Iraqi Geological

Abstract


Introduction
Ground mass movements cause landslides, subsidence, and earthquakes.Human activities like water and oil pumping, road cutting, mining, and others cause geological disasters (Poscolieri et al., 2011).The Mokattam Plateau area was affected by a number of landslides over the past few years.Since 1993, landslides have destroyed many homes, followed by landslides in 1995 and 2015 without causing any losses, while the El-Duwaiqa landslide in 2008 destroyed houses and killed five people (Shalapy, 2022).The landslides did not stop in the Mokattam plateau until today, despite the leveling of some slopes, including some of the slopes of the Ain Moussa area in the north of the plateau, where the research area is located (Fig. 1).For many years, researchers have relied on geophysical investigation methods to solve problems at engineering sites related to the exploration of geological formations, geotechnical conditions, and environmental concerns (Giocoli et al., 2015).More and more researchers around the world are using geophysical methods to learn more about landslides, including their depth, thickness, material properties, and probable slip surface (Falae et al., 2019), and studying the internal structure, soil moisture, drainage, and water circulation (Samodra et al., 2020).ERT is a popular geophysical technique utilized to study landslides (Kristyanto et al., 2017;Crawford et al., 2018;Huntley et al., 2019;Samodra et al., 2020;Tsai et al., 2021;Lapenna & Perrone, 2022).ERT is an active geophysical method that can generate a two-dimensional model of the underground electrical resistivity distribution (Loke, 1999).The research objectives of this case study are to employ ERT with a Wenner array to characterize the landslides of the Mokattam plateau through the detection of subsurface lithology, material thickness, and the possibility of exploring possible slipping surfaces in fractured Eocene limestone caused by surface water seepage.

Geologic Setting
Mokattam's terrain and geography were controlled by its geological structures and rock lithology.Topographically, the Mokattam Plateau is high-elevation and encircled by lowlands (Moustafa et al., 1991), which form outer slopes.The Mokattam digital elevation contour map shows heights from 250 to 25 m, with 110 to 200 m in the research area (Fig. 2a).Three types of natural slopes were identified (Shalapy, 2022).The first is natural slopes that are influenced by natural factors and are found on the northern and southern plateau edges, Wadi Moussa, and the northern side of Wadi El-Labbaba, spanning 2.9 km 2 and 37.5 percent of the plateau slopes.The second is the modification of slopes by grading, covering, or dumping surface leveling goods on their free surfaces.Most of them are internal slopes that needed to be modified to avoid hazards, covering 4.6 km 2 and 59 percent.The third is the used slopes on the southern side of Wadi El-Labbaba, which were graded and used as parks and golf courses, covering 0.3 km 2 and 3.5 percent.Geologically, the youngest exposed rocks in the research area are the Quaternary Wadi deposits.The oldest is the middle Eocene, which is represented by the Mokattam Formation.There are three other formations; Maadi, Guishi of the upper Eocene, and Gebel Ahmer of the Oligocene (Fig. 2b).The Maadi Formation, which comprises 69.6% of the plateau's entire area (Shalapy, 2022), consists mainly of thick layers of carbonates and thinner layers of shale, marl, and clay.Many authors have written about the Mokattam geology (Said, 1962;Elshazly et al., 1976); they conclude that the Maadi Formation consists of brownish units of fractured limestone, shale, and marl.It is located as a horizon at the top of the upper plateau and differentiated into five different lithologic units (Fig. 2b).The Guishi Formation consists of thin layers of white fossiliferous limestone with yellowish marl intercalated.The Mokattam Formation has three parts: the upper building stone, the Gizehensis limestone, and the lower building stone.
Structurally, in general, the Cairo-Suez structural region shares structural faults with Mokattam.The E and NE areas of the plateau have two monoclines and many normal faults and fractures (Fig. 2b).The fault trends are WNW-ESE, NW-SE, and E-W with dip angles between 60° and 71°.The main fault trends in the research area specifically are NW-SE and E-W (Sultan et al., 2008).

Materials and Methods
Engineering, environmental, and groundwater studies are all well-known reasons to use ERT (Sultan et al., 2008;Sevil et al., 2017;Araffa et al., 2021;Li et al., 2021;Hasan & Shang, 2022).The ERT has been used in numerous landslide studies, including (Uhlemann et al., 2017;Falae et al., 2019;Huntley et al., 2019;Sigdel & Adhikari, 2020;Ivanik et al., 2021;Uno et al., 2020;Abd El-Gawad, 2021;Lapenna & Perrone, 2022;Safani et al., 2023).In this research, the geoelectrical survey includes five ERT profiles (Fig. 2a) that were taken in the research area using the Winner electrode array (Fig. 3a) to find out more about the lithology, water seepage, and structure of the area.The multi-functional direct current method was used to do the geoelectrical survey with the SAS 1000 instrument.For the electric current and voltage measurements, as well as the profiles, the Wenner array used 32 steel electrodes that were five meters apart.The roll-along technique was used in ERT 1, which was parallel to the edge of the plateau, while the rest of the ERT profiles were on the plateau slope to provide an accurate depiction of subsurface lithology and detect possible structural features in all directions.The roll-along method was used to cover a larger area horizontally (Fig. 3b), especially for a system with a limited number of electrodes (Loke, 1999).In this technique, after the sequence of measurements is done, the cable is moved several unit electrode spacings past one end of the line.All of the measurements that involve electrodes on parts of the cable that don't overlap the original end of the survey line are done again.
The SAS4000 software tool was employed to extract apparent resistivity measurements from the resistivity meter.Subsequently, the 2D-resistivity tomography program known as "RES2DINV" was employed to handle the data and address any flawed or inadequate data points before processing the apparent resistivity data.RES2DINV is a commercial program that inverts apparent resistivity values into true resistivity.There was also an examination of Google Earth satellite pictures acquired between 2004 and 2022, which allowed the researcher to examine external factors including the growth of metropolitan areas, land use, and activities on the plateau before the landslide and its timing.

