NRT MONITORING OF THE 2004 SUBGLACIAL GRÍMSVÖTN ERUPTION (ICELAND) WITH ENVISAT-ASAR DATA

Ulrich Münzer(1), Ágúst Gudmundsson(2), Sandro Martinis(1)

(1) Department of Earth and Environmental Sciences, Section Geology, Ludwig-Maximilians-University, Luisenstraße 37, 80333 Munich (Germany), Email: ulrich.muenzer@iaag.geo.uni-muenchen.de,

s.martinis@iaag.geo.uni-muenchen.de (2) Fjarkönnun ehf, Furugrund 46, 200 Kópavogur (Iceland), Email: fjarkonn@simnet.is

ABSTRACT The aim of the project “Hazard Assessment and Prediction – Long-term Observation of Icelandic Volcanoes and Glaciers Using ENVISAT-ASAR and Other Radar Data“ (ESA, ID 142) is the monitoring of geodynamic processes related to geothermal, seismic and volcanic activity in the highly active Neovolcanic Zone of Southern Iceland. On 1 November 2004 Grímsvötn - a subglacial volcanic system beneath the western part of Vatnajökull ice cap - erupted after a dormant phase of only a few years since its last outbreaks in December 1998 and October 1996. This time, in contrast to the usual eruption characteristics, a glacial torrent started 3 days before the eruptive phase and probably triggered the eruption due to the release of overburden water pressure. ESA arranged a fast access to pre-processed ASAR data, providing images of the Grímsvötn area. The shortest interval was only five hours until an ASAR-IMS product was provided for FTP-download after the overflight of ENVISAT. Recording all possible ASAR swathes, the 2004 eruption period is covered by a data sequence with short acquisition intervals. This gave us the unique chance of NRT observation of a subglacial outbreak over the whole eruption period (1-6 November 2004). All scenes from 24 October 2004 onwards were integrated in our hazard monitoring GIS. Evaluation of the ASAR data showed that 8 days before the outbreak the exact position of the eruption site at the western rim of the caldera could be located through the glacial ice cover. 35 hours prior to the outbreak on 31 October 2004 the extent of the eruption site could be detected in detail. Radar techniques have successfully been applied to monitor the subglacial volcanic eruptions of Gjálp (30 September – 13 October 1996) and Grímsvötn (18-28 December 1998), and have now been confirmed as a suitable method during the new eruption in November 2004.

1. INTRODUCTION The major aim of the ESA-project ID 142 is to install a GIS-based early warning system for volcanic hazards in glaciated areas within the highly active Neovolcanic Zone (NVZ) of Iceland by means of ENVISAT-ASAR data and other data sets (seismic, hydrological, geological, glaciological, meteorological data, etc.) provided by our co-investigators [1]. Additionally, due to various research programs (ESA, AO2 D 116; NASDA, JERS-1/0410) a high number of multi-sensor SAR data as well as an archive of optical satellite data (Landsat MSS/TM, Spot, JERS-1/OPS, ASTER) dating back to 1973 can be used for long-term monitoring. ENVISAT uses a 35 day repeat pass near polar orbit at an altitude of approx. 800 km. Due to its characteristics, the ASAR (Advanced Synthetic Aperture Radar) instrument acquires data unaffected from weather and illumination [2]. Thus radar technology is best suited for continuous monitoring in the light of the Icelandic weather conditions. ASAR scenes of the antenna swath IS 2 (21.5°) corresponding to the incidence angle of the former ERS-1/2 AMI (Active Microwave Instrument) are continuously acquired over our test sites in Iceland (Fig. 1 b, c). Further, we are using continuous acquisition of the ASAR swath IS 5 (incidence angle 37.5°) to minimize terrain induced effects in the images. By using scenes of the swathes IS 2 and IS 5 and of ascending and descending orbits respectively, data are available approx. every nine days over the test sites [3]. This study concentrates on the highly active volcanic area under the western part of Vatnajökull ice cap (Fig. 1 a). In the following the NRT monitoring of the Grímsvötn eruption in autumn 2004 using ENVISAT-ASAR data is described.

