ORIGINAL PAPER

Near-surface seismic refraction applied to exploringsubsurface clay layer at a new mining area in southeastCairo, 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 wasconducted at a new mining area located in southeast Cairo,Egypt, to explore the subsurface clay layer for futureeconomic use in mining and cement industry. The purposeof the survey has been to provide geological and geophys-ical information because no borehole was existent in thearea under investigation. The aim of study had been toexplain the main characteristics of the subsurface layers.For this purpose, a new technique has been used to acquireand process the data. This technique provides criticalinformation to determine the depth of the subsurface layers,as well as morphology, stratigraphy, and potential locationsof the clay layer for future economic use. The thickness andgeneral shape of the clay layer in the whole area weredetermined and are illustrated in maps.

Keywords Shallow seismic refraction . P-wave seismicprofiles . Stratigraphy

Introduction

The shallow seismic refraction techniques is considered one ofthe most effective methods for determining the depth of thebedrock and the ground water, the lithology type, the lateraland vertical changes in lithology, and investigating thestructural features such as micro faults. The evaluated seismicvelocities can be used in the interpretation of lithology,structural features and the zones of solution cavities. Shallow

seismic refraction has been widely applied to detect andresolve many complicated problems within the subsurfacelayers. From the engineering vantage point, shallow seismicrefraction has been used to study bedrock foundationproperties in road tunneling, dam sites, quarries, hydroelectricpower plants, subway constructions, nuclear power plants,and many other facilities. P and S-wave velocities obtainedfrom shallow seismic refraction surveys are used to evaluatethe bedrock and determine its elastic properties.

Today, the shallow geophysical techniques have beenused to study the physical and dynamic characteristics of soiland bedrock. Many researchers have used the seismicrefraction technique to determine the characteristics of thesite (Helfrich et al. 1970; Gregory 1976; Sjogren andSandberg 1979; Dutta 1984; Kilty et al. 1986; Hatherlyand Neville 1986).

On the other hand, there are some precautions which mustbe taken into consideration when using shallow seismicrefraction technique such as profile length and source energywhich limits the depth penetration of the refraction method.Typically, a profile can only detect features at a depth of one-fifth of the survey length. Another significant limitation tothe refraction method is the so-called hidden layer problem.The seismic refraction method requires the increase ofseismic velocity with depth. It is difficult to resolve a thin,low-velocity sand/gravel bed beneath a high-velocity claylayer which is a typical case for velocity inversion. Theseismograms require careful analysis to pick the first arrivaltimes of layers. If a thin layer produces first arrivals, whichcannot be easily identified on a seismogram, the layer maynever be identified. Thus, another layer may be misinter-preted as incorporating the hidden layer. As a result, the layerthickness may increase.

In thepresent study, the shallowseismic refractionmethod isapplied to investigate the depth and the subsurface geological

Arab J Geosci (2010) 3:105–112DOI 10.1007/s12517-009-0036-2

A. K. Abd El-Aal (*) :A. A. MohamedNational Research Institute of Astronomy and Geophysics,Helwan,Cairo, Egypte-mail: dewaky@nriag.sci.eg

conditions of the clay layer at the investigated site. A typicalseismic refraction survey usually consists of field dataacquisition, processing, and interpretation.

The purpose of the study

The purpose of the current study is to explore lateral andvertical variation in the thickness of an economic clay layerat a new mining area located southeast of Cairo, Egyptthrough the construction of ground models. No boreholedata exist in the area under investigation. These models arederived from a dense, P-wave, shallow seismic refractionsurvey. The shallow seismic refraction profiles werecarefully designed to reach the clay layer. The data qualitywas good over the length of each profile, and the thicknessof the clay layer has been easily determined from refractiondata so that the general shape of the upper and lowersurfaces of the clay layer could be interpreted.

Site description

The investigated site is located southeast to Cairo betweenlatitudes 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 partof the desert which separates the Eastern Desert from theNile Valley. The area under investigation is a wadi plain. Thegeology of the area lying between Eastern Desert and NileValley southeast Cairo where the investigated site is locatedhas been well studied (Fahmy 1969; Said 1962, 1971, 1990;Hemdan 1992). Field investigations in the area reveal acomplex sequence of sedimentary rocks ranging in age fromMiocene to Quaternary. The surface of the site is mainlycomposed of Quaternary gravelly sand. According to thegeology of the area established by the above-mentionedauthors and the borehole data near the study area under

study, the subsurface layering is composed of an irregularclay layer and limestone as a bedrock layer of Miocene age.

