Microtremor array explorations were performed at seven sites in Ismailia city, Egypt, in order to obtain 1-D S-wave velocity (Vs) profiles. The Spatial Autocorrelation (SPAC) method was applied to microtremor data for the determination of phase velocity dispersion curves and estimation of 1-D Vs profiles. The estimated Vs profiles indicate that the maximum Vs is about 500-700 m/s in the deeper part (90-350 m) and the minimum Vs is about 140-200 m/s in the shallow subsurface layer. The calculated AVS30 values based on the estimated Vs profiles indicate that all the sites are categorized into site class C or D of NEHRP classification. The theoretical site amplification factors using Vs profiles show that the resonant frequencies for the sites located on the west side of Suez Canal range between 1 to 2 Hz, whereas they are lower than 1 Hz for the sites located on the east side.
Microtremor horizontal-to-vertical spectral ratios (HVSR) were also calculated for all sensor records at each site. The derived HVSR show similar curves for all the sensors at each site, indicating nearly flat layered structure beneath each site. There are no clear peaks of the observed HVSR in the frequency range higher than 1 Hz, where high signal-to-noise ratios were detected. We also calculated the theoretical HVSR of the fundamental mode Rayleigh waves using the estimated Vs structures and plotted them with the theoretical amplification factors. The dominant frequencies from the both theories are similar to each other at five sites. We tried to modify the Vs models for them, but the results show that the originally inverted structure models are acceptable, since discrepancies between the observed and theoretical phase velocities are obvious except one site. Our results show that the combination of SPAC and HVSR methods is advantageous to support the results of each method and overcome observational limitations.
Keywords: Ismailia city, SPAC method, AVS30, Site amplification factor, HVSR method