Reflection of seismic waves from attenuating and anisotropic ocean bottom sediments [thesis]

Rolf Sidler, Domenico Giardini, Klaus Holliger
2008
There is an increasing trend towards recording marine seismic data directly on the seafloor. This acquisition strategy is mainly motivated by the possibility to simultaneously measure the three components of particle motion in addition to the pressure in the water column immediately above the seabed. Such four-component (4C) seismic recordings thus allow for the recording of S-waves in marine environments and offer the prospect of decomposing the wavefield into its up-and down-going P-and
more » ... constituents. The assumptions for acquisition and processing of 4C data is today based on the ocean bottom model as a welded acoustic-elastic contact at the seabed with a homogeneous acoustic layer overlying a homogeneous elastic half-space. This may not hold in wide areas of the oceans where the seafloor typically consists of soft, water-saturated sediments characterized by having strong to very strong seismic attenuation. Moreover, cyclically changing sedimentation processes lead to layering in the sediments, thus introducing macroscopic seismic anisotropy, and overburden pressure and associated compaction effects are likely to result in a strong velocity gradients. The primary objective of this thesis was to gain a deeper understanding of the fundamental physical processes governing seismic wave propagation in such environments and to evaluate the implications arising for the acquisition and processing of 4C seabed seismic data. To this end, the effects of strong attenuation, anisotropy due to very fine layering, and pronounced velocity gradients have been investigated using advanced seismic modelling techniques. In the first part of this thesis reflection coefficients obtained with the frequency slowness method, using a pseudo-spectral solution of the anisotropic visco-elastic wave equation, are compared with analytic reflection coefficients, calculated using the plane-wave approach. The ocean bottom is modeled as a specular interface separating a viscoacoustic medium (water) and a viscoelastic solid (sediments) characterized by transverse isotropy with a vertical symmetry axis (VTI). The algorithm uses one grid for the fluid layer and another grid for the solid half-space and employs Fourier and Chebyshev differential operators in the horizontal and the vertical directions, respectively. The visco-elastic stressstrain relation is described by a Zener model. Special attention is given to the boundary conditions at the ocean bottom. For this purpose, a special domain-decomposition technique for wave propagation at the fluid/anelastic-anisotropic-solid interface was further developed. The examples considered in this study cover a water-steel interface, for which experimental data is available, a soft water/sediment interface and a stiff water/crustal rock interface. An additional finding was that the analytical plane-wave reflection coefficients may exhibit non-physical jumps and discontinuities the reasons for which were unknown. I therefore decided to investigate these non-physical jumps in more detail. To this end, the modeling code was further extended to the more general case of two VTI viscoelastic solids in welded contact and the numerical results for the reflection coefficient were compared to the corresponding analytical plane-wave solutions. The numerical solution was used to identify the most probable plane-wave solution. It could be shown that the nonphysical jumps were related to non-continuous evaluation of the complex square roots associated with the vertical slowness in the analytic plane-wave reflection coefficients in attenuating media. I found that this problem can be effectively alleviated by following the
doi:10.3929/ethz-a-005762773 fatcat:a5s3ccxtw5fp3hrmm2hmtxvxse