58 for marine sediments, 1.8–2.0 for sandstones). For a sand or sedimentary rock aquifer with a porosity of 30% and r0 ¼ 0:25 O m (typical of seawater), the formation resistivity would be in the range of 1.2–3.3 O m, whereas the same aquifer filled with fresh water (r0 ¼ 202200 O m) would result in a formation resistivity of 100–2000 O m. This large difference shows the advantages of the electromagnetic probing of fresh/salt water interfaces in coastal regions (Fitterman and Deszcz-Pan, 2001). If the formation factor, F ¼ r=r0 , which accounts for the influence of porosity on the electrical conductivity, can be estimated from borehole measurements, then the formation resistivity values obtained from the geo-electric surveys and laboratory measurements of sand samples can be used for estimation of conductivity of the pore fluid and of the geological formation. If, in addition, the relationship between conductivity and chloride concentration is known, then the chloride content in the formation can be estimated from the geoelectric data as well. The geo-electromagnetic probing can be carried out with two coils (a transmitter and a receiver)
placed on the land, on moving cars, helicopters or planes surveying the site (Stewart et al., 1994; Fitterman and Deszcz-Pan, 1998; Sengpiel and Siemon, 1998). A new geo-electromagnetic probing system based on the time domain electromagnetic sounding technology which uses only one coil has been recently developed by Ozorovich et al. (1999). The system can be used for fresh/salt water interface studies, water search tasks (groundwater table), waste and contamination studies, and monitoring changes in subsurface horizons (Ozorovich, 2001, 2002). The goal of this note to the SGD discussion is to contribute to a fundamental understanding of the physical processes taking place at the dynamic subsurface fresh/salt water interface by carrying out geo-electromagnetic studies on a small-scale which can provide a conceptual framework for understanding of interaction processes on a larger scale and over longer periods of time. The work has been carried out in the framework of the IAEA’s coordinated research project on ‘‘nuclear and isotopic techniques for the characterisation of SGD in coastal zones’’. In the present paper we discuss how a geo-electromagnetic survey can be used to produce resistivity sections of a geological formation. Using correlations between water characteristics (conductivity and chloride content) measured in the laboratory, and field measurements of the formation resistivity we estimated water quality in the studied area. 2. Survey area The Donnalucata study area (Fig. 1) is located in the South-eastern part of Sicily Island. The dominant geological characteristics of the territory identifying aquifers that discharge into the sea along the coastline were described by Aureli (1992). A limestone aquifer in which karstic phenomena have taken on a determining function in conditioning groundwater circulation assumes artesian characteristics. A second alluvial aquifer is partly supplied through lateral contacts and through artesianism by a deep aquifer. The links between tectonics morphology and groundwater circulation are evident. The supply of the main deep aquifer occurs on the Hyblaean Plateau, situated North of the Donnalucata area, which is primarily of carbonate in origin (Triassic–Jurassic and partly Cretaceous). The eastern part has been influenced by a volcanic activity, the western part was formed
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Fig. 1. Physical principle of the transient electromagnetic method (TEM). A stationary current flowing in the transmitting loop creates a primary magnetic field. After this current is shut off, a secondary current is induced in the subsurface, which creates secondary magnetic field, measured in the receiving coil.
