Seismic-while-drilling drill-bit source by ground force: Concept and application

For four decades, the use of the drill-bit seismic source has been evaluated extensively and investigated for seismic-while-drilling (SWD) purposes. After an initial diffusion phase, however, the routine application on drill-bit reverse vertical seismic profiling (RVSP) has declined because of issues related to variability in the signal-to-noise ratio of drill-bit signals obtained with different drilling conditions. Consequently, additional efforts have been dedicated to improving the recording of reference signals. Recent technology developments enable direct ground-force pilot measurements at downhole positions close to the bit, which enable reliable estimations of the far-field drill-bit-radiated signature. The knowledge of the ground force below the source in the near-field is a key aspect not only for the exploitation of the drill-bit source but also for conventional seismic vibrator sources. The SWD deconvolved results obtained by the ground force are different from those obtained by recording only motion vibration, such as acceleration. The ground force and particle velocity are linked by a complex impedance in the near-field, and together they provide dual measurements. In addition, the ground force provides signals with effective coupling conditions at the bit-rock interface in the presence of the drill-bit displacement action with penetration during rock fracturation. The analysis of the impedance at the bit source includes the drillstring reflections, which are observed in the reference drillstring pilot signal and are simulated by using an appropriate mechanical model. The processing of real seismic signals with different approaches by deconvolution and correlation using ground-force and motion signals produces improvements in the extraction of the source signal, with effective removal of the undesired pilot wavefields, and with cleaner estimations of the drill-bit seismic wavefields in a crosswell survey.