OPUS 14

Title of the article:  Determination of the seismic structure of the Earth's mantle based on tidal gravimeter recordings

The Project is funded by the National Science Centre, Poland (NCN) via the OPUS 14 research project No. 2017/27/B/ST10/01600.

Duration of the project:  26.06.2018 – 25.06.2023

Financing amount: 513 010,00 zł

Project team:

Principal investigator: dr hab. Monika Wilde-Piórko, email: monika.wilde-piorko@igik.edu.pl

Co-investigator: dr inż. Przemysław Dykowski, mgr inż. Kamila Karkowska, dr Marcin Polkowski, dr Marcin Sękowski

The main objective of the project was the determination of the seismic structure of the Earth's mantle by analysis of Rayleigh surface wave generated by earthquakes and recorded by gravimeters used to study the Earth’s tides (tidal gravimeters). This objective was made possible by a comprehensive analysis of earthquake recordings performed by different types of gravimeters, i.e. superconducting gravimeters and spring gravimeters, and by comparing these recordings with recordings of very broadband and broadband seismometers co-located with tidal stations. Within the project characteristics of 5 tidal gravimeters located in Polish observatories were also determined. This research, together with the analysis of earthquake records, has made it possible to extend the potentiality of tidal gravimeters as very broadband seismometers.

Modern tidal gravimeters have a constant amplitude transfer function and a time delay for periods longer than 100-200 s. Typical broadband seismometers, used to study the structure of the Earth's interior, record the velocity of displacement of the Earth’s surface without distortion for periods shorter than 120 s. Rayleigh surface waves with periods as long as 550 s have been observed – so there are waves whose registration is largely distorted by seismometers and very little by gravimeters. Thanks to this property of tidal gravimeters, it was possible to determine Rayleigh wave dispersion curves (fundamental mod) with periods up to 550 s, and thus to determine velocity distributions of shear waves even to a depth of 1200 km.

It was also shown that the records of superconducting gravimeters, if their transfer function is known, can be used to analyse surface waves in a period range of 20-120 s, i.e. regional models of the crust and uppermost mantle can be determined based on them. A new method has also been proposed for determining a joint group velocity dispersion curves of Rayleigh waves (fundamental mode) for observatories equipped with a broadband seismometer and a tidal gravimeter, which does not have a determined transfer function. This makes it possible to determine a shear wave velocity distributions with depth not only in the Earth’s mantle but also in the Earth’s crust.

The purchase of an absolute quantum gravimeter by the Institute of Geodesy and Cartography in 2021, additionally enabled to explore the possibility of using its recording to determine the structure of the Earth's crust and mantle. Analyses of recordings of earthquakes, and a comparison with the recordings of the broadband seismometer owned by the University of Warsaw, which was installed at the Borowa Gora Observatory as part of the project, have shown that only the strongest earthquakes are recorded by the absolute quantum gravimeter. Moreover, its records are highly distorted, which in practice makes impossible their use in seismological studies.

The results of the project will contribute to the development of knowledge in the field of Earth sciences, especially geophysics. The obtained results strongly contribute to the development of methodology concerning the study of the structure of the Earth's interior – resulted models are unique because they present the absolute seismic wave velocities in the Earth's mantle. The research methodology developed in the project can be applied to the analysis of surface wave records of any tidal gravimeters, not only of those for which 1 second data are available. The obtained mantle models allow verification of already existing models, developed on the basis of other seismic data and with the use of other methods.

The research conducted as well as the experience gathered by the team during the analysis of gravimetric recordings of earthquakes will allow in the future to develop a method for effective removing them from gravimeter records and thus improving the results of tidal analysis. In addition, the issue of the limited use of tidal gravimeter recordings is being raised more and more often within the geodetic community, which is traditionally engaged in recordings and analysis of Earth tides. The International Geodynamics and Earth Tide Service (IGETS) database under the umbrella of the International Association of Geodesy (IAG) currently holds recordings of more than 60 tidal gravimeters, which could be also used by seismologists.

