Evaluating the structural model from ambient vibration recordings and shallow seismic refraction techniques. A case stud
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ORIGINAL PAPER
Evaluating the structural model from ambient vibration recordings and shallow seismic refraction techniques. A case study of Zagazig City, Egypt A. A. Mohamed 1 & M. A. El-Eraki 2 & A. A. El-Kenawy 2 & S. I. Mostafa 2
Received: 12 August 2015 / Accepted: 15 August 2016 # Saudi Society for Geosciences 2016
Abstract This study uses the seismic refraction and noise measurements to investigate the velocity structure of the subsurface and emphasize the advantage of ambient vibration over the conventional seismic refraction technique. Field measurements were carried out at nine sites in and around Zagazig city. Shallow seismic refraction data were interpreted using the delay time method to obtain the two-dimension ground model at each site. Ambient vibration arrays are used to infer the one-dimensional compressional and shear wave velocity profiles through two main steps. The first step is to derive the dispersion curve from the recorded signals using the frequency-wavenumber method. The second is to invert the dispersion curve to obtain the site velocity profiles. The results of the compressional wave velocities obtained from seismic refraction technique showed that the subsurface consists of a number of layers ranging from two to four layers and give a good agreement with the results of the seismic wave velocities obtained from the ambient vibration arrays. The ambient vibration arrays gave a deeper depth of penetration than the other method, providing more information on the subsurface structure without any disturbance to the environment. This work provides reliable estimates of the seismic velocity structures of both shallow and deep sedimentary layers within the area of interest.
* S. I. Mostafa [email protected] 1
National Research Institute of Astronomy and Geophysics (NRIAG), Helwan, Egypt
2
Geology Department, Zagazig University, Faculty of Science, Zagazig, Egypt
Keywords Seismic refraction . Delay time . Ambient vibrations and frequency-wavenumber
Introduction Zagazig city is the capital of El Sharkia Governorate and is situated on the east of Nile Delta in the midst of a fertile district. The importance of the city is manifested in its high density of population, urbanization, and its archeological sites such as the ruins of Tell-Basta which is located 3 km southeast of the town. Also, it was affected by a number of earthquakes both historical and instrumental. The study area (Fig. 1) is located in a moderately active seismic area between latitudes 30.549°–30.64° N and longitudes 31.424°–31.56° E covering a surface area about 135 km2. It is affected by six active seismogenic zones: Abu Zabal, South-West Cairo, CairoSuez Shear Zone, Gulf of Suez, Gulf of Aqaba, and SouthEast Mediterranean Sea Zones, which have different levels of seismic activity. Hence, it is important to estimate the seismic wave velocities in subsurface layers to identify the subsurface structure for engineering applications. Seismic technique is considered as one of the most effective geophysical methods which ca
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