Directory UMM :Data Elmu:jurnal:J-a:Journal Of Applied Geophysics:Vol44.Issue2-3.2000:

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Journal of Applied Geophysics 44 2000 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

)

The Geophysical Institute of Israel, P.O. Box 2286, Holon 58122, Israel

Received 24 March 1998; accepted 20 November 1998

Abstract

Ž .

Two geophysical surveys using shallow reflection seismics and time domain electromagnetics TDEM , 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 VPm. 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

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additional layers based on the seismic interpretation improved both the inversion results misfit error and, especially, the hydrogeological significance of the TDEM interpretation.q2000 Elsevier Science B.V. All rights reserved.

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Keywords: Coastal aquifer; Seismic reflection; Time domain electromagnetics TDEM ; Integration

1. Introduction

The problem of effective management of groundwater resources is of paramount

impor-)_{Corresponding author. Tel.:} _q_{972-3-5576046; 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 ground-water is the only source of fresh ground-water in the whole coastal plain. The coastal aquifer supplies about one quarter of the country’s annual water

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V. ShtiÕelman, M. GoldmanrJournal of Applied Geophysics 44 2000 197–215 198

consumption. Like in many other regions, the aquifer suffers from severe salinization caused by seawater encroachment. The problem is fur-ther aggravated by the population growth and, consequently, the progressively increasing ex-traction 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 geophys-ical surveys.

Among various geophysical methods, seismic reflection and electromagnetic techniques seem to be the most suitable for this purpose. How-ever, each of these methods alone is efficient in solving only a specific hydrogeological prob-lem, 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 per-form 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

Ž

of the coastal plain of Israel 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 inter-pretation 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

Ž

the presence of fresh groundwater the so called

.

hydrological inversion . Unfortunately, in most cases the existence of impermeable hydrogeo-logical units, which justifies the validity of the above mentioned model, could not be concluded from the TDEM interpretation alone. In particu-lar, this problem was encountered in the Nitzanim area located in the southern part of the

Ž .

coastal plain of Israel Fig. 1 . In order to

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provide a detailed and reliable study of the coastal aquifer in the area, an integrated geo-physical survey, including shallow reflection seismics and TDEM, was carried out. The sur-vey was a part of an INCO-DC research project aimed at developing an integrative geophysical approach for rational management of groundwa-ter resources.

The seismic survey included two high-resolu-tion reflechigh-resolu-tion lines shot using the CMP tech-nique. In the past decade, the high-resolution

seismic reflection method has been developed and used in different regions for geotechnical

Ž

and environmental studies 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

Ž

groundwater-related investigations 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

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vicinity of a number of hydrogeological obser-vation 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 be-.

tween 50 by 50 m near the sea shore to 200 by 200 m in the eastern part of the profile provid-ing the penetration depth from about 100 m to approximately 300 m, respectively. In order to

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V. ShtiÕelman, M. GoldmanrJournal of Applied Geophysics 44 2000 197–215 201 Table 1

Seismic data acquisition: equipment and parameters

Recorder 48 channel EG & G seismograph, model ES-2401X

Ž .

Energy source Dynasource truck mounted vacuum accelerated heavy weight drop

Ž .

Receivers—single geophones 10 Hz vertical

Receiver spacing 2.5 m

Ž .

Shot spacing 2.5 m every station

Ž .

Shot location Near 1st receiver off-end geometry

Minimum offset 1 m

Maximum fold 24

Record length 0.5 s

Time sampling interval 0.5 ms

Analog band-pass filters 70–250 Hz

provide better vertical resolution and accuracy

Ž .

of interpretation Goldman 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 inter-pretational model consisting of a minimum pos-sible number of layers was used. After compar-ing 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 interpreta-tional 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

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V.

Shti

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elman,

M.

Goldman

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Journal

of

Applied

Geophysics

44

2000

197

–

215

202

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V.

Shti

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elman,

M.

Goldman

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Journal

of

Applied

Geophysics

44

2000

197

–

215

203

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V.

Shti

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M.

Goldman

r

Journal

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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

sand-Ž .

stone the 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 sev-eral cycles of transgressions and regressions of the sea. With each cycle, deposition environ-ment 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: calcare-ous 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. Calcare-ous sandstones are generally porCalcare-ous and hydro-logically 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.

The coastal aquifer is replenished by rain precipitation during the winter months. Ground-water flows westward, toward the sea. Usually, the groundwater level rises from west to east with a gradient of about 1 m per 1 km. Dis-charge of the aquifer is in the form of seepage along the seawaterrfreshwater interface. In cer-tain 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

Ž

the investigated site in the Nitzanim area Figs.

. Ž

1 and 2 . Three of the wells 12r0, 12rA and

.

