A Vibration Compensation Method for Absolute Gravimeters

Abstract

The laser interferometer is used to track the falling object in a freefall absolute gravimeter, which could be disturbed by the vibration from the ground. Thus, the vibration compensation method is often used to reduce the influence of the vibration. Typically, a sensor (broadband seismometer) is used to record the vibration. But the measured ‘vibration’ Nm(t) does not equal the motion of the reference corner cube N(t). Because there exists a transfer function G(s) making Nm(s) = G(s)N(s). Traditionally, G(s) is assumed to be equal to the transfer function of the sensor, which can be achieved with the help of other equipment. But the assumption is not reasonable and the process of calculating the transfer function is complicated. A novel vibration compensation method without any other equipment is proposed in this paper. In this method, G(s) is simplified to estimate N(t) using N′(t) = ANm(t − τ), which is used for compensation. The gain A and delay τ can be obtained by analysis of the data acquired by the absolute gravimeter. The experiments are conducted with the homemade absolute gravimeter T-1 and repeated for 75 times. The standard deviation of the uncompensated results is 3276 μGal (1 μGal = 1 × 10−8 m/s2), while that of the compensated results is 167 μGal. The compensation method not only achieves a reduction by nearly a factor of 20, but also can be simply used without any other equipment. The results indicate that the method basically meets the demands of absolute gravimeters. In the future, it may be applied to dynamic absolute gravity measurement and take the place of vibration isolators.