Converted (PS) waves can provide important information about shear-wave velocity and, in the presence of anisotropy, about the medium parameters responsible for both P- and S-wave propagation. Kinematics and amplitudes of reflected PS-waves, however, possess such undesirable features as moveout asymmetry, reflection point dispersal, and polarity reversal, which preclude application of conventional velocity-analysis methods to mode conversions.
Rather than using PS-wave kinematics directly, here we propose a method for reconstructing SS-wave reflection traveltimes from PP and PS data. The required preprocessing steps include picking of PP and PS traveltimes on prestack data and identification (correlation) of the PP and PS events from the same interfaces. The key idea of the method is to match the reflection slopes (horizontal slownesses) on common-receiver PP and PS gathers. This procedure allows us to find the coordinates of receivers that record PP- and PS-waves reflected at exactly the same (albeit unknown) subsurface points and to determine the shear-wave reflection traveltime tSS as a simple combination of the PP and PS traveltimes. The reconstructed SS-wave moveout can then be processed by velocity-analysis methods designed for pure reflection modes. The developed technique, however, cannot be used to compute the true amplitudes of the reflected SS-waves.
Our method has the following attractive features:
No information about the velocity field or anisotropic parameters is required to obtain SS-wave traveltimes from PP and PS data. If the input PP and PS arrivals are picked correctly and correspond to the same reflector, the method produces exact traveltimes tSS.
The estimates of traveltimes and reflection slopes are local, which makes reflection-point dispersal irrelevant.
Another consequence of the local nature of this procedure is that the portion of PS data in the vicinity of the polarity reversal (where the PS amplitudes are small) can be muted out without compromising the quality of tSS estimates for source–receiver pairs with high PS amplitudes.