essentially from carbonate sediments. The most considerable groundwater circulation appears along the zones of cracks, fractures and karst hollows. The frequent presence of carbonate series made up of outcrops of marly or argillaceous–marly interbeds, determines impermeable levels and results in occurrence of springs in the basins of the area. Along the coast, the carbonate aquifers directly unload their waters into the sea producing numerous springs observed on beaches. The groundwater also flows through the faults directly to the sea forming submarine springs, frequently observed in southern (the Donnalucata area) and eastern Sicily (the Siracuse area). 3. Methods 3.1. Geo-electromagnetic survey A portable, geo-electromagnetic sounding instrument, Mars electromagnetic sounding (MARSES), based on the time domain electromagnetic sounding technology was used during the Donnalucata 2002 experiment. Developed within the framework of space research missions, it possesses unique meth-
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odological and technological features and advantages (a portability, PC operation, low weight and low power consumption) (Ozorovich et al., 1999; Kontar et al., 2002b; Lobkovsky et al., 2003). The system is based on the transient electromagnetic method (TEM) sounding technology which enable to conduct subsurface soundings to a depth of 300 m, depending on the used frequency (lower frequencies sense deeper into the ground) and on a geological formation (deeper for a fresh water saturated zone than for a saltwater intruded zone). The physical and mathematical principles of TEM are described by Kamenetsky (1997). The basic aspects of the theory are directly related to the TEM subsurface sounding technology, restricted to the model of the homogeneous half-space. A stationary current flowing in the transmitter loop is creating a primary magnetic field. After this current is shut off, a secondary current is induced in the subsurface, which creates secondary magnetic field, measured in the receiving coil. We shall consider an asymptotic estimation of signals for late and early stages of transience. The development of the electromagnetic signal with time, E(t), for a late stage of the transience t0 ¼ t=ðm0 R2 =rÞb1, for a transmitting coil (with radius R) and a receiving coil (with radius r), lying above the homogeneous half-space with formation resistivity r, magnetic permeability of vacuum m0 and current I (Fig. 1), is described by the formula (Kamenetsky, 1997): 5=2
EðtÞ=I ¼ 0:05ðp1=2 m0 Þ=r3=2 ðr2 R2 Þt5=2 .
(2)
The signal for t=ðm0 R2 =rÞb1 does not depend on the radius of the receiving coil (roR). The formula (2) is also valid for a height h above the surface of the half-space determined by the coil. At a late stage of transience, the signal registered in the receiving antenna is caused by currents induced in the ring inside the section with the effective radius Reff and the depth H eff Reff ¼ ðtr=m0 Þ1=2 , exceeding the radius of the transmitting coil, ReffbR. The vertical magnetic field created by the coil is homogeneous within the limits of its area at hoReff, therefore registered signals which are proportional to derivative of magnetic field over time, do not depend on the place of their receiving. This property of TEM at late stages of transience determines the maximum depth of sounding, which together with good resolution (s3=2 ) is the main advantage of the method.
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At an early stage of transient (t051) and for the identical coils (R ¼ r), the signals do not depend on the resistivity of the media EðtÞ=I ¼ m0 R=ð2tÞ.
(3)
However, for a small receiving antenna (r=R51), the signal is proportional to the resistivity of the media, but does not depend on time: EðtÞ=I ¼ 3prðr2 =R3 Þ.
(4)
Usually a geo-electromagnetic probing system is using a transmitting coil, which is continuously driven by a stationary current, and a receiving coil (antenna) to measure the electromagnetic response.
The advantage of the MARSES system is that one coil can be used both as the transmitter and the antenna. The electromagnetic response is measured at several frequencies (from 1 kHz to 1 MHz). The formation resistivity vs. depth functions, obtained for different sampling points are then integrated into a model, and 3D maps of spatial distribution of formation resistivity with depth are produced (Ozorovich, 2001). The geo-electromagnetic survey area at the Donnalucata coast (Fig. 2) was situated on the sandy beach along the pier (axis Y), the coastline (axis X) and road, which closed the study area from buildings situated on the North. The sounding
Fig. 2. Two geo-electromagnetic survey areas (with antennas 60 60 m) on the beach, where measurements were carried out in the zone between the pier, the coastal line and road. Locations of manual and automated seepage metres and drilled borehole wells are also shown.
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antenna was arranged in the form of a square with side length of 60 m. The measurements of the geoelectromagnetic sections were carried out at two sites (Fig. 2), during various cycles of a tidal wave over a period of several days. Borehole wells 1 and 4, and 2 and 3 (drilled on the beach during the expedition) were inside the areas determined by the sounding antennas. Measured salinity of water in borehole wells 1, 2, 3 and 4 was 7.0, 17.0, 3.1 and 1.3, respectively.
