By 1930, the resistivity log was recorded commercially in several countries, and in 1931 the self-potential (SP) measurement was added. An electromagnetic (EM) teleclinometer was introduced in 1932, a continuous temperature log in 1933, and an anisotropy dipmeter ca. 1935. The significance of the SP was an ongoing study until about 1962. The bases for the quantitative interpretation of resistivity for saturation were formulated by 1941.The years from about 1940 into the 1960s saw the development of other basic logs--dipmeter, gamma ray, neutron, induction, microresistivity, focused resistivity, density, and acoustic velocity--as well as other miscellaneous logs. Suites of resistivity logs were evolved to provide more accurate analysis. Much effort was expended on the interpretation of shaly sands. Neutron, density, and acoustic logs yielded information on porosity, lithology, gas saturation, and/or shaliness. Availability of porosity from these logs facilitated resistivity-log interpretation.In the late 1950s, interest developed in the use of computers for log interpretation. By about 1961, dipmeter logs were being recorded on digital magnetic tape suitable for computer input, and other logs were available by about 1965. Digital recording has also facilitated transmission of logs via radio and telephone. Recently, digital systems for wellsite computations have been introduced.Recent acoustic-log developments include the use of compressional and shear-wave amplitudes and wave train logs for fracture detection, compressional and shear-wave transit times for porosity and lithology interpretation, and long-spacing devices for better values of acoustic velocity. In radioactivity logging, natural gamma-ray spectroscopy yields measures of the uranium, thorium, and potassium contributions; the neutron die-away log allows determination of saturation through casing; inelastic and capture gamma-ray spectroscopy provide various elemental determinations, including C/O and Si/Ca logs; and activation logging promises to furnish additional elemental measurements. A new log of photoelectric absorption of gamma rays responds primarily to lithology. Another new log measures the phase shift and attenuation rate of an EM wave in the formation to furnish information on the dielectric constant and shale content. Cameras, a television, and an acoustic borehole televiewer have been used for downhole inspection. Remote sensors include the ultra-long-spacing electrical log (ULSEL), pulsed radar, and the borehole gravimeter which also provides a deep-investigation density reading.Anticipated future developments include: continued diversification of types of logs; increased use of combination tools; use of digital techniques to handle more and more data; more use of data links for log transmission and of wellsite computer processing of logs; a greater premium on log accuracy and adequacy of log evaluation; more use of cased-hole logs; integration of more logs and other data in interpretations; and increased capabilities of logging equipment to operate in hostile environments.

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