SJBS (G), (2005), 6: 69-91
2D Electrical surveys to map
the interface between fresh
and saline ground water in
North Kedah and Perlis
Malaysia
E. A. EI Zein,(1) M. H. Loke and C.Y.Lee(2)
Abstract
In recent years, there has been an increasingly widespread use of
two-dimensional electrical imaging surveys for environmental,
hydrogeological and geotechnical studies. Such surveys give a
more accurate picture of the subsurface resistivity distribution in
areas with complex geology where conventional one-dimensional
resistivity sounding surveys are not adequate.
A two-dimensional electrical resistivity imaging survey using the
Wenner and Wenner-Schlumberger arrays were carried out along
the coastal plain of North Kedah and Perlis. The resistivity profiles
were conducted across the postulated transition zone to map the
line of interface between fresh and saline groundwater as well as
to delineate the aquifer boundaries along the transition zone. The
data were analysed and interpreted by using an automatic
inversion computer program which uses the smoothnessconstrained least-squares optimisation method.
The results, which were presented in the form of apparent and
model resistivity pseudosections show a groundwater aquifer of
_________________________________
(1)
Department of Geology, Faculty of Science, International
University of Africa, P.O. Box, 2469, Khartoum-Sudan.
(2)
School of Physics, University of Science, Malaysia, 11800
Minden Penang, Malaysia.
Sudan Journal of Basic Sciences
low resistivity values. The fresh water and saline water have
distinctive resistivity values. Thus the interface line can be clearly
distinguished from the resistivity sections. The interface varies
from being gradational to gentle dipping.
The aquifer is overlain by a thin soil layer, where the sandy soil is
relatively permeable and can provide an infiltration path for the
pollutants to enter the aquifer. The aquifer is identified as
unconfined to semi-confined. However the concentration of
agricultural pollution in the paddy fields is very low and does not
have significant effect on the resistivity of ground water. The
variation in the aquifer thickness was attributed to the variation in
thickness of alluvial deposits and/or to the structure and lithology
of the bedrock.
Measurements concluded along the transition zone between the
fresh and saline groundwater show that 2D resistivity imaging
surveys are important aid to map the interface line in areas with a
shallow water table and was successful in determining the water
quality which can not be obtained easily with other geophysical
techniques.
Introduction:
The State of Perlis and North Kedah lies in the northwestern region
of the Peninsular Malaysia. It encompasses an area of about 6400
km2 between the Latitudes of 5° to 6° 42 N and longitudes 100°
10 to 100° 50 E. The climate of the area is characterized by
equatorial maritime climate, with a uniform air temperature
through the year. The mean temperature is in the range of 25 to 28
°c. Rainfall occurs mostly during April to November with peak
periods in April to May and August to October with an average
annual precipitation of about 2200 mm. The driest period occurs in
December to March. Relative humidity is high and reflects the
seasonal rainfall pattern. Most of the rubber trees and rice grown
in Perlis and Kedah states are dependent on rainfall. Recently,
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efforts have been focused on groundwater exploration due to
increased demand of water supply in the area.
The complex hydrogeological conditions of North Kedah and
Perlis attracted the attention of geophysicists and hydrogeologists
for exploration of groundwater in terms of quality and quantity.
The water resources represent a major problem in the area. It has
been a rather chronic problem for so many years. A number of
organizations are involved in the search for groundwater in the
area, such as Geological Survey of Malaysia, Public Works
Department, Irrigation Department and lately University Sains
Malaysia. Recently the problem of groundwater conditions in
Perlis was reported in detail by Arafin and Lee (1988). Extensive
hydrogeological investigations have been carried out to assess
the groundwater potential of the states. Thus, groundwater seems
to be the best alternative to augment the present supply for
domestic as well as agricultural use in the states of Perlis and
Kedah.
The DC resistivity method is the most popular of all geophysical
methods as far as groundwater exploration is concerned since it is
sensitive to groundwater quality. The electrical imaging method is
a relatively new geophysical technique. It has been used to map
areas with complex geology where conventional resistivity
sounding or profiling surveys are not adequate (Griffiths and
Barker, 1993). Recently it has been successfully used for
geotechnical, environmental, and groundwater contamination.
