Transient Constant Area Surface and Downhole Wave Models - Measurement While Drilling Signal Analysis, Optimization and Design - page 123

of effective filtering based on directions only. Note that conventional methods
to filter based on frequency will not work since both signal and noise have about
the same frequency. Again, the sound speed (assuming water) is 5,000 ft/sec.
For the MWD pulse, the far right position is 1,700 ft. We want to be able to
“watch” all the pulses move in our graphics, so we enter “1710” ( >1700 below).
We also assume a transducer separation of 30 ft. This is about 10% of the
typical pulse width above, and importantly, is the length of the standpipe; thus,
we can place two transducers at the top and bottom of the standpipe. Recall that
Method 4-4 is based on derivatives. The meaning of a derivative from calculus
is “a small distance.” Just how small is small? The results seem to suggest that
10% of a wavelength is small enough.
Units: ft, sec, f/s, psi ...
Assume canned MWD signal? Y/N: y
Downward propagating noise (psi) assumed as
N(x,t) = Amplitude * cos {2Ŧ f (t + x/c)} ...
o Enter noise freq "f" (hz): 15
o Type noise amplitude (psi): 20
o Enter sound speed c (ft/s): 5000
o Mean transducer x-val (ft): 1710
o Transducer separation (ft): 30
Note, the noise amplitude is not small, but chosen to be comparable to the MWD
amplitudes, although only large enough so that all the line drawings fit on the
same graphical display. The method actually works for much larger amplitudes.
Figure 4.4b . Recovery of three step pulses from noisy environment.
After SAS14D executes, it creates two output files, SAS14.DAT and
MYFILE.DAT. The first is a text file with a “plain English” summary. The
second is a data file used for plotting. To plot results, run the program
FLOAT32, which will give the results in Figure 4.4b where an index related to
time is shown on the horizontal axis (the software will be fully integrated at a
future date).