Delineation of Near-Surface Structure in the Southern Part of 15th of May City, Cairo, Egypt Using Geological, Geophysical and Geotechnical Techniques

The integration of geological, geophysical, and geotechnical interpretation at the southern part of 15th of May City, have been used to evaluate the subsurface stratigraphy, especially the clay layer which may cause serious danger to construction. Those techniques have been used to delineate the subsurface structures as normal faults, which play a critical role on the stability of buildings. Geological setting of the area has been evaluated through the construction of a geological map from different geological sections and samples obtained from more than 30 observation points. Geophysical tools such as vertical electrical soundings (12 VESs), 2-D dipole–dipole array (7 sections), P-wave shallow seismic refraction (31 profiles) and multiple channel analysis of surface waves (31 MASW profiles) have been carried out to image the subsurface situation. Geotechnical evaluation using 26 boreholes, samples, laboratory tests and geotechnical parameters has been done at the area of interest. The geological setting demonstrates that the city had been constructed on the second and third members of Qurn Formation (Upper Eocene) composed of argillaceous limestone, marl and shale. Two normal faults are passing through the area were observed. The resistivity (VES and dipole–dipole) and seismic (P-waves and MASW) results reflect the presence of the two normal faults cross the study area, affecting the obtained section of marl, clayey marl and limestone layers. The geotechnical information indicate the presence of the normal faults and the existence of clay layer with swelling ability reaching 140%, which may cause cracks in the upper layers and/or subsidence.     

Geochemistry of the Zargoli granite: Implications for development of the Sistan Suture Zone,southeastern Iran

The Zargoli granite, which extends in a northeast–southwest direction, intrudes into the Eocene–Oligocene regional metamorphic flysch‐type sediments in the northwest of Zahedan. This pluton, based on modal and geochemical classification, is composed of biotite granite and biotite granodiorite, was contaminated by country rocks during its emplacement, and is slightly changed to more aluminous. The SiO₂ content of these rocks range from 62.4 to 66 wt% with an alumina saturation index of Shand [molar Al₂O₃/(CaO + Na₂O + K₂O)] ～ 1.1. Most of its chemical variations could be explained by fractionation or heterogeneous distribution of biotite. The features of the rocks resemble those which are typical to post‐collisional granitoids. Chondrite‐normalized rare‐earth element patterns of these rocks are fractionated at (La/Lu)_N = 2.25–11.82 with a pronounced negative Eu anomaly (Eu/Eu* = 3.25–5.26). Zircon saturation thermometry provides a good estimation of magma temperatures (767.4–789.3°C) for zircon crystallization. These characteristics together with the moderate Mg# [100Mg/(Mg + Fe)] values (44–55), Fe + Mg + Ti (millications) = 130–175, and Al–(Na + K + 2Ca) (millications) = 5–50 may suggest that these rocks have been derived from the dehydration partial melting of quartz–feldspathic meta‐igneous lower crust.     

Upscaling of CO2 vertical migration through a periodic layered porous medium: The capillary-free and capillary-dominant cases

We present an upscaled model for the vertical migration of a CO₂ plume through a vertical column filled with a periodic layered porous medium. This model may describe the vertical migration of a CO₂ plume in a perfectly layered horizontal aquifer. Capillarity and buoyancy are taken into account and semi-explicit upscaled flux functions are proposed in the two following cases: (i) capillarity is the main driving force and (ii) buoyancy is the only driving force. In both cases, we show that the upscaled buoyant flux is a bell-shaped function of the saturation, as in the case of a homogeneous porous medium. In the capillary-dominant case, we show that the upscaled buoyant flux is the harmonic mean of the buoyant fluxes in each layer. The upscaled saturation is governed by the continuity of the capillary pressure at the interface between layers. In the capillary-free case, the upscaled buoyant flux and upscaled saturation are determined by the flux continuity condition at the interface. As the flux is not continuous over the entire range of saturation, the upscaled saturation is only defined where continuity is verified, i.e. in two saturation domains. As a consequence, the upscaled buoyant flux is described by a piecewise continuous function. Two analytical approximations of this flux are proposed and this capillary-free upscaled model is validated for two cases of heterogeneity. Upscaled and cell averaged saturations are in good agreement. Furthermore, the proposed analytical upscaled fluxes provide satisfactory approximations as long as the saturation set at the inlet of the column is in a range where analytical and numerical upscaled fluxes are close.     

