Modeling of Atmospheric Gravity Effects for High-Precision Observations

Temporal variations of atmospheric density distribution induce changes in the gravitational air mass attraction at a specific observation site. Additionally, the load of the atmospheric masses deforms the Earth’s crust and the sea surface. Variations in the local gravity acceleration and atmospheric pressure are known to be corrected with an admittance of about 3 nm/s2 per hPa as a standard factor, which is in accordance with the IAG Resolution No. 9, 1983. A more accurate admittance factor for a gravity station is varying with time and depends on the total global mass distribution within the atmosphere. The Institut für Erdmessung (IfE) performed absolute gravity observations in the Fennoscandian land uplift area nearly every year from 2003 to 2008. The objective is to ensure a reduction with 3 nm/s2 accuracy. Therefore, atmospheric gravity changes are modeled using globally distributed ECMWF data. The attraction effect from the local zone around the gravity station is calculated with ECMWF 3D weather data describing different pressure levels up to a height of 50 km. To model the regional and global attraction, and all deformation components the Green’s functions method and surface ECMWF 2D weather data are used. For the annually performed absolute gravimetry determinations, this approach improved the reductions by 8 nm/s2 (-19 nm/s2 to +4 nm/s2). The gravity modeling was verified using superconducting gravimeter data at station Membach inBelgiumimproving the residuals by about 15%

Schwereregistrierungen mit dem Micro-g LaCoste gPhone 98 und dem ZLS Burris Gravity Meter B-64

Für Aufzeichnungen über längere Zeiträume sind Federgravimeter − z.B. im Vergleich zu Supraleitgravimetern − die flexibelste, technisch einfachere und vergleichsweise wirtschaftlichste Lösung, unterliegen aber dem Nachteil der Drift. Wir untersuchen die Genauigkeiten des Micro-g LaCoste gPhone-98 (g-98) und ZLS Burris Gravity Meter B-64 (B-64) des Instituts für Erdmessung (IfE). Mit beiden Instrumenten wurden Registrierungen über mehrere Monate auf verschiedene Stationen mit hohen und niedrigen seismischen Störungen durchgeführt. Das g-98 und B-64 haben dabei zeitgleich an einer Station aufgezeichnet. Untersucht wird zudem eine parallele Registrierung des B-64 mit dem GWR Instruments OSG-054 Supraleitgravimeter in Onsala (Schweden). Die Gezeitenanalyse der Datenreihen dieser beiden Instrumente zeigt, dass die Gezeitenparameter der täglichen und halbtäglichen Gezeiten mit 0.3‰ (O1, K1, M2) bis 1.7‰ (S2) übereinstimmen. Sowohl für das g-98 als auch das B-64 wurde eine Stabilität des linearen Eichfaktors von 3×10e-4 ermittelt. Im Allgemeinen verhält sich die Drift eines gPhones nach einer gewissen Einlaufphase linear. Der Betrag und die Variation der Drift des g-98 nahmen im Nutzungszeitraum nach jedem Standortwechsel ab und liegt derzeit bei etwa 90 nm/s² pro Tag. Im Gegensatz dazu zeigt das B-64 derzeit ein nicht-lineares Verhalten.Modern spring gravimeters offer – e.g., compared to superconducting gravimeters – a flexible, simpler and economical solution for the long term acquisition of gravity data, but are limited by instrumental drift. We investigate the accuracy of the Micro-g LaCoste gPhone-98 (g-98) and the ZLS Burris Gravity Meter B-64 (B-64) of the Institut für Erdmessung (IfE). Both instrument conducted recordings for several months on stations with low to high seismic noise. At one station g-98 and B-64 recorded in parallel. Additionally the B-64 recorded simultaneously with the GWR Instruments OSG-054 superconducting gravimeter in Onsala (Sweden). The tidal analysis of timeseries from these two instruments show an agreement of 0.3‰ (O1, K1, M2) to 1.7‰ (S2) for diurnal- and semidiurnal amplitude factors. The linear calibration factor of g-98 and B-64 was found to be stable at the level of 3×10e-4. The instrumental drift of a gPhone is generally linear after an initial run-in phase. The magnitude as well as the variation of g-98’s drift has decreased over time with each new setup of the instrument at a new location. Currently the drift is approximately 90 nm/s² per day. The drift of B-64, on the other hand, does currently not show a linear behaviour

An Accuracy Assessment of Absolute Gravimetric Observations in Fennoscandia

We compare a suite of absolute gravimeters used to monitor the temporal changes of gravity at a number of sites in Fennoscandia. Direct comparisons are made from simultaneous observations at selected sites within and outside of the postglacial uplift region. We also compare results at sites visited by two instruments with some separation in time. We conclude from four years of data that gravity differences are obtained within an rms error of ± 3 Gal. The data reveal no systematic biases between the instruments, but occasional shifts from one year to another are noted. We consider that annual instrument comparisons are required to ensure data integrity in a regional observing program that extends over more than a decade

Genauigkeitsuntersuchungen moderner Federgravimeter für Monitoringaufgaben in der Geophysik

