# Gamma effective

Gamma effective () is the "effective velocity ratio". In C-wave analysis, the C-wave conversion point (the CCP) is *not* at the midpoint (as with P-waves), but is at a point determined by Snell's law, at the reflector. Its location depends upon the velocity structure in the overburden, and the depth to the reflector. At great depth (relative to the total source-reciever offset) the CCP position tends asymptotically to the Asymptotic Conversion Point (the ACP).For 1-D (flat-lying, but vertically inhomogeneous and anisotropic) media, the ACP has a simple formula
^{[1]}:

where the *effective* velocity ratio is:

and the *moveout* velocity ratio is the ratio of P- and S short-spread moveout velocities:

and the *vertical* velocity ratio is the ratio of P- and S- vertical velocities.

## Reference

- ↑ Thomsen, L., 1999. Converted-Wave Reflection Seismology over inhomogeneous, anisotropic media, GEOPHYSICS, 64(3), 678-690.