First evaluation of an absolute quantum gravimeter (AQG#B01) for future field experiments

Quantum gravimeters are a promising new development allowing for continuous absolute gravity monitoring while remaining user-friendly and transportable. In this study, we present experiments carried out to assess the capacity of the AQG#B01 in view of future deployment as a field gravimeter for hydrogeophysical applications. The AQG#B01 is the field version follow-up of the AQG#A01 portable absolute quantum gravimeter developed by the French quantum sensor company Muquans. We assess the instrument's performance in terms of stability (absence of instrumental drift) and sensitivity in relation to other gravimeters. No significant instrumental drift was observed over several weeks of measurement. We discuss the observations concerning the accuracy of the AQG#B01 in comparison with a state-of-the-art absolute gravimeter (Micro-g-LaCoste, FG5#228). We report the repeatability to be better than 50 nm s−2. This study furthermore investigates whether changes in instrument tilt and external temperature and a combination of both, which are likely to occur during field campaigns, influence the measurement of gravitational attraction. We repeatedly tested external temperatures between 20 and 30 ∘C and did not find any significant effect. As an example of a geophysical signal, a 100 nm s−2 gravity change is detected with the AQG#B01 after a rainfall event at the Larzac geodetic observatory (southern France). The data agreed with the gravity changes measured with a superconducting relative gravimeter (GWR, iGrav#002) and the expected gravity change simulated as an infinite Bouguer slab approximation. We report 2 weeks of stable operation under semi-terrain conditions in a garage without temperature-control. We close with operational recommendations for potential users and discuss specific possible future field applications. While not claiming completeness, we nevertheless present the first characterization of a quantum gravimeter carried out by future users. Selected criteria for the assessment of its suitability in field applications have been investigated and are complemented with a discussion of further necessary experiments.

Analytical modelling and joint inversion of surface displacements and gravity changes at volcanoes

<p>Gravity change observations at volcanoes provide information on the location and mass change of intruded magma bodies. Gravity change and surface displacement observations are often combined in order to infer the density of the intruded materials. Previous studies have highlighted that it is crucial to account for magma compressibility and the shape of the gravity change and deformation source to avoid large biases in the density estimate. Currently, an analytical model for the deformation field and gravity change due to a source of arbitrary shape is lacking, affecting our ability to perform rapid inversions and assess the nature of volcanic unrest.  </p><p>Here, we propose an efficient approach for rapid joint-inversions of surface displacement and gravity change observations associated with underground pressurized reservoirs. We derive analytical solutions for deformations and gravity changes due to the volume changes of triaxial point-sources in an isotropic elastic half-space. The method can be applied to  volcanic reservoirs that are deep compared to their size (far field approximation). We show that the gravity changes not only allow inferring mass changes within the reservoirs, but also help better constrain location, shape and the volume change of the source. We discuss how the inherent uncertainties in the realistic shape of volcanic reservoirs are reflected in large uncertainties on the density estimates. We apply our approach to the surface displacements and gravity changes at Long Valley caldera over the 1985-1999 time period. We show that gravity changes together with only vertical displacements are sufficient to constrain the mass change and all the other source parameters. We also show that while mass change is well constrained by gravity change observations the density estimate is more uncertain even if the magma compressibility is accounted for in the model.</p>

Forty-three years of absolute gravity observations of the Fennoscandian postglacial rebound in Finland

Postglacial rebound in Fennoscandia causes striking trends in gravity measurements of the area. We present time series of absolute gravity data collected between 1976 and 2019 on 12 stations in Finland with different types of instruments. First, we determine the trends at each station and analyse the effect of the instrument types. We estimate, for example, an offset of 6.8 μgal for the JILAg-5 instrument with respect to the FG5-type instruments. Applying the offsets in the trend analysis strengthens the trends being in good agreement with the NKG2016LU_gdot model of gravity change. Trends of seven stations were found robust and were used to analyse the stabilization of the trends in time and to determine the relationship between gravity change rates and land uplift rates as measured with global navigation satellite systems (GNSS) as well as from the NKG2016LU_abs land uplift model. Trends calculated from combined and offset-corrected measurements of JILAg-5- and FG5-type instruments stabilized in 15 to 20 years and at some stations even faster. The trends of FG5-type instrument data alone stabilized generally within 10 years. The ratio between gravity change rates and vertical rates from different data sets yields values between − 0.206 ± 0.017 and − 0.227 ± 0.024 µGal/mm and axis intercept values between 0.248 ± 0.089 and 0.335 ± 0.136 µGal/yr. These values are larger than previous estimates for Fennoscandia.