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Proc. ‘Envisat Symposium 2007’, Montreux, Switzerland 23–27 April 2007 (ESA SP-636, July 2007)

NVZ

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Figure 1. (a) ENVISAT-ASAR wide swath image (30/12/2006) of Iceland with the Vatnajökull test-site, © ESA 2006; ENVISAT-ASAR acquisition modi IS 2 (b) and IS 5 (c) over the subglacial central volcano Grímsvötn.

2. TEST SITE GRÍMSVÖTN 2004 Vatnajökull, the largest Icelandic ice cap, has an expanse of about 8100 km². According to [4], its average thickness is about 400 m. The thickest ice cover of about 1000 m was found in the area of Bárdarbunga volcano in the northern part of the ice plateau. The NVZ extends beneath western Vatnajökull in a SW-NE direction. As an active spreading segment, it is responsible for the high seismic and volcanic activity. The subglacial central volcanoes Thórdarhyrna, Grímsvötn, Hamarinn, Bárdarbunga, Kverkfjöll and Öraefajökull (Fig. 2 a) are closely connected to the NVZ [4, 5]. Grímsvötn, situated in the centre of western Vatnajökull (64° 23’ N, 17° 23’ W), is among the most active volcanoes in Iceland with about 70 eruptions in historical time and an estimated volume of erupted magma of 29 km³ [5]. The Grímsvötn caldera (62 km²) is covered by 150-250 m of glacier ice. Only the southern part of the caldera rim rises above the ice cover, forming the 5 km long east-west trending ridge Grímsfjall (Fig. 2 b, c). Geothermal activity causes permanent melting at the glacier base leading to the formation of a subglacial water reservoir within the caldera. Reaching a critical level, the water pressure becomes high enough to lift the glacial dam and a subglacial melwater torrent (Icelandic: jökulhlaup)

partly empties the reservoir. The water drains over a distance of 50 km beneath the outlet glacier Skeiðarárjökull to the south, inundating vast areas of Skeiðarársandur [6]. Since the major volcanic eruption in 1934 [7, 8] jökulhlaups occurred at a frequency of 4 to 6 years. However, the Gjálp-eruption in 1996 with a released meltwater volume of about 3.2 km³ caused a changing of the physical conditions within Grímsvötn. Since that time jökulhlaups have become smaller and more frequent (1999, 2000, 2002, 2004, 2005) [9, 10, Sigurdsson 2005, personal communication]. The last subglacial Grímsvötn eruption began on 1

Nov. 2004 at about 22:00 (GMT) and lasted until 6

Nov. 2004. Long- and short-term indicators preceded the eruption. Seismicity, measured by the SIL (South Iceland Lowland) network, increased significantly since mid 2003 [11]. Additionally, GPS measurements registered a constant uplift within the caldera caused by the rising of the subglacial Grímsvötn lake [12] as well as a vertical displacement of Grímsfjall due to magma intrusion [13]. Finally on 28 Oct. 2004 episodes of harmonic tremor indicated the advancing of the jökulhlaup under Skeiðarárjökull [12]. During the jökulhlaup which lasted until 07 Nov. 2004 a total volume of 0.8 km³ was discharged from Grímsvötn caldera. This release of overburden water pressure triggered the phreato-magmatic eruption accompanied by numerous earthquake swarms. After 6 days the eruption ended.

Eruption site 1- 6 Nov. 2004

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Figure 2. (a) ENVISAT-ASAR subscene (wide swath image, 30/12/2006) of Vatnajökull with location of subglacial

central volcanoes Thórdarhyrna (1), Grímsvötn (2), Hamarinn (3), Bárdarbunga (4), Kverkfjöll (5), Godahnúkar (6), Esjufjöll (7), Öraefajökull (8), © ESA 2006; (b) ASTER subscene (27/09/2004) of Grímsvötn caldera, © NASA 2004;

