A Vertical Seismometer With Build-in Retroreflector for Absolute Gravimetry

Abstract

A free-fall absolute gravimeter uses a Mach-Zehnder interferometer to track the free-falling object. Theoretically, it needs an inertial reference point, which is a reference retroreflector keeping static in inertial frame for an accurate absolute gravimetry. Practically, the reference retroreflector is always disturbed by the ground vibration. Traditionally, a vibration correction method with a broadband seismometer is used to reduce the effect of the ground vibration. The transfer function between the reference retroreflector and the seismometer is hypothesized as a proportional element with time delay. The difference between the hypothesized and the real transfer function limits the effect of the vibration correction. On this basis, a modified method, replacing the sensitive element of a seismometer with the reference retroreflector, is proposed. The motion of the reference retroreflector is measured directly by differential parallel plate capacitance detection. A closed-loop control circuit produces feedback voltage to make the reference retroreflector track the ground vibration. The feedback voltage represents the reference retroreflector’s motion directly. Experiments show the capacitance detecting circuit detects the displacement of the reference retroreflector relative to the ground with a resolution of 0.6 nm at 500 Hz. The acceleration resolution of the homemade vertical seismometer is about 10 mGal. The sensitivity of the seismometer is 316 V/g. The damp ratio of the homemade seismometer is little, and the natural frequency of the homemade seismometer is 104 Hz by analyzing the step response of the system. The bandwidth of the system is around 175 Hz. In the future, the homemade seismometer will be applied in absolute gravimeters for hostile measurement.