Observed secular gravity trend at Onsala station with the FG5 gravimeter from Hannover (original) (raw)

New Investigation of Tidal Gravity Results

2011

Some 20 series longer than one year are now available in the GGP data bank, including 3 dual sphere instruments. To eliminate the tidal loading effects we interpolated the contribution of the smaller oceanic waves from the 8 well determined ones i.e. Q1, O1, P1, K1, N2, M2, S2 and K2. It was done for six different oceanic models: SCW80, CSR3.0, FES95.2, TPXO2, CSR4.0 and ORI96. In the diurnal band no model is decisively better than the others and a mean tidal loading vector is giving the most stable solution for the study of the liquid core resonance. In the semi-diurnal band however the SCW80 and TPXO2 models are not convenient. We are investigating mainly the diurnal waves around the liquid core resonance i.e. K1, y1 and j1. The scattering of the corrected amplitude factors for the waves O1 and K1 reaches 0.3%. and the tidal factors are determined with a precision slightly better than 0.1%. O1 is fitting perfectly to the DDW99 and MAT01 models but there is an offset of 0.1% for K1...

Intracomparison tests of the FG5 absolute gravity meters

Geophysical Research Letters, 1995

The FG5 series absohlte gravimeters have an estimated instrumental accuracy of 1-2 /zGal (1-2 X 10 -8 ms-2). A number of instrument comparisons were conducted with six FG5 instruments over a period of 9 months; the predecessor series JILA4 instrument was also used. The standard deviation of mean g values (averaged over 24-48 h), as observed by different instruments, is 1.8 /zGal. Observations taken within 48 h typically agree within 2 /zGal, and the maximum observed disagreement is 6 /zGal for two observations taken 37 days apart. Individual data sets of 100-250 measurements (16-42 min duration) show minimum standard deviations of 7-8/•Gal, placing an upper bound on instrument noise. The data are not corrected for local water-table effects or regional atmospheric loading, and thus the observed repeatability is probably an underestimate of the instrument accuracy. The observed instrument agreement is consistent with the instrumental accuracy estimate. Ignoring deficiencies in modeling environmental gravity signals, this accuracy level should allow resolution of vertical motions at the sub-centimeter level.

A new tidal analysis of superconducting gravity observations in Western and Central Europe

Contributions to Geophysics and Geodesy, 2014

Tidal analysis was applied on 1 hour gravity data acquired by a subnet of 12 superconducting gravimeters situated in western and central parts of Europe. Tidal parameters for O1 and M2 were adjusted. Most of the gravity time series were provided by the Global geodynamics project. Filter effects of the decimation process were carefully studied and taken into account where necessary. The ocean loading effect included in observed tidal parameters was removed using 8 ocean models with different spatial resolutions. Two different comparisons of corrected tidal parameters were performed. As a result, the accuracy of ocean loading correction and global calibration error were evaluated. For O1, amplitude factors are in good agreement with the hydrostatic/elastic model DDW/He while the non-hydrostatic/inelastic model DDW/NHi better describes the M2 amplitude factors. The analysis of residual vectors allowed assessing the efficiency of the used ocean loading models which is about 81 % (O1) and 97 % (M2).

New investigation of tidal gravity results from the GGP network

Some 20 series longer than one year are now available in the GGP data bank, including 3 dual sphere instruments. To eliminate the tidal loading effects we interpolated the contribution of the smaller oceanic waves from the 8 well determined ones i.. It was done for six different oceanic models: SCW80, CSR3.0, FES95.2, TPXO2, CSR4.0 and ORI96. In the diurnal band no model is decisively better than the others and a mean tidal loading vector is giving the most stable solution for the study of the liquid core resonance. In the semi-diurnal band however the SCW80 and TPXO2 models are not convenient. We are investigating mainly the diurnal waves around the liquid core resonance i.e. K 1 , y 1 and j 1 . The scattering of the corrected amplitude factors for the waves O 1 and K 1 reaches 0.3%. and the tidal factors are determined with a precision slightly better than 0.1%. O1 is fitting perfectly to the DDW99 and MAT01 models but there is an offset of 0.1% for K 1 . K1 exhibits a slight phase advance with respect to O1. From our data set we computed the FCN period and found values very close to the 429.5 days deduced from the VLBI observations.

Unified European Gravity Reference Network 2002 (UEGN02) — Status 2004

International Association of Geodesy Symposia, 2005

The last previous realisation of the Unified European Gravity Reference Network was completed in 1994 (UEGN94) covering 11 West European countries. Since that time significant work has been done e.g. in the frame of the UNIGRACE project that focused on the establishment of absolute gravity stations in Central and East European countries. The past European Subcommission of the IGGC recommended to continue with the unification of the gravity reference networks at the continental level. During the IAG2001 Scientific Assembly in Budapest, it was decided to establish a new continental gravimetric reference network. The total UEGN2002 network area covers 25 countries, the number of stations expected was around 1000, but has already exeeded this number. The data processing starts from raw absolute and relative gravity observations. The tidal corrections are computed by the ECGS in Luxembourg in a uniform manner and based on state-of-the-art models. At the current status (2004) the work is focused on 12 countries with 404 absolute and 33000 relative gravity observations at some 1500 stations. Main problems are data bugs. Hence, appropriate tools had to be developed. The paper presents details of the adjustment, earth tide corrections as well as analyses of the data and adjusted gravity values. After this partial UEGN02 adjustment, the remaining countries that also showed their interest to participate are again invited to submit data.

