Transient Constant Area Surface and Downhole Wave Models - Measurement While Drilling Signal Analysis, Optimization and Design - page 139

This completes our analysis of downhole ghost signals which, for
simplicity, assumed that area mismatches between MWD collar and drillpipe do
not exist. In Chapter 5, the “real world” internal annular collar area may be
greater than, equal to or less than the drill pipe area, thus adding modeling
complications. The results are exact and extend the capabilities reported in this
chapter. In closing, we emphasize certain “original black” versus “observed
green” signal characteristics that we have computed, e.g., see Figures 4.1e,f,g.
The green traces appear elongated and distorted relative to the intended black
signals. Very often, these effects, referred to as “phase distortion,” “leading and
trailing edge error,” “pulse lengthening,” and so on, are attributed to
nonlinearities and frequency dispersion in the telemetry channel. In the lead
author's opinion, these causes are not very likely; in fact, the green traces shown
in this chapter are the sole consequence of reflections occurring at the drillbit -
in a very linear assumed system. For this reason, few of the “fixes” offered by
more complicated signal processing schemes - relying on incorrect physical
explanations - have failed in the field. Readers of the corresponding patents
should exercise caution and carefully question underlying assumptions.
4.7 References
Oppenheim, A.V. and Schafer, R.W., Digital Signal Processing , Prentice-Hall,
New Jersey, 1975.
Oppenheim, A.V. and Schafer, R.W., Discrete-Time Signal Processing ,
Prentice-Hall, New Jersey, 1989.