A method and related apparatus are described for generating acoustic signals for use in a vibratory seismic survey, including the step of combining into a drive signal a high frequency sweep signal, which sweeps upwardly through a high frequency band during a first time interval, and a low frequency sweep signal which is of lower amplitude than the high frequency sweep signal and which sweeps upwardly through a low frequency band during a second time interval, wherein the second time interval starts during the first time interval but after the beginning thereof; and applying the drive signal to a mechanical drive system for a vibratable element. The method improves the utilization of a single vibratory source.