(c) DEM of Vatnajökull. 3. NEAR REAL TIME SAR MONITORING One of the most important developments of the ENVISAT-ASAR instrument compared to the ERS-1/2 AMI is the possibility to shift the antenna into seven different positions (IS 1 – IS 7) with incidence angles ranging from 15° to 42°. This enables the recording of the same region at an average of 2.5 days which is vital for hazard monitoring. As mentioned above SAR data of the swathes IS 2 and IS 5 are continuously acquired over the Icelandic test sites. If an eruption is imminent at Grímsvötn or other test sites all swathes of the ASAR instrument are utilised. Using FTP access to ESA Rolling Archive in Kiruna, Sweden, we were able to receive ASAR data of the test area in western Vatnajökull five hours after data acquisition. Immediately, the data were integrated into the processing chain (Fig. 3) which we apply in our early warning system using the software packages provided by ERDAS Imagine (Leica Geosystems) and RSG (Joanneum Research Center). Thus, for the first time NRT SAR-monitoring could be accomplished with ENVISAT-ASAR data during the Grímsvötn eruption in 2004. Within this process the most important step in data processing is the extraction of amplitude information as well as exact terrain-geocoding to subpixel precision.

The use of corner reflectors in Iceland’s glacier areas is a prerequisite for quick data processing because the searching of Ground Control Points (GCPs) in topographical maps (1:50,000) takes a lot of time. Their special triangular construction as well as their location in areas of small surface roughness effects a high radar backscatter. As ENVISAT and ERS-1/2 move in the same orbit, all 10 reflectors (5 for the ascending and 5 the descending orbit) constructed 1995 and 1997 within the ERS project AO2 D 116 could be used in our test area. The time needed for terrain-geocoding, GIS-integration and data analysis could be reduced to less than 60 minutes which is vital for near real time monitoring. Thus for the first time the period needed for satellite data recording to complete implementation into our hazard monitoring GIS could be reduced to less than six hours (Fig. 3). The evaluation of the ASAR data from western Vatnajökull showed that already on 24 Oct. 2004 (IS 2, desc.), i.e. 8 days before the subaerial volcanic eruption, the exact outbreak position within the Grímsvötn caldera could be determined (Fig. 4). Data interpretation proved that a depression was formed at the ice surface. This was induced by increasing ice melting at the glacier base due to the heat flux of rising magma. Radar data from 31 Oct. 2004 (IS 5, desc.)

highlighted the changes of the glacial surface more in detail. The eruption site of 2004 is located at the western end of Grímsfjall (64° 23’ 12’’ N; 17° 23’ 12’’ W). It is about 2.1 km apart from the eruption site of 1998, situated directly at the southern margin of the ice covered Grímsvötn lake. Shortly intervalled ASAR data showed that the eruption site had hardly shifted during the eruption (1/11/04 – 6/11/04). Its extension of 700 m (east-west) and 600 m (north-south) stayed almost stable. According to ASAR data from 4 Nov. 2004 (IS 5, asc.) the area covered by ashes encompasses 95 km² [14]. The two ash fans were directed NNW (348°) and NNE (23°) due to prevailing winds during the tephra fall. They covered most of Grímsvötn caldera and could be detected up to subglacial volcano Bárdarbunga in northern Vatnajökull over a distance of 18.4 km and 19.6 km respectively (Fig. 5, 6). After the abrupt end of the eruption on 6 Nov. ASAR recording was continued at regular rate.

The ASAR scene dating from 13 Nov. 2004, i.e. 7 days after the end of the eruption, clearly shows the eruption site which in the meantime had filled with meltwater (Fig. 6). The meltwater area was determined to 0.25 km² with an east-west diameter of ~680 m and a north-south diameter of ~450 m. In the ASAR scenes as well as in the oblique aerial photographs the actual vent which was created towards the end of the eruption is clearly visible. Both the almost circular crater hole with a diameter of ~50 m and the little volcanic cone around the vent can exactly be identified in the ASAR scene dating from 13 Nov. 2004. The steep and strongly crevassed slope around the eruption site causes a high backscatter in the radar scene and can therefore clearly be distinguished from the smooth ‘meltwater lake’. Due to rapid data transfer via ESA’s FTP-server (5 hours after the overflight) and quick data processing it was possible to observe the events during the subglacial volcanic eruption in near real time. Actual reports have been given to the authorities in Iceland.

