The geology of the Weser river terraces to the south of Hamelin is characterized by Quaternary sediments, consisting of thin alluvium and several meters of sand and gravels, overlying the seismic bedrock. We investigated a study area along the left Weser bank south of the village of Kirchohsen using different ambient seismic vibration techniques. Borehole logs show that the thickness of the Quaternary sediments varies from around 15 m to 33.5 m.
The fundamental frequency at a measurement point can be determined with the H/V method, which is based on the ratio between the horizontal and vertical spectra of the ambient seismic vibration signals. In the study area, the resonance frequency varies from 2.6 to 6.3 Hz, where low frequencies coincide with thicker Quaternary sediments and higher frequencies with thinner sedimentary layers. Assuming a continuity of the geological units, we can directly link the H/V frequencies to the sedimentary thickness and map the transition zone between the boreholes by H/V measurements along several profiles and additional individual points. We complement these H/V measurements by a new passive seismic array measurement and the reanalysis of older measurements in order to retrieve the dispersion curves of Rayleigh and Love waves and invert them for the shear-wave velocity profile.
The combination of the different data provides a detailed overview of the lateral changes within the shallow underground structure of the sedimentary layers of the study site.