Joint Electromagnetic-Terrain Conductivity and DC-Resistivity Survey for Bedrock and Groundwater Characterization at the New Al-Obour City, Egypt

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Introduction
Recently, Egypt is seeking urban expansion to overcome the increasing population through an ambitious plan to establish new cities in most parts of the Republic.Government policy in the last decades has concentrated on the extension of agricultural land into reclaimed desert areas accompanied by the redistribution of population, starting at the peripheries of existing cities.One of those cities is the New Al-Obour city.Planning the cities acquires to determine places of risky geological structures, as well as defining the near-surface soil, rock distributions, and geotechnical characteristics (Sultan et al., 2011;Araffa, 2013;Azmy et al., 2021).
The study area is situated at the northern side of Cairo-Ismailia Desert Road, about 32.0 km from Cairo.It is located between latitudes 30˚ 13' 54.00 '' and 30˚ 17' 0.00'' N and longitudes 31˚ 35' 2.40'' E and 31˚40' 0.00'' E. It is bounded by Belbies city northward, the Cairo-Ismailia road southward, 10 th of Ramadan city from the East and Al-Obour city from the West.It covers an area of about 38.25 Km 2 (Fig. 1).
Electromagnetic and electrical resistivity methods have been used in groundwater studies due to the strong relationship between the physical properties of aquifers and electrical conductivity (resistance and conductance), (Seidu et al., 2019).The multi-spacing electromagnetic-terrain conductivity (MSEMTC) technique is currently used to characterize the bedrock and the subsurface structures with no invasion of the ground surface (Sudhaa et al., 2009;Atya et al., 2010;Gardi et al., 2013;Araffa et al., 2014;Farag, 2015).Accordingly, these techniques were conducted in the study area from September 2018 to March 2019.
The main objectives of this study were to characterize the bedrock, detect groundwater occurrence and image both the surface and subsurface structures.It also aimed to prove the reliability, applicability, and efficiency of using such non-invasive techniques in the field such as geologic mapping.
The study area has two isolated hills (G.El Hamza and G. Um-Qamar), and the elevations in the area ranged between 50 m, in the south, and 230 m, at G. El Hamza (in north) (Fig. 2).This landform is formed as a large double plunging anticline and contains two blocks.The first is G.El Hamza, that represents the southern flank, and the second is G. Um-Qamar that represents the northern flank.These two blocks are separated by a wide plane about 4.5 km long and 2.0 km wide and occupy the crest of the anticline.This crestal part is represented by a topographically low area, although it is structurally high.The blocks consist of marine Miocene sediments topped by the non-marine Miocene sediments, but the intervening plain is made up of Oligocene sediments in addition to Quaternary sands which occupy the wadi along the plain.

Geology and Structure of the Study Area
The sedimentary rocks belonging to the Quaternary and Tertiary rock units dominate the study area as shown in Fig. 3.The Quaternary sediments are categorized into Holocene and Pleistocene sediments.Holocene is characterized by variable unconsolidated sediments like young Wadi fill and young aeolian deposits (El-Fayoumy, 1968).The Pleistocene deposits include the old deltaic deposits that consist of igneous fragments and coarse sand.Tertiary rock units composed of Pliocene deposits are divided into Hagul Formation and Wadi El Halazoni Formation (Swedan, 1991).The Hagul Formation is composed mainly of sand and calcareous sandstone interbedded with clay and sandy limestone.It unconformably overlies the marine Miocene and covers with Quaternary deposits.Wadi El Halazoni Formation is composed mainly of limestone boulders, embedded in calcareous matrix.Miocene rock units have a widespread distribution in the southern high slopes, formed sandy marls and sandy limestone.Oligocene sediments are represented by sands, gravels and volcanic basalts.Eocene fossiliferous beds (chalky, marly, dolomitic, and sandy limestone) belong to the Middle and Upper Eocene (Hashem, 1997;Mohamed and El-Sabrouty, 2014).
Structurally, the study area is affected by normal dip-slip faults.The majority of these faults are oriented towards the East-West (EW) and North-West (NW) directions.Regarding folding, the existing en-echelon folds are a symmetric and doubly plunging, which are represented by two alternative anticlines and synclines.These folds are oriented in ENE to NE direction.There are five local unconformities encountered in the study area, besides the older regional one between the Oligocene and Eocene rocks just south of the study area.These unconformities are arranged, from older to younger as follows (Hashem, 1997): 1-The Oligocene sands and gravels-basaltic flows unconformity; this surface is well represented in several localities to the western foot slope of G. El-Hamza anticline flank.