Investigation of temporal gravity variations in SG-records

Journal of Geodynamics, 2004

Not all of the gravity signals commonly observed in records of superconducting gravimeters have a physically significant meaning. Instrumental noise or signal components generated by specific processing techniques can have a strong effect. This paper analyzes selected case studies from gravity data acquired during the first period of GGP and tries to give an answer to the question of which signals are significant. It is focused on small amplitude variations in the time domain, which are compared with those of conventional spring-type gravimeters. Coherent atmospheric gravity signals due to atmospheric processes associated with strong vertical convection could be observed by different sensors. The second part deals with temporal variations of main tidal parameters, which can be seen clearly in the tidal analysis results from SG data. Synthetic case studies show that they are not strongly influenced by the analysis procedure and therefore mainly reflect ocean loading processes.

The capacity of the Scintrex Autograv CG-3M no. 4492 gravimeter for “absolute-scale” surveys

In autumn 2001, the Institut für Erdmessung (IfE), received the new Scintrex Autograv CG-3M spring gravimeter no. 4492 (SC-4492). Compared with LaCoste-Romberg gravimeters, the Scintrex instrument offers the advantage of a more simple calibration function. Due to the straightforward sensor design (no micrometer screws and gearboxes, no astatisation), a modelling of periodical calibration terms and of a higher order polynomial calibration function is not required. The costly calibration measurements can be reduced to some relative observations between two reliable absolute gravity stations. No complex calibration system should be necessary. A reliable evaluation of the SC-4492 was done with respect to the stability of the calibration, measurement accuracy and precision, drift behaviour, and gravity range dependency of the calibration factor. Measurements were performed on the Hannover vertical calibration line (200 µm/s 2 (20 mGal) range, 20-storied building, 10 µm/s 2 interval), on the Cuxhaven-Harz north-south line (3000 µm/s 2 range, 34 stations, 90 µm/s 2 interval) of the gravimeter calibration system Hannover, and in the absolute gravimetry net of the Fennoscandian land uplift area (6200 µm/s 2 range). A total gravity range of almost 0.015 m/s 2 has been covered. In addition, the gravimeter was employed in the determination of vertical gravity gradients and centring to safety points for absolute gravimetry, and the observation of the horizontal gradient field at the IfE absolute gravity reference station in Clausthal. This paper presents the results as obtained in regional, local and microgravimetric surveys. The achieved accuracies are in the order of ±10 to 100 nm/s 2 (±1 to 10 µGal). No instability of the calibration and no gravity range dependency could be proven within the order of 1•10-4. Overall, the gravity meter SC-4492 meets fully the expectations of IfE.

Long-term monitoring by absolute gravimetry: Tides to postglacial rebound

Journal of Geodynamics, 2006

Absolute gravity measurements over nearly a decade at 10 field sites in North America have provided insights on a range of phenomena from tides to postglacial rebound. In this overview we demonstrate the potential of long-term, terrestrial gravity measurements at selected sites to assist in the interpretation of temporal variations in the global gravity field from the Gravity Recovery and Climate Experiment (GRACE) mission. Absolute gravity measurements can be used to calibrate annual soil moisture effects observed by GRACE and to complement GRACE results over periods longer than the expected mission lifetime. Although the preferred terrestrial gravity monitoring system is a global network of superconducting gravimeters (Global Geodynamics Project) in combination with regular absolute gravity measurements, major advances can be made using absolute gravimetry alone. Operating the FG5 gravimeter in continuous mode at a near-shore site shows that special attention must be paid to possible sea-level related biases on gravity values as a result of near-shore hydrological effects. Where soil becomes saturated annually, simple soil moisture models can be used to remove the annual soil moisture effect from gravity, or to invert gravity observations for the soil moisture effect on space missions. Our measurements show that the phenomenon of "episodic tremor and slip" (ETS) in the Cascadia Subduction Zone is accompanied by gravity change that is most likely caused by mass redistribution and not height change. An inter-annual variation of unknown origin with a "period" of around 7 years is present at 10 North American field sites, as well as in data from Gravity Observatory, Boulder, Colorado. Correcting the long-term gravity trends for the inter-annual variation brings the trends into line with GPS vertical rates from four, co-located, continuous, GPS sites in the mid-continent and allows conclusions to be drawn on the thickness of the Laurentide ice-sheet. Crown

Tidal models in a new era of satellite gravimetry

2003

... Geod. 76, 307-316. Schrama, EJO: 1996, 'Gravity research missions reviewed in light of the indirect ocean tide potential', in RH Rapp, AA Cazenave and RS Nerem (eds.), Global Gravity Field and Its Temporal Variations, Springer, Berlin, pp. 131-140. Shum, С. К. ...