AN EXPLANATION OF OBSERVED TIME DISCREPANCIES BETWEEN CONTINUOUS AND CONVENTIONAL WELL VELOCITY SURVEYS

Abstract

Equations derived from the author’s mathematical (PL) model for the absorption of seismic energy by the earth are applied to explain statistical time discrepancies observed by Gretner between measurements using a continuous (CV) logging tool and conventional check shots. In essence, the predicted discrepancy is due to the pedestal effect that the author has fully described in an earlier paper. The magnitude of the predicted discrepancy not only agrees extremely well with the statistical observations but the corresponding theoretical Q’s for rock that are necessary to cause just such deviations are quite consistent with Q values expected in the shales and sandstones in which the observations were made. Our interpretation is also consistent with Gretner’s observation that the discrepancies are independent of the measured velocities. It is also shown that a mathematical model (TLF) based upon a truncation of a linear frequency attenuation function predicts time discrepancies just about as well as does the PL model but that the predicted values of Q at 40 hz are 60 percent larger. However, ambiguities still remain and the use of the TLF model is not recommended.