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Integration of shallow reflection seismics and time domain electromagnetics for detailed study of the coastal aquifer in the Nitzanim area of Israel
Journal of Applied Geophysics 44 Ž2000. 197–215 www.elsevier.nlrlocaterjappgeo
Integration of shallow reflection seismics and time domain electromagnetics for detailed study of the coastal aquifer in the Nitzanim area of Israel V. Shtivelman, M. Goldman
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The Geophysical Institute of Israel, P.O. Box 2286, Holon 58122, Israel Received 24 March 1998; accepted 20 November 1998
Abstract Two geophysical surveys using shallow reflection seismics and time domain electromagnetics ŽTDEM., were carried out in the Mediterranean coast of Israel. The surveys were a part of an INCO-DC research project aimed at developing an integrated geophysical approach for rational management of groundwater resources. The general objective of the surveys was a detailed study of the coastal aquifer in the area and, in particular, subdivision of the aquifer into subaquifers separated by impermeable units and evaluation of water quality within each subaquifer. The seismic survey included two reflection lines shot using the CMP technique. The lines were located in the vicinity of several hydrogeological observation wells, and the borehole information was used for correlation purposes. The results of the survey show a sequence of reflected events which can be related to impermeable units located within and below the aquifer. Based on this interpretation, the aquifer was subdivided into a number of subaquifers separated by the impermeable units. At several locations along the seismic sections, fault zones interrupting the continuity of the reflections, were mapped. The TDEM survey included nine central loop soundings located along one of the seismic lines. The sea water intrusion was clearly detected as a geoelectric unit having resistivity less than approximately 2 V P m. However, the individual TDEM interpretation based on the minimum possible number of layers not always allowed to detect fresh water bearing subaquifers beneath the impermeable layers. Inclusion of additional layers based on the seismic interpretation improved both the inversion results Žmisfit error. and, especially, the hydrogeological significance of the TDEM interpretation. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Coastal aquifer; Seismic reflection; Time domain electromagnetics ŽTDEM.; Integration
1. Introduction The problem of effective management of groundwater resources is of paramount impor-
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Corresponding author. Tel.: q972-3-5576046; Fax: q972-3-5502925; E-mail: [email protected]
tance in many regions throughout the word. It is particularly important in the areas suffering from the lack of fresh surface water and insufficient rainfalls. The Mediterranean coast of Israel is a typical example of such an area, where groundwater is the only source of fresh water in the whole coastal plain. The coastal aquifer supplies about one quarter of the country’s annual water
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consumption. Like in many other regions, the aquifer suffers from severe salinization caused by seawater encroachment. The problem is further aggravated by the population growth and, consequently, the progressively increasing extraction of water from the aquifer. For rational management of the aquifer system, a detailed study of the aquifer and its separate sub-units Žwhich can be saturated with fresh groundwater. is necessary. This may be achieved by drilling observation wells and by using surface geophysical surveys. Among various geophysical methods, seismic reflection and electromagnetic techniques seem to be the most suitable for this purpose. However, each of these methods alone is efficient in solving only a specific hydrogeological problem, but is usually unable to provide a general solution. For example, TDEM is very efficient in detecting sea water intrusion, but is usually much less successful in delineating geological structures. The seismic method, vice versa, is very efficient in solving the structural problem but is unable to distinguish between fresh and saline ground waters. Therefore, the best way
seems to apply both methods and then to perform an integrated interpretation of seismic and electromagnetic data. Although in the past a number of seismic and TDEM surveys was carried out in different parts of the coastal plain of Israel Ž Goldman et al., 1991; Schlein et al., 1992; Ben-Gai, 1995. , they were usually performed independently, and no attempt has ever been made to integrate their results. However, in many cases such an interpretation would be necessary, as may be seen from the following. During the previous TDEM surveys, it was found at several locations that beneath a very low resistivity unit, which was undoubtedly identified with sea water intrusion, there was a highly resistive layer testifying to the presence of fresh groundwater Ž the so called hydrological inversion.. Unfortunately, in most cases the existence of impermeable hydrogeological units, which justifies the validity of the above mentioned model, could not be concluded from the TDEM interpretation alone. In particular, this problem was encountered in the Nitzanim area located in the southern part of the coastal plain of Israel ŽFig. 1.. In order to
Fig. 1. Map showing schematic location of seismic lines, TDEM measurements and boreholes in the investigated area.
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provide a detailed and reliable study of the coastal aquifer in the area, an integrated geophysical survey, including shallow reflection seismics and TDEM, was carried out. The survey was a part of an INCO-DC research project aimed at developing an integrative geophysical approach for rational management of groundwater resources. The seismic survey included two high-resolution reflection lines shot using the CMP technique. In the past decade, the high-resolution
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seismic reflection method has been developed and used in different regions for geotechnical and environmental studies Ž Myers et al., 1987; Branham and Steeples, 1988; Jongerius and Helbig, 1988; Treadway et al., 1988; Miller et al., 1990; Jeng, 1995; Kourkafas and Goulty, 1996; Shtivelman et al., 1998, a. and for groundwater-related investigations Ž Birkelo et al., 1987; Geissler, 1989; Miller et al., 1989; Miller and Steeples, 1990; Bruno and Godio, 1997.. The present survey was located in the
Fig. 2. Lithological logs in the boreholes located in the vicinity of the geophysical surveys. The figures to the right of each borehole indicate depth Žin meters. from the surface.
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vicinity of a number of hydrogeological observation wells, and the borehole information was used for correlation of seismic data. The TDEM survey included nine central loop soundings carried out along seismic line GI-0082
ŽFig. 1.. The transmitter loop size varied between 50 by 50 m near the sea shore to 200 by 200 m in the eastern part of the profile providing the penetration depth from about 100 m to approximately 300 m, respectively. In order to
Fig. 3. Schematic W–E hydrogeological section across the Nitzanim area. The horizontal axis shows the distance from the sea.
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Table 1 Seismic data acquisition: equipment and parameters Recorder Energy source Receivers—single geophones Receiver spacing Shot spacing Shot location Minimum offset Maximum fold Record length Time sampling interval Analog band-pass filters
48 channel EG & G seismograph, model ES-2401X Dynasource Žtruck mounted vacuum accelerated heavy weight drop. 10 Hz Žvertical. 2.5 m 2.5 m Ževery station. Near 1st receiver Žoff-end geometry. 1m 24 0.5 s 0.5 ms 70–250 Hz
provide better vertical resolution and accuracy of interpretation ŽGoldman et al., 1994. , both shallow Geonics EM47 and deep EM37 systems were applied in the western part of the line. For interpretation of the TDEM data, the following approach was applied. First, the interpretational model consisting of a minimum possible number of layers was used. After comparing with the seismic results, additional layers were included in the initial model according to
the seismic interpretation. Such an approach led to a significant improvement of the interpretational results.
2. Hydrogeological background The Coastal Plain aquifer of Israel extends along the Mediterranean shore line from the Gaza Strip in the south to the Mount Carmel in
Fig. 4. Examples of field records from line GI-0082.
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Fig. 5. Seismic section along line GI-0082. Every forth trace is displayed; the trace spacing is 5.0 m.
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Fig. 6. Seismic section in the eastern part of line GI-0082. Every trace is displayed; the trace spacing is 1.25 m.
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Fig. 7. Seismic section in the western part of line GI-0082. Every trace is displayed; the trace spacing is 1.25 m.