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3.2. Laboratory measurements Laboratory geophysical measurements refer to the empirical characterization of physical rock properties such as porosity and permeability, and their relationship with other properties directly measured on the field sites (e.g. a formation resistivity, conductivity of pore water, salinity, etc.). The laboratory measurements were carried out using a special laboratory device, capable to measure resistivity of collected sand samples from the study are in the frequency interval from several kHz up to 1 MHz. The instrument (Fig. 3) is measuring the formation resistivity and the dielectric constant using a method of a variation of the inter-electrode distance at two frequencies of the electromagnetic field. Sand sample collected in borehole well 3 (Table 1) had the water content of 31.4% vol, while sand sample collected in borehole well 2 had higher water content, 38.8% vol The measured formation resistivities of samples 3 and 2 were 47.270.5 and 37.070.5 O m, respectively, showing a higher value at a lower dielectric permittivity (and a lower water content), documenting that the pore water in the sample A is fresh water (the chloride content and conductivity of pore water was 500 mg/l and 1000 mS/cm, respectively). On the basis of the laboratory measurements combined with field measurements of pore-water conductivity, chloride content and formation resistivity of borehole wells, it was possible to construct a dependence of the formation resistivity on the pore-water conductivity (or a chloride content, showing a mineralisation of the formation) for the sounding area (Fig. 4). Using this graph, a Table 1 Results of laboratory measurements of collected sand samples
Fig. 3. Condenser with varying interelectrode distance for measurement of resistivity and dielectric constant in the frequency range 30 KHz–500 MHz. 1—press nuts; 2—support for installation of a micrometre; 3—aperture for selection of a superfluous liquid; 4—glass cylinder; 5—mobile electrode; 6— sand sample; 7—non-mobile electrode; 8—protection electrode. Dimensions are given in mm.
Parameter
Well 3
Well 2
Porosity (%) Water content (% vol) Water content (% weight) Sand content (%) Air content (%) Dielectric permittivity (rel.units) (frequency 1 MHz) Dielectric permittivity (rel. units) (frequency 300 kHz) Formation resistivity (O m) (frequency 1 MHz) Formation resistivity (O m) (frequency 300 kHz)
42.570.4 31.470.3 20.670.3 57.5 11.1 19.370.5
38.870.2 38.870.2 23.970.1 61.2 0.0 24.270.7
19.771.7
26.375.3
47.270.5
36.970.5
47.270.6
37.170.7
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pore-water conductivity (a chloride content) can be estimated from the formation resistivity measured by the geo-electromagnetic survey. A similar inversion solution of the transfer of TEM geo-electro-
magnetic data to physical parameters was done by Fitterman and Deszcz-Pan (2001) for a study area in the South Florida.
4. Results and discussion
Fig. 4. Relationship between the formation resistivity, porewater conductivity and chloride content in pore-water (for a fraction 0.25odo0.5 mm, full water saturated sands), based on laboratory and field measurements of sands from the study area in the frequency range of 0.3–10 MHz.
The analysis of high-resolution geo-electromagnetic data (an example of a real field sounding curve with inversion results is presented in Fig. 5) obtained for a site located between wells 2 and 3 showed the presence of several horizons with various formation resistivities of subsurface waters. The first horizon (up to 5 m depth) represents a fresh water saturated zone with formation resistivity of 155 O m. From 5 to 23 m the formation resistivity decreased to 5.4 O m, so this horizon can be considered as a subsurface saltwater interface zone. The next horizon (between 70 and 85 m) with the formation resistivity of 3 O m represents again a saltwater horizon (a saltwater intrusion), which is, however, distinct from the top horizon. The top and bottom mineralized layers are separated with
Fig. 5. Example of a real field sounding curve with inversion results of the formation resistivity profile, measured between borehole wells 2 and 3.
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volcanic rock with the formation resistivity of 360 O m. On the bases of the inversion solution of all geoelectromagnetic field data, a 3D model (Fig. 6) of the spatial distribution of formation resistivities was developed for the area between sites A3 and A6 (coordinates (0;90) and (0;0) in Fig. 6 are fixed at the left bottom corner of the left antenna, and at the right bottom corner of the right antenna, respectively, Fig. 2). The top 25 m represents a zone with the formation resistivity between 60 and 330 O m (expected pore water conductivity of 150–850 mS/ cm), representing a fresh water-saturated zone, with
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water flowing from the road area to the sea. However, closer to the pier a saltwater intrusion can be observed. The pier acts as a barrier for the transport of fresh water to the sea, i.e. it has blocked a superficial drain. The saltwater horizon is located at the depth between 40 and 80 m, at the right corner of the pier (Fig. 2), having a formation resistivity between 1 and 30 O m. Below a depth of 80 m a fresh water horizon is seen again, which may represent a deeper groundwater aquifer. The geo-electromagnetic data obtained for the Donnalucata boat basin confirms observations made by the seepage metres that highest SGD
Fig. 6. A 3D image of the spatial distribution of the formation resistivity with depth in the study area between sites A3, A6 and the road (coordinates (0;90) and (0;0) are fixed at the left bottom corner of the left antenna, and the right bottom corner of the right antenna, respectively). The pier is parallel along the axis Y; the coastal line is along the axis X.
