Obtaining Scintrex CG6 Gravimeter Scale Factors with an Uncertainty Below 1· 10⁻⁵ in the Zugspitze Gravimeter Calibration System

Transportable spring gravimeters have many applications besides exploration geophysics and potential field determination, such as time-lapse measurements in hydrology and volcanology. For maximum accuracy, the time-dependent scale factor of each gravimeter must be determined before and after use. The required scale uncertainty depends on the target signals, the maximum gravity difference in the network, and the desired level of accuracy. In high alpine regions, large height differences can result in gravity differences of more than 1· 10⁶ 1 · 10 6 nm/s 2 . For a target standard deviation of $$30$$ 30 nm/s 2 , the scale factor must be determined with an uncertainty below 1· 10⁻⁵ 1 · 10 - 5 . The Zugspitze Calibration System, near Garmisch-Partenkirchen in the German Alps, consists of three absolute gravity (AG) stations that cover an elevation range of 2,200 2,200 m and a gravity range of 5,220 5,220 µm/s 2 . Due to unmodeled temporary gravity changes, primarily caused by variations in snow water equivalent and groundwater, the system has a relative stability of approximately 5· 10⁻⁵ 5 · 10 - 5 . Based on 91 measurements taken over 28 months using two Scintrex CG6 spring gravimeters in this calibration system, we present two modeling approaches that attribute the observed gravity changes to either temporal variations in the instrumental scale factor or hydrological mass variations. The results demonstrate that a scale uncertainty of 5· 10⁻⁶ 5 · 10 - 6 can be achieved. This study underscores the importance of determining the temporal gravity variations at each AG reference point of a calibration line when a scale uncertainty below 1· 10⁻⁵ 1 · 10 - 5 is required, e.g., when using transportable spring gravimeters in high alpine regions.