Abstract: Improved techniques are provided for correcting signals from a downhole sensor on a drill collar eccentrically rotating within a borehole. The corrected sensor signal may be used to generate a well log which more accurately represents the conditions which the sensor would have generated had the tool been rotating such that the spacing between the sensor and the borehole well remained constant. The sensor signals are generated at time intervals of less than half the period of rotation of the drill collar. The frequency components of the time-varying sensor signals are plotted, and the frequency component attributable to the eccentric rotation between the drill collar and the borehole may be determined. The magnitude of this frequency component may be used to generate a correcting sensor signal, which then modifies the average sensor signal so that a more accurate well log may be obtained.

Abstract: The technique according to the present invention is useful for generating a corrected well log which otherwise would include erroneous signals from downhole sensor. Such signal error may be caused, for example, by periodic movement of the drill collar axis with respect to the borehole axis, eccentric rotation of the drill collar with respect to the borehole, or a whirling or precessing of the drill collar in the borehole. According to the technique of the present invention, these sensor signals are generated at time intervals of less than one-half the period of the highest frequency of the periodic movement. Discrete sensor signals are averaged to generate an average sensor signal as a function of borehole depth. Discrete sensor signals are also recorded to generate a time-varying sensor signal profile, the magnitude of frequency components of the time-varying sensor signal profile is determined, and the average sensor signal is corrected as a function of the determined magnitude of the frequency components.