Scholte Waves in Seabed Mapping
MetadataVis full innførsel
- Master theses 
With the growing demand for renewable energy solutions in the world, floating windmills are said to be very effective and environmental friendly for ensuring future’s energy demands. Hence, robust methods for evaluating the mechanical properties of seabed sediments, e.g. for mooring and stability of offshore installations, will be necessary. Knowledge about the elastic properties of the near-surface seabed, how the seabed changes its elastic properties based on water filling and how to measure the elastic properties of the seabed may be important. Additionally, methods for determining the mechanical properties of the seabed sediments may be important to understand how noise and vibrations from moored floating windmills spread along the seabed, and potentially leaking into the ocean. In this thesis, the main objective is to present and model surface acoustic Scholte waves propagating at the fluid-solid interface. We use the rock physics based differential effective medium (DEM) model to investigate how the elastic properties of water filled silicates change as introduced to water inclusions in incremental manners. Then, we model the acoustic wavefields (i.e. negative pressure or vertical particle velocity) for an environmental model given in Johansen et.al. [1, table 2], added a solid sea ice layer on top. The wavefields are modeled using the wavenumber integration method Ocean Acoustic and Seismic Exploration Synthesis (OASES), which calculates the depth-dependent Green’s function for a selected number of frequencies and determines the transfer function at any receiver position by evaluation the wavenumber integral in cylindrical coordinates. The modeled wavefields are then compared to results from a seismic experiment conducted in Van Mijenfjorden, Svalbard in 2018. Due to aliasing effects, increasing resolution in the frequency-wavenumber domain may be important for group- and phase velocity determinations. Hence, a small study of the impact of reducing the receiver intervals, holding the total array length constant, is done. Results from this study illustrates that the νp/νs-ratio of water filled silicates are orders of magnitude higher than more compact sediments, i.e. water filled silicates looses its shear strength as water inclusions are added. Seismic survey in shallow water show prominent Scholte wave data when using an airgun submerged to water at 4 meter depth, measuring the response at the seabed.