The Limitations of Generating Broadband Signals Using Hydraulic Vibrators and How to Overcome Them

Hydraulic vibrators are the most popular source for land seismic surveys despite historically having a limited bandwidth, usually less than 3.5 octaves. At low frequencies (<10 Hz) output is limited by the amount that the reaction mass can move (the stroke). At mid-frequencies (10–80 Hz), output is limited by the hold-down. At high-frequencies (>80 Hz), the predominant limitation is the phase difference between the reaction-mass and the baseplate which is a result of the ground conditions. These limitations can be overcome to a large extent by reducing the drive level where appropriate while decreasing the instantaneous sweep rate to ensure that the energy is not reduced. This process has been shown to enable transmission of sweeps with bandwidths of more than seven octaves. It must always be kept in mind, however, that the vibrator is only half of the equation; if not adequately coupled to the surface, or if the surface is not compliant, then the energy will not be transmitted.