pure and applied geophysics最新文献

We investigate a miniaturized prototype of gravimeter named B-grav to overview different metrological questions to resolve for a system as part of a geophysical instrument package set-up on an asteroid lander. It must meter with 1% tentative accuracy of a weak 50 µm/sec² gravity field in a harsh environment with very strict limitations of inclusion in a CubeSat. With its 3D orthogonal components, our system allows to determine gravity in amplitude and in angular position without levelling. B-grav in-situ calibrations are based on centrifugal torques and electrostatic forcing. Simulation of asteroid gravity field in laboratory is applied to the pendulum set-up in a vertical position to reject the Earth gravity field. Expertise gained with the design of B-grav was applied for the development at the Royal Observatory of Belgium of the gravimeter GRASS.

A literature review on historic atmospheric radionuclide monitoring data associated with nuclear explosions is presented. It covers reports on tests conducted by four countries between 1964 and 1996 and those related to the six announced nuclear tests of the Democratic People’s Republic of Korea between 2006 and 2017. Most of these nuclear explosions occurred in the atmosphere, but observation of nuclear debris from the venting of underground nuclear tests was also found. The review is limited to off-site monitoring. Many observations were done at large distances, including several nuclear explosions detected in multiple locations, generally in the same hemisphere. The measurement methods developed over time, and many different fission and activation products were identified. Examples are given, and further suggestions are made about how this data set is of value for validating and enhancing methods based on radionuclide analysis and related atmospheric transport simulations to identify and characterize the source of an event relevant to atmospheric radioactivity monitoring for the Comprehensive Nuclear Test Ban Treaty.

It is well known that significant amounts of radioxenon radionuclides are released from Medical Isotope Production Facilities and to a lesser extent from Nuclear Power Plants (NPP). These emissions cause a background in the atmosphere that is often detected by noble gas systems of the International Monitoring System (IMS) operated by the Comprehensive Nuclear-Test-Ban Treaty Organization Preparatory Commission for nuclear explosion monitoring. In addition to those facilities, the operation of a Spent Nuclear Fuel (SNF) reprocessing plant may possibly also contribute to the IMS observations, but this has not yet been investigated. Even after long cooling time, the short-lived radioxenon isotopes are present in spent fuel due to spontaneous fission with the isotopes ²⁴⁴Cm and ²⁴⁰Pu being the main contributors. The SNF reprocessing process can promptly release the whole radioxenon inventory if there is no retention system. The aim of this work is to investigate the possible radioxenon emission during SNF reprocessing caused by spontaneous fission of heavy elements. Two independent methods are applied to determine the radioxenon releases. One approach is to use the published release of ¹³¹I as a proxy. The other is to analyse the parameters of reprocessed spent fuel to determine the content of ²⁴⁴Cm and ²⁴⁰Pu and with this information to estimate the radioxenon inventory. It turns out that the estimated maximum release of ¹³³Xe is of the order of GBq/day which is almost as high as the average discharge on an NPP site. Assuming the absence of an effective retention system that prevents the release of radioxenon into the environment, the results of the calculations show that industrial scale reprocessing plants should be considered as a weak but not negligible source of radioxenon.

Isotopic activity ratios of the relevant radionuclides detected at radionuclide stations within the International Monitoring System (IMS) for the Comprehensive Nuclear-Test-Ban Treaty (CTBT), are crucial for characterizing release events under assumed scenarios. This analysis involves considering radioactive gases initially released from an underground nuclear event, resulting in radionuclide concentrations in a plume of air passing over an IMS station that are subsequently sampled by that station. Modeling this process requires considering the post-detonation radionuclide evolution of an assumed underground nuclear explosion, simulating atmospheric transport, and finally, collecting and measuring samples. Activities collected in the samples are determined through spectrum analysis of sample measurements, and the activity concentrations are then estimated by assuming a constant concentration during sampling. While this assumption holds true for radionuclides with relatively longer half-lives, exceeding 7 times the sampling duration, challenges arise for isotopes with short half-lives, such as ¹³⁵Xe with half-life of 9.14 h compared to the 12-h collection duration for some noble gas systems. Therefore, this study investigates decay correction during sampling using two approaches: 1) Interval constant concentration: the collection duration is divided into multiple intervals, with a constant concentration assumed in each interval; 2) Decaying concentration: the activity collected in the sample is derived based on an analytical solution to the ordinary differential equations governing the activity decay and ingrowth. The impact of these approaches on isotopic activity ratios is demonstrated in three cases with short half-lives: ¹⁴⁰Ba/¹⁴⁰La, and ^133mXe/¹³³Xe, and ¹³⁵Xe/¹³³Xe. The decay correction on the ratio ¹³⁵Xe/¹³³Xe might be approximately 2 for 24-h sampling, and 1.2 for the shortest collection duration of 6 h. For the pair of ^133mXe to ¹³³Xe, the ingrowth correction might be negligible compared to the relatively large value of the ratio ¹³³Xe/^133mXe, which exceeds 10 in a few days after the detonation. For the ratio ¹⁴⁰La/¹⁴⁰Ba, the contribution from the ingrowth of parent-daughter decay is approximately 0.35 for a collection duration of 24 h.

