Signal Processor Design and Additional Noise Models - Measurement While Drilling Signal Analysis, Optimization and Design - page 176

Figure 6.8b. Typical frequency spectrum for 12 Hz carrier signal.
The physical properties of the MWD source signal 'p are also of interest.
There is lack of clarity on signal strength as a function of frequency at the
present time. For example, Montaron, Hache and Voisin (1993) and Martin et al
(1994) suggest that the amplitude of the pressure pulse created is roughly
independent of frequency at higher frequencies. On the other hand, Su et al
(2011) shows experimentally that 'p decreases with increasing frequency when
the flow rate is fixed. Additional investigation is needed to determine the exact
dependence of signal strength on frequency and flow rate. This empirical
knowledge would supplement the wave interaction models developed in
Chapters 2-5.
Other sources of “noise,” or more accurately, uncertainty, arise in signal
processing. Boundary condition differences associated with “hard versus soft
rock” may be responsible for ”closed versus open” downhole end reflections
that confuse analysis. At other times, reports do not distinguish between
positive versus negative pulsers, which create completely different reflection
and wave propagation patterns. And still, some rig operators have indicated
“loss of signal” at higher depths that can be attributed to mechanical
malfunctions under high pressure - effects that have nothing to do with signal
processing. All of these are valid concerns, but because of their diverse nature,
illustrate why the design of a general signal processor very challenging.
6.9 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.