Absolute gravity measurements at the iGrav047 site on Helgoland (GFZ Potsdam, AWI facility) with the Hannover gravity meter FG5X-220 in June 2021

Absolute gravity observations were performed with the Hannover FG5-220 absolute gravimeter on Helgoland in Juli 2021Deutsche Forschungsgemeinscha����� (DFG)/Collaborative Research Centre/Project-ID 434617780 – SFB 1464/E

Absolute und relative gravity measurements at ILL Grenoble with the Hannover absolute gravity meter FG5X-220 (Oct. 2021) and the relative meters Scintrex CG-3#4493 and CG-6#171 (Aug. and Oct. 2021)

Before, during and after the experiment 3-14-415 (PF2 UCN) “Weak Equivalence Principle Test with Neutrons” at ILL Grenoble, gravimetric measurements were performed. The PF2 experiment lasted from 24th of August to 13th of October 2021

Absolute gravity measurements at the iGrav047 site on Helgoland (GFZ Potsdam, AWI facility) with the Hannover gravity meter FG5X-220 in July 2020

Absolute gravity measurements with the Hannover gravity meter FG5X-220 were perfomed on the island Helgoland in June 2021Deutsche Forschungsgemeinschaft (DFG)/Exzellenzstrategie des Bundes und der Länder/EXC‐2123 QuantumFron������ers – 390837967/E

Modeling of Atmospheric Gravity Effects for High-Precision Observations

Temporal variations of atmospheric density distribution induce changes in the gravitational air mass attraction at a specific observation site. Additionally, the load of the atmospheric masses deforms the Earth’s crust and the sea surface. Variations in the local gravity acceleration and atmospheric pressure are known to be corrected with an admittance of about 3 nm/s2 per hPa as a standard factor, which is in accordance with the IAG Resolution No. 9, 1983. A more accurate admittance factor for a gravity station is varying with time and depends on the total global mass distribution within the atmosphere. The Institut für Erdmessung (IfE) performed absolute gravity observations in the Fennoscandian land uplift area nearly every year from 2003 to 2008. The objective is to ensure a reduction with 3 nm/s2 accuracy. Therefore, atmospheric gravity changes are modeled using globally distributed ECMWF data. The attraction effect from the local zone around the gravity station is calculated with ECMWF 3D weather data describing different pressure levels up to a height of 50 km. To model the regional and global attraction, and all deformation components the Green’s functions method and surface ECMWF 2D weather data are used. For the annually performed absolute gravimetry determinations, this approach improved the reductions by 8 nm/s2 (-19 nm/s2 to +4 nm/s2). The gravity modeling was verified using superconducting gravimeter data at station Membach inBelgiumimproving the residuals by about 15%

Report on levelling and GNSS results for stations on the MPQ campus in Garching

This technical report describes the levelling and GNSS (Global Navigation Satellite System) results for stations on the MPQ (Max-Planck-Institut für Quantenoptik) campus in Garching, aiming at the derivation of relativistic redshift corrections for novel clock comparison experiments. The underlying observations were carried out mainly in the year 2016, but supplementary information and data were also considered until the end of 2018. The (relative) accuracy of the levelled heights within the internal network on the MPQ campus is estimated to be better than 1 – 2 mm, which is based on the raw double-run levelling discrepancies and loop misclosures involving also stations on rooftops of buildings. The accuracy of the GNSS (ellipsoidal) heights is estimated to be better than 1 cm. The consistency between the levelled and GNSS heights was evaluated internally by approximating the quasigeoid by a horizontal plane as well as externally by comparing with a gravimetric quasigeoid model, yielding maximum residuals of only 2 – 3 mm