Seismic field instruments

In the present study, we used the Geometric Strataviewsystem. 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 paperrecord, or saved on floppy disk.

Power source

The Strataview operates from a nominal 12 V DC.

Geophone cables

The geophone cable is a multi-conductor cable withconnectors molded at intervals along the cable. There is astandard cable used for refraction surveys, consisting of 12takeouts (geophone connections) at selected intervals.

Geophones

The moving-coil geophone is the basic vibration sensor.The coil and its support spring oscillate with a naturalfrequency, which is specified for all geophones. The usefulseismic information is generally called signal. An undesir-able vibration (from wind, vehicle traffic, airplanes, surfacewaves) is called noise. Improving the signal-to-noise ratiois very important in seismic exploration. Geophone fre-quencies are chosen such as to provide adequate signals inthe frequency band found in the seismic data and not at the

Fig. 1 Location map of theinvestigated site

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frequencies of the noise signals. Most of noise types tend tobe low frequency. In the present study, geophones withnatural frequency around 40 Hz were used.

Energy source

A power-assisted weight drop (180 kg) was used to generateseismic waves. Weight drop systems were some of theearliest and in some areas most successful, non-explosiveseismic sources ever used. In the current study, the systemused a 180-kg weight mounted on the back of a large truckwhich was allowed to free fall to the ground, thus generatingseismic waves. To increase the efficiency for the weight dropsystem 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 inrefraction work are: (1) forward and reverse profilesconsisting of a pair of shot points (SP) which surround acommon geophone spread, (2) split profiles consisting of asingle shot point surrounded by a pair of geophone spread,and (3) in-offset profiles consisting of shot points at differentdistances on both sides of a common geophone spread.

In this study, a new technique has been used to acquireand process the data. A number of detectors were placed on

the ground along a straight line through the shot points todetect the direct and refracted waves. This technique isknown as profile-shooting technique (it includes all thecommon type techniques in one profile). This technique ismainly used to determine the velocity and thickness ofsubsurface layers by picking the first arrival (P-wave),which is generated by weight drop as a source of seismicenergy. To investigate more detailed topography of the claylayer, a specific geometry is used for P-wave acquisitionconsisting of five shots fired for every single profile. Thefirst shot is a normal shot within a 5-m distance beforegeophone 1 (geophone 1 at zero m), the second shot at a82.5 m distance between geophones 6 and 7, the third oneis a midpoint shot at a 172.5 m distance between geophones12 and 13, the fourth shot at a 262.5 m distance betweengeophones 18 and 19, and the last shot is a reverse shot at a350 m next to geophone 24 (Fig. 2). The length of everyseismic profile is 345 m containing 24 geophones and thegeophone interval is 15 m. This geometry providessufficient coverage to produce a seismic refraction stackedprofile.

The area under investigation is divided into eight majorseismic lines (Fig. 3) covering all the studied area. Seismiclines 1 and 4 comprise 3 profiles in each line. Seismic line8 consists of two profiles whereas seismic lines 2, 3, 5, 6and 7 include only one profile in each line. Data is recordedon a stacking seismograph with no preacquisition filtersapplied. The P-wave source is the weight drop. Thecommon recording parameters are listed in Table 1. Figure 4shows an example of the unprocessed raw seismograms forone complete profile. P-wave shot gathers are characterizedby coherent noise in the low-frequency range.

Fig. 3 P-wave seismic profilesat the investigated site

Fig. 2 Profile-shooting tech-nique used in the study

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Refraction analysis and interpretation

Several techniques have been established for seismicrefraction interpretation, each depends on the character ofthe refractor. These techniques can broadly be grouped into

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 andJovanovich 1985; Whiteley 2002, 2004), (4) inversion andtomography method (Boschetti et al. 1996; Hecht 2003;Hofmann and Schrott 2003; Roach 2003; Schuster andQuintus-Bosz 1993; Sheehan et al. 2005; Watanabe et al.1999; Wright 2006; Zhang and Toksöz 1998). Intercept−timemethods can be done with a pencil and calculator or, at most,a spread sheet program. Reciprocal-time methods vary froma simple version to a generalized version, which taxes mostpersonal computers. Ray-tracing, inversion and, topographymethods require significant computational resources. Thedetails and basic equations of all these methods are found inmost geophysical text books.

