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Integrated Geophysical Survey for Site Investigation at a New Dwelling Area, Egypt Eslam Elawadi1, Gad El-Qady2,3, Ahmed Nigm1, Fathy Shaaban2 and Keisuke Ushijima3 1 Nuclear Materials Authority (NMA) of Egypt, Cairo, Egypt 2 National Research Institute of Astronomy and Geophysics (NRIAG), Cairo, Egypt 3 Earth Resources Engineering Dept., Kyushu University, Fukuoka, 812-8581, Japan ABSTRACT An integrated geophysical survey was carried out in a new dwelling area at 15-May town, southeast Cairo, Egypt. The buildings in this area are intensively affected by dangerous cracks that cause structural instability. The survey aimed to image the shallow subsurface structures, including karstic features, and evaluate their extent, as they may cause rock instability and lead to cracking of the residential buildings. Resistivity profiling (2-D), using a dipole-dipole array and ground penetrating radar (GPR) surveys were carried out along seven parallel traverses extending about 150 meters between the buildings blocks. Additional measurements using a Schlumbereger array and very low frequency electromagnetic (VLF-EM) methods were conducted. The acquired data were processed and interpreted integrally to elucidate the shallow structural setting of the site. Integrated interpretation led to the delineation of hazard zones rich with karstic features in the area. Most of these karstic features are associated with vertical and sub-vertical linear features such as faults, fracture zones, and geologic contacts. These features are the main reason of the rock instability that resulted in potentially dangerous cracking of residential buildings.

Introduction 15-May town is one of the new urban areas in Greater Cairo zone, Egypt. It is located 35 km southeast of Cairo, and about 5 km from the industrial zone in Helwan town. Starting in 1979, 15-May town was established to provide affordable housing for people with limited income as well as to accommodate the population expansion of the industrial zone in Helwan. This town is constructed on a limestone plateau that forms the eastern bank of River Nile. Excavation and engineering works in different locations of this plateau reveal the presence of shale and marl intercalation, faults, fracture zones and frequent karstic features such as cavities, voids and sinkholes. These features can result in potentially dangerous collapse of roads and houses. The investigated site (Fig. 1) is located in the southern part of 15-May town where the buildings are severely damaged by crack systems (Fig. 2). Investigating this site to identify the reasons for these cracks can guide the maintenance processes used to increase the safety factor of the buildings. Moreover, delineating fracture zones and karstic features in the area can assist in the future planning for the construction of new buildings. Environmental and engineering applications of surface geophysical techniques have gained wide interest

in the last few decades. This is evidenced by the intensive research and exploration works using DC resistivity, ground penetrating radar and electromagnetic techniques (Roth et al., 2000; Schoor, 2002; Chamberlain et al., 2000; Fisher et al., 1983). These methods are quick, inexpensive and use non-invasive means to provide information about the subsurface properties, depth to bedrock, location and distribution of conductive fluids, location and orientation of fractures and faults (Reynolds, 1997). The objective of the present work is to apply the relevant surface geophysical methods to investigate the subsurface setting of the area. This can lead to mapping the fracture patterns of the site and locating any geological structures that may cause cracking in buildings and increase the hazards in the study area. Geological Background and Site Characteristics The investigated site consists mainly of a limestone plateau extending east of the River Nile. This plateau is covered mainly with Wadi Garawi and El-Qurn Formations of middle Eocene age. These formations (120 m thick) consist of chalky and marly limestone intercalated with shale, sandy marls and shale banks (Strougo, 1985; Said, 1990). Structurally, the area is

JEEG, December 2006, Volume 11, Issue 4, pp. 249–259

Journal of Environmental and Engineering Geophysics eego-11-04-03.3d 8/11/06 01:54:05

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Figure 1. Location of the investigated site showing the locations of the survey lines and resistivity soundings.

affected by three sets of faulting systems striking mainly in NW-SE , E-W, and NE-SW directions. All of them are normal faults, with dip ranges from 7u to 40u, where the beds are locally dragged. In addition to faults, at least two sets of joints dissect the area. These joints trend in E-W and NW-SE directions (Moustafa et al., 1985). The surface of the investigated site (Fig. 1) is covered with houses, gardens, and streets, which mask the surface geological features. Development also limits the choice of geophysical survey methods (to avoid the effect of building materials and facilities) and restricts the survey lines to be along the streets. Excavation, construction and quarrying processes, which are located very near to the investigated site, show many karstic features, such as cavities and sinkholes (Fig. 2). These features are mainly associated with fracture zones, faults and joints that accelerate their formation (Farag and Ismail, 1959).

