Developments in the field of well logging over the last 25 years are reviewed. Surface and borehole instrumentation have evolved significantly, taking advantage of modern digital and analog integrated circuits. Most open-hole petroleum well logs are now recorded digitally. Digital logs are also frequently acquired in cased-hole petroleum, mineral, and geotechnical applications. Nuclear well-log measurements have become accepted and reliable. New measurements include borehole compensated density and neutron-porosity, sidewall epithermal neutron-porosity, and most recently litho-density. The neutron decay log, developed early in the 25-year period, has undergone a number of major improvements since its introduction. Probes which make spectral measurements of natural gamma-ray emission, and gamma-ray emission from neutron interactions with matter have also been developed. Resistivity measurements are now made with probes which combine three or more sensors each with different depths of investigation so that information about the borehole invasion profile can be acquired. Acoustic logging methods have expressed major developments and improvements. The compensated sonic measurement was introduced early in the period along with the cement bond logging method. Interest in measurement of shear-wave velocity has produced new direct shear-wave measurements as well as improved acoustic probes for full-waveform acoustic logging.Other interesting or promising methods which have been developed or improved during the period include the borehole televiewer, the borehole gravimeter, and the nuclear magnetic resonance log. The digital computer provides powerful capabilities for well-log analysis both at the well site and in the office. Analysis of complex sand-shale and carbonate formations using two or more logs in a simultaneous solution of a litho-porosity model is now routine. Powerful signal processing techniques are being applied to 'deconvolve' well logs, to enhance or synthesize images of the wellbore, and to estimate or extract information from full-waveform acoustic logs.While new or improved measurements have been introduced and log analysts now have access to powerful computers and graphic work stations, understanding of the petrophysical significance of the measurements lags behind the basic hardware measurement and interpretation technology.

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