Conclusions

We have shown that the minimum-phase helical factorization of the Helmholtz equation can be applied recursively to extrapolate waves up to 90$^\circ$. The hope is that a recursive extrapolator may image steeper dips than an explicit extrapolator with the same cost. Unfortunately, however, technical problems related to the factoring of functions with roots near the unit circle, are currently preventing this method from competing effectively with conventional extrapolation algorithms.