CEOSEJI '98 Proceedil1p, Ceo!. Soc. ll{a!aY,lia BlI!!. 45, Decelllber /999; pp . 197-205
Ninth Regional Congress on Geology, Mineral and
Energy Resources of Southeast As.ia - G EOSEA '98
GEOSEA '98
17 - 19 August 1998 • Shangri-La Hotel, Kuala Lumpur, Malaysia
The use of geoelectrical imaging surveys for the delineation of
different subsurface geological and man-made features
S.S. ABDUL NASSIR AND C.Y. LEE
School of Physics
Universiti Sains Malaysia
11800 Penang, Malaysia
Abstract: Geoelectrical imaging surveys are becoming increasingly important exploration tools in
various shallow geotechnical and environmental investigations. The wide range of resistivity variations
between the anomaly and the host has made it an attractive physical parameter for subsurface exploration.
New field concepts and techniques have been developed for situations where an understanding of
subsurface conditions in detail is required . The Multi-scale survey technique has greatly enhanced the
horizontal coverage and the resolution of the array used . Moreover it has partially suppressed the effect
of electrical noise on the field measurements as well as strengthened the voltage signal to noise ratio of
the Wenner-Schlumberger array by u sing multiple dipole lengths . Interpretation of the resulting
resistivity cross-section has enabled the clear identification of the subsurface features .
INTRODUCTION
Geophysical methods are routine procedures
to delineate geological structures and other
subsurface phenomena (Dahlin, 1996; Zeng and
McMechan,1997). Proper usage ofthese methods
leads to an increase in the resolution of the
resulting subsurface section.
Electrical
tomography (Griffiths and Barker, 1993), also
known as the electrical imaging method, has the
ability to image subsurface features by examining
its resistivity distribution. In this study, the use of the geoelectrical
imaging technique for the delineation of different
subsurface geological and man-made features is
explored. The new field survey concepts and
techniques that emerged to improve the data
acquisition are given in some detail. The results
obtained during field surveys employing the
Wenner and Wenner-Schlumberger electrode
arrays on resistivity traverses in several sites to
detect and map a variety of subsurface features
are described.
DESIGN OF THE FIELD SURVEY
Electrical tomography surveys are normally
-carried out by deploying electrodes in a line
perpendicular to the strike ()fthe subsurface target
which needs to be imaged. The data acquisition is
accomplished by a computer-controlled system
(Griffiths et al., 1990) whereby a series of resistivity
measurements are made in constant-separation
profiles with the electrode spacing being increased
in each successive traverse (Fig. 1). The data
collected from these surveys are then processed
(Loke and Barker, 1996) to be interpreted
accurately.
It has been found (Abdul N as sir, 1997) that the
conventional electrical tomography survey
techniques would be impractical and uneconomic
particularly for long resistivity traverses that are
usually carried out for environmental and
geotechnical studies. The Multi-scale survey
technique- has been developed to handle such
surveys, since in this technique a series of
measurements are carried out using different
electrode spacings over the same survey line .
The sparse data points produced by these
surveys are then compiled to build up the final
pseudo-section (Fig. 2). As the Multi-scale survey
technique provides more opportunities to extract
valuable information from the subsurface than ever
before, ?- broader picture with detailed information
on shailow and deeper layers of the surveyed area
is obtained from these surveys.
In the Wenner-Schlumberger array, the
potential dipole spacing is increased after multiple
numbers of depth levels (Edwards, 1977) depending
on the geology of the area, the electrode spacing
and the amount of current injected to the ground
(Abdul Nassir, 1997). An increase in the current
198
S.S. ABDUL NASSIR AND C.Y. LEE
electrode spacing while keeping potential dipole
length constant leads to an increase in the
measuring cross section area beneath the electrode
setup. This will result in a decrease of the
measurable voltage signal, since the later is
inversely proportional to the cross sectional area
between the outermost electrodes (Mooney, 1980).
When the resistivity surveys are conducted in
noisy areas, the current signal introduced into the
ground via current electrodes has to be as high as
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possible in order to secure a good signal to noise
ratio. Meanwhile the duty time of the DC-current
pulsation as well as the digital stacking of
measurements collected at each electrode setup also
have to be increased so as to filter the field
measurements from electrical noise disturbances
(Hoogervorst, 1975). Therefore, the effect of
electrical noise on the resistivity measurements
would be suppressed, and thus conserves the
measurable voltage signal as a smooth square waveform.
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Figure 1o Schematic diagram of the sequence of measurements used to build up a 2D
resistivitypseudosection made by a computer-controlled multi-electrode Wenner system.
The apparent resistivity datum pOints arrangement
E1
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Figure 2o The measurement sequences for building up a 2D-resistivity pseudo-section
of the Wenner array by using the Multi-scale survey technique.
