  Relation between dips of post-stack migrated and unmigrated data is well
known and easy to derive. A similar relation between dips of pre-stack migrated
and unmigrated constant offset data is not available in literature and is
calculated here. Since migration outputs for multiple offsets are available,
the formula valid for all offsets gives us a tool to check the correctness of
the migration velocity model. The formula is also used to study how a reflector
with constant dip gets curved in constant offset gathers. Further, a handy
formula for prestack migration aperture is derived and its variations with
offset, depth and dip angle studied.
