Updated Absolute Gravity Rate of Change Associated With Glacial Isostatic Adjustment in Southeast Alaska and Its Utilization for Rheological Parameter Estimation

In Southeast Alaska (SE-AK), rapid ground uplift of up to 3 cm/yr has been observed associated with post-Little Ice Age glacial isostatic adjustment (GIA). Geodetic techniques such as global navigation satellite system (GNSS) and absolute gravimetry have been applied to monitor GIA since the last 1990s. Rheological parameters for SE-AK were determined from dense GNSS array data in earlier studies. However, the absolute gravity rate of change observed in SE-AK was inconsistent with the ground uplift rate, mainly because few gravity measurements from 2006 to 2008 resulted in imprecise gravity variation rates. Therefore, we collected absolute gravity data at six gravity points in SE-AK every June in 2012, 2013, and 2015, and updated the gravity variation rate by reprocessing the absolute gravity data collected from 2006 to 2015. We found that the updated gravity variation rate at the six gravity points ranged from −2.05 to −4.40 μGal/yr, and its standard deviation was smaller than that reported in the earlier study by up to 88 %. We also estimated the rheological parameters to explain the updated gravity variation rate, and their optimal values were determined to be 55 km and 1.2 × 10^19 Pa s for lithospheric thickness and upper mantle viscosity, respectively. These optimal values are consistent with those independently obtained from GNSS observations, and this fact indicates that absolute gravimetry can be one of the most effective methods in determining sub-surface structural parameters associated with GIA accurately. Moreover, we utilized the gravity variation rates for estimating the ratio of gravity variation to vertical ground deformation at the six gravity points in SE-AK. The viscous ratio values were obtained as −0.168 and −0.171 μGal/mm from the observed data and the calculated result, respectively. These ratios are greater (in absolute) than those for other GIA regions (−0.15 to −0.16 μGal/mm in Antarctica and Fennoscandia) because glaciers in SE-AK have melted more recently than in other regions.

Insights into Seismogenetic Areas in Central Italy from Combined Absolute Gravity and GNSS Measurements

In this study we present and discuss gravity and ground deformation variations, at different time scales, observed in a wide mesh absolute gravity and GNSS network set up in central Italy. The network was installed in the area affected by the 2009 (L’Aquila; Mw 6.1) and 2016 (Amatrice-Norcia; Mw 6.0 and 6.5) seismic activity, in order to verify if gravity and ground deformation variations could be related to seismic effects. The new network includes 5 stations distributed between the Lazio, Umbria, and Abruzzo regions. From 2018 to 2020 three campaigns were carried out using the transportable Micro-g LaCoste FG5#238 and the portable Micro-g LaCoste A10#39 absolute gravimeters and completed with two simultaneous GNSS measurements. Topographic instruments, measurement and analysis techniques enabling accurate measurements in the positioning of the stations and to control their variations over time were applied. The high reliability and accuracy of the absolute gravity data gathered, after being corrected for known effects, showed a negative short-term (2018–2020) pattern throughout the area, up to −30 µGal. Since some stations of the new network coincided with benchmarks already measured in the past, an analysis of long-term gravity changes was carried out and a fair degree of stability was observed in two stations, while positive large variations, of approximately 70 and 157 µGal, were recorded in the other two stations in the time intervals 1954–2020 and 2005–2010, respectively. On the other hand, variations highlighted by GNSS height measurements were all below 3 cm. Here, the first long-lasting gravity measurements carried out with absolute gravimeters in a seismic area in Italy are presented, providing meaningful geophysical information. The obtained results, in terms of availability of a combined absolute gravity and GNSS network, definition of data acquisition and analysis procedures, as well as creation of a high quality data archive, lay the foundations for a multidisciplinary approach towards improving the knowledge of this seismogenetic area of Italy.

Review on Vibration Isolation Method for Atomic Interference Gravimeter

The cold atomic interference absolute gravimeter is an ultra-precision instrument for measuring absolute gravity acceleration. At present, the highest measurement accuracy can reach the order of micro gamma. It has important application value and research significance in many disciplines, such as geophysics, resource exploration and assisted navigation. Because of its ultra-high precision, the ultra-low frequency micro-vibration noise on the ground has become one of the important factors affecting its accuracy, and it is also the bottleneck of the further development of gravimeter. Firstly, based on the theoretical and experimental results, this paper analyzes the vibration isolation requirements of atomic interference gravimeter. Secondly, it summarizes the research progress of atomic interference gravimeter isolation system and introduces three main isolation methods: passive vibration isolation, active vibration isolation and vibration compensation. Finally, the future development direction of atomic interference gravimeter isolation technology is analyzed and prospected.