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the north, reaching the width of 10–15 km eastward. The aquifer consists of the Quaternary sequence of marine and continental deposits composed predominantly of calcareous sandstone Žthe Kurkar unit. resting on an erosional contact. The sequence starts with the Calabrian regional regression which terminates the Saqiye marine regime. The accumulated succession of sediments which follows bears evidence of several cycles of transgressions and regressions of the sea. With each cycle, deposition environment shifted from west to east and back. The lithologic variability observed in the Quaternary sequence reflects different types of sediments characteristic of a certain environment: calcareous sandstone, sandy limestone, sand, clay, silt, conglomerate and loam. In the west, the aquifer consists of marine deposits, while in the east, the deposits are of continental origin. Calcareous sandstones are generally porous and hydrologically conductive and considered aquifers, whereas clays are impermeable and act as aquicludes. In the places where the clays are thick and extensive enough, they divide the aquifer into distinct subunits. The aquifer rests on impermeable black shales and clays of the Saqiye group of Pliocene–Miocene age.
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The coastal aquifer is replenished by rain precipitation during the winter months. Groundwater flows westward, toward the sea. Usually, the groundwater level rises from west to east with a gradient of about 1 m per 1 km. Discharge of the aquifer is in the form of seepage along the seawaterrfreshwater interface. In certain areas, over-exploitation of the aquifer has caused inland seawater encroachment to more than 1 km from the sea shore. A number of observation wells are located at the investigated site in the Nitzanim area ŽFigs. 1 and 2.. Three of the wells Ž 12r0, 12rA and 12r1. penetrate the entire aquifer down to the Saqiye group which is about 160 m deep in this area. In the western part of the area Ž wells 12r0 and 12rA. marine sediments are penetrated, whereas in the eastern part Ž well 12r1. continental sediments were encountered. In Fig. 2, various Kurkar subaquifers encountered in the wells are designated by capital K with corresponding indices. According to hydrological data, the water table level in the area is about 2–3 m above the mean sea level ŽMSL.. The sea water penetrates into the aquifer up to about 1.0 km from the coastal line.
Fig. 8. Seismic section along line GI-0083. Every forth trace is displayed; the trace spacing is 5.0 m.
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A schematic W–E hydrogeological section across the investigated area is represented in Fig. 3.
3. Seismic survey The high-resolution seismic reflection survey carried out in the Nitzanim area included two seismic lines shot using conventional P-wave technique Žlines GI-0082 and GI-0083, Fig. 1.. The length and locations of the lines were chosen in accordance with the survey’s target and local surface conditions. The coordinates and elevations along the lines were measured using a differential GPS. The equipment and parameters used in the survey are presented in Table 1. The overall quality of the acquired seismic data was good, as can be seen from Fig. 4, showing an example of three typical field records from line GI-0082. The acquired seismic data were processed at the Geophysical Institute of Israel ŽGII. processing center using the industry-standard PROMAX package. Seismic time sections for the reflection lines are presented in Figs. 5–8. The horizontal axis on the sections shows station numbers while the vertical axis is two-way travel time in milliseconds. The sections are related to a horizontal datum placed at the MSL. The part of the sections located above the datum appears at negative times. The boreholes located in the vicinity of the lines and line intersections are marked above each section. Since no velocity information from boreholes was available in the investigated area, no attempt at depth conversion of the sections was made. However, rough estimates of the elevations of the reflected events appearing on the sections were made on the basis of the velocities obtained from seismic refraction surveys carried out in different parts of the Coastal Plain. According to the refraction data, the velocity of P waves in the Kurkar unit Žcalcareous sandstone. is about 2000 mrs. Since below the MSL the section is represented mainly
by the Kurkar unit Ž Fig. 2. , this velocity can be used to estimate the elevation of the reflected events appearing below the datum. Based on this velocity, positive reflection times in milliseconds roughly correspond to negative elevations in meters. 3.1. Line GI-0082 (Fig. 5) The line runs in a NW–SE direction and is about 2.6 km long Ž Fig. 1. . Four boreholes located in the vicinity of the line, are marked above the seismic section represented in Fig. 5. The borehole data shown in Fig. 2 were used for correlation of the seismic data. Seismic section along the line shows a sequence of reflected events which can be traced down to times of about 250 ms Ž corresponding to depths of about 270–300 m. . One of the most prominent features on the section is an anomalous zone in the central part of the line between stations 520–600. The zone subdivides the section laterally into three parts with different character of seismic data. The continuity of the reflectors appearing on both sides of the zone, is clearly interrupted within the zone. This disturbance zone may possibly be related to a fault system crossed by the line. However, a more probable interpretation seems to be that in this region the line crosses a Kurkar ridge running parallel to the shore. If this ridge existed in the time of deposition and separated sea to the west of it from land to the east, it might explain the continental character of the sediments in the eastern part of the area Ž as encountered in borehole 12r1. versus the marine sediments in the western part Žboreholes 12r0, 12rB and 12rA.. For convenience of interpretation, the section was divided into two parts Ž shown to a larger scale in Figs. 6 and 7. and each part was considered separately. The seismic events interpreted as corresponding to various Kurkar layers within the aquifer were marked by thick white lines and designated by capital K with the corresponding indices, whereas the reflector as-
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sociated with the Top Saqiye interface confining the aquifer was marked by thin black line. 3.1.1. The eastern part of the line The region to the east of the disturbance zone ŽFig. 6. is characterized by a relatively simple and clear subsurface picture. The uppermost strong reflector appearing above the datum at times varying between y50 ms to y10 ms is apparently related to the interface between the upper dry sand layer and the underlying Kurkar unit. The velocity in the sand layer is about 400 mrs, so that the depth to the interface varies from almost the surface to about 16 m below the surface. The high amplitudes of the reflector are due to the large velocity contrast above and below the interface. The dominant frequency along the reflector is about 100 Hz. Below the interface, a sequence of reflected events with a general westward dip can be detected. These events are apparently related to various layers within the aquifer. The apparent dominant frequency of the reflections is about 75 Hz. Taking the average velocity of 1500 mrs gives the dominant wavelength of 20 m. The thicknesses of many loam and clay units encountered in the boreholes ŽFig. 2. do not exceed 5 m, i.e., a quarter of the dominant wavelength. Therefore, we cannot expect that separate reflections from top and bottom of such layers can be detected on our sections; rather, some interferential effect from the whole layer will be obtained. In other words, we can possibly detect the presence of such a thin layer in the section but will be unable to estimate its vertical extension. The relatively low apparent dominant frequencies of the reflections Žabout 75 Hz as compared to 100 Hz of the upper reflector. may also indicate the interferential character of the events. The reflections appearing in Fig. 6 below the datum down to times of about 120 ms, were correlated to the lithological data from borehole 12r1 ŽFig. 2. located about 170 m southward of station 980. The lower reflector appearing at time of about 120 ms, is apparently related to the Top Saqiye interface penetrated by the bore-
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hole at elevation of about y132 m. This interface corresponds to the erosional contact between the Kurkar unit and the underlying thick impermeable clays of the Saqiye group. Previous seismic surveys carried out in the Coastal Plain ŽSchlein et al., 1992; Ben-Gai, 1995. showed that this interface usually appears as a clear marker on seismic sections. In Fig. 6, this reflector can be traced westward of the borehole up to station 600. The reflectors above the Top Saqiye interface may be correlated to the loam and clay units embedded within the aquifer below the MSL. For example, consider these reflectors in the vicinity of borehole 12r1. The reflector appearing at time of about 100 ms, is apparently related to the top of the lower Kurkar layer K 14 penetrated at elevation of y109 m. The layer has a general inclination to the west and can be traced up to station 600. The reflector appearing at time of about 80 ms, may be correlated to the loam layer encountered at elevation of y76 m. Therefore, the reflector may be associated with the top of the Kurkar layer K 13. The layer can be traced westward up to station 600. The two reflectors appearing at times of about 40 ms and 65 ms, may be correlated to the top and bottom of the loam layer encountered at elevations of y48 m and y62 m. The corresponding time interval below the loam may be associated with the Kurkar layer K 12 . This layer can be traced westward up to station 670 where it seems to pinch out. The upper reflector appearing at times of about 25 ms, cannot be correlated to any unit in borehole 12r1. Probably this reflector is related to a thin local loam lense not reaching the borehole. The reflector seems to lift up to the west and disappear in the vicinity of station 800.
