- Open Access
Delineation of the subsurface structures and basement surface of the Abu-Rodaym area, Southwestern Sinai, using ground magnetic data
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2012
- Received: 24 January 2012
- Accepted: 17 December 2012
- Published: 23 August 2013
The present study deals with the analysis of data from a ground magnetic survey that was conducted in the Abu-Rodaym area of the Southwestern Sinai Peninsula, Egypt. This survey was carried out to delineate the subsurface structural framework, and to identify the thickness of the sedimentary basin of the study area. Locating these structures, and determining the localities of maximum sedimentary thicknesses that consist mainly of sandstone, serves as a preliminary process in exploring the confined aquifer beneath the surface of the Abu-Rodaym area. This will greatly benefit Bedouins who suffer greatly from a lack of water in the driest region in the country. The processing, analysis, and interpretation, of the total intensity magnetic data shows that there are three sets of faults striking mainly in the N-S, NW-SE, and NE-SW, directions. The depth to the basement surface was found to fluctuate from about 45 m, to more than 100 m, beneath the ground surface. It was also found that the variations in magnetic observations were produced by the striking structures that are mainly responsible for the variations in thicknesses of the sedimentary rocks in the area.
- Ground magnetic
- sedimentary basin
A magnetic survey is a powerful tool for delineating the geology (lithology and subsurface structure) of buried basement terrain. Such a survey maps the variation of the geomagnetic field, which occurs due to changes in the percentage of magnetite in the rock. It reflects the variations in the distribution and type of magnetic minerals below the Earth’s surface (Mekonnen, 2004). Magnetic minerals can be mapped from the surface to greater depths in the rock crust depending on the dimension, shape, and magnetic property, of the rock. Sedimentary formations are usually nonmagnetic and, consequently, have little effect, whereas mafic and ultramafic igneous rocks exhibit a greater variation and are useful in exploring the bedrock geology concealed below cover formations (Mekonnen, 2004).
The main objectives of this study are to delineate the trends of the subsurface structures, to determine the depth to the basement surface, and to study the groundwater potentialities of the study area.
To achieve these goals, a ground magnetic survey of the Abu-Rodaym area was conducted. The magnetic survey data were subjected to a quantitative interpretation that involved some geophysical processing and interpretational techniques, including: (1) reducing the total intensity magnetic data to the north magnetic pole; (2) isolating the magnetic data into their residual, and regional, components, using Fast Fourier Transformation (FFT) techniques; (3) Euler deconvolution to delineate the subsurface structures from the RTP regional magnetic map; and (4) two-dimensional magnetic forward modeling along two selected RTP magnetic profiles.
Structurally, the southwestern part of Sinai is strongly tectonized where faults, folds, as well as joints, are the main structural features. El Rakaiby and El Aassy (1990) concluded that the N-S, ENE-WSW, NE-SW and NNW-SSE are the main fault trends in the Um Bogma area. Al Shami (1994) studied the main structural setting of Wadi Allouga at the South of the Abu-Rodaym area and concluded that the area was subjected to two phases of compressional forces; the older of these was acting in the E-W direction, while the more recent was acting in the N-S direction. He added that these structural features played the most important role in the mode of occurrence of radioactive mineralization in this area.
Al-Shami (2003) classified the lithologic succession of Paleozoic rocks that cover the study area into seven formations arranged from the oldest; the Sarabit El-Khadim, Abu-Hamata and Adadia Formations (the so-called “lower sandstone series”). This is followed unconformably upwards by the Um-Bogma Formation (the “Carboniferous limestone series”), and overlain by the El-Hashash, Magharet El-Mayah, and Abu-Zarab Formations, which represent the Abu-Thora formation according to Kora (1984), or the “upper sandstone series” by Barron (1907).
The processing techniques, including spectral analysis, were applied. The data processing passed through various steps, starting by converting the corrected TMI data from the space domain to a frequency domain, using Fast Fourier Transformation (FFT). Then, the reduction to the north magnetic pole (RTP) map was derived from the total magnetic intensity data, using inclination and declination values of 44.3° and 2.45°, respectively (Rabie, 2011).
Reduction to the pole transforms an anomaly into the anomaly that would have been observed if the magnetization and regional field were vertical (as if the anomaly was measured at the north magnetic pole). A symmetric body produces a symmetric anomaly at the magnetic poles. Hence, reduction to the pole is a way to remove the asymmetries caused by a non-vertical magnetization, or regional field, and to produce a simpler set of anomalies to interpret (Dobrin and Savit, 1988).
