A new gravity laboratory in Ny-Ålesund, Svalbard

Transcription

1 J. Geod. Sci. 2017; 7:18 30 Research Article Open Access K. Breili*, R. Hougen, D. I. Lysaker, O. C. D. Omang, and O. B. Tangen A new gravity laboratory in Ny-Ålesund, Svalbard Assessment of pillars and implications for geodynamical applications DOI /jogs Received January 3, 2017; accepted March 8, 2017 Abstract: The Norwegian Mapping Authority (NMA) has recently established a new gravity laboratory in Ny-Ålesund at Svalbard, Norway. The laboratory consists of three independent pillars and is part of the geodetic core station that is presently under construction at Brandal, approximately 1.5 km north of NMA s old station. In anticipation of future use of the new gravity laboratory, we present benchmark gravity values, gravity gradients, and final coordinates of all new pillars. Test measurements indicate a higher noise level at Brandal compared to the old station. The increased noise level is attributed to higher sensitivity to wind. We have also investigated possible consequences of moving to Brandal when it comes to the gravitational signal of present-day ice mass changes and ocean tide loading. Plausible models representing ice mass changes at the Svalbard archipelago indicate that the gravitational signal at Brandal may differ from that at the old site with a size detectable with modern gravimeters. Users of gravity data from Ny-Ålesund should, therefore, be cautious if future observations from the new observatory are used to extend the existing gravity record. Due to its lower elevation, Brandal is significantly less sensitive to gravitational ocean tide loading. In the future, Brandal will be the prime site for gravimetry in Ny-Ålesund. This ensures gravity measurements collocated with space geodetic techniques like VLBI, SLR, and GNSS. Keywords: Glacial isostatic adjustment, gravimetry, loading, Ny-Ålesund *Corresponding Author: K. Breili: Geodetic Institute, Norwegian Mapping Authority, NO-3507 Hønefoss, Norway and Faculty of Science and Technology, Norwegian University of Life Sciences (NMBU), NO-1432 Ås, Norway, brekri@kartverket.no R. Hougen, D. I. Lysaker, O. C. D. Omang, O. B. Tangen: Geodetic Institute, Norwegian Mapping Authority, NO-3507 Hønefoss, Norway 1 Introduction Since 1991, regular gravity observations have been conducted in Ny-Ålesund at Svalbard, Norway. The observations are of particular scientific interest because Glacial Isostatic Adjustment (GIA) in this region results from both past and present day ice mass changes (PDIMC). Together, these two effects make observations from Svalbard valuable to better understand how ice melting on different time scales impacts geodetic observations [15]. Also, Svalbard is one of the regions that has experienced the greatest temperature increase during the last three decades [10, 22]. It is therefore a key area to monitor in order to map effects of global warming like e.g. increased glacial mass loss. The gravity laboratory in Ny-Ålesund is collocated with several other geodetic instruments, i.e. Global Navigation Satellite System (GNSS) receivers, a Very Long Baseline Interferometry (VLBI) antenna, a Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) ground beacon, and a tide gauge. Knowledge of gravity increases the value of these instruments because gravity offers independent control of secular height changes deduced from geodetic measurements [30]. The gravimetric data from Ny-Ålesund include more than ten absolute gravity (AG) campaigns and time series from two superconducting gravimeters (SG). The comprehensive amount of data results from a joint effort involving a number of institutions and instruments (see Table 1 for an overview). Results are previously reported in e.g. [30, 31, 25, 15, 14], and [16]. Both studies of GIA and the control of height time series from space geodetic measurements require consistent gravity observations over a long period of time, preferably AG-campaigns in combination with a record of continuous SG measurements. In Ny-Ålesund most gravity campaigns have been conducted at the Norwegian Mapping Authority s (NMA) geodetic observatory officially opened in 1995 [27]. Only provisional facilities were available for the first AG-campaigns. A small wooden shed covered with a tarpaulin provided shelter from precipitation and winds for both the 1991 and 1998 AG-campaigns. Also, the 1991 campaign was measured at a separate pillar that was later 2017 K. Breili et al., published by De Gruyter Open. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License.

