Arab J Geosci (2010) 3:105–112
DOI 10.1007/s12517-009-0036-2
ORIGINAL PAPER
Near-surface seismic refraction applied to exploring
subsurface clay layer at a new mining area in southeast
Cairo, Egypt
A. K. Abd El-Aal & A. A. Mohamed
Received: 10 September 2008 / Accepted: 20 January 2009 / Published online: 7 March 2009
# Saudi Society for Geosciences 2009
Abstract A near-surface seismic refraction survey was
conducted at a new mining area located in southeast Cairo,
Egypt, to explore the subsurface clay layer for future
economic use in mining and cement industry. The purpose
of the survey has been to provide geological and geophysical information because no borehole was existent in the
area under investigation. The aim of study had been to
explain the main characteristics of the subsurface layers.
For this purpose, a new technique has been used to acquire
and process the data. This technique provides critical
information to determine the depth of the subsurface layers,
as well as morphology, stratigraphy, and potential locations
of the clay layer for future economic use. The thickness and
general shape of the clay layer in the whole area were
determined and are illustrated in maps.
Keywords Shallow seismic refraction . P-wave seismic
profiles . Stratigraphy
Introduction
The shallow seismic refraction techniques is considered one of
the most effective methods for determining the depth of the
bedrock and the ground water, the lithology type, the lateral
and vertical changes in lithology, and investigating the
structural features such as micro faults. The evaluated seismic
velocities can be used in the interpretation of lithology,
structural features and the zones of solution cavities. Shallow
A. K. Abd El-Aal (*) : A. A. Mohamed
National Research Institute of Astronomy and Geophysics,
Helwan,
Cairo, Egypt
e-mail: [email protected]
seismic refraction has been widely applied to detect and
resolve many complicated problems within the subsurface
layers. From the engineering vantage point, shallow seismic
refraction has been used to study bedrock foundation
properties in road tunneling, dam sites, quarries, hydroelectric
power plants, subway constructions, nuclear power plants,
and many other facilities. P and S-wave velocities obtained
from shallow seismic refraction surveys are used to evaluate
the bedrock and determine its elastic properties.
Today, the shallow geophysical techniques have been
used to study the physical and dynamic characteristics of soil
and bedrock. Many researchers have used the seismic
refraction technique to determine the characteristics of the
site (Helfrich et al. 1970; Gregory 1976; Sjogren and
Sandberg 1979; Dutta 1984; Kilty et al. 1986; Hatherly
and Neville 1986).
On the other hand, there are some precautions which must
be taken into consideration when using shallow seismic
refraction technique such as profile length and source energy
which limits the depth penetration of the refraction method.
Typically, a profile can only detect features at a depth of onefifth of the survey length. Another significant limitation to
the refraction method is the so-called hidden layer problem.
The seismic refraction method requires the increase of
seismic velocity with depth. It is difficult to resolve a thin,
low-velocity sand/gravel bed beneath a high-velocity clay
layer which is a typical case for velocity inversion. The
seismograms require careful analysis to pick the first arrival
times of layers. If a thin layer produces first arrivals, which
cannot be easily identified on a seismogram, the layer may
never be identified. Thus, another layer may be misinterpreted as incorporating the hidden layer. As a result, the layer
thickness may increase.
In the present study, the shallow seismic refraction method is
applied to investigate the depth and the subsurface geological
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conditions of the clay layer at the investigated site. A typical
seismic refraction survey usually consists of field data
acquisition, processing, and interpretation.
Arab J Geosci (2010) 3:105–112
study, the subsurface layering is composed of an irregular
clay layer and limestone as a bedrock layer of Miocene age.
Seismic field instruments
The purpose of the study
The purpose of the current study is to explore lateral and
vertical variation in the thickness of an economic clay layer
at a new mining area located southeast of Cairo, Egypt
through the construction of ground models. No borehole
data exist in the area under investigation. These models are
derived from a dense, P-wave, shallow seismic refraction
survey. The shallow seismic refraction profiles were
carefully designed to reach the clay layer. The data quality
was good over the length of each profile, and the thickness
of the clay layer has been easily determined from refraction
data so that the general shape of the upper and lower
surfaces of the clay layer could be interpreted.
In the present study, we used the Geometric Strataview
system. The system consists of:
Strataview 24-channel digital seismographs
The digital signal is stored in a semiconductor memory,
where it can be viewed on a display, plotted on a paper
record, or saved on floppy disk.
Power source
The Strataview operates from a nominal 12 V DC.
