Detection of Magnetically Susceptible Dyke Swarms in a Fresh Coastal Aquifer

Detection of Magnetically Susceptible Dyke Swarms in a Fresh Coastal Aquifer

MOHAMED H. KHALIL1

Abstract—Groundwater constitutes the main source of

freshwater in Shalatein, on the western coast of the Red Sea, in

Egypt. The fresh aquifer of Shalatein is intensively dissected by

shallow and deep faults associated with the occurrence of dykes

and/or dyke swarms. In this context, synthesis of electrical

resistivity, ground magnetics, and borehole data was imple-

mented to investigate the freshwater aquifer condition, locate the

intrusive dykes and/or dyke swarms, and demarcate the potential

freshwater zones. Nine Schlumberger VES’s with maximum

current electrode half-spacing (AB/2) of 682 m were conducted.

The subsurface was successfully delineated by general four

layers. The fresh aquifer of the Quaternary and Pre-Quaternary

alluvium sediments was effectively demarcated with true resis-

tivities ranged from 30 to 105 Xm and thickness ranged between

20 and 60 m. A ground magnetic survey comprised 35 magnetic

profiles, each 7 km in length. Magnetic data interpretation of the

vertical derivatives (first and second order), downward continu-

ation (100 m), apparent susceptibility (depth of 100 m), and

wavelength filters (Butterworth high-pass of wavelengths

\100 m and Band-Pass of wavelengths 30–100 m) successfully

distinguished the near surface structure with five major clusters

of dyke swarms, whereas filters of the upward continuation

(300 m) and Butterworth low-pass (wavelengths [300 m) clearly

reflected the deep-seated structure. The computed depth by the

3D Euler deconvolution for geological contacts and faults

(SI = 0) ranged from 14 to 545 m, whereas for dyke and sill

(SI = 1), it ranged from 10 to 1,095 m. The western part of the

study area is recommended as a potential freshwater zone as it is

characterized by depths [100 m to the top of the dykes, higher

thickness of the fresh aquifer (45–60 m), depths to the top of the

fresh aquifer ranging from 25 to 40 m, and higher resistivities

reflecting better freshwater quality (70–105 Xm).

Key words: Dyke swarms, around magnetics, VES, vertical

derivatives, apparent susceptibility, wavelength filters, continuation

filters, 3D Euler deconvolution.

1. Introduction

In shallow fresh coastal aquifers dissected by faults

and fractures, investigating the aquifer potentiality

associated with the detection of possible subsurface

dykes intrusion is a challenge for hydro-geophysicists

(PRASAD et al. 1996; PATIL and RAO 2002; DURAISWAMI

2005). Dykes can extend vertically and laterally for

long distances and impede the flow of groundwater

(IZUKA and GINGERICH 1998). Dykes can intersect at

various angles and compartmentalize the more per-

meable rock in which ground water can be impounded

(TAKASAKI and MINK 1985). Dykes may lower the

overall rock porosity and permeability (MEINZER 1930).

However, dykes could improve the potential yield of

the aquifer where it contains more fractures than the

host rocks (SINGHAL and GUPTA 1999). Pumping tests in

Botswana indicate that dykes thicker than 10 m serve

as groundwater barriers, but those of smaller width are

permeable as they develop cooling joints and fractures

(BROMLEY et al. 1994).

Shalatein (as a case study), also spelled Shalateen

or Shalatayn, is the southernmost city of Egypt. It lies

on the western coast of the Red Sea and marks the

administrative boundary between Egypt and Sudan

(Fig. 1). Shalatein, like most of the coastal Egyptian

cities, is suffering from an acute shortage of fresh-

water. Nevertheless, it is one of the top priority areas

proposed by the Egyptian government for develop-

ment, new urbanization, and tourism due to its

strategic location. Currently, groundwater constitutes

the main source of economic freshwater in Shalatein

(SADEK 2004). Interestingly, the fresh aquifer of

Shalatein is intensively dissected by shallow and deep

faults (NANO et al. 2002) associated with the possible

occurrence of dykes and/or dyke swarms (EL-BAYO-

UMI and GREILING 1984; EL AMAWY et al. 2000). The

annual rainfall (less than 150 mm) constitutes the

main freshwater recharge source for the aquifer. The

arid climate, absence of freshwater resources, and

complex geological subsurface condition in Shalatein

necessitate the proper identification of potential fresh

aquifer zones.

1 Geophysics Department, Faculty of Science, Cairo Uni-

versity, Giza, Egypt. E-mail: [email protected]

Pure Appl. Geophys.