Conceptual background
Electromagnetic induction (EMI) is one of the oldest geophysical methods, and it involves measuring electric and magnetic components created by the primary field (PF) caused by natural or artificial sources of EM field (Farag, 2005).MSEMTC survey procedures are techniques of frequency-domain electromagnetic (FDEM) induction.In a typical FDEM survey, an EM field is generated using a transmitter coil, this field propagates into the subsurface.Eddy currents are induced in the subsurface, while the EM wave propagates through the ground.These eddy currents then produce a secondary field which is recorded using a receiver.This method detects the subsurface electrical conductivity using the phase and magnitude of the secondary EM field.The PF travels above and below the ground, and then creates electric currents in the subsurface conductor.These currents become secondary fields that change the PF.The resulting field will be different from the PF in direction, phase, and intensity and can be interpreted to detect the conductor existence (Sharma, 1997).
EM34-3 measuring system (Geonics, Limited, Canada) consists of receiver and transmitter coils that work at three different frequencies: high frequency (6400 Hz) for shallow penetration, an intermediate frequency (1600 Hz), and low frequency (400 Hz) for deep penetration.Both coils are 10.0 meters away from each other for shallow exploration, 20.0 m for intermediate and 40.0 m for deep penetration.Quantity measured: apparent conductivity in milliSiemens/meter (mS/m).There are two chief modes operations: the first is the horizontal dipole mode (HDM) where the coils lie vertically; the second is vertical dipole mode (VDM) where these coils are positioned horizontally.The VDM system was used in the study area along every profile using the three frequencies 6400, 1600 and 400 Hz.The maximum investigation depth is ranged between 50.0 and 55.0 m, using the largest inter-coil spacing (40.0 m) that is corresponding to the lowermost operating frequency (400 Hz).
Reliable sixteen MSEMTC profiles and eleven Vertical Electrical Soundings (VES) (Fig. 2) were conducted in the field between September 2018 and March 2019.These profiles have direction NW-SE, where they are perpendicular to the structure direction of the study area.
A Geonics EM 34-3 conductivity meter was used to acquire the data.The eleven VES's is located at the location of the EM profiles to integrate and correlate between the results interpreted from each method for enhancing the accuracy, determining more information about the near-surface layers, getting the best real subsurface geological model and determining the groundwater aquifers and it extensions through the study area.
The terrain conductivity data viewing may be sufficient for interpretation.However, the measured EMTC where plotted as pseudo-sections (mS/m) (Figs.4b and 5b).These conductivities where then converted into electrical resistivities (Ohm.m) and plotted versus the equivalent depths (Figs.4c and 5c) according to Spies (1989) and Farag (2005).They were plotted as two-dimensional pseudo-sections.Finally, Geologic cross sections along the profiles are done (Figs.4d and 5d).
This semi-quantitative interpretation introduced a rational model (s) for 1D smoothed inversion trails in 2D by the 'FreqEM TM -0.1/2006' software (Geotomo Software, Malaysia) and specially-designed spreadsheet package 'EMCalcView−XL−1.0'according to Farag, 2005.The linearized inversion used the scheme in an iterative least-squares optimization way (Keofoed et al., 1972;Jupp andVozoff, 1975 Guptasarma andSingh, 1997) to reduce the function which regularized the concluding 1D solution.Topographic data were executed through the inversion trials.The final inversion outcomes match the measured data by 10.2-11.5 RMS %.