GEOSEA '98 ProceeiJingJ (GSM Bull. 43)
THE USE OF GEOELECTRICAL IMAGING SURVEYS FOR THE DELINEATION OF DIFFERENT SUBSURFACE FEATURES
FIELD EXAMPLES
In this section, the application of electrical
tomography using the Wenner and WennerSchlumberger arrays for the mapping of several
subsurface features is given. The proposed field
techniques were incorporated into these surveys to
lower the cost of survey, enhance the measurable
voltage signals and increase the horizontal coverage
and resolution of the resistivity sections.
Granite bedrock survey
A 2D-electrical imaging survey using the
Wenner-Schlumberger array was conducted on a
slope to the east of the Student Affairs Office in the
Universiti Sains Malaysia campus (Fig. 3).
According to an outcrop exposed near the survey
site, the geology of the surveyed area consists of
two main layers. The top layer consists of a light
to dark brown silty clay with a little fine to coarse
sand and gravel while the second layer or bedrock
is made up of weathered granite.
The resistivity model section (Fig. 4) clearly
resolves the topography of granite bedrock, since it
shows up as an undulating high resistivity zone in
the resistivity section. The low resistivity plume
on the top left side of the resistivity section is due
to the embankment material that had been used to
pave the road crossing the area while the low
resistivity zone at the other end of the inverse
section represents the original surface layer. This
layer varies in thickness but in general it gradually
becomes thicker as it progresses slope down" from
the middle of the section.
Salt water intrusion survey
The survey site is located east of Yan, Kedah
State in Malaysia (Fig. 5). The salt water intrusion
boundary was earlier located by the ID resistivity
sounding and borehole surveys conducted by the
Geological Survey of Malaysia. Due to the inherent
capability of the electrical imaging method for
detecting lateral changes in pore-water salinity, a
2D survey using the Wenner array was conducted
to map the salt-fresh water intrusion boundary.
The survey line has an almost north-west southeast orientation, which is perpendicular to the
postulated intrusion boundary. The fresh water
and saline water boundary in the Wenner inverse
model (Fig. 6) is clearly shown as a steeply dipping
curved boundary between the fresh and saline zone.
The high resistivity zone corresponds to the
fresh water. The source of fresh water is believed
to be rainwater descending from the Gunung Jerai
catchment area (Bradford, 1972) towards the lower
areas covered by paddy fields. The low resistivity
December 1999
199
zone corresponds to the sea water intrusion from
the Straits of Malacca.
Underground pipe survey
The survey was conducted over an underground
metal pipe located at the Convocation field in the
USM campus (Fig. 7). The area in the vicinity of
the surveyed site has a number of electrical noise
sources that affect the resistivity measurements
and have the ability to redistribute electrical
current.
The Wenner-Schlumberger model resistivity
section (Fig. 8) shows the correct shape and location
of the metal pipe, which is indicated by a low
resistivity area in the inverse section. The highly
resistive regions above the anomaly are due to a
layer of dry silty clayey !,)and with gravel. The
highly resistive region on the right side of the
inverse model is due to the effect of an underground
sewerage pipe crossing that area at the end of the
survey line.
CONCLUSION
The electrical tomography method reveals
significant details of the subsurface structures, and
produces a more accurate 2D geological picture.
The proposed techniques have proved to be a rapid,
economic and valuable tool for delineating localized
features including man-made voids and for precise
mapping of the salt-water intrusion boundary and
bedrock profile. It appears to be an effective
approach to producing a broader picture of
subsurface structures with information from various
depths.
ACKNOWLEDGEMENT
The authors wish to thank the Geophysics
Laboratory assistants of School of Physics,
Universiti Sains Malaysia, for aiding in the
fieldwork. Abdul N assir would like to thank the
Public Services Department, Malaysia (JPA) for
the scholarship provided.
REFERENCES
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EDWARDS, L.5., 1977. A modified pseudo-section for resistivity
DAHLIN,
200
S.S. ABDUL NASSIR AND C.Y. LEE
Figure 3. Granite bedrock survey. Map showing the location of the survey line at the USM-Campus.
Undulating Granite Bedrock Survey
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fine to coarse sand
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Figure 4. Granite bedrock survey. The Wenner-Schlumberger inverse section with topography.
CEOSEA '98 Proceedill.IJJ ( CSM BuLL. 43)
THE USE OF GEOELECTRICAL IMAGING SURVEYS FOR THE DELINEATION OF DIFFERENT SUBSURFACE FEATURES
Groundwater intrusion survey
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Figure 5. Salt water intrusion survey, Map showing the location of the
resistivity survey lines near Yan, Kedah State (after the ministry of works
and public utilities report, 1983),
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ofthe resistivity survey line.
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GEOSEJI '98 Proceedill.q,/ (GSM BII!!. 45)
THE USE OF GEOELECTRICAL IMAGING SURVEYS FOR THE DELINEATION OF DIFFERENT SUBSURFACE FEATURES
203
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and induced-polarization. Geophysics, 42, 1020-1036.
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Mnnllscript received 18 August 1998
December I!J!J!J