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Australian Journal of Basic and Applied Sciences, 7(1): 344-357, 2013 ISSN 1991-8178
Corresponding Author: Sayed Hemeda, Lecturer, Conservation Department, Faculty of Archaeology, Cairo University
E-mail: hemeda@civil.auth.gr
344
Electrical Resistance Tomography (ERT) Subsurface Imaging for Non- destructive
Testing and Survey in Historical Buildings Preservation
Sayed Hemeda
Lecturer, Conservation Department, Faculty of Archaeology, Cairo University
Abstract: The paper presents the application of non-pervasive electrical resistance tomography (ERT)
subsurface imaging surveys for the rehabilitation and strengthening of Habib Sakakini’s Palace (1897
AC) in Cairo, Egypt. The use of several high-resolution geoelectrical methods derived from the field
survey techniques proved to be very effective in the Non-Destructive Testing and survey of
architectural heritage. In particular the application of a tomographical approach allowed us to obtain
subsurface images of the cross-sections of the bearing soil with complex layers and structures that
clearly show the presence of eventual anomalies. Some experiments with geoelectrical tomographic
techniques gave very interesting results also when working on historical buildings that seemed a
hostile environment for geoelectrics. This is very interesting also due to the velocity of the
measurements and of the data processing: this means short times and low costs. The use of micro-
geophysical techniques offers many advantages with respects to some “classical” techniques under
different angles: velocity of execution, non-pervasiveness and costs. The results of ERT are compared
to ground penetrating radar (GPR) – they are just as detailed but are often easier to interpret, at a lower
cost.
Key words: El-Sakakini palace, Cairo, Electrical Resistance Tomography (ERT), Architectural
Heritage Preservation, Non-destructive testing, survey.
INTRODUCTION
The application of geophysics to archaeology dates back to the early 1950s. The rapidly evolving
technology over the past 20 years has made the geophysical approach a reliable investigative and survey tools,
both before and during excavation and restoration processes. Geophysical prospecting allows the physical
parameters of the subsoil to be mapped in large-scale reconnaissance surveys. In specific cases, it can provide
useful information on the depth and shape of buried structures. One of the most commonly applied techniques of
geophysical surveying is the electrical resistivity tomography (ERT) that provide the measurement of the
specific electrical resistance of soil (Mol and Preston, 2010; Papadopoulos and Sarris, 2011; Tsourlos and
Tsokas, 2011). ERT method is suitability in detecting walls, cavities and other structures at differing depths
(Monteiro and Senos Matia 1987; Papadopoulos et al., 2010). The use of resistivity in identification of walls has
long been a common practice (Sarris and Jones 2000; Drahor et al., 2008; Tsokas et al., 2008; Tsokas et al.,
2009; Berge et al., 2011., G. Leucci and Greco, 2012).
The aim of the electrical resistivity tomography (ERT) technique is to scan the subsurface along the
survey line using a selected electrode array. An automatic electronic system collects data quickly in order to
observe the pseudosection along the investigation line in the field (Drahor, 2006., Yunus and Mehmet 2007)
Geophysical methods can assist the engineers in solving the problem through the detection of different
physical properties of the soil, by sending a physical property and receiving it again (such as current, sonic
wave, EM wave etc). Electrical Resistivity Tomography (ERT) is a technique that can detect and characterize
layers by exploiting resistivity contrasts between different layers using electrical current.
The present study helps in understanding the amount of hazards and risk that EL Sakakini suffers, by
imaging the subsurface of the soil. EL Sakakini is located within Cairo city which is considered moderate
hazard but high risk seismic zone with moderate seismicity. ERT is a two-dimensional (2-D) model where the
resistivity changes in the vertical direction, as well as in the horizontal direction along the survey line. ERT can
help monitoring the soil properties and dynamics of saltwater intrusion in coastal zones such as West Nile Delta
Project. It can also define soil water salinity and aquifer properties.
The study demonstrates the efficiency of the electrical methods to map subsurface conductive zones. ERT
Geophysical data were calibrated using soil geotechnical drilling data and then interpreted based on this basis.
The results illustrate the potential of electrical resistivity methods to separate different layers and monitor the
subsurface conditions based on electrical resistivity.
Aust. J. Basic & Appl. Sci., 7(1): 344-357, 2013
345
Fig. 1: General outside view of El-Sakakini Palace in Cairo, Egypt.
2. Scope of Work:
The main objectives of this study is to separate the soil conditions in continuous 2D-image of the resistivity
distribution, both laterally and in depth Finally compare this with the drilling data to detect soil homogeneity
and possible anomalous features.