3.1.2. The western part of the line In this region, the seismic section Ž Fig. looks much more complicated. A sequence reflected events with various local dips can detected below the datum down to times
7. of be of
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Fig. 9. Preliminary 1-D interpretation of the TDEM data.
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about 250 ms. The continuity of the events is clearly interrupted at a number of locations which were interpreted as shallow faults and marked on the section by thin lines. The faults have a form of flower structures and are apparently related to strike-slip motions with minor vertical displacements. Consider a sequence of reflections in the vicinity of borehole 12rA ŽFig. 2. located near station 365. Correlation with the borehole data shows that the reflector appearing at times of about 130 ms, may be related to the top Saqiye interface penetrated at elevation of y143 m. This reflector can be traced up to station 515 to the east ŽFig. 6. and up to station 200 to the west. Its extension further to the west is unclear, apparently due to decreasing thickness of the lowermost Kurkar layers, as can be seen in borehole 12r0. The reflector appearing at time of about 115 ms, may be correlated to the top of the Kurkar
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layer K A6 penetrated in the borehole at elevation of y122 m. The reflector appearing at time of about 90 ms may be correlated to the top of the relatively thick clay layer penetrated at elevation of y86 m. It is difficult to detect and trace reflections from two thin Kurkar layers K A4 and K A5 below the clay layer, although some indications on their presence as a single unit can be found in the section. Two upper reflectors appearing at times of about 40 ms and 60 ms, may be correlated to two thin clay layers penetrated at elevations of y40 m and y53 m. These reflectors separate three upper Kurkar layers K A1, K A2 and K A3. In the seismic section, the event associated with K A3 layer clearly pinches out in the vicinity of station 280, while K A2 layer can be traced further to the west. The sequence of events at the beginning of the section Žin the vicinity of station 20. can be
Fig. 10. Pseudo-2D resistivity cross-section in the western part of seismic line GI-0082 Žpreliminary interpretation..
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correlated to borehole 12r0 ŽFig. 2. located about 135 m southward of the beginning of the line. Here the identification of the Top Saqiye interface is problematic, probably due to a relatively small thickness of the lower Kurkar layers K 05 and K 06 . The strong reflector appearing at time of about 100 ms, may possibly be related to the top of the thick clay layer penetrated at elevation of y110 m. The reflector appearing at time of about 80 ms, may be related to the clay–loam layer penetrated at elevation of y79 m. The time interval between the above two reflectors may be associated with the Kurkar layers K 03 and K 04 . Tracing these layers to the east indicates that they may be apparently related to layer K A2 mapped in the vicinity of borehole 12rA. The reflector appearing at time of about 50 ms, may be related to a thin clay unit penetrated at elevation of about y48 m; this reflector apparently separates two upper Kurkar layers K 01 and K 02 . The layers may be traced Žalbeit somewhat problematically. up to borehole 12rB Žstation 155. where they seem to correspond to layers K B1 and K B2 . Other events appearing above and below the reflector, seem to be uncorrelated to the borehole data; probably, these events correspond to local clay lenses which do not reach the borehole. 3.2. Line GI-0083 (Fig. 8) This line runs in a SW–NE direction and is about 1.9 km long ŽFig. 1. . In the vicinity of borehole 12rA it crosses line GI-0082, as marked above the section. The general character of seismic section along the line is similar to that of line GI-0082. The section shows a sequence of almost horizontal or very gently dipping reflected events. The reflector related to the Top Saqiye interface can be identified at times of about 130–140 ms; above the interface, the event corresponding to Kurkar layers K 06 and K 07 can be traced almost along the entire section. In the southern part of the line, Kurkar
layer K A3 can be detected. Continuous tracing of the layer along the section is somewhat problematic, possibly due to geometric or facial lateral changes along the corresponding geological units. In the region between stations 510 and 580, the continuity of all reflectors is clearly interrupted. Here the line apparently crosses a fault zone, as marked on the section. Additional, smaller faults were mapped in the vicinity of stations 250–300.
4. TDEM survey Nine TDEM stations were established roughly along seismic line GI-0082 ŽFig. 1. . All the collected data were processed and interpreted using the Interpex TEMIX-XL 1-D interpretation package ŽInterpex, 1996.. The interpretation results for all nine soundings are shown in Fig. 9. No a priori information has been used at this stage. The initial model for each inversion was obtained by applying first Occam’s inversion ŽConstable et al., 1987. and then reducing to the possible minimum the number of layers in the recovered smooth model. According to the above described hydrogeological setting of the area, the interpreted resistivities can be divided into four groups: Ø Very low resistivities Žless than 3 V P m.. These resistivities are typical for saline water saturated lithologies Ž both aquifers and aquicludes.. Ø Low resistivities Ž between 3 to 8 V P m.. These resistivities are characterizing either brackish water saturated aquifers or aquicludes Žmainly clays. . Ø Moderate resistivities Žbetween 8 and 15 V P m.. Typical for fresh water saturated aquifers and some aquicludes Ž loams.. Ø High resistivities Žgreater than 15 V P m.. These resistivities are typical for either dry or fresh water saturated aquifers. The lateral extent of sea water intrusion into the aquifer can be estimated from the TDEM
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measurements alone just taking into account the depth to the aquifer base Žtop Saqiye., which is very consistent along the whole profile ŽFigs. 2 and 3. . According to the borehole data the depth to the aquifer base is approximately 160 m in the whole test area. This means that if the very low resistivity unit is located at depths shallower than 160 m, it represents sea water intrusion, otherwise it is identified with the Saqiye
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clays underlying the aquifer. Fig. 9 clearly indicates that sea water intrusion terminates somewhere between stations N3 and N4 Ž i.e., at a distance of approximately 800 m from the sea. . Thus, according to the preliminary TDEM interpretation the aquifer is fully saturated with fresh water eastward of station N4. Unfortunately, due to an insufficient resistivity contrast, the depth to the water table cannot be obtained from
Fig. 11. Preliminary Žabove. and final Žbelow. 1-D interpretations of the TDEM data at station N4.
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the TDEM measurements. Therefore the most interesting part of the profile for evaluating the quality Žsalinity. of groundwater is located between stations N1 and N4. Fig. 10 shows the pseudo-2D resistivity cross-section of the west part of the profile compiled from appropriate 1-D resistivity vs. depth models ŽFig. 9.. Beneath station N3 one can see two moderately resistive layers which can be identified with fresh water saturated subaquifers. However, the lateral extension of the lower subaquifer is unclear since it was not detected at stations N2 and N4. It seams it is terminated somewhere between stations N3 and N4 by an aquiclude layer having resistivity of approximately 5 V P m, while between N3 and N2 the subaquifer probably becomes saturated with saline water. Thus, according to the preliminary TDEM interpretation, the lower subaquifer saturated with fresh groundwater is confined to an area around station N3.
