Measurement While Drilling, 2nd Edition

Chapter 18Wave-Based Directional Filtering

18.1 Background.

In high-data-rate MWD mud pulse telemetry, surface reflections at the mud pump and desurger can “confuse” standpipe transducers as wave cancellations and reinforcements contaminate intended upgoing signals. MWD pressure waves reflect without sign and shape change at pump pistons; also, reflections at desurgers can lead to synchronization loss since elastic membranes may unrecognizably distort signals (e.g., square waves can reflect can with exponential-like form). In addition, mud pump noise traveling opposite to the signal, typically many times the upcoming signal strength, introduce further reductions to signal-to-noise ratio. Thus, the use of robust “directional filtering” with multiple transducers, ideally employing minimal waveform information with the fewest practical constraints, is motivated. Here applications for two schemes from Chapter 4 are evaluated under severe operational conditions using synthetic signal and noise data. In practice, the algorithms would be used with frequency, wavelet, white noise and other filters. It is important to recognize that filtering based on direction of origin requires at least two transducers. Because data rates will be adjustable in field applications, for instance, lower rates for deeper wells or high attenuation environments, transducer spacings must likewise be flexible – spacings and time samples must be chosen accordingly as the host transmission frequency used. The ...