Optical processing of underground sound waves in seismic exploration for oil.

When coherent light from a laser beam is passed through a transparency on which pictorial information is impressed, the information itself acts as a complex grating to produce a Fraunhofer diffraction pattern which is the two-dimensional Fourier transform, or power spectrum, of the information in the picture. By properly obstructing the light in the transform, one can filter out undesired spatial frequencies or directions of inclinations in the original picture. The most widespread commercial application of this principle has been to the analysis and filtering of seismic reflection records made in exploration for oil. Removal of undesired noise from seismic recordings can often be accomplished more economically by optical filtering than by more conventional digital and electrical analog techniques. Examples are demonstrated where optical filters have substantially improved the quality of seismic data. Use of optical transforms for analysis and solution of geological problems is also illustrated. The most unique advantages of optical processing are in economy, flexibility, and convenience of monitoring. Principal disadvantages are limited dynamic range, and difficulty in constructing filters that behave as gradational rather than step functions.