A multi-component seismic experiment was conducted by deploying a 4-C ocean-bottom cable in a shallow water canal. The aim of the experiment was to investigate the feasibility of acquiring shear-wave information in shallow marine environment. In the recorded data, we observe airgun generated shear-wave reflections. This observation is validated by a model study. The presence of shear-wave reflection is caused by the low frequency of the source in relation to the water depth. Furthermore, converted waves have also been identified based on two "attributes" namely the particle polarization and the normal moveout velocity. Comparison with synthetic data suggests the presence of waves converted at a reflector as well as at the water bottom.
Shear-wave information is often necessary for the characterization of the shallow subsurface and for the retrieval of geotechnical parameters. During the last three decades, it has been shown by many that the S-wave velocity is much more sensitive to changes in lithology and mechanical properties than the P-wave velocity. In marine setting, the S-wave information can be obtained from seismic measurements. Different methods exist towards this end. Gerhmann et al. (1984) developed a system which directly generates and records horizontally polarised Swaves in shallow water areas. The system consists of a modified airgun capable of generating horizontal shear stress and a 3-component geophone system placed at the water bottom. The authors showed an example of marine shear-wave refraction profiling in the Baltic Sea. Although this system proved to be useful, it is has not been much used.
For structural mapping P-waves are still the most successful in marine environment. Therefore people have been inverting for the S-wave velocity from PP-reflections. However, Riedel et al. (2001) showed that large uncertainties are involved in the estimation of the S-wave velocity. These uncertainties are attributed to the insensitivity of the P-wave reflection amplitude to this parameter in shallow marine sediments.
Another approach to obtain S-wave information has been proposed by Caiti et al. (1994) and it involves recording surface waves using a receiver array laid down on the water bottom sediments. The recorded surface waves are analysed and inverted to obtain S-wave profiles (Caiti et al. 1994 and Park et al., 2000). However, the depth of these profiles is limited and the accuracy and resolution are depthdependent. The S-wave velocity can also be retrieved from converted waves i.e., waves converted from compressional to shear mode. In hydrocarbon exploration, these waves have been applied successfully for many years. For shallow marine environment, modeling studies showed that there are two angles where maximum S-wave conversion can be expected (el Allouche et al. 2008), one at moderate angles (between 40 and 50 degrees) and one at post-critical angels. An appropriate way to detect converted waves is to use an ocean-bottom cable configuration.
In this abstract, we show the results of a multi-component seismic experiment conducted by deploying a 4-C oceanbottom cable in a shallow water canal. An analysis of the components suggests the presence of source generated shear-waves as well as converted waves in the data.