Hurbanovo, as the only station for continuous monitoring of gravity changes in Slovakia, is equipped with a relative spring gravimeter gPhoneX#108. The gravity station was established in 2019 and has been a part of the International Geodynamics and Earth Tide Service (IGETS) since 2021. In addition to gravity measurements, several complementary measurements at this station are carried out, including Global Navigation Satellite Systems (GNSS) for geodetic positioning co-located with Interferometric Synthetic Aperture Radar (InSAR) corner reflectors, seismometer for seismic monitoring, meteorological measurements including atmospheric pressure, temperature, and precipitation, as well as hydrological measurements of soil moisture and groundwater levels. Our contribution includes a fundamental statistical and correlation analysis of the temporal data collected through these diverse techniques and sensors, highlighting foundational insights obtained from the investigation. Results demonstrate the benefit of thermal insulation of the gPhoneX instrument but also reveal the increased anthropogenic noise at the Hurbanovo site coming most likely from the nearby traffic. Despite of the noisy environment gravimeter can capture significant precipitation events and larger groundwater variations.

Absolute gravity time series from Antarctica are used to study the viscoelastic gravity change and deformation due to Glacial Isostatic Adjustment (GIA) after the Holocene deglaciation. Here we present the three-decades long absolute gravity (AG) time series at the Finnish Antarctic Research Station Aboa. A gravity increase of nearly 50 (upmu)Gal is observed. Comparisons of the gravity trend with the land uplift observed in the Aboa GPS station time series and with GIA model predictions show that GIA can’t explain the observed gravity increase. We use satellite gravimetry and altimetry, GPS measurements, and modelling to interpret the gravity increase. A regional mass increase around Aboa is observed with satellite gravimetry. Satellite altimetry shows positive surface elevation change in the region over the last three decades. GPS-based surface elevation change measurements in the vicinity of Aboa also point to increase snow and ice volume. Increased precipitation in Dronning Maud Land in the 2000s is noted in the literature. Modelling of the direct attraction due to added mass on the ice sheet around Aboa yields gravity change comparable to what is observed in the time series. Consequently the apparent explanation to the gravity increase is the positive mass balance of the seasonal snow close to the gravity laboratory and of the surrounding ice sheet. Increased direct attraction and elastic ground deformation overshadow the viscoelastic GIA signal in the absolute gravity time series. Conversely, absolute gravity time series at Aboa can be used as an independent observation of the mass increase.

Non-Tidal Atmospheric Loading (NTAL) plays a crucial role in the precision and reliability of GNSS-based positioning and geophysical interpretations, particularly in high-latitude regions, sensitive to atmospheric dynamics. This investigation examines the influence of non-tidal atmospheric loading on GNSS time series and velocities derived from them for high-latitude regions. With a dataset from 2020 to 2023, we process a GNSS network across northern Europe, focusing on the Finnish permanent GNSS network (FinnRef). Using GAMIT/GLOBK software, where corrections are applied at the observation level, we incorporate a new atmospheric grid model derived from the European Centre for Medium-Range Weather Forecasts (ECMWF) numerical weather data. This model provides higher spatial resolution compared to previously available models in GAMIT/GLOBK. Temporal variability of NTAL-corrected GNSS time series is reduced by 17% in the vertical component, and by 8% and 2% in the north and east components, respectively, across the FinnRef network. Additionally, our results highlight that NTAL correction lowers vertical trend uncertainty by an average of 33.5%. Besides evaluating metrics such as spectral power density (PSD) and annual amplitude variation, we observe that the spectral index of the vertical component drops from − 1.44 to − 0.9, indicating reduced long-term noise correlation. We also compare this observation-level approach with an alternative method that applies NTAL corrections at the raw-data level and find that the observation-level correction shows slightly better performance. These results demonstrate that significant improvements in the stability of GNSS time series can be expected after NTAL application, especially in the vertical component.

High accuracy repeated gravity observations within Vrancea active seismic zone, where upper mantle seismicity occurs within full intra-continental environment and no evidence on an active subduction, have succeeded to outline the overall lowering of the gravity over the epicentres area, unexpectedly associated with a local relative subsidence of topography, superposed on the overall trend of Carpathians uplift following denudation and erosion of the mountains catena. A particular gravity decrease related to seismicity was also unveiled by successive gravity campaigns conducted prior and after some significant earthquakes (M 5+). As the vertical deformation of the crust may not explain the gravity change in the area, attempts have been made to model and understand genesis of the mass deficit responsible for the observed gravity. Solutions provided by gravity inversion and some 2D and 3D forward modelling have been interpreted in terms of vertical stretching of the upper part of the crust under the gravity pull generated by eclogitization of the lower crust penetrating the upper mantle. Given the subduction-related scenarios, often used to explain the Vrancea seismicity, may not offer the mechanical environment to justify the lithosphere stretching, some alternative, non-conventional geodynamic models like Rayleigh–Taylor gravitational instability, or the unstable triple-junction are suggested to explain the phenomenon.