5. Integrated interpretation of seismic and TDEM results In order to verify the above results, a combined interpretation of the TDEM and seismic data was performed. For this purpose, the western part of seismic line GI-0082 was used. The locations of the TDEM stations are marked above the seismic section ŽFig. 7.. According to the seismic interpretation, the lower Kurkar units designated as K A6 and K A7 extend from a vicinity of TDEM station N2 continuously through stations N3 and N4 further eastward Ž Fig. 7. . The moderately resistive layer detected beneath station N3 at approximately the same elevation Ž y120 m. is most likely identified with these units saturated with fresh water. If this assumption is correct, the data collected at station N4 where the lower subaquifer was not detected, should be reinterpreted. In order to make the TDEM interpreta-
Fig. 12. Pseudo-2D resistivity cross-section and appropriate borehole data in the western part of seismic line GI-0082 Žfinal interpretation..
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tion consistent with the seismic results, an additional moderately resistive layer has been included in the initial model used for the TDEM inversion. This led to the solution shown in Fig. 11. For the sake of convenience, the figure also shows the results of the preliminary interpretation. The uncertainty of the model parameters can be roughly estimated by using the so called linear equivalence analysis Ž Goldman et al., 1994.. The best fit models are shown by solid lines in the resistivity vs. depth sections in Fig. 11. Dashed lines in the figure represent alternative models for which the misfit error is only slightly greater than for the best fit model. As could be expected, both the resistivity and thickness of the lower moderately resistive layer are poorly resolved and its inclusion in the interpreted model is only justified by the existence of the independent seismic interpretation. It should be emphasized that the reinterpretation was based on seismic results only, without being biased by any borehole information. The misfit error of the final interpretation decreased from approximately 3% obtained in the preliminary interpretation to slightly more than 2%. But the most important result is that the final interpretation of the TDEM data now becomes consistent with the independent seismic interpretation and ultimately with the borehole data available. The final resistivity cross-section accompanied by the borehole data is shown in Fig. 12. One can see that the location of the lower moderately resistive unit roughly coincides with the lower subaquifer in well 12rA. Note that the coincidence is rather poor due to the above mentioned uncertainty in the layer parameters. The resistivity of the aquiclude layer separating two subaquifers varies laterally from approximately 5 V P m in the eastern part of the section to slightly more than 2 V P m in its western part. This lateral variation can be explained by different salinities of water within the aquiclude: the closer to the sea, the higher salinity Ži.e., the lower resistivity. is expected. It should be noted that this variation may explain the above mentioned difference in the interpre-
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tation of the TDEM data at points N3 and N4. The much smaller resistivity contrast between the aquiclude and underlying subaquifer at point N4 as compared to N3 may be the reason why this subaquifer was not detected at N4 during the preliminary interpretation. The boundary drawn within the very low resistivity unit between approximately 1.5 V P m and 2.5 V P m can be most likely identified with the boundary between the sea water saturated aquifer and aquiclude. Finally, the boundary between the moderately and highly resistive layers and very low resistivity unit exactly coincides with the freshwaterrseawater interface detected in wells 12r0 and 12rB. The interface was not detected in well 12rA obviously because of the impermeable clays appearing somewhere in the vicinity of station N3. Comparison of the seismic and TDEM sections with the geological cross-section compiled from the borehole data shows some discrepancy in the layer geometry in the area between boreholes 12rA and 12rB. Specifically, while the geological cross-section obtained by linear interpolation between the boreholes shows monotonous inclination of all layers westward ŽFig. 3., both geophysical sections show a structural high in the corresponding region Ž Figs. 7 and 12.. Such a situation can be encountered over a region with sparse borehole control. In this case, seismic sections can provide a continuous and, therefore, more diagnostic image of the subsurface.
6. Conclusions The geophysical surveys carried out in the Nitzanim area provided an important information necessary for a detailed study of the aquifer in the area. The seismic sections obtained along the reflection lines display a sequence of reflected events down to times of about 250 ms Žabout 270–300 m depth.. Correlation of the seismic
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data with borehole information shows that the reflections can be related to various impermeable units located within and below the aquifer. By tracing the reflectors along the sections, the geometry and lateral extension of the units can be estimated. Based on this interpretation, the aquifer can be subdivided into a number of subaquifers separated by the impermeable units, as it has been done using the borehole data alone ŽFig. 3. . At several locations along the sections, disturbance zones apparently related to shallow faults were mapped; this mapping may have implications for the hydraulic continuity of the various aquifers in the region. The TDEM survey resulted in a pseudo-2D resistivity cross-section located along the western part of seismic line GI-0082. The section clearly shows sea water intrusion appearing as a highly conductive geoelectric unit. A number of fresh water saturated subaquifers can be detected in the section. However, the lower subaquifer was not always revealed by the routine conventional TDEM inversion. The reinterpretation of the TDEM data based on the combined use of seismic and electromagnetic results, enabled us to successfully solve the above inversion problem thus considerably improving the hydrogeological significance of the geophysical results. The integrated approach to the interpretation of the geophysical data can be applied for solution of similar problems in other areas.
Acknowledgements This work was carried out in the framework of the INCO-DC project financed by the European Commission Žcontract aIC18CT96-0122.. The authors are grateful to E. Fleisher and D. Gilad for fruitful discussions of hydrogeological aspects of the project and to M. Ezersky for the technical assistance.
