Impact of environmental phenomena on continuous relative gravity measurements performed in urban area

The relative monitoring gravimeter gPhoneX #108 was installed in the building of Faculty of Civil Engineering of the Slovak University of Technology in Bratislava in the beginning of 2016. Main purpose of the paper is to analyze how external environmental phenomena generating non-gravitational signal, such as strong human activity, ambient noise and tilting of observing site due to solar radiation, can influence the quality of relative gravity measurements and their suitability for geodynamic applications. For this purpose, we accomplished spectral analysis of gravity data with different sampling rate, examination of instrumental drift, and tidal analysis of hourly gravity records. Our study showed that the gPhoneX #108 gravity measurements are adversely affected by the tilting of the building, where the gravimeter is located. This effect produces a significant non gravitational signal with characteristic daily and seasonal variation depending on the weather conditions. The instability of the observing site also generates a strong non-gravitational signal and corrupts the estimation of tidal parameters mainly in a diurnal tidal band. The investigation of the instrumental drift proved its irregular character due to the changes of temperature in the operating room. Another limitation factor for detection of small gravity changes is the level of ambient noise with the average daily amplitude of about 100 nm s^?2. Obtained experimental results can be useful for planning and performing the relative gravity measurements in a noisy environment, or in the case of an instable observing site. The most significant non-gravitational signal with variable daily and seasonal influence was caused by the instability of the observing site. In order to minimize this influence we recommend regular calibration of the built-in tiltmeters.