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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
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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. q 2000 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.: q972-3-5576046; Fax: q972-3-5502925; E-mail: markiprg.energy.gov.il
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 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. Ž
. PII: S 0 9 2 6 - 9 8 5 1 9 8 0 0 0 5 3 - 6
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
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. which 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
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coastal plain of Israel Fig. 1 . In order to
Fig. 1. Map showing schematic location of seismic lines, TDEM measurements and boreholes in the investigated area.
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 reflection 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 Ž
. borehole indicate depth in meters from the surface.
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 Ž
. Fig. 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
Fig. 3. Schematic W–E hydrogeological section across the Nitzanim area. The horizontal axis shows the distance from the sea.
Table 1 Seismic data acquisition: equipment and parameters
Recorder 48 channel EG G seismograph, model ES-2401X
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Energy source Dynasource truck mounted vacuum accelerated heavy weight drop
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Receivers—single geophones 10 Hz vertical
Receiver spacing 2.5 m
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Shot spacing 2.5 m every station
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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
