Geology Reference
In-Depth Information
2.2.3 High-data-rate continuous wave telemetry
.
Sirens have offered relatively high data rates in the past. But it is
important to observe that axially reciprocating positive pulsers and rapidly
opening-and-closing negative pulsers can also be used to provide waves for
continuous
wave
telemetry. The rationale for selecting one pulser type over
another, in the past, was based on power and erosion considerations and, in this
sense, on limitations in mechanical design and performance. However, if the
wave properties of signal production used in continuous wave generation can be
harnessed by taking advantage of constructive interference in order to
significantly increase source strength without incurring power and erosion
penalties, a game-changing means for high-data-rate telemetry and high
resolution well logging would become reality.
The importance of wave motions in telemetry physics cannot be
deemphasized. For existing positive and negative pulsers, signal detection and
decoding at the surface presently involve the monitoring of slowly changing
pressure levels - a relatively elementary endeavor. At high data rates, new
classes of subtle surface signal processing problems arise and we describe these
complications next - but again, these same subtle effects offer the potential for
significant gain if their physics can be intelligently and robustly harnessed.
Consider the problem of signal generation. Whether we use positive
pulsers, negative pulsers or mud sirens, waves are created that travel both uphole
and downhole. Trade journals (and even scientific papers) normally depict only
the upgoing wave, which ideally is correlated with valve displacement or
velocity (which is, in turn, driven by assumed strings of “0's” and “1's dictated
by logging sensors). In reality, the down-going wave reflects “at the drillbit” to
travel uphole; this wave will add to newly created up-going waves, and the wave
that ultimately travels uphole contains the intended signal plus “ghost
reflections” of created data from earlier times.
The above paragraph only “scratches the surface” insofar as complications
are concerned. Depending on the data stream transmitted (that is, the detailed
motion history of the valve), the geometry of the bottomhole assembly, the mud
sound speed, the transmission frequency, the pulser type and the telemetry
scheme, the results of constructive and destructive wave interference can lead to
good versus bad signals, but more than likely, simply unpredictable signals. In
designing a reliable continuous wave telemetry system, it is important to
understand in detail the physics of downhole wave interactions as this
understanding is critical to surface signal processing - these can have disastrous
operational consequences but they can also be used to advantage.
We have casually noted “reflection at the bit,” but we again emphasize that
reflections at the drillbit represent only those at one obvious reflector. In fact,
changes in acoustic impedance are found at drillpipe-collar junctions for the
downgoing mud flow and at other area changes; they are additionally found, for
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