Geophone cables
Site description
The investigated site is located southeast to Cairo between
latitudes 29° 39′ 40″ and 29° 39′ 50″N and longitudes 31° 23′
26″ and 31° 24′ 07″E (Fig. 1). The site constitutes a small part
of the desert which separates the Eastern Desert from the
Nile Valley. The area under investigation is a wadi plain. The
geology of the area lying between Eastern Desert and Nile
Valley southeast Cairo where the investigated site is located
has been well studied (Fahmy 1969; Said 1962, 1971, 1990;
Hemdan 1992). Field investigations in the area reveal a
complex sequence of sedimentary rocks ranging in age from
Miocene to Quaternary. The surface of the site is mainly
composed of Quaternary gravelly sand. According to the
geology of the area established by the above-mentioned
authors and the borehole data near the study area under
Fig. 1 Location map of the
investigated site
The geophone cable is a multi-conductor cable with
connectors molded at intervals along the cable. There is a
standard cable used for refraction surveys, consisting of 12
takeouts (geophone connections) at selected intervals.
Geophones
The moving-coil geophone is the basic vibration sensor.
The coil and its support spring oscillate with a natural
frequency, which is specified for all geophones. The useful
seismic information is generally called signal. An undesirable vibration (from wind, vehicle traffic, airplanes, surface
waves) is called noise. Improving the signal-to-noise ratio
is very important in seismic exploration. Geophone frequencies are chosen such as to provide adequate signals in
the frequency band found in the seismic data and not at the
Arab J Geosci (2010) 3:105–112
107
Fig. 2 Profile-shooting technique used in the study
frequencies of the noise signals. Most of noise types tend to
be low frequency. In the present study, geophones with
natural frequency around 40 Hz were used.
Energy source
A power-assisted weight drop (180 kg) was used to generate
seismic waves. Weight drop systems were some of the
earliest and in some areas most successful, non-explosive
seismic sources ever used. In the current study, the system
used a 180-kg weight mounted on the back of a large truck
which was allowed to free fall to the ground, thus generating
seismic waves. To increase the efficiency for the weight drop
system in the current study the following steps were done.
1. Repeating the drops and adding records together.
2. Accelerating the weight under a force greater than that
of gravity.
3. Dropping the weight from a greater height.
Data acquisition
The most common types of profiles that can be used in
refraction work are: (1) forward and reverse profiles
consisting of a pair of shot points (SP) which surround a
common geophone spread, (2) split profiles consisting of a
single shot point surrounded by a pair of geophone spread,
and (3) in-offset profiles consisting of shot points at different
distances on both sides of a common geophone spread.
In this study, a new technique has been used to acquire
and process the data. A number of detectors were placed on
Fig. 3 P-wave seismic profiles
at the investigated site
the ground along a straight line through the shot points to
detect the direct and refracted waves. This technique is
known as profile-shooting technique (it includes all the
common type techniques in one profile). This technique is
mainly used to determine the velocity and thickness of
subsurface layers by picking the first arrival (P-wave),
which is generated by weight drop as a source of seismic
energy. To investigate more detailed topography of the clay
layer, a specific geometry is used for P-wave acquisition
consisting of five shots fired for every single profile. The
first shot is a normal shot within a 5-m distance before
geophone 1 (geophone 1 at zero m), the second shot at a
82.5 m distance between geophones 6 and 7, the third one
is a midpoint shot at a 172.5 m distance between geophones
12 and 13, the fourth shot at a 262.5 m distance between
geophones 18 and 19, and the last shot is a reverse shot at a
350 m next to geophone 24 (Fig. 2). The length of every
seismic profile is 345 m containing 24 geophones and the
geophone interval is 15 m. This geometry provides
sufficient coverage to produce a seismic refraction stacked
profile.
The area under investigation is divided into eight major
seismic lines (Fig. 3) covering all the studied area. Seismic
lines 1 and 4 comprise 3 profiles in each line. Seismic line
8 consists of two profiles whereas seismic lines 2, 3, 5, 6
and 7 include only one profile in each line. Data is recorded
on a stacking seismograph with no preacquisition filters
applied. The P-wave source is the weight drop. The
common recording parameters are listed in Table 1. Figure 4
shows an example of the unprocessed raw seismograms for
one complete profile. P-wave shot gathers are characterized
by coherent noise in the low-frequency range.
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Arab J Geosci (2010) 3:105–112
Table 1 Seismic refraction data recording parameters
P-wave acquisition
Recording system
Geophone interval
Sampling interval
Record length
Recording format
Geophones
Source
Geometrics StrataView
15 m
0.125 ms
256 ms
SEG-2
40 Hz (vertical)
Weight drop 180 kg
Refraction analysis and interpretation
Several techniques have been established for seismic
refraction interpretation, each depends on the character of
the refractor. These techniques can broadly be grouped into
Fig. 4 Examples of seismograms collected at the studied
area
the following four categories: (1) intercept−time method
(Adachi 1954; Barton and Barker 2003; Hales 1958;
Hagedoorn 1959), (2) reciprocal or delay-time method
(Leung 1995, 1997, 2003; Palmer 1980; Wyrobek 1956;
Sjogren 2000) and (3) ray-tracing method (Jones and
Jovanovich 1985; Whiteley 2002, 2004), (4) inversion and
tomography method (Boschetti et al. 1996; Hecht 2003;
Hofmann and Schrott 2003; Roach 2003; Schuster and
Quintus-Bosz 1993; Sheehan et al. 2005; Watanabe et al.
