The Sources of Errors in Slowness Measurements and an Evaluation of Full Waveform Compensation Techniques

Abstract

ABSTRACT The sources of errors in slowness, or ∆t, measurements are analyzed in this paper. Using synthetic waveforms generated by the finite difference method and a scaled model, the slownesses of compressional, shear and Stoneley waves are evaluated for examples of bedding, cave-in, fractured formation, eccentered and tilted tools. The depth-derived borehole compensation (DDBHC) and other means of improving the accuracy of slowness estimation are applied to the full waveform slowness estimation. The effect of washouts on slowness estimates is extremely large if borehole compensation is not applied. Bed boundaries cause moderate error in slowness measurements and can be minimized by compensation. Simulation shows that the tool eccentering and tilting have only minor effect on compressional and shear slownesses, and no effect on the Stoneley wave. The shear slowness is disturbed more by discontinuities than compressional and Stoneley waves. The slowness is most affected when the receiver array is straddling the discontinuity and relatively more affected after the array passes the discontinuity than before entering it. The conventional DDBHC method produces fairly good results for full waveform slowness estimation. Better results can be obtained, especially for shear slowness estimates, if some preprocessing is done on the waveforms and if the weighting on the receiver and transmitter slownesses used in the conventional DDBHC method is modified according to the results of model simulations and earlier studies. For borehole washouts, more scaled model and simulation work is needed for developing better compensation algorithms.