� 2013 Springer Basel

DOI 10.1007/s00024-013-0696-4 Pure and Applied Geophysics

One of the main obstacles raised during the dril-

ling of water wells in the study area is the occurrence

of dykes. Two boreholes (BH-1 and BH-2) were

drilled in the area (Fig. 1). BH-2 encountered the

dykes at a depth of 11 m from the surface and didn’t

reach the fresh aquifer. On the other hand, BH-1

didn’t encounter any dykes and reached the top of the

fresh aquifer at a depth of 21 m from the surface.

Therefore, prior to drilling, it is essential to locate any

subsurface intrusive dykes and/or dyke swarms. In

this context, synthesis of electrical resistivity, ground

magnetics, and borehole data was implemented to

investigate the freshwater aquifer condition, locate

the intrusive dykes and/or dyke warms, and demar-

cate the potential freshwater zones.

2. Geological and Structural Setting

The basement rocks in the Eastern Desert, Egypt

constitute a part of the Arabian Nubian Shield of the

Precambrian (SABET 1972). The south of the Eastern

Desert is characterized by widespread ophiolitic

melange rocks associated with extensive metasedi-

ments of oceanic character (GREILING et al. 1994).

Calc-alkaline metavolcanics characteristic of island

arcs or volcanic arcs of active continental margins

were also recognized (KRONER et al. 1987).

The study area (Shalatein) in the southern of the

Eastern Desert is bounded by latitudes 2564220 and

2571399N and longitudes 752082 and 759154E

(Datum: WGS-84, Projection: UTM), covering an

area of about 49 km2 (Fig. 1). Shalatein is occupied

by Neoproterozoic Pan-African (Late Proterozoic

Precambrian) basement rocks including metamorphic

and intrusive assemblages, and is unconformably

overlain by Cretaceous sandstones (AKAAD and NO-

WEIR 1980). Both Precambrian and Cretaceous rocks

are excavated by Red Sea rifting of the Tertiary

basalts (NANO et al. 2002). The Late Proterozoic

Precambrian basement rocks cover large parts of the

area, forming high to medium relief of weathered

blocky mountains and hills (SADEK et al. 1996). The

metamorphic assemblage comprises dismembered

ophiolitic metamorphosed ultramafic rocks and

island-arc calc-alkaline metavolcanics (the most

predominant). On the other hand, the intrusive

assemblage rocks include gabbro-diorite, syn-tectonic

tonalite–granodiorite, and late-tectonic monzogra-

nites-alkali feldspar granites (KRONER et al. 1987).

AKAAD and EL RAMLY (1961) pointed out that these

intrusive plutons are not all of the same age, but they

are of different members, intruded over a lapse of

time. These plutonites are followed by the formation

of pegmatites, aplites, felsites, and quartz vein (EL

SHAZLY 1977). Intrusive assemblage rocks occur in

Figure 1A location map of Shalatein, western coast of the Red Sea, Eastern Desert associated with the topographic contour, VES, magnetic profiles,

and boreholes locations (QuickBird, June 2012; Datum: WGS-1984: Projection: Universal Transverse Mercator (UTM)). (Faults compiled

after NANO et al. 2002; SADEK 2004)

M. H. Khalil Pure Appl. Geophys.

the form of plutonites or batholiths with round to oval

outlines (KRONER et al. 1987). The study area is tra-

versed by Wadi Gahlya which drains eastwards to the

Red Sea and is filled with Quaternary alluvial sedi-

ments (Fig. 1).

The subsurface structures and tectonism of the

study area are related to the Gulf of Suez and the Red

Sea tectonics. These structures extend from the

basement rocks upwards into the sedimentary

sequences and divide the area into several major and

minor faulted blocks of varying lengths and trends

(EL-BAYOUMI and GREILING 1984). The predominant

directions of the fault systems in the area were

addressed (e.g., SABET 1972; EL SHAZLY 1977; AGHA

1981; GASS 1981; HASSAN and HASHAD 1990; EL

AMAWY et al. 2000; NANO et al. 2002) into the fol-

lowing categories: the Meridional (N–S) trend, the

Mediterranean Sea (E–W) trend, the Gulf of Suez-

Red Sea (NW–SE) trend, the Najd (WNW–ESE)

trend, the Aqaba (NNE–SSW) trend, Trans-African

(NE–SW) trend, and the Syrian Arc (ENE) trend.

3. Methodology

3.1. Geoelectric Resistivity Survey

Geoelectric resistivity is one of the most signif-

icant variables reflecting the physical properties in a

complicated sedimentological environment (KHALIL

et al. 2008). It depends mainly on the lithology, water

content, porosity, and ionic concentration of the pore

fluid (KHALIL 2009, 2012a). Direct current (dc)

vertical electrical soundings (VES) method measures

the apparent resistivity (qa) of the subsurface (e.g.,

REYNOLDS 1997; KHALIL 2010), which inverted to

develop a model of the subsurface structure and

stratigraphy in terms of its electrical properties (e.g.,

HALIHAN et al. 2005; OLDENBORGER et al. 2007;

DESCLOITRES et al. 2008; KHALIL 2010, 2012a, b).

