Seasonal Gravity Research Articles

Interference of ocean and land mass changes in seasonal crustal deformation of coastal stations: A case study in northern Australia

In tropical regions, alternation of rainy and dry seasons causes significant land hydrological mass changes. It brings “landward” seasonal crustal movements of coastal global navigation satellite system (GNSS) stations in rainy seasons. Here, we report that the coastal region of the Gulf of Carpentaria (GOC), northern Australia, behaves oppositely, i.e., they move “oceanward” during austral summer months despite large precipitations on land. Here we study seasonal gravity changes there with satellite gravimetry and analyze seasonal movements of GNSS stations in that region. During the austral summer, hydrological mass increases in nearby land areas. In addition to that, sea water mass within the gulf also increases due to east-southeastward seasonal winds. It has larger amplitudes than on land, and its peak comes a few months earlier, resulting in seasonal displacements of coastal GNSS stations driven predominantly by ocean mass changes. In this study, we emphasize the importance of seasonal horizontal crustal movements, i.e., the oceanward displacements of coastal stations there in austral summer clearly indicate the dominance of the excessive ocean mass over those on land. Such seasonal mass changes within GOC have been largely overlooked because of insufficient spatial resolution of satellite gravimetry. This study presents a model case for interpreting complicated seasonal surface deformation caused by significant seasonal load changes on land and ocean, often found in coastal areas of half-closed oceans in the world.

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

Continuous high-resolution gravimetry is increasingly used to monitor mass distribution changes in volcanic, hydrothermal or other complex geosystems. To quantify the often small target signals, gravity contributions from, e.g. atmospheric mass changes, global and local hydrology should be accounted for. We set up three iGrav superconducting gravity meters for continuous monitoring of the Þeistareykir geothermal field in North Island. Additionally, we installed a set of hydrometeorological sensors at each station for continuous observation of local pressure changes, soil moisture, snow and vertical surface displacement. We show that the contribution of these environmental parameters to the gravity signal does not exceed 10 µGal (1 µGal = 10–8 m s−2), mainly resulting from vertical displacement and snow accumulation. The seasonal gravity contributions (global atmosphere, local and global hydrology) are in the order of ± 2 µGal at each station. Using the environmental observations together with standard gravity corrections for instrumental drift and tidal effects, we comprehensively reduced the iGrav time-series. The gravity residuals were compared to groundwater level changes and geothermal mass flow rates (extraction and injection) of the Þeistareykir power plant. The direct response of the groundwater levels and a time-delayed response of the gravity signal to changes in extraction and injection suggest that the geothermal system is subject to a partially confined aquifer. Our observations indicate that a sustainable “equilibrium” state of the reservoir is reached at extraction flow rates below 240 kg s−1 and injection flow rates below 160 kg s−1. For a first-order approximation of the gravity contributions from extracted and injected masses, we applied a simplified forward gravity model. Comparison to the observed gravity signals suggest that most of the reinjected fluid is drained off through the nearby fracture system.

Earth\u2019s gravity from space

Satellite gravimetry began with the launch of the satellites Sputnik 1 and 2 in 1957. During the following 43 years, more and more details were discovered and the models of the Earth’s gravity could be refined. Methods improved and more and more satellite orbits and ground stations were added in the analysis, employing more advanced and precise measuring techniques. A new era started with the dedicated gravimetry missions CHAMP (2000–2010), GRACE (2002–2017), and GOCE (2009–2013). The methods of satellite-to-satellite tracking and satellite gradiometry resulted in a substantial improvement of our knowledge of the Earth’s gravity field in terms of accuracy and its spatial and temporal variations. There are three basic ways of using gravity and geoid models in Earth sciences and geodesy. First, in solid Earth physics, the highs and lows of the field are investigated in comparison with an idealized Earth, e.g., a hydrostatic equilibrium figure. In particular, in South America, Africa, Himalaya and Antarctica the gravity field is known much better now, due to GOCE and lead to an improved understanding of the continental crust and lithosphere. Second, in oceanography, the geoid serves as surface in equilibrium, a hypothetical ocean at rest. The ocean topography is the deviation of the actual ocean surface, measured by satellite altimetry, from this reference. The ocean topography serves as a new and independent input to ocean circulation modeling and leads to an improved understanding of ocean transport of mass, heat, and nutrients. Similarly, geodetic heights of the land surface will soon be referred to the geoid, leading to globally consistent heights and enabling the removal of existent systematic deformations and offsets of national and continental height systems. Third, the GRACE time series of monthly gravity models, reflecting seasonal, inter-annual and long-term gravity changes, became one of the most valuable data sources of climate change studies.

