Converted-wave seismic exploration: A tutorial
Robert R. Stewart, James E. Gaiser, R. James Brown, Donald C. Lawton
Multicomponent seismic recording (measurement with vertical and horizontal geophones and possibly a hydrophone or microphone) captures the seismic wavefield more completely than with conventional single element techniques. In the last several years, multicomponent surveying has developed rapidly - allowing creation of converted-wave or P-S images that make use of a downgoing P wave converting to an upgoing S wave at the deepest point of penetration. Survey design for P-S surveys is similar to that of P waves, but must take into account the P-S wave's asymmetric raypath. P-S surveys use conventional sources but have several times more recording channels. Some special processes for P-S analysis include anisotropic rotations, shear receiver statics, asymmetric and anisotropic binning, shifted hyperbolic velocity analysis, P-S to P-P time transformation, P-S DMO, pre- and post-stack migration, and stacking velocity and reflectivity inversion for S velocities.
Current P-S sections are approaching (and in some cases exceeding) the quality of conventional P-P seismic data. Interpretation of P-S sections uses elastic ray tracing, synthetic seismograms, correlation with P-wave sections and depth migration. Numerous applications for P-S sections have arisen including sand/shale differentiation, carbonate identification, definition of interfaces with low P-wave contrast, anisotropic analysis, imaging through gas zones, and reservoir monitoring. Marine converted-wave analysis using 4-C recordings (a three-component geophone plus a hydrophone) has generated some remarkable images. Imaging through high-velocity layers (e.g. salt, basalt, permafrost) and in regions of significant structure also look promising. Development of the P-S method has taken about 15 years, but has now entered into its commercial phase.