F. Trabelsi, A. Ben Mammou, J. Tarhouni, C. Piga, G.P. ... - SWIM Site

Salt Water intrusion characterisation in the
Coastal Aquifer of Nabeul Hammamet Using
Geophysical Methods
(1). Fatma Trabelsi , (1). Abdallah Ben Mammou, (2) .Jamila Tarhouni, (3). Carlo Piga,
(3) Gian Piero Deidda , (3). Gaetano Ranieri
Abstract— A Time Domain Electromagnetic (TDEM) survey
was carried out on the Mediterranean coast of Tunisia (Nabeul
Hammamet), in order to delineate salt water intrusion into
coastal fresh water aquifer detected by drilling wells.
The survey was also carried out to determine the geometry of the
fresh water aquifer and to identify the continuity between the
aquifer system of Nabeul Hammamet and the contiguous aquifer
system on the northern side (coastal aquifer of Korba).
One dimensional interpretation of the TDEM data was
correlated to existing data from borehole logs (Gamma Ray,
Spontaneous Potential, Resistivity) .
The results indicate that TDEM survey was able to map the
interface between fresh and salt water and estimate depth to
basement when siting new monitoring wells.
Index Terms Coastal aquifer, Salt water intrusion, TDEM,
Tunisia.
I. INTRODUCTION
ocated in the North -Eastern part of Tunisia in Cap Bon
L peninsula, the largely populated Nabeul-Hammamet area
(Figure 1) is considered an interesting agricultural sector
with important urban, tourist and industrial activities. The
steady increase in water consumption has severely depleted
the shallow aquifer resulting in declining water levels, salt
water encroachment, and dewatering in some areas of this
coastal plain.
The resistivity of a water bearing formation is highly
dependent upon porosity, degree of saturation, and water
salinity [19]. Electrical surveys may be used to map aquifers
and water quality zones. The inherent capabilities of the
Manuscript received September 29,2006. This work was supported in part by
a cooperative ground water studies between the Faculty of Engineering of
Cagliari in Italy, the National Institute of Agronomy (INAT) and a Faculty of
Sciences (FST) in Tunisia, in framework of Interlink project of Italian
Ministry of Research. The project is also supported by the Regional
Government of Sardinia (L.R.19/96).
(1). Geology Department, Faculty of Sciences of Tunis (FST), University El
Manar, Tunisia (e-mail: trabelsifatma@gmail.com)
(2). Genie Rural Department , National Agriculture Institute of Tunis (INAT),
University 7 Novembre, Tunisia
(3) Dipartimento di Ingegneria del Territorio-Università di Cagliari- Italy
electrical geophysical methods to detect changes in water
resistivity make them highly sensitive to the fresh-saline water
interface found in coastal regions. Several authors have
described the subject in detail [11, 13, 15].
In the survey area, the Time Domain Electro Magnetic
(TDEM) method was employed to determine electrical
resistivity stratification with depth.
The main aim of this work was to detect, identify and
delineate aquifers by exploiting the expected large contrasts in
resistivity between fresh water saturated rocks (high and
intermediate resistivity) and salt water contaminated or clayey
materials (low resistivity).
Other aims were to verify the continuity between the aquifer
of Nabeul- Hammamet and the contiguous aquifer on the
northern side: the coastal aquifer of Korba. For this survey,
TDEM soundings enabled to verify the presence of path-ways
for migration of contaminated groundwater by salt water
intrusion.
Fig. 1. Location Map of Nabeul Hammamet area
Proceedings 1st SWIM-SWICA Joint Saltwater Intrusion Conference, Cagliari-Chia Laguna, Italy - September 24-29, 2006
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I. GEOLOGICAL & HYDROGEOLGICAL BACKGROUND
A. Geological Background
Northeastern Tunisia is characterised by NE-SW to NNE-
SSW folds and by faults of various directions, forming shear
corridors. These faults delimit compartments with different
deformation geometries [3].