Parallel Observations with Three Superconducting Gravity Sensors During 2014–2015 at Metsähovi Geodetic Research Station, Finland

The new dual-sphere superconducting gravimeter (SG) OSG-073 was installed at Metsahovi Geodetic Fundamental Station in Southern Finland in February 2014. Its two gravity sensors (N6 and N7) are side by side, not one on top of the other as in other earlier dual-sensor installations. The old SG T020 has been recording continuously since 1994–2016. This instrument is situated in the same room at a distance of 3 m from the dual-sphere SG. T020 observed simultaneously for 1 year with N6 and for 15 months with N7. The gravity signals observed by N6 and N7 are very similar, except for the initial exponential drift. We have calculated the power spectral density to compare the noise level of these instruments with other low noise SGs. In this paper we present the observed differences in the gravity time series of T020 and OSG-073, induced by local hydrology. We have observed a clear 10–20 nms-2 difference in the seasonal gravity variations of OSG- 073 and T020. We have found clear gravity differences due to transient effect of heavy precipitation. In addition, we compare the remote effect on gravity due to variations in the Baltic Sea level and total water storage in Finland to the observed gravity signal. We also present modeling results of gravity variations due to local hydrology.

Nonlinear Drift of the Spring Gravimeter Caused by Air Pressure from the Kunming GS15 Gravimeters

Abstract In order to monitor and correct the meteorological factors of the spring gravity meter, the characteristics of the time varying gravity changes caused by meteorological factors were analyzed. Kunming GS15 gravity meter from 2007 continuous gravity observation has been carried out with the sampling rate of the pressure observation. In this study, we first compare the effects of 4 types of gravity meter and 3 different types of stations on the gravity observed seasonal gravity signals. It is indicated that the observed seasonal gravity signal of the cave is only 1/10, and there is a constant temperature and constant pressure device. Compared with the same time, the gravity signal of the gravity signal is about 100 times smaller. The influence of the pressure load of the gravity meter is tested by using the theory of pressure. The results show that only the actual value of 2cpd - 3cpd pressure load varies from -0.395 to -0.280×10-8ms-2 , and the 1cpd to 1 cpm periodic partial type gravity meter is also in accordance with the law of gravity and air pressure. And with the characteristics of time lag. In this paper, the nonlinear zero drift parameters of the linear regression model with time lag and the time series of the GS15 gravity meter are used to simulate the nonlinear zero drift parameters of the gravity meter. The results show that the gravity signal contains time lag 35 hours, and the air pressure admittance is 0.8 × 10-8ms-2/mbar. The correlation can reach 79%. The gravity changes signal and satellite gravity as well as the gravity water load signal of the land water model are the same as the gravity water load signal in the autumn as the minimum value, and the seasonal variation of the maximum value of gravity in summer.

Seasonal and static gravity field of Mars from MGS, Mars Odyssey and MRO radio science

We present a spherical harmonic solution of the static gravity field of Mars to degree and order 120, GMM-3, that has been calculated using the Deep Space Network tracking data of the NASA Mars missions, Mars Global Surveyor (MGS), Mars Odyssey (ODY), and the Mars Reconnaissance Orbiter (MRO). We have also jointly determined spherical harmonic solutions for the static and time-variable gravity field of Mars, and the Mars k2 Love numbers, exclusive of the gravity contribution of the atmosphere. Consequently, the retrieved time-varying gravity coefficients and the Love number k2 solely yield seasonal variations in the mass of the polar caps and the solid tides of Mars, respectively. We obtain a Mars Love number k2 of 0.1697 ± 0.0027 (3-σ). The inclusion of MRO tracking data results in improved seasonal gravity field coefficients C30 and, for the first time, C50. Refinements of the atmospheric model in our orbit determination program have allowed us to monitor the odd zonal harmonic C30 for ∼1.5 solar cycles (16 years). This gravity model shows improved correlations with MOLA topography up to 15% larger at higher harmonics (l = 60–80) than previous solutions.

