Array measurements of S‐wave velocities from ambient vibrations

SUMMARY The S-wave velocity is a very important factor in local hazard assessment. Direct measurement with conventional methods is very costly and therefore inexpensive and efficient methods are needed to make local hazard assessment more feasible. Techniques based on the analysis of recordings of ambient vibrations from small-scale arrays of sensors have become popular recently. One technique that is favoured by several research groups is the extraction of the dispersion curve by estimation of the f ‐k spectrum and its inversion for the S-wave velocity structure. This paper presents the results from an application based on high-resolution beamforming applied to the vertical component of the measurements. Synthetic ambient vibrations generated with a 2-D finite-difference code are used to illustrate and test the application. By superposition of random signals it is shown that the dispersion curve can be extracted even if 60 per cent of the wavefield consists of spatially uncorrelated signals. The errors in the phase velocities amount to less than 10 per cent. The dispersion curve can be extracted from the fundamental frequency of resonance upwards. Data from two real measurements are presented—from one site close to a city and another site within an industrial complex. The inverted S-wave velocity structures agree with reference data for the sites. The rule of thumb for the resolution of the method is confirmed.

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