Ambiguities in interpretation always occur and

therefore it is very necessary to calibrate the observed

VES data with the available borehole data to assign

proper resistivity ranges for the various lithologic

units (KHALIL 2006; KHALIL et al. 2008).

In the study area, nine Schlumberger VES’s with

maximum current electrode half-spacing (AB/2) of

682 m were conducted to demarcate the fresh aquifer.

This spacing was adequate to penetrate down to the

target layer (10–40 m depth) of the sand and gravel

water aquifer (Quaternary alluvial). A reconnaissance

field survey was conducted to locate the proper

locations of the resistivity measurements (VES’s)

according to the availability on the ground and

boreholes (Fig. 1) in order to calibrate the measured

resistivity data (Fig. 2a). Resistivity measurements

were carried out using a high accuracy digital signal-

enhancement resistivity-meter (GISCO USA,

ABEM-TERRAMETER, SAS 1000). Field data were

processed and interpreted by the methodology dis-

cussed by KHALIL (2012a) and geoelectric modeling

was carried out by IPI2Win (BOBACHEV 2002). The

maximum root mean square (RMS) error of the

resulting models is 4.3 %. Furthermore, the inter-

preted VES stations, geological subsurface studies,

and borehole data in the study area were integrated to

produce two geoelectric cross-sections, profile-1 and

2 (Fig. 2b, c).

3.2. Ground Magnetic Survey

Magnetic anomalies have tangibly proven out-

standing capabilities in delineating and depth

estimation for the subsurface structure especially

where rock outcrop are scares and/or absent (e.g.,

VOGT et al. 1982; CORDELL et al. 1985; BOURNAS et al.

2003; VERDUZCO et al. 2004; BEIKI et al. 2010; KHALIL

2012b). Magnetically susceptible intrusive dykes are

a potential source of magnetic anomalies (e.g.,

MURTHY et al. 1980; BLAKELY and SIMPSON 1986;

KARA et al. 2003; PILKINGTON and KEATING 2004;

WIJNS et al. 2005). The interpretability of the

magnetic anomalies could be enhanced substantially

by various filtering techniques (e.g., HINZE and ZIETZ

1985; LIDIAK et al. 1985; URQUHART and STRANGWAY

1985; YARGER 1985; COOPER and COWAN 2006;

STAVREV 2006; FURNESS 2007; KHALIL 2012b). In the

wave number domain, wavelength and wavelets

filters applying discrete Fourier-transform algorithms

(BHATTACHARYYA 1965; BRIGHAM 1974) play signifi-

cant roles in the separation of the shallow and deep

magnetic source anomalies (e.g., BOTT 1973; HINZE

and ZIETZ 1985; ROBERTS et al. 1989; PAOLETTI et al.

2007).

Detection of Magnetically Susceptible Dyke Swarms

Figure 2a VES-5 correlated with the adjacent borehole (BH-1); b the geoelectric cross-section profile-1; c the geoelectric cross-section profile-2 (based

on the VES and geological subsurface studies). Surface elevations (m ? MSL) are based on GPS with an accuracy ±5–10 m


In the study area, magnetic data were collected

digitally using a GSM-19T proton magnetometer

with sensitivity 0.15 nT at 1 Hz, resolution 0.01 nT,

and absolute accuracy ±0.2 nT. Ground magnetic

survey comprised 35 magnetic profiles, each of 7 km

length (Fig. 1). Observations were carried out at a

height of 1 m above the ground surface with a

sampling rate of 10 s and spatial resolution of 100 m.

The diurnal variations in the total magnetic field data

were monitored every 2 min with an EG&G Geo-

metrics G-856A base station magnetometer located

off the study area. The package of Geosoft Oasis

Montaj (2007) was used to apply various filters and

depth estimation.

3.2.1 Diurnal Correction

Regardless of the type of survey and instrument used,

magnetometer observations are affected by a number

of sources in addition to the signature of the

subsurface; therefore, the data must be handled by

some kind of correction (BARANOV 1957; ANDREASON

and ZIETZ 1962). Diurnal correction has to be carried

out to account for the temporal variations of the

geomagnetic field, which are primarily caused by

particle and electromagnetic radiation from the sun

perturbing the ionosphere of the earth, and thus the

geomagnetic field. The temporal variations have a

wide range of period and amplitude. Typical variation

Figure 3a Computed resistivity (Xm), b computed depth (m), and c computed thickness (m) of the fresh subsurface aquifer


during a normal day is some tens of nanotesla (nT).