2 A new gravity laboratory in Ny-Ålesund 19 Table 1: Institutions and instruments used to observe gravity in NyÅlesund: Bundesamt für Kartographie und Geodäsie (BKG), Finnish Geospatial Research Institute (FGI), European Center for Geodynamics and Seismology (ECGS), Ecole et Observatoire des Sciences de la Terre (EOST), National Astronomical Observatory of Japan (NAOJ), Norwegian Mapping Authority (NMA), and Norwegian University of Life Sciences (NMBU). The JILAg-5h campaign in 1991 was conducted at a pillar approximately 10 m north-west of Kongepunktet. The total uncertainty (1-sigma) of the JILAg-5, FG5, and A10 gravity values is typically 7, 2, and 11 µgal, respectively. All gravity values are at height 1.20 m above top of marker. Epoch 1991 May 1998 July 2000 July 2001 August 2002 August 2004 June 2007 June 2010 August 2012 May 2012 December 2013 May April Instrument JILAg-5h FG5-101 FG5-206 FG5-101 FG5-216 FG5-206 FG5-206 FG5-206 FG5-206 FG5-226 FG5-226 FG5-206 A GWR C39 igrav-12 Institution FGI and NMA BKG EOST BKG ECGS EOST EOST EOST EOST NMA and NMBU NMA and NMBU EOST NMA NAOJ and NMA NMA planned to be operational in 2018 and 2021, respectively, whereas the gravity laboratory is now ready for use. It is located in a designated wooden building and has three independent pillars made of armed concrete and anchored to the bedrock (see Fig. 1). In addition to the pillars, the laboratory is equipped with a wind gauge, a GNSS receiver is located 10 m away, and a thermistor string monitors the temperature at several depths through the ground down to the bedrock. Gravity (µgal) * * * * ** ** Not available ** Adopted from [29], but transferred to h = 1.20 m Adopted from [31], but transferred to h = 1.20 m * Adopted from [16], but transferred to h = 1.20 m ** Processed from raw data. used for a PRARE (Precision Range and Range-Rate Equipment) ground station. Hence, this pillar was not appropriate for later gravity measurements. In stead, a new site for gravity was established on a concrete construction approximately 10 m to the south. This base has been used for all later gravity measurements, and it was transformed into a designated gravity laboratory in 1999 when it was supplied with walls and roof. Today, the laboratory facilitates three zones for gravity. The center zone, called Kongepunktet (King s point), is dedicated to AG-measurements while the SG GWR-C39 permanently occupied the northeastern zone from 1999 to A third zone was established for the new SG igrav-12 in 2013 and the SG GWR-C39 was switched off and dismantled in April A new geodetic core station is presently under construction at Brandal, 1.5 km north of the old station (see Fig. 2). When completed, the new station will include a twin VLBI antenna, a telescope for satellite laser ranging (SLR), several GNSS-receivers, as well as a gravity laboratory and a SG. The VLBI antenna and the SLR telescope are Figure 1: Upper: A wooden construction fulfilling Norwegian standards for low-energy buildings houses the new gravity laboratory. Middle: Schematic of the gravity laboratory. A, B, and C indicate the three pillars. Courtesy LPO-architects. Lower: East-west terrain profile through pillar B at Brandal.

3 20 K. Breili 2 Analysis of gravity observations m Figure 2: Map of Ny-Ålesund. Red triangular markers: new and old gravity sites (Kongepunktet and Brandal). Black stars: new and old VLBI antennas. Yellow marker: DORIS ground beacon. Green square: Location of future SLR-telescope. Blue pentagon: Tide gauge. The black lines are geolines indicating possible active faults. The red shaded areas indicate the settlement of Ny-Ålesund, roads, and the air-field. The new gravity laboratory and the first measurements from the new pillars are the focus of this study. First we address measurements collected during April 2016 with an A10 absolute gravimeter. The main motivation for conducting these first measurements was to prepare for future gravity campaigns in the new laboratory. This implies the establishment of benchmark gravity values, vertical gravity gradients, and official coordinates and heights for the new pillars. We also address the pillars suitability for future high accuracy gravity measurements. In particular, we analyze the noise level in the A10 measurements, and check how the pillars respond to winds. Then we go on to discuss consequences of moving from Kongepunktet to Brandal with respect to the gravitational signals that arise due to PDIMC and ocean tide loading (OTL). Our results are the first reported from the new geodetic core station and may be helpful for operators of gravimeters in Ny-Ålesund. In this study, we use AG campaigns conducted in April 2016 with the A10-42 absolute gravimeter owned and operated by the NMA. For comparison and completeness, published and reprocessed gravity values obtained at Kongepunktet by FG5 instruments are also provided (see Table 1). Both the A10 [5, 18] and the FG5 [21] are free-fall gravimeters with similar working principles, i.e. they allow gravity to be estimated from time-distance pairs describing the trajectory of a free-falling test mass inside a vacuum chamber. The A10 is designed for field measurements; it is robust, and easy to move from one site to the next. This is advantageous for testing a new gravity laboratory where several pillars can be occupied successively. Unfortunately, the A10 s robustness is at cost of accuracy. It has an accuracy of better than 10 µgal (1 µgal = 10 8 m/s 2 ) at field sites, assuming that instrument standards are checked [5]. In contrast, the FG5 provides measurements with an accuracy of typically 1 2 µgal and allows gravity changes to be estimated with a precision of 0.5 µgal/yr after 10 years of annual measurements [35]. The raw AG-observations were processed with the g- software (version 9) provided by Micro-g LaCoste, the manufacturer of the A10 and the FG5 gravimeters. Each drop was corrected for Earth tides, polar motion, ocean tide loading (OTL), and atmospheric loading (ATL). For computing Earth tide corrections, we used the ETGTAB model [33, 3], and corrections for the change in the Earth s centripetal acceleration due to polar motion were computed from final polar coordinates provided by the International Earth Rotation and Reference Systems Service (IERS). The OTL-corrections were computed from the FES2004 [12] global ocean tide model by an internal routine of the g-software. The effect of atmospheric loading was computed by multiplying the difference between nominal pressure and observed pressure at each drop with an admittance factor. At Kongepunktet the admittance factor was set to µgal/hpa, adopted from [31]. This is an empirical parameter estimated from SG-observations simultaneously with tidal parameters. Because variations in air pressure influence sea level, it is possible that the admittance factor partly captures the effect of non-tidal loading. The empirical parameter may, therefore, not be appropriate at Brandal which has a different relative location to the sea. Instead, we opt to use the conventional admittance factor of 0.3 µgal/hpa when processing the gravity observations collected at Brandal.