SOME FEATURES OF CURRENT TECHNOGENIC MOVEMENTS OF THE EARTH’S CRUST

We describe the history of studying the current crustal movements by various methods and discuss technogenic effects recorded at large water-reservoir zones and mineral deposits in Siberia. Initially, classical surveying techniques aimed to obtain high-accuracy ground-based measurements of height, tilt and direction. Modern geodesy techniques and methods for measuring absolute gravity are now available to investigate displacement, deformation, tilt and other phenomena taking place on the Earth’s surface. These methods are used to estimate kinematic parameters of the crust areas (e.g. rates of subsidence and horizontal movements) and to monitor fluid motions in mineral deposits. Such data are critical for ensuring a proper management of the mineral deposits. In this article, we analyse technogenic processes observed in the Ust Balyk oil-gas field, the Zapolyarny gas deposit, the water-reservoir zone at the Sayano-Shushenskaya hydroelectric power station (SSHPS) on the Yenisei river, and large open-pit mines in the Kuzbass basin. Our analysis is based on surface displacement rates estimated from the data collected in different periods of observations at large man-made facilities. In the study of the hydro technical objects, we estimated the displacement rates at 5.0 mm per year. In the northern areas of the West Siberian petroleum basin, subsidence rates amounted to 20–25 mm per year in the early 2000s. These estimates were supported by the high-accuracy gravity measurements showing an increase up to 6–7 microGal per year in the oil-gas field development areas. We assess a possibility of triggering effects related to weak seismicity due to a high stress accumulation rate (1 KPa per hour) in the SSHPS area. A connection between earth tides and catastrophic events, such as gas emissions in high amounts on mining sites, is discussed. Having analysed the surface monitoring records taken in South Primorye in September 2017, we conclude that underground nuclear explosions in North Korea in this period did not cause any significant displacement of the surface in this most southerly region of the Russian Far East territories.

The semiannual oscillation (SAO) in the tropical middle atmosphere and its gravity wave driving in reanalyses and satellite observations

Gravity waves play a significant role in driving the semiannual oscillation (SAO) of the zonal wind in the tropics. However, detailed knowledge of this forcing is missing, and direct estimates from global observations of gravity waves are sparse. For the period 2002–2018, we investigate the SAO in four different reanalyses: ERA-Interim, JRA-55, ERA-5, and MERRA-2. Comparison with the SPARC zonal wind climatology and quasi-geostrophic winds derived from Microwave Limb Sounder (MLS) and Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) satellite observations show that the reanalyses reproduce some basic features of the SAO. However, there are also large differences, depending on the model setup. Particularly, MERRA-2 seems to benefit from dedicated tuning of the gravity wave drag parameterization and assimilation of MLS observations. To study the interaction of gravity waves with the background wind, absolute values of gravity wave momentum fluxes and a proxy for absolute gravity wave drag derived from SABER satellite observations are compared with different wind data sets: the SPARC wind climatology; data sets combining ERA-Interim at low altitudes and MLS or SABER quasi-geostrophic winds at high altitudes; and data sets that combine ERA-Interim, SABER quasi-geostrophic winds, and direct wind observations by the TIMED Doppler Interferometer (TIDI). In the lower and middle mesosphere the SABER absolute gravity wave drag proxy correlates well with positive vertical gradients of the background wind, indicating that gravity waves contribute mainly to the driving of the SAO eastward wind phases and their downward propagation with time. At altitudes 75–85 km, the SABER absolute gravity wave drag proxy correlates better with absolute values of the background wind, suggesting a more direct forcing of the SAO winds by gravity wave amplitude saturation. Above about 80 km SABER gravity wave drag is mainly governed by tides rather than by the SAO. The reanalyses reproduce some basic features of the SAO gravity wave driving: all reanalyses show stronger gravity wave driving of the SAO eastward phase in the stratopause region. For the higher-top models ERA-5 and MERRA-2, this is also the case in the lower mesosphere. However, all reanalyses are limited by model-inherent damping in the upper model levels, leading to unrealistic features near the model top. Our analysis of the SABER and reanalysis gravity wave drag suggests that the magnitude of SAO gravity wave forcing is often too weak in the free-running general circulation models; therefore, a more realistic representation is needed.

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.

The semiannual oscillation (SAO) in the tropical middle atmosphere and its gravity wave driving in reanalyses and satellite observations

Gravity waves play a significant role in driving the semiannual oscillation (SAO) of the zonal wind in the tropics. However, detailed knowledge of this forcing is missing, and direct estimates from global observations of gravity waves are sparse. For the period 2002–2018, we investigate the SAO in four different reanalyses: ERA-Interim, JRA-55, ERA-5, and MERRA-2. Comparison with the SPARC zonal wind climatology and quasi-geostrophic winds derived from Microwave Limb Sounder (MLS) and Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) satellite observations show that the reanalyses reproduce some basic features of the SAO. However, there are also large differences, depending on the model setup. Particularly, MERRA-2 seems to benefit from dedicated tuning of the gravity wave drag parameterization and assimilation of MLS observations. To study the interaction of gravity waves with the background wind, absolute values of gravity wave momentum fluxes and drag derived from SABER satellite observations are compared with different wind data sets: the SPARC wind climatology, data sets combining ERA-Interim at low altitudes and MLS or SABER quasi-geostrophic winds at high altitudes, as well as data sets that combine ERA-Interim, SABER quasi-geostrophic winds, and direct wind observations by the TIMED Doppler Interferometer (TIDI). In the lower and middle mesosphere SABER absolute gravity wave drag correlates well with positive vertical gradients of the background wind, indicating that gravity waves contribute mainly to the driving of the SAO eastward wind phases and their downward propagation with time. At altitudes 75–85 km, SABER absolute gravity wave drag correlates better with absolute values of the background wind, suggesting a more direct forcing of the SAO winds by gravity wave amplitude saturation. Above about 80 km SABER gravity wave drag is mainly governed by tides rather than by the SAO. The reanalyses reproduce some basic features of the SAO gravity wave driving: All reanalyses show stronger gravity wave driving of the SAO eastward phase in the stratopause region. For the higher-top models ERA-5 and MERRA-2 this is also the case in the lower mesosphere. However, all reanalyses are limited by model-inherent damping in the upper model levels, leading to unrealistic features near the model top. Our analysis of the SABER and reanalysis gravity wave drag suggests that the magnitude of SAO gravity wave forcing is often too weak in the free-running general circulation models, therefore, a more realistic representation is needed.