3. El-Sakakini Palace:
The site of Habib Sakakini’s palace is considered from the important sites that have attracted the attention
of a lot of historicans, especially at the Mamluk’s and Ottoman’s er. The location of that palace anciently was a
water pond that was located at the eastern side of Egyptian gulf besiding Sultan Bebris Al-Bondoqdary mosque
as that pond was so called “Prince Qraja al-Turkumany pond”. That pond had acquired the extent of importance
when it had been filled down by Habib Sakakini at 1892, where he had constructed his famous palace in 1897.
The rococo style and quite fancy architecture of the palace is said to be taken from an Italian palace that
Habib Pasha saw and fell in love with. He then ordered an Italian company to create a replica of it in Cairo. The
architectural rococo style of the facades was common at the time in Europe, but quite rare in Egypt.
He chose quite an impressive site for his home. The palace stands high in a focal location where eight main
roads radiate out, hence making it the center point of the zone. Acquiring such a distinguished location normally
would have not been an easy task at the time. However his good connection with the Khedive allowed him such
privileges.
EL Sakakini is located within Cairo city, in a crowded area full of people and neighboring buildings. The
soil is covered with asphalt, and this was a difficulty when electrodes were planted in the ground. This palace is
hidden in the middle of quite a busy area. Upon arrival, one instantly wonders what the urban surroundings must
have looked like in 1897, at the time of its construction.
4. Summary of Electrical Resistivity Tomography Method:
Electrical resistivity tomography method (ERT) is a more accurate two-dimensional (2-D) model, where
resistivity changes in the vertical direction, as well as in the horizontal direction along the survey line is imaged.
It is assumed that resistivity does not change in the direction that is perpendicular to the survey line. In many
situations, particularly for surveys over elongated geological bodies, this is a reasonable assumption.
In normal 1D resistivity measurements, a voltage is injected in the soil via two current electrodes, causing
current to flow through the soil, and the voltage drop between the inner electrodes is measured using a sensitive
voltmeter or complex geophysical instruments (Figure 2). The electrical resistivity is determined for a certain
depth (this depth is function of the spacing between electrodes) using equations like that resent in Figure 2.
In 2D ERT resistivity method, the previous normal 1D resistivity measurement is repeated several times at
different places and different depths to make a complete image to the subsurface. This is done by shifting the
measurements with distance forward along a profile, and also by increasing the separation between electrodes to
Aust. J. Basic & Appl. Sci., 7(1): 344-357, 2013
346
increase the depth. The overall process is to get measuring resistivity points that give 2D resistivity image to the
subsurface. This ERT or 2D is done by large number of electrodes, 25 or more, connected to a multi-core cable.
A laptop microcomputer together with an electronic switching unit is used to automatically select the relevant
four electrodes for each measurement (Figure 3).
Fig. 2: Shape of injected current (A), and example of equations used to determine soil electrical resistivity (B)
in 1D method (Wenner array), Where a is the separation between electrodes, ∆V is the potential
difference and i is the current injected in the soil.
For example, Figure 3 shows a possible sequence of measurements for the Wenner electrode array for a
system with 20 electrodes. In this example, the spacing between adjacent electrodes is “a”. The first
measurement is made by using two current electrode C1 & C2, and two potential electrodes P1 & P2. These are
done in the electrode positions 1, 2, 3, 4 the measured point depth is about 20% 25% of the total length (3a).
When using electrode positions 2, 3, 4, 5 and so on to the final position 17, 18, 19, 20, then you finished the first
depth of measurements at level n=1 (Figure3).
The measurements are now repeated but electrode separation is increased to increase the depth level. For
example if we increased separation between electrodes into “2a”, the second row of measurements at n=2 is
measured. The process is repeated until we get resistivity values for the whole image (pseudosection) and all
desired depths from depth level n=1 to n=6 (Figure 3).
5. Instrument Used:
The present study was done using the Sting R1 instrument (Figure 4 & Table-1). This is an Advances
geoscience instrument manifactured in USA (http:///www.agiusa.com). The instrument proved to be very
(A)
(B)
Aust. J. Basic & Appl. Sci., 7(1): 344-357, 2013
347
effective for resistivity measurements in the Egyptian soil and worldwide. The instrument has the ability to
reduce erros based on one or more of the folowing items:
-Stacking readings to reduce erros
-Calculates the stadared deviation of readings and errors to be accepted or repeated by user.
-Inject high current in the ground (10-500 mA)
-Injected high voltage in the ground (200-400 volts)
-Modify the shape of the injected function by changing the time between +ve and –ve directions (Figure-5) to
avoid soil chargability and electrolytic polarization.
