Hans-Georg Scherneck

Continuous GPS measurements of postglacial adjustment in Fennoscandia 1.Geodetic results

[1] Data collected under the auspices of the BIFROST GPS project yield a geographically dense suite of estimates of present-day, three-dimensional (3-D) crustal deformation rates in Fennoscandia [Johansson et al., 2002]. A preliminary forward analysis of these estimates [Milne et al., 2001] has indicated that models of ongoing glacial isostatic adjustment (GIA) in response to the final deglaciation event of the current ice age are able to provide an excellent fit to the observed 3-D velocity field. In this study we revisit our previous GIA analysis by considering a more extensive suite of forward calculations and by performing the first formal joint inversion of the BIFROST rate estimates. To establish insight into the physics of the GIA response in the region, we begin by decomposing a forward prediction into the three contributions associated with the ice, ocean, and rotational forcings. From this analysis we demonstrate that recent advances in postglacial sea level theory, in particular the inclusion of rotational effects and improvements in the treatment of the ocean load in the vicinity of an evolving continental margin, involve peak signals that are larger than the observational uncertainties in the BIFROST network. The forward analysis is completed by presenting predictions for a pair of Fennoscandian ice histories and an extensive suite of viscoelastic Earth models. The former indicates that the BIFROST data set provides a powerful discriminant of such histories. The latter yields bounds on the ( assumed constant) upper and lower mantle viscosity (nu(UM), nu(LM)); specifically, we derive a 95% confidence interval of 5 x 10(20) less than or equal to nu(UM) less than or equal to 10(21) Pa s and 5 x 10(21) less than or equal to nu(LM) less than or equal to 5 x 10(22) Pa s, with some preference for (elastic) lithospheric thickness in excess of 100 km. The main goal of the ( Bayesian) inverse analysis is to estimate the radial resolving power of the BIFROST GPS data as a function of depth in the mantle. Assuming a reasonably accurate ice history, we demonstrate that this resolving power varies from similar to 200 km near the base of the upper mantle to similar to 700 km in the top portion of the lower mantle. We conclude that the BIFROST data are able to resolve structure on radial length scale significantly smaller than a single upper mantle layer. However, these data provide little constraint on viscosity in the bottom half of the mantle. Finally, elements of both the forward and inverse analyses indicate that radial and horizontal velocity estimates provide distinct constraints on mantle viscosity.

Climate monitoring using GPS

Abstract We present results on long-term trends of integrated precipitable water vapor (IPWV) over the Scandinavian region based on data from the Swedish permanent Global Positioning System (GPS) network, obtained during the period August 1993 to the end of 2000. We assess the magnitude of the effects on the estimated IPWV caused by antenna radome changes by comparisons with other independent techniques, such as microwave radiometry and radiosondes. The agreement between the techniques is at 1 mm level for the IPWV content and at 0.1 mm/yr for the estimated linear trend. Using the IPWV differences between the techniques, we assess the effects of radome changes to be in the interval 0–1.8 mm depending on the type of radome used. The estimated trends of IPWV over Scandinavia show a general increase of 0.1–0.2 mm/yr, and are more pronounced in the south–west region. We also estimate trends based on summer and winter periods. We find them to be larger for the winter periods compared to the summer in the southern parts and the opposite in the northern regions of Scandinavia.

Vertical crustal motion observed in the BIFROST project

Abstract This paper reports from investigations on the robustness of estimated rates of intraplate motion from the continuous GPS project BIFROST (Baseline Inferences from Fennoscandian Rebound Observations, Sealevel and Tectonics). We study loading effects due to ocean, atmosphere and hydrology and their impact on estimated rate parameters. We regularly find the admittance of a modelled perturbation at less than fifty percent of the full effect. We think that the finding relates to a difficult noise situation at all periods, and that a satisfying model for the dominating noise source has not been found yet. An additional reason for low admittance is found in the mapping process of the no-fiducial network solution into a conventional reference frame.

