The evolution of the Earth’s surface is driven by external and internal forces, the latter of which can only be studied indirectly. Knowledge about the structure of the Earth’s interior is very important for modeling and predicting the processes occurring at the surface. This study presents a new concept of joint analysis of the gravimetric and seismometric recordings of earthquakes for determining the seismic structure of the Earth down to the depth of 1250 km. The proposed method allows the use of gravimetric data without the known full transfer function of the instrument. Group velocity dispersion curves of the fundamental mode of Rayleigh waves up to the period of 550 s are measured based on the joint analysis of the recordings of superconducting gravimeter and broadband seismometers operating at the same location in five testing sites in Europe, allowing for the exploration of a broader response for incoming seismic waves. Averaged dispersion curves for earthquakes around the world for each site are inverted by the weighted linear inversion and Monte Carlo methods to estimate the distribution of shear-wave seismic velocity in the Earth’s mantle. A comparison of the deterministic and probabilistic inversion methods can excellently demonstrate surface waves’ ability to determine the Earth’s mantle structure. The inversion results are compared with the global ak135 seismic model (Kennett et al. 1995) to verify the proposed method.

This study presents the application of broadband seismic recordings in improving the quality of tidal gravimetric recordings, especially during strong earthquakes. Four sites in Western and Central Europe, where broadband seismometer and tidal gravimeter are collocated, are selected. The data together with transfer functions of instruments from the Black Forest (Germany), Membach (Belgium), and Rochfort (Belgium) are downloaded from the IRIS database. The GWR iGrav-027 superconducting gravimeter is fully operational at the Borowa Gora Geodetic-Geophysical Observatory of the Institute of Geodesy and Cartography (Poland) since late April 2016. Later, the REF-TEK broadband seismometer was installed in the cooperation with the University of Warsaw. The Observatory, also temporally hosts a spring gravimeter, LCR ET-26 (owned by the Warsaw University of Technology). The seismometric and gravimetric signals of high coherence are selected for each site. The complex transfer functions in frequency domain are found by the least square minimization of the seismometric and seismic-corrected gravimetric signal. The transfer functions are calculated when the coherence is significant for two cases: with and without observed earthquake recordings. Next, the estimated transfer functions and the ongoing seismometric recordings are used to improve the quality of tidal gravimetric recordings. The presented method shows a significant reduction of the noise level of gravimetric recordings in a frequency range of seismometer’s operability. In order to obtain reliable models of the Earth’s structure based on analysis of surface waves recorded by tidal gravimeters, the careful estimation of gravimeters’ transfer functions in the seismic frequency range is necessary. The transfer functions of iGrav-027 and ET-26 were determined by the step response method during the experiments in 2018-2019. Now, thanks to the presented research, their quality can be verified. The limitation of this verification can be tested on recordings of Black Forest, Membach and Rochfort with very well-determined and documented transfer functions of tidal gravimeters. This work has been done in the research project No. 2017/27/B/ST10/01600 financed from the funds of the Polish National Science Centre.

Gravimetric data show excellent capabilities in long-period seismology. Tidal gravimeters can detect surface waves of periods even up to 500-600 s, while a typical broad-band seismic sensor, due to its mechanical limitation, can detect them only up to the periods of 200-300 s. Consequently, gravimetric data can complement seismic recordings for longer periods, depending on what seismometer the station is equipped with and what seismometer’s cut-off period is. A superconducting gravimeter can act as a single-dimension (only vertical component) of a very broad-band seismometer. We selected over a dozen stations worldwide with co-located typical broad-band seismic sensors and superconducting gravimeters. A time series from broad-band seismometers have been downloaded from Incorporated Research Institutions for Seismology (IRIS) database. The raw gravimetric data (1-Hz or 1-min) are available in the International Geodynamics and Earth Tide Service (IGETS) database. Some of the data were made available courtesy of the station’s operators. This study presents a joint analysis of the gravimetric and seismometric data to determine group-velocity dispersion curves of Rayleigh surface waves. We created a database of recordings of earthquakes for all stations and instruments. Following, we calculated the individual group-velocity dispersion curves of fundamental-mode Rayleigh waves. Simultaneous seismic and gravity recordings at the same location allow exploring a broader response for incoming seismic waves. In this way, one joint group-velocity dispersion curve of Rayleigh surface waves for a broader range of periods has been estimated for all stations. All curves were then inverted by linear inversion and Monte Carlo methods to calculate a distribution of shear-wave seismic velocity with depth in the Earth’s mantle. This work was done within the research project No. 2017/27/B/ST10/01600 financed from the Polish National Science Centre funds.

