Scholte-wave tomography for shallow-water marine sediments

We determine the 3-D in situ shear-wave velocities of shallow-water marine sediments by extending the method of surface wave tomography to Scholte-wave records acquired in shallow waters. Scholte waves are excited by air-gun shots in the water column and recorded at the seafloor by ocean-bottom seismometers as well as buried geophones. Our new method comprises three steps: (1) We determine local phase-slowness values from slowness-frequency spectra calculated by a local wavefield transformation of common-receiver gathers. Areal phase-slowness maps for each frequency used as reference in the following step are obtained by interpolating the values derived from the local spectra. (2) We infer slowness residuals to those reference slowness maps by a tomographic inversion of the phase traveltimes of fundamental Scholte-wave mode. (3) The phase-slowness maps together with the residuals at different frequencies define a local dispersion curve at every location of the investigation area. From those dispersion curves we determine a model of the depth-dependency of shear-wave velocities for every location. We apply this method to a 1 km(2) investigation area in the Baltic Sea (northern Germany). The phase-slowness maps obtained in step (2) show lateral variation of up to 150 per cent. The shear-wave velocity models derived in the third step typically have very low values (60-80 m s(-1)) in the top four meters where fine muddy sands can be observed, and values exceeding 170 m s(-1) for the silts and sands below that level. The upper edge of glacial till with shear-wave velocities of 300-400 m s(-1) is situated approximately 20 m below sea bottom. A sensitivity analysis reveals a maximum penetration depth of about 40 m below sea bottom, and that density may be an important parameter, best resolvable with multimode inversion.