DC Resistivity Method The DC resistivity method is well established theoretically and many case studies have been published (e.g., Edwards, 1977; Telford et al., 1990). Two electrode configurations were used in this study: 2-D profiling using the dipole-dipole array, and vertical electrical soundings using the Schlumberger array. The apparent resistivity value was calculated according to the formula {ra 5 KDV/I}, where K is the geometrical factor, which depends on the electrode configuration and separations. In the dipole-dipole configuration {K 5 pn (n + 1)(n + 2)a}, where a is the distance between the two potential electrodes and na is the distance between the adjacent current and potential electrodes. In Schlumberger configuration {K 5 pn(n + 1)a} where (n + 1/2)a 5 AB/2, (Ward, 1990). The measurements were performed using the Syscal-R2 instrument of IRIS - Instruments Company. The dipole-dipole array was chosen because it has low EM coupling between the current and potential circuits (Loke, 1998). In addition it is very sensitive to horizontal changes in resistivity, which makes it a good choice for mapping vertical structures, such as faults, dikes and cavities (Loke, 1998). This array has been successfully used in many areas to detect karstic structures such as sinkholes, voids and fractures (e.g., Roth et al., 2000; Schoor, 2002). Seven 2-D resistivity profiles (L1 to L7) were carried out between the building blocks (Fig. 1). The electrode spacing was 4 m and (n) was changed from 1 up to 7 times the dipole length to obtain adequate depth of investigation and lateral resolution. Before current injection, the electrode contact resistance (RS) was checked to assure the lowest possible value. The injected current was automatically determined and applied by the equipment. Then the measurements were stacked three times for each point along the profile to enhance data quality. The measured apparent resistivity data were inverted to create a model for the subsurface resistivity using an iterative smoothness-constrained least squares inversion, Res2Dinv (Loke and Barker, 1996; Loke, 1998). This scheme needs no previous knowledge of the subsurface; the initial guess model is constructed directly from field measurements. In the inversion of this data set, the robust inversion (Claerbout and Muir, 1973) was used. This choice is suitable for detecting fractures and faults because it sharpens the linear features such as faults, dikes and contacts (Loke, 2001). The pseudosections of the measured and calculated apparent resistivity as well as the section of the inverted resistivity model for L1 are displayed in Fig. 3,


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Figure 2. Photographs of cracks that appeared in the buildings at the site (top), and the cavities and sinkholes encountered near to the site (bottom). as an example. Figure 4 shows a collective 3-D view of the inverted resistivity models for all of the profiles (L1 to L7). The inversion process converged with an RMS error varying between 3 and 7. There was not much difference in the inverted section for the last 3 iterations, so we choose the sections with the lowest RMS error. Generally, the inverted resistivity sections show that the site is characterized by relatively low resistivity background ranging from 5 to 22 ohm-m with an average value of about 15 ohm-m. These low resistivity values may refled the infiltration of surface water used for the irrigation of the gardens located at the southern part of the site and between the buildings blocks. The resistivity section of profile L1 (Fig. 3) showed two distinct anomalies that here relatively high resistivity values at a distance approximately equal to 50 and 76 m from the starting point of measurements. The first one (190 Ohmm) was located at five meters depth and shows shallower extension in the profile L2 and deeper extension in L7. The second anomaly (more than 300 Ohm-m) was located at 3.2 m depth, and also shows extension in L2 and L7. Similar anomalous zones were detected in the rest of the profiles with relatively high resistivity values. According to the available geological information, these anomalies are probably related to hard, cherty limestone

blocks or air-filled cavities. Moreover, linear abrupt changes in the resistivity distributions noticeable in the sections are mostly related to contacts between hard and fractured wet limestone as well as the other linear structures. Three vertical electrical soundings (VES) were conducted in the southern part of the site to investigate the vertical changes in the resistivity distribution. The VES stations were located along survey line L7 with 50 m separation (Fig. 1) and the current electrode halfspacing (AB/2) ranged from 1.0 up to 200 m in successive steps. The data were inverted using a 1-D inversion technique applying Occam’s method (Constable et al., 1987). In this method, the depth to the bottom of each layer is logarithmically spaced between the minimum and the maximum depth (according to AB spacing); then the resistivity values are initialized to the average value and iteratively adjusted to produce the smoothest model that fits the data. The inverted 1-D resistivity values (Fig. 5) show that the shallow subsurface of the site (up to 20 meter depth) is composed of a relatively low resistivity formation, which is probably soft and saturated with water. This interpretation correlates well with the surface geology and results of the dipole-dipole survey. This conductive formation overlies higher resistivity bedrock (Fig. 5).


