Data example and modelling study of P-wave azimuthal anisotropy potentially caused by isotropic velocity heterogeneity

Azimuthal variation in traveltimes is increasingly being used as a tool for reservoir characterization and fracture detection. One issue in interpreting the results of azimuthal normal moveout (NMO) analyses is to determine whether the apparent observed anisotropy is due to intrinsic azimuthal anisotropy or, in reality, is primarily caused by lateral velocity variations. A field data example is presented where lateral heterogeneity is suspected as the main cause of an observed azimuthal NMO anomaly. Modelling studies of isotropic media containing lateral velocity variations were performed to investigate the possibility that the observed anomaly in the field data could be caused solely by isotropic lateral velocity variation. In both the modelled and field data, the patterns observed in the apparent velocity anisotropy are very distinctive with obvious correlations between ellipse parameters (fast and slow velocities and azimuth of the fast velocity) and the structure of the lateral velocity variation. The inverted RMS and interval NMO ellipses also show a very high degree of correlation both spatially and temporally, extending below the heterogeneous layer.