Mud Acoustics in Modern Drilling - Wave Propagation in Drilling, Well Logging and Reservoir Applications - page 292

Figure 5.9. Conceptual telemetry schemes for MWD sources.
In a true sense, the “smoke signal method” used by American Indians
during the last century represents an early prototype telemetry scheme. Modern
methods number in the hundreds, as simple literature searches will show, on
scanning communications journal publications. Figure 5.9 shows, conceptually,
several types of telemetry methods available to MWD designers. The top trace
illustrates pulse width modulation , while the second demonstrates phase shift
ke yi n g (or, "PSK"). The third introduces FM frequency modulation , while the
fourth shows PSK modulation due to a low torque MWD motor. Downhole
hardware limitations clearly, in this example, inflict profound consequences on
surface signal processing.
5.5.3.9 Additional practical considerations.
We have described many of the design variables likely to affect telemetry
rate, but additional practical issues remain. These include the effects of
mudpump pistons with worn O-rings, unsteady flow rates due to positive
displacement effects of duplex and triplex pumps, loss of signal synchronization
due to drill bit bounce and momentary axial shock, attenuation and frequency
dispersion in the modern viscoelastic muds used in horizontal wells, and real-
time signal processing capabilities and limitations in view of allowed costs.
Lear and Dareing (1990) considered the effect of drillstring vibrations on MWD
pressure pulse signals. Vibrations of the metal at the bit face or mud motor face
will excite the mud column and introduce high frequency background noise. In
this sense, drillstring axial vibrations and MWD acoustics are coupled; thus,
integration of the algorithms described in here and in Chapter 4 should provide a
first step in understanding the physics of the solids-fluids interaction.