References Ben-Gai, Y., 1995. Sequence stratigraphy of the PlioPleistocene in the continental margin of the Southern Levant. PhD Thesis, Tel Aviv Univ. Birkelo, B.A., Steeples, D.W., Miller, R.D., Sophocleous, M., 1987. Seismic reflection study of a shallow aquifer during a pumping test. Ground Water 25, 703–709. Branham, K.L., Steeples, D.W., 1988. Cavity detection using high resolution seismic reflection methods. Mining Engineering 40, 115–119. Bruno, P.P.G., Godio, A., 1997. Environmental risk assessment of a shallow aquifer in ‘Piana Campana’ ŽItaly.: a field comparison between seismic refraction and reflection methods. European Journal of Environmental and Engineering Geophysics 2, 61–76. Constable, S.C., Parker, R.L., Constable, C.G., 1987. Occam’s inversion: a practical algorithm for generating smooth models from electromagnetic sounding data. Geophysics 52, 289–300. Geissler, P.E., 1989. Seismic profiling for groundwater studies in Victoria, Australia. Geophysics 54, 31–37. Goldman, M., Gilad, D., Ronen, A., Melloul, A., 1991. Mapping of seawater intrusion into the coastal aquifer of Israel by the time domain electromagnetic method. Geoexploration 28, 153–174. Goldman, M., Du Plooy, D., Eckard, M., 1994. On reducing ambiguity in the interpretation of transient electromagnetic sounding data. Geophys. Prosp. 42, 3–25. Interpex, 1996. TEMIX-XL User’s manual. Interpex Ltd., Golden, Colorado. Jeng, Y., 1995. Shallow seismic invest of a site with poor reflection quality. Geophysics 60, 1715–1726. Jongerius, P., Helbig, K., 1988. Onshore high-resolution seismic profiling applied to sedimentology. Geophysics 53, 1276–1283. Kourkafas, P., Goulty, N.R., 1996. Seismic reflection imaging of gypsum mine working at Sherburn-in-Elmet, Yorkshire, England. European Journal of Environmental and Engineering Geophysics 1, 53–63. Miller, R.D., Steeples, D.W., Brannan, M., 1989. Mapping a bedrock surface under dry alluvium with shallow seismic reflections. Geophysics 27, 1528–1534. Miller, R.D., Steeples, D.W., 1990. A shallow seismic reflection survey in basalts of the Snake River Plain, Idaho. Geophysics 55, 761–768. Miller, R.D., Steeples, D.W., Myers, P.B., 1990. Shallow seismic reflection survey across the Meers fault, Oklahoma. Bull. Geolog. Soc. of Am. 102, 18–25. Myers, P.B., Miller, R.D., Steeples, D.W., 1987. Shallow seismic reflection profile of the Meers fault, Comanche County, Oklahoma. Geophys. Res. Lett. 15, 749–752. Schlein, N., Buchbinder, B., Ben-Gai, Y., 1992. Sequence stratigraphy of the Pliocene Yafo Formation: a high
V. ShtiÕelman, M. Goldmanr Journal of Applied Geophysics 44 (2000) 197–215 resolution pilot survey in five selected areas along the coastal plain. Israel, IPRG Report No. 204r45r91. Shtivelman, V., Frieslander, U., Zilberman, E., Amit, R., 1998. Mapping shallow faults at the Evrona playa site using high resolution reflection method. Geophysics 63, 1257–1264. Shtivelman, V., Ron, H., Israeli, A., 1998a. Construction
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site investigation using shallow seismic reflection survey at the Margaliot site in the Upper Galilee, Israel. European Journal of Environmental and Engineering Geophysics 3, 63–74. Treadway, J.A., Steeples, D.W., Miller, R.D., 1988. Shallow seismic study of a fault scarp near Borah Peak, Idaho. J. Geophys. Res. 93, 6325–6337.
Integration of shallow reflection seismics and time domain electromagnetics for detailed study of the coastal aquifer in the Nitzanim area of Israel V. Shtivelman, M. Goldman
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The Geophysical Institute of Israel, P.O. Box 2286, Holon 58122, Israel Received 24 March 1998; accepted 20 November 1998
Abstract Two geophysical surveys using shallow reflection seismics and time domain electromagnetics ŽTDEM., were carried out in the Mediterranean coast of Israel. The surveys were a part of an INCO-DC research project aimed at developing an integrated geophysical approach for rational management of groundwater resources. The general objective of the surveys was a detailed study of the coastal aquifer in the area and, in particular, subdivision of the aquifer into subaquifers separated by impermeable units and evaluation of water quality within each subaquifer. The seismic survey included two reflection lines shot using the CMP technique. The lines were located in the vicinity of several hydrogeological observation wells, and the borehole information was used for correlation purposes. The results of the survey show a sequence of reflected events which can be related to impermeable units located within and below the aquifer. Based on this interpretation, the aquifer was subdivided into a number of subaquifers separated by the impermeable units. At several locations along the seismic sections, fault zones interrupting the continuity of the reflections, were mapped. The TDEM survey included nine central loop soundings located along one of the seismic lines. The sea water intrusion was clearly detected as a geoelectric unit having resistivity less than approximately 2 V P m. However, the individual TDEM interpretation based on the minimum possible number of layers not always allowed to detect fresh water bearing subaquifers beneath the impermeable layers. Inclusion of additional layers based on the seismic interpretation improved both the inversion results Žmisfit error. and, especially, the hydrogeological significance of the TDEM interpretation. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Coastal aquifer; Seismic reflection; Time domain electromagnetics ŽTDEM.; Integration
1. Introduction The problem of effective management of groundwater resources is of paramount impor-
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Corresponding author. Tel.: q972-3-5576046; Fax: q972-3-5502925; E-mail: [email protected]
tance in many regions throughout the word. It is particularly important in the areas suffering from the lack of fresh surface water and insufficient rainfalls. The Mediterranean coast of Israel is a typical example of such an area, where groundwater is the only source of fresh water in the whole coastal plain. The coastal aquifer supplies about one quarter of the country’s annual water
0926-9851r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 9 8 5 1 Ž 9 8 . 0 0 0 5 3 - 6
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consumption. Like in many other regions, the aquifer suffers from severe salinization caused by seawater encroachment. The problem is further aggravated by the population growth and, consequently, the progressively increasing extraction of water from the aquifer. For rational management of the aquifer system, a detailed study of the aquifer and its separate sub-units Žwhich can be saturated with fresh groundwater. is necessary. This may be achieved by drilling observation wells and by using surface geophysical surveys. Among various geophysical methods, seismic reflection and electromagnetic techniques seem to be the most suitable for this purpose. However, each of these methods alone is efficient in solving only a specific hydrogeological problem, but is usually unable to provide a general solution. For example, TDEM is very efficient in detecting sea water intrusion, but is usually much less successful in delineating geological structures. The seismic method, vice versa, is very efficient in solving the structural problem but is unable to distinguish between fresh and saline ground waters. Therefore, the best way
seems to apply both methods and then to perform an integrated interpretation of seismic and electromagnetic data. Although in the past a number of seismic and TDEM surveys was carried out in different parts of the coastal plain of Israel Ž Goldman et al., 1991; Schlein et al., 1992; Ben-Gai, 1995. , they were usually performed independently, and no attempt has ever been made to integrate their results. However, in many cases such an interpretation would be necessary, as may be seen from the following. During the previous TDEM surveys, it was found at several locations that beneath a very low resistivity unit, which was undoubtedly identified with sea water intrusion, there was a highly resistive layer testifying to the presence of fresh groundwater Ž the so called hydrological inversion.. Unfortunately, in most cases the existence of impermeable hydrogeological units, which justifies the validity of the above mentioned model, could not be concluded from the TDEM interpretation alone. In particular, this problem was encountered in the Nitzanim area located in the southern part of the coastal plain of Israel ŽFig. 1.. In order to
Fig. 1. Map showing schematic location of seismic lines, TDEM measurements and boreholes in the investigated area.
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provide a detailed and reliable study of the coastal aquifer in the area, an integrated geophysical survey, including shallow reflection seismics and TDEM, was carried out. The survey was a part of an INCO-DC research project aimed at developing an integrative geophysical approach for rational management of groundwater resources. The seismic survey included two high-resolution reflection lines shot using the CMP technique. In the past decade, the high-resolution
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seismic reflection method has been developed and used in different regions for geotechnical and environmental studies Ž Myers et al., 1987; Branham and Steeples, 1988; Jongerius and Helbig, 1988; Treadway et al., 1988; Miller et al., 1990; Jeng, 1995; Kourkafas and Goulty, 1996; Shtivelman et al., 1998, a. and for groundwater-related investigations Ž Birkelo et al., 1987; Geissler, 1989; Miller et al., 1989; Miller and Steeples, 1990; Bruno and Godio, 1997.. The present survey was located in the
Fig. 2. Lithological logs in the boreholes located in the vicinity of the geophysical surveys. The figures to the right of each borehole indicate depth Žin meters. from the surface.
