The present study deals with the analysis and interpretation of the results of field geophysical survey and laboratory geophysical measurements. The study of the magnetic and electrical methods was selected because the beach sands contain many minerals that have magnetic and electric properties. Analysis and interpretation of the field and laboratory magnetic and geoelectric maps demonstrated that the investigated beach-alluvial deposits can be subdivided according to their magnetic and geoelectric properties into three main zones striking nearly parallel to the shoreline of the Mediterranean Sea at the study area. The northern zone is more enriched in black sands than the central or southern zones. Field and laboratory magnetic susceptibility measurements provided very useful maps for the concentration of heavy minerals. The deep-seated magnetic response was calculated at an average depth of 239.6 m, while the near-surface magnetic responses were computed at average depths of 9.1, 57.9, and 81.8 m, respectively. The correlation between the geophysical features, recorded on the total magnetic field intensity, the electric resistivity, the IP chargeability, and the calculated metal factor, was found to agree to a great extent. The heavymineral concentration was found to decrease with depth. However, the heavyminerals show parallel zones below the surface, suggesting similar sedimentation environments.
The alluvial-beach placer deposits, East Rosetta, Mediterranean Sea Coast, Northern Egypt, are known for their enhanced natural radiation environment, due to the presence of radiogenic heavy minerals, such as monazite and zircon, which contain Th and U in their chemical composition [
Map of Northern Egypt showing the location of the study area.
The main objectives of the present study are evaluation of surface extensions of the beach-alluvial deposits, using field magnetic and susceptibilities survey, as well as laboratory magnetic and geoelectric measurements; mapping the spatial distribution patterns of zones of heavy mineral concentrations; identification of the depositional phases; delineation of the subsurface (shallow and deep seated) magnetic sources.
Figure
Schematic diagram for all the techniques in the study area.
Placer sand deposits in the northern Nile Delta, especially in Koam Mashaal area, have received the attention of several researchers since the early years of the 20th century. The most recent researches that carried out studies on these placer sands include Hedrick and Waked [
The placer deposits, East Rosetta, have high mineral productivity, which can be easily mined. El Askary and Frihy [
The ground magnetic survey was conducted along a set of equally spaced (100 m) parallel traverses oriented N-S, a direction that is perpendicular to the general extension of the Mediterranean Sea Coast in the study area. The general strike of the lithological units in the study area was determined according to the general elongation of the black-sand lenses occurring along the Mediterranean Sea Coast, which approximately extends in an E-W direction. Magnetic measurements were made at regular intervals of 10 m. The survey data were tied along one tie line perpendicular to the direction of the survey traverses. A base station was placed at a reference point, selected far from artificial magnetic disturbances within the surveyed area. The total magnetic-intensity measurements, collected at the survey stations, were regarded as positive or negative deviations from the ones at the reference point (base station).
The ground raw magnetic survey measurements were subjected to essential corrections, including diurnal as well as tie-line corrections. On the other hand, the terrain correction was not applied to the magnetic data, since the surveyed area is characterized by its smooth terrain and insignificant elevation differences between survey stations. The geomagnetic correction was calculated at the central point of the study area, where the International Geomagnetic Reference Field (IGRF) reaches 43,519.12 nT, the inclination angle attains 45.89°, and the declination angle equals 2.78° at 31°27′05′′N lat. and 30°33′00′′E long. in October 30, 2001 [
The total magnetic-field intensity measurements were carried out using a portable proton-precession magnetometer, model PMG-1, Geofyzika Brno, Czech Republic, having a sensitivity of 0.1 nT and a measuring range from 25,000 to 100,000 nT (PGM-1 Manual, 1997). Another magnetometer of the same type was used as a base station for automatic monitoring and recording of diurnal variations in the Earth’s magnetic field. The measuring time was set for one minute at the base station as a matter of diurnal variation and storage capacity of the instrument.
The ground magnetic survey data, including spectral frequency analysis and isolation of magnetic anomalies, were carried out using the software of Magnetic Data Analysis (MDA2-Ver 1.97; [
The pocket susceptibility meter, type KT-6, Geofyzika Brno, Czech Republic, is designed for quick-field measurements of magnetic susceptibility of outcropping rocks, drill cores, and larger pieces of rocks. The sensitivity of the equipment is
A 12-channel global positioning system (GPS) instrument was used to set up the survey grid with a Universal Transverse Mercator (UTM) coordinate system, using World Grid (WG84) as datum. All samples were put in plastic bags and labelled by their UTM positions.
The laboratory geoelectric (resistivity and chargeability) measurements were applied to the 966 quartered representative samples, taken from large samples collected in the field at depths up to 50 cm. The large samples were taken along 32 equally spaced (100 m apart) profiles, at equally spaced (50 m apart) stations. These profiles were oriented in an N-S direction, covering about 5.4 km2. The samples were taken by pushing a rigid plastic tube (5 cm in diameter and 70 cm in length) into the ground up to 70 cm in depth. The collected samples were quartered using John’s splitter.