Investigating seasonal gravity wave activity in the summer polar mesosphere

The NASA Aeronomy of Ice in the Mesosphere (AIM) satellite is the first spaceborne mission dedicated to studying high-altitude (~83km) Polar Mesospheric Clouds (PMCs). Since its launch in 2007, the Cloud Imaging and Particle Size (CIPS) instrument onboard AIM has obtained large-field, high resolution (25km2/pixel) images of the PMCs, enabling a unique investigation of mesospheric gravity wave activity in the summer polar mesosphere where previous measurements have been sparse. In this study, we have analyzed 12 consecutive seasons of AIM/CIPS PMC albedo data to determine the statistical properties of medium and large horizontal scale (>100km) gravity waves present in the PMC data. Over 60,000 wave events with horizontal scale-sizes ranging up to >2000km have been identified and measured, revealing a wealth of wave events particularly in the ~300–800km range where our analysis sensitivity is largest. These data are ideal for investigating the intra-seasonal, inter-annual and hemispheric variability of these waves as observed over the whole summer polar cap regions. Throughout this 6 year study, the wave activity in the southern hemisphere was found to be consistently 10–15% higher than in the northern hemisphere and both the northern and southern hemisphere wave activity was determined to decrease systematically (by ~15%) during the course of each summer season. This decrease agrees well with previous seasonal stratospheric studies of variations in the wave energy, suggesting a direct influence of the lower atmospheric sources on polar mesospheric dynamics. Very similar and consistent results were also found from season to season in both hemispheres providing new information for gravity wave modeling and dynamical studies of the high-latitude summer-time mesosphere.

Decadal geodetic variations in Ny-Ålesund (Svalbard): role of past and present ice-mass changes

The geodetic rates for the gravity variation and vertical uplift in polar regions subject to past and present-day ice-mass changes (PDIMCs) provide important insight into the rheological structure of the Earth. We provide an update of the rates observed at Ny-Alesund, Svalbard. To do so, we extract and remove the significant seasonal content from the observations. The rate of gravity variations, derived from absolute and relative gravity measurements, is −1.39 ± 0.11 μGal yr −1 . The rate of vertical displacements is estimated using GPS and tide gauge measurements. We obtain 7.94 ± 0.21 and 8.29 ± 1.60 mm yr −1 , respectively. We compare the extracted signal with that predicted by GLDAS/Noah and ERA-interim hydrology models. We find that the seasonal gravity variations are well-represented by local hydrology changes contained in the ERA-interim model. The phase of seasonal vertical displacements are due to non-local continental hydrology and non-tidal ocean loading. However, a large part of the amplitude of the seasonal vertical displacements remains unexplained. The geodetic rates are used to investigate the asthenosphere viscosity and lithosphere/asthenosphere thicknesses. We first correct the updated geodetic rates for those induced by PDIMCs in Svalbard, using published results, and the sea level change due to the melting of the major ice reservoirs. We show that the latter are at the level of the geodetic rate uncertainties and are responsible for rates of gravity variations and vertical displacements of −0.29 ± 0.03 μGal yr −1 and 1.11 ± 0.10 mm yr −1 , respectively. To account for the late Pleistocene deglaciation, we use the global ice evolution model ICE-3G. The Little Ice Age (LIA) deglaciation in Svalbard is modelled using a disc load model with a simple linear temporal evolution. The geodetic rates at Ny-Alesund induced by the past deglaciations depend on the viscosity structure of the Earth. We find that viscous relaxation time due to the LIA deglaciation in Svalbard is more than 60 times shorter than that due to the Pleistocene deglaciation. We also find that the response to past and PDIMCs of an Earth model with asthenosphere viscosities ranging between 1.0 and 5.5 × 10 18 Pa s and lithosphere (resp. asthenosphere) thicknesses ranging between 50 and 100 km (resp. 120 and 170 km) can explain the rates derived from geodetic observations.

