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An iGrav superconducting gravimeter has been installed on the small offshore island of Helgoland in the North Sea approximately at sea level with the overall aim of high-accuracy determination of regional ocean tide loading (OTL) and non-tidal ocean loading (NTOL) signals. For validation, a second gravimeter (gPhoneX) has been setup to observe temporal gravity variations in parallel on the upper land of Helgoland. The first part of this study covers the determination of regional OTL parameters 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, the regional OTL parameters from both gravimeters agree at the level of a few 0.1 nm/s². The second part of this study shows the gravity residuals that are largely dominated by regional NTOL signals. Driven by strong westerly and easterly winds, the water level fluctuates between 1.15 m above mean high water and 1.50 m below mean low water, inducing gravity increases of up to 30 nm/s² and decreases of down to -40 nm/s², respectively, during the analysed time period. These high-accuracy gravity observations will be used as “ground truth” for the calibration and validation of ocean tide models and satellite gravimetry from GRACE Follow-On as well as corresponding de-aliasing products. Finally, the SG network in Central Europe will strongly benefit from an improved knowledge of non-tidal ocean loading signals of the North Sea., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

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

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

Absolute gravimeters are used in geodesy, geophysics and physics for a wide spectrum of applications. Stable gravimetric measurements over timescales from several days to decades are required to provide relevant insight into geophysical processes. Users of absolute gravimeters participate in comparisons with a metrological reference in order to monitor the temporal stability of the instruments and determine the bias to that reference. However, since no measurement standard of higher-order accuracy currently exists, users of absolute gravimeters participate in key comparisons led by the International Committee for Weights and Measures. These comparisons provide the reference values of highest accuracy compared to the calibration against a single gravimeter operated at a metrological institute. The construction of stationary, large-scale atom interferometers paves the way for a new measurement standard in absolute gravimetry used as a reference with a potential stability up to 1 nm/s 2 at 1 s integration time. At the Leibniz University Hannover, we are currently building such a very long baseline atom interferometer with a 10-m-long interaction zone. The knowledge of local gravity and its gradient along and around the baseline is required to establish the instrument’s uncertainty budget and enable transfers of gravimetric measurements to nearby devices for comparison and calibration purposes. We therefore established a control network for relative gravimeters and repeatedly measured its connections during the construction of the atom interferometer. We additionally developed a 3D model of the host building to investigate the self-attraction effect and studied the impact of mass changes due to groundwater hydrology on the gravity field around the reference instrument. The gravitational effect from the building 3D model is in excellent agreement with the latest gravimetric measurement campaign which opens the possibility to transfer gravity values with an uncertainty below the 10 nm/s2 level., Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659, Bundesministerium für Bildung und Forschung http://dx.doi.org/10.13039/501100002347, Niedersächsisches Ministerium für Wissenschaft und Kultur http://dx.doi.org/10.13039/501100010570, https://www.bipm.org/kcdb

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.

