Optimizing the Generation and QC of Low-Dwell Sweeps

Low-dwell sweeps are now currently used in production, as a result of mechanical enhancements in vibrators and custom sweep methodologies. Nonetheless, the low-dwell ramp-up required to preserve the target force at low frequencies increases sweep duration and can impact crew productivity. The ramp-ups must be carefully fine-tuned to respond to this new paradigm, thanks to vibroseis controllers and QC computation evolution.

At low frequencies, the classic ground-force zero-crossing feedback loop becomes indeed obsolete while system non linearity and the associated distortion increase. Vibroseis controllers need to perform a higher rate ground-force command, and be adapted to provide optimum gain and phase to accurately fit the sweep ground-force to the desired pilot.

Low-dwell sweeps and vibrator limitations can be simulated prior to production thanks to adapted tools that take into account factors such as the initial phase. Such simulations show that two identical sweeps, differing only on the initial phase, can exceed or not the vibrators’ physical limitations, consequently allowing the sweep optimization and validation. While rarely done in practice for low-dwell ramp-ups, QC is essential to properly monitor sweeps. Computation methods evolve: the use of an appropriate window and normalized QC values results in accurate and exploitable QC data (Patent pending).