VELOCITY AND FREQUENCY FILTERING OF SEISMIC DATA USING LASER LIGHT

Abstract

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. With suitable lenses the diffraction pattern can be converted back 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. The equipment developed for this processing incorporates special design features to combine high optical resolution, precise discrimination of moveouts and frequencies, limitation in the length of the overall optical path to permit the use of a short optical bench, and visual monitoring by use of a microscope or a closed‐circuit TV system. Filter elements consist of wedges mounted on a rotary stand for velocity rejection, wires of various diameters for band stop frequency rejection, and plates bounded by knife edges for low‐pass filtering. The technique is applicable to most problems encountered in seismic prospecting where spurious events obscure desired reflections. The most frequent application so far has been the removal of multiple reflections. The method has turned out to be highly useful for eliminating noise, regardless of origin, which interferes with reflections whenever the noise consists of traveling events, even though fragmental, which have different apparent velocities from the reflections. The method has also been effective in solving structural problems in tectonic areas by removing diffractions or, alternatively, by enhancing them at the expense of the reflections to delineate faults and other sources of diffraction. Ringing or reverberation can often be attenuated or eliminated in marine shooting by passing reflection frequencies that are less than the lowest observed harmonic of the fundamental reverberation frequency. Examples are shown of transforms and/or filtered sections illustrating these applications. A particularly valuable feature of this optical processing system is the ease of monitoring the results. The facility with which this can be done gives the technique distinct advantages over digital or analog methods, where the geophysicist loses contact with his results while processing is under way. Optical filtering also offers an intrinsically more economical approach to seismic data processing because hundreds of information channels can be handled n a single photographic operation.