The first author thanks Nicola Bienati, Paul Fowler, and Ilya Tsvankin
for fruitful and stimulating discussions. A special acknowledgment
goes to Giuseppe Drufuca and the members of the DSP(GEO) research
group at Politecnico di Milano for their support. The second author
acknowledges partial financial support from ExxonMobil and
RPSEA and thanks Tariq
Alkhalifah, Will Burnett, and Yang Liu for helpful
discussions. Both the authors are indebted to Alexey Stovas and
the anonymous reviewer for their excellent reviews. Their suggestions
and insightful remarks have improved the clarity of the paper.