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fluxes were observed at site A3, up to 35 cm/day (Taniguchi, 2005). In situ measurements of salinity and 222Rn concentration at this site showed that the discharged water is salty water (salinity 38.7, the highest one in the boat basin) with low 222Rn concentration (0.2 kBq/m3) (Povinec et al., 2005b). This would indicate that the SGD observed at this site is represented by a re-circulated seawater, in agreement with the formation resistivity profile shown in Fig. 5. The geo-electromagnetic measurements were carried out during several days at various tide levels. They revealed a spatial variability of the fresh/salt water interface with time, showing an inverse relationship with the daily tide. A negative correlation between SGD and sea level changes, as observed by measurements of seepage rates in the Donnalucata boat basin have been attributed to the tidal effect, when a greater hydraulic gradient is observed at low tide, and a smaller one at high tide, causing increasing SGD fluxes during decreasing sea level, and opposite (Taniguchi, 2005). The tidal effect observed in the spatial distribution of formation resistivities (Fig. 5) showed a nonlinear transformation of the boundary of the fresh/salt water interface with time in the process of its spreading offshore. 5. Conclusions The geo-electromagnetic sounding in combination with nuclear and isotopic techniques is a useful tool for investigation of SGD, as it can provide additional information on the spatial and temporal fluctuations of the fresh/salt water interface in the coastal zone. Several observations were made on the basis of the inspection of the geo-electromagnetic data obtained for the Donnalucata coast:
The high-resolution geo-electromagnetic profile showed the presence of several horizons with various formation resistivities of the geologic media. The geo-electromagnetic data confirms the observations made by the seepage metres that highest SGD fluxes were observed in the central part of the basin, as well as in situ measurements of salinity and 222Rn concentration, which showed that the discharged water is recirculated seawater. The 3D spatial distribution of formation resistivities with depth helped to identify in the area close to the pier a saltwater intrusion, as the pier
acts as a barrier for the transport of fresh water to the sea. Below a depth of 80 m a fresh water horizon is seen again, which may represent a deeper groundwater aquifer. The geo-electromagnetic measurements have revealed spatial and temporal variability of the fresh/salt water interface in the coastal zone. The measured formation resistivities were in inverse relationship with the daily tide, showing a nonlinear transformation of the boundary of the fresh/salt water interface in the process of its spreading offshore with time.
The geo-electromagnetic surveys helped to expand information on spatial and temporal variations in the fresh/salt water interface, its structural and geological properties, which are not possible to obtain using other techniques. Thus we have an opportunity to construct complex measures in the salt/fresh water interface, as a synergy of in situ isotope and geophysical measurements. Further developments in the geo-electromagnetic techniques are in progress which include a new software for interpretation of 3D data and modelling of the spreading of the fresh/salt water interface with time, as well as development of a new survey technique based on simultaneous measurements with four sounding loops for high-resolution spatial and temporal studies of geo-electromagnetic sections. References Aureli, A., 1992. The submarine springs in Sicily. Report of the Instituto di Geologia e Geofisica Universita di Catania, Sicily, 21pp. Burnett, W.C., Chanton, J., Christoff, J., Kontar, E.A., Krupa, S., Lambert, M., Moore, W., O’Rourke, D., Paulsen, R., Smith, C., Smith, L., Taniguchi, M., 2002. Assessing methodologies for measuring groundwater discharge to the ocean. EOS 83, 122–123. Fitterman, D., Deszcz-Pan, M., 1998. Helicopter EM mapping of saltwater intrusion in Everglades National Park, Florida. Exploration Geophysics 29, 240–243. Fitterman, D., Deszcz-Pan, M., 2001. Saltwater intrusion in Everglades National Park, Florida, measured by airborne electromagnetic surveys. First International conference on Saltwater Intrusion and Coastal Aquifers Monitoring, Modeling and Management. 23–25 April, Essaouira, Morocco, pp. X1–X7. Kamenetsky, F.M., 1997. Transient geo-electromagnetics. GEOS, Moscow. In: Burnett, W.C., Chanton, J.P., Kontar, E.A. (Eds.), Submarine Groundwater Discharge, Biogeochemistry, vol. 66, 2003. Kluwer Academic Publishers, The Netherlands, 202pp.
