Effects of density and velocity changes on the correlation of P-P and P-S reflection events

Alexandru Vant, R. James Brown

Assuming that the SEG polarity standard is followed, there will still be cases in which events on the P-P and P-S section have opposite apparent polarities, here called "the unusual situation". This situation makes the correlation and interpretation processes more difficult. To reduce this ambiguity we modelled seismic responses from a wide range of geologically plausible two-layer interface types using acoustic and seismic P- and S-wave velocities and densities measured at confining pressures equivalent to depths up to 2000 m.

The Zoeppritz equations were used to calculate the exact reflection coefficients for a 20 degree angle of incidence and elastic parameters corresponding to a depth of 1000 m. The results were displayed in plots of RPP versus RPS. In order to investigate the polarity consistency with offset for some of the interfaces mentioned above, we created synthetic velocity and density logs and used them to obtain pseudo-zero-offset gathers together and synthetic stacks.

The unusual polarity situation was found to be associated with geological situations where not all the rock parameters change in the same direction (e.g. velocities increase and density decreases) across the interface. Moreover, in these cases, the changes in elastic parameters are relatively small across the interface and the reflection coefficients are also small when compared to their theoretical range. Although there are exceptions, the polarity generally does not change with offset up to an offset-to-depth ratio of 1.

Later, we exemplify the PP-PS correlation problems that can be created by missing density or shear-wave-sonic logs and also show that the above unusual polarity conditions remain unchanged for interfaces situated at shallower (500 m) and greater (2000 m) depths.