Conversely, disturbed day variations are irregular and

extreme, comprising up to several hundreds of

nanotesla, and are probably associated with a mag-

netic storm (ANDREASON and ZIETZ 1962). In the study

area, the observed magnetic data were corrected for

diurnal variations of the magnetic field by subtracting

the base station observations from the field observa-

tions and referenced to an arbitrary station. No

correction was made for the spatial variations of the

core-derived magnetic field because of the limited

size of the study area.

3.2.2 Reduction to the Pole (RTP)

The anomalies of the measured magnetic field are

usually shifted from the centers of their magnetic

sources due to the inclination and declination of the

induced magnetization vector from the magnetic poles

(MENDONCA and SILVA 1993). Therefore, a reduction to

pole (RTP) transformation is typically applied to the

total magnetic data to minimize the polarity effects

(BLAKELY 1995) and align the peaks and gradients of

magnetic anomalies directly over their sources (BARA-

NOV and NAUDY 1964). Assuming that all the observed

magnetic anomalies are due to induced magnetic

effects, pole reduction can be calculated in the

frequency domain (GRANT and DODDS 1972). In the

study area, RTP was applied to the diurnally corrected

data (Fig. 4). The parameters used to produce the RTP

map are 40� inclination and 3.5� declination.

3.2.3 Energy Spectrum (Radially Averaged Power

Spectrum)

Spectral analysis of magnetic data has been used

extensively to estimate the depth to the top of magnetic

sources (e.g., BHATTACHARYYA 1966; SPECTOR and

GRANT1970; CIANCIARA and MARCAK 1976; STEFAN and

VIJAY 1996; KHALIL 2012b). The logarithm of the power

of the signal at each wavelength can be plotted against

wavelength (regardless of direction) to produce a power

spectrum curve. The radial averaged energy spectrum

(power spectrum) represents the spectral density aver-

aged for all grid elements. SPECTOR and GRANT (1970)

stated that the depth factor invariably dominates the

shape of the radially averaged power spectrum of the

magnetic data. Depth estimation from potential field

using power spectra requires a realistic assumption of

the statistical properties of the source distributions

(STEFAN and VIJAY 1996). A typical energy spectrum of

magnetic data exhibits three components; a deep source

component, a shallow source component, and a noise

component. In the study area, the power spectrum was

calculated for the RTP magnetic data to estimate the

depths of the shallow and deep magnetic sources.

3.2.4 Vertical Derivatives

The vertical derivative filters compute the vertical

rate of change in the magnetic field (MCGRATH et al.

1970). The derivative frequency suppresses the long

wavelengths and enhances the shallowest geological

sources (RAVAT 1996). Particularly, first and second

vertical derivatives emphasize higher gradient, pro-

vide sharper resolution, and approximate shape

outlines of near surface magnetic sources (HOOD

et al. 1979). Noteworthy, derivative filters enhance

the high wavenumber components of the spectrum

(PETERS 1949; SILVA and HOHMANN 1983), therefore, it

Figure 4Contour map of the diurnally corrected total magnetic field

intensity (nT), observed at 1-m height, Shalatein


is often necessary to apply an additional low-pass

filter to remove high wavenumber noise (ROEST and

PILKINGTON 1993; PESONEN et al. 1994). In the study

area, the first and second vertical derivatives were

calculated for the magnetic RTP data (Fig. 5) and

additionally enhanced by applying the low-pass

wavenumber filtered for different wavelengths [25,

[50, [100, and [200 m.

3.2.5 Continuations Filtering

Continuation methods project the observed anomaly

field to higher elevations (upward continuation) or to

lower elevations (downward continuation) (YARGER

1985). One of the main advantages of the continu-

ation methods is that the character of the geopotential

field anomaly is retained as long as the continuation

doesn’t extend into the sources. Therefore, qualitative

interpretations can be performed on the results of

anomaly continuation (KELLER et al. 1985).

3.2.5.1 Downward Continuation Downward con-

tinuation is used to enhance the responses from sources

at a depth by effectively bringing the plane of mea-

surement closer to the source. By this way, anomalies

will have less spatial overlap and be more easily dis-

tinguished from each other (YARGER 1985). Downward

continuation emphasizes the components of higher

wave-number, increases the anomaly resolution of the

individual sources (KELLER et al. 1985; KHALIL 2012b),

and provides a more accurate determination of both

horizontal and vertical extents of near-surface mag-

netic sources (KELLER et al. 1985; BOSCHETTI et al.

2001). Nevertheless, downward continuation usually

increases high amplitude and short wavelength noise

from shallow sources. Therefore, it is usually recom-

mended to apply an additional Butterworth low-pass

filter to eliminate noise in the processed data (BOSCH-

ETTI et al. 2001). In the study area, the RTP magnetic

data were downward continued to 25, 50, 100, 150, and

200 m and filtered by Butterworth low-pass filter to

eliminate noise.

