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When coherent light from a laser beam is passed through a transparent reduction of a variable-density or variable-area record section, the seismic signals act as an optical grating to produce a diffraction pattern which is the two-dimensional Fourier transform of the section itself. By using suitable lenses, the diffraction pattern can be converted into an image of the original section. By obstructing portions of the pattern corresponding
to particular frequencies or dips on the section, one can remove such frequencies or dips from the reconstructed image. This is the basis for a new technique of optical filtering which United Geophysical Corporation has designated as LaserScan.
With this optical processing technique, the first step is an examination of the spectrum of the data. This technique lends itself well to the analysis and evaluation of the information on the seismic record sections being processed. Such a study commonly suggests optimum positions for filtering settings. Examples are given of this, and of the effects of some of the suggested filtering.
A number of seismic record sections are shown before and after filtering. The various examples show the enhancement of reflection data previously confused or concealed by the undesired events which are filtered out during the optical processing. Unwanted events are rejected by taking advantage of the differences between their dips (move out) and (or) frequencies and those of the desired reflections. Examples are discussed in which LaserScan techniques have been applied:
Reject multiple reflections which override or obscure genuine reflections dipping in different directions.
Remove diffractions and (or) reflected refractions.
Attenuate high velocity noise events.
Eliminate high frequency interference. LaserScan is very effective as a frequency filter because of the sharp cut-off slopes obtained.
Because hundreds of information channels can be processed in a single photographic operation, optical filtering has proved to be an efficient and economical method of frequency and velocity filtering.
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