Sverrir Gudmundsson

Three‐dimensional surface motion maps estimated from combined interferometric synthetic aperture radar and GPS data

[1] We provide a technique to efficiently produce high-resolution three-dimensional surface motion maps by combining information about the motion of the Earth’s surface from interferometric observations of synthetic aperture radar images and repeated Global Positioning System (GPS) geodetic measurements. Unwrapped interferograms, showing pixel-wise change in range from ground to satellite, and sparse values of threedimensional movements are required as input. The problem of finding the full threedimensional motion field is separated into two two-dimensional problems. Initially, the vertical component of the deformation field and its horizontal component in the look direction of the satellite are found. Later, the look direction component is resolved into north and east components. Initial values for the motion fields are assigned to each pixel of interferograms from interpolation of available GPS observations. These values are then updated and optimized by comparison with the interferograms and the GPS observations. An additional constraint is an assumption of a smoothly varying motion field. Markov random field-based regularization and simulated annealing algorithm are used for the optimization. The technique is applied to create surface motion maps for the Reykjanes Peninsula, SW Iceland. INDEX TERMS: 0933 Exploration Geophysics: Remote sensing; 0910 Exploration Geophysics: Data processing; 1206 Geodesy and Gravity: Crustal movements—interplate (8155); 8150 Tectonophysics: Evolution of the Earth: Plate boundary—general (3040); 8155 Tectonophysics: Evolution of the Earth: Plate motions—general; KEYWORDS: remote sensing, motion maps, deformation, data fusion, interferometry, GPS

Ice-volume changes, bias-estimation of mass-balance measurements and changes in subglacial lakes derived by LiDAR-mapping of the surface of Icelandic glaciers

Abstract Icelandic glaciers cover ∼11 000 km2 in area and store ∼3600 km3 of ice. Starting in 2008 during the International Polar Year, accurate digital elevation models (DEMs) of the glaciers are being produced with airborne lidar. More than 90% of the glaciers have been surveyed in this effort, including Vatnajökull, Hofsjökull, Myrdalsjökull, Drangajökull, Eyjafjallajökull and several smaller glaciers. The publicly available DEMs are useful for glaciological and geological research, including studies of ice-volume changes, estimation of bias in mass-balance measurements, studies of jökulhlaups and subglacial lakes formed by subglacial geothermal areas, and for mapping of crevasses. The lidar mapping includes a 500-1000 m wide ice-free buffer zone around the ice margins which contains many glacio-geomorphological features, and therefore the new DEMs have proved useful in geological investigations of proglacial areas. Comparison of the lidar DEMs with older maps confirms the rapid ongoing volume changes of the Icelandic ice caps which have been shown by mass-balance measurements since 1995/96. In some cases, ice-volume changes derived by comparing the lidar measurements with older DEMs are in good agreement with accumulated ice-volume changes derived from traditional mass-balance measurements, but in other cases such a comparison indicates substantial biases in the traditional mass-balance records.

Glacier-volcano interactions deduced by SAR interferometry

Glacier-surface displacements produced by geothermal and volcanic activity beneath Vatnajokull ice cap in Iceland are described by field surveys of the surface topography combined with interferograms acquired from repeat-pass synthetic aperture radar images. A simple ice-flow model serves well to confirm the basic interpretation of the observations. The observations cover the period October 1996-January 1999 and comprise: (a) the ice-flow field during the infilling of the depressions created by the subglacial Gjalp eruption of October 1996, (b) the extent and displacement of the floating ice cover of the subglacier lakes of Grimsvotn and the Skafta cauldrons, (c) surface displacements above the subglacier pathways of the jokulhlaups from the Gjalp eruption site and the Grimsvotn lake, (d) detection of areas of increased basal sliding due to lubrication by water, and (e) detection of spots of temporal displacement that may be related to altering subglacial volcanic activity. At the depression created by the Gjalp eruption, the maximum surface displacement rate away from the radar decreased from 27 cm d -1 to 2 cm d over the period January 1997-January 1999. The observed vertical displacement of the ice cover of Grimsvotn changed from an uplift rate of 50 cm d -1 to sinking of 48 cm d -1 , and for Skafta cauldrons from 2 cm d -1 to 25 cm d -1 .