Earth Fissuring and Land Subsidence in Western Saudi Arabia

The present investigation deals with the engineering geological evaluation of earth fissuring associated with land subsidence in Wadi Al-Yutamah. The investigations include surface mapping and sampling, in situ and laboratory soil testing, water well inventory, geophysical survey, and monitoring of open fissures and the level of the wadi floor.The earth fissures in the area developed as a result of land subsidence due to man-induced water level declines caused by pumping water from the wadi aquifer above a safe yield. This situation has produced a compaction of underlying unconsolidated sediments and formed hair fissures above ridges and steps of buried surface bedrock. These hair fissures enlarged later after flood erosion and possible enhancement with hydrocompaction.The wadi soil in the study area consists mainly of silt of low plasticity, low density and high void ratio and it was classified as loess like material and collapsing soil. The settlement in the area is greatly increased by excessive wetting under constant pressures. The calculated coefficients of subsidence (collapse) show that the wadi soils were considered to pose moderate problems when wetted.Monitoring of the existing open earth fissures using extensometers indicates that the width of the fissure increases after flooding or rain falls. Monitoring of the ground level using GPS techniques, shows a good relation between the declination of the water table and the subsidence of the ground of the wadi floor.     

Spectral characteristics of natural and artificial seismic events in the Lop Nor test site, China

Seismic discriminants based on the spectral seismogram and spectral magnitude techniques have been tested to discriminate between three events; a nuclear explosion which took place in Lop Nor, China with m _b 6.1 and two earthquakes from the closest area with m _b 5.5 and 5.3, respectively. The spectral seismogram of the three events shows that the frequency content of the nuclear explosion differs from that of the earthquakes where the P wave is richier in high frequency content in the nuclear explosion than the corresponding earthquakes. It is also observed that the energy decays more rapidly for the nuclear explosion than for the earthquakes. Furthermore, the spectral magnitudes reveal significant differences in the spectra between the nuclear explosion and the two earthquakes. The observed differences appear to be quite enough to provide a reliable discriminant. The estimated stress drop from the magnitude spectra indicates a higher stress drop of the nuclear explosion relative to the earthquakes of the same tectonic region.     

Gravity contributions to the understanding of salt tectonics from the Jebel Cheid area (dome zone, Northern Tunisia)

Detailed gravity measurements integrated with geological data were computed to constrain the mechanisms that were active during the emplacement of the Triassic evaporite-bearing folds of Jebel Cheid from the salt-dome zone in the Atlassic region. The gravity analysis consists in mapping the contrasting gravity responses: complete Bouguer anomaly, residual anomaly and derivative maps; the main results display a positive amplitude gravity anomaly as the response of Triassic evaporite bodies and important NE–SW-trending features at the boundaries between the Triassic outcrops and their enveloping strata. In contrast with gravity calculations of a salt dome structure usually resulting in negative gravity anomaly models, the Jebel Cheid clearly expresses a positive gravity anomaly; furthermore, this result is supported by synthetic gravity interpretation.     

Estimating the near surface amplification factor to minimize earthquake damage: a case study at West Wadi Hagoul area, Egypt

The damage caused by earthquake occurrences in different localities all over the world necessitates the evaluation of the underground and/or sub-soil structure for strategic projects. The case study is the proposed area for a new factory construction that lies 3 km west of Suez Cement factory, in the area between Kottamiya and Ain Sokhna. The shallow seismic refraction survey was carried out through 10 major lines (grid lines), comprising 30 seismic profiles. The microtremor measurements were done at 25 locations at the grid intersection points. The obtained compressional (P) and horizontal shear (SH) wave velocities, with the horizontal to vertical (H/V) spectral ratios, are used to deduce the subsurface structures, to evaluate the elastic and dynamic properties and the transfer function at both the surface and bedrock. The results illustrate that there is a good correlation between the fundamental frequencies deduced through the theoretical transfer function and the H/V spectral ratio. The maximum value of ground motion amplification factor of the study area is 3.3 at a corresponding frequency of 7.8 Hz.     

Seismological and GPS constraints on Sinai sub-plate motion along the Suez rift

This work presents new seismological and Global Positioning System (GPS) results aimed at understanding the nature and rate of strain associated with the opening of the Suez rift that separates the Sinai sub-plate from the African plate. The Sinai sub-plate has played a significant role in the tectonic evolution of the northern Red Sea and the Eastern Mediterranean region. Most small, moderate and large earthquakes occur within belts associated with the geologically documented borders of this sub-plate including the Dead Sea fault (DSF) system in the east, the Cyprian arc (CA) in the north, and the Suez rift (SR) to the southwest. The DSF and CA are well defined; however, the SR is only partially defined. Earthquake foci distribution supports the idea that the SR is seismically active, and this earthquake activity cannot be ignored throughout the kinematics evaluation of northern Red Sea region. The earthquake activity is relatively higher in the southern part of the SR and gradually decreases northward. The high seismicity is mainly attributed to the presence of the Sinai triple junction. Earthquake focal mechanisms in the SR are dominated by oblique normal faulting with left-lateral strike-slip components on NW trending fault planes consistent with regional kinematics. The extensional semi-principal stress axes derived from fault plane solutions are oriented NNE-SSW in good agreement with the current stress field obtained from borehole breakouts along the SR as well as results from GPS surveying. Recent survey-mode GPS observations provide evidence for coherent northerly motion of the Sinai sub-plate that varies between 2 and 5 mm/yr. Moreover, strain analysis indicates that the southern SR is dominated by extension while its northern segment is characterized by constriction, inconsistent with earthquake focal mechanisms and regional tectonic models. The overall northward motion of the Sinai sub-plate indicates that slab-pull rather than ridge-push is the dominant force controlling regional kinematics. Based on the low rate of extension and lack of oceanic crust, the SR can be considered an incipient plate boundary between the Sinai sub-plate and the Nubian plate.     