ARTICLE IN PRESS E.A. Kontar, Y.R. Ozorovich / Continental Shelf Research 26 (2006) 843–851 Kontar, E.A., Burnett, W.C., Povinec, P.P., 2002a. Submarine groundwater discharge and its influence on hydrological trends in the Mediterranean Sea. Proceedings of the CIESM Workshop: Tracking Long Term Hydrological Change in the Mediterranean Sea, 22–24 April 2002, Monaco. pp. 109–114. Kontar, E.A., Ozorovich, Yu.R., Salokhiddinov, A., Azhigaliyev, Ye.B., 2002b. Study of groundwater–seawater interactions in the Aral Sea Basin. Proceedings of the International Conference on Low-lying Coastal Areas—Hydrology and Integrated Coastal Zone Management, 9–12 September 2002, Bremerhaven, Germany. pp. 225–230. Lobkovsky, L.I., Kontar, E.A., Garagash, I., Ozorovich, Yu.R., 2003. Monitors and methods for investigation of submarine landslides, seawater intrusion and contaminated groundwater discharge as coastal hazards. In: Beer, T., Ismail-Zadeh, A. (Eds.), Risk Science and Sustainability: Science for Reduction of Risk and Sustainable Development of Society. Kluwer Publishers, NATO, pp. 191–207. Ozorovich, Yu., 2001. Operational system for groundwater and pollution determination and monitoring. In: A New Hydrology For the A Thirsty Planet. Proceedings of IAHS Scientific Assembly, 18–27 July 2001, Maastricht, The Netherlands. Ozorovich, Yu.R., 2002. Operational system for groundwater and pollution determination and monitoring into the coastal zones. Proceedings of the SWIM17—17th Salt Water Intrusion Meeting, May 2002, Delft. Ozorovich, Y.R., Linkin, V.M., Smythe, W., 1999. Mars electromagnetic sounding experiment—MARSES. Proceedings of LPI Conference, Houston. pp. 97–101.
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Povinec, P.P., Aggarwal, P.K., Aureli, A., Burnett, W.C., Kontar, E.A., Kulkarni, K.M., Moore, W.S., Rajar, R., Taniguchi, M., Comanducci, J.-F., Cusimano, G., Dulaiova, H., Gatto, L., Groening, M., Hauser, S., Levy-Palomo, I., Oregioni, B., Ozorovich, Y.R., Privitera, A.M.G., Schiavo, M.A., 2005a. Characterisation of submarine groundwater discharge (SGD) offshore south-eastern Sicily—IAEA-UNESCO SGD collaboration. Journal of Environmental Radioactivity (submitted for publication). Povinec, P.P., Comanducci, J.-F., Levy-Palomo, I. Oregioni, B., 2005b. Monitoring of submarine groundwater discharge along the Donnalucata coast in the south-eastern Sicily using underwater gamma-ray spectrometry. Continental Shelf Research, this issue, doi:10.1016/j.csr.2005.12.009. Sengpiel, K.P., Siemon, B., 1998. Examples of 1-D inversion of multifrequency HEM data from 3-D resistivity distributions. Exploration Geophysics 29, 133–141. Stewart, D.C., Anderson, W.L., Grover, T.P., Labson, V.F., 1994. Shallow subsurface mapping by electromagnetic sounding in the 300 kHz to 30 MHz range: model studies and prototype system assessment. Geophysics 25, 1201–1210. Stieglitz, T., 2005. Submarine groundwater discharge into the near-shore zone of the Great Barrier reef, Australia. Marine Pollution Bulletin 51, 51–59. Stieglitz, T., Ridd, P.V., Hollins, S., 2000. A small sensor for detecting animal burrows and monitoring burrow water conductivity. Wetlands Ecology and Management 8, 1–7. Taniguchi, M., 2005. Submarine groundwater discharge measured by seepage meters in Sicilian coastal waters. Continental Shelf Research, this issue, doi:10.1016/j.csr.2005.12.002.