12r1 penetrate the entire aquifer down to the Saqiye group which is about 160 m deep in this

Ž

area. In the western part of the area wells 12r0

.

and 12rA marine sediments are penetrated,

Ž .

whereas in the eastern part well 12r1 conti-nental sediments were encountered. In Fig. 2, various Kurkar subaquifers encountered in the wells are designated by capital K with corre-sponding indices.

According to hydrological data, the water table level in the area is about 2–3 m above the

Ž .

mean sea level MSL . The sea water penetrates into the aquifer up to about 1.0 km from the coastal line.

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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

Ž .

technique lines GI-0082 and GI-0083, Fig. 1 . The length and locations of the lines were cho-sen 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 parame-ters 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-stan-.

dard 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 millisec-onds. 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 conver-sion 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 m. rs. Since below the MSL the section is represented mainly

Ž .

by the Kurkar unit 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 mil-liseconds roughly correspond to negative eleva-tions in meters.

( ) 3.1. Line GI-0082 Fig. 5

The line runs in a NW–SE direction and is

Ž .

about 2.6 km long 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 se-quence of reflected events which can be traced

Ž

down to times of about 250 ms corresponding

.

to depths of about 270–300 m . One of the most prominent features on the section is an anoma-lous zone in the central part of the line between stations 520–600. The zone subdivides the sec-tion laterally into three parts with different char-acter of seismic data. The continuity of the reflectors appearing on both sides of the zone, is clearly interrupted within the zone. This distur-bance 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

Ž

eastern part of the area as encountered in

bore-.

hole 12r1 versus the marine sediments in the

Ž .

western part boreholes 12r0, 12rB and 12rA . For convenience of interpretation, the section

Ž

was divided into two parts shown to a larger

.

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as-( )

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 fre-quency 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

Ž .

boreholes Fig. 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 ef-fect from the whole layer will be obtained. In other words, we can possibly detect the pres-ence of such a thin layer in the section but will be unable to estimate its vertical extension. The relatively low apparent dominant frequencies of

Ž

the reflections about 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

Ž .

12r1 Fig. 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-hole at elevation of about y132 m. This inter-face corresponds to the erosional contact be-tween the Kurkar unit and the underlying thick impermeable clays of the Saqiye group. Previ-ous seismic surveys carried out in the Coastal

Ž .

Plain Schlein 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 appear-ing at time of about 100 ms, is apparently related to the top of the lower Kurkar layer K14

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 . The layer can13

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 . This layer can be traced12

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

Ž .

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Fig.

9.

Preliminary

1-D

interpretation

of

the

TDEM

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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 appar-ently related to strike-slip motions with minor vertical displacements.

Consider a sequence of reflections in the

Ž .

vicinity of borehole 12rA Fig. 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

Ž .

the east Fig. 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

layer K_A6 penetrated in the borehole at eleva-tion 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 KA4 and KA5 be-low 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 KA1, KA2 and KA3. In the seismic section, the event associated with KA3 layer clearly pinches out in the vicinity of station 280, while KA2 layer can be traced further to the west.

The sequence of events at the beginning of

Ž .

the section in the vicinity of station 20 can be

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Ž .

correlated to borehole 12r0 Fig. 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 rela-tively small thickness of the lower Kurkar lay-ers K₀₅ and K . The strong reflector appearing₀₆ at time of about 100 ms, may possibly be related to the top of the thick clay layer pene-trated 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 ofy79 m. The time interval between the above two reflectors may be associated with the Kurkar layers K03 and K . Tracing these04

layers to the east indicates that they may be apparently related to layer KA2 mapped in the vicinity of borehole 12rA. The reflector appear-ing 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 K01 and K . The layers02

Ž

may be traced albeit somewhat

problemati-. Ž .

cally up to borehole 12rB station 155 where they seem to correspond to layers KB1and KB2. Other events appearing above and below the reflector, seem to be uncorrelated to the bore-hole data; probably, these events correspond to local clay lenses which do not reach the bore-hole.

( ) 3.2. Line GI-0083 Fig. 8

This line runs in a SW–NE direction and is

Ž .

about 1.9 km long Fig. 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 se-quence of almost horizontal or very gently dip-ping 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 K06

and K07 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 prob-lematic, possibly due to geometric or facial lateral changes along the corresponding geologi-cal units.

In the region between stations 510 and 580, the continuity of all reflectors is clearly inter-rupted. 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

Ž .

along seismic line GI-0082 Fig. 1 . All the collected data were processed and interpreted using the Interpex TEMIX-XL 1-D

interpreta-Ž .

tion package Interpex, 1996 . The interpreta-tion 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

inver-Ž .

sion Constable 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 di-vided into four groups:

Ž .

Ø Very low resistivities less than 3 VPm .

