Advanced System Summary and Modern MWD Developments - Measurement While Drilling Signal Analysis, Optimization and Design - page 279

reflector. The use of constant frequency methods again eliminates phase
randomness. A frequency f at the source will lead to an identical surface
frequency f for whatever wave interactions are present (nonlinearities will, of
course, add harmonics, but these are damped by the telemetry channel). We
note that, in practice, it is not necessary to use the model of Chapter 2 during
field operations. A simpler “self-optimizing” procedure would have the pulser
sweep frequencies from, say 0 to 100 Hz, with amplitudes monitored from the
surface. Suppose high and low amplitudes are associated with the frequencies
f h and f l . Then, the pulser can be instructed to operate with the frequency
sequence “ f h - f l - f h - f l - f h - f l - ” and so on using a downlink procedure
employing, say, mudpump flow variations or a surface-based siren device.
Multiple frequencies, say “40-50-60-70 Hz,” may provide more than simple 0's
and 1's by adding 2's and 3's. Of course, additional bandwidth is possi ble by
clever encoding, a subject not addressed in this topic.

10.1.3 Sirens in tandem.

Our use of FSK plus constructive interference serves twofold purposes:
simplified signal processing and additional signal strength without incurring
erosion and power penalties. A second type of constructive wave interaction is
possible for signal enhancement which can be used in addition to our telemetry
scheme. Signal reinforcement is accomplished by placing two (or more) sirens
or positive pulsers in series as suggested in Figure 10.4. The mechanical system
would be designed so that the volume between the two pulsers never closes
completely, so as to isolate contained fluid from that in the drilling channel. In
fact, all sirens would open and close “in step,” that is, if one is 10% open, so are
the others. This allows acoustic signals to superpose and thus reinforce each
other. This implementation is important to transmissions from deep wells.
Other uses for “sirens in series” are possible. For instance, high data rates can
be achieved if individual sirens operate at different frequencies. Since the
drilling channel is linear, these transmissions act independently and are
processed without difficulty. This usage, of course, involves mechanical
complexities beyond the scope of our present discussion.

Figure 10.4. Section view, a pair of ganged or tandem mud sirens
(e.g., see U.S. Patent No. 5,583,827 for details).