The near-surface groundwater aquifer that threatened the Great
Pyramids of Giza, Egypt, was investigated using integrated geophysical
surveys. A total of 10 electrical resistivity imaging, 26 shallow seismic
refraction, and 19 ground-penetrating radar surveys were conducted in the Giza
Plateau. Collected data for each method were evaluated by state-of-the art
processing and modeling techniques. A three-layer model depicts the
subsurface layers and better delineates the groundwater aquifer and water
table elevation. The resistivity of the aquifer layer and seismic velocity vary
between 40 and 80
In recent years, the 4500-year-old Great Pyramids of Giza (GPG),
Cheops (Khufu), Chephren (Khafre), Menkaure, and the Great Sphinx have been threatened
by a rising groundwater table resulting from water leakage from the
suburbs, irrigation canals, and mass urbanization surrounding the GPG. This
problem promoted the need to use nondestructive near-surface geophysical
techniques integrated with available borehole hydrogeological data to
investigate and characterize groundwater occurrences in the GPG. The GPG
is located in the southwestern part of the greater Cairo region (Fig. 1).
Geologically, the Giza Plateau is composed mainly of white limestone,
cream and yellow argillaceous limestone, and dark grey dolomitic limestone of
middle–upper Eocene age. The plateau rocks are commonly interbedded with thin
marl layers in their upper part, which dips about 5–10
Location map of the study area at the pyramid plateau.
Geologic map of the Giza Plateau, Egypt (modified after Yehia, 1985).
Many geophysical studies have been carried out in the GPG, mostly for archaeological exploration and investigations (e.g., Dobecki, 2005; Abbas et al., 2009, 2012). Geophysical studies have made an effective contribution to characterizing groundwater aquifers, especially geoelectrical resistivity, seismic refraction, and ground-penetrating radar techniques. Sharafeldin et al. (2017) studied the groundwater table in the GPG using combined electrical resistivity imaging (ERI) and ground-penetrating radar (GPR). The present work implemented an integration of electrical resistivity imaging (ERI), shallow seismic refraction (SSR), and ground-penetrating radar (GPR) techniques to depict the groundwater table and characterize the aquifer in the Giza pyramid area. The locations of different surveys conducted in the GPG are illustrated in Fig. 4.
Groundwater aquifers affecting the Giza Plateau (El-Arabi et al., 2013).
Locations for the profiles and techniques used along the different parts of the Giza Plateau.
Two-dimensional electrical resistivity imaging surveys by tomographic inversion are usually carried out using a multielectrode system with 24 electrodes or more connected to a multicore cable (Griffiths and King, 1965). A Syscal-Pro resistivity meter from IRIS Instruments, France, was deployed at the site of the GPG using a 24 multielectrode dipole–dipole array configuration with 5 m electrode spacing. The length of geophone spread is 120 m for each profile and reaches more than 30 m as the maximum depth of investigation, considering the offset shots. A total of 10 ERI profiles were created to characterize the resistivity of subsurface layers to delineate the groundwater aquifer (Fig. 4). The topographic elevation of each electrode is considered along the ERI profile and linked to the Res2Dinv program. The acquired ERT data were processed using Prosys II software from IRIS Instruments to filter and exterminate bad and noisy data acquired in the field and produce pseudo-resistivity sections. The Res2Dinv software was used to invert the collected data along the conducted ERT profiles (Loke, and Barker, 1996; Loke, 2012). This software works by automatically subdividing the subsurface of the desired profile into several rectangular prisms and then applies an iterative least-squares inversion algorithm to solve a nonlinear set of equations to determine the apparent resistivity values of the assumed prisms while decreasing the misfit values between the predicted and the measured data. Table 1 shows the main characteristics of the ERI profiles, including information such as profile location, length, measured points, and expected depth of penetration.
Main characteristics of ERI profiles.
Seismic refraction is widely used in determining the velocity and depth of
weathering layers, with static corrections for deeper reflection data. It is
also employed in civil engineering for bedrock investigations and large-scale construction.