Previous geophysical and geological studies were carried out in
the area by Scrivenor (1919 and 1931), Wilbourn, (1922,1924),
Alexandar (1964); Jones (1964 and 78); Arafin and Lee (1988); and
hydrogeological studies by Ministry of Works and Public Utilities
(1983). These studies concluded that there is no regionally
extensive aquifer system and the ground water occurrence
depends on local geological conditions. The marine alluvial
deposits along the coastal plain, which represent the groundwater
aquifer, were invaded by saline seawater. The saline water
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extends to about 20 km. from the sea towards the land. An
encounter with fresh and saline groundwater interface resulted in
the cancellation of 15 wells in southern Perlis. This study is
intended to apply electrical imaging surveys to map the
postulated fresh and saline groundwater interface more
accurately and to delineate the aquifer boundaries along the
transition zone which extends along the coastal plain of North
Kedah and Perlis.
General Geology and Hydrogeology:
The geological succession of the region was described by (Jones,
1978). The stratigraphic sequence in the area starts with four main
sedimentary sequences of Paleozoic to Mesozoic age. The
Machinchang Formation, Kupang Pasu and Singa Formation, and
Chuping Formation. These sedimentary formations were laid
down in a geosynclinal environment and uplifted by orogenic
forces, which produced major folds with a north-south trend
accompanied by thrusting and major plutonism (granitic
intrusion).
The basement rocks are unconformably overlain by two posttectonic formations of Tertiary age (Bukit Arang coal beds). On the
top of the stratigraphic sequence rest the Pleistocence and Recent
strata (coastal and raised alluvium). Fig. (1) summarizes the
stratigraphic sequence in the study area. A previous gravity
survey in the region (Arafin, 1988) delineated a major
sedimentary basin of Bukit Arang Tertiary beds with maximum
depth of over 800 m. The hydrogeology of the region can be
expected to be complex because of the structurally and
stratigraphically complicated geology of the area. Therefore,
there is no extensive aquifer system, and groundwater occurrence
depends on local geological conditions. The main sources of
ground water are the alluvial, limestone and fractured clastic
formations. The average depth to the water table in wells varies
between 2 and 4 meters and it rarely exceeds 10 meters. Table.1
summarizes the hydrogeological significance of geological
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formations in the region. Groundwater in Kedah and Perlis are of
relatively good quality but some potentially high yielding wells
have been abandoned due to poor quality or contaminated
groundwater (Ministry of Works and Public Utilities, 1983).
However, in the coastal plain lenses of fluvial sand which occurs
within the Quaternary to Recent deposits are of moderate to high
yields but water quality is brackish to saline. The bedrock below
the marine alluvium also contains water of poor quality. The
marine alluvium aquifer along the coastal plain is intruded by
saline seawater. The transition zone or the line of interface
between saline/ fresh groundwater was postulated previously by
Ministry of Works and Public Utilities (1983).
2-D electrical imaging survey:
Electrical resistivity surveys have been employed for many
decades in routine hydrogeological investigations of many types.
The resistivity method is capable of determining the groundwater
quality and quantity, i.e. the aquifer thickness, and whether the
water is saline, brackish, fresh or contaminated with toxic waste
(Zohdy, 1974). However, the method has not developed much in
the previous decade, except for Barker’s (1981) offset Wenner
configuration. Recent development in field acquisition of electrical
tomography data and in data-inversion technology have resulted
in the production of electrical images that more accurately mirror
the subsurface than is possible with conventional resistivity
surveys (Griffith et al. 1990; Loke and Barker, 1994,1995). The
technique is particularly suitable for shallow site investigations
where understanding of complex subsurface structure in detail is
important, and so it is well suited for hydrogeological and
environmental studies.
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Fig. (1) Geological Map of North Kedah and Perlis.
(After Geological Survey Malaysi).
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The resistivity of groundwater depends on the concentration of the
electrolytes present. Fresh ground waters generally have
resistivity values of about 10 to 100 .m. In comparison, saline
water which contains sodium chloride and other natural salts has a
significantly lower resistivity of about 0.15 to 5 .m. Many soluble
chemicals (which might be associated with groundwater pollution
problems) produced electrolytes which can also cause a
significant reduction in the subsurface resistivity.
