Modeling gravimetric signatures of third-degree ocean tides and their detection in superconducting gravimeter records

We employ the barotropic, data-unconstrained ocean tide model TiME to derive an atlas for degree-3 tidal constituentsincluding monthly to terdiurnal tidal species. The model is optimized with respect to the tide gauge data set TICON-td that isextended to include the respective tidal constituents of diurnal and higher frequencies. The tide gauge validation shows a rootmean-square (RMS) deviation of 0.9–1.3mm for the individual species. We further model the load tide-induced gravimetric signals by two means (1) a global load Love number approach and (2) evaluating Greens-integrals at 16 selected locations of superconducting gravimeters. The RMS deviation between the amplitudes derived using both methods is below 0.5 nGal (1 nGal = 0.01 nm/s2 ) when excluding near-coastal gravimeters. Utilizing ETERNA-x, a recently upgraded and reworked tidal analysis software, we additionally derive degree-3 gravimetric tidal constituents for these stations, based on a hypothesis-free wave grouping approach.We demonstrate that this analysis is feasible, yielding amplitude predictions of only a few 10 nGal, and that it agrees with the modeled constituents on a level of 63–80% of the mean signal amplitude. Larger deviations are only found for lowest amplitude signals, near-coastal stations, or shorter and noisier data sets

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

Updated OCTAS geoid in the northern North Atlantic - OCTAS07

A new gravimetric geoid (OCTAS07v2) is generated using Stokes’ formula with gravity data as input. As local gravity data, a combination of land gravity data, new and old airborne gravity data, and adjusted marine gravity data has been used. All marine gravity data has been error screened and quality assured by removing dubious data and adjusting the data when necessary. Voids in the gravity data distribution were patched with gravity data from satellite altimetry. The OCTAS07v2 geoid was estimated using the remove-compute-restore technique. The long-wavelength signal of the local gravity data was reduced using a Wong-Gore modified Stokes’ function. The long-wavelength part was represented by a global gravity field model based on GRACE data. The OCTAS07v2 geoid model was combined with the OCTAS07 MSS model to create a synthetic Mean Dynamic Topography (MDT) model. In comparison with the OCCAM MDT, our new synthetic MDT model gave a std. dev. of the residuals of 11 cm. A comparison to the main northern North Atlantic currents show many similar features

The quality role of height system and geoid model in the realization of GNSS heighting

V okviru projekta Vzpostavljanje evropskega prostorskega referenčnega sistema v Sloveniji je bila izračunana tudi ploskev testnega geoida. V prispevku bo predstavljena primerjava določitve geoidnih višin iz geoida 2000 in testnega geoida iz leta 2010. Analizo natančnosti ploskev geoida Slovenije smo opravili s primerjavo geoidnih višin, ki jih dobimo kot razlike merjenih eliposoidnih in nadmorskih višin (»merjene geoidne višine«) in tistih, ki jih interpoliramo iz modelov. Primerjali smo »merjene« in interpolirane geoidne višine na 352 GNSS/nivelman kontrolnih točkah. Z izračunom ploskve testnega geoida smo pridobili možnost predhodne analize kakovosti prihodnjega geoida, ki je zelo pomembna za izvajanje GNSS-višinomerstva v geodetski praksi.This paper presents a quality analysis and comparison of two height reference surfaces. The first is the actual geoid model from the year 2000, and the second is the test geoid model determined in the frame of the project Establishment of the European Reference System in Slovenia. Quality analysis is based on the comparison of geoid heights determined from measured ellipsoidal and mean-sea-level heights and geoid heights interpolated from the model. A comparison was made on 352 GNSS/levelling points

Absolute gravity observations in Norway (1993–2014) for glacial isostatic adjustment studies: The influence of gravitational loading effects on secular gravity trends

AbstractWe have compiled and analyzed FG5 absolute gravity observations between 1993 and 2014 at 21 gravity sites in Norway, and explore to what extent these observations are applicable for glacial isostatic adjustment (GIA) studies. Where available, raw gravity observations are consistently reprocessed. Furthermore, refined gravitational corrections due to ocean tide loading and non-tidal ocean loading, as well as atmospheric and global hydrological mass variations are computed. Secular gravity trends are computed using both standard and refined corrections and subsequently compared with modeled gravity rates based on a GIA model. We find that the refined gravitational corrections mainly improve rates where GIA, according to model results, is not the dominating signal. Consequently, these rates may still be considered unreliable for constraining GIA models, which we trace to continued lack of a correction for the effect of local hydrology, shortcomings in our refined modeling of gravitational effects, and scarcity of observations. Finally, a subset of standard and refined gravity rates mainly reflecting GIA is used to estimate ratios between gravity and height rates of change by ordinary and weighted linear regression. Relations based on both standard and refined gravity rates are within the uncertainty of a recent modeled result

Modeling gravimetric signatures of third-degree ocean tides and their detection in superconducting gravimeter records

We employ the barotropic, data-unconstrained ocean tide model TiME to derive an atlas for degree-3 tidal constituents including monthly to terdiurnal tidal species. The model is optimized with respect to the tide gauge data set TICON-td that is extended to include the respective tidal constituents of diurnal and higher frequencies. The tide gauge validation shows a root-mean-square (RMS) deviation of 0.9–1.3 mm for the individual species. We further model the load tide-induced gravimetric signals by two means (1) a global load Love number approach and (2) evaluating Greens-integrals at 16 selected locations of superconducting gravimeters. The RMS deviation between the amplitudes derived using both methods is below 0.5 nGal (1 nGal =0.01nms2) when excluding near-coastal gravimeters. Utilizing ETERNA-x, a recently upgraded and reworked tidal analysis software, we additionally derive degree-3 gravimetric tidal constituents for these stations, based on a hypothesis-free wave grouping approach. We demonstrate that this analysis is feasible, yielding amplitude predictions of only a few 10 nGal, and that it agrees with the modeled constituents on a level of 63–80% of the mean signal amplitude. Larger deviations are only found for lowest amplitude signals, near-coastal stations, or shorter and noisier data sets

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