It is also used in groundwater investigations, the
detection of fracture zones in hard rocks, examining stratigraphy and
sedimentology, detecting geologic faults, evaluating karst conditions, and
hazardous waste disposal delineation (Steeples, 2005; Stipe, 2015). A
refraction technique is widely developed for characterizing groundwater
tables (Grelle and Guadagno, 2009). Particularly, unsaturated soil
followed by saturated soil can be separated by a refracting interface (Haeni,
1988). The seismic velocity values for the depth estimation of the
groundwater can be used as an indicator for water saturation. The values of
A total of 26 SSR profiles were acquired at GPG (Fig. 4). An OYO McSEIS-SX
seismograph with 24 geophones and channels was deployed at the GPG site to
collect seismic refraction data with geophone spacing of 5 m. Sledge
hammers weighing 10 kg with an iron–steel plate are used to generate seismic
SSR and GPR profiles in Nazlet El-Samman village.
To test the accuracy of the resulting tomographic models, these models were used to calculate the arrival-time curve that was compared with the measured arrival time and RMS errors between the two results, which are calculated and illustrated as an example on a modeled seismic profile as blue and black segments in the SSR3 graphs (Fig. 5). Table 2 shows the main characteristics of the SSR profiles, including information such as profile location, length, measured points, and expected depth of penetration.
Main characteristics of SSR profiles.
GPR is a noninvasive and effective geophysical technique to visualize the
near-surface structure of the shallow subsurface and is widely used to solve
environmental and engineering problems (Jol and Bristow, 2003; Comas et al.,
2004; Neal, 2004). GPR is a site-specific technique that imposes a vital
limitation on the quality and resolution of the acquired data (Daniels,
2004). The GPR surveys were carried out using a 100 MHz shielded antenna
from
MALA ProEx GPR. A total of 19 GPR profiles were performed along selected
locations in the study area (Fig. 4). The GPR profiles range in lengths from
40–200 m, according to the space availability, with a total GPR survey length of
about 2.5 km. Wheel calibration was carried out near the Great Sphinx along
30 m of distance, and the velocity used for calibration is
100 m
Main characteristics of GPR profiles.
The ERI profile data represented in Table 1 are used to invert the 2-D
resistivity models of the GPG site. Five selected profiles are presented in
Figs. 6, 7, 8, 9, and 10 to characterize the resistivity model in the
different archaeological sites that are potentially threatened by groundwater hazards.
The interpreted profiles show that the subsurface of the area is
composed of three layers: the surface layer consists mainly of sands and
gravels and some exposures of hard limestone and marl. The resistivity values
show a wide range of variation between 40
Average interpreted groundwater elevations to the nearest eight installed piezometers (modified after AECOM, 2010).
All seismic profiles (Figs. 5 to 10) show a three-layer model for the
subsurface succession with the inverted velocities and thicknesses. The topmost
layer exhibits a velocity range of 500–1000 m s
ERI, SSR, and GPR profiles for the Sphinx and the Sphinx Temple.
ERI, SSR, and GPR profiles in the Valley Temple of Khafre and central field of Mastaba.
All interpreted GPR profiles (Figs. 5 to 10) detect the surface layer that is characterized by strong reflection, and the thickness is good match to that resulting from seismic interpretation. The top of the second layer shows weak reflections due to the water content through capillary seepage. Despite that, the water table in most GPR sections has been traced and is in good agreement with the SSR and ERI results. Due to the signals attenuated below the water table and the depth limitation of the GPR, the third layer cannot be defined in most sections.
ERI, SSR, and GPR profiles in the tomb of Queen Khentkawes.
ERI, SSR, and GPR profiles in the Valley Temple of Menkaure.
The integrated interpretation of the SSR, ERI, and GPR surveys supports a
three-layer model assumed to represent the subsurface succession with the
inverted velocities, resistivities, and thicknesses. The collaboration of
different geophysical techniques that are susceptible to different physical
properties can minimize the ambiguity associated with a separate technique.
