HR: 0800h
AN: S31B-1054    [Abstracts]
TI: Modelling Borehole Wave Propagation using a 3D Variable-Grid Finite-Difference Scheme
AU: * Toomey, A
AF: Earth Science Division, Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley, CA 94720 United States
AU: Daley, T M
AF: Earth Science Division, Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley, CA 94720 United States
AU: Wang, X
AF: CSIRO Petroleum, ARRC, PO Box 1130, Technology Park, Bentley, WA 6102 Australia
AB: Generating accurate synthetic seismograms for wave propagation in a three-dimensional fluid-filled borehole is difficult using conventional uniform-grid finite-difference schemes. The low-velocity modes generated in the borehole require very fine sampling to avoid numerical dispersion. The need for accurate discretisation of the borehole and casing also calls for fine grid spacing. In uniform-grid finite-difference schemes this fine grid spacing must be used throughout the model, which restricts such models to small dimensions, unrealistic velocity structures, or long wavelengths due to computer memory limitations. Using a variable-grid finite-difference scheme improves computational efficiency by partially avoiding oversampling of the wavefield in high velocity materials. The grid spacing is tailored to the velocity model, allowing the use of fine grid spacing inside the borehole and coarser grid spacing in the rock formation. We have implemented the variable-grid scheme of Pitarka (1999) in a parallel, 3D anisotropic finite-difference code, allowing large models with complex velocity structure to be tackled. First, the accuracy of this scheme for simple plane-layered geometries is verified by comparison with results obtained using the pseudo spectral method. The suitability of the scheme for acoustic logging problems is then assessed: finite-difference results for monopole and dipole sources in an open borehole are compared with analytical results obtained using the real axis integration method. Finally, the method is applied to an investigation of the modes generated in a borehole penetrating a fractured zone. Data from single well seismic surveys has suggested that this method holds promise for locating gas-filled fracture zones (Majer et al, 1997). An anomalous event observed in single-well data from the Newberry well is thought to indicate the presence of a fractured zone at some distance from the well. We use the variable-grid finite-difference scheme to investigate whether a fractured zone may act as a type of low-velocity wave guide which traps energy, giving rise to the observed high amplitude low-velocity arrivals.
DE: 7260 Theory and modeling
DE: 5104 Fracture and flow
DE: 0902 Computational methods, seismic
DE: 0915 Downhole methods
SC: Seismology [S]
MN: 2004 AGU Fall Meeting