Das Verbundprojekt SIMULTAN erforscht die Früherkennung für Instabilität, Unruhe und Kollaps von Erdfällen. Der neuartige Forschungsansatz kombiniert strukturelle, geophysikalische, petrophysikalische und hydrologische Kartierungsmethoden, die von Sensorentwicklung und mulit-skaliger Überwachung flankiert werden, und umfasst eine Informationsplattform. Kollapsprozesse an Erdfällen finden generell in den obersten wenigen 100 Metern der Erdkruste statt. Individuelle Prozesskomponenten können einfach sein und verstanden werden. Aber es wechselwirken auch Prä-Kollapsprozesse und Vorläufer auf unterschiedlichen raum-zeitlichen Skalen und mit kleinen Variationen miteinander. Dies erfordert innovative, multi-skalige Beobachtungen, Analysen und integrierte Früherkennungskonzepte, besonders für urbane Bereiche, die bisher noch nicht vollständig entwickelt verfügbar sind und auch noch nicht als automatische operationelle Systeme arbeiten. Zur Identifizierung und Quantifizierung von Subrosionsbereichen zeigten sich bohrlochseismische Verfahren mit kombinierten P- und S-Wellen als zielführend. Als besonderer Indikator, der sich auch für Langzeitmonitoring eignet, hat sich dabei das ungewöhnliche Konversionsverhalten der Wellen in Subrosionszonen gezeigt. Neue Prozessingverfahren zur Detektion kleiner seismischer Ereignisse und für emergente Einsätze unterstützen den Ansatz von angepassten Arraymessungen. Geodätisch-gravimetrische Überwachungsnetze sind auch unter urbanen Bedingungen geeignet, durch Subrosion verursachte Oberflächendeformationen und Massenverlagerungen räumlich-zeitlich zu erfassen und zu überwachen. Nivellements liefern bezüglich der Oberflächendeformation höchste Genauigkeiten; zugleich sind sie vergleichsweise kostengünstig zu realisieren. Die Integration von GNSS hat in Verbindung mit dem Nivellement die räumliche Auflösung im Untersuchungsgebiet zusätzlich gestützt und hat das Potenzial, diese großräumig zu kontrollieren. Gravimetrische Messungen sind mit einem sehr hohen Aufwand verbunden. Die Kombination oberflächengeophysikalischer Verfahren mit einer vertikal hochaufgelösten direct push-basierten Erkundung im Raum mit einem maßgeschneiderten hydrogeologischen Monitoring bietet eine zuverlässige Grundlage für numerische Prozessmodellierungen und ermöglichen es, erdfallrelevante Prozesse abzubilden und zu erfassen. Karstaquifer- und geomechanische Modellierungen konnten die Prozesse in den Fokusgebieten erfolgreich abbilden, sobald diese mit realistischen Werten unterlegt wurden. Es zeigt sich somit insgesamt, dass sowohl flächenhafte Messungen und zeitlich wiederholte Kampagnen sehr vorteilhaft sind, da sie es erlauben, Fehler zu verringern und beanspruchte Bereiche einzugrenzen. Erst dieses integrierte Vorgehen ermöglicht es, potenzielle Wegsamkeiten im Untergrund hochauflösender als bisher abzubilden und zu bewerten. Als solches stellt es ein Instrumentarium zur Charakterisierung und angepassten Überwachung von Untergrundbereichen im Allgemeinen zur Verfügung. Diese Ergebnisse und Szenarien stehen der Öffentlichkeit auf einer Informationsplattform zur Verfügung (http://simultan.gfz-potsdam.de). BMBF - Bundesministerium für Bildung und Forschung - Projektträger Jülich/Sonderprogramm GEOTECHNOLOGIEN (Frühwarnsysteme)/03G0843 (A bis J)/EU

Subrosion, d.h. die Auslaugung und Verlagerung leicht lösbarer Gesteine im Untergrund, ist ein weltweites Phänomen. Sie führt zu lokalen Massenverlusten und damit zur Entwicklung von Erdfällen in Form von Subrosionssenken bis hin zu Kollapsereignissen, was im urbanen Raum ein erhebliches Risiko für Infrastruktur und Menschenleben darstellt. Dieser Beitrag beschäftigt sich mit der geodätisch-gravimetrischen Überwachung zweier Erdfallgebiete in Bad Frankenhausen und Hamburg-Flottbek. In lokalen Monitoringnetzen wurden über einen Zeitraum von mehreren Jahren regelmäßig wiederholte Schwere- und Höhenmessungen durchgeführt, um die Massenverlagerungen im Untergrund und damit verbundene Subsidenz zu erfassen. In beiden Messgebieten wurden Absenkungen um Millimeter bis wenige Zentimeter über den Untersuchungszeitraum erfasst. Außerdem wurden lokal signifikante, zeitliche Schwereänderungen von 1-2 µGal/a beobachtet. Als wichtig erwiesen sich hierbei sowohl die Korrektur des Einflusses zeitlicher Variationen im Bodenwassergehalt auf die Schweremessungen, wie auch die Kombination von Relativ- und Absolutgravimetrie zur Stabilitätskontrolle der großräumigen Schwerereferenz. Wenngleich die beobachteten Schwereänderungen klein sind und sich aus der Konfiguration der Messnetze, den urbanen Umgebungsbedingungen und der Mehrdeutigkeit der Gravimetrie Limitationen ergeben, ist die Durchführbarkeit von gravimetrischen Wiederholungsmessungen in Subrosionsgebieten sinnvoll und erfolgreich.