The resistivity measurements are normally made using a system
with 4 electrodes planted into the ground surface. Current is
injected into the ground using 2 electrodes and the resulting
potential difference on the ground surface is measured at the
other 2 electrodes. From the current (I) and potential difference
(V) values, an apparent resistivity  a  values can be calculated
which is given by
a  KV I
Where K is the geometric factor which depends on the type of
electrode array used (Keller and Frischknicht, 1966). The
apparent resistivity  a  is related in a complex manner to the true
subsurface resistivity (Dey and Morrison, 1979).
Electrical tomography (also referred to electrical properties of the
subsurface by passing an electrical current along many different
paths and measuring the associated voltages. Such surveys are
usually carried out using a large number of electrodes, 25 or
more, connected to a multicore cable. A laptop microcomputer is
used to automatically select the relevant 4 electrodes for each
measurement. The apparent resistivity data points from such a
survey are normally arranged and contoured in a pictorial form
Fig. (2) as a pseudosection (Hallof, 1957). The inversion of the data
collected by this type of survey produces 2D picture of the
electrical resistivity data as developed by Loke and Baker, (1994).
However, there are many factors affecting the image results. One
of the important factors is the type of the electrode array used. For
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hydrogeological investigations, the Wenner and WennerSchlumberger (Pazdirek, and Blaha, 1996) array are the most used.
Suitable electrode configuration used where the electrical
electrodes spacing are ranged from 1 to 10 meters, depending on
the depth ranges of investigation required.
Measurements and data processing:
Field measurements were carried out to cover the area of alluvial
deposits along the coastal plain of North Kedah and Perlis, which
began in mid January till mid December 1997 using a Campus
Geopulse Imager system (Serial No. 144). A multicore cable of 5
meters spacing with 25 electrodes was used. The laptop is
connected to Geopulse via the RS 232 socket, and the survey was
run automatically using the image 25 software for collecting the
data. Both modes of survey, traverse and roll-along with
specifically created parameter files were performed during the
course of this work Fig (2).
A total of 32 electrical imaging profiles were performed during
the course of this study, using the Wenner and WennerSchlumberger electrode configuration. To ensure a penetration
depth of up to 30 m, a maximum electrode spacing of 40m was
used. The resistivity profiles constructed in an E-W direction
across the postulated transition zone between fresh and saline
groundwater interface Fig. (3). The spread and extent of the
resistivity profiles were restricted due to marshy land, Paddy
fields, hills, thick bushes or jungle which are very common in
equatorial countries like Malaysia, as well as buildings and roads.
Care was taken in the fieldwork to avoid measurements near
power lines and underground pipes.
The apparent resistivity data were modeled using the 2-D INV
program which is an automatic inversion computer program
(Loke, 1995) to obtain the 2-D resistivity model. The
pseudosections were plotted in the field for data quality control, to
choose suitable positions to run the survey and for an initial
interpretation of the data. A root-mean-square (RMS) error of less
than 5% was obtained in most cases, which indicates that the data
are of high quality.
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Geophysical interpretation:
In the interpretation of data from geophysical surveys, all the
available geophysical and geological data should be used so as to
achieve the best results. The resistivity data presented here is
only part of the 32 resistivity profiles measured over the study
area. In the interpretation of the model sections, the following
sources of information were used: the general geology and
hydrogeology of the survey area, results from borehole surveys, a
knowledge of typical resistivity values of rocks, soil, and
groundwater and earlier resistivity surveys by (Arafin, 1988).
The anomalies in the resistivity profiles are correlated with the
groundwater occurrence (in terms of quality and quantity), the
aquifer boundaries, and the fresh/saline ground water interface
respectively. The saline groundwater has a lower resistivity value
than fresh ground water due to the higher concentration of NaCl
and other ions which reduce the resistivity of the saline water. A
higher salinity also increases the density of groundwater.
Therefore the saline water usually underlies the fresh water within
the aquifer. Correlation of resistivity values and the water quality
is given in Table 2.
Since the Wenner-Schlumberger data set includes all the Wenner
array measurements, only the result for the WennerSchlumberger array will be shown in the following discussion. The
resistivity profiles were used as control points for mapping the
line interface in detail.
Table 2. Correlation of resistivity values and groundwater
quality.
