Stories from the Field,Fundamental Questions and Solutions - Measurement While Drilling Signal Analysis, Optimization and Design - page 38

Comments: It is imperative that long wind tunnels model reality. In actual
MWD field application, a downhole pulser creates an upward propagating wave
that reflects at a solid mudpump piston (much slower piston movement in
progress is irrelevant to this reflection). Pressure waves impinging at solid
piston boundaries will reflect with their signs or polarities unchanged. In the
setup shown, the far end of the long wind tunnel is open to the atmosphere and
pressures will reflect with opposite signs and return toward the source. To
minimize this effect, signal strengths were controlled so that unrealistic
reflections do not affect other transducer measurements (a special wind tunnel
has since been designed which exhausts wind differently while allowing
impingement at a solid boundary). Interestingly, theory shows that pressure
waves impinging at a solid boundary locally double in magnitude. This was
verified in experiments, suggesting that pressure transducers mounted near the
mud pump could provide improved signal to noise ratios - this idea has been
applied to the “one hundred feet hose” amplifier described elsewhere in this
topic, a significant invention that has been field tested and patented. Figures
1.5q and 1.5r show work sessions captured during wind tunnel testing.
Figure 1.5q. Early prototype tool.
Figure 1.5r. At work in the test shed.