Für Aufzeichnungen über längere Zeiträume sind Federgravimeter - z.B. im Vergleich zu Supraleitgravimetern - die fexibelste, technisch einfachste und vergleichsweise wirtschaftlichste Lösung, unterliegen aber dem Nachteil der Drift. Wir untersuchen die Genauigkeiten des Micro-g LaCoste gPhone-98 und ZLS Burris Gravity Meter B-64 des Instituts für Erdmessung. Mit beiden Instrumenten wurden Registrierungen über mehrere Monate auf fünf Stationen mit hohen und niedrigen seismischen Störungen durchgeführt. Untersucht wird zudem eine parallele Registrierung des ZLS mit dem Supraleitgravimeter in Onsala (Schweden). Die Gezeitenanalyse gleichzeitiger Datenreihen beider Instrumente zeigt, dass die Gezeitenparameter der täglichen und halbtäglichen Gezeiten mit 0.04% (O1,K1,M2) bis 0.16% (S2) übereinstimmen. Sowohl für das g-98 als auch das B-64 wurde eine Stabilität des linearen Eichfaktors von 3x10e-4 ermittelt. Im Allgemeinen verhält sich die Drift eines gPhones nach einer gewissen Einlaufphase linear. Der Betrag und die Variation der Drift des g-98 nahm im Nutzungszeitraum nach jedem Standortwechsel ab und liegt derzeit bei etwa 90 nm/s^2 pro Tag. Im Gegensatz dazu zeigt das B-64 deutlich ein nichtlineares Verhalten. Das Leibniz-Institut für Angewandte Geophysik besitzt das gPhoneX-129 seit Ende 2012. Anhand einer acht Monate langen parallelen Registrierung mit dem g-98 werden diese Geräte unterschiedlicher Generationen verglichen. Diese erste Langzeitregistrierung des gX-129 zeigt die Entwicklung der Gerätedrift eines fabrikneuen Gerätes im Vergleich zu einem bereits seit 3 Jahren im Gebrauch befi ndlichen Gerätes. Insbesondere die Abweichungen von einem linearen Driftverhalten erschweren die Unterscheidung zwischen Signal und Drift. Zusätzlich wird der Einfluss unterschiedlicher Datenerfassungsraten (1 und 7 Hz) untersucht. Die Qualität der Zeitreihen der drei Gravimeter wird durch eine Gezeitenanalyse bewertet sowie die Langzeitstabilität der Instrumente getestet

Observing Fennoscandian Gravity Change by Absolute Gravimetry

The Nordic countries Norway, Sweden, Denmark and Finland are a key study region for the research of glacial isostasy, and, in addition, it offers a unique opportunity for validating and testing the results of the GRACE experiment. Over a period of five years, the expected life time of GRACE, a temporal geoid variation of 3.0 mm is expected in the centre of the Fennoscandian land uplift area, corresponding to a gravity change of about 100 nm/s2. This is expected to be within the detection capabilities of GRACE. With terrestrial absolute gravimetry, the gravity change due to the land uplift can be observed with an accuracy of \ub110 to 20 nm/s2 for a 5-year period. Thus, the terrestrial insitu observations (ground-truth) may be used to validate and test the GRACE results.Since 2003, absolute gravity measurements have been performed in Fennoscandia at about 30 stations covering Norway, Sweden, Finland and Denmark. Four groups with FG5 absolute gravimeters (BKG, FGI, IfE, UMB) are engaged to survey the uplift network annually by a mutually controlled procedure. Nearly all absolute stations are colocated with permanent GPS stations. From the 2003 and 2004 comparisons between the instruments, an overall accuracy of \ub130 nm/s2 is indicated for a single absolute gravimeter and a single station determination. This is in full agreement with the project goal

Observing Fennoscandian Gravity Change by Absolute Gravimetry

The Nordic countries Norway, Sweden, Denmark and Finland are a key study region for the research of glacial isostasy, and, in addition, it offers a unique opportunity for validating and testing the results of the GRACE experiment. Over a period of five years, the expected life time of GRACE, a temporal geoid variation of 3.0 mm is expected in the centre of the Fennoscandian land uplift area, corresponding to a gravity change of about 100 nm/s2. This is expected to be within the detection capabilities of GRACE. With terrestrial absolute gravimetry, the gravity change due to the land uplift can be observed with an accuracy of \ub110 to 20 nm/s2 for a 5-year period. Thus, the terrestrial insitu observations (ground-truth) may be used to validate and test the GRACE results.Since 2003, absolute gravity measurements have been performed in Fennoscandia at about 30 stations covering Norway, Sweden, Finland and Denmark. Four groups with FG5 absolute gravimeters (BKG, FGI, IfE, UMB) are engaged to survey the uplift network annually by a mutually controlled procedure. Nearly all absolute stations are colocated with permanent GPS stations. From the 2003 and 2004 comparisons between the instruments, an overall accuracy of \ub130 nm/s2 is indicated for a single absolute gravimeter and a single station determination. This is in full agreement with the project goal

CCM.G-K2 key comparison

International audienceIn November 2013 an International Key Comparison, CCM.G-K2, was organized in the Underground Laboratory for Geodynamics in Walferdange. The comparison has assembled 25 participants coming from 19 countries and four different continents. The comparison was divided into two parts: the key comparison that included 10 NMIs or DIs, and the pilot study including all participants. The global result given by the pilot study confirms that all instruments are absolutely coherent to each other. The results obtained for the key comparison confirm a good agreement between the NMI instruments

CCM.G-K2 key comparison

In November 2013 an International Key Comparison, CCM.G-K2, was organized in the Underground Laboratory for Geodynamics in Walferdange. The comparison has assembled 25 participants coming from 19 countries and four different continents. The comparison was divided into two parts: the key comparison that included 10 NMIs or DIs, and the pilot study including all participants. The global result given by the pilot study confirms that all instruments are absolutely coherent to each other. The results obtained for the key comparison confirm a good agreement between the NMI instruments. Main text. To reach the main text of this paper, click on Final Report [http://www.bipm.org/utils/common/pdf/final_reports/M/G-K2/CCM.G-K2.pdf] . Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/ [http://kcdb.bipm.org/] . The final report has been peer-reviewed and approved for publication by CCM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA)