Near-surface seismic refraction applied to exploring subsurface clay layer at a new mining area in southeast Cairo, Egypt

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1 Arab J Geosci (2010) 3: DOI /s 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 dewaky@nriag.sci.eg 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

$A typical seismic refraction survey usually consists of field data acquisition, processing, and interpretation.$

2 106 Arab J Geosci (2010) 3: conditions of the clay layer at the investigated site. A typical seismic refraction survey usually consists of field data acquisition, processing, and interpretation. 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. Site description The investigated site is located southeast to Cairo between latitudes and N and longitudes and 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 study, the subsurface layering is composed of an irregular clay layer and limestone as a bedrock layer of Miocene age. Seismic field instruments 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 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 Fig. 1 Location map of the investigated site

Weight drop systems were some of the earliest and in some areas most successful, non-explosive seismic sources ever used.

3 Arab J Geosci (2010) 3: 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 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 m distance between geophones 12 and 13, the fourth shot at a 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. Fig. 3 P-wave seismic profiles at the investigated site

$108 Arab J Geosci (2010) 3:105 112 Table 1 Seismic refraction data recording parameters Recording system Geophone interval Sampling interval Record length Recording format Geophones Source Refraction$

4 108 Arab J Geosci (2010) 3: Table 1 Seismic refraction data recording parameters Recording system Geophone interval Sampling interval Record length Recording format Geophones Source Refraction analysis and interpretation P-wave acquisition Geometrics StrataView 15 m ms 256 ms SEG-2 40 Hz (vertical) Weight drop 180 kg Several techniques have been established for seismic refraction interpretation, each depends on the character of the 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 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. Fig. 4 Examples of seismograms collected at the studied area

$Arab J Geosci (2010) 3:105 112 109 The travel time(distance curves are constructed based on refracted waves from subsurface layering interfaces.$

5 Arab J Geosci (2010) 3: 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. 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 P- wave 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 Fig. 5 Travel time and ground model of Line 1 (Profiles 1, 2 and 3)

$110 Arab J Geosci (2010) 3:105 112 Table 2 Velocity model obtained from refraction method Line No Profile No Velocity of first layer (m/s) Velocity of second layer (m/s) Velocity of third layer (m/s)$

6 110 Arab J Geosci (2010) 3: Table 2 Velocity model obtained from refraction method Line No Profile No Velocity of first layer (m/s) Velocity of second layer (m/s) Velocity of third layer (m/s) thickness of first layer thickness of second layer Min. (m) Max. (m) Min (m) Max. (m) ,995 2, ,925 2, ,945 2, ,959 2, ,990 2, ,954 2, ,980 2, ,907 2, 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

Arab J Geosci (2010) 3:105 112 111 Fig. 7 a Contour map showing the depth to the top of the limestone bedrock.

7 Arab J Geosci (2010) 3: 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 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. Conclusions Near-surface seismic refraction surveys can provide information about the subsurface velocity profile and subsurface structure. They can also provide detailed knowledge of 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. 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

8 112 Arab J Geosci (2010) 3: qualified cadres during the field work as well as all other facilities required for this research. References Adachi R (1954) On a proof of fundamental formula concerning refraction method of geophysical prospecting and some remarks. Kumamoto J Sci 2:18 23 Barton R, Barker N (2003) Velocity imaging by tau-p transformation of refracted traveltimes. Geophys Prospect 51: Bassiouni MA, Abdelmalik WM, Boukhary M (1974) Litho- and biostratigraphy of middle and upper Eocene rocks in the Minia- Beni Suef reach of the Nile Valley, Egypt. 6th Afr. Micropaleontol. Colloq., Tunis: Boschetti F, Dentith MC, List RD (1996) Inversion of seismic refraction data using genetic algorithms. Geophysics 61: Dutta NP (1984) Seismic refraction method to study the foundation rock of a Dam. Geophys Prospect 32: Egyptian National Seismological Network (2004) Evaluation of clay layer using shallow seismic refraction technique at south east Cairo, Egypt. Internal report submitted to clay company Fahmy SE (1969) Biostratigraphy of Paleogene deposits in Egypt. Proc. 3rd Afr. Micropaleontol. Colloq. Cairo: Gregory AR (1976) Fluid saturation effects on dynamic elastic properties of sedimentary rocks. Geophysics 44: Hagedoorn JG (1959) The plus minus method of interpreting seismic refraction sections. Geophys Prospect 7: Hales FW (1958) An accurate graphical method for interpreting seismic refraction lines. Geophys Prospect 6: Hatherly PJ, Neville MJ (1986) Experience with the generalized reciprocal method of seismic refraction interpretation for shallow seismic engineering site investigation. Geophysics 51: Hecht S (2003) Differentiation of loose sediments with seismic refraction methods potentials and limitations derived from case studies. In: Schott L, Hordt A, Dikau R (eds) Geophysical applications in geomorphology. 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