Results and Discussion
Through the interpretation of the ERT profiles (Fig. 4), it was found that there are two layers: the first is a high-geoelectrical resistivity layer of fractured limestone covered in some places by backfill sediments, and the second is a low-geoelectrical resistivity layer of shale below the limestone.In between the two layers is a zone that is holding the weight of water leaking from the surface; it has a medium resistivity value.First, there is a layer of fractured limestone on the surface that has high geoelectrical resistivity and is covered by a small amount of backfill sediment.This layer has weak zones that represent cracks through which surface water seeps and also works to reduce the resistivity value in places of fractures, which appear on the longitudinal ERT in brown while the rest of the layer appears in orange and red (Fig. 4).The resistivity of this layer ranges from 55 to 400 Ω.m and its thickness ranges from 9 to 18 meter.Second, under the limestone, there is another layer consisting of shale; its resistivity is relatively low, ranging from 0.5 to 8 Ω.m, and it is represented on the ERT in blue.Third, there is a zone between the limestone and the shale characterized by relatively medium geoelectrical resistivity values ranging from 8 to 55 Ω.m represented in green and yellow, and it is bearing the water leaking from the surface.Because the surface of the shale layer changes in elevation as deduced from the ERT results, the water moves and collects at the lowest point on the shale, and with the passage of time and increasing surface leakage, the water tries to escape to the outside over the shale layer, and this may cause landslides in the upper part of fractured limestone, as happened in some parts of the research area.It is also noted through the results of the ERT profiles on the slope (from ERT2 to ERT5) that the slope area differs in its resistivity, as the slope was settled by backfilling in some of its parts with the beginning of urban expansion in the area.These sections usually have an inconsistent and dissimilar composition, which aids in the cause of landslides.In addition to a geoelectrical study of the area, nearly 20 years' worth of Google Earth satellite photos were looked at to find possible places where water from sources like garden irrigation, swimming pools, and sewage systems could leak to the surface (Fig. 5).This is thought to be one of the side effects of urbanization over time.Since June 2004, the region has seen limited urban expansion according to Google Earth image, and the second picture, since November 2010, shows increasing urban expansion, as well as garden areas and the presence of some swimming pools near the edge of the plateau (Figs 5b  and c), and this has continued until the end of 2018 (Fig. 5d), when the government removed all of the swimming pools in the area for some reasons, including safety.Despite this, there are traces of water leakage that appeared in late 2021 (Fig. 5e), and limited landslides occurred as the image shows.Previous analysis of geoelectrical data and comparison with available satellite images showed that the landslides were caused by fractured layers of limestone near the surface that let groundwater in.These layers are above an impermeable layer of shale that holds the water above it.The water then moves in the direction of the shale's slope to gather at the deepest point on the shale's surface (Fig. 6).The collecting water creates weaknesses through which water exits, increasing the likelihood of landslides, which is exactly what happened.If the same conditions are present, this might happen again at other points along the plateau's edge.

Conclusions
Based on the previous analysis of the ERT profiles and satellite images, it is clear that landslides in the research area were caused by many factors, such as:

•
The nature of the layers consists of fractured limestone over the shale layer.
• Leakage of surface water through the new urban areas occurs because the water gets in through the cracks in the fractured limestone layer on the surface.In general, water collects above the impermeable shale layer.This water gathers at the lowest point above the shale, which is most likely to cause landslides because of the water that has leaked through the surface.
• It is also noted through the results of the four profiles on the slope that the slope area differs in its resistivity, as the slope was settled by fill consisting of incoherent sediments of different composition in some parts with the beginning of urban expansion in the region, which led to its easy disintegration and landslide, especially with superficial water leakage.
• From what we've seen so far, we can conclude that it is possible to find places where landslides are likely to happen by finding places where water pools above the shale layer along the plateau and using engineering methods to fix them.

Fig. 1 .
Fig.1.Google Earth Image Showing the Location of the Study Area.

Fig. 2 .
Fig.2.a) Digital elevation contour map of the Mokattam plateau and research area with ERT locations marked; b) Geological map of the Mokattam area, including the research area (modified fromAraffa et al., 2021)

Fig. 3 .
Fig.3.a) A sketch showing the arrangement of electrodes for a 2-d electrical Wenner Survey and the sequence of measurements used to build up a pseudo section; b) The roll-along technique is used to extend the survey area horizontally, especially for systems with few electrodes (after Loke, 1999).

Fig. 5 .
Fig.5.A timeline of Google Earth images from 2004 to 2022 shows how the number of cities, gardens, and swimming pools near the edge of the plateau keeps growing, in addition to the landslide and ERT locations that are labeled in 5f.

Fig. 6 .
Fig.6.ERT-1 results integrated with Google Earth and field images demonstrating the landslide cause as a result of collecting seeping water above the impermeable shale layer.