Evaluation of high frequency vibrator response

Abstract

Accurate analysis of the motion of a commercial high frequency hydraulic vibrator commonly used for near-surface applications demonstrated that the rigid body assumption of the weighted-sum approximation is not valid throughout the vibrator's operational frequencies. This study reveals significant response variability across the baseplate, which is dependent on accelerometer position with respect to radial position and internal baseplate structure. Consequently, the baseplate cannot be considered a point source of propagation, which complicates optimizing source operations to increase data fidelity. In an effort to optimize the source signature approximation to increase data fidelity, simultaneous acquisition of baseplate acceleration and pressure beneath the baseplate provided a means to directly compare the response of strategically placed accelerometers to the true ground force. This study concludes that the most representative approximation occurs when multiple accelerometers are positioned on the baseplate to average the baseplate motion. In addition, this study found that the IVI Minivib I is incapable of providing measurable seismic energy at frequencies over 200 Hz due to opposing baseplate and reaction mass phase. Based on this observation, it is clear the design of the baseplate needs to be modified by adding extra weight and rigidity to the driven structure. Increasing rigidity of the baseplate will reduce source generated harmonic distortion caused by baseplate flexure resulting in a more uniform response across the baseplate and a more accurate ground force approximation. Additionally, the opposing phase relationship between the baseplate and the reaction mass could be remediated by increasing the baseplate weight resulting in an increase in energy above 200 Hz.