Table-1 give complete specification of the instrumen used.
Fig. 3: The main components and arrangement of electrodes for a 2-D electrical survey (ERT) Wenner array,
and the sequence of measurements used to build up a resistivity image for the subsurface pseudosection
for depth levels n=1 to n=6.
Fig. 4: Components of ERT measuring instrument A: a Fully automatic SuperSting resistivitymeter, 400-800
Volts, Deep penetration instrument, USA manifactured, B: Switch box and C: swift unit.
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348
Table 1: Complete technical specifications for instrument, used in this study (Sting R1 USA).
Fig. 5: Shematic diagram showing used reversible wave, and possibility to increase Time delay to avoid
electrolytic polarization (Sting R1 instrument, used in this study).
6. Electrode Array:
There are many types of ERT electrode arrays (Figure 6). Such as, Wenner array, Schlumberger array and
Dipole-Dipole array (Figure 6). Dipole-dipole array was used in this study due to its signal strength and
horizontal resolution. The arrangement of the electrodes in Dipole-Dipole array is shown in figure 7. The
spacing between the current electrodes pair, C2-C1, is given as “a” which is the same as the distance between
the potential electrodes pair P1-P2. This array has another factor marked as “n” in Figure 7. This is the ratio of
the distance between the C1 and P1 electrodes to the C2-C1 (or P1-P2) dipole separation “a”. For surveys with
Aust. J. Basic & Appl. Sci., 7(1): 344-357, 2013
349
this array, the “a” spacing is initially kept fixed and the “n” factor is increased from 1 to 2 to 3 or more in order
to increase the depth of investigation. The sensitivity function plot is shown in Figure 8. It shows that the largest
sensitivity values are located between the C2-C1 dipole pair, as well as between the P1-P2 pair. This means that
this array is most sensitive to resistivity changes between the electrodes in each dipole pair.
Thus the dipole-dipole array is very sensitive to horizontal changes in resistivity. That means that it is good
in mapping vertical structures, such as dykes and cavities. It also has better horizontal data coverage than the
Wenner.
To overcome the problem of the delay of current due to increase in n, we have increased the “a” spacing
between the C1-C2 (and P1-P2) dipole pair to reduce the drop in the potential when the overall length of the
array is increased, also to increase the depth of investigation.
Figure 9 shows two different arrangements for the dipole-dipole array with different “a” and “n” factors.
The actual measuring array and points used in this study is shown in figure 10, we have used 152data points in
each conducted ERT imaging profile.
Fig. 6: Common electrode arrays used in resistivity surveys.
Fig. 7: Dipole-Dipole array used in electrical resistivity imaging at West Nile Delta project site. Two current
electrodes C1, C2 (Or B, A) and two potential electrodes P1, P2 (Or M, N) are used.
Aust. J. Basic & Appl. Sci., 7(1): 344-357, 2013
350
Fig. 8: The sensitivity function or patterns for the (a) Wenner (b) Wenner-Schlumberger and (c) dipole-dipole
arrays.
7. Field Procedure:
We have used the Diploe-Diploe array with 28 electrodes cable, with 6.25 electrode spacing or totally
168.75 m. A total of 152-216 data points were taken at each profile. The data points were arranged over
different depths as shown in figure 10.
8. The Raw Data:
Four ERT profiles were conducted beside EL Sakakini Palace to detect the soil Geoelectrical variations
(Figure 11). Each profile contains 152data points as mentioned above. The data acquired in this study were
processed using the RES2DINV software (2012 version). Two profiles were made to 90 m and two to 30 m to
detect the deep and shallow properties (About 40 m, Edwards 1970). Raw data were subjected to resistivity
check in the beginning of each profile to assure good planting and connection of electrodes with the soil, and to
assure low surface resistance. Generally the surface resistance is moderate compared with other sites due
Aust. J. Basic & Appl. Sci., 7(1): 344-357, 2013
351
nearness of clayey contents and ground water. The data were collected passed on double check for each point,
standard deviation was also controlled in almost all cases to approach the zero value (Error < 1 %).
Fig. 9: Data configuration for a complete dipole-dipole using different electrode separation “a” and “n” factor.
Note that increasing the spacing between current and potential electrodes result in total increases to the
investigation depth.
Fig. 10: Common electrode array used in ERT profiles at WND, using 28 electrodes, 6.25m electrode
separation, 152-216 data points. Depth of investigation is about 42m (Edwards 1977).
Table 2: Total length of the four acquired ERT profiles at El Sakakini palace area.