Conceptual background
The geo-electric resistivity methods may be used for profiling or sounding.Sounding presents a means to determine the vertical changes in the electrical properties.Sounding data Interpretation reveals the thickness and depth of subsurface layers with different resistivities (Zhody et al., 1974).The apparent resistivity represents the result of a resistance (R) and a geometric factor (K) for a certain electrode array, according to the expression in equation ρa =K*R =K (ΔV/I).The geometric factor takes into consideration the geometric configuration of electrodes and contributes a term that has the length unit (meters).Apparent resistivity (  ) has units of ohm-meters (Reynolds, 1997).The resistivity method depends on station measurements, where electrodes must be in electrical and physical contact with the ground.There are several types of electrode configuration arrays utilized in the resistivity measurements (Lowrie, 2007).The present field measurements were conducted using ABEM Terrameter SAS 300 resistivity meter along eleven Vertical Electrical Soundings using either Schlumberger or Wenner arrays (Figs. 7  and 8).The Shlumberger array was used through four VES's (VES-01, VES-05, VES-06 and VES-10) with maximum AB/2 reached to 800 m apart, while the Wenner array was used through seven VES's (VES-02, VES-03, VES-04, VES-07, VES-08, VES-09 and VES-11) with electrode spacing (a) ranged between 1 and 100 m as shown in Fig. 2. IpI2 win software V3.1.2 (2008) was used for the quantitative interpretation of these VES's.Schlumberger array geometrical factor (K) can be defined in equation 1 and Wenner array geometrical factor (K) can be defined in equation 2.

Data analysis and interpretation
Measured data were analyzed and interpreted using Moscow State University IPI2win V3.1.2(2008) resistivity sounding interpretation software and Golden Surfer 16 software.After collecting the apparent resistivity data, the measured data were imported in the IPI2win software.Log -log graph for the AB/2 with its apparent resistivity point of measurement.This graph represented the observed apparent resistivity curve for each VES.Forward modeling inversion for the measured data were done to get the best fitting between the calculated and measured apparent resistivity data by an initial model from the previous geological information, geological outcrops at the study area.
Figs. 9, 10, and 11 display the measured (black line) and calculated (red line) resistivity data for eleven VES's respectively.The results of inverted models represented by the thicknesses and true resistivities of the geo-electrical layers.Root Mean Square error (RMS) value measures the error percentage between calculated and measured apparent resistivity curves.The RMS values for eleven VES's are were ranged between 1.92% and 17.10%.
Fig. 12 reveals the interpreted true resistivity of eleven VES's with the same datum at elevation (+130 m).The true resistivity ranged from 1-5000 ohm.m, the red color represents the conductive rock units while the blue colors represent the resistive one.It is noticed that at the same datum there are highly variations in the true resistivity values and that reflects that the study area affected by several structures (faults and folds).These results ensure the results of measured the EMTC data and the surface geology in the study area.
Fig. 13 represents the Lithologic interpretation for the Shlumberger VES's (VES-01, VES-05, VES-6 and VES-10) with same elevation datum (+130 m).The obtained results gave an approximate elevation of water table ranged between +70 m at VES-01 and +60 m at VES-10.The water table was found in high-resistive Lower Oligocene (Gabal Ahmar Fm.) which is chiefly composed of sand and gravels.

