Monika Wilde-Piorko

and 5 more

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.
This study presents the novel application of tidal gravimetric data in the determination of phase-velocity dispersion curves of Rayleigh surface waves. Tidal gravimeters are capable to detect surface waves of periods even up to 500−600 s, while a typical broad-band seismic sensor can detect them up to the periods of 200−300 s due to its mechanical limitation. The capability of using tidal gravimeters as long period seismometers has already been analyzed since the late ’90s. These researches were mainly focused on the comparison of the power spectral density of noise level of gravimeters and seismometers and the analysis of normal modes of the largest earthquakes recorded by gravimeters. The first steps in the calculation of group velocity dispersion curves of Rayleigh surface waves were carried out lately. The Wiener deconvolution technique was used to calculate the phase-velocity dispersion curve by a two-stations method, from two pre-processed traces recorded in two different stations, located at the same great circle that the epicentre. The Incorporated Research Institutions for Seismology (IRIS) database were searched in terms of availability of at least 1 Hz data recorded by a co-located broad-band seismometer and tidal gravimeter. Additionally, the IRIS database provides the information about the transfer function of instruments, what is necessary in the two-stations method. Only, three sites met the above conditions: Black Forest, Membach and Rochfort, all located in Western and Central Europe. The compatibility of gravimetric and seismic data in the period range of 20-100 s are shown. Tidal gravimeters, because of their higher sensitivity, can better detect weaker earthquakes, which results in a higher number of recordings. However, to explore all advantages of gravimetric recordings of earthquakes, especially up to the periods of 500-600 s, above analysis must be applied to tidal gravimeters with higher inter-station distances and evaluated transfer functions. This work was done in the research project No. 2017/27/B/ST10/01600 financed from the funds of the Polish National Science Centre.