In the Nabeul Hammamet catchment area, is a NW-SE
trending structure corresponding to the Grombalia trough
covered by the Quaternary series [5]. Beneath these series,
several authors have proposed a graben structure limited by
two major faults to explain the thickening of the Mio-Plio-
Quaternary series in this trough [2, 6, 26]. In the southern part
of the trough, the Hammamet Fault extends over a length of 5
km. In the subsurface, this fault represents a major deep polyphased
strike-slip fault represented by a flower structure [2, 4,
6, 8, 14].
This structure gives the zone a form of syncline bordered
NW-wards by major faults.
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Fig. 2. Structural map of Nabeul Hammamet area
Fig. 3. Geological map of study area
B. Hydrogeological Background
The Nabeul-Hammamet area ( Figure 1) covers a surface area
of about 308 km 2 , a length of 34 km and a width ranging from
8 to 16 km. The climate is semi-arid with mean annual
precipitation at Nabeul city of 400 mm and mean temperature
of 19°C.
Surface water resources are very limited. The area was
supplied by two main temporary rivers (wadi) with extended
catchment areas, serving to ensure natural recharge of the
aquifer systems. Groundwater resources constitute the main
source of water supply for agricultural, tourist and industrial
activities.
Former studies of the basin provided other hydrogeological
information. Essentially composed of Plio-Quaternary
deposits, the shallow aquifer displays several lithologies:
sands, sandstones, limestones, and sandy clays. These deposits
rest on the Pliocene Nabeul clay.
The deeper aquifers are embedded in detritus formations of
Oligo-Mio-Pliocene age.
Aquifers are presumably hosted in the sedimentary formations
of the Quaternary of Nabeul and in the Plio-Quaternary and
Upper Oligocene of Hammamet.
II. METHODS
A. TDEM soundings
TDEM method has been utilized successfully in
hydrogeological surveys, delineating aquifers structures and
contamination 13, 16, 28]. The earth is energized by abruptly
shutting off the current in the transmitter. According to
Faraday’s Law, currents are induced in the subsurface,
decaying with time and producing secondary magnetic field
that creates a measurable voltage in the receiver [27]. In the
coincident loop configuration, only one loop serves as
transmitter and receiver during the time-on and time-off
period of the current, respectively. Unlike other geophysical
sounding methods where the receiver-transmitter array must
be expanded to explore deeper parts of the geoelectric section,
the depth of investigation for the TDEM method depends on
length of recording window [24].
Measurement of the signal at later times provides information
at greater depths. The intensity of the eddy currents at specific
times and depths is determined by the bulk conductivity of
subsurface rock units and the fluids contained therein. Depth
of investigation depends on the time interval after shutoff of
the primary current, because later the receiver subsequently
senses eddy currents at progressively greater depths [13, 21,
22]. The method is insensitive to inhomogeneities in the
surface layers; most common reason for poor quality data sets
in all electromagnetic methods. Among the EM methods, this
method has the best lateral and vertical resolution with regards
to highly conductive targets [17]. A variety of data
interpretation techniques have been proposed.
These include the imaging techniques [20, 30], conductive
finite plate models [18, 25] and inversion methods that use 1D
horizontal layered earth model [7, 9, 12, 29].
A TDEM survey consists of TEM FAST 48 battery powered
transmitter (4 s–16 ms), connected to a single-turn,
Proceedings 1st SWIM-SWICA Joint Saltwater Intrusion Conference, Cagliari-Chia Laguna, Italy - September 24-29, 2006

ungrounded, insulated cable in the form of a square loop with
sides of between 25 and 300m.
In the present work, we conducted a large scale TDEM
survey involving 55 soundings (Figure 4) with the coincident
loop configuration (50×50m and 100 ×100m square loops at
respectively 50m and 100m spacing) aligned on a rectangular
grid. We performed 1D interpretation scheme [10] and the
interpreted results have been correlated with availabe
geological and geophysical data, providing useful information
about the local hydrogeological conditions.
Fig. 4. Location of TDEM stations in the study area
III. RESULTS
A. TDEM Results
Time Domain Electromagnetic (TDEM) soundings were
interpreted using one-dimensional (1-D) inversion. The initial
model for each inversion was obtained by first applying
Occam’s inversion [9] and then reducing to the minimum
possible the number of layers in the recovered smooth model.
