HR: 10:30h
AN: S32A-01    [Abstracts]
TI: How Forgetful are Seismic Waves ?
AU: * Milkereit, B
EM: bm@physics.utoronto.ca
AF: University of Toronto, Dept. of Physics, 60 St. George Street, Toronto, ON, M5S 1A7 Canada
AB: 3D surface seismic and vertical seismic profiling (VSP) techniques can be employed to image crustal structures in complex geological settings. The effects of heterogeneities on seismic wave propagation can be described in terms of different propagation regimes (Wu, 1989): quasi-homogeneous for heterogeneities too small to be seen by seismic waves, Rayleigh scattering, Mie scattering and small-angle scattering. These scattering regimes cause characteristic amplitude, phase and travel time fluctuation, which can be used to obtain estimates of scale length. Horizontal resolution of exploration seismic data is often discussed in terms of Fresnel zone. For surface and VSP data, the Fresnel radius increases with increasing depth of investigation. In addition, the lateral resolution is limited by the effective frequency content of the seismic signal. Based on strong contrast in petrophysical data, crustal exploration targets (such as gas-hydrates, permafrost or massive sulfide ores) should make strong P-wave, S-wave and converted wave reflectors against most background velocity models. In the context of realistic geological models, 3D numerical simulations are required to better assess elastic wave interactions with high acoustic impedance targets. In addition, it is important to study the influence of composition and shape of high acoustic impedance targets on the full scattered wavefield through a series of numerical modeling experiments based on the 3D elastic finite-difference (FD) method. Massive sulfide ores consisting of the end-member sulfide minerals pyrite, sphalerite, and galena, which span the full range of observed P- and S- wave velocities and densities in ore rocks, as well as gabbro inclusions, are investigated for different shapes which represent the complex morphologies often observed for ore deposits. 3D FD modeling reveals that large ore deposits lead to a strong and complex scattering response that is often dominated by shear-wave events (Bohlen et al., 2003). For example, the analysis of FD snapshots of forward and backward scattered compressional waves for a simple low velocity sphere (massive sulfide) shows scattering coda generated by the target and significant travel time delays for the direct (forward scattered) wavefield. At late travel times, however the direct wave "heals". The integration of petrophysical data and 3D elastic modeling studies demonstrate that directional scattering and wavefront healing place important constraints on how to handle static corrections and azimuth binning in 3D seismic datasets. References Bohlen, T., C. Mller and B. Milkereit, Elastic seismic wave scattering from massive sulfide orebodies: On the role of composition and shape, in: Hardrock Seismic Exploration, SEG, Tulsa, 86-102, 2003. Wu, R.J., Seismic wave scattering, in: Solid Earth Geophysics, Editor: D.E. James, 1166-1187, 1989.
DE: 0669 Scattering and diffraction
DE: 0689 Wave propagation (4275)
DE: 0900 EXPLORATION GEOPHYSICS
DE: 0935 Seismic methods (3025)
SC: Seismology [S]
MN: 2005 Joint Assembly