Abstract
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.