Harmonic Analysis: Six-Segment Downhole Acoustic Waveguide - Measurement While Drilling Signal Analysis, Optimization and Design - page 49

2.2 MWD Telemetry Concepts Re-examined
The Measurement-While-Drilling literature has, for almost four decades,
classified telemetry methods in a simple-minded manner according to three
well-known categories: “positive pulser,” “negative pulser,” and “mud siren,”
the latter synonymously referred to as “continuous wave telemetry” (continuous
transmissions are, of course, possible with positive and negative pulsers). These
are conceptually illustrated in Figures 2.1, 2.2 and 2.3, respectively, where only
the upgoing signals (as in conventional publications) are shown. It turns out that
this representation is valid at very low frequencies, as we will demonstrate from
our more general model. For now, we begin by explaining accepted
conventional views and their original rationale, and then, re-examine extensions
to the physical problem in the high-data-rate context in each instance.
Figures 2.1, 2, 3. Positive, negative and siren pulsers (left to right).
2.2.1 Conventional pulser ideas explained .
Positive pulsers are essentially poppet valves that slowly plug and unplug
small orifice openings by axial motions. When the orifice closes, the speed of
the fluid column upstream reduces significantly while pressure slowly increases
- a pressure that returns to ambient conditions when the valve fully opens. For
this reason, this signal generator is known as a “positive pulser.” Positive versus
ambient hydrostatic pressures are used to communicate “0's” and “1's” to the
surface. It is clear that such pulsers require high levels of mechanical power as
well as the ability to withstand significant levels of erosion.