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vicinity of a number of hydrogeological observation wells, and the borehole information was used for correlation of seismic data. The TDEM survey included nine central loop soundings carried out along seismic line GI-0082
ŽFig. 1.. The transmitter loop size varied between 50 by 50 m near the sea shore to 200 by 200 m in the eastern part of the profile providing the penetration depth from about 100 m to approximately 300 m, respectively. In order to
Fig. 3. Schematic W–E hydrogeological section across the Nitzanim area. The horizontal axis shows the distance from the sea.
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Table 1 Seismic data acquisition: equipment and parameters Recorder Energy source Receivers—single geophones Receiver spacing Shot spacing Shot location Minimum offset Maximum fold Record length Time sampling interval Analog band-pass filters
48 channel EG & G seismograph, model ES-2401X Dynasource Žtruck mounted vacuum accelerated heavy weight drop. 10 Hz Žvertical. 2.5 m 2.5 m Ževery station. Near 1st receiver Žoff-end geometry. 1m 24 0.5 s 0.5 ms 70–250 Hz
provide better vertical resolution and accuracy of interpretation ŽGoldman et al., 1994. , both shallow Geonics EM47 and deep EM37 systems were applied in the western part of the line. For interpretation of the TDEM data, the following approach was applied. First, the interpretational model consisting of a minimum possible number of layers was used. After comparing with the seismic results, additional layers were included in the initial model according to
the seismic interpretation. Such an approach led to a significant improvement of the interpretational results.
2. Hydrogeological background The Coastal Plain aquifer of Israel extends along the Mediterranean shore line from the Gaza Strip in the south to the Mount Carmel in
Fig. 4. Examples of field records from line GI-0082.
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Fig. 5. Seismic section along line GI-0082. Every forth trace is displayed; the trace spacing is 5.0 m.
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Fig. 6. Seismic section in the eastern part of line GI-0082. Every trace is displayed; the trace spacing is 1.25 m.
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Fig. 7. Seismic section in the western part of line GI-0082. Every trace is displayed; the trace spacing is 1.25 m.
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the north, reaching the width of 10–15 km eastward. The aquifer consists of the Quaternary sequence of marine and continental deposits composed predominantly of calcareous sandstone Žthe Kurkar unit. resting on an erosional contact. The sequence starts with the Calabrian regional regression which terminates the Saqiye marine regime. The accumulated succession of sediments which follows bears evidence of several cycles of transgressions and regressions of the sea. With each cycle, deposition environment shifted from west to east and back. The lithologic variability observed in the Quaternary sequence reflects different types of sediments characteristic of a certain environment: calcareous sandstone, sandy limestone, sand, clay, silt, conglomerate and loam. In the west, the aquifer consists of marine deposits, while in the east, the deposits are of continental origin. Calcareous sandstones are generally porous and hydrologically conductive and considered aquifers, whereas clays are impermeable and act as aquicludes. In the places where the clays are thick and extensive enough, they divide the aquifer into distinct subunits. The aquifer rests on impermeable black shales and clays of the Saqiye group of Pliocene–Miocene age.
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The coastal aquifer is replenished by rain precipitation during the winter months. Groundwater flows westward, toward the sea. Usually, the groundwater level rises from west to east with a gradient of about 1 m per 1 km. Discharge of the aquifer is in the form of seepage along the seawaterrfreshwater interface. In certain areas, over-exploitation of the aquifer has caused inland seawater encroachment to more than 1 km from the sea shore. A number of observation wells are located at the investigated site in the Nitzanim area ŽFigs. 1 and 2.. Three of the wells Ž 12r0, 12rA and 12r1. penetrate the entire aquifer down to the Saqiye group which is about 160 m deep in this area. In the western part of the area Ž wells 12r0 and 12rA. marine sediments are penetrated, whereas in the eastern part Ž well 12r1. continental sediments were encountered. In Fig. 2, various Kurkar subaquifers encountered in the wells are designated by capital K with corresponding indices. According to hydrological data, the water table level in the area is about 2–3 m above the mean sea level ŽMSL.. The sea water penetrates into the aquifer up to about 1.0 km from the coastal line.
Fig. 8. Seismic section along line GI-0083. Every forth trace is displayed; the trace spacing is 5.0 m.
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A schematic W–E hydrogeological section across the investigated area is represented in Fig. 3.
3. Seismic survey The high-resolution seismic reflection survey carried out in the Nitzanim area included two seismic lines shot using conventional P-wave technique Žlines GI-0082 and GI-0083, Fig. 1.. The length and locations of the lines were chosen in accordance with the survey’s target and local surface conditions. The coordinates and elevations along the lines were measured using a differential GPS. The equipment and parameters used in the survey are presented in Table 1. The overall quality of the acquired seismic data was good, as can be seen from Fig. 4, showing an example of three typical field records from line GI-0082. The acquired seismic data were processed at the Geophysical Institute of Israel ŽGII. processing center using the industry-standard PROMAX package. Seismic time sections for the reflection lines are presented in Figs. 5–8. The horizontal axis on the sections shows station numbers while the vertical axis is two-way travel time in milliseconds. The sections are related to a horizontal datum placed at the MSL. The part of the sections located above the datum appears at negative times. The boreholes located in the vicinity of the lines and line intersections are marked above each section. Since no velocity information from boreholes was available in the investigated area, no attempt at depth conversion of the sections was made. However, rough estimates of the elevations of the reflected events appearing on the sections were made on the basis of the velocities obtained from seismic refraction surveys carried out in different parts of the Coastal Plain. According to the refraction data, the velocity of P waves in the Kurkar unit Žcalcareous sandstone. is about 2000 mrs. Since below the MSL the section is represented mainly
by the Kurkar unit Ž Fig. 2. , this velocity can be used to estimate the elevation of the reflected events appearing below the datum. Based on this velocity, positive reflection times in milliseconds roughly correspond to negative elevations in meters. 3.1. Line GI-0082 (Fig. 5) The line runs in a NW–SE direction and is about 2.6 km long Ž Fig. 1. . Four boreholes located in the vicinity of the line, are marked above the seismic section represented in Fig. 5. The borehole data shown in Fig. 2 were used for correlation of the seismic data. Seismic section along the line shows a sequence of reflected events which can be traced down to times of about 250 ms Ž corresponding to depths of about 270–300 m. . One of the most prominent features on the section is an anomalous zone in the central part of the line between stations 520–600. The zone subdivides the section laterally into three parts with different character of seismic data. The continuity of the reflectors appearing on both sides of the zone, is clearly interrupted within the zone. This disturbance zone may possibly be related to a fault system crossed by the line. However, a more probable interpretation seems to be that in this region the line crosses a Kurkar ridge running parallel to the shore. If this ridge existed in the time of deposition and separated sea to the west of it from land to the east, it might explain the continental character of the sediments in the eastern part of the area Ž as encountered in borehole 12r1. versus the marine sediments in the western part Žboreholes 12r0, 12rB and 12rA.. For convenience of interpretation, the section was divided into two parts Ž shown to a larger scale in Figs. 6 and 7. and each part was considered separately. The seismic events interpreted as corresponding to various Kurkar layers within the aquifer were marked by thick white lines and designated by capital K with the corresponding indices, whereas the reflector as-