In October of 2021 the AQG-B07 absolute quantum gravimeter has been installed at the Borowa Góra Geodetic Geophysical Observatory. Since its installation the instrument is under ongoing evaluation performance and testing at Borowa Góra as well as in other locations in Poland. Within the first months of the AQG-B07 operation periodic continuous measurements were conducted at Borowa Góra as well as multiple gravity determinations on gravity stations at the gravimetric laboratory. Gravity values obtained with the AQG-B07 were compared with those from the A10-020 absolute gravimeter using the record of the iGrav-027 superconducting gravimeter to evaluate the offset between those absolute instruments. Continuous gravity records allowed to evaluate the short term stability of the AQG-B07 against the expected behaviour of the instrument. By the end of 2021 a 12 day record have been collected with a broadband seismometer recording side by side which allowed to evaluate the noise characteristic of residual gravity values collected with the AQG-B07 gravimeter. In January of 2022 the gravimeter was operating in Warsaw in the premises of the Institute of Geodesy and Cartography located in an urban area. Gravity measurements conducted in more active micro seismic environment allowed to gain a better perspective on the performance of the AQG-B07. Absolute gravity determinations were simultaneously done with the A10-020 gravimeter at the same location. Evaluations of noise as well as analysis against the seismometer, in particular to test the ability of the AQG-B07 to record earthquakes for determination of the seismic structure of the Earth's mantle from the analysis of surface waveforms, were performed in the framework of the research project No. 2017/27/B/ST10/01600 financed from the funds of the Polish National Science Centre.

In June of 2018 a project for the establishment of a modern permanent Absolute Gravity Network on the island of Ireland was initiated by the National Mapping Agency of Ireland, Ordnance Survey Ireland (OSi) with the cooperation of Institute of Geodesy and Cartography (IGiK), and Land and Property Services (LPS) in Northern Ireland. The project assumes conducting absolute gravity surveys of the network using the A10 absolute gravimeter on approximately 60 stations homogenously distributed on the island of Ireland.Data processing includes time variable corrections for body tides, barometric pressure, polar motion as well as ocean tidal loading. For Ireland the ocean tidal loading effect can reach peaks of between 400 nm/s2 on the west coast and 200 nm/s2 on the east coast. This effect is significant and up to now the authors are unaware of previous historical data or tidal gravity records being performed in Ireland. Hence it was considered as a valid component of the overall Absolute Gravity Project to evaluate the current situation with ocean tidal loading effect in Ireland using gravimetric tidal records in order to validate available ocean tidal loading models.In order to assure the most optimal use of ocean tidal model as well as minimize the errors of including ocean tidal correction in absolute gravity processing the LaCoste&Romberg model G spring gravimeter was installed at OSi headquarters in Phoenix Park, Dublin, Ireland. Over a continuous period of 28 months gravity record with more than 99% data completeness at near 2Hz sampling rate was conducted.The project data was acquired through using a self-programmed Raspberry Pi computer allowing for automatic download and remote access to the data.A set of CSR, DTU, EOT, FES, GOT, TPXO (ocean tide loading provider – Chalmers, http://holt.oso.chalmers.se/loading/) ocean tidal loading models were used in a joint analysis with the collected tidal record. Analysis included performing tidal adjustment of the gravity data in the ETERNA 3.40 (ET34-X-V73) as well as comparison of IAG (International Association of Geodesy) recommended model combinations with the collected data.Recommendations by the project team as to which of the ocean tidal models is most suitable to be used in Ireland for the purpose of absolute gravity measurements were made.

The regional key comparison of absolute gravimeters, EURAMET.M.G-K3 and the simultaneously organized additional comparison, was held in Germany at the Geodetic Observatory Wettzell of the German Federal Agency for Cartography and Geodesy in the spring of 2018. Here we present the list of the participants who actually performed measurements during the comparison, the data submitted by the operators as well as the results of the determination of the gravity as a function of height at the comparison sites. The measurement strategy is briefly discussed and the results of the data harmonization is documented. Finally, the results of the constrained least squares adjustment are presented including the degrees of equivalence of each gravimeter and the key comparison reference values. Main text To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

