HR: 15:25h
AN: S32C-08    [PDF]
TI: Anisotropic Parameter Estimation From PP and PS Waves in 4-Component Data
AU: * Traub, B
EM: bt@bgs.ac.uk
AF: British Geological Survey, Murchison House, West Mains Road, Edinburgh, EH9 3LA United Kingdom
AU: * Traub, B
EM: bt@bgs.ac.uk
AF: University of Edinburgh, Grant Institute, West Mains Road, Edinburgh, EH9 3JW United Kingdom
AU: Li, X
EM: xyl@bgs.ac.uk
AF: British Geological Survey, Murchison House, West Mains Road, Edinburgh, EH9 3LA United Kingdom
AU: Scrutton, R
EM: Roger.Scrutton@ed.ac.uk
AF: University of Edinburgh, Grant Institute, West Mains Road, Edinburgh, EH9 3JW United Kingdom
AB: In recent years we have seen that in many cases anisotropy cannot be neglected in seismic processing routines. Furthermore we know that sea-bed properties play an important role in marine seismics, particularly for sea-bed multicomponent seismic, because they significantly influence the amplitudes, phases and travel times of seismic waves. \\ In this work we investigate how reliably we can extract anisotropy parameters from P- and PS-wave data using near-surface layer multicomponent data from the Alba Field, North Sea. Near-surface data is affected by apparent anisotropy due to a vertical velocity compaction gradient. Furthermore, we can see phase reversals in the pressure wave data set which will lead to erroneous anisotropy parameter values. Thus, the imaging of the target area is affected. We extract the P-wave anisotropy parameter $\eta$ (Alkhalifah and Tsvankin, 1995) which can be related to Thomsen's $\epsilon$ and $\delta$ formulation (Thomsen, 1986) by $\eta=(\epsilon-\delta)/(1+2\delta)$ from P-wave data and compare the result with reference values from a model, built according to well, VSP and surface data. As the parameters show differences up to an order of magnitude we use PS-wave data to estimate the effect of anisotropy. The effect of anisotropy on the converted waves in a single layer can be described by the parameter \begin{equation*} \chi=(\gamma_0-1)\gamma_{eff}^2\eta, \,\,\, with\,\,\gamma_{eff}=\frac{\gamma_2^2}{\gamma_0}, \end{equation*} where $\gamma_0$ is the vertical velocity ratio, $\gamma_2$ the rms velocity ratio and $\gamma_{\text{eff}}$ the effective velocity ratio. This equation is true for a single TIV layer. In this case $\eta$ can be calculated from $\chi$ and vice versa. Thus, estimating $\chi$ from converted-wave data we can compute $\eta$ for the near-surface events. The resulting parameters fit the reference values much better and show values of the same order of magnitude.
DE: 0900 EXPLORATION GEOPHYSICS
DE: 0935 Seismic methods (3025)
SC: Seismology [S]
MN: 2003 Fall Meeting