Elastic-wave sensitivity analysis for seismic monitoring

Effective and reliable reservoir monitoring is critically important for optimizing oil/gas production and ensuring safe geological carbon sequestration. It requires an optimal seismic sensor deployment that uses a minimum number of sensors to record the most significant information resulting from reservoir property changes. The monitoring survey has typically been designed using conventional seismic-wavefield illumination analyses. However, seismic-wavefield illumination cannot alone yield an optimum seismic acquisition for effectively monitoring reservoir property changes. We introduce a new approach for designing optimal seismic monitoring surveys by analyzing sensitivities of elastic wavefields with respect to reservoir property changes. The method is based on differentiating the elastic-wave equation with respect to geophysical parameters. The resulting equations are solved using a finite-difference scheme. Numerical studies demonstrate that seismic survey designs based on elastic-wave sensitivity analysis can be very different than those based on elastic-wavefield illuminations under the same optimal criteria. Sensitivity analyses can also be used to investigate whether a VSP or a surface seismic survey is more effective for monitoring P- or S-wave speed changes in a target region.