Transient Constant Area Surface and Downhole Wave Models - Measurement While Drilling Signal Analysis, Optimization and Design - page 133

is the one that is actually sent uphole. Using data from the green line only, we
recover the blue signal, which is seen to be identical to the black signal, the
intended signal. The green signal does not look like the black signal, but has the
appearance of two sine waves separated by an interval of silence.

4.6 Method 4-6. Downhole reflection and deconvolution at the bit,
waves created by MWD dipole source, bit assumed as perfect open
end or zero acoustic pressure reflector software reference,
DPOPEN*.FOR ) .

4.6.1 Software note.
In Method 4-5, the drillbit is assumed as a solid reflector, meaning “small
drillbit nozzles” subject to cautions previously noted; it can also mean “not so
small” nozzles, but drilling into very hard rock. In Method 4-6, larger drillbit
nozzles are considered, that is, the drillbit is taken as an acoustically opened
zero pressure boundary condition. This is probably the typical situation, as
explained in detail in Chapter 2. When “p 2 L,t” is known, 'p(t) is solved by
our DPOPEN*.FOR model, which is derived from DELTAP*.FOR. The
present method also enables excellent signal recovery.

4.6.2 Physical problem.
The physical problem considered here is same as that in Method 4-5
(where the bit is a solid reflector) except that the drillbit is now assumed as an
open-ended acoustic reflector.

Schematic

"1" "2"

Drillbit
x = 0

MWD
pulser
x = L

Upgoing
signal to
surface

x

Figure 4.6a. Open end bottom reflection, waves from dipole source.