Application of ultra‐precise GRACE Follow-On LRI measurements for validation of the state-of-the-art static gravity field models and examination of sub-monthly time-variable gravity signals

<p>The test Laser ranging interferometer (LRI) on the GRACE Follow-On satellites provides complementary inter-satellite ranging measurements to the baseline K-band microwave ranging (KBR) system that can be used to examine standard, and create novel, GRACE-FO data products.  We first calculated the KBR and LRI inter-satellite ranging residuals using dynamic orbits computed from non-gravitational accelerations, a static gravity field model and other background geophysical models like ocean tides. To accurately quantify the improvement by LRI, we directly examined the inter-satellite ranging residuals in the time and frequency domains. The frequency-domain analysis reveals that LRI enhances the accuracy of gravity measurements by ~1 order of magnitude over 60-200 CPR (10-37 mHz) frequencies with the signal dominated by static gravity field of the Earth. The time-domain analysis shows that LRI is capable of detecting static gravity signals as small as a few 0.1 nm/s<sup>2</sup> in 100-200 CPR frequency band. We made use of such LRI data acquired in 2019 to validate the state-of-the-art gravity field models GGM05S, GGM05C, GOCE-TIM-R6e, EIGEN-6C4, ITSG-Grace2018s and GOCO06s. We found that LRI data can identify subtle un-/mis-modeled static gravity signals in these models in the spectral as well as spatial domains, and thus, suggest how the next generation of gravity field models could be improved. We also examined the high‐frequency (sub-monthly) variations of the Argentine Gyre using LRI measurements along with satellite altimetry data. Through comparison of measured gravity change by LRI with synthetic gravity change from altimetry sea surface data (evaluated at GRACE Follow-On altitude), we clearly demonstrate how the high-frequency Argentine Gyre signal is fully captured by instantaneous LRI measurements by individual data arcs, but not in the monthly mean Level-2 data. Such along-orbit analyses of LRI data could be employed for, among others, validation of high-frequency non-tidal ocean models used in GRACE and GRACE Follow-On de-aliasing products.</p> <p> </p>

Improved IMU Preintegration with Gravity Change and Earth Rotation for Optimization-Based GNSS/VINS

IMU preintegration technology has been widely used in the optimization-based sensor fusion framework, in order to avoid reintegrating the high-frequency IMU measurements at each iteration and maintain the ability of bias correction when bias estimation changes. Since IMU preintegration technology was first proposed, several improved versions have been designed by changing the attitude parameterization or the numerical integration method in the most current related research. However, all of these versions have failed to take the change of gravity and the earth rotation into consideration. In this paper, we redesign the IMU preintegration algorithm in which the earth rotation and gravity vector are calculated from the geodetic position. Compared with the covariance matrix form, in this paper, the uncertainty of the preintegrated IMU measurements is propagated in the form of a square root information matrix (SRIM) for better numerical stability and easy use in the optimization-based framework. We evaluate the improved IMU preintegration algorithm by using the dataset collected by our sensor platform equipped with two different-grade IMUs. The test results show that the improved IMU preintegration algorithm can cope well with the gravity change and earth rotation. The earth rotation must be taken into consideration for the high-grade IMU that can effectively sense the earth rotation. If the change of gravity is omitted, the root-mean-square error (RMSE) of the horizontal attitude is about 1.38 times greater than the geodetic displacement. Additionally, the positioning RMSE does not increase obviously within a limited range, which means tens of kilometers and several hundred meters for the low-grade and high-grade IMU used in the experiment, respectively.

Evaluation of the capacities of a field absolute quantum gravimeter (AQG#B01)

Quantum gravimeters are a promising new development allowing for continuous, high-frequency absolute gravity monitoring while remaining user-friendly and transportable. In this study, we present experiments carried out to assess the capacity of the AQG#B01 in view of future deployment as a field gravimeter for hydro-geophysical applications. The AQG#B01 is the field version follow-up of the AQG#A01 portable absolute quantum gravimeter developed by MuQuans. We assess the instrument's performance in terms of stability (absence of instrumental drift), sensitivity in relation to other gravimeters, and hydrogeological mass changes. We discuss the observations concerning the accuracy of the AQG#B01 in comparison with a state-of-the-art absolute gravimeter (Micro-g-LaCoste, FG5#228). Repeatability is tested by instrument displacement between close-by measurement positions. We report the repeatability to be better than 50 nm s−2. No significant instrumental drift was observed over several weeks of measurement. This study furthermore investigates whether changes of instrument tilt and external temperature and combination of both, which are likely to occur during field campaigns, influence the measurement of gravitational attraction. We repeatedly tested external temperatures between 20 and 30 °C and did not find any significant effect. As an example of a geophysical signal, a 100 nm s−2 gravity change is detected with the AQG#B01 after a rainfall event at the Larzac geodetic observatory (Southern France). The data agreed with the gravity changes measured with a superconducting relative gravimeter (GWR, iGrav#002) and the expected gravity change simulated as an infinite Bouguer slab approximation. We close with operational recommendations for potential users and discuss specific possible future field applications. While not claiming completeness, we nevertheless present the first characterisation of a quantum gravimeter carried out by future users. Crucial criteria for the assessment of its suitability in field applications have been investigated and are complemented with a discussion of further necessary experiments.