Effects of horizontal acceleration on the superconducting gravimeter CT #036 at Ishigakijima, Japan

Yuichi Imanishi,Hiroshi Ikeda,Kazunari Nawa,Yoshiaki Tamura

doi:10.1186/s40623-018-0777-9

Abstract

In the gravity sensor of a superconducting gravimeter, a superconducting sphere as a test mass is levitated in a magnetic field. Such a sensor is susceptible to applied horizontal as well as vertical acceleration, because the translational degrees of freedom of the mass are not perfectly limited to the vertical direction. In the case of the superconducting gravimeter CT #036 installed at Ishigakijima, Japan, horizontal ground acceleration excited by the movements of a nearby VLBI antenna induces systematic step noise within the gravity recordings. We investigate this effect in terms of the static and dynamic properties of the gravity sensor using data from a collocated seismometer. It is shown that this effect can be effectively modeled by the coupling between the horizontal and vertical components in the gravity sensor. It is also found that the mechanical eigenfrequency for horizontal translation of the levitating sphere is approximately 3 Hz.

Highlights

In the superconducting gravimeter (SG) (Prothero and Goodkind 1968; Goodkind 1999; Hinderer et al 2015), the elastic restoring force as in mechanical spring-type gravimeters is replaced with a magnetic levitation force exerted on a superconductor
In this paper, we investigate this effect in a systematic way from the viewpoint of a gravity sensor based on magnetic suspension
In this paper, we investigated the possible effects of horizontal acceleration on the SG operated at Ishigakijima from the viewpoint of a gravity sensor based on magnetic suspension

Summary

Introduction

In the superconducting gravimeter (SG) (Prothero and Goodkind 1968; Goodkind 1999; Hinderer et al 2015), the elastic restoring force as in mechanical spring-type gravimeters is replaced with a magnetic levitation force exerted on a superconductor. Because the vertical restoring force as given by Eq (12) decreases for finite r, tilting the SG causes a horizontal displacement of the sphere from the central axis and a downward motion of the sphere, i.e., an apparent increase in the gravity (GWR Instruments 1985) When adjusting the supporting magnetic field, we measure the displacement of the levitating sphere in response to a constant vertical force generated by a 10 mA current. This is called the “magnetic gradient,” a parameter that corresponds to the inverse of the stiffness of the spring. It is noted that the precondition for the analysis ( fH ≫ fV) is satisfied because fV is on the order of 0.1 Hz

Findings

Discussion

Conclusions