3.2.5.2 Upward Continuation Upward continuation

projects the observed magnetic anomaly to higher

elevations and therefore serves as low wavenumber

pass filter (YARGER 1985). Upward continuation

attenuates the high wavenumber anomalies associated

with near surface sources allowing an enhanced

interpretation of deep magnetic sources (LIDIAK et al.

1985). Noteworthy, upward continuation is consid-

ered a clean filter because it produces almost no side

effects that may require the application of other filters

or processes to correct (ARTZATE et al. 1990). In the

study area, the magnetic RTP data (Fig. 5) were

upward continued to 50, 100, 200, and 300 m.

3.2.6 Apparent Susceptibility

How easily a body can be magnetized is determined

by its magnetic susceptibility (ROSE et al. 1996). The

magnetic susceptibility of rocks is strongly variable

and depends extremely on lithology. This variation of

susceptibility does not only exist between different

rock types, but large variations also occur within a

given rock type (HOOVER and WILLIAMS 2007).

Basement rocks have usually high susceptibilities

due to their high magnetite content (iron), whereas

sedimentary rocks have much lower susceptibilities

(HOOVER et al. 2008).

Figure 5Contour map of the reduced to the pole (RTP) magnetic data,

Shalatein


In the study area, the apparent magnetic suscep-

tibility was calculated by the package of Geosoft

Oasis Montaj (2007) to a depth of 50, 75, 100, and

150 m. The employed filter of the apparent suscep-

tibility is a compound that performs a reduction to the

pole, downward continuation to the source depth,

correction for the geometric effect of a vertical square

ended prism, and division by the total magnetic field

to yield susceptibility. The susceptibility filter calcu-

lates the apparent magnetic susceptibility of the

magnetic sources using the following assumptions;

the magnetic field has had the International Geomag-

netic Reference Field (IGRF) removed, there is no

remanent magnetization, all magnetic response is

caused by a collection of vertical and square-ended

prisms of infinite depth extent. The result is presented

in unit of CGS electromagnetic (e.m.u.).

3.2.7 Wavelength Filtering

Wavelength filtering can be designed to remove either

the high wavenumber noise (due to small, near surface

sources) or the longer wavelength component (due to

the regional anomalies) with a low-pass and a high-pass

filters, respectively (PESONEN et al. 1994). The signif-

icance of this technique depends on the proper choice of

the cut-off wavelength used in the filter design (DEAN

1958; ZURFLUEH 1967). The calculated radially-aver-

aged power spectrum (Fig. 6) is used effectively to

estimate the cutoff wavelengths. Wavelength filtering

is usually performed in the frequency domain (ROEST

and PILKINGTON 1993).

3.2.7.1 Butterworth High-Pass Filter (BWHP) The

Butterworth high-pass filter is one of the most useful

filtering processes that can be applied to attenuate the

longer wavelength component of the regional anom-

alies and enhance the shorter wavelength of the near

surface anomalies (LEITE and LEAO 1985; LIDIAK

et al. 1985). Noteworthy, the degree of Butterworth

filter roll-off is controlled, while leaving the central

wavenumber fixed. Usually, filter order ‘‘1’’ is

employed for a very gentle roll-off, whereas order

‘‘20’’ or higher is employed for a much sharper

Figure 6Radially averaged power spectrum and depth estimate of the RTP magnetic data at Shalatein


roll-off (default value is ‘‘8’’). In the study area, a

BWHP filter of roll-off order ‘‘8’’ and wavelengths

\50, 100, 150, and 200 m were designed to pass the

shallower anomalies.

3.2.7.2 Butterworth Low-Pass Filter (BWLP) The

Butterworth low-pass filter is applied to enhance the

longer wavelength anomalies derived from deeper

anomalous sources and attenuate the shorter wave-

length of the near surface anomalies (ROEST and

PILKINGTON 1993; PESONEN et al. 1994). In the study

area, a BWLP filter of roll-off order ‘‘8’’ and wave-

lengths [50, 100, 200, and 300 m were designed to

reveal deep-seated causative structures.

3.2.7.3 Band-Pass Filter This filter can be used to

pass or reject a range of wavenumbers from the data.

However, applying such a cutoff filter to an energy

spectrum almost invariably introduces a significant

amount of ringing (Gibb’s Phenomena). Therefore,

usually it is recommended to apply a smoother filter such

as the Butterworth filter (PESONEN et al. 1994). In the

study area, band-pass filter augmented by a Butterworth

filter was used to separate the magnetic anomalies pro-

duced at source depths between 30 and 100 m.