Three months of local sea level derived from reflected GNSS signals

By receiving Global Navigation Satellite System (GNSS) signals that are reflected off the sea surface, together with directly received GNSS signals (using standard geodetic‐type receivers), it is possible to monitor the sea level using regular single difference geodetic processing. We show results from our analysis of three months of data from the GNSS‐based tide gauge at the Onsala Space Observatory (OSO) on the west coast of Sweden. The GNSS-derived time series of local sea level is compared with independent data from two stilling well gauges at Ringhals and Gothenburg about 18 km south and 33 km north of OSO, respectively. A high degree of agreement is found in the time domain, with correlation coefficients of up to 0.96. The root‐mean‐square differences between the GNSS‐derived sea level and the stilling well gauge observations are 5.9 cm and 5.5 cm, which is lower than for the stilling well gauges together (6.1 cm). A frequency domain comparison reveals high coherence of the data sets up to 6 cycles per day, which corresponds well to the propagation of gravity waves in the shallow waters at the Kattegat coast. Amplitudes and phases of some major tides were determined by a tidal harmonic analysis and compared to model predictions. From the GNSS‐based tide gauge results we find significant ocean tidal signals at fortnightly, diurnal, semi‐diurnal, and quarter‐diurnal periods. As an example, the amplitudes of the semi‐diurnal M2 and the diurnal O1 tide are determined with 1σ uncertainties of 11 mm and 12 mm, respectively. The comparison to model calculations shows that global ocean tide models have limited accuracy in the Kattegat area.

Investigations of Fennoscandian glacial isostatic adjustment using modern sea level records

We present a new method for determining the rates from a network of regional tide gauge data. The method involves using a model for the sea level observations (annual averages of relative sea level) that explicitly includes a term for the interannual sealevel variations, which are assumed to be constant for all sites in the network. The resulting simultaneous analysis of all the sea level observations yields sea level rates which are biased by the unknown temporal slope of the interannual variations. By differencing sea level rate estimates with respect to one site, “site-referenced” sea level rates are calculated which are free from this bias. The differencing procedure also removes any signal due to a common sea level rate, such as might be associated with eustatic sea level rise. The resulting site-referenced sea level rates therefore in principle reflect only the relative vertical crustal motions of the tide gauge sites with respect to the geoid. These site-referenced sea level rates represent a potentially more accurate data set than have previously been used for sea level studies. We have used these rates to investigate glacial isostatic adjustment (GIA) in Fennoscandia. Specifically, we have used the site-referenced sea level rates within the Baltic Sea to estimate adjustments to the ice history and Earth model which are used to calculate vertical rates of adjustment due to GIA, a method never before used for Fennoscandia. The parameters selected were lithospheric thickness, the viscosities of (assumed isoviscous) upper and lower mantles, and a Fennoscandian ice thickness scaling parameter. We present the results of this estimation procedure for a specific test ice model. We also find evidence that the sea level record for the site nearest the center of uplift, Furuogrund, may be contaminated by systematic errors of unknown origin. Data from this site have previously been used to determine a rate for the maximum uplift that we find is perhaps 1 mm yr−1, or ∼10%, too large.

The BIFROST project: GPS determined 3-D displacement rates in Fennoscandia from 800 days of continuous observations in the SWEPOS network

Abstract This presentation summarizes results obtained from 800 days of operation of SWEPOS, the Swedish permanent GPS array. Located in the area of the Fennoscandian land uplift, SWEPOS is the main measuring device for the BIFROST project, aiming primarily at determination of crustal motions and changes in global sea level in conjunction with postglacial rebound. As a future prospect the project will contribute to determine present-day changes in the oceanic water budget as well as strive for a more complete understanding of the viscoelastic structure of crust and mantle. We show solutions of site positions obtained from the daily processing of the GPS observations. We determine their variations in time, discerning them from plate or frame orientation, and discuss a number of perturbation effects. Among these the possibility of random walk monument motion is considered. The feasibility of single-site solutions rather than baseline pairs is demonstrated. On the basis of 800 days of operation of the permanent network, data analysis is able to confirm the general rebound pattern. Comparison with both modelling results and evidence from mareograph and precise levelling shows high rates of vertical surface displacement from GPS measurements in the centre of the Pleistocene glaciation region. We discuss the possibility of rate biases due to problems with the terrestrial reference frames.

Mobile quantum gravity sensor with unprecedented stability

Changes of surface gravity on Earth are of great interest in geodesy, earth sciences and natural resource exploration. They are indicative of Earth systems mass redistributions and vertical surface motion, and are usually measured with falling corner-cube- and superconducting gravimeters (FCCG and SCG). Here we report on absolute gravity measurements with a mobile quantum gravimeter based on atom interferometry. The measurements were conducted in Germany and Sweden over periods of several days with simultaneous SCG and FCCG comparisons. They show the best-reported performance of mobile atomic gravimeters to date with an accuracy of

Repeated absolute gravity measurements for monitoring slow intraplate vertical deformation in western Europe

\mathrm{39\,nm/s^2}

BIFROST project : 3-D crustal deformation rates derived from GPS confirm postglacial rebound in Fennoscandia

and long-term stability of

Observing Fennoscandian Gravity Change by Absolute Gravimetry

\mathrm{0.5\,nm/s^2}