4 A new gravity laboratory in Ny-Ålesund 21 Table 2: EUREF89 coordinates, ellipsoidal heights, and levelled heights for Kongepunktet, the tide gauge, and the three pillars at Brandal. Add 0.22 m to the levelled heights in order to obtain heights above present mean sea level. Latitude Longitude Ellipsoidal Levelled ( N) ( E) height (m) height (m) Brandal-A Brandal-B Brandal-C Kongepunktet Tide gauge In total five pillars were observed with the A10, i.e. Kongepunktet, three pillars at the new gravity laboratory, and next to the tide gauge in the harbor. The campaigns at Kongepunktet will serve as references because the behavior of this pillar is well known from a number of previous AG campaigns and a 17 years long SG-record. We consider Kongepunktet as a most suitable place for both AG and SG. The three pillars at Brandal line up from southwest to northeast and are denoted A, B, and C. The campaign at the tide gauge was conducted with a view to calibrate e.g. ship-born gravimeters visiting Ny-Ålesund. Its gravity value and coordinates are included for completeness, but this site will not be discussed further. All pillars, except the one at the tide gauge, were observed several times. In total 24 campaigns have been analyzed, each campaign including from 4 to 100 sets with typically 120 drops in each set. 3 Final coordinates, heights, and benchmark gravity values For all gravity sites, coordinates and heights are important for computing accurate corrections for Earth tides, OTL, and polar motion, and for modelling the gravitational effects of e.g. hydrology, snow and ice mass changes. The positions and ellipsoidal heights of the new pillars at Brandal were determined by trigonometric surveying relative to nearby markers occupied by GNSS. A similar approach was applied at the tide gauge. Kongepunktet was positioned relative to the GNSS-station NYA1 by compass and measuring tape. The pillars were also levelled. There is no official height system for Ny-Ålesund. Instead, all levelled heights refer to the tide gauge benchmark (TGBM) which is also the Norwegian Polar Institute s (NPI) fundamental benchmark for Svalbard. The official height of the TGBM is m above mean sea level calculated for an unknown epoch. Due to vertical land motion and sea-level changes, the TGBM is m above a recent update of mean sea level calculated from observations over the period 1996 to That means, add 0.22 m to the reported levelled heights in order to achieve heights above present mean sea level. Table 2 lists the final coordinates, ellipsoidal heights, and levelled heights. Benchmark gravity values were established by computing averages of successful A10 campaigns conducted in April They are listed in Table 3 and we opt to report gravity at two different heights; 1) at the top of the pillar s marker (h =0 m); and 2) at the typical datum height used for FG5 absolute gravity measurements (h =1.20 m). Table 2 also lists the vertical gravity gradients that were used to transfer gravity to the two datum heights (h = 0 m and h = 1.20 m). All gradients were observed with a LaCoste and Romberg relative gravimeter during the AG campaigns in April The gradients were estimated from repeated measurements of the gravity difference between the floor and at height approximately 1.20 m above the floor. Ten repetitions yielded gradients with a standard error of 0.02 to 0.03 µgal/cm. Non-linear gradients may introduce additional uncertainty when transferring gravity between different heights. Therefore, the best practice is to measure a quadratic gradient (i.e. gravity differences between three levels) when establishing gradients at new stations. However, this issue was not investigated for the pillars in Ny- Ålesund. Notice that the gravity differences between the pillars change when the datum height is altered. This is an effect due to the pillars slightly different gradients. The A10 results can be assess by comparing the A10 value at Kongepunktet to gravity obtained with FG5s, see Table 1. The campaign conducted with FG5-226 in May 2013 is the one closest in time and with a published gravity value. It differs by 2 µgal from the A10 result. The difference increases to 6 µgal if we project the result of FG5-226 to May This was done by making use of the average annual gravity rate of 1.39 µgal/yr reported in [16].