3.2.8 Depth Estimation by 3D Euler Deconvolution

The depth of a magnetic source is of great value in

geological/geophysical interpretation of subsurface

structure (e.g., NAUDY 1971; REID et al. 1990; BOURNAS

et al. 2003). Nevertheless, estimation of source depth

from magnetic field data is a complex task and the

reliability of the resulting values is uncertain due to

imperfect representation of the geological model,

insufficient data sampling, unknown magnetization

values, and nonuniqueness of the inverse problem

(e.g., BLAKELY and SIMPSON 1986; BARBOSA et al. 1999;

GRAUCH et al. 2001; LI XIONG 2003; GRAUCH et al. 2004;

BEIKI et al. 2010).

The objective of the 3D Euler deconvolution

process is to produce a map showing the locations

and the corresponding depth estimations of geologic

sources of magnetic anomalies in a two-dimensional

grid (REID et al. 1990). The 3D Euler method is based

on Euler’s homogeneity equation, which relates the

magnetic field and its gradient components to the

location of the sources, by the degree of homogeneity

(SI: structural index) (THOMPSON 1982). The Euler

Deconvolution method is applicable to all geologic

models (faults, magnetic contacts, dykes, sills, etc.)

and it is insensitive to magnetic remanence and

geomagnetic inclination and declination (THOMPSON

1982). A solution is only recorded if the depth

uncertainty of the calculated depth estimate is less

than a specified threshold and the location of the

solution is within a limiting distance from the center of

the data window (WHITEHEAD and MUSSELMAN 2008). In

the study area, the package of Geosoft Oasis Montaj

(2007) was used to estimate the depth from the total

magnetic intensity grid by the 3D Euler Deconvolu-

tion. The depth was calculated for SI = 0 and 1, which

corresponded to geological contacts (faults) and dykes,

respectively, with a chosen depth uncertainty B5 %.

4. Results and Discussion

4.1. Geoelectric Resistivity Survey

Figure 2a illustrates sample (VES-5) of the

inverted data correlated with the adjacent borehole

(BH-1). Figure 2b and c exhibit geoelectric cross-

sections; profile-1 and profile-2, respectively. A

general four subsurface layers were recognized in

the area, from top to bottom as follows:

A surface layer characterized by mixture of dry

Quaternary alluvium sediments silt, sand, and gravel

boulders (Wadi deposits with heterogeneous nature).

This layer is characterized by resistivities ranging

from 581 to 1,738 Xm, and thicknesses ranging

from 2 to 4 m.

A second layer characterized by dry Quaternary

alluvium sediments of sand and gravel with variable

size. This layer is characterized by resistivities

ranging from 182 to 391 Xm, and thicknesses

ranging between 5 and 17 m.

A third layer of Quaternary and Pre-Quaternary

alluvium sediments (sand and gravel). This layer

represents the fresh aquifer and is characterized by

resistivities ranging from 30 to 105 Xm and thick-

nesses ranging between 20 and 60 m.

A fourth bottom layer represents the salt water

bearing formation of Quaternary and Pre-


Quaternary sediments of silt and sand stone. The

resistivities of this layer are very low and range

between 3 and 6 Xm.

Fresh aquifer potentiality could be recognized

toward the western part of the area where a gradual

increase could be observed in the resistivities

(70–105 Xm), depths (25–40 m), and thickness

(45–60 m) (Fig. 3a, b, and c, respectively).

4.2. Ground Magnetic Survey

Figure 4 exhibits the contour map of the diurnally

corrected total magnetic field (nT) at Shalatein. Foci

sharp gradients could be observed at different loca-

tions in the central toward eastern parts of the area.

The RTP (Fig. 5) minimized the polarity effects and

aligned the peaks and gradients of the diurnally

corrected magnetic anomalies (Fig. 4) directly over

their sources. Inspection of the RTP map revealed

well-pronounced high magnetic anomalies with

intense magnetic variations characterized by different

patterns and trends. The central (North–South) and

northeastern parts of the area are characterized by

large variations in the magnetic amplitudes with steep

gradient anomalies. Most of these anomalies are

characterized by a round to oval shape with a big

aerial extent and are elongated mostly in the NS,

NW–SE, and NE–SW directions. In contrast, the

western part reflects low magnetic amplitude with

moderate gradient and comprises two main oval

anomalies trending NW–SE and NE–SW (Fig. 5).

The radially averaged power spectrum (Fig. 6)

revealed two linear segments corresponding to the

long (deep-seated magnetic sources) and short (shal-

low-seated magnetic sources) wavelength

components. These linear segments were used to

deduce the average depths to the tops of the shallow

(25–300 m) and deep (300–780 m) causative mag-

netic sources. Although, these depths are average

estimates and don’t reflect high accuracy, they are

nevertheless useful in the design of many filters (e.g.,

wavelength and continuations filtering).

Figure 7a, b Contour maps of the first and second vertical derivatives magnetic data filtered by the low-pass wavenumber (wavelengths [100),

respectively, Shalatein


Inspection of the first and second vertical deriv-

atives filtered by the low-pass wavenumber revealed

sharper resolution for the near surface causatives

sources (especially the second vertical derivative).

