Microtremor measurements used to map thickness of soft sediments

Abstract

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 (H/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 suggests 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 resonant frequency of the subsoil. The practical relevance of the investigation is illustrated by means of cross sections, constructed from results of the microtremor analyses, which provide a convincing image of the surficial structure of the areas investigated.

Evaluating landslide response in seismic and rainfall regime: A case study from the SE Carpathians, Romania

“…The thickness (h) of the loose/soft material is consecutively estimated using the shear-wave velocity (Vs) and resonance frequency (fo) using the following equation (Murphy et al 1971;Ibs-von Seht & Wohlenberg 1999); h=Vs/ (4*fo) Eq. 1…”

An Empirical Correlation between the Residual Gravity Anomaly and the H/V Predominant Period in Urban Areas and Its Dependence on Geology in Andean Forearc Basins

“…Depending on the geology, this may or may not correspond to the peak associated with the longest period. Additionally, the resulting predominant period (T 0 ) can be used to estimate the thickness of the soil deposit (H) combined with the harmonic mean shear-wave velocity (V s ) of the site using the Equation ( 1) (e.g., [26]). This relation assumes 1-D wave propagation, and just as in the case of gravity inversion, it involves a particular treatment in each city.…”

“…However, a correlation between both methods, if it exists, has not been thoroughly studied, and thus very little information is available on whether one method could be an estimator of the other. A large part of available studies deals with the relationship between H/V frequency and depth to the basement derived from other techniques, such as boreholes or other seismic data [26][27][28][29][30][31][32]. Some other authors incorporate gravity information in their studies, to estimate the sediment thickness, e.g., [21,23,24], but these studies are not orientated to determining a correlation between both techniques, but rather they use the information to reach other objectives.…”

Ambient vibration measurements in the Southern Rhine Graben close to Basle

“…vs Layer thickness vs power Avg. vs after Kind (2002) after Kind (2002) derived from f H/V law model Using the alternative power-law depth models given by Ibs van Seht and Wohlenberg (1999) and Parolai et al (2002), the depth estimates (110-220 m and 130-270 m, respectively) eastern direction. Finally, to the East of the fault zone two dominant peaks at high frequencies around 8 Hz suggest the existence of a shallow dominant seismic contrast, which was also recently validated by the S-wave seismic experiments (U. Polom, pers.…”