WELL SURVEYS AND THE CALIBRATION OF VELOCITY LOGS*

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.