Migration Algorithm

We use a 2-D Extended Split-Step prestack depth migration algorithm Stoffa et al. (1990) with three so-called ``reference velocities'' to handle lateral velocity variation. Image sampling in depth is 16 meters, which is probably a bit coarse for the regions of the image above the basalt, but adequate for sub-basalt imaging.

The migration algorithm outputs angle domain common image gathers (ADCIG) for velocity analysis. in fact, the ``angle'' axis here is parameterized in terms of offset ray parameter, p_h, which is linked to reflection opening angle $\theta$ by the following relationship Prucha et al. (1999).  

$$\frac{\partial{t}}{\partial{h}} = p_{h} = \frac{ 2 \sin{\theta} \cos{\phi}}{V(z,\bold m)}$$ (1)

$\phi$ is the reflector dip angle, and $V(z,\bold m)$ is the interval velocity in the neighborhood above the reflector point. Sava and Fomel (2000) show an efficient method for direct computation of ADCIGs as a function of opening angle. In both cases, the ADCIGs are easier to interpret for angle-dependent phenomena than are offset-domain common image gathers, a fact that we feel will improve our ability to identify converted waves in the presence of complex multiples and refracted waves.

We summarize some important parameters for the migration below:

• Input data CMP spacing: binned to 25 m (from nominal of 12.5 m).
• Image CMP sampling: 1024 samples @ 25 m (26 km total).
• Image Depth sampling: 325 samples @ 16 m (5.2 km total).
• Frequencies imaged: 3-34 Hz.
• ADCIG sampling: 120 samples @ $8\times10^{-6}$ sec/m.