Western Atlas International, Inc.

Goran Milan Stankovic

Uwe Karsten Albertin

A hybrid wavefront generator for bridging shadow zones for use with Kirchhoff migration. A graticule of travel-time grid points is mapped over a velocity model of an area of interest. The data assigned to each travel-time grid point are initialized using a unique code. By use of a conventional travel-time generator, wavefront travel times are calculated for the respective travel-time grid points. The calculated travel time, if valid, replaces the unique code previously resident therein. Wavefront travel times are re-evaluated for each travel-time grid point using a finite-difference travel-time solver. The quantity resident in each travel-time grid point is examined. If the quantity found is the unique initialization code, the unique initialization code is replaced by the re-evaluated finite-difference travel time. If a valid wavefront travel time is found, no further action takes place.

Method for extrapolating traveltimes across shadow zones

An array of output image points is distributed over a velocity model of a subsurface region of interest. A fan of rays is shot from each of a plurality of acoustic source points along the surface, the angular separation between the rays being equal to .pi./n, where n is the number of rays. A family of traveltime wavefronts is erected at selected time intervals, .DELTA.t, along the ray trajectories. The resulting wavefront map is superimposed over the array of output image points. Sub-arrays of output image points will lie within discrete traveltime cells, the widths of which are defined by the separation between adjacent rays paths and the heights are defined by the wavefront separation distance. Ray segments, wavefront segments and output image points are identified by a velocity layer index. A velocity interface that may cut across a traveltime cell, divides the cell into two parts, each having a different layer index. The traveltime to be assigned to a selected output image point is determined by projection from a nearby wavefront segment that has the same layer index as that of the output image point.

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Houston, TX, United States of America

Goran Milan Stankovic