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Figure 3. Dipole-dipole resistivity sections along survey line L1; (top) measured apparent resistivity, (middle) calculated apparent resistivity, and (bottom) inverted model resistivity section. Ground Penetrating Radar (GPR) Survey Ground penetrating radar (GPR) is a high-frequency electromagnetic technique; commonly applied to solve most of the engineering and environmental problems. A GPR system radiates short pulses of high-frequency EM energy into the ground from a transmitting antenna. The velocity of EM waves is related to the dielectric permittivity of subsurface materials. When this wave encounters the interface between two materials having different dielectric permittivity, a portion of the energy is reflected back to the surface, where it is detected by a receiver antenna and transmitted to a control unit for processing and display. Depth of penetration of radar waves is a function of antenna frequency, dielectric constant and the electrical conductivity of the soils. However, low-frequency antennas achieve greater depth of penetration than those of high frequency, but they have poor spatial resolution. Conductive soils, such as clays, attenuate the radar waves much more rapidly than resistive soils such

as dry sand and resistive rock (Davis and Annan, 1989). A raw estimation of depth can be obtained by multiplying the one-way travel time with the average propagation velocity of the GPR pulse inside the geological formation. The GPR method has been increasingly used for shallow subsurface mapping because of its capability to provide a high resolution image of the near surface discontinuity and heterogeneity (Reynolds, 1997). The GPR survey, using SIR-2000 instrument, was directed to inspect the upper ten meters of the study area. The GPR data were collected along the same resistivity profiles L1 to L7 (Fig. 1). The GPR data were acquired using a 200 MHz mono-static antenna applying time windows of 120 ns, with 20 scans per meter and 512 samples per scan. The data were processed using the Reflex program (version 2.1.1). Several processing steps were applied to each radar record separately such as, background removal, band bass (1-D and 2-D) filters, median filter and gain control.


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Figure 4.

3-D view of the inverted resistivity sections for the survey lines (L1 to L7).

A radar record was collected over a known cave in the study area to determine the dielectric constant of the soil and convert travel time to depth on the radar sections (Fig. 6). The average velocity of the electromagnetic wave propagation was 0.1 m/ns for this area. After Inspection of the radar sections, four radar records were selected for representation. Figure (7) represents a segment of the radar record conducted along profile L4. This figure shows two significant anomalous. The first one is located at a horizontal distance of 94 m along the profile, and represents a cave at 2 m depth, while, the second is located at a horizontal distance of 80 m and represents an extended fracture through the limestone rocks filled with wet soil. Figure 8 displays a segment of the radar record along profile L7; this section contains a GPR reflection which might be interpreted as an extension of a cave at the horizontal distance of 75 m along the profile, while the arrows point to cracks filled with surface water.

Figure 5. 2-D view of the inverted 1-D resistivity models for the vertical electrical sounding.


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Figure 6. Radar record conducted over an excavated cave in the study area, as a case study to determine the dielectric constant for the study area. Figure 9 shows a part of the radar section along profile L9. Inspection of this section reveals the presence of small cave signatures at horizontal distances from 200 to 220 m. Very Low Frequency (VLF) Electromagnetic Survey Very low frequency electromagnetics (VLF-EM) is an inductive technique, which uses electromagnetic signals of radio waves from remote military transmitters. These signals have frequency ranges between 15

Figure 7.

and 30 kHz, and are propagated between the surface of the earth and the ionosphere causing the primary field. When there is a localized conductor, such as water-filled fractures, the primary field induces secondary currents inside that conductor, and these currents generate a secondary field which is different in phase and superimposed on the primary field. Thus, measuring the total field (primary and secondary) using VLF-EM receiver on the surface of the earth can help in detecting the conductive elongated structures, geological contacts

The radar record conducted over the profile L4.


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Figure 8.


like altered zones, faults, and conductive dykes (Guerin et al., 1994; McNeill and Labson, 1990). The depth of investigation of the method is controlled by the geometry and dimensions of the explored body, the resistivity of the host medium and the anomalous bodies and the used frequency. Generally, no absolute rule can be given but the penetration depth is commonly more than 30 m (Fraser, 1969).

Figure 9.

In the present work, VLF data were collected along a single profile (L7, Fig. 1) at a location far from the sources of noise. The receiver used in this study is TVLF unit (IRIS- Instruments, 1993). This receiver enables automatic filtering and digital stacking to enhance the observed signal and calculate the quality coefficient. The VLF survey was carried out in the tilt (magnetic) mode. The tilt angle mode is used to prospect



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Figure 10.