5 22 K. Breili Table 3: Gravity at top of the marker (g 0 ) and at 1.20 m above top of marker (g 1.20 ) at Kongepunktet, next to the tide gauge, and the three pillars at Brandal. The total uncertainties are calculated by summarizing in quadrature the squared standard error of the average gravity value, the squared standard error of the model-corrections (Earth tides, ocean tide loading, polar motion, atmospheric loading), the squared standard error of the transfer of gravity to the datum height, laser and clock errors, system type error (10 µgal), and setup-error (3 µgal). The rightmost column lists the observed vertical gravity gradients and their standard errors. The epoch is April 2016 for all gravity values. Site g 0 g 1.20 dg/dh (µgal) (µgal) (µgal cm 1 ) Brandal-A ± ± ±0.03 Brandal-B ± ± ±0.03 Brandal-C ± ± ±0.03 Kongepunktet ± ± ±0.03 Tide gauge ± ± ±0.02 Also the projected value is within the expected accuracy of the A10, but it indicates that the measurements of the A10-42 are biased high. This tendency is consistent with findings at Hønefoss, Norway, where both A10-42 and FG5-226 measurements exist. Here, A10-measurements just before and after the campaigns in Ny-Åleusund are approximately 5 µgal higher than gravity observed with FG5-226 in September Closer agreement between the two instruments is obtained by subtracting the difference at Hønefoss from the A10 measurements in Ny-Ålesund. We remark that systematic biases may arise among gravimeters of the A10 type due to inaccurate instrument standards, i.e. the frequency of the gravimeter s rubidium oscillator and the wavelength of the ML-1 laser. The A10-42 was purchased in October 2015, and neither the oscillator nor the laser were recalibrated before of the measurements in Ny- Ålesund. 4 Assessment of pillars The new pillars at Brandal are constructed differently from Kongepunktet and the building itself has a different design. We therefore assess whether the measurements at the new pillars have a noise level comparable with that observed at Kongepunktet. The noise level in gravity measurements is not constant. It varies with time due to e.g. changing weather, different sea states, earthquakes, and varying human activity in and around the gravity station. In addition, the type of gravimeter in play is of significance because all types of gravity measurements imply a filter dampening or isolating the gravimeter from undesired ground motions. For an absolute gravimeter, a superspring realizes the filter. It is a mechanical system that mimics a spring with a long natural period (30 seconds for the A10 and seconds for the FG5). In principle, the superspring isolates the test mass from ground motions at frequencies higher than the natural frequency [18], but in practice higher frequency signals also influence the measurements. Because the noise level varies, as described above, we compare the measurements at the new pillars to the campaigns at Kongepunktet. The number of studies incorporating gravimetric data from this pillar have proven that Kongepunktet is well suited for both absolute gravimetry as well as the deployment of superconducting gravimeters. We assume that a comparable noise level at the new pillars implies that also they are well suited for gravimetry. On the other hand, the opposite may indicate problems. The overall noise level at each pillar was assessed by averaging the drop to drop scatter over all sets collected at each pillar. The drop to drop scatter, or simply the drop scatter, is defined as the standard deviation of a group of free fall experiments. With the A10, the drop scatter is typically calculated from 120 free fall experiments. We used only measurements under favorable conditions, i.e. calm winds, modest sea states, and low human activity inside the gravity laboratory. In addition, sets with extraordinary enlarged drop scatter due to earthquakes were omitted. Table 4 summarizes the drop scatters. They range from approximately 20 to 40 µgal, i.e. at the level expected for an A10 under laboratory conditions [18]. Still, the spread among the pillars and the standard deviation at each pillar indicate that the noise level varies considerably. Applying Welch s unequal variances t-test [37], it was found that the average drop scatter at Kongepunktet is significantly (at the 95 % level) lower compared to the three pillars at Brandal. At the same time, Brandal-C has a significantly higher average drop scatter than all other pillars. We em-

6 A new gravity laboratory in Ny-Ålesund 23 phasize that this result only applies to the A10 measurements conducted in April Measurements under e.g. other weather conditions and sea states may lead to different conclusions. Also, the precision of the final gravity value will be the drop scatter divided by the square root of the number of measurements. Hence, increased noise level can be compensated by increasing the number of measurements. Wind speed measurements indicate that variable winds may be the origin of some of the spread in the drop scatters at both Kongepunktet and Brandal. The wind speed was measured with a 20-second sampling rate next to Kongepunktet and with 15-minutes sampling rate at Brandal. Unfortunately, the low sampling rate at Brandal does not allow variation in the drop scatter during short campaigns to be investigated. Initial tests indicate that the wind speed data at Brandal have a coefficient of correlation of 0.67 to the wind speed data at Kongepunktet. Hence, we consider the wind data from Kongepunktet as a quite good proxy for the conditions at Brandal and opt to use wind speed data from Kongepunktet for all pillars due to its superior temporal resolution. Drop scatter [µgal] Drop scatter [µgal] Kongepunktet Brandal B Wind speed [m/s] Brandal A Brandal C Wind speed [m/s] Figure 3: Drop scatter vs. wind speed at Kongepunktet (upper left), Brandal-A (upper right), Brandal-B (lower left), and Brandal-C (lower right). Figure 3 shows drop scatter versus wind speed at Kongepunktet, Brandal-A, B, and C. The drop scatters grow slightly for increasing wind speeds at all pillars. This emerges also from the coefficients of correlation (between wind speed and drop scatter) listed in Table 4, ranging from 0.57 to The highest correlation was found at Kongepunktet. This was