Geoelectrical exploration for groundwater in Al Maha Forest,Ain Jouhra,Morocco

An area of about 30 km² located in Ain Jouhra, south of Rabat, Morocco, was the subject of a geoelectric resistivity investigation. The main goal of the investigation was the assessment of the groundwater potential of the uppermost aquifer. The aquifer conditions such as depth, thickness and boundaries were also investigated. The obtained apparent resistivity curves were first analysed qualitatively and classified using simple curve shapes. Thereafter, the data were converted to resistivity and thickness pairs semi‐quantitatively by means of master curves and then quantitatively by computer modelling using ATO and Winsev software (Zohdy, 1989; Zohdy and Bisdrof, 1989). Lithological control from the available single well with a stratigraphic log aided in the correlation of the resistivity values to different rock units. Three different AB‐spacing iso‐resistivity maps, an isopach map of the main groundwater‐bearing horizon, the depth to the aquifer substratum map and five geoelectric cross‐sections were constructed. The interpretation of these soundings indicates the presence of an unconfined to semi‐confined sandy aquifer with relatively important extent and varying thickness. The maximal thickness of the aquifer is recorded in the central part of the investigated area and is thinning southwards to pinch out farther to the south. Geophysical as well as field data indicate a hydraulic connection between the upper and deeper aquifers. Indeed, the two aquifers are separated from each other by a marly substratum that is indicated throughout the area by the lowest values of the interpreted true resistivity. The value of this resistivity varies laterally, most likely due to the lateral variation in the shale‐to‐sand ratio. The altitude of the substratum decreases towards the north, and increases southwards. Regarding the availability of the groundwater in the study area, zones with high potential are theoretically expected to occur in the central part where the transversal resistance is greatest. However, sufficient water supply and high flow rates from wells intended to produce restrictively from the most upper aquifer are not likely to exist. This conclusion, which seems to be very pessimistic, is evidenced from two real field and experimental observations. The first is the rapid fall of the level of Gharnoug lake, despite the ongoing feeding by three wells. Hence, the amount of water level drop cannot be accounted for by the evaporation alone. That means that the deeper aquifer is continuously draining the upper aquifer at a high flow rate. Very low rates are recorded in all the wells that penetrated only the upper aquifer, the exception being the well that reached deeper into the lower aquifer. The flow rate in this lower aquifer measured 18 litre s^?1. Copyright © 2007 John Wiley & Sons, Ltd.     

An efficient numerical model for incompressible two-phase flow in fractured media

Various numerical methods have been used in the literature to simulate single and multiphase flow in fractured media. A promising approach is the use of the discrete-fracture model where the fracture entities in the permeable media are described explicitly in the computational grid. In this work, we present a critical review of the main conventional methods for multiphase flow in fractured media including the finite difference (FD), finite volume (FV), and finite element (FE) methods, that are coupled with the discrete-fracture model. All the conventional methods have inherent limitations in accuracy and applications. The FD method, for example, is restricted to horizontal and vertical fractures. The accuracy of the vertex-centered FV method depends on the size of the matrix gridcells next to the fractures; for an acceptable accuracy the matrix gridcells next to the fractures should be small. The FE method cannot describe properly the saturation discontinuity at the matrix–fracture interface. In this work, we introduce a new approach that is free from the limitations of the conventional methods. Our proposed approach is applicable in 2D and 3D unstructured griddings with low mesh orientation effect; it captures the saturation discontinuity from the contrast in capillary pressure between the rock matrix and fractures. The matrix–fracture and fracture–fracture fluxes are calculated based on powerful features of the mixed finite element (MFE) method which provides, in addition to the gridcell pressures, the pressures at the gridcell interfaces and can readily model the pressure discontinuities at impermeable faults in a simple way. To reduce the numerical dispersion, we use the discontinuous Galerkin (DG) method to approximate the saturation equation. We take advantage of a hybrid time scheme to alleviate the restrictions on the size of the time step in the fracture network. Several numerical examples in 2D and 3D demonstrate the robustness of the proposed model. Results show the significance of capillary pressure and orders of magnitude increase in computational speed compared to previous works.