Reliable models of in‐situ shear‐wave velocities of shallow‐water marine sediments are important for geotechnical applications, lithological sediment characterization, and seismic exploration studies. We infer the 2D shear‐wave velocity structure of shallow‐water marine sediments from the lateral variation of Scholte‐wave dispersion. Scholte waves are recorded in a common receiver gather generated by an air gun towed behind a ship away from a single stationary ocean‐bottom seismometer. An offset window moves along the common receiver gather to pick up a local wavefield. A slant stack produces a slowness–frequency spectrum of the local wavefield, which contains all modes excited by the air gun. Amplitude maxima (dispersion curves) in the local spectrum are picked and inverted for the shear‐wave velocity depth profile located at the center of the window. As the window continuously moves along the common receiver gather, a 2D shear‐wave velocity section is generated. In a synthetic example the smooth lateral variation of surficial shear‐wave velocity is well reconstructed. The method is applied to two orthogonal common receiver gathers acquired in the Baltic Sea (northern Germany). The inverted 2D models show a strong vertical gradient of shear‐wave velocity at the sea floor. Along one profile significant lateral variation near the sea floor is observed.

Seismic velocities (Vp and Vs) of compressional (P‐) and shear (S‐) waves are important parameters for the characterization of marine sediments with respect to their sedimentological and geotechnical properties. P‐ and S‐wave velocity data of near‐surface marine sediments (upper 9 m) of the continental slope of the Barents Sea are analysed and correlated to sedimentological and geotechnical properties. The results show that the S‐wave velocity is much more sensitive to changes in lithology and mechanical properties than the P‐wave velocity, which is characterized by a narrow range of values. The correlation coefficients between S‐wave velocity and silt and clay content, wet bulk density, porosity, water content and shear strength are higher than 0.5 while the correlation coefficients of P‐wave velocity and the same parameters are always lower than 0.4. Although the relationship between Vs and clay content has been widely described, the data show that Vs is better correlated with silt content than with clay content for the sediments of the area investigated. However, they show different trends. While Vs increases with increasing clay content, it decreases with increasing silt content.

Four methods for the determination of Q in marine sediments are compared: two traditional methods, i.e. the risetime and the spectral ratio method, and two newly established ones, the spectrum modeling and the wavelet modeling method. In the latter one Q and the reflection time T are determined simultaneously, which gives a much better accuracy for T than reading it from the seismogram. The risetime and the spectral ratio methods are used for obtaining Q directly from the data. The principle of the modeling methods is to calculate the effect of absorption and dispersion on a reference wavelet or its spectrum for various values of Q, and the best fit between the observed and the calculated data leads to the optimum result. Numerical tests on synthetic data show that a precision of more than 25% for data containing noise or superposed arrivals can hardly be achieved; in any case, wavelet modeling is the superior method. Application to data from a vertical reflection profile in the Baltic Sea yields Q in the range of 15–100 for different layers, which is to be expected in the sedimentary environment of this area. The computations were performed in the Computer Center of Kiel University. The authors thank R. Meissner for his comments on the manuscript.

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ZusammenfassungDie Kenntnis der elastischen Eigenschaften mariner Sedimente, insbesondere die Scherwellengeschwindigkeit, ist eine wichtige Information für die Untersuchung der Stabilität von Meeresböden. Dies ist insbesondere für geotechnische Anwendungen im flachmarinen Bereich von Bedeutung, aber auch für die Untersuchung von Hangstabilitäten der Kontinentalrän-der und für die Gashydratforschung von Interesse.Seismische Experimente in der Ostsee sowie die Analyse eines Datensatzes aus der Nordsibirischen Laptevsee konnten zeigen, dass dispersive seismische Wellenfelder mit Hilfe einer nahe der Meeresoberfläche geschleppten seismischen Quelle (Airgun) angeregt werden können. AbstractThe knowledge of the shear wave velocity structure of shallow marine sediments is an important information for the assessment of sediment stability. This is of interest for marine geotechnical applications as well as investigations of sea-floor stability of continental shelves and margins and in gashydrate research. The derivation of seismic velocities with special interest on the shear wave velocity from the analysis and inversion of dispersive seismic waves is investigated in this work.Dispersive waves are excited by surface towed airguns and the acquisition was achieved with two different configurations. Firstly, the stationary-receiver method comprises of an ocean bottom seismometer station (OBS) and excitation with a surface towed airgun. Secondly, a towed-acquisition system includes a streamer towed at ten meters above the sea bed and excitation with an airgun. Two different wave types could be observed with either of the two acquisition systems. Interface waves, namely of the Scholte wave type, propagating along the sea floor and guided waves in the water column, denoted as acoustic guided waves despite of their sediment interaction at the sea bed. The both wave types were observed in certain environments indicating the limitations in the acquisition of these wave types depending on the sediment properties. These limitations as well as the difference in dispersion sensitivity to variations of the seismic properties give rise for separate treatment of both wave types. While the dispersion sensitivity of Scholte waves as well as other interface waves is dominant for shear wave velocity variations, the variation of the dispersion of acoustic guided waves is affected by variations of compressional and shear wave velocity as well as density to a smaller extend. The difference in velocity and frequency range of interest for both wave types, i.e. the ranges of most variations in the dispersion characteristics, also affect the required acquisition configuration and parameters. While a long offset recording (exceeding 800 m) is required for acoustic guided waves, the fulfillment of the spatial sample criteria sometimes limits the feasibility to adequately acquire the dispersive wavefield. This is most dominant for the Scholte wave measurements in very soft marine sediments.The derivation of the shear wave velocity structure was inve...