expected since this pillar is in the immediate vicinity of the wind gauge. At the same time, the drop scatters are in general lower at Kongepunktet, indicating that this pillar is less sensitive to winds compared to Brandal. The coefficient of regression between drop scatter and wind speed supports this assumptions. At the 95 % level, it is significantly smaller at Kongepunktet. Among the pillars at Brandal, the drop scatters from Brandal-C have the largest coefficient of correlation and the highest coefficient of regression. The latter parameter is significantly larger than at Brandal-A and B. We conclude that Brandal-C is more sensitive to winds compared to the other pillars. Kongepunktet s low sensitivity to local sources of seismic noise was somewhat unexpected because the floor is resting directly on the concrete base the gravimeter is placed on. The concrete base is a heavy construction with horizontal dimensions of 4 4 m and a thickness of 0.5 m. It is placed on top of two iron posts forming a cross and grouted bolts anchor the whole construction to bedrock 4 m below the ground. On the other side, the pillars at Brandal are higher and more narrow, i.e. they have a top surface of m and heights of 7 to 9 m. With its foundation, the construction at Kongepunktet is more massive compared to the pillars at Brandal. We believe this is the key to the low sensitivity to local sources of seismic noise at this site. 5 Implications on geodynamic applications Moving from Kongepunktet to Brandal may have implications on the signals from PDIMC and OTL recorded by a gravimeter. We investigate this in depth by modelling the gravitational effects of these processes separately at both sites and then evaluate the differences. On the other hand, GIA from past ice-mass changes is usually modelled as a long spatial wavelength process. For instance, [31] modelled the effect over Svalbard up to the degree 180 in terms of spherical harmonics, which corresponds to a spatial resolution of approximately 222 km at the Earth s surface. Hence, we expect negligible differences at the two stations in the signal from past ice-mass changes. 5.1 The effect of PDIMC The effect of surface loads on geodetic observations can be calculated by convolving an elastic Green s function with a function describing the spatial distribution of the load (see e.g. [6]). The method has been previously used to as-

7 24 K. Breili Table 4: Mean drop scatters and their standard deviation at each pillar. CC and CR are the coeflcients of correlation and regression between drop scatter and wind speed, respectively. Site Mean Std deviation CC CR Number of (µgal) (µgal) (µgal s m 1 ) sets Kongepunktet ± Brandal-A ± Brandal-B ± Brandal-C ± sess the effect of PDIMC on geodetic observations in Ny- Ålesund [11, 14, 16, 31]. Standard Green s functions presuppose loads and observation points located on a spherical Earth. Because gravimeters observe the vertical component of the gravitational vector from the load, this is not viable for calculating the gravitational attraction from nearby elevated loads [17, 2]. Common solutions are 1) to use specialized Green s functions that depend on the relative height difference between the load and the gravity stations [17] or 2) separately compute the Newtonian attraction by e.g. modelling the load as right rectangular prisms and then use Eq. 8 in [20] to compute the attraction from each prism. In this study, we will use the latter method to calculate the attraction and combine it with an elastic Green s function based on the Gutenberg-Bullen Earth model as tabulated in Table A3 in [6]. With this approach, the elastic response computed with the Green s function includes the effect of the vertical displacement of the observation point due to the load, and the effect of redistributed masses due to the deformations. The annual PDIMC on the glaciers at the Svalbard archipelago is calculated from five ice models adopted from [11] and summarized in Table 5. The ice models are defined over five zones illustrated in Fig. 4. All models assign uniform thinningrates to all glaciers in zone 2 to 5, i.e. 1 meter water equivalent per year (mweq/yr) for Model 0 and 0.25 mweq/yr for Model 1 to 4. In the zone surrounding Ny-Ålesund, zone 1, empirical relationships between the elevation and the thinning rates are used for Model 1 to 4. The relationships are based on GPS profiles from 1996 and 2004 at the glacier Kongsvegen, and aerial photographs (1990) and SPOT satellite images (2007) from the glacier Holtedalsfonna [11, 8]. Model 0 uses a uniform thinning rate of -1 mweq/yr also for zone 1 while Model 2 differs from Model 1 in that the elevation dependent height changes are averaged over zone 1. In Model 3 and 4, different glacial dynamics at Holtedalsfonna (defined as the area N to N and E to E, see Fig. 4) are investigated. Figure 4: The DEM covering glaciated areas of Svalbard used to compute the effect of PDIMC at Kongepunktet and Brandal. The colors represent elevation above sea level, ranging from 0 to 1800 m. The numbers (1 to 5) and the dashed lines indicate the five zones used to define the ice models and the black rectangle outlines Holtedalsfonna. The total annual ice mass change varies between the models. With a uniform thinning rate of 1 mweq/yr, the total annual mass loss is 35.2 Gt/yr. It ranges from 11.6 to 11.8 Gt/yr for Model 2 to 4, i.e. in the middle of the range of 5 to 18 Gt/yr derived from Gravity Recovery and Climate Experiment (GRACE) satellite solutions [15]. We consider, therefore, Model 1 to 4 to be more realistic than Model 0. Model 0 is still relevant because there is a linear relationship between the total mass in the model and the gravitational effects as long as the thinning rate is uniform. This implies that the effect of any other uniform thinning rate can be found by scaling the results of Model

