Full Elastic Waveform Inversion: Future of Quantitative Seismic Imaging

eismic reflection data are acquired at a very high cost. Conventional processing (stacking<br>and migration) provides very high-quality image of the sub-surface, but does not provide<br>quantitative measure of the physical properties of the sub-surface. Amplitude versus offset<br>(AVO) analyses can be used to estimate P and S-wave impedances. Since the method is local,<br>i.e. assumes 1D media, linear approximation to the reflection coefficient, and ignores<br>interference effects, the results are very approximative. Over the last ten years, we have<br>developed a suite of 1D and 2D full waveform inversion. We have particularly focused on the<br>use of wide-aperture data, containing near- and post-critical angle reflections, which helped to<br>constrain medium scale features of the velocity model, allowing convergence towards the<br>global minimum. The algorithm has been applied to surface seismic reflection, ocean bottom<br>cable and walk-away VSP data. Both vertical (Vz) and horizontal (Vx) particle velocity<br>records aree used, allowing fine-scale estimation of both P- and S-waves velocities.<br>The 2D elastic waveform inversion scheme is based on Shipp and Singh (2002) and<br>Freudenreich et al. (2002), utilizing a finite-difference solution to the 2D elastic wave<br>equation (Levander, 1988) operating in the time-distance domain. The aim of the scheme is<br>to model shot gathers accurately, and to use the residual between observed and modeled<br>wavefields to update the velocity model appropriately using a conjugate gradient method.<br>Both Vp and Vs may be inverted for, whilst density is coupled empirically with Vp.