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 (

Senkungsvorgänge der Erdoberfläche stellen vor allem in urbanen Gebieten eine deutliche Gefährdung dar. Oftmals sind diese Senkungen durch Massenvariationen im Untergrund verursacht und führen zu langsamen, aber auch abrupten Einsenkungen der Erdoberfläche. Im Rahmen des Verbundprojektes SIMULTAN (Sinkhole Instability: MULti-scale monitoring and ANalysis) wird die Untersuchung dieser Phänomene auf Grundlage geophysikalisch-geodätischer Methoden durchgeführt, um ein tieferes Verständnis der komplexen Zusammenhänge zwischen Auslaugungsprozessen im Untergrund und den eintretenden Effekten auf der Erdoberfläche zu gewinnen. In dem hierzu angelegtem integrierten geodätisch-gravimetrischen Überwachungsnetz werden die Methoden Gravimetrie, Nivellement und GNSS zusammengeführt und die Ergebnisse der zeitlichen Variationen bzw. auftretenden Deformationen in einem urbanen Umfeld auf Signifikanz hin untersucht. Basierend auf den GNSS-Kampagnen, die in den Jahren 2015-2017 in Hamburg Groß Flottbek durchgeführt wurden, lassen sich Erkenntnisse gewinnen, auf welcher Ebene und in welchem Umfang die einzelnen Methoden und deren Kombination erfolgreich eingesetzt werden können. Es wird aufgezeigt, dass die Kombination zu sehr guten Übereinstimmungen führt und Senkungen von 1,5-2 mm durch alle Methoden nachweisbar sind, obwohl die innerstädtische Kubatur eine komplexe Vielzahl an Herausforderungen an eine präzise GNSS-Auswertung aufweist.

For the first time, we present a complete, processed compilation of all repeated absolute gravity (AG) observations in the Fennoscandian postglacial land uplift area and assess their ability to accurately describe the secular gravity change, induced by glacial isostatic adjustment (GIA). The data set spans over more than three decades and consists of 688 separate observations at 59 stations. Ten different organizations have contributed with measurements using 14 different instruments. The work was coordinated by the Nordic Geodetic Commission (NKG). Representatives from each country collected and processed data from their country, respectively, and all data were then merged to one data set. Instrumental biases are considered and presented in terms of results from international comparisons of absolute gravimeters. From this data set, gravity rates of change (⁠g˙⁠) are estimated for all stations with more than two observations and a timespan larger than 2 yr. The observed rates are compared to predicted rates from a global GIA model as well as the state of the art semi-empirical land uplift model for Fennoscandia, NKG2016LU. Linear relations between observed g˙ and the land uplift, h˙ (NKG2016LU) are estimated from the AG observations by means of weighted least squares adjustment as well as weighted orthogonal distance regression. The empirical relations are not significantly different from the modelled, geophysical relation g˙=0.03−0.163(±0.016)h˙⁠. We also present a g˙-model for the whole Fennoscandian land uplift region. At many stations, the observational estimates of g˙ still suffer from few observations and/or unmodelled environmental effects (e.g. local hydrology). We therefore argue that, at present, the best predictions of GIA-induced gravity rate of change in Fennoscandia are achieved by means of the NKG2016LU land uplift model, together with the geophysical relation between g˙ and h˙⁠.

