Polarity flip

The polarity inversion visible at zero-offset in converted wave data (PS) is an intrinsic property of the shear wave displacement Danbom and Domenico (1988). In a constant velocity medium, the vector displacement field produces opposite movements in the two geophones at either side of the source (Figure 1). This leads to the polarity flip in the seismic gather.

 

pflip Figure 1 Polarity inversion in converted waves seismic data. +g and -g correspond to positive and negative polarity in a common shot gather. Modified from Tatham and McCormack (1991).

For media with more complex velocity, the normal incidence ray determines the location of the polarity flip. For flat reflectors in v(z) media, and in areas with constant $\gamma$, the normal incidence ray emerges at the surface at zero-offset. However, in general, the P and S-wave ray paths corresponding to the normal-incidence (zero-amplitude) ray will not necessarily emerge at the surface at the same point. Figure 2 illustrates this for the case of a dipping layer and a non-constant $\gamma$.

This path deviation produces a polarity reversal at non-zero offset in the data space. In areas of complex structure, the picking of this polarity flip point is difficult; however, in the angle domain (model space), this point is a uniquely determined function of the P-velocity, S-velocity, and reflector dip; therefore, it is easy to correct the polarity flip in the model space.

 

pflip2 Figure 2 Polarity flip problem for a dipping layer and a non-constant $\gamma$.