The geophysical industry is in the midst of a digital revolution. Digital seismic recorders are replacing analog field systems; digital data processing techniques are now widely used; and many publications have appeared on the principles of digital filtering and the design of optimum filters, particularly multichannel filters. Far less material has appeared on parameter and spectrum estimation and on time-varying procedures. Analysis using optical diffraction has found considerable favor. Although the dynamic range reported is only about 40 db, the ability to handle entire record sections has resulted in close scrutiny of this nondigital analysis method. Several nondynamite seismic sources have been developed for land use; while in marine exploration, more powerful sources and multidetector cables have led to penetrations reported to be up to eight thousand ft. Common reflection point record stacking methods are being used on about 75 percent of seismic held crews. Strong interest continues in the development of mobile magnetometers with increased sensitivity. A growing use of shipborne gravity meters has resulted from precisions of two or three mgal at a ship speed of 10 knots. Digital computers are being used to apply terrain and elevation corrections and to aid in separating the gravity field into regional trends and anomalies. Digital recorders for use with dipmeters and other well logging tools are available in many areas. A system for recording on a single tape up to eleven logging curves from separate surveys is under active development. Analog computers at the well produce for visual interpretation computed curves that emphasize hydrocarbon anomalies. A new logging system, neutron lifetime logging, promises to locate oil behind pipe. Improved design of acoustic, gamma-gamma density, and dipmeter tools minimize borehole effects.

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