LA MESURE DE LA VITESSE VERTICALE PAR LE CAROTTAGE CONTINU (CVL); PRECISION DES MESURES ET DISCUSSIONS DE QUELQUES RÉSULTATS*

The introduction of the CVL and the “core speed tester” (or ultra sonic velocity meter) have enabled the geophysicist to complement large‐interval, average velocity measurement of sedimentary formations (obtained by conventional well velocity surveys, “Gardner” velocity shots, etc.) by small interval, detailed study of the velocity of particular strata, in situ or in the laboratory. The significance of laboratory velocity measurements on cores is affected by irreversible alterations of the core. At first the integrated time curve of the CVL, calibrated with a skeleton conventional velocity survey, seemed to assure the precision of the interval velocity. Later it was realised that variations of the “Delay Time” (compensation for Mud Travel Time in the single receptor CVL), failure to identify the first energy arrival, etc., could cause errors in the interval velocity measured. These errors can be, detected and reduced by increasing the number of control points of the well survey, running logs down and up the hole, overlapping, and also by the comparison of the Continuous Velocity Logs for neighbouring wells and for various strata in a given well.

Despite these errors, the C VL has been found extremely useful, not only for the determination of average velocity, the identification of reflecting strata, and of refraction markers, but also for geological correlations between wells, the determination of maximum porosity and detailed studies of the parameters affecting the velocity of sedimentary rocks.

The relation between interval velocity and porosity is discussed and a hypothesis that the low velocity of argillaceous, low‐porosity limestones may in part be due to the total fluid contained is put forward.