Signal Processor Design and Additional Noise Models - Measurement While Drilling Signal Analysis, Optimization and Design - page 169

6.4.2 High data rate effects.
At higher data rates, that is, those achieved with rapidly varying sinusoidal
or periodic waveforms, this is not the case. Signals do not necessarily interfere
constructively because of phase differences.
Figure 6.4b. Pressure versus “t” at given “x.”
In Figure 6.4b, model PSURF*.FOR is used to illustrate our point. The
clean, constant frequency, blue signal enters the drillpipe and the red signal is
measured uphole (zero attenuation is assumed in this example). At first, silence
(zero pressure) is found because the signal has not reached the standpipe
transducer. Then the wave arrives with an amplitude initially identical to that of
the blue trace. This signal proceeds to the mudpump and reflects. In this case, it
diminishes the pressure measured at the standpipe due to destructive interference
arising from phasing effects. Short wavelengths make signal processing more
difficult. In this example, the (upper) blue line represents the intended signal,
but it is the (lower) red trace that is actually recorded at the standpipe
transducer. Note the kink in the (bottom) red curve at t = 0.7 sec. The multiple
transducer methods of Chapter 4, of course, will remove the foregoing
destructive interference effects to recover the blue trace.