Geology Reference
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it must create antisymmetric pressure signals and, at the same time, allow up and
downgoing waves to transparently pass through it and interfere. It is also
necessary to emphasize that wave refraction and reflection methods for very
high frequencies (associated with very short wavelengths) are inapplicable. The
solution, it turns out, lies in the use of mathematical forcing functions, an
application well developed in earthquake engineering and nuclear test detection
where long seismic waves created by local anomalies travel in multiple
directions around the globe only to return and interfere with newer waves.
Wave propagation subtleties are also found at the surface at the standpipe.
We have noted that (at least) two sets of signals can be created downhole for a
single position-modulated valve action (multiple signals and MWD drill collar
reverberations are actually found when area mismatches with the drill pipe are
large). These travel to the surface past the standpipe transducers. They reflect
not only at the mudpump, but at the desurgers. For high-frequency, low
amplitude signals (e.g., those due to existing sirens), desurgers serve their
intended purpose as the internal bladders “do not have enough time” to distort
signals. On the other hand, for low-frequency, high amplitude signals (e.g.,
positive and negative pulsers), the effects can be disastrous: a simple square
wave can stretch and literally become unrecognizable.
Thus, robust signal processing methods are important. However, most of
the schemes in the patent literature amount to no more than crude “common
sense” recipes that are actually dangerous if implemented. These often suggest
“subtracting this, delaying that, adding the two” to create a type of stacked
waveform that improves signal-to-noise ratio. The danger lies not in the
philosophy but in the lack of scientific rigor: true filtering schemes must be
designed around the wave equation and its reflection properties, but few MWD
schemes ever are. Moreover, existing practices demonstrate a lack of
understanding with respect to basic wave reflection properties. For example, the
mud pump is generally viewed with fear and respect because it is a source of
significant noise. It turns out that, with properly designed multiple-transducer
signal processing methods, piston induced pressure oscillations can be almost
completely removed even if the exact form of their signatures is not known. In
addition, theory indicates that a MWD signal will double near a piston interface,
which leads to a doubling of the signal-to-noise ratio. Placing transducers near
pump pistons works: this has been verified experimentally and suggests
improved strategies for surface transducer placement.
1.2.4 New telemetry approach.
A nagging question confronts all designers of high-data-rate mud pulse
systems. If sirens are to be the signal generator of choice (say, if lowered
torques enable faster direction reversals), how does one overcome their
inherently weaker signal producing properties? The Joukowski formula “p =
UUc” provides an exact solution from one-dimensional acoustics stating that the
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