Geology Reference
In-Depth Information
sensitivities are readily available and need to be used together with the
directional cancellation schemes developed here). Our results, summarized in
Figure 10.9, demonstrate that 10 bps is possible, and how they might be
accomplished by using constructive wave interference, increasing transducer
sensitivity, and by altering drilling system properties.
SG CP C DIA L P0 P Fcrit
A 1.7 50 3000 4 25000 145.0 0.4 13.8
E 1.7 50 3000 4 25000 246.5 0.4 16.4
F 1.7 50 3000 4 25000 493.0 0.4 20.1
G 1.7 50 3000 5 25000 493.0 0.4 31.4
H 1.7 20 3000 5 25000 493.0 0.4 78.5
I 1.7 20 3000 5 35000 493.0 0.4 40.1
J 2.0 40 4000 5 35000 246.5 0.4 34.1
K 2.0 40 4000 5 35000 246.5 0.2 41.9
Figure 10.9. Critical frequencies, hypothetical MWD tools.
Software reference, MWDFreq.vbp and datarate*.for are used for critical frequency analysis.
Results are based on attenuative acoustic wave model allowing fluid flow in pipe.
10.1.7
Ghost signals and echoes.
While the dangers of surface reflections are well known, we have
repeatedly emphasized the existence of downhole reflections which, if not
properly addressed, can destructively interfere with upgoing signals, or
introduce drill collar reverberations, or both. Interestingly, this problem was
also identified in “An Overview of Acoustic Telemetry” by Drumheller (1992)
in the context of drillpipe telemetry research conducted at Sandia National
Laboratories. Many authors, until then, had assumed simply that materials
waves created downhole simply traveled upwards. Drumheller remarks,
“Unfortunately, this over-simplified picture is extremely misleading. As the
early results of this project illustrate, real hammer blows in real drillstrings do
not result in this kind of response at all. The first complication which arises is
that unless the hammer blow occurs exactly at the top or bottom end of the
drillstring, two pulses are generated. One pulse travels up the drillstring while
the other travels down. If the hammer is placed near the bottom of the
drillstring, the downward traveling pulse will quickly reflect off the drill bit and
follow directly behind or possibly overlap the leading upward-traveling pulse.
This results in an unwanted echo of the original pulse. In a similar fashion, if
the receiver is near the top of the drillstring, the two pulses will pass the
receiver, reflect off the top of the drillstring, and pass the receiver again. This
process will continue creating more echoes until attenuation weakens the echoes
to an undetectable level.” In the time-domain view, echoes create false pulses
which are indistinguishable from and confused with the true data pulse. This
has been our assertion throughout this topic, however, we have put these echoes
to good use by having them reinforce (as opposed to canceling) upgoing waves.
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