The carried out low-pass wavelengths were [25, 50,

100, and 200 m, as the maximum depth to the bottom

of the fresh aquifer did not exceed 80 m. Figure 7a

and b shows the first and second vertical derivatives

filtered by the low-pass wavenumber (wavelengths

[100 m), respectively. Five major clusters of dyke

swarms (A, B, C, D, and E) were significantly

pronounced (Fig. 7a, b).

The downward continuation to 25, 50, and 100 m

filtered by Butterworth low-pass filter emphasized

significantly the higher wave-number components

and enhanced the anomaly of the individual shallow

magnetic sources. Whereas, the downward continued

to 150 and 200 m revealed less resolution and

distortion, respectively, of the same sources which

indicated extension of the continuation into these

sources. Figure 8a exhibits the downward continua-

tion to 100 m, in which, the five major clusters of

Figure 8a Contour map of the downward continuation 100 m magnetic data filtered by Butterworth low-pass filter. b Contour map of the upward

continuation 300 m magnetic data, Shalatein

Figure 9Contour map of the apparent magnetic susceptibility down to a

depth of 100 m, Shalatein


dyke swarms (A, B, C, D, and E) were clearly

identified in the central (NS) toward eastern parts of

the area. On the other hand, the western part appeared

free of any anomalies except in the northwestern

portion. A significant correlation could be observed

between the downward continuation to 100 m

(Fig. 8a) and the first and second vertical derivatives

(Fig. 7a, b).

The upward continued maps 50, 100, 200, and

300 m exhibited the changes in anomaly character

with increasing observation of source distance. They

revealed increasing attenuation and broadening of the

high wavenumber anomalies. As such, the upward

continuation up to 300 m (Fig. 8b) revealed a large

and homogenous anomaly caused by deep structure,

undistorted by the local, high amplitude, and high

gradient of the shallow magnetic sources. A homog-

enous gradual increase in the magnetic amplitude

could be recognized toward the northeastern

direction.

Figure 9 shows the apparent magnetic suscepti-

bility down to a depth of 100 m. The central (NS),

eastern, and northeastern parts revealed high suscep-

tibilities indicating dykes swarms of high magnetite

content, whereas the majority of the western part

revealed much lower susceptibilities, reflecting sed-

imentary rocks free from dykes at this depth.

Nevertheless, a gradual increase in the susceptibilities

could be recognized in the northwestern direction

indicating probable dykes. Five major clusters of

dyke swarms (A, B, C, D, and E) were clearly

observed and positively conformed to the prior

results.

The Butterworth high-pass filter (BWHP) of

wavelengths \50, 100, 150, and 200 m effectively

enhanced the depiction and isolation of the shallow

magnetic sources. As such, Fig. 10a shows the

BWHP filter of wavelengths \100 m where it is

distinguished by the number of positive and negative

anomalies with high and moderate frequencies,

respectively. Most of the pronounced anomalies are

characterized by short wavelengths, sharp amplitude,

a round to oval shape, and clearly located the detailed

positions of the dyke swarms. Remarkably, the

Figure 10a Contour map of the Butterworth high-pass wavenumber filtered (wavelengths \100 m) magnetic data overlaid by shallow-seated faults.

b Contour map of the Butterworth low-pass wavenumber filtered (wavelengths [300 m) magnetic data overlaid by deep-seated faults,

Shalatein


detected anomalies (Fig. 10a) clarified significantly

the near-surface structure with trends that conformed

to the addressed shallow fault systems (Fig. 1).

In contrast, the Butterworth low-pass filter

(BWLP) of wavelengths [50, 100, 200, and 300 m

proved high-resolution capabilities in attenuating the

shorter wavelength of the near surface anomalies and

emphasizing the longer wavelength derived from

deeper sources. As such, Fig. 10b shows the BWLP

filter of wavelengths [300 m, which was quite

amenable to delineate the causatives deep-seated

structure with large and homogenous anomalies.

Noteworthy, the detected anomalies of the BWHP

and BWLP filters (Fig. 10a, b) distinguished suc-

cessfully the near-surface and deep structure,

respectively. The trends of the detected faults

conformed positively to the addressed shallow and

deep fault systems (Fig. 1); A (Meridional NS trend),

B (Aqaba NNE–SSW trend), C (combination of

Meridional NS and Gulf of Suez-Red Sea NW–SE

trends), D (Meridional NS trend), and E (Gulf of

Suez-Red Sea NW–SE trend).

The band-pass filter succeeded effectively in

isolating and enhancing the anomaly wavelengths

associated with the magnetic geologic sources laid

between 30 and 100 m (Fig. 11). Five major clusters

of dyke swarms (A, B, C, D, and E) were significantly

pronounced and conformed to the aforementioned

results.

