Dip-moveout correction of PS data
In principles of dip-moveout correction we learned that the DMO impulse response is in the form of an ellipse (Figure 5.1-12). We also learned that the aperture of the DMO operator depends on the reflector dip, the source-receiver separation, the reflection time of moveout-corrected common-offset data, and the velocity above the reflector. Since shear-wave velocities are generally lower than compressional-wave velocities, given that all other factors are the same, the DMO impulse response associated with the PS data will be different in shape as compared to the DMO impulse response associated with the conventional PP data shown in Figure 5.1-12. Specifically, the slower the velocity the more the action of the DMO operator (principles of dip-moveout correction). This means that the symmetric form of the DMO ellipse associated with the PP data is replaced with an asymmetric form as shown in Figure 11.6-38 . Additionally, the resulting curve is shifted laterally to the CCP location indicated by the dotted trajectory in Figure 11.6-38b (Alfaraj, 1993).
Figure 11.6-38 Impulse responses of a DMO operator for (a) PP data, and (b) PS data (adapted from Alfaraj, 1993) for a given source (S) and receiver (R) separation. The horizontal axis is the midpoint axis and the vertical axis is the event time after NMO correction. The dotted curve represents the common-conversion-point (CCP) trajectory as in Figure 11.6-31b.
Figure 11.6-39 (a) Common-midpoint (CMP) raypath geometry for PP-data, and (b) common-conversion-point (CCP) raypath geometry for PS-data. Adapted from .
Figure 11.6-32 Geometry of a common-conversion-point (CCP) raypath used to derive the reflection traveltime equation (74) for the PS-wave.
The PP DMO impulse response for variable velocity v(z) increasing with depth can be formed by squeezing the PP DMO impulse response for constant velocity . Similarly, the asymmetric shape of the PS DMO impulse response shown in Figure 11.6-38b may be formed by squeezing the PP DMO impulse response on the source side and stretching it on the receiver side (Alfaraj, 1993).
Another important difference between the DMO corrections of the PP and PS data is in respect to the reflection-point dispersal (Section E.1). We learned, again in principles of dip-moveout correction, that the DMO correction removes the reflection-point dispersal along a dipping reflector. With the PP data, reflection-point dispersal is an issue only in the case of a dipping reflector. However, with the PS data, reflection-point dispersal takes place even for the case of a flat reflector. This is illustrated in Figure 11.6-39 where the raypaths of Figure 11.6-32 have been adapted to the CMP geometry. Note that you need to apply DMO correction to the PS data even in the absence of dip to remove the reflection-point dispersal (Alfaraj, 1993). Expressed differently, implicit to the PS DMO correction is CCP binning that involves mapping of amplitudes along the dotted trajectory in Figure 11.6-38b.
- den Rooijen, 1991, den Rooijen, H. P. G. M., 1991, Stacking og P – SV reflection data using dip moveout: Geophys. Prosp., 39, 585–598.
- Hale and Artley, 1992, Hale, D. and Artley, C., 1992, Squeezing dip-moveout for depth-variable velocity: Geophysics, 58, 257–264.
- 4-C seismic method
- Recording of 4-C seismic data
- Gaiser’s coupling analysis of geophone data
- Processing of PP data
- Rotation of horizontal geophone components
- Common-conversion-point binning
- Velocity analysis of PS data
- Migration of PS data