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sociated with the Top Saqiye interface confining the aquifer was marked by thin black line. 3.1.1. The eastern part of the line The region to the east of the disturbance zone ŽFig. 6. is characterized by a relatively simple and clear subsurface picture. The uppermost strong reflector appearing above the datum at times varying between y50 ms to y10 ms is apparently related to the interface between the upper dry sand layer and the underlying Kurkar unit. The velocity in the sand layer is about 400 mrs, so that the depth to the interface varies from almost the surface to about 16 m below the surface. The high amplitudes of the reflector are due to the large velocity contrast above and below the interface. The dominant frequency along the reflector is about 100 Hz. Below the interface, a sequence of reflected events with a general westward dip can be detected. These events are apparently related to various layers within the aquifer. The apparent dominant frequency of the reflections is about 75 Hz. Taking the average velocity of 1500 mrs gives the dominant wavelength of 20 m. The thicknesses of many loam and clay units encountered in the boreholes ŽFig. 2. do not exceed 5 m, i.e., a quarter of the dominant wavelength. Therefore, we cannot expect that separate reflections from top and bottom of such layers can be detected on our sections; rather, some interferential effect from the whole layer will be obtained. In other words, we can possibly detect the presence of such a thin layer in the section but will be unable to estimate its vertical extension. The relatively low apparent dominant frequencies of the reflections Žabout 75 Hz as compared to 100 Hz of the upper reflector. may also indicate the interferential character of the events. The reflections appearing in Fig. 6 below the datum down to times of about 120 ms, were correlated to the lithological data from borehole 12r1 ŽFig. 2. located about 170 m southward of station 980. The lower reflector appearing at time of about 120 ms, is apparently related to the Top Saqiye interface penetrated by the bore-
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hole at elevation of about y132 m. This interface corresponds to the erosional contact between the Kurkar unit and the underlying thick impermeable clays of the Saqiye group. Previous seismic surveys carried out in the Coastal Plain ŽSchlein et al., 1992; Ben-Gai, 1995. showed that this interface usually appears as a clear marker on seismic sections. In Fig. 6, this reflector can be traced westward of the borehole up to station 600. The reflectors above the Top Saqiye interface may be correlated to the loam and clay units embedded within the aquifer below the MSL. For example, consider these reflectors in the vicinity of borehole 12r1. The reflector appearing at time of about 100 ms, is apparently related to the top of the lower Kurkar layer K 14 penetrated at elevation of y109 m. The layer has a general inclination to the west and can be traced up to station 600. The reflector appearing at time of about 80 ms, may be correlated to the loam layer encountered at elevation of y76 m. Therefore, the reflector may be associated with the top of the Kurkar layer K 13. The layer can be traced westward up to station 600. The two reflectors appearing at times of about 40 ms and 65 ms, may be correlated to the top and bottom of the loam layer encountered at elevations of y48 m and y62 m. The corresponding time interval below the loam may be associated with the Kurkar layer K 12 . This layer can be traced westward up to station 670 where it seems to pinch out. The upper reflector appearing at times of about 25 ms, cannot be correlated to any unit in borehole 12r1. Probably this reflector is related to a thin local loam lense not reaching the borehole. The reflector seems to lift up to the west and disappear in the vicinity of station 800.
3.1.2. The western part of the line In this region, the seismic section Ž Fig. looks much more complicated. A sequence reflected events with various local dips can detected below the datum down to times
7. of be of
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Fig. 9. Preliminary 1-D interpretation of the TDEM data.
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about 250 ms. The continuity of the events is clearly interrupted at a number of locations which were interpreted as shallow faults and marked on the section by thin lines. The faults have a form of flower structures and are apparently related to strike-slip motions with minor vertical displacements. Consider a sequence of reflections in the vicinity of borehole 12rA ŽFig. 2. located near station 365. Correlation with the borehole data shows that the reflector appearing at times of about 130 ms, may be related to the top Saqiye interface penetrated at elevation of y143 m. This reflector can be traced up to station 515 to the east ŽFig. 6. and up to station 200 to the west. Its extension further to the west is unclear, apparently due to decreasing thickness of the lowermost Kurkar layers, as can be seen in borehole 12r0. The reflector appearing at time of about 115 ms, may be correlated to the top of the Kurkar
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layer K A6 penetrated in the borehole at elevation of y122 m. The reflector appearing at time of about 90 ms may be correlated to the top of the relatively thick clay layer penetrated at elevation of y86 m. It is difficult to detect and trace reflections from two thin Kurkar layers K A4 and K A5 below the clay layer, although some indications on their presence as a single unit can be found in the section. Two upper reflectors appearing at times of about 40 ms and 60 ms, may be correlated to two thin clay layers penetrated at elevations of y40 m and y53 m. These reflectors separate three upper Kurkar layers K A1, K A2 and K A3. In the seismic section, the event associated with K A3 layer clearly pinches out in the vicinity of station 280, while K A2 layer can be traced further to the west. The sequence of events at the beginning of the section Žin the vicinity of station 20. can be
Fig. 10. Pseudo-2D resistivity cross-section in the western part of seismic line GI-0082 Žpreliminary interpretation..
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correlated to borehole 12r0 ŽFig. 2. located about 135 m southward of the beginning of the line. Here the identification of the Top Saqiye interface is problematic, probably due to a relatively small thickness of the lower Kurkar layers K 05 and K 06 . The strong reflector appearing at time of about 100 ms, may possibly be related to the top of the thick clay layer penetrated at elevation of y110 m. The reflector appearing at time of about 80 ms, may be related to the clay–loam layer penetrated at elevation of y79 m. The time interval between the above two reflectors may be associated with the Kurkar layers K 03 and K 04 . Tracing these layers to the east indicates that they may be apparently related to layer K A2 mapped in the vicinity of borehole 12rA. The reflector appearing at time of about 50 ms, may be related to a thin clay unit penetrated at elevation of about y48 m; this reflector apparently separates two upper Kurkar layers K 01 and K 02 . The layers may be traced Žalbeit somewhat problematically. up to borehole 12rB Žstation 155. where they seem to correspond to layers K B1 and K B2 . Other events appearing above and below the reflector, seem to be uncorrelated to the borehole data; probably, these events correspond to local clay lenses which do not reach the borehole. 3.2. Line GI-0083 (Fig. 8) This line runs in a SW–NE direction and is about 1.9 km long ŽFig. 1. . In the vicinity of borehole 12rA it crosses line GI-0082, as marked above the section. The general character of seismic section along the line is similar to that of line GI-0082. The section shows a sequence of almost horizontal or very gently dipping reflected events. The reflector related to the Top Saqiye interface can be identified at times of about 130–140 ms; above the interface, the event corresponding to Kurkar layers K 06 and K 07 can be traced almost along the entire section. In the southern part of the line, Kurkar
layer K A3 can be detected. Continuous tracing of the layer along the section is somewhat problematic, possibly due to geometric or facial lateral changes along the corresponding geological units. In the region between stations 510 and 580, the continuity of all reflectors is clearly interrupted. Here the line apparently crosses a fault zone, as marked on the section. Additional, smaller faults were mapped in the vicinity of stations 250–300.