8 A new gravity laboratory in Ny-Ålesund 25 Table 5: Ice models used to calculate PDIMC. Holtedalsfonna is defined as the glaciated area within N to N and E to E. m ice is the annual mass change in each model. Ice-model m ice Model description Gt/yr h = 1.0 mweq/yr for all zones Gt/yr Zone 1: h depends on the elevation. Zone 2 to 4: h = 0.25 mweq/yr Gt/yr Similar to Model 1, but the height changes of zone 1 are averaged Gt/yr Similar to Model 1, but no glacial dynamics at Holtedalsfonna Gt/yr Similar to Model 1, but h = 0.29 mweq/yr at Holtedalsfonna. 0. For instance, the PDIMC signal calculated for Model 0 can be scaled to fit the average thinning rate of Svalbard as observed by e.g. ICESat [19] or GRACE [15]. See Table 5 for a summary of the ice models and [11] for more details. To run the ice models, it is necessary to know the elevations and outlines of the ice-covered areas. We used digital elevation models (DEM) provided by the NPI [24]. For Brøggerhalvøya, where Ny-Ålesund is located, a DEM with a spatial resolution of 5 5 m was applied. For the rest of the zones the spatial resolution was m. Both DEMs were resampled to fit the outlines of glaciers as they appear in the map data of Svalbard in scale 1: [23]. With these outlines, the ice-models cover in total km 2 and include glaciers at the islands Spitsbergen, Nordaustlandet, Edgeøya, Barentsøya, Kvitøya, Prins Karls Forland, and all smaller islands found in the map data (see Fig. 4). Finally, before computing the gravitational effects, the spatial resolution of the DEMs was resampled with the software QGIS [28]. It was set to m at Brøggerhalvøya, m for the rest of zone 1, and m for zone 2 to zone 5. Initial runs indicate that these choices ensure proper processing time and precision when computing the attraction from nearby loads. The gravitational effects of PDIMC for the five icemodels are listed in Table 6. Firstly, we remark that the PDIMC signal at Kongepunktet calculated for a uniform thinning rate of 1 mweq/yr (Model 0) is 0.12 µgal/yr larger than the result of a similar study by [14]. We attribute the difference to the glacier outlines, which in the present study are more complete and have higher spatial resolution. Figure 5 illustrates the situation at Brøggerhalvøya. In this region, the ice model in [14] does not include Mørebreen, Røysbreen, Trongskarbreen, and Vestre Lovenbreen. These glaciers have a total area of approximately 8 km 2 and they contribute by 0.15 µgal/yr for a uniform thinning rate of 1 mweq/yr, i.e. of the same order as the gap between the two studies. This is not a criticism of [14], but illustrates the urgency of complete and accurate Figure 5: Glacier outlines at Brøggerhalvøya, i.e. the glacier outlines in the S100 map database of the NPI (blue) are overlain by reconstructed outlines used by [14] (orange). Notice that all features in the orange set are also included in the blue set. glacier outlines, especially in the zone closest to the gravity laboratories. Comparing the annual PDIMC signal at Kongepunktet to that at Brandal reveals differences for all ice models. Table 6 shows that the attraction components contributes most to the differences, while the elastic components are virtually the same for all ice models at the two sites. Addressing the total effect (elastic + attraction), the smallest differences are found for Model 0 and Model 2, i.e. the two models using uniform thinning rates in zone 1. For the models served with height-dependent thinning rates, the difference between Brandal and Kongepunktet is larger, around 0.15 µgal/yr for all of them. It was somewhat surprising that largest differences in the PDIMC signal were found for these models, because they imply only a third of the mass change in Model 0. Still, closer investigations prove that the mass changes taking place at the closest glacier are larger in these models than those of Model 0. The results in Table 6 indicates that varying the glacial dynamics at Holtedalsfonna 15 to 40 km away from Ny-

9 26 K. Breili Table 6: The annual gravitational effect of PDIMC at Kongepunktet and Brandal calculated for five ice models. g A and g E are the gravitational attraction of the ice masses and the elastic effect, respectively. Ice Site g A g E g A + g E Difference model (µgal/yr) (µgal/yr) (µgal/yr) (µgal/yr) 0 Kongepunktet Brandal Kongepunktet Brandal Kongepunktet Brandal Kongepunktet Brandal Kongepunktet Brandal Differential effect of PDIMC [µgal] Model 0 Model 1 Model 2 Model 3 Model Distance to ice load [km] Figure 6: Differential PDIMC signal for five ice-models. In the near field, Model-1, 3, and 4 are not distinguishable. Ålesund affects the two study points similarly. This is supported by Fig. 6 that shows largest contribution to the differences from ice-masses within 15 km from the study point. Farther away, only negligible differences arise. We remark that the attraction component and the elastic component almost balances for all models, resulting in a weak total gravitational PDIMC signal. As a consequence, the differences between the two stations are larger than the PDIMC signal itself for Model 1, 3, and 4. With a view to the trend in the observed gravity record, the differences in the annual PDIMC signal are small. A significant negative trend characterizes the time series of gravity from Kongepunktet. Combining data from the GWR-C39 with six AG-campaigns, [25] estimated the average annual rate to 1.77 ± 0.12 µgal/yr for the period September 1999 to September [16] reported an updated estimate. They included more data and removed the seasonal signal in both the SG-record and the AG-campaigns, resulting in an annual trend of 1.39 ± 0.11 µgal/yr for the period January 2000 to June The differences in the annual PDIMC signal between Brandal and Kongepunktet amount to 3 to 11 percentage of the average rates, but for Model 1, 3, and 4 it is slightly larger than the uncertainty of both trend estimates. The differences are, therefore, at the limit of what detectable in a trend analysis focusing on the average rate. They are also smaller than 0.5 µgal/yr which is the maximal rate uncertainty [32] recommend for crustal deformation studies. [25] argue that the change in gravity is best described as piecewise linear trends ranging from 0.23 to 3.22 µgal/yr. The differences between the two sites are of the same order as observed year-to-year variations. Corresponding variation is also detected in the height time series from a collocated GNSS-station. [11] attributed the variation in the vertical rate to varying rates in PDIMC, and it is likely that the variation in the gravity rates have the same origin. The fact that the variation in PDIMC manifests in geodetic observations, allows for using geodetic results as constraints on e.g. in-situ mass balance measurements. Nevertheless, users of gravity data from Ny- Ålesund should be cautious if they try to extend the gravity record from Kongepunktet with future gravity observations at Brandal; there is a chance that the shift in the location may significantly influence the rate estimates and disturb PDIMC analysis if the gravity laboratory s position is not modelled properly. 5.2 The effect of ocean loading Kongepunktet and Brandal are both coastal stations, located 120 m and 250 m from the sea, respectively. We therefore expect gravity measurements at both stations to be in-