Water quality
Fresh water
Saline water
Brackish (Week saline)
Resistively range (Ohm.m)
10 – 25
<5
5 – 10
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Profile 4
This profile is along the irrigation canal running westward from
Jelutong Village upto about 1.2 km west Pendang road as shown in
Fig. (3). It represents the longest profile done in this survey, of
about 980m. The data set has 1536 data points for WennerSchlumberger array, and 197 electrodes.
The 2-D pseudosection inversion model is shown in Fig. (4) with
RMS errors of 4.2% after 5 iterations. It shows a low resistivity zone
(3-22  m) of dark blue and blue color region, with a lower
resistivity value of 3.3-5  m in the western section of the profile
indicating a saline water intrusion. The resistivity value increases
towards the eastern end of the profile near a hilly region (light
blue zone) showing a gradational contact between fresh and
saline groundwater at a distance of about 700m from the base
point. The abnormal increase of the resistivity value at the western
end of the profile is explained by the intrusion of the fresh water
from the Pendang river which is not far from this area.
The water table is at a shallow depth of 1.2 to 1.5m indicating a
direct recharge from the Paddy field water. Therefore the aquifer
can be described and the groundwater might have been contaminated by fertilizers and pesticides used in the Paddy field. The
thickness of the aquifer is about 40 to 45m. The bedrock shows an
undulating topography, with relatively high resistivity value of
more than 200  m (dark and dark reddish zone). The weathered
zone (yellow and reddish) having resistivity values between 40 to
50  m. The large differences between the resistivity of the water
horizon and the bedrock indicate massive bedrock (Kubang Pasu
formation).
Profile 6
This profile is located along the canal near the paved road east of
Tg. Musang Village (Langer Area).It is trending East-West and has
a length of 240m Fig. (3). The data set has 325 datapoints.
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The inversion model pseudosection is shown in Fig. (5) with an
RMS error 3.0%.
The ground water aquifer has low resistivity value 407 to 12m
and is 18 to 20m in thickness. There is a prominent change in
resistivity value along the aquifer zone. The low resistivity
anomaly zone on the western portion of this zone is due to the
saline groundwater. The fresh water zone is on the eastern end of
the pseudosection. The interface between fresh and saline water is
nearly vertical and at about 80 m from the base station.
Profile 4
Fig. (4). Inverse Model pseudosection for Profile 4. Jelutong
village-north Pendang.
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Fig. (5) Inverse Model Pseudosection for Profile 6, Tg. Musang
Village – East Langar.
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Profile 22
This profile is along the road running westwards from Bt. Kodiang
up to about 1.5 km east of the railway line in the Kodiang Area
Fig. (3). It has a length of 200m with a total of 255 data points with
41 electrodes positions. The inverse model pseudosection Fig. (6)
after 5 iterations has an RMS error of 1.9% for the WennerSchlumberger data seta.
The bedrock in this area is mainly Chuping limestone which is
exposed as isolated hills such as at Bt. Kodiang and Bt. Kechil. It
has a resistivity of up to 250  m indicating shallow and/or massive
Chuping limestone.
The main features on the pseudosection are a low resistivity water
horizon (4 to25  m). The lower resistivity value at the western end
of the profile is due to incursion of saline water into the aquifer.
The resistivity becomes slightly higher towards the Kodiang hills
(up to 25  m) due to recharge of fresh water from uphill. The two
resistivity zones in the east and west has a sharp contact about
120m from base point. This suggests rapid change in water
quality. Therefore, the interface can be accurately traced along
this point.
Profile 32
This profile is located west of the Setul mountain series near the
sea. It runs eastward towards the hills and north of Wai Kechil
Village to the west of Kangar Fig. (3). It has a length of 200m. The
data set has 255 data points with 41 electrode positions.
The inverse model section Fig. (7) has an RMS error of 1.2%. The
groundwater aquifer shows a continuous zone of low resistivity
values. It has a resistivity of 6 to 32  m. The large difference in
resistivity values between the groundwater and the bedrock
suggests an impermeable Setul Formation bedrock. The interface
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between fresh and saline groundwater in this area is controlled by
the impermeable Setul mountain series, and it is probably located
to the east of Wai Kechil Village and north of Tok Wang Raja
Village.
Discussion of the Results:
The detailed interpretation of resistivity pseudosection shows a
ground water aquifer of low resistivity value (0.25 to 30  m). The
fresh water in this zone shows higher resistivity values than water.
