Harmonic Analysis: Six-Segment Downhole Acoustic Waveguide - Measurement While Drilling Signal Analysis, Optimization and Design - page 78

Figure 2.7. “Typical” bottomhole assembly with mud motor.
Insofar as the signal propagating up the drillpipe is concerned, the positive
effects of constructive interference always obtain at 1 Hz or less; enough of the
downward wave propagating toward the bit apparently reflects upward to
positively superpose in phase with new upgoing waves. As noted earlier, the
transmission efficiency associated with low data rate transmissions is typically
90% or more and source position is unimportant. But at 12 Hz, the signal
decreases as the pulser moves away from the mud motor; however, this spatial
trend reverses itself at high frequencies. The magnitudes of the pressure wave in
the borehole annulus show consistently low values, typically 1-6 % of the pulser
'p, with higher values achieved at the lower frequencies. These results are
consistent with numbers quoted by workers in MWD gas influx detection. We
caution that the above results apply only to the bottomhole assembly studied in
Figure 2.7, and then, only to our use of water as the drilling fluid; the reader
should not generalize these results to other situations.