10 A new gravity laboratory in Ny-Ålesund 27 fluenced by OTL, i.e. the combined effect of the direct attraction from the ocean tides and the gravitational effects resulting from the elastic deformation of the Earth due to the ocean load [6]. Kongepunktet is the most elevated of the two stations and the vertical component of the gravitational vector to the local ocean tides is therefore expected to be stronger at Kongepunktet than at Brandal. As a consequence, empirical tidal coefficients derived from the SG record at Kongepunktet may not be appropriate at Brandal. We investigate this by comparing tidal constants estimated from the SG-record at Kongepunktet to predicted tidal constants at both Kongepunktet and Brandal. The total tidal signal is the sum of OTL and solid Earth tides (SET). OTL can be predicted in a similar way as the PDIMC effect, but with a model representing the ocean tides replacing the ice model. For ocean tides, Green s functions for a spherical Earth are appropriate, but the effect of elevated observation points should be taken into account [1]. We follow the methodology and use the same software as discussed in [26] who computed OTL and nontidal loading (NTL) at coastal gravity stations in Norway. With this methodology, OTL was computed by convolving the Green s function with a global ocean tide (GOT) model resampled to higher spatial resolution within a spherical distance of 1 degree from the gravity laboratory. For Ny- Ålesund, we used a detailed coastline provided by the NPI [23] to decide whether cells in the ocean model are located on land or not. We opt to use the two GOTs FES2012 [12] and NAO99b [13] because [26] have previously demonstrated that corrections based on these models strongly reduced the long periodic residuals in measurements at AG sites along the Norwegian coast. The OTL predictions were expressed as amplitudes and Greenwich phase lags. To compute the total tidal signal, these sinusoids were combined with corresponding sinusoids representing SET generated by the g-software with the ETGTAB model [33, 3]. The predicted tidal signal was compared to observed tidal waves estimated from hourly SG observations covering the interval from September 1999 to December We used the software Tsoft [36] in this step. Table 7 lists both the observed and predicted amplitudes and phases for the eight constituents that the ocean tides primarily influence on. The differences between the predicted and observed tidal waves are significantly smaller at Kongepunktet than at Brandal. At Kongepunktet all predicted amplitudes are within 0.3 µgal from the observed amplitudes for both FES2012 and NAO99b. The phases agree within 9, except for N2 where the predicted phase for FES2012 differs by 29 from the estimated value. Larger differences are found at Brandal. For M2, the difference is 0.8 µgal and 0.9 µgal for the predictions based on NAO99b and FES2012, respectively. Also the phases deviate more at Brandal, up to 53. The larger differences suggest that tidal parameters estimated from SG data at Kongepunktet should not be used to correct gravity observations at Brandal. We also computed the elastic effect and the attraction component of a one-meter uniform sea level anomaly within 10 km from Ny-Ålesund. At Kongepunktet the attraction is 3.52 µgal while it is 0.66 µgal at Brandal. The elastic response is virtually the same, i.e µgal at Kongepunktet and 0.22 µgal at Brandal. The actual impact of a local sea level anomaly can be calculated by summarizing the two effects and multiplying the sum by the actual water level above mean sea level as observed by e.g. the tide gauge. The mean high water in Ny-Ålesund is 0.46 m, i.e. increasing gravity by approximately 1.7 µgal at Kongepunktet and 0.4 µgal at Brandal. We conclude that Brandal is less sensitive for the ocean tides and NTL compared to Kongepunktet and attribute the differences mainly to the lower elevation of Brandal which reduces the direct attraction from the ocean strongly. Rising Brandal (artificially) to Kongepunktet s height, increases the amplitudes of the semi-diurnal and diurnal constituents to the same level as that of Kongepunktet. Likewise, the attraction from a one-meter uniform sea level anomaly increases to 3.0 µgal which differ by 0.52 µgal from the value at Kongepunktet. The different elevation of the two stations is also the origin of the quite large phase differences observed for some constituents. Again, rising Brandal makes the local tides more influential and the phase differences considerably smaller. This indicates that the local tides are slightly out of sync with the global contribution. In addition to the ocean, variable water level in a nearby lake (Brandallaguna) may influence on gravity. The lake is enclosed from the ocean by a ridge of gravel and has an area of approximately 0.3 km 2. It is located 60 m away and 10 m below the gravity laboratory (see lower part of Fig. 1). We believe it is not influenced by the tides, but visual observations suggests that its water level has seasonal variations due to changing influx from streams and through the surrounding loose fill. Using a similar approach as the one used for NTL, the total effect of a 1 m increase in water level was calculated to 1.41 µgal at Brandal. This is of the same magnitude as the entire error budget of a FG5 [21] and more than one magnitude larger than the sensitivity of a SG [9]. On the other side, the effect is 0.01 µgal at Kongepunktet, i.e. negligible. We anticipate that the water level of the lake in the future will be monitored routinely, allowing gravity reductions to be computed.