Three-dimensional glacier surface motion maps at the Gjálp eruption site, Iceland, inferred from combining InSAR and other ice-displacement data

Abstract We use topographically corrected interferograms, repeated global positioning system observations of locations of stakes and time series of elevation data to produce time series of high-resolution three-dimensional (3-D) ice surface motion maps for the infilling of the ice depression created by the 1996 subglacial eruption at the Gjálp volcano in Vatnajökull, Iceland. The ice inflow generated uplift in the central parts of the depression. During the first months, the uplift was much reduced by basal melting as the subglacial volcano cooled. For those motions surface-parallel ice flow cannot be assumed. The 3-D motion maps are created by an optimization process that combines the complementary datasets. The optimization is based on a Markov random-field regularization and a simulated annealing algorithm. The 3-D motion maps show the pattern of gradually diminishing ice flow into the depression. They provide a consistent picture of the 3-D motion field, both spatially and with time, which cannot be seen by separate interpretation of the complementary observations. The 3-D motion maps were used to calculate the cooling rate of the subglacial volcano for the first year after the eruption. First an uplift rate resulting solely from the inflow of ice was calculated from inferred horizontal motions. Basal melting was then estimated as the difference between the calculated uplift generated by the inflow of ice, and the observed uplift that was the combined result of ice inflow and basal melting. The basal melting was found to decline from 55 m3 s–1 (due to power of 18 GW) in January 1997 to 5 m3 s–1 (2GW) in October 1997.

Response of Eyjafjallajökull, Torfajökull and Tindfjallajökull ice caps in Iceland to regional warming, deduced by remote sensing

We assess the volume change and mass balance of three ice caps in southern Iceland for two periods, 1979–1984 to 1998 and 1998 to 2004, by comparing digital elevation models (DEMs). The ice caps are Eyjafjallajökull (ca. 81 km2), Tindfjallajökull (ca. 15 km2) and Torfajökull (ca. 14 km2). The DEMs were compiled using aerial photographs from 1979 to 1984, airborne Synthetic Aperture Radar (SAR) images obtained in 1998 and two image pairs from the SPOT 5 satellites high-resolution stereoscopic (HRS) instrument acquired in 2004. The ice-free part of the accurate DEM from 1998 was used as a reference map for co-registration and correction of the vertical offset of the other DEMs. The average specific mass balance was estimated from the mean elevation difference between glaciated areas of the DEMs. The glacier mass balance declined significantly between the two periods: from −0.2 to 0.2 m yr−1 w. eq. during the earlier period (1980s through 1998) to −1.8 to −1.5 m yr−1 w. eq. for the more recent period (1998–2004). The declining mass balance is consistent with increased temperature over the two periods. The low mass balance and the small accumulation area ratio of Tindfjallajökull and Torfajökull indicate that they will disappear if the present-day climate continues. The future lowering rate of Eyjafjallajökull will, however, be influenced by the 2010 subglacial eruption in the Eyjafjallajökull volcano.

Localized uplift of Vatnajökull, Iceland: Subglacial water accumulation deduced from InSAR and GPS observations

We report on satellite and ground-based observations that link glacier motion with subglacial hydrology beneath Skeiðararjokull, an outlet glacier of Vatnajokull, Iceland. We have developed a technique that uses interferometric synthetic aperture radar (InSAR) data, from the European Remote-sensing Satellite (ERS-1/-2) tandem mission (1995–2000), to detect localized anomalies in vertical ice motion. Applying this technique we identify an area of the glacier where these anomalies are frequent: above the subglacial course of the river Skeiðara, where we observed uplift of 0.15–0.20md during a rainstorm and a jokulhlaup, and subsidence at a slower rate subsequent to rainstorms. A similar pattern of motion is apparent from continuous GPS measurements obtained at this location in 2006/07. We argue that transient uplift of the ice surface is caused by water accumulating at the glacier base upstream of an adverse bed slope where the overburden pressure decreases significantly over a short distance. Most of the frictional energy of the flowing water is therefore needed to maintain water temperature at the pressure-melting point. Hence, little energy is available to enlarge water channels sufficiently by melting to accommodate sudden influxes of water to the base. This causes water pressure to exceed the overburden pressure, enabling uplift to occur.

Glacier winds on Vatnajokull ice cap, Iceland, and their relation to temperatures of its lowland environs

Abstract During the ablation season, the ice cap Vatnajökull (8100 km2) develops its own microclimate that we describe by meteorological data collected during the summers of 1994-2003. Persistent glacier winds are generated down the melting ice cap, whose variations in speed can be related empirically to the temperature fluctuations of the lowland environs of the ice cap. This suggests that climate warming would be accompanied by stronger glacier winds down the outlets of Vatnajökull, producing stronger turbulent fluxes that might amplify the melting rates in the lower ablation areas.