R We There Yet?

I compare stress and strain sensitivity of vertical seismic velocity using two rock physics models. Third order elasticity relates tensor strain changes to changes in anisotropic velocity. The R-factor rock physics model (linking vertical velocity change to change in vertical strain, Hatchell and Bourne, 2005) can be derived from third order elasticity by neglecting horizontal strain changes and subsequent Taylor approximation. In the overburden, the influence of horizontal stress and strain changes on vertical velocity increases the predictions for the R-factor. Inside the reservoir the influence of horizontal stress changes decreases estimates for the R-factor. This may explain the observation that the R-factor in field observation of time-lapse seismic data is consistently larger in the overburden than in the reservoir.