A comprehensive study on the largest Amplitude traceable short gravimetric calibration line in China

Zhenghua Liu ^1,2* Jianing Gou ³ Ji Li ^1,2* Junjie Hu ^1* Xin Zhang ^3*

• ¹ Hubei earthquake agency, Wuhan, China
• ² Institute of Seismology, China Earthquake Administration, Wuhan, Hebei Province, China
• ³ Wuhan Institute of Seismic Metrology and Measurement Engineering Co., Ltd., Wuhan, China, Wuhan, Hebei Province, China

Relative gravimeter must be calibrated periodically on a gravimetric calibration line with a tiny uncertainty. We created a new gravimetric calibration baseline with a 326 μ Galmilligal gravity difference in Jiugong Mountain, central China. The baseline consisted of four absolute gravimetry stations and eight basic relative gravimetry stations. Firstly, we carried out metrological parameters calibration (laser length and atomic clock) for FG5X(#265) and A10(#057) gravimeter with beat frequency experiment and atomic clock comparison respectively. Then we compared absolute gravity measurement from A10#057 and FG5X#265. We obtained a normalized error (i.e., the ratio of the gravity difference over the expended uncertainty of the difference) of 0.58, indicating that the discrepancy between both measurements lies within the measurement uncertainties Then, the comparison of absolute gravimetry with A10 and FG5 Gravimeter shows the normalized error is 0.58, indicating that the absolute gravimetry shows low bias between A10 and FG5. In order to precisely characterize the 4 absolute gravity stations, we conducted i) precise geodetic coordinate measurements, ii) vertical and horizontal gravity gradients, and iii) we estimated the seismic background noise level And the geodetic coordinates, vertical and horizontal gravity gradients, and seismic background noise level of gravity points have been measured carefully using eight GNSS receivers, two Burris gravimeter and a Güralp 40TDE seismometer respectively. Finally, we have conducted relative gravimetric survey using Scincrex CG6(#236 and #238) constrained by FG5X and A10 absolute gravimeter to fulfill metrological traceability of the calibration line. The findings show that the basic relative station's and absolute stations' respective errors on the gravitational acceleration are 3 μGal and 5 μGal. For each gravity difference, the average biases between the FG5X-265 and CG-6 gravimeter are included in the uncertainty of the absolute observations.