Main achievements of the project:

  1. Determination of the transfer function of one superconducting gravimeter and four spring gravimeters by the 'step response' method, including determination of zeros and poles of the transfer function as well as error estimation.
  2. Demonstration that joint recordings made by a broadband or very broadband seismometer and a superconducting gravimeter can be used to determine the phase characteristics of the gravimeter and that the seismometric data can be used to improve the quality of the superconducting gravimeter recordings over the operating frequency range of seismometer.
  3. Developing the procedure for processing of the gravimetric data for its using in the analysis of earthquake recordings.
  4. Developing a database of earthquake recordings performed by gravimeters and seismometers, including the development of a formula for determining parameters of earthquakes which can be recorded by gravimeters and a procedure for determining saturation levels of gravimeters.
  5. Demonstration that the time shift of a gravimeter can be determined based on the correlation of wave packages (quasi-monochromatic signals) of Rayleigh waves (fundamental mod) recorded by a broadband or very broadband seismometer and a superconducting gravimeter or absolute quantum gravimeter.
  6. Demonstration of compatibility of group velocity dispersion curves of Rayleigh (fundamental mod) determined from seismometric and gravimetric data in a period range of 20-120 s and determination of group velocity dispersion curves from gravimetric data in a period range of 300-550 s (outside the range of the broadband seismometer).
  7. Demonstration of compatibility of phase velocity dispersion curves (fundamental mod) determined from seismometric and gravimetric data in a period range of 20-90 s, and determination of phase velocity dispersion curves from gravimetric data up to a 300 s wave period. Demonstration of the need to use a correctly determined transfer function or time delay (depending on the period interval considered) in phase velocity dispersion curves estimation from gravimetric data.
  8. Development of a method for the determination of joint seismometric and gravimetric group velocity dispersion curves of Rayleigh waves (fundamental mod), enabling the determination of group velocity dispersion curves above a period of 2 s, in the absence of knowledge of the gravimeter transfer function and the availability of 1 s gravimetric data.
  9. Development of models of the Earth's mantle based on linear and Monte Carlo inversion of group and phase velocity dispersion curves resulted from gravimeter and seismometer recordings for selected areas of Europe, South America, Asia and Australia. Quantitative demonstration by checkerboard tests the depth ranges of the reconstruction of the vertical anomalies of the medium on the basis of linear inversion of group velocity dispersion curves of Rayleigh waves (fundamental mod). Demonstration that the Earth's upper mantle can only be characterised by 2-3 layers when it is determined based on the inversion of group velocity dispersion curves of Rayleigh waves (fundamental mod),

References:

Dykowski, P., Kryński, J., Wilde-Piórko, M., Sękowski, M. (2018): Assessment of iGrav-027 superconducting gravimeter for validation of gravity variations based on atmospheric and hydrological models. International Symposium Gravity, Geoid and Height Systems "GRAVITY FIELD OF THE EARTH” 2018, Copenhagen, Denmark (oral).

Karkowska K., Wilde-Piórko M., (2019): How can gravimeters improve recordings of earthquakes? European Geosciences Union General Assembly 2019, Vienna, Austria (oral).

Wilde-Piorko M., Dykowski P., Karkowska K., Olszak T., Polkowski M., Sekowski M. (2019): Determination and Verification of Relative Gravimeters’ Transfer Function. International Union of Geodesy and Geophysics General Assembly 2019 (IUGG 2019); Montreal, Canada (oral);

Wilde-Piorko M., Karkowska K. (2019): Improving the Determination of Earth’s Mantle Structure by Continuous Gravimetric Records. International Union of Geodesy and Geophysics General Assembly 2019; Montreal, Canada (oral).

Karkowska K., Wilde-Piórko M., (2019): How can gravimeters improve the determination of Earth’s mantle structure? 20th Czech-Polish Workshop on Recent Geodynamics of Central Europe and 2nd Symposium of the Committee on Geodesy of the Polish Academy of Sciences, Jakuszyce, Poland (oral).

Karkowska K., Wilde-Piórko M., (2020): Determination of the Earth’s Structure Based on Long-Period Surface Wave Recordings of Tidal Gravimeters. AGU Fall Meeting 2020, USA, on-line (oral).