Most of the sites yield sounding curves with a monotonic
decrease in apparent resistivity with time. Such curve behavior
undoubtedly proves the presence of a low resistivity layer in
the base of the section. At some sites it was necessary to
include a thin layer of very low resistivity within the top
resistive layer to be consistent with the data.
Figure 5 shows some typical examples of TDEM sounding
curves from locations close to the coast (sites F16 and F15)
and farther inland (sites F12, F13, F7, F6, F3, F8 and F9). The
quantitative interpretation of the TDEM measurements is,
however hampered by the equivalence problem, which means
that several different layered models may give practically the
same resistivity curve.
To choose the model that best represents actual subsurface
conditions, supplementary data such as well logs are required.
In the study area there are more than 200 drilled wells. The
static water level is generally found at a few meters above
mean sea level.
Fig. 5. Examples of model of resistivity vs. depth generated by TEM- FAST
smooth modelling procedure corrected to sea level
1) Comparison of borehole Log and TDEM resistivity
Both TDEM soundings from the surface and borehole
resistivity logs measure vertical geoelectrical sections.
Comparison of the two provides important information (i) to
define the position of the transition zones within individual
coastal plain aquifers, (ii) to interpret the geoelectrical section
derived from TDEM data.
We present results of TDEM, which are correlated to borehole
logs (SP, GR, and Resistivity). TDEM soundings were
conducted at existing drilled wells wherever possible, or at a
maximum distance of 1km from wells allowing correlation of
apparent resistivity with borehole geophysical logs. TDEM
survey can then be extrapolated to other parts of the coastal
plain between research stations.
The TDEM resistivity soundings show good correlation with
the borehole resistivity, SP and GR Logs. The sequence and
thickness of layers, however, and the relative values of high
and low resistivity, are consistent between two records.
Field example 1
The presence of dispersed or interbedded clay will suppress
the bulk resistivity of an aquifer, as shown by the example in
Figure 6 where the relatively high Gamma Ray count in the
borehole log reflects the clay-rich nature of the aquifer in this
area. Low resistivity measurements on both the TDEM
sounding and borehole resistivity log represent the combined
effects of clay content and high salinity conditions within the
aquifer.
Proceedings 1st SWIM-SWICA Joint Saltwater Intrusion Conference, Cagliari-Chia Laguna, Italy - September 24-29, 2006
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Fig. 6. Field Example 1
Field Example 2
In this example (Figure 7), the TDEM sounding shows good
correlation with the borehole log. The depth of the low
resistivity layer coincides with a water level of the borehole
(-9.5m) which is considered as the fresh water aquifer.
The second low resistivity zone is apparent below the depth of
(-6m) with very low resistivity. This layer consists of
sandstone with clay and is attributed to the saline water
infiltration. Additional TDEM surveys in the study area
indicate that up coning of salt water may be occurring due to
excessive withdrawals from local supply wells.
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Fig. 7. Field example 2
2) Geoelectrical sections 2D
On the basis of the geological and geophysical data of
hydrogeological drillings and oil wells, and the results of
correlation between borehole logs and TDEM resistivity
soundings, the aquifer system of Nabeul Hammamet is
considered as a multi-layer aquifer..
In order to understand the distribution of the permeable area,
the geoelectrical sections have been constructed.
Proceedings 1st SWIM-SWICA Joint Saltwater Intrusion Conference, Cagliari-Chia Laguna, Italy - September 24-29, 2006

The section contributes (i) to defining the geometry of the
aquifer, in particular in the northeastern part of Nabeul, (ii) to
identifying the structures and (iii) a salt water wedge.
Geoelectrical models plotted in pseudo 2-D sections, using
interpolation to acquire a better presentation of the subsurface
of the area, shows the presence of three aquifers formation.
In Figure 8, the geoelectrical section (1) obtained, shows a
very low resistivity layer between 1-5 Ohm-m attributed to
aquifer formation at greater depths, primarily due to salt water
encroachment.