Table 1 Seismic refraction data recording parameters

P-wave acquisition

Recording system Geometrics StrataView

Geophone interval 15 m

Sampling interval 0.125 ms

Record length 256 ms

Recording format SEG-2

Geophones 40 Hz (vertical)

Source Weight drop 180 kg

Fig. 4 Examples of seismo-grams collected at the studiedarea

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The travel time(distance curves are constructed based onrefracted waves from subsurface layering interfaces. Figure 5shows the first-arrival travel time curves and ground modelsfor seismic line 1 at the investigated area. The reciprocalmethod and regression of raw corrected arrivals followedby a series of ray-tracing and model adjustments iterations(SIP seismic identification program) had been applied todetermine 3-layered velocity vs. depth structures. Thecalculated P-wave velocities and thickness for the con-ducted profiles are listed in Table 2. The uppermost layer(layer 1) is a thin surface layer with a thickness rangingfrom 0.5 m to 7 m and is characterized by low P-wavevelocity ranging from 704 m/s to 799 m/s. This layer isinterpreted as gravelly sand according to surface fieldinvestigation and an old mining site near the study area.

The contact between layer 1 and layer 2 is characterized bya high acoustic impedance contrast due to the high secondP-wave velocity ranging from 1907 m/s to 1,995 m/s and isinterpreted as the upper surface of clay layer. The deepestlayer (layer 3) is interpreted as bedrock (limestone) with P-wave velocities ranging from 2,817 m/s to 2,897 m/sconsistent with the expected values for Miocene limestonebedrock in the area. The depth of this layer is consistentwith the bedrock depth expected from limited boreholesnear the area under investigation. The depths of the uppersurface of clay layer and limestone layer are shown inFigs. 6 and 7, respectively. Note that there is a significantincrease in velocity from surface to bedrock. Theserefraction analyses provide simplified P-velocity vs. depthrules for this site. It must be realized, however, that it is

Fig. 5 Travel time and groundmodel of Line 1 (Profiles 1, 2and 3)

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Table 2 Velocity model obtained from refraction method

Line No Profile No Velocity of firstlayer (m/s)

Velocity of secondlayer (m/s)

Velocity of thirdlayer (m/s)

thickness offirst layer

thickness ofsecond layer

Min. (m) Max. (m) Min (m) Max. (m)

1 1+2+3 704 1,995 2,881 0.25 12 9 53

2 5 730 1,925 2,870 0.25 5 13 34

3 6 733 1,945 2,820 0.25 4 16 35

4 7+8+9 770 1,959 2,817 1 12 16 45

5 10 765 1,990 2,897 1 8 11 32

6 11 711 1,954 2,831 3 6 7 19

7 12 730 1,980 2,895 3 10 10 25

8 4+13 799 1,907 2,832 3 10 25 45

Fig. 6 a Contour map showingthe depth to the top of the claylayer. b 3D map showing the topof the clay layer

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likely to have velocity variations within these layers thatcannot be determined from first-arrival data. In particular,refraction analysis cannot identify the presence of a low-velocity “hidden” layer within the section. Previous studiesin lithology types and seismic velocities of subsurfacelayers established in the old mines around and near the areaindicate that lower velocity sediments often do not exist inthe area (Bassiouni et al 1974; Egyptian National Seismo-logical Network 2004; Fahmy 1969; Hemdan 1992). On theother hand, differences in seismic velocity for each bedfrom one profile to another are largely due to uncertaintiesin the first arrivals picks or to the presence of unconsoli-dated sediments and small structures in each profile.

Conclusions

Near-surface seismic refraction surveys can provide infor-mation about the subsurface velocity profile and subsurfacestructure. They can also provide detailed knowledge of

subsurface geometry and can be used to identify andquantify acoustic impedance boundaries within the over-burden and at the bedrock contact. In this study, therefraction models identify a high-velocity P-wave contrastwithin the overburden. The overburden–clay contact hasidentified in this survey, in addition to the clay thicknessthat has been determined. The refraction sections show theoverburden-bedrock contact to be essentially curved andnot flat (see Fig. 7) which may be attributed to the nature ofdepositional environment (Hemdan 1992). As noted above,the velocity structure determined from the refractionanalysis cannot include the existence of possible low-velocity layers. As a first approximation based on theground model, I have obtained the lateral and verticalvariations in thickness and p-wave velocity in the secondlayer (clay layer) for economic calculations in future.

Acknowledgments The author is grateful to the National ResearchInstitute of Astronomy and Geophysics (NRIAG), Egypt for theprovision of the shallow seismic refraction instruments, and the

Fig. 7 a Contour map showingthe depth to the top of thelimestone bedrock. b 3D mapshowing the top of the limestonebedrock

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qualified cadres during the field work as well as all other facilitiesrequired for this research.

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