Noise levels of superconducting gravimeters at seismic frequencies

Abstract

Summary Until recently superconducting gravimeters (SGs) have been used principally in tidal studies (periods 6–24 hr) due to their high sensitivity and low drift rates. This paper considers the performance of these instruments as long‐period seismometers, particularly in the normal mode band (periods 1–54 min). To judge their suitability in providing useful information to seismology, it is important to determine their noise characteristics compared to other established instruments such as spring gravimeters. We compare several continuously recording instruments: the SGs in Esashi (Japan), Wuhan (China), Strasbourg (France) and Cantley (Canada) and the spring gravimeter ET‐19 and seismometer STS‐1 at the Black Forest Observatory (BFO, Germany). We also include non‐permanent instruments, the SG102 at BFO as well as the ET‐18 in Metsähovi (Finland). The five quietest days out of the available records are stacked to obtain the power spectral density of the noise in the frequency band 0.05–20 mHz (50 s to 6 hr). Our reference is the New Low Noise Model designed for seismometers. Only at the BFO site were there several instruments that could be compared; even so, in order to obtain the best individual data for each instrument the records selected were not simultaneous. The noise characteristics of the different instrument–site combinations are compared, leading to conclusions about site selection, instrument modifications and the recent potential of SGs to contribute to seismic normal mode studies. We refer to our previous work on the seismic noise magnitude, a summary statistic derived from the power spectral density which has been used to rank the performance of instrument–site combinations.

Results of Gravity Observations Using a Superconducting Gravimeter at the Tibetan Plateau

“…The EFO are classified as spheroidal and toroidal oscillations, and the former are associated with cubical dilation and gravity changes which can be detected by the high-precision gravimeters installed on the ground. It was proven that the SG played an important role in structuring the long-period seismogram (Banka and Crossley 1999;Van Camp 1999;Lei et al 2005;Park et al 2005). On March 11, 2011, the magnitude 9.0 Tohoku-Oki Earthquake released such large energy that the Pacific tsunami was generated and the EFO were excited.…”

“…It is now the best instrument to survey the temporal gravity variations in the world. It has the potential to detect almost all signatures with periods ranging from serveral seconds related to coseimic movements to several years related to the variations in the Earth's rotation, even the phenomena associated with the secular tectonic movements of local crust, such as the the Earth's free oscillations (Banka and Crossley 1999;Van Camp 1999;Lei et al 2005;Park et al 2005), the Earth's tides (Sun et al 2001;Xu et al 2004a), the load effects of barometric pressure (Sun and Lou 1998), the nearly diurnal resonance (Defraigne et al 1994;Xu et al 2002), translational oscilations of the solid inner core (Smylie 1992;Courtier et al 2000;Rosat et al 2003;Xu et al 2010), Polar motion (Loyer et al 1999;Xu et al 2004b), secular crust deformation due to earthquakes or other reasons Imanishi et al 2004;Richter et al 2004;Xu et al 2008) and so on. As a result, a significant scientific project, i.e., the Global Geodynamics Project, has been carried out since 1997 in order to investigate global and local dynamic problems using continuous gravity data from a worldwide network of superconducting gravimeters Crossley 2004;Crossley and Hinderer 2009).…”

First analyses of the iOSG-type superconducting gravimeter at the low noise underground laboratory (LSBB URL) of Rustrel, France

“…A standard procedure was introduced by [6,7] to compare noise levels of various gravimetric sites. Since other worldwide SG data are available at a sampling interval of one minute, we down-sample the raw one-second iOSG-24 data to one minute using a low-pass filter with a cut-off period of two minutes.…”

“…Since other worldwide SG data are available at a sampling interval of one minute, we down-sample the raw one-second iOSG-24 data to one minute using a low-pass filter with a cut-off period of two minutes. Then we consider daily time windows and remove solid tides for an elastic reference Earth model from the data as in [7]. A local atmospheric pressure reduction is also applied using a nominal admittance of −3 nm/s 2 /hPa.…”

On the noise characteristics of time series recorded with nearby located GPS receivers and superconducting gravity meters

“…Noises in GPS observations have already been widely discussed in the papers by Zhang et al (1997), Mao et al (1999), Williams et al (2004), Amiri-Simkooei et al (2007), Teferle et al (2008), Bos et al (2008), Acta Geod Geophys Santamaria-Gomez et al (2011) or Klos et al (2015). Concerning gravimetric observations, Banka and Crossley (1999) compared several continuously recording instruments: SGs, spring gravimeters and seismometers to determine noise characteristics. They statistically averaged the observations to obtain and investigate power spectral density (PSD) of the noise in the frequency band from 0.05 to 20 mHz.…”