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 Ž
. GII 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
Ž
. calcareous sandstone is about 2000 mrs. 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
. scale in Figs. 6 and 7
and each part was considered separately. The seismic events inter-
preted as corresponding to various Kurkar lay- ers within the aquifer were marked by thick
white lines and designated by capital K with the corresponding indices, whereas the reflector 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
Ž .
Fig. 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 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 . The layer can
13
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 traced
12
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
. In this region, the seismic section Fig. 7
looks much more complicated. A sequence of reflected events with various local dips can be
detected below the datum down to times of
Fig. 9.
Preliminary 1-D
interpretation of
the TDEM
data.
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 penetrated in the borehole at eleva-
A6
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 K
and K be-
A4 A5
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 K
, K and K
. In
A1 A2
A3
the seismic section, the event associated with K
layer clearly pinches out in the vicinity of
A3
station 280, while K layer can be traced
A2
further to the west. The sequence of events at the beginning of
Ž .
the section in the vicinity of station 20 can be
Ž .
Fig. 10. Pseudo-2D resistivity cross-section in the western part of seismic line GI-0082 preliminary interpretation .
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 K
and K . The strong reflector appearing
05 06
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 of y79 m. The time interval between
the above two reflectors may be associated with the Kurkar layers K
and K . Tracing these
03 04
layers to the east indicates that they may be apparently related to layer K
mapped in the
A2
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 K
and K . The layers
01 02
Ž may be traced
albeit somewhat problemati- .
Ž .
cally up to borehole 12rB station 155 where they seem to correspond to layers K
and K .
B1 B2
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 K
06
and K can be traced almost along the entire
07
section. In the southern part of the line, Kurkar layer K
can be detected. Continuous tracing
A3
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
