Chapter 21 - Effect of Spatial Sampling on Time-lapse Seismic Monitoring in Random Heterogeneous Media

Abstract

Abstract From the viewpoint of spatial sampling in time-lapse seismic surveys, we reveal the effects of scattered waves on subsurface monitoring using a numerical simulation of the seismic wavefield and comparing the different responses of the final section by applying two different types of data processing: conventional CMP (Common Mid-Point) stacking and poststack migration. We consider a seismic waveform consisting of incoherent scattered waves generated by random isotropic heterogeneity, which is a noise-like wavefield caused by multiple scattering of seismic waves. We employed the finite-difference method for modeling a two-dimensional acoustic wavefield in random heterogeneous media. Our numerical experiments indicate that the highly dense spatial sampling does not improve resolution of the difference section between base and monitor surveys when the subsurface structure contains random heterogeneity. We demonstrate the existence of a small but significant difference by subtracting two sections with different spatial sampling. This small difference is attributed to two factors: (1) the truncation artifact which is due to geometrical limitation and that cannot be practically prevented during data acquisition and (2) the residuals due to improper amplitude balancing before subtraction. This difference causes numerous complicated events over the whole section in heterogeneous media even when the interval of spatial sampling becomes shorter than the Nyquist sampling interval and degrades seismic difference sections if the spatial sampling interval of the base survey is not identical to that of the monitor survey.