Variability of Scholte-wave dispersion in shallow-water marine sediments

Models of in situ shear-wave velocities of shallow-water marine sediments are of importance for geotechnical applications, sediment characterization, and seismic exploration studies. Here pseudo-2D shear-wave velocity models are inferred from the lateral variation of Scholte-wave dispersion at five different geological sites in the Baltic Sea (northern Germany). To explore Scholte-wave dispersion and the lateral variability of shear-wave velocities, Scholte waves were excited by air gun shots in the water layer and recorded by stationary ocean-bottom-seismometers or buried geophones. We analyze the recorded seismograms in a common-receiver-gather using offset-windowed, multichannel dispersion analysis. The observed local slowness-frequency spectra for the different study sites vary significantly with respect to excitation amplitudes and phase slownesses of different modes, as well as the excited frequency range. The excitation amplitudes are influenced by the local shear-wave velocity structure, absorption, length of Scholte-wave travel path, and the elevation of the source above the sea floor. The inverted shear-wave velocities range from 50 m/s to 600 m/s. Directly at the sea bottom, shear-wave velocities of 50 m/s for fine muddy sand and 300 m/s for glacial till were inferred. The maximum vertical gradient was + or -80% (mean 250 m/s) within a depth range of 40 m, and horizontally + or -33% (mean 350 m/s) within 300 m distance. The layer boundaries in the inverted shear-wave velocity models are in good agreement with high-frequency, zero-offset compressional-wave reflections. However, it was not possible to acquire the fundamental Scholte mode above very soft, unconsolidated sediment with shear-wave velocities smaller 50 m/s. The analysis of synthetic data shows that this is due to the elevation of the source and the receiver response function.