Gravity effect of water storage changes in a weathered hard-rock aquifer in West Africa: results from joint absolute gravity, hydrological monitoring and geophysical prospection

Advances in groundwater storage monitoring are crucial for water resource management and hydrological processes understanding. The evaluation of water storage changes (WSC) often involve point measurements (observation wells, moisture probes, etc.), which may be inappropriate in heterogeneous media. Over the past few years, there has been an increasing interest in the use of gravimetry for hydrological studies. In the framework of the GHYRAF (Gravity and Hydrology inAfrica) project, 3 yr of repeated absolute gravity measurements using a FG5-type gravimeter have been undertaken at Nalohou, a Sudanian site in northern Benin. Hydrological data are collected within the long-term observing system AMMA-Catch. Once corrected for solid earth tides, ocean loading, air pressure effects, polar motion contribution and non-local hydrology, seasonal gravity variations reach up to 11 μGal, equivalent to a WSC of 260-mm thick infinite layer of water. Absolute temporal gravity data are compared to WSC deduced from neutron probe and water-table variations through a direct modelling approach. First, we use neutronic measurements available for the whole vertical profile where WSC occur (the vadose zone and a shallow unconfined aquifer). The RMSD between observed and modelled gravity variations is 1.61 μGal, which falls within the error bars of the absolute gravity data. Second, to acknowledge for the spatial variability of aquifer properties, we use a 2-D model for specific yield (Sy) derived from resistivity mapping and Magnetic Resonance Soundings (MRS). The latter provides a water content (θMRS) known to be higher than the specific yield. Hence, we scaled the 2-D model of θMRS with a single factor (α). WSC are calculated from water-table monitoring in the aquifer layer and neutronic measurements in the vadose layer. The value of α is obtained with aMonte-Carlo sampling approach, minimizing the RMSD between modelled and observed gravity variations. This leads to α = Sy/θMRS = 0.63 ± 0.15, close to what is found in the literature on the basis of pumping tests experiments, with a RMSD value of 0.94 μGal. This hydrogeophysical experiment is a first step towards the use of time-lapse gravity data as an integrative tool to monitor interannual WSC even in complicated subsurface distribution.

Gravity effect of glacial ablation in the Eastern Alps – observation and modeling

Abstract. Absolute gravity measurements have been regularly performed in the Austrian Eastern Alps since 1985. A gravity increase of 300 nm s−2 has been observed so far. The gravity trend is explained by ablation effects within surrounding glaciers. Ice thickness changes derived from 3 successive glacier inventories of 1969, 1997 and 2006 are used for quantitative 3-D modeling based on rectangular prisms with basis areas of ≤ 8 m × 8 m. Local topographic changes due to man-made mass displacements close to the measuring site are modeled by a polyhedron approach. Two-thirds (2/3) of the observed gravity increase can be explained by the ablation model response and man-made effects. A positive trend of about 100 nm s−2 remains. The origin of the residual trend remains open. Correcting for geodynamical processes like Alpine uplift or postglacial deformation is expected to cause a slight increase of this trend. The observed gravity signal shows seasonal gravity variations as well, which are probably due to snow cover effects but cannot be quantified due to the lack of appropriate snow cover information.

Seasonal gravity changes estimated from GRACE data

Abstract: Since 2002, the GRACE program has provided a large amount of high-precision data, which can be used to detect temporal gravity variations related to global mass re-distribution inside the fluid envelop of the surface of the Earth. In order to make use of the GRACE data to investigate earthquake-related gravity changes in China, we first studied the degree variances of the monthly GRACE gravity field models, and then applied decor-relation and Gaussian smoothing method to obtain seasonal gravity changes in China. By deducting the multi-year mean seasonal variations from the seasonal maps, we found some earthquake-related gravity anomalies.

