Communications – electrical: acoustic wave systems and devices – Seismic prospecting – Well logging
Patent
1994-10-13
1996-12-24
Moskowitz, Nelson
Communications, electrical: acoustic wave systems and devices
Seismic prospecting
Well logging
367 28, 367 32, 364422, G01V 140, G06F 1548
Patent
active
055879667
ABSTRACT:
Methods for determining an optimal frequency band for processing of flexural waves according to bias-corrected STC or QSTC are disclosed. A preliminary estimate of the formation shear slowness S and a determination of the borehole diameter D are first obtained, and the flexural waves are processed in a frequency band having a center frequency f.sub.c which is a function of preliminary estimate of S and the product of that estimate of S and diameter D. Where the preliminary estimate of S indicates a slow formation, f.sub.c is chosen as 0.6/SD. Where the estimate indicates a fast formation, f.sub.c is chosen as 0.2/SD. In other or all formations, a smooth function such as [0.2+((0.8/.pi.) arctan ((S-100)/100))/SD] can be used to choose the center frequency. Alternatively, if errors in the environmental parameters can be estimated statistically, they can be used to select an optimum center frequency. Once f.sub.c is chosen, the filter parameters which define the bandwidth of the filter used in the Fourier transform of QSTC and in bias-corrected STC are preferably normalized by scaling the bandwidth BW to a specified fraction of f.sub.c ; BW=f.sub.c /Q. Similarly, the window length T.sub.w used in semblance processing is preferably normalized to scale with f.sub.c so that the processing window contains the same number of cycles regardless of the frequency of the center frequency.
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Kimball Christopher V.
Scheibner David J.
Gordon David P.
Jeffery Brigitte L.
Moskowitz Nelson
Schlumberger Technology Corporation
Smith Keith G. W.
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