Sonic Logging

Published in Petroleum Transactions, AIME, Volume 216, 1959, pages 106–114. Abstract The principle, the equipment and field operation of sonic logging are described. The two-receiver system produces logs independent of hole size and mud. Field experience is given and forms the basis for the interpretation of the log. The derivation of porosity values from measured velocities is discussed, according to the type of formations (hard formations, compacted sands, and unconsolidated formations). The time-average equation proposed by M.R.J. Wyllie, A.R. Gregory and L.W. Gardner is used as a basis for the computation of porosity in limestones, cemented sandstones and compacted sands. Variations in the lithologic character of limestones do not seem to change the porosity calibration markedly. The compaction of sands is related to the compaction of shale adjacent to them; and, thus, the shale velocity is used for the establishment of empirical relations for the computation of porosity in unconsolidated formations. Various formulas are tentatively presented to account for shale and fluid content. Field experience demonstrates that considerable attenuation of sonic energy takes place in unconsolidated formations, particularly when gas bearing, and in fractured formations. Unusually large attenuation produces skipped cycles, a feature easily recognized. The application of sonic logging to structural studies is featured, showing its possible integration with the dipmeter. Correlation and its application to the interpretation of seismic surveys are reviewed.The paper is illustrated with field examples. Introduction Sonic logging is the recording of the time required for a sound wave to traverse a definite length of formation. Sonic travel-times are inversely proportional to the speed of sound in the various formations.The speed of sound in subsurface formations depends upon the elastic properties of the rock matrix, the porosity of the formations and their fluid content and pressure. Below the "weathered" or low-velocity layer extending from 50 to 100 ft or so below the surface, sound velocities may range from about 6,000 ft/sec in shallow shales to as much as 24,000 ft/sec in dolomites. In hard formations (well cemented and/or compacted), the sonic log reflects the amount of fluid in the formations; hence, it correlates well with their porosity.