Measurements in the Earth mode frequency range by an electrostatic sensing and feedback gravimeter

A gravirneter designed to use an electrostatic null-seeking servo is described. The position sensor is a capacitor plate system. A phase-sensitive detection system is used, and feedback forces are applied to the mass via the same capacitor plate system. The design of the system was such that all aspects of its behavior, including the effects of all internal noise sources could be calculated and measured. With this instrument noise was studied in the frequency range 1 cph to 30 cph. The results are described along with an analysis of the instrumental noise sources and measurements of their power spectrums. The lowest instrumental noise level 10−22 (δg/g)2/cph can be attributed to rectification of internal instrument modes driven by high-frequency seismic noise and to the effects of aliasing of high-frequency seismic noise. Measurements show that internal noise sources (temperature fluctuations, convection, and electronic noise) are two orders of magnitude too low to account for the observed noise level.