Thus the interface can be clearly distinguished. The interface
varies from a gradational to a sharp contact. It is influenced by the
topography in the bedrock and the aquifer thickness. The
resistivity profile (P to P ) in the survey area were used as
1
32
control points for mapping the saline and fresh water interface
boundary.
The groundwater has the following resistivity values: Fresh water
are characterised by a resistivity value of 10 to 20 ohm.m or more.
The resistivity values of 5 to 10 ohm.m is identified with weakly
saline (brackish) water. The value of less than 5 ohm.m is
interpreted as groundwater contaminated by saline sea water.
This value may be less than 0.1 .m in areas near the sea, such as
profiles 30,31 and 32, west of Kangar. Similar relations were
described by Morris (1965) on the basis of geological sounding
diagrams and data drilling carried out in Pakistan, Libya and
Egypt.
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Profile 22
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Profile 32
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Similar relations were obtained from chemistry data from
boreholes. Fresh water has low electrical conductivity values (550
to 650 micro mhos/cm; i.e. resistivity values of 18.2 to 14.7 .m
and low chloride contents (0.3 to 1meq/1). While the brackish
water has relatively higher conductivity values (1000 to micro
mhos/cm or resistivity values of 8.9 to .m) and high chloride
contents of about 4.5 meq/1. However there is no borehole data in
the saline water zone. The depth to the bedrock in the Arau area is
also confirmed by boreholes.
On the other hand, there are slightly lower values of resistivity in
areas with fresh ground water, especially in the wet Paddy Field
areas. This may be due to a higher saturation of aquifer.
The groundwater aquifer is overlain by a thin layer and water
table is at a depth of 1 to 3m. Therefore, the aquifer varies from
unconfined to semiconfined, and the thickness varies from 10
meters (Profile 22) to 35 metres north (Profile 6), in exceptional
cases up to 40m. This indicates that the aquifers are localized in
the alluvial deposits, and the variation in aquifer thickness is
attributed to the variation in thickness of alluvium deposits and/or
to the structure, lithology and the nature of the weathered zone of
the bedrock. The bedrock varies from impermeable Setul
Formation and Chuping Limestone Formation in the north to the
fractured Kubang Pasu Formation and Sungai Patani Formation to
the south of the area which is shallower, more weathered and
fractured. The resistivity of such bedrock in the area varies
between 45 and 350  m.
Conclusion:
This study is intended to confirm the postulated transition zone of
fresh and saline groundwater and to map the extent of the aquifer
and the boundary conditions along this zone. The 2-D resistivity
imaging technique is suitable for shallow site investigation. It was
successful in giving an accurate image for the groundwater
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quality. The correlation between water quality and resistivity
values is determined.
The resistivity profiles were used as control points to map the
interface more accuratelly. The Wenner and WennerSchlumberger arrays proved to be the most useful electrode
arrays for groundwater exploration. They were successfully used
to delineate the interface and the aquifer boundaries. The
distinctive resistivity contrast between the top dry soil layer and
the impermeable bedrock compared with alluvial aquifer makes
the resistivity method an appropriate tool for hydrogeological
mapping. The interface in many of the 2-D resistivity
pseudosections is interpreted as a transition zone, where the
gradational nature of the resistivity contrast between fresh and
saline groundwater makes it difficult to find a sharp boundary,
such as along Profile 6 (P6) in the North Pendang area.
Borehole drilling and hydrochemical analysis would be
recommended to confirm the resistivity results.
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(1) Arafin, M.S., 1988. Geophysical investigation of the
hydrogeology of Perlis. Ph.D. Thesis (unpublished), Uni.
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(2) Arafin, M.S. and Lee, C.Y., 1987. Application of Resistivity
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(3) Arafin, at the Sixth World Water Congress, Ottawa University,
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(6) Dey, A. and Morrison, H.F., 1979. Resistivity modeling for
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(15) Loke, M.H. and Barker, R.D., 1996. Rapid least-squares
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(16) Ministry of Work and Public Utilities Report-Development of
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(17) Morris, D.B. 1964. The application of resistivity methods to
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(18) Pazdirek, O. and Blaha, V., 1996. Examples of resistivity
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