Profile Name
Length (m)
(1) ERT1 30
(2) ERT2 30
(3) ERT3 90
(4) ERT4 90
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352
Short electrical
Tomography profile
Long electrical
Tomography profie
ERT-3
ERT-4
ERT-2 ERT-1
Fig. 11: Base map showing the 4 electrical resistivity tomography profiles conducted at EL Sakakini Palace site
(Please refer to table 2).
Fig. 12: Arrangement of blocks used in inversion process by RES2DINV program together with the data points
in the pseudosection.
9. Data Processing & Modeling:
Data processing and modeling were done using the RES2DINV Geotomo software (2012 version).
RES2DINV is a computer program that automatically determines the 2D resistivity model for the data obtained
Aust. J. Basic & Appl. Sci., 7(1): 344-357, 2013
353
(Griffiths and Barker 1993). The program makes inversion by dividing the original data into rectangular bocks
(Figure 12). The blocks are tied to the distribution of the data, while the depth of the blocks is equal to depth of
investigation (Edwards 1977). The program uses the forward modeling to calculate the apparent resistivity
values, and a non-linear least-squares optimization technique for inversion routine (deGroot-Hedlin and
Constable 1990, Loke and Barker 1996a). The program supports both the finite-difference and finite-element
forward modeling techniques. An example of the program output for observed, calculated apparent resistivity
and the final model obtained after inversion is given in Figure 13.
Fig. 13: The observed and calculated apparent resistivity pseudosection for profile ERT-1 together with the
model obtained by inversion process using RES2DINV program.
The Following Processing Steps were Applied to the Raw Data Obtained:
1-Noisy data were removed from each profile (-ve values)
2-Bad datum points for joined profiles were checked and removed (bad or wrong unexpected resistivity values
for fixed levels) (Figure 14)
3-A trial for the inversion data is made and an initial model is performed
4-RMS error between the observed and calculated apparent resistivity is calculated
5-Bad data points with large RMS errors is cut-off from original data
6-Final inversion model with lest RMS errors accepted geologically is produced otherwise steps 2-6 are repeated
until final approved model is attained.
10. Results:
EL Sakakini Palace is located at Cairo City at Ghamra district. The site is almost flat.
The soil of EL Sakakini Palace is of low to moderate electrical resistivity ranging between 2- 400 Ohm.m.
When compared to other soils exist in Egypt. This is most probably due to the nearness ground water level
which exist at depth 0.5-1 m. and existence of a saline source of water which affect the electrical resistivity (< 2
Ohm.m). The following is a description to the four electrical resistivity profiles conducted at EL Sakakini
Palace.
Profile Ert-1 (Figure-15):
In general this profile is composed of two main layers:
1- A first layer composed of silty clay, limestone fragments, pottery fragments and calc extends to depth of 5 m
and possesses average resistivity of about 30-450 Ohm.m. With remarkable, hot colors (yellow to purple).
2-The second layer is composed of silty clay mixed with sand in some places, starting from average depth of 5m
and extended to the end of the section (7m) and possesses average resistivity between 2-20 Ohm.m with
remarkable blue color.
Aust. J. Basic & Appl. Sci., 7(1): 344-357, 2013
354
Fig. 14: Raw data with few bad data points removed in the processing steps before final model calculations.
Profile Ert-2 (Figure-16):
Profile ERT-2 show mainly the same features like profile ERT-1 it composed of two main layers:
1- A first layer composed of silty clay, limestone fragments, pottery fragments and calc extends to depth of 5 m
and possesses average resistivity of about 30-450 Ohm.m. With remarkable, hot colors (yellow to purple).
2-The second layer is composed of silty clay mixed with sand in some places, starting from average depth of 5m
and extended to the end of the section (7m) and possesses average resistivity between 2-20 Ohm.m with
remarkable blue color. The only difference between these two profiles is the variation of resistivity which is
most probably due to the change in soil composition in different directions.
Profile Ert-3 (Figure-17):
Profile ERT-3 show three main layers:
1- A first layer composed of silty clay, limestone fragments, pottery fragments and calc extends to depth of 5 m
and possesses average resistivity of about 30-450 Ohm.m. With remarkable, hot colors (yellow to purple).
2-The second layer is composed of silty clay mixed with sand in some places, starting from average depth of 5m
and extended to 15 m depth. In the left part the soil is relatively possesses low electrical resistivity between 2-20
Ohm.m with remarkable blue color. In the right part the soil show a higher resistivity ranging between 50-200
Ohm.m. This could be attributed to the exceedance of sand component over the clay component in right
direction.
