The Effect of Shallow Quaternary Deposits on the Shape of the H/V Spectral Ratio

In the last two decades, the horizontal-to-vertical (H/V) spectral ratio of seismic noise technique has been widely used for site-effect estimation and geophysical exploration through the soil fundamental frequency. Usually, only one peak is observed in the H/V spectral ratio, but in some cases, a second peak can also be obtained. Nevertheless, to date, the peaks at higher frequencies are rarely studied in detail. Geological and geophysical data are especially needed to better explain the presence of this second peak, which normally is neglected. An extensive survey of H/V measurements was conducted in the Llobregat river delta, located to the south of Barcelona. At most sites, two clear peaks were identified: one at low frequencies (< 1 Hz) and the other at higher frequencies (> 1 Hz). To understand this behaviour, a seismic noise array and active surface wave measurements have been conducted to obtain a shear-wave velocity profile (V (s)) up to the bedrock. Two impedance contrasts have been detected: the first one at a shallow depth and the second one between the soft sedimentary cover and the bedrock. During the modelling process, the theoretical H/V computed from the obtained V (s) models fits well with the experimental H/V peaks. The results from this study show that the structure of shallow quaternary layers can clearly change the shape of the H/V ratio, producing two clear peaks in some situations. In this case, the contact between the low-velocity clay layer and the gravels with a high seismic wave velocity produces a shallow impedance contrast related to the second peak observed in the H/V ratio. Comprehension of these secondary peaks could avoid a misreading of the soil fundamental frequency that could produce errors in a site-effect evaluation or in the calculation of the bedrock depth. Finally, we show that passive seismic techniques provide the quaternary overburden and bedrock geometry in urban areas and allow for the limitations of other geophysical techniques in these environments to be overcome.