A theoretical investigation of average H/V ratios

The mode summation method and a finite difference technique are applied to investigate the spectral ratio between the horizontal and vertical componen ts (H/V ratio) of ambient vibrations and to explore the variation of the re sonance frequency and the amplitude and shape of polarization as a function of the structure and the source positions. Layered structural models are u sed by assuming a large number of sources distributed around a receiver, wi th shallow source depths that are randomly assigned. We identify stable par ts of the H/V ratios that are independent of the source distance and are do minated by the ellipticity of the fundamental-mode Rayleigh wave in the fre quency band between the fundamental frequency of resonance of the unconsoli dated sediments and the first minimum of the average H/V ratio. The ellipti city in this frequency band is determined by the layering of the sediments. The numerical simulations are compared with observations at a site where th e thickness and velocity structure of the unconsolidated sediments are know n from S-wave and surface wave measurements. Two methods are applied to com pute the H/V ratio, the classical method in the frequency domain and a meth od based on frequency-time analysis that allows us to locate P-SV wavelets in the time-series. The main problem in comparing synthetics with observati ons is the contribution of SH waves in the observed H/V ratios. We propose a method to minimize these effects and the effects of the superposition of different incoming P-SV waves. An inversion scheme is applied to the stable parts of the observed H/V ratio, based on a genetic algorithm, to retrieve the S-wave velocity structure from a single ambient vibration record.