Mohamed Metwaly

A hydrogeophysical study to estimate water seepage from northwestern Lake Nasser, Egypt

Estimating the water seepage from Lake Nasser, southern Egypt, into the adjacent Nubian sandstone aquifer is one of the main factors influencing the water balance of the lake. Up till now, there has been no information about seepage values, particularly in the northwestern part of the lake, due to a complete absence of boreholes. The present study is an approach using the time domain electromagnetic (TDEM) technique to estimate the hydraulic parameters of the shallow Nubian sandstone aquifer which are essential in water seepage calculations. The average porosity of the Nubian sandstone aquifer is calculated using bulk resistivities deduced from a TDEM model by applying the Archie formula. As calculated porosity values are consistent with laboratory measurements, accordingly the void ratio is estimated from the calculated porosity. The calculated void ratio in combination with the grain size diameter of the shallow part of the Nubian sandstone is used to estimate hydraulic conductivity. Then Darcy’s low is applied to calculate the seepage value of the lake water in the adjacent Nubian sandstone aquifer. The main result of the current study shows that the anticipated water seepage value is 2.6 × 10 6 m 3 yr –1 . This value seems reasonable in comparison with the total seepage value inferred from isotope studies around Lake Nasser.

Tracing subsurface oil pollution leakage using 2D electrical resistivity tomography

This study provides an evaluation of the subsurface oil contaminant flow in an oil production field at the Wadi Feiran fan, along the Gulf of Suez, Egypt. The 2D electrical resistivity tomography technique has been applied along four profiles surrounding the spill location of oil dump site. Theoretical soil resistivity response versus fluid resistivity for different cation exchange capacity values has been studied. Then, a physical model for the current situation of flowing the oil over the groundwater (sea invaded water) of saturated fluvial sediments has been introduced. It was supposed that the crude oil has higher resistivity values and turned to be conductive far away from the oil spill source under the effect of microbial biodegradation. Such conditions of contaminated distribution and its resistivity values have been considered during the inversion and interpretation of the acquired data sets. The processed 2D resistivity profiles reveal that oil plumes are accumulated in the seawater invasion direction, whereas it has low signatures along the profiles parallel to the shore line.

Delineation of landslide endangered areas and mapping their fracture systems by the pressure probe method

A new method, the so-called pressure probe (Pre-P) method, has been developed for detecting and characterizing mechanically weak zones which may not be visible from the surface and which may occur, e.g. due to landslides. On a high bank at Dunaszekcső, Hungary, the fracture system of the loess landslide area was investigated by large resolution applying this method and proved that (1) cracks as small as 2- to 3-cm wide are detectable; (2) the fractures follow each other almost periodically; and (3) on the side of the fractures towards the slump, there are less fractured zones whose width correlates with the width of the given fracture. We also demonstrated that on the passive side of the clearly visible fracture, (1) there are also fractures along which future rock displacement is expected; (2) these fractures are at least as wide as the active side fractures; and (3) the blocks there are about twice as wide as those on the active side. A block several meters wide is expected to fall before the main mass movement. The Pre-P method seems to be the most powerful tool to map the fracture system of such landslides because of its speed, simplicity of application, cost and interpretation. The Pre-P profiles and maps of the fracture system of a landslide enable to understand landslide evolution and delineate endangered areas earlier than by other methods.

Tracing buried pipelines using multi frequency electromagnetic

Abstract In this paper the application of multi frequency electromagnetic techniques to locate buried pipelines is described. The survey site has two pipelines of SUMED, one of the world chokepoints. At desert or arid areas, regular geophysical surveys usually are difficult to carry out. EM techniques could be the best among geophysical techniques to be used for this target at these conditions. The EM survey was performed using a GEM-300 multi-frequency electromagnetic profiler. It is of handheld electromagnetic induction-type that measures in-phase and quadrature terrain conductivity without electrodes or direct soil contact. An area of 60 × 15 m was surveyed, that supposed SUMED pipeline existed. Six different frequencies, typically 2025, 2875, 4125, 5875, 8425, 12,025 Hz, have been used simultaneously. The slice maps for in-phase and conductivity distribution at each frequency could help to trace the extension of the pipeline. Two pipelines were traced successfully with 20 m spacing of each others.