In the near surface down to depths of over 20m, exists a layer
formation with a resistivity between 7-20 Ohm-m, indicative
of fresh water. It coincides with a thick high resistivity zone
on the borehole record.
But in some geoelectrical sections, a high resistivity layer with
25-55 Ohm-m indicative of fresh water in the shallower
aquifers, is readily apparent near the surface. It coincides with
a thick high -resistivity zone on the borehole record. This high
resistivity zone just appears in the TDEM sounding in fault
zones. This variation of resistivity correlated to structural data
gives good agreement for the presence of these faults and
graben boundary definition. TDEM sounding within the
graben shows continuity of the aquifer formation overlain by
thin sediments.
On the other hand hydrostratigraphic details are almost
completely masked by a low resistivity contrast between fresh
water aquifers and clays and by the lateral effect of fault
displacements in this areas. Consequently, this fresh aquifer
cannot be detected due to lack of resolution.
Fig. 8 . Geoelectrical section (1)
Fresh water, salt water transition zones and the geometry of
the salt wedge in the aquifer are shown in the geoelectrical
sections by vertical and lateral variations in resistivity within
individual aquifers. The geoelectrical section of lateral profile
(Figure 9) parallel to the coast shows a very low resistivity
layer, according to the well log.
The top layer consists of an aquifer formation, hence, the
low resistivity is attributed to the saline water infiltration.
Fig. 9. Geoelectrical section (2)
The question always asked by hydrogeologists about the
Nabeul Hammamet aquifer, is whether continuity and
communication exists within this aquifer and the contiguous
aquifer on the northern-eastern side (coastal aquifer of Korba).
A cross section (Figure 10), shows continuity of the two
aquifers and probably the presence of fault detected in
sounding b5, interpreted as the limit of this aquifer.
Fig. 10. Geoelectrical Section (3)
3) 3D geoelectrical section
A 3D model was constructed using EVS (Environmental
Visualisation Software) data representation.
The program is able to extract data from the entire volume
investigated (i.e. resistivities higher, lower than fixed value)
and reconstruct by a 3D interpolation, the plume or the
extension of a layer in the volume.
Proceedings 1st SWIM-SWICA Joint Saltwater Intrusion Conference, Cagliari-Chia Laguna, Italy - September 24-29, 2006
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Session 9 Geophysics
Fig.11. 3-D Model of resistivity layer between 1 and 5 Ohm-m
Figure 11 shows the plume of a resistivity layer ranging
from 1-5 Ohm-m. Comparison between the results of TDEM
soundings, borehole logs and 2-D geoelectrical sections,
shows that for the TDEM soundings near the coast, this layer
can be attributed to salt water intrusion. For the TDEM
soundings carried out in inland areas this low resistivity can
be attributed to salt clay deposits and it can be due to intensive
pumping from wells in the area.
Figure 12 shows the continuity of resistivity layer ranging
between 7 and 25 Ohm-m versus elevation. This layer is
attributed to the second aquifer formation, and confirms
results of the geoelectrical section (3).
In general, these interpretation results are a good agreement
with the expected resistivity values for the layers appearing in
the well log, and the 2-D sections.
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Fig.12. 3-D Model of resistivity layer between 7 and 25 Ohm-m
IV. CONCLUSION
TDEM surveys provide an efficient, and semi-quantitative
method of evaluating ground water resources in coastal
aquifers, at fairly high vertical resolution, to a depth down to
few hundred meters, on both local and regional scales.
Comparison between TDEM soundings and borehole logs
provides more information about fresh water formation and
the structural configuration of the aquifer.
In our work, we delineate the salt water intrusion areas,
identify a subsurface aquifer formation and determine the
hydrogeological limit between the Korba and Nabeul-
Hammamet aquifers.
ACKNOWLEDGMENT
This work was conducted as part of cooperative ground water
studies between the Faculty of Engineering at Cagliari
University in Italy, the National Institute of Agronomy
(INAT) and the Faculty of Sciences of Tunis (FST). “F. A.
Fatma Trabelsi thanks all the research team at the geophysics
laboratory at the Faculty of Engineering at Cagliari University
(Italy).
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