High-frequency parasitic modes of superconducting gravimeters |
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| Authors: | Yuichi Imanishi |
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| Institution: | (1) Ocean Research Institute, The University of Tokyo, 1-15-1, Minamidai, Nakano, Tokyo 164-8639, Japan |
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| Abstract: | The superconducting gravimeter (SG) has a long-period instrumental noise called the parasitic mode at periods around 100 s,
whose precise mechanism has not yet been identified. In this paper, another instrumental noise is detected at much higher
frequencies by analyzing the high-rate gravity channel of two SGs in Japan. This is also a parasitic oscillation, characterized
by frequencies on the order of 1 Hz and very high Q values. Detailed spectra indicate that the noise actually consists of two modes with small frequency separations. Based on
a simple theory on the rotational motions of the superconducting sphere in the gravity sensor, the observed modes are tentatively
identified as rotational oscillations of the sphere about two orthogonal axes in the horizontal plane. Interactions between
the parasitic modes are investigated using the spectra acquired on an earthquake, to conclude that the low-frequency parasitic
mode is likely to be a rotational motion of the sphere about the vertical axis. |
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| Keywords: | Superconducting gravimeter Instrumental noise Parasitic mode Rotational motion |
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