COMMON IMAGE GATHER APERTURE MUTE

Recording geometries limit the maximum offset ray parameter. However, our migration can put energy in part of the image gathers beyond this maximum aperture. Therefore a mute needs to be applied to this part of the image gathers. The maximum P_h is a function of the coefficients on the right side of equation (1) and is given by equation (3):

$$P_h(max) \; = \; \frac{2 \tan^{-1}(\frac{h_{max}}{z})\cos \phi_{max}}{V(z,{\bf m})} \;,$$ (3)

where h_max is the maximum half offset, $\phi_{max}$ is the maximum expected geologic dip, z is depth, and $V(z,{\bf m})$ is the midpoint interval velocity function. This simple relation is a reasonable approximation, but to truly define the migration aperture, ray-tracing needs to be used to account for the wavefield path above the reflector. Figure 7 shows identical image gathers, with the right panel having the mute applied. As expected, the deeper reflectors have a reduced maximum P_h.

The main use of this mute is to zero points outside of the migration aperture so that they are not used in our least-squares fit to find AVO intercept and AVO gradient Gratwick (2001).

 

ph_mute
Figure 7 Identical image gathers. Right shows the result of the P_h mute.