Performance of three iGrav superconducting gravity meters before and after transport to remote monitoring sites

Abstract

Summary High spatial and temporal resolution of gravity observations allows quantifying and understanding mass changes in volcanoes, geothermal or other complex geosystems. For this purpose, accurate gravity meters are required. However, transport of the gravity meters to remote study areas may affect the instrument's performance. In this work, we analyse the continuous measurements of three iGrav superconducting gravity meters (iGrav006, iGrav015 and iGrav032), before and after transport between different monitoring sites. For four months, we performed comparison measurements in a gravimetric observatory (J9, Strasbourg) where the three iGravs were subjected to the same environmental conditions. Subsequently, we transported them to Þeistareykir, a remote geothermal field in North Iceland. We examine the stability of three instrumental parameters: the calibration factors, noise levels and drift behaviour. For determining the calibration factor of each instrument, we used three methods: First, we performed relative calibration using side-by-side measurements with an observatory gravity meter (iOSG023) at J9. Second, we performed absolute calibration by comparing iGrav data and absolute gravity measurements (FG5#206) at J9 and Þeistareykir. Third, we also developed an alternative method, based on intercomparison between pairs of iGravs to check the stability of relative calibration before and after transport to Iceland. The results show that observed changes of the relative calibration factors by transport were less than or equal to 0.01 per cent. Instrumental noise levels were similar before and after transport, whereas periods of high environmental noise at the Icelandic site limited the stability of the absolute calibration measurements, with uncertainties above 0.64 per cent (6 nm s-2 V-1). The initial transient drift of the iGravs was monotonically decreasing and seemed to be unaffected by transport when the 4K operating temperatures were maintained. However, it turned out that this cold transport (at 4K) or sensor preparation procedures before transport may cause a change in the long-term quasi-linear drift rates (e.g. iGrav015 and iGrav032) and they had to be determined again after transport by absolute gravity measurements.

Environmental and anthropogenic gravity contributions at the Þeistareykir geothermal field, North Iceland

“…In addition to environmental gravity contributions, artefacts of the gravity meter like instrumental drift or self-noise may decrease the accuracy of the target signal (Crossley et al 2004;Rosat and Hinderer 2018). With respect to long-term drift rates, Schäfer et al (2020) showed the impact of transporting a superconducting gravimeter. Therefore, an accurate estimation and reduction of instrumental drift and environmental contributions is essential before we can interpret the gravity residuals accurately with respect to a specific geophysical phenomenon.…”

Environmental And Anthropogenic Gravity Contributions At The Þeistareykir Geothermal Field, North Iceland

“…5 we compare the time series of gravity residuals of iGrav006, iGrav015 and iGrav032, before and after the combined environmental reductions of global effects, soil water content, snow water equivalent and vertical surface displacement. The initial residuals (before the aforementioned environmental reductions were applied) have been derived by reducing Earth and ocean tides, polar motion, local pressure and instrumental drift, as well as removing spikes and offsets (caused by earthquakes and other disturbances) from the original gravity time series (Schäfer et al 2020). Reductions of the individual environmental gravity contributions for each iGrav are shown in Figures A5 to A7 in the Appendix.…”

“…In addition to environmental gravity contributions, artefacts of the gravity meter like instrumental drift or self-noise may decrease the accuracy of the target signal (Crossley et al 2004;Rosat and Hinderer 2018). With respect to long-term drift rates, Schäfer et al (2020) showed the impact of transporting a superconducting gravimeter. Therefore, an accurate estimation and reduction of instrumental drift and environmental contributions is essential before we can interpret the gravity residuals accurately with respect to a speci c geophysical phenomenon.…”

“…In June 2019, we started continuous measurements with iGrav015 at the central station, in the transient area between geothermal extraction and injection. Schäfer et al (2020) provide further details about the gravity station setup and the continuous observation chronology. Gravity observations are subject to various environmental contributions (e.g.…”

Technical note: Introduction of a superconducting gravimeter as novel hydrological sensor for the Alpine research catchment Zugspitze

“…Gravimetric methods are already applied in the RCZ. Episodic AG observations were carried out since 2004 with a FG5 absolute gravimeters by the Leibniz University Hannover (LUH) for the analysis of long-term gravity changes at Mount Zugspitze and for a long-range gravimeter calibration base (Timmen et al, 2006;Peters et al, 2009). Timmen et al (2021) estimated a geophysical trend of −20 nm s −2 yr −1 , with an uncertainty of 3 nm s −2 yr −1 (single standard deviation 1σ ) from AG observations between 2004 and 2019 as a consequence of alpine mountain uplift and hydrological mass loss.…”