a vibrator seismic source has a hydraulic system , including a pad , for applying reciprocating forces to the ground and sensor means for generating a feedback signal representative of the force applied to the ground . circuitry is provided for generating a pilot signal representative of the desired force to be applied to the ground . the pilot and feedback signals are single sideband shifted to increase both frequencies by a predetermined frequency . the high frequency pilot and feedback signals have their phase difference conventionally detected and provided to a digital computer for phase shifting the signal for controlling the vibrator so that the feedback signal is in phase with the pilot signal . in another embodiment , the instantaneous phase of each signal is computed continuously by a digital computer . the control signal digital representation is corrected to correspond with the pilot digital representation and an analog output signal is generated to provide the proper analog control signal .