The ELREC-2 instrument, induced polarization of low power system (IP-L system), manufactured by IRIS Instruments, France, was used in this study to measure the electric resistivity and induced polarization (IP) parameters on the 966 quartered representative samples collected from the field as previously mentioned. The measuring system consists of three units: a sample holder frame, an IP-L low-power time domain transmitter, and an ELREC-2 as a time-domain IP receiver [
Environmental magnetism deals with the magnetic properties of natural iron oxides as a tool for understanding and interpreting the processes in sedimentary systems [
Frequency analysis of the potential field data using the computer software that implements the fast Fourier transform (FFT) has now become a routine practice. The fast Fourier transform is a computational tool which facilitates signal analysis such as power spectrum analysis and filter simulation by means of digital computers. It is a method for efficiently computing the discrete Fourier transform of a series of data samples. It has become a widely used tool for interpretation of potential field data, especially for depth estimation. This approach has been developed by many workers (e.g., [
The recorded total magnetic-field intensity measurements in the study area ranges from 35 to 115 nT. The magnetic intensities are roughly arranged in two zones parallel to the shoreline, orienting in an NW-SE direction (Figure
Filled colour contour map of the total magnetic intensity of Koam Mashaal area, East Rosetta, Mediterranean Sea Coast, Northern Egypt.
The local power spectrum of the ground magnetic data for the study area.
Frequencies of magnetic anomalies could reveal the depths of their magnetic sources. The regional magnetic-field intensity map (Figure
Filled colour contour map of the regional component of magnetic-field intensity of Koam Mashaal area, East Rosetta, Mediterranean Sea Coast, Northern Egypt at an average depth
An important interpreting technique in geophysical exploration is the residual mapping, in which local anomalies are separated from regional ones. According to Burger [
Filled colour contour map of the residual component of magnetic-field intensity of Koam Mashaal area, East Rosetta, Mediterranean Sea Coast, Northern Egypt at an average depth
Filled colour contour map of the residual component of magnetic-field intensity of Koam Mashaal area, East Rosetta, Mediterranean Sea Coast, Northern Egypt at an average depth
Filled colour contour map of the residual component of magnetic-field intensity of Koam Mashaal area, East Rosetta, Mediterranean Sea Coast, Northern Egypt at an average depth
As a result of the presence of the earth’s magnetic field, rocks containing magnetic minerals show induced magnetizations. The constant of proportionality between the inducing field and the magnetization is known as the magnetic susceptibility (
The magnetic susceptibility (
Magnetic susceptibility (k
Filled colour contour map of field magnetic susceptibility (k
The measurements of laboratory magnetic susceptibility (k
Filled colour contour map of laboratory magnetic susceptibility (k
The filled colour contour maps for field and laboratory magnetic susceptibility measurements (Figures
From the visual examination of all geophysical magnetic contour maps, it is noticed that the contours are elongated in two main directions, one parallel to the seashore, that is, in the N 75°E direction, and the other is oriented in the NW-SE direction. The values always point to low concentrations of heavy minerals towards the shore line and to the south of the study area. These observations could be interpreted as follows.
When the area is covered by standing sea water and its slow motion in winter seasons, the denser heavy minerals go down and the light ones float up by gravity. In the summer seasons when the seawater retreats, and the wind blows in an NW-SE direction; the light sand minerals are transported away from the shore to the south of the study area. Hence, the heavy minerals are concentrated by this way in an NW-SE direction, beside the main action of the sea waves (which act similarly to the action of a Wilfley table on the concentration of the heavy minerals) in a direction parallel to the sea shore (N 75°E direction). There are two other evidences for these two directions. The first one is the satellite image of the Rosetta area (Figure
Northern Egypt Landsat colour composite image showing the location of the studied Koam Mashaal beach area, East Rosetta, Mediterranean Sea Coast.
Electrical measurements are among the most difficult of all geophysical methods to interpret quantitatively, because of the complex theoretical bases of the technique. Electric IP quantitative interpretation is considerably more complex than the electric resistivity method. Much electric IP interpretation is, however, only qualitative [
The recorded electric resistivity measurements range from 0.1 to 200 Ohm·m. The electric resistivity map (Figure
Filled colour contour map of the electric resistivity (
The low electric IP chargeability values agree well with the high electric resistivity values at the narrow zone, located in both central and eastern parts of the study area as well as its northern part (Figure
Filled colour contour map of the electric chargeability
Similar to the distribution of the IP chargeability, and electric resistivity values, the high values of the calculated metal factor (Figure
Filled colour contour map of the calculated metal factor (MF) values from laboratory measurements, Koam Mashaal area, East Rosetta, Mediterranean Sea Coast, Northern Egypt. Logarithmic contour interval = 0.2.
The correlation between the geophysical features recorded on the maps of total magnetic field intensity, magnetic susceptibility, electric resistivity, IP chargeability and calculated metal factor of the study area was found to agree to a great extent. Field and laboratory magnetic and geoelectric maps demonstrate that the investigated beach-alluvial deposits of Koam Mashaal area can be subdivided into three main zones striking nearly parallel to the shoreline of the Mediterranean Sea, Northern Egypt. The northern zone is more enriched in black sands than the central or southern zones. The heavy mineral concentration was found to decrease with depth. However, the heavy minerals also show some parallel zones below the surface, suggesting similar sedimentation environments. The deep-seated magnetic response was interpreted to lie at an average depth of 239.6 m, while the near-surface magnetic responses were interpreted to lie at average depths of 9.1, 57.9, and 81.8 m, respectively. The recorded electric resistivity measurements range from 0.1 to 200 Ohm·m. The high electric IP chargeability values coincide well with the high values of the calculated metal factor, low electric resistivity values, and high magnetic susceptibility values. These tools are very useful and are recommended for the study and investigation of black-sand beach deposits everywhere.