On the efficiency of barometric arrays to improve the reduction of atmospheric effects on gravity data

Abstract

Air pressure reduction in local gravity measurements is usually done by means of an empirical transfer function (barometric admittance) between pressure and gravity both measured at the same site. In the local zone (<50 km) pressure can change rapidly, but is spatially coherent, so pressure observations at the gravity site are sufficient except when a front is passing through the local zone. When large horizontal gradients affect the gravity station, the local zone becomes a critical area for which it may be useful to have more detailed pressure data. One way to model the local atmosphere is to use an array of sensors located around the gravity station. We model the barometric local effect on gravity data using superconducting gravity (SG) data, collected in Strasbourg and barometric records in five sites around the SG station at distances ranging between 10 and 60 km. Six months of gravity and air pressure records are analysed both in the time and frequency domains. The admittance factors for each station are computed by means of different numerical analyses and their time and frequency behaviour investigated. The results demonstrate that in normal weather conditions a single barometer can be enough to account for the pressure effect originating in the local zone and the pressure data from an array are redundant. The use of the barometric array pressure data is found to improve the reduction of the air pressure effects in only one time span among those selected. The improvement is significant at low frequency, while an artificial increase in the residual gravity derives from the array reduction in the high-frequency band (>3.0 cycles/day). The atmospheric reduction from the barometric array is also compared to global loading computations from ECMWF-4Dvar surface pressure data, having a sampling rate of 3 h and a spatial resolution of 0.5°. We also tested a hybrid method based on a spline interpolation of joint ECMWF-4Dvar surface pressure data and local observations collected on the barometric array. The findings are discussed in light of the main features (dimension, geometry and instrumental accuracy) of the barometric array.