Porosity - Lithology From Shear Seismic
Complex transmission and mode conversion phenomena occur at the interface between two media, as compressional or shear wave energy passes through it. When a compressional wave strikes an interface, the incident energy is distributed over four distinct waves:
   1. transmitted compressional wave, Pt
   2. reflected compressional wave. Pr
   3. converted transmitted shear wave, PSt
   4. converted reflected shear wave, PSr

 


Reflected and transmitted wave modes

The amplitude of each of these components versus incidence angle is shown below. Notice the dramatic change in reflected energy at angles above the critical angle.

If an S-wave reaches the interface, converted S-waves, SPt and SPr, are also created. For seismic surveys, SH is the component of the shear wave (SPr) perpendicular to the vertical plane containing the seismic line. SV is the component in the plane. The direction of particle motion for the various modes is shown below.


Reflected and transmitted wave modes

When SH is in the interface plane, there is no conversion of SH-waves into P- and SV-waves and inversely. This is why SH seismic records are more simple as a rule than the P or SV records.

Interference from shear waves on conventional CDP seismic stacking is avoided by adequate velocity analysis, since shear waves are much slower than compressional and have much higher normal moveout. The typical velocity regime is shown below.


Shear amplitude and velocity

By suitably gathering and velocity filtering seismic traces, the compressional and shear arrivals can be separated from each other. The interval velocity from compressional and shear sections can be computed from the stacking velocity of each. Poisson's ratio can be computed and displayed from:
_____1: PR = ((Vc / Vs)^2 - 2) / (2 * (Vc / Vs)^2 - 1)
 

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