Correcting gravimeters and tiltmeters for atmospheric mass attraction using operational weather models

Abstract The Newtonian attraction of the atmosphere is a major source of noise in precise gravimetric measurements. A major part of the effect is eliminated using local air pressure records and constant admittance factors. However, vertical mass shifts under constant surface pressure or distant pressure anomalies are not covered by this technique although they affect the gravimeter. In order to improve the atmospheric correction and to evaluate the horizontal components of attraction as well, the Newtonian attraction is computed based on the spatial density distribution derived from three-dimensional weather models. Operational models from the German Weather Service (DWD) of various scales were used, supplemented by a global data set from the European Centre of Medium Weather Forecast (ECMWF) for comparison. The low temporal resolution and the improper point-mass assumption in the near field are tackled by a cylindrical local model by computing the attraction analytically based on local air pressure records with high temporal resolution. It is shown that a height of at least 50 km and global coverage is required to meet a threshold of 1 nm/s2. Neglecting the upper atmosphere leads to an overestimation of the seasonal gravity signal. At distances greater than 10 ° the time consuming three-dimensional computation can be replaced by a two-dimensional surface pressure approach without significant error. The results show differences up to 20 nm/s2 as compared to the linear regression method. The three-dimensional atmospheric correction significantly reduces noise in the time series, giving more insight into other signals such as hydrological effects or deformation processes.

Validating global hydrological models by ground and space gravimetry

The long-term continuous gravity observations obtained by the superconducting gravimeters (SG) at seven globally-distributed stations are comprehensively analyzed. After removing the signals related to the Earth’s tides and variations in the Earth’s rotation, the gravity residuals are used to describe the seasonal fluctuations in gravity field. Meanwhile, the gravity changes due to the air pressure loading are theoretically modeled from the measurements of the local air pressure, and those due to land water and nontidal ocean loading are also calculated according to the corresponding numerical models. The numerical results show that the gravity changes due to both the air pressure and land water loading are as large as 100×10−9 m s−2 in magnitude, and about 10×10−9 m s−2 for those due to the nontidal ocean loading in the coastal area. On the other hand, the monthly-averaged gravity variations over the area surrounding the stations are derived from the spherical harmonic coefficients of the GRACE-recovered gravity fields, by using Gaussian smoothing technique in which the radius is set to be 600 km. Compared the land water induced gravity variations, the SG observations after removal of tides, polar motion effects, air pressure and nontidal ocean loading effects and the GRACE-derived gravity variations with each other, it is inferred that both the ground- and space-based gravity observations can effectively detect the seasonal gravity variations with a magnitude of 100×10−9 m s−2 induced by the land water loading. This implies that high precision gravimetry is an effective technique to validate the reliabilities of the hydrological models.

Repeated Absolute Gravity Measurements and Seasonal Gravity Changes in the Kamioka Mine, Gifu, Japan

Repeated Absolute Gravity Measurements and Seasonal Gravity Changes in the Kamioka Mine, Gifu, Japan

On models of Mars’ interior and amplitudes of forced nutations: 1. The effects of deviation of Mars from its equilibrium state on the flattening of the core–mantle boundary

