Hannu Koivula

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.

Widespread low rates of Antarctic glacial isostatic adjustment revealed by GPS observations

Bedrock uplift in Antarctica is dominated by a combination of glacial isostatic adjustment (GIA) and elastic response to contemporary mass change. Here, we present spatially extensive GPS observations of Antarctic bedrock uplift, using 52% more stations than previous studies, giving enhanced coverage, and with improved precision. We observe rapid elastic uplift in the northern Antarctic Peninsula. After considering elastic rebound, the GPS data suggests that modeled or empirical GIA uplift signals are often over?estimated, par t icularly the magnitudes of the signal maxima. Our observation that GIA uplift is misrepresented by modeling (weighted root?meansquares of observation?model differences: 4.9–5.0 mm/yr) suggests that, apart from a few regions where large ice mass loss is occurring, the spatial pattern of secular ice mass change derived from Gravity Recovery and Climate Experiment (GRACE) data and GIA models may be unreliable, and that several recent secular Antarctic ice mass loss estimates are systematically biased, mainly too high.

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.

Vertical velocities in Finland from permanent GPS networks and from repeated precise levelling

Abstract Postglacial rebound is a long-studied phenomenon in Fennoscandia, and the general features of contemporary vertical motion (0–8 mm/year relative to mean sea level) are well known from tide gauges and repeated precise levelling. GPS on permanent stations has proved to be a powerful tool in studies of crustal motion, capable of detecting small trends in a fraction of the time required by the classical methods. We determine vertical velocities from 5 years of data in the permanent Finnish GPS network FinnRef®. We compare them with velocities derived from three precise levellings spanning nearly hundred years, and from tide gauge records. From the comparison, both FinnRef velocities and levelled velocities appear to be accurate to 0.4 mm/year (one-sigma). The isobases (lines of equal velocities) are less elongated towards northeast than in geophysical models of the rebound. However, the processing of nearly the same GPS data in BIFROST using different methods produces velocities that disagree with FinnRef more than levelling does. The levelled velocities are between the two GPS results and do not resolve the conflict.

On the periodicity of GPS time series

We have computed GPS time series of different resolutions in the Finnish permanent GPS network FinnRef. In the Lomb periodigrams we can distinguish an annual period (Makinen et al. 2000) but also a diurnal period. The annual period is visible in all components, including the computed baseline length between two stations. The amplitude of a period is a function of the baseline length, thus behaving like a scale error.

Scale variation of GPS time series

We give an overview of time series analyses of permanent GPS stations using solutions of the IGS and FinnRef® networks. Lomb periodograms show in most cases a statistically significant annual period both in station coordinates and inter-station distances. In regional networks the scale of the whole network changes periodically, and in some cases there is also a secular trend. There are several possible causes of scale variations, which may not be separable in the data. These include computational artefacts, periodic systematic errors in satellite orbits, signal path delay variations, and geophysical causes like loading and postglacial rebound. We discuss possible reasons, their significance, and their consequences on high-precision GPS observations. Additional constraints, e.g. time series from the superconducting gravimeter, are also discussed.

Tide gauge monitoring using GPS

In this work we have studied accuracy of GPS for tide gauge stability monitoring as well as possibilities for observing the absolute sea level rise of the Baltic Sea with GPS and tide gauge time series. Our determination give the average sea level rise for the long, up to 120 years, time series 1.9 + 1.0 mm/year, and when corrected for the geoid rise, 1.6 + 1.0 mm/ year. Rates of recent years are even eight times higher. The possibility to detect minor changes in tide gauge benchmark height with GPS may be limited by GPS-related errors, which can be up to several mm even in a 10 km baseline.

USE OF THE FINNISH PERMANENT GPS NETWORK (FINNNET) IN REGIONAL GPS CAMPAIGNS

The Finnish Geodetic Institute has established a network of 12 permanent GPS-stations in Finland (FinnNet). The planning of the network started during winter 92/93. The first stations were installed in 1994 and the last one was built in the autumn of 1996. The stations are equipped with geodetic Ashtech Z-12 receivers and Dorne Margolin type antennas. All the stations are working fully automatically. The network is used as a 1st order reference frame in Finland for GPS users, for crustal dynamics studies and for providing RTCM corrections in real time applications.

Finnish permanent GNSS network: From dual-frequency GPS to multi-satellite GNSS

The Finnish Geodetic Institute (FGI) is building a new multi-satellite GNSS network in Finland. The renewal of the old GPS-only network FinnRef is funded by the Ministry of Agriculture and Forestry. The new network will consist of 19 stations collecting data from GPS, Glonass, Galileo and later also from Compass satellites. The network stations will be build on stable bedrock sites, equipped with Javad Delta-G3T receivers and individually calibrated Javad Dorne Margolin choke ring antennas covered with SCIGN radomes. Data will be transferred both as hourly files and real-time streams to the FGI and distributed to the users, co-operation partners and research institutions.

P3: A Public Precise Positioning Service based on a national GNSS network

In this article we present the P3 (Public Precise Positioning) Service project where network GNSS data are combined with a consumer-grade receiver. P3 utilizes the Finnish national FinnRef GNSS network which is operated by the Finnish government and available free of charge to the general public. The project investigates the use of network real-time kinematics (RTK) and real-time precise point positioning (PPP) techniques in order to achieve a horizontal positioning accuracy better than 0.5 meters using a low-cost GNSS receiver. An online survey was conducted to identify potential users and applications of the service, and initial positioning results presented in the paper support the feasibility of the goal.