Filtering the magnetic data is an essential process prior to analysis and interpretation. The objective of the filter is to condition the data set and to render the resulting presentation in such a way as to make it easier to interpret the significance of anomalies in terms of their geological sources (Bird, 1997). Therefore, the most effective way to filter the data is with an understanding of the geologic control and the desired filtered results. Several filtering techniques can be performed in the frequency domain. However, one of the most traditional filters, used in the potential field, is the separation of long (deep), and short (shallow), wavelength anomalies. The success of this technique depends on the proper choice of cut-off wavelength used in the filter design. The cut-off wavelengths, and information about the contribution of the short and long wavelengths in the spectrum, can be obtained from the calculated radially-averaged power spectrum of the data.
Depending upon the magnetic maps, the RTP regional and the RTP residual components (Figs. 7 and 8), and the Euler deconvolution map (Fig. 9), the following results can be obtained. The Abu-Rodaym area is intersected by three sets of faults, shown in Fig. 10. Two sets represent normal faults, with one trending in the N-S and the other trending in the NW-SE. The third set represents a group of strike-slip faults trending mainly towards the NE-SW direction.
These sets of faults were mapped using different rules and signatures of mapping faults from the magnetic data. Some of these rules used in mapping these faults depend on the following considerations: the contacts between any two different textures; the contact between any two different magnetic gradients; steep and gentle gradients that reflect shallower, and deeper, magnetic blocks; sharp dislocation, or bends, in magnetic contour lines, especially if this is in the 90° range; the difference in magnetic intensities along a contact or lineament; and the relative positioning of similar magnetic anomalies, both in size and amplitude, that suggests a fault with a horizontal displacement only, rather than a vertical displacement (Rabie, 2011).
The existence of the first two sets of normal faults play the main role in creating graben structures G1, G2, G3, and G4 (Fig. 10), which occupy many parts of the study area, and represent sedimentary basins. Also, these basins seem to be open outside the area to the north and the south. Furthermore, the depth to the basement rocks at graben (G3), sited in the central part, is shallower than that under graben (G1) located at the eastern part of the study area. It is to be noted that these normal faults (Fig. 10), have been suggested to be of Miocene age, and parallel to the two complementary shear fractures of the Suez and Aqaba fault trends of Oligo-Miocene age (Said, 1990).
Considering the increasing thickness of the sedimentary succession in the study area, where the graben structures exist, the probability of the existence of groundwater bearing aquifers in these localities can be considered fair. These localities can be regarded as suitable basins in which ground-water aquifers could occur within the lower sandstone series, or the Abu-Thora formation. The best locations to drill for groundwater are at the boundaries of these grabens within the fault plane that surround these structures, because a fault plane always represents weak and open paths for groundwater, in this part of the Southeastern Sinai. Accordingly, it is logical to assume that such broad magnetic anomalies shown on the regional magnetic map (Fig. 7) can be related to basement rocks rather than any magnetic intrusion.
In order to calculate the depths to the basement surface, and to determine its susceptibility in the study area, two intersected profiles, (A−A′) and (B−B′), were applied on the regional reduction to the pole magnetic map using the GM-SYS program (Fig. 7).
These two models emphasize that the magnetic variations in the basement rocks are mainly related to the variation in depth to the top of the different basement blocks, rather than its lithological composition, because it reflects the same value of magnetic susceptibility of about 0.013 (SI units).
The ground magnetic survey that was conducted in the Abu-Rodaym area has yielded good information about the subsurface structures, and their gross framework, which did not show any evidence on the surface. It reveals that the study area is intersected by three sets of faults trending entirely in the N-S and NW-SE directions. These two faults are considered as normal faults. The third set, trending in the NE-SW direction, are considered as strike-slip faults. The results of 2D magnetic modelling indicated that, the depth to the surface of the basement rocks varied from 45 m to about 110 m, and proved that the variation in the magnetic field in the area was caused only by the striking faults rather than the variation in the lithological constituents of the basement. The study is characterized by graben structures, which represent suitable places to drill for groundwater-bearing aquifers, but carrying out a detailed ground geoelectrical survey over these graben locations is recommended in order to locate probable aquifers by following up the resistivity variation of the different rocks.
The authors thank the Exploration Division of the Nuclear Materials Authority (NMA) for providing the proton precession magnetometer (model PMG-1) and the Geosoft Oasis Montaj software package that was used for processing and mapping the magnetic data. The authors also appreciated the input of Dr. Ahmed A. Ammar, Dr. El-Sayed M. El-Kattan, and Dr. Ba-her M. Gheith, Professors of Applied Geophysics for supporting and reviewing this manuscript.
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