Stories from the Field,Fundamental Questions and Solutions - Measurement While Drilling Signal Analysis, Optimization and Design - page 26

Figure 1.5b.2. Multiple transducer array for surface noise removal
(top, accommodating multiple frequencies; bottom, one configuration used).
Low amplitudes need not be achieved by bringing the rotor to a complete
stop. If a high-amplitude is associated with, say 60 Hz, then a useful low-
amplitude candidate can be found at, say 55 Hz, as suggested by Figures
1.2b,c,d. Thus, FSK can be efficiently achieved while minimizing the effects of
mechanical inertia. Rotor torque reduction, while an objective in wind tunnel
analysis, is useful, but need not be the main design driver in our approach.
In order to make constructive wave interference work, the siren must be
located as close to the most significant bottom reflector, normally the drillbit, as
possible (intervening waveguides, e.g., mud motors, resistivity-at-bit subs, and
so on, support wave transmission). Thus, the siren is placed beneath the turbine
in the MWD collar, in contrast to existing designs. This reduces the time needed
for waves to meet and reinforce. Figure 1.2a shows a “mud motor.” This
acoustic element may, in fact, represent a resistivity-at-bit sub. Calculations
show that 10 bits/sec can be accomplished provided this section is
approximately fifteen feet in length or less. Tests confirm that long waves pass
effortlessly through turbines without reflection. Detailed waveguide analyses
suggest that signal gains of 1.5-2.0 are doable using single-siren designs alone.
PSK methods, again, are undesirable because they result in wave cancellations
and ghost signals that hinder signal processing.