Soft Sediment Characterization using Seismic Techniques at Beni Suef City, Egypt

Abstract

It is well known that the local geological characteristics in terms of topographic setting and the existence of soft sediments over bedrock may affect earthquake waves and cause seismic amplification. These effects are called “site effects”. Microtremors which provide an efficient practical tool for site effects estimation were recorded at 43 sites in Beni Suef City, Egypt. The recorded seismic signals were analyzed using the Horizontal-to-Vertical Spectral Ratio (HVSR) method. The targeted site parameters are the fundamental frequency ( f0) and the corresponding amplitude of seismic waves ( A0). Selected H/V curves with clear peak frequency have been inverted to infer the S-wave velocity profile of the underlying sediments. Information about subsurface sediments needed for the inversion process was extracted from available boreholes data. Moreover, the estimated values of f0 and A0 have been used for a preliminary calculation of the seismic vulnerability index ( Kg) which represents an indicator of soil liquefaction potentiality in the event of future earthquakes at the study area. The estimated H/V curves reveals significant variations in f0 and A0 parameters, reflecting variations in the soil characteristics (thickness and type) at the study area. The estimated values of f0 are (0.4–3.7 Hz), and commonly decrease from east to west. The A0 values vary from flat H/V curves (without any clear peak) at rock sites to 7.8 near to the Nile River and in the cultivated areas. The obtained velocity profiles could investigate S-wave structure down to 200 m depth. The estimated Kg varies from 10 to more than 50 μstrain/gal. The highest Kg values are noticed in the west and northwest in the study area were the soft sediments exist with considerable thickness, while the smallest Kg values are noticed in the south east where limestone and stiff soil occur near the ground surface.