Hydraulic limitations, non‐rigidity of the baseplate as well as variable characteristics of the ground constantly distort the downgoing energy output by vibrators. Therefore, a real time feedback control must be performed to continuously adjust the emitted force to the reference pilot signal. This ground force is represented by the weighted sum of the reaction mass and the baseplate accelerations. It was first controlled with an amplitude and phase locked loop system, poorly reactive and sensitive to noise. Later on, new vibrator electronics based on a digital model of the vibrator were introduced. This model is based on the physical equations of the vibrator and of the ground. During an ‘identification’ process, the model is adjusted to each particular vibrator. Completed by a Kalman adaptive filter to remove the noise, it computes ten estimated states via a linear quadratic estimator. These states are used by a linear quadratic control to compute the torque motor input and to compare the ground force estimated from the states with the pilot signal. Test results using downhole geophones demonstrate the benefit of filtered mode operation.

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).

The development of high‐productivity seismic recording using the vibroseis method over the last 30 years is treated from a Sercel perspective. The simultaneous use of vibrators has been highly dependent on real‐time field computing capabilities such as those delivered by the correlator‐stackers in the early 1980s. A second step forward in the mid 1980s–early 1990s was related to digital vibrator electronics, which provides efficient management and quality control of fleets of vibrators. However, it was only in the late 1990s with the advent of real‐time satellite positioning (GPS) of these fleets that alternate or simultaneous sweeping was commonly used in production. During the last ten years, thanks to GPS timing, continuous data recording and innovative simultaneous sourcing methodologies, vibroseis has been able to reach unexpected levels of productivity. As a result, the cost per seismic trace has dropped, enabling denser spatial sampling and associated seismic imaging improvement.

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