Optimization Driven Model-Space Approach for Gas Clouds Using Full Waveform Inversion
Authors: Prajapati Srichand , Ghosh Deva P , Bin Wan Yusoff Wan Ismail 
Published: May 2016

Abstract
Exact quantitative information from seismogram based on full-wavefield modeling with non-linear wave equation method enhanced by computational cost and time over-burden, conventional full waveform inversion (FWI) method is a very challenging. Due to the limited accuracy, lack of adequate and accurate initial velocity model, and the approximate modeling of the wave-physics complexity, local minima does not prevent towards the convergence of misfit function. An alternative simultaneous-source encoding inversion technique is proposed. With the use of proposed algorithm, successfully recoverd the true geological information though inversion, however, the use of simultaneous source introduces the crosstalk artifacts which are efficient at higher frequencies and minimum at low frequencies, which we can clearly see in the model fit at top but it has minimal effect on whole dataset. However, we successfully recovered the true geological structure.

Introduction
Efficient computation of high capability in subsurface imaging, full-waveform inversion remains an elusive area of research. Full waveform inversion is based on the minimization of the misfit between observed and estimate data after each optimization to achieve satisfactory results with reduced misfit (Tarantola, 1984; 1986). The estimation of misfit and gradient are linearly dependant on each seismic source in the acquired data with a band-limited wavelet (like, a Ricker wavelet). Computational complexity of wavefield is proportional to the size of model and acquisition geometry, which simulate the over burden costs and time. Computational cost of full waveform inversion is a major issue to estimate the true velocity model for large-scale problem such as gas clouds, where the wave are attenuated and acoustic velocity decrease drastically with time. In the Malaysian (Malay) basin, the geophysical challenges are numerous, like imaging thin sands, imaging below gas clouds and below carbonates, understanding wave propogation in effective media. Gas clouds problems are very dominant in Malay basin and its challenges are well explained by Ghosh et al., (2010). An example of such imaging problem in East Malaysia production field, where in one part there is no gas leakage and perfect imaging (Figure 1a), where as in another part of the same field with similar structural play (Figure 1b) is a serious "Seal" breach in gas leakage resulting P-wave masking.
Use of OBC shear wave approach to solve the wipeout issue,1)Considering P-wave attenuation, using Q Migration (Reilly et al., 2008)2) Equivalent solution as 1) but using Scattering theory (Ghazali, 2011)3) Full waveform inversion improving P image
They manifest most commonly as a vertical "chimney' below where the seismic P-wave energy is wiped out making the reflectors with strong internal short period multiples and non-hyperbolic moveout, the probable possible causes are,a) Multiple scattering wavefield due to complicated faulting.b) Wells are difficult to penetrate in gas clouds due to poor seismic imaging and lack of true geological knowledge.c) Few attentation was made using 2D acoustic full waveform inversion to image gas cloud (Prieux et al., 2009)