Shear‐wave velocities in shallow marine sediments

Abstract

Shear-wave velocities in shallow (0 to 130 m deep) marine sediments are obtained from offshore Brazil. The data, acquired in-situ, consisted of both direct velocity measurements and indirect (geotechnical) predictions. Expressions to correlate VS and depth for shallow marine sediments were derived empirically. We find that VS=48z 0.44 (z is in m and VS in m/s) fits the data closely. This equation describes the sediment shear velocity in the 130 m directly below the ocean floor. This expression predicts values similar to those by Hamilton (1976), but matches very shallow data more closely.

Characterization of seabed properties from Scholte waves acquired on floating ice on shallow water

“…The estimated V p / V s ratios are orders of magnitude higher than usually encountered in seismic analysis. This is evidence of very loose sediments and consistent with other studies (Ayres and Theilen ; Rodrigues‐Suarez and Stewart ). Considering the sediments made up of silt, clay and water, a two‐step DEM model was used to convert inverted S‐wave velocities to porosity data.…”

“…The reason for the latter is that, in order to maintain the porosity, the water‐filled porosity of the clay needs to increase when porous clay is replaced by silt grains, which have no porosity. The S‐wave velocity modelling is consistent with data of Ayres and Theilen () who studied mechanical properties of seabed samples from the Barents Sea, and Rodrigues‐Suarez and Stewart () who undertook direct S‐wave velocity measurements of the upper 130 m of seabed sediments outside Brazil. The modelled S‐wave velocities versus porosity curves in Figure now facilitate to predict the porosities of the upper sediments using the S‐wave velocities inverted from the Scholte waves.…”

Shear wave statics using receiver functions

“…We focus on seabed seismic data because both source‐side and P ‐wave statics are considered insignificant for this environment. More importantly, since the shear wave velocities in the seabed are extremely low, with V p / V s velocity ratios up to 10 in the upper 25 m (Hamilton 1976; Rodriguez‐Suarez & Stewart 2000), the events in the receiver function are dominated by the shear wave traveltime and hence are a direct measure of the S statics. When the P ‐wave traveltime through the near surface cannot be neglected, the P statics must be found separately and added to the traveltime differences given by the receiver functions.…”