Geoelectric resistance scanning on parts of Abydos Cemetery Region, Sohag Governorate, Upper Egypt

The ancient Egyptian Abedju (original name of the present Abydos) locality is a famous archaeological site southwest of Balyana town, Sohag Governorate, Upper Egypt. It is located to the west of the agricultural land of the Nile Valley. The locality represents one of the most important burial grounds for kings and high court dignitaries in ancient Egypt. Shallow geophysical techniques are considered as one of the most accurate and cost-effective methods used in archaeological prospecting and are considered as rapid and safe techniques in detecting a wide range of buried archaeological features. The geoelectric resistance scanning technique, using the Geoscan RM-15 Resistance Meter, is applied with a twin-electrode con- figuration at three sites of the unexcavated localities of Abydos area. This technique proved to be a useful means of exploring the sites through mapping the subsurface burial inhomogeneities resulting from the marked resistance contrast between the buried stone and brick walls and the covering fill of sand, silt and clay. Interesting different buried features are imaged and displayed on maps and three-dimensional representations to guide the archaeological excavation programmes at the sites of Umm El-Qaab, Shunet El-Zebib and Kom El-Sultan within the Abydos region. At the Umm El-Qaab site the combined interpretation of the tomography resistance ranges and maps indicate buried walls and rubble as well as an amorphous background area. At the Shunet El-Zebib site, the texture of the images, which show scattered and crowded high-resistance anomalies, confirms the exist- ence of buried tombs at the site, which is surrounded by defence walls. Thus, this site was not a fortress or a palace, but a special cemetery for higher dignitaries. Kom El-Sultan, however, shows a concentration of high-resistance anomalies at the northern part. Thus, these may be related to walls and rubble related to a nearby elevated temple. Copyright * c 1999 John Wiley & Sons, Ltd.

Increasing the effectiveness of electrical resistivity tomography using γ11n configurations

A new array type, the γ11n arrays are introduced in this paper, in which the sequence of the current(C) and potential (P) electrodes is CPCP and the distance between the last two electrodes is n times the distance between the first two ones and that of the second and third one. These arrays are called quasi null arrays because they are – according to their array and behaviour – between the traditional and null arrays. It is shown by numerical modelling that in detection of small-effect inhomogeneities these configurations may be more effective than the traditional ones including the optimised Stummer configuration. Certain γ11n configurations – especially the γ112, γ113 and γ114 – produced better results both in horizontal and vertical resolution investigations. On the basis of the numerical studies, the γ11n configurations seem to be very promising in problems where the anomalies are similar to the numerically investigated ones, that is they can detect and characterise, for example,tunnels, caves, cables, tubes, abandoned riverbeds or discontinuity in a clay layer with greater efficacy than those of the traditional configurations. γ11n measurements need less data than traditional configurations therefore also the time demand of electrical resistivity tomography (ERT) measurements can be shortened by their use.

Characterisation of a landslide by its fracture system using Electric Resistivity Tomography and Pressure Probe methods

We have studied a slowly moving loess landslide along the River Danube in South Hungary. In contrast with other efforts, we aimed to determine its fracture system. Due to the homogeneous composition of the loess, it seems to be the only possibility to get information about the landslide and its further evolution. Beside of the well-known Electrical Resistivity Tomography (ERT) the so-called Pressure Probe (PreP) method was applied to characterise the supposedly dense fracture system. This method was developed to detect and characterise mechanically weak zones, which may not visible from the surface, and may occur e.g. due to landslides. Fracture zones had been especially well localised by the ERT, enabling the prediction of the positions of future rupture surfaces and thus also the delineation of the endangered zones. PreP was able to give a very detailed image about the surface projection of the fractures. Both methods proved to be good to characterise the fracture system of such a landslide area. Geophysical predictions have been verified also in reality: the mass movements occurred about 1½ years after the measurements. Therefore, to provide early risk warnings and to avoid damage to constructions or endangering human life, the application of the ERT and PreP methods is highly recommended.

