In this paper an outline of a servohydraulic-type vibrator which we designed as a trial is described. The vibrator is capable of producing high peak force to 1 ton and broadband frequencycontrolled sweeps of both variable amplitude and sweep rate in vertical as well as horizontal vibration direction. Methods of forced vibration tests are discussed giving a description of the electrical-hydraulic method of controlling forced vibration force. Some preliminary tests on forced vibration tests of reinforced concrete footing are carried out to check the performance of the vibrator. It is shown that methods of signal enhancement are successfuly applied in the field during data acquisition as well as in computer data processing. In order to investigate the effect of sweep period on the frequency response, of a subsurface irregular ground, two different sweep period, i.e. 9 sec and 300 sec, of sweep generation ranged from 5 Hz to 30 Hz are applied. Compared those 'results, we have shown that shorter sweep period, i. e. 9 sec, is adequate to aquire a steady-state response of the ground which is convenient in computer data processing.