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The purpose of Dip Move Out is to replace a nonzero offset data set
by a zero offset data set. Both data sets are to be equivalent in the
sense that both data sets result in the same subsurface picture after
migration.
The prestack migration response to a nonzero offset impulse is the locus
Q(x,z) for which all traveltimes SQR are equal. Such a locus is called
isochrone. In a constant velocity medium an isochrone corresponding to
the twoway traveltime t_{n} is an ellipse with its foci at the location of
source and receiver. The major and minor axes (A and B) of the ellipse are
 
(10) 

B^{2} = A^{2}h^{2}.

(11) 
The ellipse equation is
 
(12) 
The poststack migration response to a zero offset impulse of time t_{z} is
the wavefront at the time t_{z}/2 after initiation. In the case of constant velocity
medium it is a circle with midpoint at the source receiver position B(b,0)
and radius t_{z}V/2:
 
(13) 
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Stanford Exploration Project
11/18/1997