Prediction of the location of future rupture surfaces of a slowly moving loess landslide by electrical resistivity tomography

ABSTRACT A slowly moving loess landslide along the River Danube in South Hungary was studied using electrical resistivity tomography. Our aim was to determine the fracture system of the study site. Due to the homogeneous composition of the loess, it seems to be the only possibility to get information about the landslide and its further evolution. The applicability of the electrical resistivity tomography technique for such a supposedly dense fracture system was studied by numerical modelling, and the results have been verified in the field. The dip of the fractures could not always been observed, and they could not be explored deeply. However, it was possible to map their surface projection to get the desired information about the structure of the landslide. Fracture zones could have been especially well localized, enabling the prediction of the positions of future rupture surfaces and thus the delineation of the endangered zone. Although the area outside of the already subsided one is not endangered yet, the area which has already started to move is going to break into two. Parts of the about 5 m wide blocks at the front of the landslide may fall or slide down anytime. A large area was assumed to move as one unit. Most of our predictions have been verified by the mass movements that occurred about one and half years after the measurements. The electrical resistivity tomography method proved to be a good tool to characterize the fracture system of such a landslide area, enabling the prediction of future rupture surfaces and also delineation of the endangered area. Its use is therefore highly recommended to monitor landslides to provide early risk warnings to avoid damage to constructions or endangering human life.

Combined Inversion of Electrical Resistivity and Transient Electromagnetic Soundings for Mapping Groundwater Contamination Plumes in Al Quwy'yia Area, Saudi Arabia

Time-domain electromagnetic (TEM) soundings and vertical electrical soundings (VES) were conducted to assess the subsurface groundwater contamination in Al Quwy’yia area through tracing the conductivity changes caused by leachate contamination of groundwater. Most of the contamination sources are coming from the incomplete sanitary system and the illegal dumping of wastewater along the lowland area outside the town. Contamination of the groundwater poses a major threat because most of the local inhabitants rely on ground water to supply up to 60% of their water needs for various life activities. TEM and VES data sets have been acquired along four longitudinal profiles to cover the most urban area of Al Quwy’yia. The combined inversion of VES and TEM data sets increase the resolving certainty of the subsurface resistivity models. The basement rock has the highest resistivity values, whereas the uncontaminated limestone has moderate resistivity response in comparison with the contaminated zones, which have a lower resistivity response. The constructed cross-sections and resistivity slice maps along the area provide valuable information about the shallow seepage from the septic tanks, as well as the deep infiltration from the dump site at the southern part of the study area.

Mapping the Fracture System of a Landslide by Pressure Probe Method

Mechanically weak zones produced by rock realignment in the subsurface may give very useful information, but may not be directly seen from the surface. A new method, the so-called pressure probe (Pre-P) method was developed for such problems. Here an application of this method is presented on the example of a landslide. The fracture system of the investigated landslide area could have been investigated by very large resolution. We know from these investigations among others that: 1. there are consolidated zones on the slope side of the fractures whose broadness correlates with the broadness of the given fracture; 2. both the large and the small fractures follow each other periodically; 3. the future slide faces already exist. The here presented Pre-P method seems to be the most favourable tool to map the fracture system of such kind of landslides according to its resolution capacity (even 2-3 cm wide fracture proved to be detectable by this method), speed, simplicity of its application, the interpretation of the data and its costs. The fracture system map made using Pre-P data offers the possibility to better understand landslides and to delineate the not-yet-endangered areas before other methods could do it.