Mud Acoustics in Modern Drilling - Wave Propagation in Drilling, Well Logging and Reservoir Applications - page 289

5.5.3.6 Mud pulse telemetry - the transmission channel.
In the foregoing discussion, we spoke of signal generation at the source
point, and assumed that, once created, signals were discernible to receivers at
the surface. This is not the case, since reflections within the transmission
channel introduce complications that affect reception. The nature of the
reflections depends, to a large extent, on the type of pulser used.
Reflection problems near the pulser

Figure 5.7 illustrates the reflection
modes associated with poppet valves, negative pulsers and mud sirens,
respectively, from left to right. In the first schematic for poppet valves, a
created positive pulse is shown traveling uphole, while its complementary
antisymmetric pressure pulse travels downhole. This latter pressure signal
reflects at the solid boundary shown, and turns upward with the same negative
sign. If the pulse length or duration is long enough, it interferes with the tail end
of the upgoing positive pulse being generated, thus causing destructive
interference of the created signal. A similar process applies during the opening
cycle of the poppet pulser operation. Negative pulsers act differently. Because
the pressures created at the source point are symmetric about it, thus having the
same sign, reflection at the bottom and subsequent superposition enhances the
tail end of the created signal, leading to constructive interference . Mud siren
operation is even more complicated. While sirens physically behave like
positive pressure pulsers, it is clear that at the higher frequencies that they are
usually operated at, both constructive and destructive interference are possible.
Such acoustic effects can degrade field operations, but they can, of course, also
be applied to advantage. We have assumed a solid boundary for discussion
purposes. Actually, the drillbit (having nozzles) will function as an open-end
reflector under many circumstances; in fact, MWD signals are routinely
detected in the annulus, where their presence (or absence) is taken as an
indicator of gas influx . In the more general models developed in Measurement
While Drilling Signal Analysis, Optimization and Design (Chin et al , 2014), a
general waveguide model is developed that does not presuppose that the
reflection mechanism is known. In addition to these effects, partial reflections
and transmissions at area discontinuities (e.g., at the pipe-to-collar interface)
will affect transmitted signal quality. It is clear that high-data-rate mud pulse
telemetry systems of the future, whether they are poppet, negative pressure, or
siren in variety, will require a detailed understanding and intelligent application
of acoustic source properties as elucidated here and in Chapter 1.

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