Gravity field modelling for optical clock comparisons

A coordinated programme of clock comparisons is carried out within the EMRP-funded project “International Timescales with Optical Clocks” (ITOC), aiming at a validation of the uncertainty budgets of the new optical clocks in view of an optical redefinition of the SI second. Based on Einstein’s general relativity theory, clocks are affected by the gravitational field and the velocity of the clocks. For an Earth-bound clock at rest, the corresponding relativistic redshift effect is directly related to the (geodetic) gravity potential. As optical clocks are now targeting a relative accuracy of 10^{-18}, corresponding to a sensitivity of about 0.1 m^2/s^2 in terms of the geopotential or 0.01 m in height, precise knowledge of the gravity potential is required at the respective clock sites. Alternatively, optical clocks may also be employed for deriving the gravity potential (denoted as “chronometric levelling” or “relativistic geodesy”) and hence offer completely new options for geodetic height determination. The ITOC project involves clock sites at the national metrological institutes in France, Germany, Italy and the United Kingdom. In order to determine the gravity potential with best possible accuracy at these sites, two approaches are considered, namely geometric levelling and GNSS ellipsoidal heights in combination with a gravimetric (quasi)geoid model. Additional absolute and relative gravity observations were carried out around the clock sites and then used to compute an updated quasigeoid model. The general strategy, the work undertaken so far, the update of the quasigeoid models as well as corresponding differences and accuracies are presented. The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union

Geodetic-Gravimetric Monitoring of Mountain Uplift and Hydrological Variations at Zugspitze and Wank Mountains (Bavarian Alps, Germany)

In 2004, first absolute gravity (AG) measurements were performed on the top of Mt. Zugspitze (2 sites) and at the foot (1 site) and top (1 site) of Mt. Wank. Mt. Wank (summit height 1780 m) and Mt. Zugspitze (2960 m) are about 15 km apart from each other and belong geologically to different parts of the Northern Limestone Alps. Bridging a time span of 15 years, the deduced gravity variations for Zugspitze are in the order of −0.30 μm/s2 with a standard uncertainty of 0.04 μm/s2. The Wank stations (foot and top) show no significant gravity variation. The vertical stability of Wank summit is also confirmed by results of continuous GNSS recordings. Because an Alpine mountain uplift of 1 or 2 mm/yr cannot explain the obtained gravity decline at Zugspitze, the dominating geophysical contributions are assumed to be due to the diminishing glaciers in the vicinity. The modelled gravity trend caused by glacier retreat between epochs 1999 and 2018 amounts to −0.012 μm/s2/yr at both Zugspitze AG sites. This explains more than half of the observed gravity decrease. Long-term variations on inter-annual and climate-relevant decadal scale will be investigated in the future using as supplement superconducting gravimetry (installed in 2019) and GNSS equipment (since 2018)

Geodetic monitoring of subrosion-induced subsidence processes in urban areas: Concept and status report

The research project SIMULTAN applies an advanced combination of geophysical, geodetic, and modelling techniques to gain a better understanding of the evolution and characteristics of sinkholes. Sinkholes are inherently related to surface deformation and, thus, of increasing societal relevance, especially in dense populated urban areas. One work package of SIMULTAN investigates an integrated approach to monitor sinkhole-related mass translations and surface deformations induced by salt dissolution. Datasets from identical and adjacent points are used for a consistent combination of geodetic and geophysical techniques. Monitoring networks are established in Hamburg and Bad Frankenhausen (Thuringia). Levelling surveys indicate subsidence rates of about 4-5 mm per year in the main subsidence areas of Bad Frankenhausen with a local maximum of 10 mm per year around the leaning church tower. Here, the concept of combining geodetic and gravimetric techniques to monitor and characterise geological processes on and below the Earth's surface is exemplary discussed for the focus area Bad Frankenhausen. For the different methods (levelling, GNSS, relative/absolute gravimetry) stable network results at identical points are obtained by the first campaigns, i.e., the results are generally in agreement. © 2017 Walter de Gruyter GmbH, Berlin/Boston 2017