These resistivities are typical for saline water

Ž

saturated lithologies both aquifers and

.

aquicludes .

Ž .

Ø Low resistivities between 3 to 8 VPm .

These resistivities are characterizing either brackish water saturated aquifers or

Ž .

aquicludes mainly clays .

Ž

Ø Moderate resistivities between 8 and 15 VP

.

m . Typical for fresh water saturated aquifers

Ž .

and some aquicludes loams .

Ž .

Ø High resistivities greater than 15 VPm .

These resistivities are typical for either dry or fresh water saturated aquifers.

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measurements alone just taking into account the

Ž .

depth to the aquifer base top Saqiye , which is

Ž

very consistent along the whole profile Figs. 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 shal-lower than 160 m, it represents sea water intru-sion, otherwise it is identified with the Saqiye

clays underlying the aquifer. Fig. 9 clearly indi-cates that sea water intrusion terminates

some-Ž

where between stations N3 and N4 i.e., at a

.

distance of approximately 800 m from the sea . Thus, according to the preliminary TDEM inter-pretation 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

Ž . Ž .

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the TDEM measurements. Therefore the most interesting part of the profile for evaluating the

Ž .

quality salinity of groundwater is located be-tween 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.

Ž .

depth models Fig. 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 VPm, while between N3 and

N2 the subaquifer probably becomes saturated with saline water. Thus, according to the prelim-inary TDEM interpretation, the lower sub-aquifer saturated with fresh groundwater is con-fined to an area around station N3.

5. Integrated interpretation of seismic and TDEM results

In order to verify the above results, a com-bined interpretation of the TDEM and seismic data was performed. For this purpose, the west-ern part of seismic line GI-0082 was used. The locations of the TDEM stations are marked

Ž .

above the seismic section Fig. 7 .

According to the seismic interpretation, the lower Kurkar units designated as KA6 and KA7

extend from a vicinity of TDEM station N2 continuously through stations N3 and N4 further

Ž .

eastward Fig. 7 . The moderately resistive layer detected beneath station N3 at approximately

Ž .

the same elevation 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 sub-aquifer was not detected, should be reinter-preted. In order to make the TDEM

interpreta-Ž

Fig. 12. Pseudo-2D resistivity cross-section and appropriate borehole data in the western part of seismic line GI-0082 final

.

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tion consistent with the seismic results, an addi-tional moderately resistive layer has been in-cluded 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 interpreta-tion. The uncertainty of the model parameters can be roughly estimated by using the so called

Ž

linear equivalence analysis 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 alterna-tive 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 exis-tence of the independent seismic interpretation. It should be emphasized that the reinterpretation was based on seismic results only, without be-ing biased by any borehole information. The misfit error of the final interpretation decreased from approximately 3% obtained in the prelimi-nary 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 inter-pretation and ultimately with the borehole data available. The final resistivity cross-section ac-companied by the borehole data is shown in Fig. 12. One can see that the location of the lower moderately resistive unit roughly coin-cides 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 5VPm in the eastern part of the

section to slightly more than 2 VPm in its

western part. This lateral variation can be ex-plained by different salinities of water within the aquiclude: the closer to the sea, the higher

Ž .

salinity i.e., the lower resistivity is expected. It should be noted that this variation may explain the above mentioned difference in the

interpre-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_VPm

and 2.5 VPm 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 coin-cides with the freshwaterrseawater interface de-tected in wells 12r0 and 12rB. The interface was not detected in well 12rA obviously be-cause of the impermeable clays appearing some-where in the vicinity of station N3.

Comparison of the seismic and TDEM sec-tions with the geological cross-section compiled from the borehole data shows some discrepancy in the layer geometry in the area between bore-holes 12rA and 12rB. Specifically, while the geological cross-section obtained by linear in-terpolation between the boreholes shows monotonous inclination of all layers westward

ŽFig. 3 , both geophysical sections show a struc-. Ž

tural high in the corresponding region 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 contin-uous and, therefore, more diagnostic image of the subsurface.

6. Conclusions

The geophysical surveys carried out in the Nitzanim area provided an important informa-tion necessary for a detailed study of the aquifer in the area.

The seismic sections obtained along the re-flection lines display a sequence of reflected

Ž

events down to times of about 250 ms about

.

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data with borehole information shows that the reflections can be related to various imperme-able 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

Ž .

alone Fig. 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 west-ern 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 de-tected in the section. However, the lower sub-aquifer was not always revealed by the routine conventional TDEM inversion.

The reinterpretation of the TDEM data based on the combined use of seismic and electromag-netic 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 solu-tion of similar problems in other areas.

Acknowledgements

This work was carried out in the framework of the INCO-DC project financed by the

Euro-Ž .

pean Commission contract a_{IC18CT96-0122 .}

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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