A10 absolute gravimeters are in active use for more than 10 years. This type of gravimeter has proven to be a very efficient tool for absolute gravity determinations, both in field and in laboratory conditions. In order to get full assessment of the gravimeter performance, study on the total uncertainty budget is required. It is especially important when the gravity determinations with the A10 are to be compared with gravity determinations using other types of absolute gravimeters, e.g. during absolute gravimeter comparison campaigns such as ICAG, ECAG or other similar local surveys. With the uncertainty estimated, the reliability of the A10 gravimeter required for gravity control establishment can further be evaluated. Uncertainty budget components provided by the manufacturer need to be revised in order to estimate the experimental uncertainty of gravity determinations with the A10. The budget itself can be divided into three main components: correction/reduction models, instrument related issues, and statistical uncertainty. As the A10 gravimeter is used in laboratory and field conditions two uncertainty estimates were considered. In addition, the sensitivity of the A10 gravimeter with respect to local hydrology has been discussed. Studies and estimates were performed based on numerous surveys with the A10-020 on the stations of the gravity control in Poland. The most interesting material comes from the repeated regular absolute gravity determinations with the A10-020 at three stations in Borowa Gora Geodetic-Geophysical Observatory. Additional information is provided by absolute gravimeter comparison campaigns and calibrations of metrological parameters performed from October 2008.

In November 2013 an International Key Comparison, CCM.G-K2, was organized in the Underground Laboratory for Geodynamics in Walferdange. The comparison has assembled 25 participants coming from 19 countries and four different continents. The comparison was divided into two parts: the key comparison that included 10 NMIs or DIs, and the pilot study including all participants. The global result given by the pilot study confirms that all instruments are absolutely coherent to each other. The results obtained for the key comparison confirm a good agreement between the NMI instruments. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by CCM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

In November 2013 an International Key Comparison, CCM.G-K2, was organized in the Underground Laboratory for Geodynamics in Walferdange. The comparison has assembled 25 participants coming from 19 countries and four different continents. The comparison was divided into two parts: the key comparison that included 10 NMIs or DIs, and the pilot study including all participants. The global result given by the pilot study confirms that all instruments are absolutely coherent to each other. The results obtained for the key comparison confirm a good agreement between the NMI instruments.

The Institute of Geodesy and Cartography in Warsaw, Poland, operates the A10-020 absolute portable gravimeter since November 2008. Numerous gravity measurements with the A10-020 gravimeter, conducted under both laboratory and field conditions, provide a unique material for the estimation of accuracy as well as reliability of the determined gravity. Monthly measurements conducted with the A10-020 at the Borowa Gora Geodetic-Geophysical Observatory, north of Warsaw, provided a 2 year long time series at two laboratory sites and one field station. They have been analysed in terms of their internal consistency and compliance with the previous measurements performed with a few other absolute gravimeters (mainly FG5). The results of a number of calibrations of both, the rubidium oscillator and the polarization-stabilized laser of the A10-020 were considered in the analysis. The effect of applying the frequency standard as well as laser calibration data on the quality of gravity determined was investigated. In addition, the impact of weather conditions as well as variability of metrological parameters on surveyed gravity was taken into consideration when evaluating accuracy and reliability of gravity survey with the A10 gravimeter.

Numerous global gravity field models (GGFMs) have resulted from the satellite gradiometry mission GOCE. Validation is indispensable to test the performance of the new-generation models. For this purpose independent datasets of terrestrial data are very often used.

The A10 absolute gravimeter is the first fully operational equipment to perform absolute gravity determinations in field conditions. A long time series of gravity determinations with the A10-020 performed since 2008 on a monthly basis on three stations in Borowa Gora Observatory provides an invaluable data source for quality estimation of the meter and its performance. Data from regular metrological calibrations of both, linear-polarized and stabilized laser and rubidium oscillator of the A10-020 are a complementary material for the analysis of the gravimeter performance. In May 2012 a measurement campaign at nearly 15 points was conducted to test and verify the developed methodology of absolute gravity survey with the A10 for establishing a new gravity control in Poland. Measurements were performed at absolute gravity stations of current Polish gravity control and their eccentric points. The obtained results were analyzed considering different types of station monumentation. At five laboratory stations the A10-020 results were compared with the recent FG5-230 determinations. The comparison included unification of vertical gravity gradient determinations as well as metrological parameters. At all occupied stations the vertical gravity gradient had been determined with two LaCoste & Romberg gravimeters with the use of a special stand made in the Institute of Geodesy and Cartography. The importance of vertical gravity gradient determination for the establishment of the new gravity control is discussed. The experience with the A10-020, including its suitability for modernization and re-measurement of gravity control in Finland, Sweden, Norway, and Denmark proves its high efficiency and accuracy. Furthermore it allows to develop a complete methodology for the establishment of a new type of gravity control.