In a static approach the effects of deviation of Mars from its equilibrium state on the flattening of the core–mantle boundary (CMB) are studied. The calculations are performed for a set of interior structure models with 50- and 100-km thick crust and averaged crustal density varying in the range of 2700–3200 kg m −3, which are based on new values for the moment of inertia and the elastic tidal Love number k 2 s defined by Konopliv et al. [Konopliv, A.S., Yoder, C.F., Standish, E.M., Yuan, D.-N., Sjorgen, W.L., 2006. A global solution for the Mars static and seasonal gravity, Mars orientation, Phobos and Deimos masses, and Mars ephemeris. Icarus, 182, 23–50]. The models differ in density contrast at the crust–mantle boundary and the core radius. The observational data constrain the radius of a liquid core to be within 1700–1800 km. For joint interpretation of gravity and topography data an outer surface of a hydrostatical model is chosen as a reference surface. The Green's functions (or loading factors) technique for the case of a single anomalous density wave located at two depth level is used to compute the second-degree CMB deformation, and then the CMB flattening. Two types of models have been calculated: an elastic model and a model with an elastic lithosphere and weakened layers below it (relaxed values of shear modulus); the effective shear modulus of the mantle is reduced in comparison with an elastic one and in extreme case it is approaching zero everywhere except the elastic lithosphere. Non-equilibrium state of Mars results in three-axiality of the core–mantle boundary. For an elastic case the deformation of CMB leads to the large decrease of the semiaxis b, going through the central region of Tharsis rise, by 660–780 m, and the increase of the equatorial semiaxis a by 240–300 m, and the polar axis c by 400–490 m. Tharsis rise plays a constitutive role for the CMB deformation. The second-degree harmonics in the expansion of non-equilibrium part of gravity field and topography are sensitive, first of all, to the properties of the crust in the Tharsis region. Non-hydrostatic contributions decrease the flattening of the CMB by about 5–6%. The effect of loading increases when the value of shear modulus decreases by a factor of 10, but as the shear modulus is decreasing up to zero the flattening is approaching its hydrostatic value.

Seasonal gravity wave activity observed with the Equatorial Atmosphere Radar and its relation to rainfall information from the Tropical Rainfall Measuring Mission

Five years of tropospheric data below 12 km from the Equatorial Atmosphere Radar (EAR) are examined for seasonal and interannual gravity wave activity. Results are compared with data from the Tropical Rainfall Measuring Mission (TRMM) satellite. A semiannual cycle of vertical wind variances inside and above convection is found. Maxima occur at the equinoxes and minima at the solstices. This semiannual cycle is also observed in the TRMM storm height and surface rainfall. Half of the vertical wind variance above convection is due to waves with periods of less than 2 h, with the rest coming from waves with periods of less than 24 h. On average, 78% of the total horizontal wind variance above convection is due to waves with periods of less than 24 h. The TRMM rainfall at 2 km altitude is greater than the EAR surface rainfall, suggesting that the EAR site is not representative of regional rainfall conditions. Momentum fluxes are calculated and shown to be dominated by multiday processes, although standard deviations are greater than the mean values.

A global solution for the Mars static and seasonal gravity, Mars orientation, Phobos and Deimos masses, and Mars ephemeris

With the collection of six years of MGS tracking data and three years of Mars Odyssey tracking data, there has been a continual improvement in the JPL Mars gravity field determination. This includes the measurement of the seasonal changes in the gravity coefficients (e.g., J ¯ 2 , J ¯ 3 , C ¯ 21 , S ¯ 21 , C ¯ 31 , S ¯ 31 ) caused by the mass exchange between the polar ice caps and atmosphere. This paper describes the latest gravity field MGS95J to degree and order 95. The improvement comes from additional tracking data and the adoption of a more complete Mars orientation model with nutation, instead of the IAU 2000 model. Free wobble of the Mars' spin axis, i.e. polar motion, has been constrained to be less than 10 mas by looking at the temporal history of C ¯ 21 and S ¯ 21 . A strong annual signature is observed in C ¯ 21 , and this is a mixture of polar motion and ice mass redistribution. The Love number solution with a subset of Odyssey tracking data is consistent with the previous liquid outer core determination from MGS tracking data [Yoder et al., 2003. Science 300, 299–303], giving a combined solution of k 2 = 0.152 ± 0.009 using MGS and Odyssey tracking data. The solutions for the masses of the Mars' moons show consistency between MGS, Odyssey, and Viking data sets; Phobos GM = ( 7.16 ± 0.005 ) × 10 −4 km 3 / s 2 and Deimos GM = ( 0.98 ± 0.07 ) × 10 −4 km 3 / s 2 . Average MGS orbit errors, determined from differences in the overlaps of orbit solutions, have been reduced to 10-cm in the radial direction and 1.5 m along the spacecraft velocity and normal to the orbit plane. Hence, the ranging to the MGS and Odyssey spacecraft has resulted in position measurements of the Mars system center-of-mass relative to the Earth to an accuracy of one meter, greatly reducing the Mars ephemeris errors by several orders of magnitude, and providing mass estimates for Asteroids 1 Ceres, 2 Pallas, 3 Juno, 4 Vesta, and 324 Bamberga.

