Precise Correction of Gravity Data for the Effect of Local Atmospheric Loading at Beijing Seismic Station, China

Abstract

The gravity effect due to local atmospheric loading is an important signal and correction in various geometrical applications. In this paper, we calculated loading effects on gravity using Farrell’s method with the gravity loading Green’s function from the Preliminary Reference Earth Model (PREM). The elastic part of the gravity loading Green’s function was calculated using a linear combination of Love numbers and Legendre polynomial. The direct part of the gravity loading Green’s function was calculated using Newtonian gravitational theory. Gravity Green’s functions are the sum of the abovementioned two Green’s functions (the elastic and direct parts). The integral elevation used to calculate air mass was 86 km. In this paper, we conducted integration only over local areas (0.5° angular distance from the gravity station). Under dry and wet air conditions, the difference in the gas constant R was 6.052 J/kg K, and the corresponding gravity change was approximately 1.3 µGal. The gas constant variation under relative humidity between 40 and 80% at the Beijing seismic station was 0.76 J/kg K, and the gravity variation was 0.1 µGal. The gas constant of dry air was selected to compute the direct part of the Green’s function given the limited gravity variation in this range of relative humidity. The atmospheric gravity Green’s functions were convolved with the observed pressure; the influence did not exceed 10 µGal within 50 km of the gravity station. The influence of temperature, station height, and topography on gravity was mostly in the near region of the station (angular distance smaller than 0.1°), with the maximum impact from 1000-m station height (4.68 µGal), followed by 1000-m topographic height (2.93 µGal). The impact of temperature at −30°was 0.15 µGal.