4. TDEM survey Nine TDEM stations were established roughly along seismic line GI-0082 ŽFig. 1. . All the collected data were processed and interpreted using the Interpex TEMIX-XL 1-D interpretation package ŽInterpex, 1996.. The interpretation results for all nine soundings are shown in Fig. 9. No a priori information has been used at this stage. The initial model for each inversion was obtained by applying first Occam’s inversion ŽConstable et al., 1987. and then reducing to the possible minimum the number of layers in the recovered smooth model. According to the above described hydrogeological setting of the area, the interpreted resistivities can be divided into four groups: Ø Very low resistivities Žless than 3 V P m.. These resistivities are typical for saline water saturated lithologies Ž both aquifers and aquicludes.. Ø Low resistivities Ž between 3 to 8 V P m.. These resistivities are characterizing either brackish water saturated aquifers or aquicludes Žmainly clays. . Ø Moderate resistivities Žbetween 8 and 15 V P m.. Typical for fresh water saturated aquifers and some aquicludes Ž loams.. Ø High resistivities Žgreater than 15 V P m.. These resistivities are typical for either dry or fresh water saturated aquifers. The lateral extent of sea water intrusion into the aquifer can be estimated from the TDEM
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measurements alone just taking into account the depth to the aquifer base Žtop Saqiye., which is very consistent along the whole profile ŽFigs. 2 and 3. . According to the borehole data the depth to the aquifer base is approximately 160 m in the whole test area. This means that if the very low resistivity unit is located at depths shallower than 160 m, it represents sea water intrusion, otherwise it is identified with the Saqiye
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clays underlying the aquifer. Fig. 9 clearly indicates that sea water intrusion terminates somewhere between stations N3 and N4 Ž i.e., at a distance of approximately 800 m from the sea. . Thus, according to the preliminary TDEM interpretation the aquifer is fully saturated with fresh water eastward of station N4. Unfortunately, due to an insufficient resistivity contrast, the depth to the water table cannot be obtained from
Fig. 11. Preliminary Žabove. and final Žbelow. 1-D interpretations of the TDEM data at station N4.
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the TDEM measurements. Therefore the most interesting part of the profile for evaluating the quality Žsalinity. of groundwater is located between stations N1 and N4. Fig. 10 shows the pseudo-2D resistivity cross-section of the west part of the profile compiled from appropriate 1-D resistivity vs. depth models ŽFig. 9.. Beneath station N3 one can see two moderately resistive layers which can be identified with fresh water saturated subaquifers. However, the lateral extension of the lower subaquifer is unclear since it was not detected at stations N2 and N4. It seams it is terminated somewhere between stations N3 and N4 by an aquiclude layer having resistivity of approximately 5 V P m, while between N3 and N2 the subaquifer probably becomes saturated with saline water. Thus, according to the preliminary TDEM interpretation, the lower subaquifer saturated with fresh groundwater is confined to an area around station N3.
5. Integrated interpretation of seismic and TDEM results In order to verify the above results, a combined interpretation of the TDEM and seismic data was performed. For this purpose, the western part of seismic line GI-0082 was used. The locations of the TDEM stations are marked above the seismic section ŽFig. 7.. According to the seismic interpretation, the lower Kurkar units designated as K A6 and K A7 extend from a vicinity of TDEM station N2 continuously through stations N3 and N4 further eastward Ž Fig. 7. . The moderately resistive layer detected beneath station N3 at approximately the same elevation Ž y120 m. is most likely identified with these units saturated with fresh water. If this assumption is correct, the data collected at station N4 where the lower subaquifer was not detected, should be reinterpreted. In order to make the TDEM interpreta-
Fig. 12. Pseudo-2D resistivity cross-section and appropriate borehole data in the western part of seismic line GI-0082 Žfinal interpretation..
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tion consistent with the seismic results, an additional moderately resistive layer has been included in the initial model used for the TDEM inversion. This led to the solution shown in Fig. 11. For the sake of convenience, the figure also shows the results of the preliminary interpretation. The uncertainty of the model parameters can be roughly estimated by using the so called linear equivalence analysis Ž Goldman et al., 1994.. The best fit models are shown by solid lines in the resistivity vs. depth sections in Fig. 11. Dashed lines in the figure represent alternative models for which the misfit error is only slightly greater than for the best fit model. As could be expected, both the resistivity and thickness of the lower moderately resistive layer are poorly resolved and its inclusion in the interpreted model is only justified by the existence of the independent seismic interpretation. It should be emphasized that the reinterpretation was based on seismic results only, without being biased by any borehole information. The misfit error of the final interpretation decreased from approximately 3% obtained in the preliminary interpretation to slightly more than 2%. But the most important result is that the final interpretation of the TDEM data now becomes consistent with the independent seismic interpretation and ultimately with the borehole data available. The final resistivity cross-section accompanied by the borehole data is shown in Fig. 12. One can see that the location of the lower moderately resistive unit roughly coincides with the lower subaquifer in well 12rA. Note that the coincidence is rather poor due to the above mentioned uncertainty in the layer parameters. The resistivity of the aquiclude layer separating two subaquifers varies laterally from approximately 5 V P m in the eastern part of the section to slightly more than 2 V P m in its western part. This lateral variation can be explained by different salinities of water within the aquiclude: the closer to the sea, the higher salinity Ži.e., the lower resistivity. is expected. It should be noted that this variation may explain the above mentioned difference in the interpre-
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tation of the TDEM data at points N3 and N4. The much smaller resistivity contrast between the aquiclude and underlying subaquifer at point N4 as compared to N3 may be the reason why this subaquifer was not detected at N4 during the preliminary interpretation. The boundary drawn within the very low resistivity unit between approximately 1.5 V P m and 2.5 V P m can be most likely identified with the boundary between the sea water saturated aquifer and aquiclude. Finally, the boundary between the moderately and highly resistive layers and very low resistivity unit exactly coincides with the freshwaterrseawater interface detected in wells 12r0 and 12rB. The interface was not detected in well 12rA obviously because of the impermeable clays appearing somewhere in the vicinity of station N3. Comparison of the seismic and TDEM sections with the geological cross-section compiled from the borehole data shows some discrepancy in the layer geometry in the area between boreholes 12rA and 12rB. Specifically, while the geological cross-section obtained by linear interpolation between the boreholes shows monotonous inclination of all layers westward ŽFig. 3., both geophysical sections show a structural high in the corresponding region Ž Figs. 7 and 12.. Such a situation can be encountered over a region with sparse borehole control. In this case, seismic sections can provide a continuous and, therefore, more diagnostic image of the subsurface.
6. Conclusions The geophysical surveys carried out in the Nitzanim area provided an important information necessary for a detailed study of the aquifer in the area. The seismic sections obtained along the reflection lines display a sequence of reflected events down to times of about 250 ms Žabout 270–300 m depth.. Correlation of the seismic
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data with borehole information shows that the reflections can be related to various impermeable units located within and below the aquifer. By tracing the reflectors along the sections, the geometry and lateral extension of the units can be estimated. Based on this interpretation, the aquifer can be subdivided into a number of subaquifers separated by the impermeable units, as it has been done using the borehole data alone ŽFig. 3. . At several locations along the sections, disturbance zones apparently related to shallow faults were mapped; this mapping may have implications for the hydraulic continuity of the various aquifers in the region. The TDEM survey resulted in a pseudo-2D resistivity cross-section located along the western part of seismic line GI-0082. The section clearly shows sea water intrusion appearing as a highly conductive geoelectric unit. A number of fresh water saturated subaquifers can be detected in the section. However, the lower subaquifer was not always revealed by the routine conventional TDEM inversion. The reinterpretation of the TDEM data based on the combined use of seismic and electromagnetic results, enabled us to successfully solve the above inversion problem thus considerably improving the hydrogeological significance of the geophysical results. The integrated approach to the interpretation of the geophysical data can be applied for solution of similar problems in other areas.
Acknowledgements This work was carried out in the framework of the INCO-DC project financed by the European Commission Žcontract aIC18CT96-0122.. The authors are grateful to E. Fleisher and D. Gilad for fruitful discussions of hydrogeological aspects of the project and to M. Ezersky for the technical assistance.
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