VLF data measured along the profile L.7.

for conductive elongated structures and geological contacts like altered zones, faults and conductive dikes. In this mode, it is convenient to operate with a transmitter which is located in the supposed strike (645u) of the proposed target. In such configuration, the primary magnetic field which is perpendicular to the structure produces a flux which attains a maximum across the structure, and provides the largest possible anomaly. Accordingly, two international stations are used to transmit the signal; they are Saint Assise station, France (16.8 kHz) and the GB station, UK (22.3 kHz). The measured tilt angle (real component) and ellipticity (imaginary or quadratur) for each frequency were presented in the form of stacked profiles (Fig. 10). The VLF data were interpreted qualitatively to determine the location of subsurface structures in comparison with the other methods. VLF anomalies were detected at 40, 140, 150, and, possibly, at 200 m horizontal location, in both frequencies. These anomalies were interpreted as conductive fracture zones. Integrated Interpretation Integrated interpretation of the acquired geophysical data is summarized in a schematic cross section showing the interpreted structures (Fig. 11). Two major high-resistivity anomalous zones (shown in light gray tone) are delineated from the resistivity sections with the expected horizontal extension dipping to the south. The

existence of these anomalies is confirmed by coinciding dome-shaped reflection events in the GPR section. The GPR signature of these sources supports the interpretation of these anomalous zones, which reflect the presence of massive bodies. One of these zones is very near to the surface; therefore, we could manually inspect it by digging that encountered a hard limestone rock. Accordingly, many minor high-resistivity zones with local horizontal extensions were also interpreted to indicate the presence of resistive, hard, limestone boulders. A group of low-resistivity zones (shown in dark gray) are shown in the profiles L3, L4, L5 and extensively in L7. These zones were interpreted to be pockets of shale and/or limestone fragments partially saturated with water seepage from irrigation. Some anomalous zones were delineated only from the GPR sections (shown in white), characterized by scattering, bending or distortion of the reflection events. These zones were inferred to correspond to karstic features that were too small to be detected by the resistivity survey with the chosen survey geometry. The dominant feature in the interpreted sections is the vertical to nearly-vertical fracture zones, contacts and joints detected by all geophysical methods. These features, common in this type of limestone, could be seen clearly in many sections and in a limestone quarry very near to the investigated site. Most of the karstic features were associated with these fractures (Fig. 2).


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Figure 11.

3-D view of the interpreted cross sections from the geophysical survey.

Therefore, these fractures and fault-like features are considered to be the main hazard for the construction of buildings in the study area. The hazard presented by these fractures has been greatly increased by two factors. The first is the uncontrolled use of the surface water for irrigation in the main garden as well as gardens between the buildings that activate the dissolution processes and facilitate movements along the fracture planes. The

second is the use of dynamite in the limestone quarry very near to the town. Summary and Recommendations In the present study, an integrated geophysical survey was carried out in a dwelling site at 15-May town, south Cairo, Egypt. The new buildings at this site


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258 Journal of Environmental and Engineering Geophysics were severely damaged by a series of cracks that affect their stability. The objective of this study was to investigate the shallow subsurface of the area to outline its geological structures, and to determine whether such structures caused these cracks. Three surface geophysical methods were used in this study: 2-D resistivity profiling using dipole-dipole and Schlumberger arrays, ground penetrating radar (GPR), and very low frequency electromagnetics VLF-EM. The geophysical data collected using these methods were processed and interpreted to image the subsurface structures of the investigated site. Two high-resistivity anomalous areas were delineated by electrical resistivity and GPR methods in the southern part of the study area. Based on the geophysical signature of these anomalies and digging at the location of the shallower one, these anomalous areas were interpreted as hard limestone blocks. A group of low-resistivity spots were delineated and interpreted as pockets of shale and/or limestone fragments partially saturated with water. Moreover, some anomalous zones were delineated in the GPR data that we believe reflect small karstic features. The prominent structure in the site is the vertical to near-vertical fracture zones, contacts and joints that were detected with all three geophysical methods. With the uncontrolled use of surface water for irrigation and the frequent massive dynamite explosions used in the limestone quarry nearby, the detected fracture zones and karstic features can be considered to be the main cause of the cracks that appeared in the new buildings at this site. Therefore, we recommend the following safeguards to increase the safety factor of the buildings in the neighborhood: 1. Controlling the irrigation rate and quantity of the green strip in the town. 2. Controlling the frequency and intensity of the dynamite explosions used in the limestone quarry near the town. 3. Detailed geophysical study, especially at the new proposed zones in 15-May town before constructing the houses. Acknowledgments We are indebted to all the staff of the Exploration Geophysical Laboratory in Kyushu University for their contribution and support during this work. The DC resistivity and VLF surveys were conducted using NMA facilities, while NRIAG staff could help in acquiring GPR data with NRIAG facilities, sincere thanks are due to both NMA and NRIAG. We appreciate the thoughtful comments made by the anonymous reviewers and their constructive criticism that improved the manuscript to its present status.