11 28 K. Breili Table 7: Observed and predicted gravity tides at Kongepunktet and Brandal. All phases are local and lag with the plus sign. Wave Observations Predictions NAO99b Predictions FES2012 Predictions NAO99b Predictions FES2012 Kongepunktet Kongepunktet Kongepunktet Brandal Brandal A[µGal] ϕ A[µGal] ϕ A[µGal] ϕ A[µGal] ϕ A[µGal] ϕ M S N K K O P Q Drop scatter A10 42 [µgal] Drop scatter FG5 226 [µgal] Figure 7: The drop scatter of simultaneous A10 and FG5 measurements at Ås (red) and Hønefoss (blue), Norway. 6 Summary and concluding remarks The main objectives of the present study have been to establish final coordinates for the new pillars, present benchmark gravity values, assess Brandal as a site for precise gravity measurements, and detect possible differences between Brandal and Kongepunktet when it comes to the signal from PDIMC and OTL. We have detected differences between Kongepunktet and Brandal. Exploring five plausible ice-models, we have shown that the annual gravity rate due to PDIMC may change by up to 0.15 µgal/yr. The results in Table 6 indicate that the difference depends on the ice-model in play. Unfortunately, we have not been able to validate the ice models with independent data. It is an open question whether the empirical relation between elevation and mass changes used by Model 1-4 is appropriate for the closest glaciers at Brøggerhalvøya. We argue that the accuracy of the ice-models is not critical for the main conclusions of this study. What is important, is that we have proven that differences of a magnitude detectable with today s most accurate gravimeters may arise using plausible ice models. With an ice-model given, the uncertainty in the calculated gravitational signal results mainly from errors in the Earth model. An impression of the contribution of the Earth model to the uncertainty can be found by recalculating the gravitational signal with a different Earth model. Comparison of results obtained with the Preliminary Reference Earth Model [4] and those listed in Table 6 identify only differences of some thousands of a microgal. In reality, ice-mass changes will vary from year to year, cf. the variable annual gravity rates reported in [25]. [10] report recent warming at Spitsbergen driven by sea ice decline, higher sea surface temperature, and a general background warming. They find that the temperature has increased in all seasons, but with the strongest increase in the winter months. The warming has been associated with increased precipitation [7]. Regional climate model simulations suggests substantial increase of temperature and that precipitation will increase by a few percentages at the southwest of Spitsbergen forward to 2100 [7]. It is not clear how this will influence on the glacial mass balances, but a plausible scenario is longer summers and associated increased annual thinning rates. Hence, actual future masschanges may introduce completely different gravitational signals and gravity differences than those reported above. As an example, scaling the thinning rates of Model 1 by two within zone 1, doubles the difference in the PDIMC signal between Kongepunktet and Brandal.

12 A new gravity laboratory in Ny-Ålesund 29 Moving from Kongepunktet to Brandal, the noise level in our test measurements increases significantly. The potential problem of increased noise is a manageable issue. The desired precision can still be achieved by increasing the number of observations in a campaign. Also, the experienced noise level is at the level expected under laboratory conditions, at least under moderate wind speeds and at low activity in the building. Nevertheless, the parts of the building surrounding the gravity pillars at Brandal are under reconstruction. The new design will include shelters protecting the pillars from wind stress and only soft-type insulation in the gaps between the floor and the pillars. We expect that these modifications will reduce the pillars sensitivity to winds and human activity and consequently improve the noise level further. We also remark that Brandal with its low elevation has a more fortunate location with respect to disturbing signals created by OTL and NTL. If not precisely corrected, these processes have the potential to increase the scatter in the measurements, and thereby interfere with the analysis of geodynamical processes like e.g. GIA [34]. For future gravity campaigns, it is likely that other gravimeters will be employed, e.g. instruments of the FG5- type. Due to their superior accuracy, the FG5 is more appropriate for detecting smaller changes in the gravity field. The FG5 is similar to the A10 in its working principles, but has a superspring with a different natural frequency and a longer drop-chamber with a different design. It is possible that the FG5 and A10 respond differently to ground motions. The question is then whether the assessment based on an A10 is relevant. We tested this at the two stations Ås and Hønefoss at Norway mainland. Figure 7 shows how the drop scatters of the two instruments follow each other for simultaneous measurements. The overall correlation between the two instruments is 0.91, and it is 0.36 at Hønefoss and 0.65 at Ås. The positive coefficients of correlation suggest that sites suitable for A10 measurements are suitable also for FG5 measurements, and vice versa. Based on this, we envisage Brandal as a favourable location also for FG5 measurements. We envisage that Brandal will be the prime site for future gravimetry observations in Ny-Ålesund. At Kongepunktet, the VLBI antenna will eventually be dismantled after the new core station is completed, while GNSS will be kept operational for an indefinite time. In addition, measurements at Brandal ensure collocated instruments with no geological faults in between. In contrast, faults exists in between Kongepunktet and Brandal (see Fig. 2). This may potentially introduce disturbing effects when combining data from the two sites. In the coming years, we recommend that gravity campaigns should involve measurements at both sites. This will allow further exploration of the differences between the two sites and will contribute to a better understanding of PDIMC and GIA at the Svalbard archipelago. Acknowledgement: The authors are thankful to Jon Glenn Gjevestad at the Norwegian University of Life Sciences (NMBU) for making at disposal the FG5-226 absolute gravimeter and the LaCoste & Romberg (G-761) relative gravimeter. The NMA s crew in Ny-Ålesund is kindly acknowledged for in-kind contributions. The FES2012 ocean tide model is produced by Noveltis, Legos, and CLS Space Oceanography Division and distributed by Aviso, with support from CNES, at We also thank the two anonymous reviewers for fruitful comments. References [1] K. Breili. Ocean tide loading at elevated coastal gravity stations. Kart og Plan, 69(3): , [2] K. Breili and B. R. Pettersen. Effects of surface snow cover on gravimetric observations. J. 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