Figure 12a illustrates the depth estimated in the

study area computed by the 3D Euler deconvolution

for SI = 0 (geological contacts and faults). A signif-

icant distribution and dense clustering of the obtained

solutions along the rims of the anomalies could be

observed. The corresponding depth estimations are in

the range of 14 to 545 m. A good conformity is

observed with the pre-defined shallow- and deep-

seated faults (Figs. 1, 11a, b); Fig. 12b illustrates the

depth estimation computed for SI = 1 (dyke and sill).

Although, a few scattered clusterings were observed,

the predominated obtained solution is characterized

by a meaningful distribution and dense clustering

conformed to the pre-defined five major clusters of

dykes swarms (A, B, C, D, and E). The corresponding

depth estimations are in the range of 10 to 1,095 m.

Noteworthy is that the western part of the study area

is characterized by depths [100 m to the top of the

sources dykes. The obtained depth results are in good

agreement with the BH-2, which encountered the

dykes at a depth of 11 m, and BH-1, which didn’t

encounter any dykes and reached the top of the fresh

aquifer at a depth of 21 m.

5. Conclusion

One of the main obstacles that arises during the

drilling of development water wells in Shalatein, on

the western coast of the Red Sea, Egypt (Fig. 1), is

the occurrence of dykes. In this context, synthesis of

electrical resistivity, ground magnetics, and borehole

data was implemented to determine the freshwater

aquifer condition, locate the intrusive dykes and/or

dyke swarms, and demarcate the potential freshwater

zones.

Nine Schlumberger VES’s with maximum current

electrode half-spacing (AB/2) of 682 m were

Figure 113D map of the band-pass wavenumber filtered (wavelengths

30–100 m) magnetic data, Shalatein


conducted (Fig. 1). The interpreted VES stations,

geological subsurface studies, and borehole data

(BH-1 and BH-2) in the study area were integrated to

produce two geoelectric cross-sections (profile-1 and

2) with four general subsurface layers (Fig. 2). The

fresh aquifer of the Quaternary and Pre-Quaternary

alluvium sediments (sand and gravel) was effectively

demarcated with resistivities that ranged from 30 to

105 Xm and thicknesses that ranged between 20 and

60 m (Figs. 2, 3).

A ground magnetic survey comprised 35 magnetic

profiles, each 7 km in length. Observations were

carried out at a height of 1 m above the ground sur-

face with sampling rate of 10 s. The observed

magnetic data were diurnally corrected (Fig. 4) and

reduced to the pole (Fig. 5). Magnetic data interpre-

tation of the vertical derivatives (first and second

order), downward continuation (100 m), apparent

susceptibility (depth of 100 m), wavelength filters

(Butterworth high-pass of wavelengths \100 m and

Band-Pass of wavelengths 30–100 m) successfully

distinguished the near surface structure with five

major clusters of dyke swarms (A, B, C, D, and E)

(Figs. 7a, b, 8a, 9, 10a, 11, respectively). Filters of

the upward continuation (300 m) and Butterworth

low-pass (wavelengths [300 m) clearly reflected the

deep-seated structure (Figs. 8b, 10b, respectively).

Worth noting is the fact that the trends of the detected

faults conformed positively to the addressed shallow

and deep fault systems (Fig. 1).

In the study area, depth was computed using the

3D Euler deconvolution. The computed depth for

geological contacts and faults (SI = 0) ranged from

14 to 545 m (Fig. 12a) with a good conformity to the

addressed shallow and deep-seated faults. In contrast,

the computed depth for dyke and sill (SI = 1) ranged

from 10 to 1,095 m with a distribution patterns and

dense clustering conformed to the pronounced five

major clusters of dyke swarms (A, B, C, D, and E)

(Fig. 12b). Remarkably, the depth results are in good

agreement with the BH-2, which encountered the

dykes at depth of 11 m, and BH-1, which didn’t

Figure 123D Euler deconvolution applied to the total magnetic intensity calculated for SI = 0 and 1, a and b, respectively, with depth uncertainty

B5 %, Shalatein


encounter any dykes and reached the top of the fresh

aquifer at a depth of 21 m.

Therefore, the western part of the study area is

recommended for the drilling of development fresh-

water wells as it is characterized by depths[100 m to

the top of the dykes, higher thicknesses of the fresh

aquifer (45–60 m), depths to the top of the fresh

aquifer ranging from 25 to 40 m, and higher resis-

tivities reflecting better freshwater quality

(70–105 Xm).

Acknowledgments

The author would like to express his sincere thanks

and deep appreciation to the staff of the geophysical

department, Cairo University. Sincere thanks are

given to anonymous reviewers.

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Detection of Magnetically Susceptible Dyke Swarms in a Fresh Coastal Aquifer