Geology Reference
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When the diminished wave signal reaches the surface, it reflects at the
mudpump, which may be a solid or open-end reflector (we assume that, for high
enough carrier wave frequencies, the effects of desurger distortion are
unimportant), and attenuates downward. The present model computes the
pressure signal that is obtained at any point along the drillpipe - the main
positions of interest, of course, are those along the standpipe. It is assumed that,
because of attenuation, multiple reflections do not occur in the drilling channel.
6.4.1 Low-data-rate physics.
To understand the significance of the model in this section, let us consider
first a relatively long rectangular pulse, namely, the 0.5 sec duration rectangular
waveform considered previously in Method 4-1, Run 1. In this example, the
reflection occurs at the mudpump modeled as a solid reflector and pump noise is
omitted for clarity. This waveform nominally represents a 1 bit/sec data rate, a
low rate typical of present MWD tools.
Figure 6.4a . Wide signal - low data rate.
Figure 6.4a shows pressure measurements at a standpipe transducer. The
incident upgoing assumed signal (black) is a broad pulse with a width of about
0.5 sec. The red curve is the reflection obtained at a solid reflector with no
attenuation assumed; there is very little shifting of the red curve relative to the
black curve, since the total travel distance to the piston is very short. The
transducer will measure the superposition of incident and reflected signals which
broadly overlap. This superposition appears in the green curve - it is about
twice the incident signal due to constructive wave interference - moreover, it
does not cause any problems and actually enhances signal detection. The blue
curve is the signal extracted from data using only the green curve and the
algorithm of Method 4-1. Colors above are, respectively, black, red, green and
blue, starting from the bottom curve. The key conclusion from Figure 6.4a is
obvious: at low data rates, constructive wave interference is always found at
positive displacement mudpumps and always enhances signal detection .
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