Vibration error compensation algorithm in the development of the laser interference absolute gravimeter

Measurement error arising from vibration interference is recognized as the primary obstacle limiting the accuracy and stability of laser interference absolute gravimeters. The present work addresses this issue by proposing a global search optimization algorithm that determines the optimal absolute value of gravity based on the measured time-displacement coordinates of a falling body and the signal obtained from the passive vibration isolation system of the inertial reference corner-cube in a laser interference absolute gravimeter. Results of numerical calculations conducted under vibration interference conditions with added white noise resulting in a signal-to-noise ratio of 40 dB demonstrate the following. (1) The accuracy and standard deviation of the gravimeter obtained using the proposed algorithm are −0.04 μGal (1 μGal = 1 × 10−8 m/s2) and 0.24 μGal, respectively, while those values obtained by the standard least-squares solution are 10.19 μGal and 154.11 μGal, respectively. (2) The resolution of the test results shows that the average response of the reference value of acceleration due to gravity superimposed by a disturbance of 1.00 μGal is 1.01 μGal using the proposed algorithm and 0.87 μGal using the standard least-squares solution.

Time and tide: pendulum clocks and gravity tides

Tidal fluctuations in gravity will affect the period of a pendulum and hence the timekeeping of any such clock that uses one. Since pendulum clocks were, until the 1940s, the best timekeepers available, there has been interest in seeing if tidal effects could be observed in the best performing examples of these clocks. The first such observation was in 1929, before gravity tides were measured with spring gravimeters; at the time of the second (1940–1943), such gravimeters were still being developed. Subsequent observations, having been made after pendulum clocks had ceased to be the best available timekeepers and after reliable gravimeter measurements of tides, have been more of an indication of clock quality than a contribution to our knowledge of tides. This paper describes the different measurements and revisits them in terms of our current knowledge of Earth tides. Doing so shows that clock-based systems, though noisier than spring gravimeters, were an early form of an absolute gravimeter that could indeed observe Earth tides.

Repeated absolute gravity measurements on a dense network at Campi Flegrei – a reliable tool for volcano monitoring

Since 1981, relative gravity measurements have routinely carried out at the Campi Flegrei caldera, a densely populated area. The gravity network also includes two absolute stations periodically measured with a laboratory absolute gravimeter, which does not permit field measurements. At the end of 2014, the Osservatorio Vesuviano, Section of Napoli of the Istituto Nazionale di Geofisica e Vulcanologia (INGV-OV), acquired a portable absolute gravimeter that allows field operations on outdoor sites. Therefore, in 2015 a dense absolute gravity network was established in Campi Flegrei. This will permit an advanced approach for volcano monitoring. The net consists of 36 stations, 34 of which located inside the caldera and placed upon or very close to gravity stations belonging to the relative network. Five surveys were carried out on June 2015, on February and November 2017, on October 2018 and on October 2019. The comparison with height changes suggests that significant Δg are partly due to the uplift occurred over the same time intervals and mostly to shallow processes associated to the dynamic of the local hydrothermal system. The comparison with the gradients observed during the last large uplift (1982–1984) and the following subsidence (1985–2003) confirms this observation. These results suggest that the present activity may be due to a transient or pulsating phenomenon as the alternating recharge/discharge of fluids in the surface hydrothermal system. Gravity changes detected by absolute measurements are in good agreement with those obtained by relative ones, and confirms the feasibility of this methodology for volcano monitoring. Finally, they also encourage replacing the relative networks with absolute ones, with all the consequent advantages.

Raman-Laser System for Absolute Gravimeter Based On 87Rb Atom Interferometer

The paper describes a Raman-laser system with high performance for an absolute gravimeter that was based on 87Rb atom interferometer. As our gravimeter is a part of the standard acceleration of gravity of China, the Raman lasers’ characteristics should be considered. This laser system includes two diode lasers. The master laser is frequency locked through the frequency-modulation (FM) spectroscopy technology. Its maximum frequency drift is better than 50 kHz in 11 h, which is measured by home-made optical frequency comb. The slave laser is phase locked to the master laser with a frequency difference of 6.8346 GHz while using an optical phase lock loop (OPLL). The phase noise is lower than −105 dBc/Hz at the Fourier frequency from 200 Hz to 42 kHz. It is limited by the measurement sensitivity of the signal source analyzer in low Fourier frequency. Furthermore, the power fluctuation of Raman lasers’ pulses is also suppressed by a fast power servo system. While using this servo system, Raman lasers’ pulses could be fast re-locked while its fast turning on again in the pulse sequence. The peak value fluctuation of the laser power pulses is decreased from 25% to 0.7%, which is improved over 35 times. This Raman-laser system can stably operate over 500 h, which is suited for long-term highly precise and accurate gravity measurements.