Published in Petroleum Transactions, AIME, Volume 213, 1958, pages 207–212. Introduction This paper discusses the characteristics of the velocity logs now available to the petroleum industry, and some of their advantages and limitations. The velocity log was designed as an aid to the geophysicist to determine velocity layering in the earth and to provide fast, accurate and economical vertical travel time data. After a number of logs had been made, studies were carried out on their use in correlating strata in adjacent wells. It was found that adjacent strata usually show sufficient velocity contrast to make it possible to correlate these from well to well, even in areas where electric or radioactive logs fall. In addition, velocity logs have been useful in the determination of porosity and thickness of reservoir rock. These uses for velocity logs require an understanding of the characteristics of the log in response to the changes in velocity and in borehole conditions encountered normally in logging. General Discussion of Instrumentation The measurement made in velocity logging is more accurately described as a measurement of nine rather than velocity. Ideally the log is obtained by measuring the length of time required for compressional sound waves to travel through a fixed distance in the formation. In practice, the time measurement may be made accurately but the distance through which the waves have traveled may vary slightly. Changes in borehole diameter are responsible largely for the difficulty in maintaining a fixed distance, especially if the diameter variations are so abrupt and frequent that the borehole instrument cannot follow contours of the hole. Several service companies have velocity logging equipment available or under development which have different transmitters, receivers, and measuring systems. While these differences are important, there are actually only two systems in use at the present time (Fig. 1). The first system initiates a pulse at a transmitter and detects it at a single receiver. The transmitter and receiver are separated by a fixed distance, called the spacing. The time measured is that between the initiation of the pulse and the first arrival of acoustical energy at the receiver.