3- The third layer is composed mainly of sand and gravel; it extends to depth between 15-20 m and has average
resistivity between 70-300 Ohm.m
Profile Ert-4 (Figure-18):
Profile ERT-4 show three main layers:
1- A first layer composed of silty clay, limestone fragments, pottery fragments and calc extends to depth of 5 m
and possesses average resistivity of about 16-100 Ohm.m. With remarkable, hot colors (yellow to brown).
2-The second layer is composed of silty clay mixed with sand in some places, starting from average depth of 5m
and extended to 15 m depth. In the right part the soil is relatively possesses low electrical resistivity between 2-
30 Ohm.m with remarkable blue color. In the left part the soil show a higher resistivity ranging between 70-300
Ohm.m.
3- The third layer is composed mainly of sand and gravel; it extends to depth between 15-20 m and has average
resistivity between 100-300 Ohm.m
Aust. J. Basic & Appl. Sci., 7(1): 344-357, 2013
355
-7
-6
-5
-4
-3
-2
-1
0
D
e
p
t
h
(
m
)
024681012141618202224262830
Distance (m)
-502345811162333486999143206297427738
Resistivity
Ohm.m
Fill Fill
Sity clay to sandy sity clay
0
Fig. 15: Electrical resistivity tomography inversion model for profile ERT-1. The hachured lines are average
depth of layers as detected from drilling data (for profile location see Figure 11).
-7
-6
-5
-4
-3
-2
-1
0
D
e
p
t
h
(
m
)
024681012141618202224262830
Distance (m)
-502345811162333486999143206297427738
Resistivity
Ohm.m
Sity clay to sandy sity clay
Fill Fill
0
Fig. 16: Electrical resistivity tomography inversion model for profile ERT-2. The hachured lines are average
depth of layers as detected from drilling data (for profile location see Figure 11).
-20
-15
-10
-5
0
D
e
p
t
h
(
m
)
-20
-15
-10
-5
0
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
Distance (m)
-502345811162333486999143206297427750
0
Resistivity
Ohm.m
Fill
Sity clay to sandy sity clay
Sand & Gravel
Fig. 17: Electrical resistivity tomography inversion model for profile ERT-3. The hachured lines are average
depth of layers as detected from drilling data (for profile location see Figure 11).
Aust. J. Basic & Appl. Sci., 7(1): 344-357, 2013
356
-20
-15
-10
-5
0
-20
-15
-10
-5
0
D
e
p
t
h
(
m
)
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
Distance (m)
0234581
1
16 23 3
3
4
8
69 99 14
3
20
6
297 42
7
73
8
Resistivity
Ohm.m
Fill
Sity clay to sandy sity clay
Sand & Gravel
Fig. 18: Electrical resistivity tomography inversion model for profile ERT-4. The hachured lines are average
depth of layers as detected from drilling data (for profile location see Figure 11).
11. Conclusion:
The soil found at EL Sakakini Palace composed mainly of four layers.
1- A first layer composed of silty clay, limestone fragments, pottery fragments and calc extends to depth of 5 m
and possesses average resistivity of about 10-450 Ohm.m. With remarkable, hot colors (yellow to Purple).
2-The second layer is composed of silty clay mixed with sand in some places, starting from average depth of 5m
and extended to 15 m depth. In the right part the soil is relatively possesses low electrical resistivity between 2-
30 Ohm.m with remarkable blue color. In the left part the soil show a higher resistivity ranging between 70-300
Ohm.m. This could be attributed to the existence of mores and component over the clay component in the left
direction. It is worth mentioning here that the direction of ERT-4 is opposite to ERT-3
3- The third layer is composed mainly of sand and gravel; it extends to depth between 15-20 m and has average
resistivity between 100-300 Ohm.m
Based on the electrical resistivity measured the soil show no homogeneous effects. Some parts are saturated
show high resistivity interbedded with low and vise versa. For example the second layer shows different
resistivity values in the left and right parts. This is most probably due to the change in soil composition and
change from silty or clay into more sand component. The other important thing is that the soil is saturated with
water in shallow level (0.5-1 m) which constitutes a bad weathering effect to the EL Sakakini Palace. A last
notice to be taken into consideration is the high salinity of the soil in some parts (< 2 Ohm.m), this indicate a
high corrosion effects to the base of the foundation. This must be taken into consideration in engineering
solution to isolate the corrosion of the base.
ACKNOWLEDGMENT
The present paper is part of project entitled "Risk assessment and seismic response analysis of architectural
heritage in Egypt" funded by Cairo University and under the management of Dr Sayed Hemeda, the author of
the research paper and the main investigator of the project. I'd like to thank Dr/ Mohamed Gamal, geophysics
Department, Cairo University for his great efforts and help through the investigations.
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