1999; Wright 2006; Zhang and Toksöz 1998). Intercept−time
methods can be done with a pencil and calculator or, at most,
a spread sheet program. Reciprocal-time methods vary from
a simple version to a generalized version, which taxes most
personal computers. Ray-tracing, inversion and, topography
methods require significant computational resources. The
details and basic equations of all these methods are found in
most geophysical text books.
Arab J Geosci (2010) 3:105–112
The travel time(distance curves are constructed based on
refracted waves from subsurface layering interfaces. Figure 5
shows the first-arrival travel time curves and ground models
for seismic line 1 at the investigated area. The reciprocal
method and regression of raw corrected arrivals followed
by a series of ray-tracing and model adjustments iterations
(SIP seismic identification program) had been applied to
determine 3-layered velocity vs. depth structures. The
calculated P-wave velocities and thickness for the conducted profiles are listed in Table 2. The uppermost layer
(layer 1) is a thin surface layer with a thickness ranging
from 0.5 m to 7 m and is characterized by low P-wave
velocity ranging from 704 m/s to 799 m/s. This layer is
interpreted as gravelly sand according to surface field
investigation and an old mining site near the study area.
Fig. 5 Travel time and ground
model of Line 1 (Profiles 1, 2
and 3)
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The contact between layer 1 and layer 2 is characterized by
a high acoustic impedance contrast due to the high second
P-wave velocity ranging from 1907 m/s to 1,995 m/s and is
interpreted as the upper surface of clay layer. The deepest
layer (layer 3) is interpreted as bedrock (limestone) with Pwave velocities ranging from 2,817 m/s to 2,897 m/s
consistent with the expected values for Miocene limestone
bedrock in the area. The depth of this layer is consistent
with the bedrock depth expected from limited boreholes
near the area under investigation. The depths of the upper
surface of clay layer and limestone layer are shown in
Figs. 6 and 7, respectively. Note that there is a significant
increase in velocity from surface to bedrock. These
refraction analyses provide simplified P-velocity vs. depth
rules for this site. It must be realized, however, that it is
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Arab J Geosci (2010) 3:105–112
Table 2 Velocity model obtained from refraction method
Line No
1
2
3
4
5
6
7
8
Profile No
1+2+3
5
6
7+8+9
10
11
12
4+13
Velocity of first
layer (m/s)
704
730
733
770
765
711
730
799
Fig. 6 a Contour map showing
the depth to the top of the clay
layer. b 3D map showing the top
of the clay layer
Velocity of second
layer (m/s)
1,995
1,925
1,945
1,959
1,990
1,954
1,980
1,907
Velocity of third
layer (m/s)
2,881
2,870
2,820
2,817
2,897
2,831
2,895
2,832
thickness of
first layer
thickness of
second layer
Min. (m)
Max. (m)
Min (m)
Max. (m)
0.25
0.25
0.25
1
1
3
3
3
12
5
4
12
8
6
10
10
9
13
16
16
11
7
10
25
53
34
35
45
32
19
25
45
Arab J Geosci (2010) 3:105–112
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Fig. 7 a Contour map showing
the depth to the top of the
limestone bedrock. b 3D map
showing the top of the limestone
bedrock
Conclusions
subsurface geometry and can be used to identify and
quantify acoustic impedance boundaries within the overburden and at the bedrock contact. In this study, the
refraction models identify a high-velocity P-wave contrast
within the overburden. The overburden–clay contact has
identified in this survey, in addition to the clay thickness
that has been determined. The refraction sections show the
overburden-bedrock contact to be essentially curved and
not flat (see Fig. 7) which may be attributed to the nature of
depositional environment (Hemdan 1992). As noted above,
the velocity structure determined from the refraction
analysis cannot include the existence of possible lowvelocity layers. As a first approximation based on the
ground model, I have obtained the lateral and vertical
variations in thickness and p-wave velocity in the second
layer (clay layer) for economic calculations in future.
Near-surface seismic refraction surveys can provide information about the subsurface velocity profile and subsurface
structure. They can also provide detailed knowledge of
Acknowledgments The author is grateful to the National Research
Institute of Astronomy and Geophysics (NRIAG), Egypt for the
provision of the shallow seismic refraction instruments, and the
likely to have velocity variations within these layers that
cannot be determined from first-arrival data. In particular,
refraction analysis cannot identify the presence of a lowvelocity “hidden” layer within the section. Previous studies
in lithology types and seismic velocities of subsurface
layers established in the old mines around and near the area
indicate that lower velocity sediments often do not exist in
the area (Bassiouni et al 1974; Egyptian National Seismological Network 2004; Fahmy 1969; Hemdan 1992). On the
other hand, differences in seismic velocity for each bed
from one profile to another are largely due to uncertainties
in the first arrivals picks or to the presence of unconsolidated sediments and small structures in each profile.
112
qualified cadres during the field work as well as all other facilities
required for this research.
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