The present invention relates to marine seismic exploration, and more particularly to marine seismic exploration in which a seismic source is coupled to the ocean floor to generate acoustic pulses. 
In present seismic exploration, acoustic pulses are generated by seismic sources, propagate through the earths crust, are reflected by subsurface interfaces and detected upon the return to the surface.  In marine exploration, seismic sources have taken the form of explosive charges and airguns.  However, both of these types of seismic sources have had deleterious effects on marine life.  As a result, a hydraulic vibrator had been developed.  The hydraulic vibrator used in marine exploration is similar to that used in land based seismic exploration.  This type of seismic source has been found to have less deleterious effects on marine ecosystems. 
In seismic pulse generation, it is beneficial to be able to generate pulses over a wide frequency range.  In this regard, the use of hydraulic vibrators includes a problem in the range of frequencies generated.  In general, a hydraulic vibrator system includes a hydraulic power plant, a hydraulic cylinder, hydraulic circuitry and structural members designed to operate over a range of frequencies.  Stroke and flow requirements for low frequency operation necessarily are exclusive of high frequency operation due to their size and mass.  Similarly, stroke and flow design requirements concommitant with high frequency propagation exclude the applicability of these vibrator systems from use in low frequency systems.