The sedimentary structure in the Zafarraya basin, located in the south of Spain, is studied by using the horizontal-to-vertical spectral ratio (hvsr) for ambient noise. To improve the reliability of the results, the stability of the hvsr measurement was checked by using a time-dependent analysis of the noise records taking into account only clear peaks. The resonant frequency varies through the basin and does not ever coincide with the maximum of the horizontal power spectrum.
Several fits between the frequency of the main peak in the hvsr for microtremors and the depth to the bedrock are obtained for data from 17 sites, where geotechnical information was available. The relation derived using a scheme based on surface waves was considered to be the most reliable one. Thus, a function describing the average velocity versus depth for the sedimentary cover of the basin is also derived under assumptions of lateral homogeneity and smooth vertical variations of velocity due to age and/or confining pressure.
The measurement of microtremors at 86 uniformly distributed points provides a map of the sedimentary thickness based on the previously calculated relationships, showing values greater than 200 m at the deeper zones.