ALBERT

Notes: Since 1955, the continuous velocity measurements in bore holes, made with one, later with more receivers, have become indispensable for the geophysicist, and can be useful to the geologist and the production engineer. From the beginning we have found that the amplitudes of the signals received vary in the course of a single operation. The variations have for a long time been considered as a disturbing phenomenon. It was tried to suppress them by changing the instrument gain.With increasing precision of the velocity measurements we have sometimes observed anomalies that were hard to explain, discrepancies between the continuous velocity logs and the conventional well velocity survey, as well as between continuous velocity measurements made with different spacings. These phenomena could be observed in cased as well as in uncased holes. The first attempts to explain these phenomena by considering the velocities only, did not lead to an acceptable theory. A further study of these anomalies of the continuous velocity log has shown that in reality they were caused by strong variations of the amplitude which affected the velocity measurements. This result has led us to study the amplitude variations in open as well as in cased holes.Up to now, the characteristics of the apparatus used in France and in the Sahara have permitted amplitude measurements with a single receiver only. In addition to the “cement bond log” in cased holes, it seems that in open holes correlations can be observed between the measured amplitude and a) the lithology; b) the fluid contained in the pores; c) the continuity of the matrix or, conversely, the permeability. However, it is now certain that numerous factors affect the amplitudes of the signals simultaneously.In order to study the attenuation of ultrasonic waves in the different formations traversed by a deep well, we have studied the deformations of the signal in the course of its path in the mud and in the formation, restricting ourselves to the P waves. The factors to be taken in consideration are: a) the geometric energy dispersion, inversely proportional to the square of the distance from the source; b) attenuation by absorption and scattering in the mud and in the formation; c) the transmission coefficients at the transition from mud to formation and conversely, as well as at the transition between two formations having different velocities; d) phenomena of signal composition that are due to the fact that the receivers and the transmitters have a non-infinitesimal length; these last mentioned phenomena have considerable influenc eon the amplitudes of the received signals.Finally, the practical method for obtaining an attenuation log in an open hole requires the measurement of the ratio of the amplitudes of the first arrivals at the two receivers. Some examples show the importance of amplitude measurements made with two receivers.

Notes: In the last two years several double-receiver Velocity Logs (V-Logs) have come into service. Theoretically, in this type of log the effect of the travel time in the mud and in the invaded or altered formations, responsible for the “Lag” (or delay times) in single receiver V-Logs, could be eliminated. Thus the integration of the interval times would give the “real” vertical time as a function of depth. In practice, several authors have already noted that even for V-logs with two receivers the integrated times differ from the results of the conventional well surveys.We consider here some of the geometrical, geological and instrumental factors that affect the time measurement of double receiver Velocity Logs, as well as the possibility that the discrepancy between the V-logs and the conventional well survey could in some cases be due to faults, dip, folding, or the intrusion of high velocity layers near the well. Some examples are given, and the question of the polarity of first arrivals in well surveys is discussed.At present the discrepancies between the integrated times of even double receiver V-Logs and the results of the well surveys remain too great to permit us the economy of eliminating the well survey. Furthermore, for the study of these discrepancies we must continue to shoot a considerable number of calibration points. We have, however, succeeded in reducing greatly the cost of the conventional survey by reorganising our well shooting methods. These methods, as well as the problems of surface corrections and the choice of datum plane they entail, are discussed.

Notes: 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.