SSR is sensitive to elastic properties and density that clearly depict the
subsurface layer boundaries as well as the groundwater table associated with
a velocity of 1500–1600 m s
ERI, SSR, and GPR profiles in the causeway to Menkaure Pyramid.
Groundwater hazards were detected in some locations that have archaeological importance; these locations are Nazlet El-Samman village, the Great Sphinx, Sphinx Temple, Valley Temple of Khafre, central field of Mastaba, and Khafre causeway.
A cross section using the ERT data shows how the groundwater elevation changes from the Sphinx to Menkaure Pyramid.
Groundwater elevation map from the geophysical data gathered in Giza Plateau and piezometer groundwater levels measured by Cairo University.
Table 4 shows a comparison of the groundwater table elevation data recorded by piezometers installed by Cairo University during the year 2018 in the Wadi Temple and Sphinx area (AECOM, 2010). The interpreted water table elevation resulted from the nearest conducted geophysical surveys. There is a relatively small difference among the results, and differences might be related to the pumping effect when the surveys were conducted during 2016, as well as the tolerance in the geophysical data and variation in contact surface between the wet and saturated zones.
Figure 11 represents a cross section using the interpreted results of SSR,
ERI, and GPR data to illustrate the difference in groundwater table
elevation between the Great Sphinx and the small pyramids of Menkaure. It
indicates an increase in groundwater elevation from west to east. The MW6 borehole drilled
at a ground elevation of 65 m detected a water table at an elevation of 15 m
(AECOM, 2010). The water table to the west might be considered a perched
water table due to leakage from mass urbanization, surface runoff, and
capillary fracture seepage. Figure 12 represents the compiled groundwater
table elevation contour map from the geophysical surveys and groundwater
table levels measured from piezometers installed by Cairo University
(AECOM, 2010). The present geophysical surveys proved that the pumping
systems installed by Cairo University (2008) and AECOM (2010) have lowered the
groundwater levels and there is a need for more pumping to
compensate for the recharge of water leakage from the surrounding area
of the Sphinx. A threat comes from the eastern edge where the Nazlet El-Samman suburb,
a golf course, and the Sound and Light Gardens will raise the groundwater level, decrease
the outflow from the archaeological area, and
consequently raise the water level at the site. A new threat along the
western edge of the area is due to the mass urbanization studied by Bekhit et
al. (2013), who stated that increasing the head along the western boundary by
1 m reduces the outflow from the western boundary by about
1120 m
The integrated interpretation of ERT, SSR, and GPR surveys was performed at
the Great Pyramids of Giza site to successfully investigate the groundwater
aquifer and water table elevation and assist
hazard mitigation. An integrated interpretation of three-layer models
is assumed to depict the subsurface layers and a better delineation of the aquifer
layer. The surface layer is composed of sands and gravels with a seismic velocity
of 500–1000 m s
The data used in this study are available from the corresponding author upon request.
SMS proposed the paper idea, performed the data interpretation, and edited the paper; KSE proposed the paper idea and contributed to reviewing; MASY collected the field data and performed the data processing; HK provided expertise on the SSR data processing and interpretation and contributed to reviewing; ZED participated in all the fieldwork and shared in the field data, processing, and preparing the figures; NS contributed to reviewing this paper.
The authors declare that they have no conflict of interest.
The authors would like to thank Jothiram Vivekanandan, the chief executive editor, Nicola Masini, the associate editor, and all expert reviewers for their constructive comments that improved our paper. The critical comments of anonymous reviewers largely improved the paper. The Geophysics Department, Cairo University, furnished all facilities to conduct the research. IIE-SRF funded the scholarship of Sharafeldin M. Sharafeldin hosted by the Geophysical Engineering Department, KTU, Turkey. The Supreme Council of Antiquities, Egypt, granted us permission to conduct the surveys and their assistance is gratefully acknowledged. Edited by: Nicola Masini Reviewed by: four anonymous referees