Geology Reference
In-Depth Information
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Figure 5.7.
Downhole pulser reflection models.
5.5.3.7 The transmission channel uphole.
As if the downhole environment were not challenging enough, we
introduce additional complications arising from the remainder of the
transmission channel. The pressure transducers shown in the schematic of
Figure 5.8 will receive signals that consist of the wave signals described above,
plus their downward reflections from the mud pump or desurger, plus mud
pump noise, drilling noise and general random noise. The surface receiver must
take into account all of these non-ideal conditions, and “deconvolve” (or,
“unscramble”) the raw data in such a way as to recover the original information-
bearing P
s
(t) or V
s
(t) waveform. Pump noise and MWD reflections do not
constitute true noise in the communications sense, since they are not random:
they represent coherent waves with well defined acoustic properties. Thus, they
can be filtered using wave-based software methods, many of which are
described in the open patent literature. These range from techniques that
subtract complete “snapshots” of the mud pump signature from the total
received signal, to conventional filters that eliminate pump noise based on
frequency spectrum, and to more novel filters that discriminate on the basis of
direction rather than frequency. Such “directional filters” are motivated in the
now classic paper of Clayton and Engquist (1977), which gave wave-equation-
based differential operators that were capable of accurately filtering or selecting
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