The Vibrator and its Interaction with the Ground

Throughout the decades of its use, there has always been a requirement to obtain an accurate estimate of the propagating wave emitted by the vibrator mechanism. This is even more relevant in today's recording environment of a low signal amplitude and high noise when employing a single source, single receiver, and 'overlapping signals' from simultaneous acquisition techniques. The ground force estimate (or its time derivative) is assumed by many to be a representation of the seismic wave (its source signature) that is transmitted into the surrounding medium. However, from published articles, many authors have questioned this assumption noting significant phase and amplitude differences between the ground force and the signal received at a remote sensor, especially at higher frequencies. Having an accurate source signature estimate at each location has significant ramifications beyond improving the quality of acquisition (and processing) that today is heavily reliant on 'shooting blind' into single channel nodes: it implies that the vibrator-earth model used is an accurate representation of the changing ground conditions at each shot-point whose properties can then be determined. In addition, an accurate vibrator-earth model may also lead to improvements in the design of vibrators for increased amplitude (greater signal) of the propagating wave. In this paper, we discuss the interaction between the vibrator mechanism and the surrounding media, the forces and energies generated during a sweep, re-examine the assumptions made in the ground force models and show that current source signature estimates are not an accurate representation of the propagating wavelet.