Integrative geodetic-gravimetric approach for investigating the longterm stability in the sinkhole area of Hamburg-Flottbek. Bundesministerium für Bildung und Forschung/Geotechnologien, SIMULTAN/03G0843A/EU

Integrative geodetic-gravimetric approach for investigating the longterm stability in the sinkhole area of Hamburg-Flottbek. Bundesministerium für Bildung und Forschung/Geotechnologien, SIMULTAN/03G0843A/EU

International audience; The frequency stability and uncertainty of the latest generation of optical atomic clocks is now approaching the one part in 1018 level. Comparisons between earthbound clocks at rest must account for the relativistic redshift of the clock frequencies, which is proportional to the corresponding gravity (gravitational plus centrifugal) potential difference. For contributions to international timescales, the relativistic redshift correction must be computed with respect to a conventional zero potential value in order to be consistent with the definition of Terrestrial Time. To benefit fully from the uncertainty of the optical clocks, the gravity potential must be determined with an accuracy of about 0.1m2s−2, equivalent to about 0.01 m in height. This contribution focuses on the static part of the gravity field, assuming that temporal variations are accounted for separately by appropriate reductions. Two geodetic approaches are investigated for the derivation of gravity potential values: geometric levelling and the Global Navigation Satellite Systems (GNSS)/geoid approach. Geometric levelling gives potential differences with millimetre uncertainty over shorter distances (several kilometres), but is susceptible to systematic errors at the decimetre level over large distances. The GNSS/geoid approach gives absolute gravity potential values, but with an uncertainty corresponding to about 2 cm in height. For large distances, the GNSS/geoid approach should therefore be better than geometric levelling. This is demonstrated by the results from practical investigations related to three clock sites in Germany and one in France. The estimated uncertainty for the relativistic redshift correction at each site is about 2×10−18.

Sinkholes are slowly to suddenly developing subsidence structures at the surface, caused by solution of rocks. Their hazard potential is large, especially in urban areas. In the joint research project SIMULTAN (Sinkhole Instability: integrated MULTi-scale monitoring and ANalysis) the investigation and surveillance of sinkhole areas by a combination of geophysical, petrophysical, geodetical and hydrological methods contributes to the development of an early warning system of instability, as well as, in combination with modeling, to an improved process understanding. We focus on a region in the city of Hamburg where the subsidence areas Wobbe See and Flottbek Markt are located. Subsidence rates are in the order of 1 mm/a and presumably originate from solution processes related to the Othmarschen-Langenfelde Diapir. Following an integrative geodetic-gravimetric approach, surface deformation and mass change due to subrosion are observed, likewise with the objective of surveillance of the sinkhole area. Quarterly repeated levelling and gravimetry campaigns at 8 stations (since 01/2016) provide first results. On the one hand, they indicate a trend to subsidence in the Wobbe See area, in agreement with results of GNSS measurements, and on the other hand, mainly seasonal variations of elevation differences emerge up to now around Flottbek Markt, that can be correlated with hydrology. Can we detect potential mass changes in urban areas? This is investigated by repeated high-precision gravity measurements. The results are reproducible and significant temporal variations of gravity differences of up to 12 Gal (std. dev. 1 Gal). These variations comprise hydrological induced seasonal variations as well, due to local conditions at the points, like topography and different ground sealing. By tying the local, relative measurements to a regional reference system large-scale stability control is enabled. Regional SAPOS stations are linking the local, relative GNSS network and monitor their stability. Results of ionosphere free linear combination prove the stability of the local reference. The coordinate system ETRS89 in combination with the vertical datum DHHN2016 are the common geodetic datum for all combined geophysical approaches. As local urban conditions are challenging, new approaches like adaptive, dynamic elevation masks and extended recording periods are applied. Concerning gravity, annual absolute measurements are realized at one reference station, which supports the assumption of hydrological induced effects in the observed gravity changes. The intended modelling and correction of the hydrological induced gravity effect allows to separate possibly masked small effects of mass redistribution due to solution and sinkhole development. In case of observable gravity change, e.g. the mass dependent gravity gradient dg/dh can indicate mass change.

Das HITec ist für die Wissenschaft der Geodäsie und der Physik eine weltweit einzigartige Laborumgebung. Dort soll durch die Simulation verschiedenster Umweltbedingungen die Entwicklung von MultiSensorSystemen (MSS) der nächsten Generation praktisch realisiert werden. Forscherteams vom Institut für Erdmessung (IfE) und dem Geodätischen Institut Hannover (GIH) berichten.