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If we assume that the slowness model has a constant value *m*_{0} in the region
around source position (*x*_{s},*z*_{s}), we can use the following initial
conditions for the local paraxial ray method. The traveltimes are
The ray directions are specified by the angles of rays
to the vertical, as follows:
We can also find that
and
If the slowness varies in this region, the paraxial ray method should be used
to find the initial conditions.

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Stanford Exploration Project

12/18/1997