Pulling the downhole signal out of pump noise

In what the surface pressure transducer records, the downhole data is usually far smaller than the noise built up from pump strokes, drillstring vibration and reflections off the surface manifold. Whether the data comes out intact is decided in the decoder.

Where the noise comes from

• Pump strokes — Periodic noise whose frequency follows the stroke rate, and the largest single contributor by energy. It is also the most regular, so adaptive cancellation deals with it well.
• Drillstring and mud motor — Broadband noise tied to rotary speed, changing in real time with the drilling state.
• Surface manifold reflections — Acoustic reflections off the standpipe, kelly hose and surface lines make the same pulse appear at the transducer more than once.
• Downhole attenuation and dispersion — A long lateral and viscous mud together reduce pulse amplitude and spread the waveform, so adjacent symbols overlap.

The decoding chain

Pre-processing (detrending, outlier removal) → noise modelling and cancellation (adaptive cancellation for the pumps) → matched filtering and pulse detection → symbol synchronisation → decoding and error correction → validation and output. Symbol synchronisation is the critical step: once sync is lost, everything after it is wrong, while the pressure trace still looks entirely normal and gives nothing away.

What the surface software has to do

• Manual sync — When the decoder loses sync it can be rebuilt by hand, without shutting the pumps down and re-circulating.
• Spectrum view — Live noise spectrum, so the source and frequency of the noise can be identified at the wellsite, with the history kept for later analysis.
• Decode playback — Raw pressure data is kept for the whole run and can be re-decoded with an improved algorithm to recover surveys lost earlier.