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This paper presents a new algorithm for calculating the Green's function
of first arrivals in two dimensions
using asymptotic ray approximation.
The traveltimes and amplitudes associated with the approximate
Green's function are computed on a regular grid using a finite difference
scheme. This algorithm is efficient and stable.
Examples with different types of slowness models show that
for the traveltime calculation, the algorithm can handle
very sharply-contrasted slowness models, and for the amplitude calculation,
the algorithm requires the slowness models to smooth.
I expect that the algorithm and its extension to 3-D will be useful
in Kirchhoff prestack depth migration and in traveltime tomographic
inversion. I also plan to apply this algorithm to the conversion of migrated
time sections to depth sections and to residual depth migration.

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** Previous:** Amplitudes
Stanford Exploration Project

12/18/1997