References Chamberlain, T.A., Sellers, W., Protector, C., and Coard, R., 2000, Cave detection in limestone using ground penetrating radar: Journal of Archaeological Science, 27, 957–964. Claerbout, J.F., and Muir, F., 1973, Robust modeling with erratic data: Geophysics, 38, 826–844. Constable, S.C., Parker, R.L., and Constable, C.G., 1987, Occam’s Inversion: A practical algorithm for generating smooth models from electromagnetic sounding data: Geophysics, 52, 289–300. Davis, J.L., and Annan, A.P., 1989, Ground penetrating radar for high resolution mapping of soil and rock stratigraphy: Geophysical Prospecting, 37, 531–551. Edwards, L.S., 1977, A modified pseudo section for resistivity and IP: Geophysics, 42(5), 1020–1036. Farag, I., and Ismail, M., 1959, Contribution to the stratigraphy of the Wadi Hof area (northeast Helwan): Bull. Fac. Sci., Cairo Univ., 34, 147–168. Fisher, G., Le Quang, B.V., and Muller, I., 1983, VLF ground survey-A powerful tool for the study of shallow twodimensional structures: Geophys. Prosp., 31, 977–991. Fraser, D., 1969, Contouring of VLF-EM data: Geophysics, 34, 958–967. Guerin, R., Tabbagh, A., and Andrieux, P., 1994, Field and/or resistivity mapping in MTVLF and implications for data processing: Geophysics, 59, 1695–1712. Iris Instruments, 1993, T-VLF operating manual, Orleans, France. Loke, M.H., and Barker, R., 1996, Rapid least-squares inversion of apparent resistivity pseudo sections by a quasi-Newton method: Geophysical Prospecting, 44, 131–152. Loke, M.H., 1998, RES2DINV, Rapid 2D resistivity and IP inversion using least-squares methods, User manual, Austin Tex., Advanced Geosciences, Inc., 66 pp. Loke, M.H., 2001, Constrained time lapse resistivity imaging inversion. The Environmental and Engineering Geophysical Society SAGEEP 2001 Symposium Program, March 2001, Denver: 34. McNeill, J.D., and Labson, V.F., 1990, Geological mapping using VLF radio fields, in Nabighian, M. (ed.), Electromagnetic Methods in Applied Geophysics, 2, B, Tulsa, Oklahoma: Society of Exploration Geophysicists, 521–640. Moustafa, A., Yehia, A., and Abdel Tawab, S., 1985, Structural setting in the area east of Cairo, Maadi and Helwan: Middle East Res. Centre, Ain Shams Univ., Sci. Res. Series, 5, 40–64. Reynolds., and John, M., 1997, An introduction to applied environmental geophysics, West Sussex, England: John Wiley & Sons Ltd., 796 pp. Roth, M.S.J., Nyquist, J.E., and Guzas, B., 2000, Locating subsurface voids in karst: A comparison of multielectrode earth resistivity testing and gravity testing: Proceedings of SAGEEP’ 2000, 359–365. Said, R., 1990, Geology of Egypt, Balkema Pub., Rotterdam, Netherlands.


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259 Elawadi et al.: Geophysical Survey at a New Dwelling Area, Egypt Schoor, M.V., 2002, Detection of sinkholes using 2D electric resistivity imaging: Journal of Applied Geophysics, 50, 393–399. Strougo, A., 1985, Eocene stratigraphy of the eastern greater Cairo (Gebel Mokattam-Helwan) area: Middle East Res. Centre, Ain Shams Univ., Sci. Res. Series, 5, 1–39.

Telford, W.M., Geldart, L.P., and Sheriff, R.E., 1990, Applied geophysics (2d ed.), New York: Cambridge University Press, 770 pp. Ward, S., 1990, Geotechnical and environmental geophysics. SEG publications, investigations in Geophysics No. 5. V. I, 389 pp.


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