Abstract: In April 2008 an ambient vibration measurement campaign was conducted in Cádiz Bay (South of Spain) in the framework of the RISTE-COSTE project. Two different techniques were applied in order to study the site effects from predominant soil types in six nearby towns: Cádiz, Chiclana, Rota, Puerto Real, San Fernando and El Puerto de Santa María.
The first technique considered was the H/V method (also known as Nakamura’s technique) to identify the natural frequency of the soil deposit. In order to apply this method, five sites were chosen from every town, all of them located in free field condition. The equipment used to obtain the records was a CityShark digitizer with two Lennartz, triaxial sensors of 5 s. Different time window lengths were used to obtain the records varying from 10 to 20 minutes depending on site conditions. The second technique was based on the set up of an array (array technique) consisting of 11 sensors (two concentric circles with 5 sensors each one plus one central sensor) with a maximum diameter of 100 m. The soil profile characteristics (Vs and thickness of each layer) for each site were obtained first using the FK and SPAC methods and then performing the inversion of the dispersion and autocorrelation curves using the “neighbor algorithm”.
The results of H/V technique showed predominant high fundamental frequencies in the majority of sites (from 4 to 12 Hz), which is confirmed in some cases by the results of array technique. The array technique showed clear definition of Vs and thickness for the layer closer to the surface and this is used for the numerical simulations to characterize soil amplification.
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Authors:
Schimdt V., Macau A., Figuras S., Benjumea B., Goula X.