Study of the seasonal gravity signal in superconducting gravimeter data

Thanks to their high sensitivity and their long-term stability, superconducting gravimeters (SG) are able to record surface gravity changes on a wide frequency band (periods from a few seconds to secular variations). We focus in this presentation on the seasonal gravity changes measured by about 20 worldwide SG. We model all well-known sources of long-term gravity changes, i.e. solid Earth tides, polar motion and length-of-day as well as global atmospheric, tidal and non-tidal ocean loading effects. These corrections lead to gravity residuals characterized by a strong seasonal signal with an amplitude of a few microgals. We compare these residuals with loading estimates from global hydrology (snow and soil-moisture) models. For more than half of the analysed SG, we are able to show a good correlation between the gravity residuals and the estimated continental water storage loading effects. For the other instruments, the discrepancies may be associated with local hydrology effects, which cannot be taken into account in global continental water storage models. © 2005 Elsevier Ltd. All rights reserved.

Gravity tide and seasonal gravity variation at Ny-Ålesund, Svalbard in Arctic

Abstract We have analyzed 4 years of gravity data (September 1999–August 2003) obtained from the superconducting gravimeter at Ny-Alesund, Svalbard in Arctic. The tidal analysis results indicate that the time variation in amplitude and phase of the short-period gravity tides may be due to the seasonal variation of the ocean tides, especially in the semidiurnal band. The gravity residuals, which were reduced from the following three effects from the original data: (1) the observed short- and long-period tides (up to the annual Sa wave) including the loading and attraction of ocean tide, (2) the air pressure and (3) the polar motion, show clear correlation with the computed hydrological effects (i.e. the effects of soil moisture and snow) in both the amplitude and phase. However, we found that the gravity data reveal a problem in the snow model at Ny-Alesund that was used for the computation. The vertical motion obtained from the continuous GPS measurements carried out near the gravity station shows seasonal variations that are consistent with the gravity data at least in their sign.

Long-term and seasonal gravity changes at the Strasbourg station and their relation to crustal deformation and hydrology

Abstract The time-varying gravity is observed at the Strasbourg station with super-conducting gravimeters (SG) since 1987, with a first record from 1987 to 1996 (GWR T005) and a second one in continuity from 1996 till now (GWR C026). The long-term behaviour of the SG is constrained by regular absolute gravity (AG) measurements, which are performed in parallel since 1989, first with the JILAg-5 instrument and later on with the FG5#206. Moreover, a permanent GPS station, which belongs to the French geodetic network, has been installed at the end of 1999. We will show that the AG measurements suggest that the gravity is slowly increasing in time at a rate of about 1.6 μGal/year, and besides, exhibits a quasi-annual component of several μGal variable amplitude. We present an analysis of the GPS data obtained at the SG station in the Rhine graben as well as at another regional station in the Vosges mountains distant by about 40 km in order to constrain the gravity contribution due to the vertical displacement of the station in the earth's gravity field (geometrical part). The tectonic context of the region is described and our first results from our two data sets of limited duration suggest a small subsidence of our station in the graben; however, this fact needs further confirmation when more geodetic data will be available. We also analyze the water table changes beneath the station (local scale) and in the Alsatian Plain (regional scale) to estimate the hydrological contributions from ground water to the gravity residual signal, and we show that some similarities exist between the gravity residuals and the hydrological contributions, especially for the seasonal terms. Other missing contributions of annual period (air mass motion in the atmosphere, ocean circulation, continental hydrology) have to be considered.