Wilde-Piórko M., Karkowska K., (2020): Determination of Phase-Velocity Dispersion Curves of Rayleigh Surface Waves from Tidal Gravimetric Recordings of Earthquakes. AGU Fall Meeting 2020, USA, on-line (poster), DOI: https://essopenarchive.org/doi/full/10.1002/essoar.10506023.1.

Dykowski P., Krynski J., Sekowski M., Wilde-Piorko M., Olszak T, (2021): Evaluation of the gravity reference function at the Borowa Gora Observatory. IAG Scientific Assembly 2021, Beijing, China, on-line (oral).

Karkowska K., Wilde-Piórko M., Dykowski P., Olszak T., Sękowski M., Polkowski M., (2021): Determination of the Earth's mantle structure based on a joint analysis of gravimetric and seismometric earthquake recordings at the Borowa Gora Geodetic-Geophysical Observatory. IAG Scientific Assembly 2021, Beijing, China, on-line (poster).

Wilde-Piorko M., Karkowska K., Dykowski P., Sekowski M., Polkowski M., (2021): Determination of group and phase-velocity dispersion curves of Rayleigh surface waves from tidal gravimetric recordings of earthquakes. IASPEI-IAGA Scientific Assembly 2021, Hyderabad, India, on-line (oral).

Wilde-Piorko M., Dykowski P., Olszak T., Karkowska K., Sękowski M., Polkowski M., (2021): Improving the Quality of Tidal Gravimetric Recordings. AGU Fall Meeting 2021, New Orlean, USA, on-line (poster), DOI: https://essopenarchive.org/doi/full/10.1002/essoar.10509926.1.

Karkowska K., Wilde-Piorko M., El-Sayed El-Sharkawy A.M.M., Meier T., Dykowski P., Sękowski M., (2021): Phase- and Group-velocity Maps from Gravimetric and Seismometric Recordings of Earthquakes. AGU Fall Meeting 2021, New Orlean, USA, on-line (poster).

Karkowska K., Wilde-Piórko M., Dykowski P., (2022): Analysis of earthquakes recordings of tidal gravimeters in the period range of 10-1000 s. Acta Geodynamica et Geomaterialia 19(1): 79–92, DOI: 10.13168/AGG.2021.0043.

Dykowski P., Arnal M., Menoret V., Sękowski M., Karkowska K., Wilde-Piorko M., Krynski J., (2022): First results from the AQG-B07 absolute quantum gravimeter. European Geosciences Union General Assembly 2022 (EGU 2022), Vienna, Austria, (oral).

Karkowska K., Wilde-Piórko M., Dykowski P., Sękowski M., Polkowski M., (2022): Exploring the Earth's mantle structure based on joint gravimetric and seismometric group-velocity dispersion curves of Rayleigh waves. European Geosciences Union General Assembly 2022 (EGU 2022), Vienna, Austria, (oral).

Dykowski P., Arnal M., Menoret V., Sekowski M., Wilde-Piórko M., Krynski J., (2022): Testing the capabilities of the AQG-B07 absolute quantum gravimeter. Gravity Geoid and Height Systems 2022, Austin, USA, 12-14 September 2022 (oral).

Karkowska K., Wilde-Piórko M., (2022): Determination of Earth’s structure based on intermediate-period surface wave recordings of tidal gravimeters: a case study. Earth, Planets and Space 74: 150, DOI: https://doi.org/10.1186/s40623-022-01712-4.

Wilde-Piorko M., Karkowska K., Dykowski P., Sękowski M., Polkowski M., (2023): Determination of the seismic structure of the Earth’s mantle based on joint analysis of gravimetric and seismometric data. International Union of Geodesy and Geophysics General Assembly 2023 (IUGG 2023), Berlin, Germany, (poster).

Karkowska K., Wilde-Piórko M., Dykowski P., Sękowski M., Polkowski M., (2023): Determination of the seismic structure of the Earth based on joint analysis of gravimetric and seismometric data - the case study. Geology Geophysics and Environment (after review).