Focusing energy

Our imaging condition provides a third idea for decreasing cost. As we go down in depth, energy focuses at zero offset. The outer offsets will either have no energy, or energy that we don't care about. As we step down in depth, we can decrease our offset domain. With this final savings, we can rewrite our cost function as:

$$C \approx \sum_{f} \sum_{nz(f)} nh(z)*nx log(nh) + nx*nh(z) log(nx) + CEXP(nx,nh(z))) .$$ (4)