Results and Discussion
The results of EMTC data and VES interpretation were deduced using 'EMCalcView−XL−1.0 and IpI2 win software V3.1.2.The 2D transformed-inverted smoothed-resistivity sections, resulting by EMTC by a variable inter-coil spacing at several operating frequencies beneath survey profiles, causing a maximum resolution of depth ranged between 50 and 55 m.There are high variations in the true resistivity values that show the main structure style of the study area, this structure is represented by the double plunging anticline intersected by two major faults and forming the main horst in the area according to the integrated interpretation of EM, VES's and surface geology.The interpreted faults through the EMTC profiles confirm the general geological setting of the study area where: The predominant direction is N 45° E which is related to the Gulf of Aqaba, this direction is confirmed by the faults F1, F2, F4, F10, F13, F14 and F15.While, the direction E-W trend is related to the Mediterranean tectonics and confirmed by the faults F5, F6, F7, F8, F9, F11 and F12.Finally, the direction N 45°W is correlated to the Gulf of Suez trend, this direction does not appear on the profiles but exist in the geological map.It can be stated through the results of the EM method that the area was affected chiefly by the Gulf of Suez, as well as, the Gulf of Aqaba, River-Nile system and Mediterranean tectonics Syrian arc system.According to the integration between the results of VES's and EM with surface geology, there are eight folds in the study area as follows: The first plunging syncline is located in SE part of the study area.The next three plunging anticlines; two of them are located in the NW part and the third is placed in the SE part of the study area.The following three double plunging anticlines; two of them located in the NW part and the third is situated in the middle of the study area.The last two non-plunging anticlines are situated in southern part of the area.
The encountered geo-electric sequence (Figs. 4 and 5) has shown up with fourteen rock units belong to Quaternary, Upper, Middle, Lower Miocene, Upper and Lower Oligocene ages (Table 1).These rock units are variable in thickness, composition, and resistivity.These geo-electric sequences have appeared with low-resistive thin Quaternary (Wadi Fillings/ RU [A]), composed mostly of yellow fine/coarse-grained sand.These surface deposits' thickness is diverse laterally, from 0.5 to 15 m.These deposits partly cover the high/very high-resistive Upper Miocene (Hagul Fm./RU [B]), which is composed mainly of yellowish brown, sand, gravel and sandstone.Its thickness can be 30 m. and 70 m at VES 4 and VES 1, respectively.Its whole thickness surpasses the present maximum depth of investigation (around 65 m).

Conclusions
The current MSEMTC and vertical electrical sounding resistivity measurements might be utilized inexpensively and efficiently in delineating the bedrock characterization and revealing the geo-electric sequence at the New Al-Obour City, Northeastern Cairo, Egypt.
Finally, the current study inspires applying the MSEMTC and vertical electrical sounding resistivity methods to view the horizontal and vertical resistivity structures created by subsurface geology.These methods may also aid in designing an optimum geotechnical program for other new cities in Egypt.

Fig. 3 .
Fig. 3. Geological map of presence survey site and geological cross-section passing in the direction SE-NW through the study area

Fig. 4 .
Fig. 4. (a) Typical measured conductivity data versus offset distance, through a variable inter-coil spacing; (b) terrain conductivity pseudo-section; (c) 2D transformed-inverted smoothed-resistivity section; (d) geologic cross-section interpreted from present EM-TC data and the outcrop surface geology along profile 1.The used symbols are described as in Fig. 2

Fig. 6
Fig. 6 shows maps for inter-coil spacing +1.0 m elevated 10.0 m, 10.0, 20.0 and 40.0 meters inter-coil spacing corresponding to frequency of 6400, 6400, 1600 and 400 Hz respectively.The conductivity values ranged from 1 -85.0 mS/m, the red color represents the conductive rock units while the blue color represents the resistive one.It is noticed that the NW and south parts of the maps (+1.0 m elevated 10.0 m, 10.0 m inter-coil spacing) reflects the conductive zones and NE part is obtained in the maps of 20.0, 40.0 m inter-coil spacing and that reflects the surface geology and the area structure.

Fig. 5 .
Fig. 5. (a) Typical measured conductivity data versus offset distance, through a variable inter-coil spacing; (b) terrain conductivity pseudo-section; (c) 2D transformed-inverted smoothed-resistivity section; (d) geologic cross-section interpreted from present EM-TC data and the outcrop surface geology along profile 9.The used symbols are described as in Fig. 2

Fig. 6 .
Fig. 6.(a) Measured conductivity contour map for varying inter-coil spacing +1.0 m elevated 10.0 meters and for corresponding frequency of 6400 Hz; (b) conductivity map for inter-coil spacing 10.0 meters corresponding to frequency of 6400 Hz; (c) conductivity map for inter-coil spacing 20.0 meters corresponding to frequency of 1600 Hz; (d) conductivity map for inter-coil spacing 40.0 meters corresponding to frequency of 400 Hz.