A Gravimetric Support Network for Very Long Baseline Atom Interferometry

With the introduction of portable atom interferometers (AI), a genuinely independent method for the determination of g is available for the first time since the introduction of laser interferometer based instruments. Current AIs offer several advantages and already reach the accuracy of classical sensors. Additionally, a small number of stationary experiments were implemented for research in fundamental physics and geodesy. These instruments, extending the free fall distance of atoms to several meters, allow for longer evolution times of the wave function, thereby increasing the sensitivity of the AI compared to decimetres in portable devices. The construction of an AI with a 9 m interaction zone is currently being completed at Leibniz University Hannover. The knowledge of g and its gradient is required for the evaluation of systematic effects and uncertainties in AI experiments. Therefore, a gravimetric control network connected to one absolute gravimeter pier was established and repeatedly observed during the construction of the Very Long Baseline Atom Interferometry facility (VLBAI). Before the installation of the instrument, this network included the central axis of the VLBAI and one vertical off-axis parallel profile. The latter profile can also be observed during operation of the VLBAI. The effect of local gravity changes, e. g., hydrology, is comparable to 1 nm/s² on both axes. The gravimetric measurements serve as a reference during initial tests of the VLBAI. Repeated observations in the future will be used to characterize the effect of local hydrology and other mass variations along the vertical axis. A model of the research building and groundwater level monitoring supplements the gravimetric network. As the VLBAI is capable of measuring g and its vertical gradient with higher accuracy (<1 nm/s²) than classical instruments, the model will be used to transfer g to a gravimetry laboratory for gravimeter comparisons. We present our strategy for gravimetric control of the VLBAI. This will provide a reference at first and will later be used to establish the VLBAI as a reference for gravimeter comparisons. The results of the first gravimetric campaigns and the comparison with the model of the VLBAI environment show an agreement within the instrumental uncertainties of the relative gravimeters used

A superconducting gravimeter on the island of Heligoland for the high-accuracy determination of regional ocean tide loading signals of the North Sea

The superconducting gravimeter GWR iGrav 047 has been installed on the small offshore island of Heligoland in the North Sea approximately at sea level with the overall aim of high-accuracy determination of regional tidal and non-tidal ocean loading signals. For validation, a second gravimeter (gPhoneX 152) has been setup within a gravity gradiometer approach to observe temporal gravity variations in parallel on the upper land of Heligoland. This study covers the determination of regional ocean tide loading (OTL) parameters based on the two continuous gravimetric time-series after elimination of the height-dependent gravity component by empirical transfer functions between the local sea level from a nearby tide gauge and local attraction effects. After reduction of all gravity recordings to sea level, both gravimeters provide very similar height-independent OTL parameters for the eight major diurnal and semidiurnal waves with estimated amplitudes between 0.3 nm s−2 (Q1) and 11 nm s−2 (M2) and RMSE of 0.1–0.2 nm s−2 for 2 yr of iGrav 047 observations and a factor of 2 worse for 1.5 yr of gPhoneX 152 observations. The mean absolute OTL amplitude differences are 0.3 nm s−2 between iGrav 047 and gPhoneX 152, 0.4 nm s−2 between iGrav 047 and the ocean tide model FES2014b and 0.7 nm s−2 between gPhoneX 152 and FES2014b which is in good agreement with the uncertainty estimations. As by-product of this study, OTL vertical displacements are estimated from the height-independent OTL gravity results from iGrav 047 applying proportionality factors dh/dg for the eight major waves. These height-to-gravity ratios and the corresponding phase shifts are derived from FES2014b. The OTL vertical displacements from iGrav 047 are estimated with amplitudes between 0.4 mm (Q1) and 5.1 mm (M2) and RMSE of 0.1–0.7 mm. These OTL amplitudes agree with FES2014b within 0.0 (M2) and 0.8 mm (K1) with a mean difference of 0.3 mm only. The OTL amplitudes from almost 5 yr of GNSS observations show deviations of up to 6 mm (M2) compared to vertical displacements from both iGrav 047 and FES2014b, which suggests systematic effects included in the estimation of OTL vertical displacements from GNSS. With the demonstrated accuracy, height-independent sensitivity in terms of gravity and vertical displacements along with the high temporal resolution and the even better performance with length of time-series, iGrav 047 delivers the best observational signal for OTL which is representative for a large part of the North Sea