Microtremor measurements used to map thickness of soft sediments

The observations about the behavior of microtremor spectra presented here show that noise measurements can be used as a powerful tool to determine the thickness of soft cover layers. The most suitable method for this determination is Nakamura' s technique, which is the ratio of the horizontal-component noise spectrum and that of the vertical component (I-I/V spectrum). The frequency of the main peak in these spectral ratios correlates well with the sediment thickness at the site. Using an extensive database of microtremor measurements carried out in the western Lower Rhine Embayment (Germany), it was possible to show that this correlation is clearly valid for a wide range of thickness, namely, from tens of meters to more than 1000 m. A simple formula was derived that, for the sediments to be found in the area investigated, directly calculates the cover thickness from the frequency of the main peak in the H/V spectrum. A comparison with calculated resonant frequencies sug- gests the relation derived from the noise measurements depending on the velocity depth function of the shear wave. Classical spectral ratios are shown to be strongly influenced by the noise level and are therefore less reliable in determining the res- onant frequency of the subsoil. The practical relevance of the investigation is illus- trated by means of cross sections, constructed from results of the microtremor anal- yses, which provide a convincing image of the surficial structure of the areas investigated.