ASAR data acquisition of test site Vatnajökull (west)ASAR data acquisition of

test site Vatnajökull (west)Esa receiving station

Kiruna/SwedenEsa receiving station

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Downlink fromsatellite

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Downlink fromsatellite

Preprocessing

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University of Munich,LMU

University of Munich,LMU

Icelandic Co-I Ágúst Gudmundsson,

Fjarkönnun ehf.

Download of raw data

Import of SAR data

Extraction of amplitudeinformation

GCP file of thecorner reflectors

Improvement of sensorand orbit parameters

DEM integration

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GIS-integration

Data analysis

Download of raw data

Import of SAR data

Extraction of amplitudeinformation

GCP file of thecorner reflectors

Improvement of sensorand orbit parameters

DEM integration

Terrain-geocoding

GIS-integration

Data analysis

Briefing to icl. authorities

~ 5 hours ~ 1 hour

Figure 3. Scheme of NRT - monitoring during the Grímsvötn eruption in November 2004.

Grímsvötncaldera

Eruptionsite

Figure 4. Terrain-geocoded ENVISAT-ASAR subscene (IS 5, desc.) dating from 31 October 2004, 35 hours ahead of the eruption on 1 November 2004, 22:00 GMT, © ESA 2004.

Eruption site (18 – 28 Dec. 1998)

Figure 5. Terrain-geocoded ENVISAT-ASAR subscene (IS 5, desc.) dating from 4 November 2004, during the eruption (1 – 6 Nov. 2004) with the ash-fans in the Grímsvötn caldera. The 3-days old eruption site is marked with a circle, ©

ESA 2004.

Figure 6. Terrain-geocoded ENVISAT-ASAR subscene (IS 6, desc.) dating from 13 November 2004, 7 days after the end of the eruption, © ESA 2004; oblique aerial photographs of the eruption site, dating from 7 December 2004 (lower left

corner, © Gudbjörnsson 2004) and from 4 August 2005 (upper left corner, © Münzer 2005). 4. SUMMARY AND OUTLOOK The former ERS project AO2 D 116 had already shown that radar remote sensing enabled early warning of the subglacial volcanic eruption Gjálp (30/9/ - 13/10/1996) and Grímsvötn (18 - 28/12/1998) [10]. ENVISAT ASAR shows essential advantages when installing an early warning system for subglacial volcanoes. The independence of the emitted microwaves from weather and illumination conditions as well as the short observation intervals due to the 7 recording modi of ASAR are of high importance for continuous monitoring. In autumn 2004 ASAR data were successfully used to predict a new eruption at Grímsvötn. 35 hours before the outbreak on 1 November the exact localisation of the eruption site could have been derived. This is crucial to determine potential routes of the ensuing meltwater torrent by the use of hydrological data in the hazard monitoring GIS. In co-operation with ESA, the time span needed from satellite recording up to data processing and implementation into the hazard monitoring GIS could be reduced to less than six hours during the Grímsvötn eruption in Nov. 2004. Present research aims at automating the SAR processing chain – a prerequisite on the way of pushing the monitoring close to a real time service. Progress was made by an automated detection of the installed

corner reflectors [15]. This avoids the time-consuming manual measurement of ground control points. ACKNOWLEDGEMENT We are grateful to ESA for their generous and fast supply with radar scenes within the ENVISAT project (ID 142) via the ESA Rolling Archive in Kiruna. We would like to thank J. Raggam, W. Hummelbrunner and K. H. Gutjahr at the Joanneum Research Center Graz for special software development. A grant for a doctoral candidate was given by the Bavarian Research Foundation (DPA – 37/04). Thanks are due to the Icelandic Meteorological Office (Vedurstofa) for the supply with seismic SIL-data and to the Icelandic Research Council (RANNIS) for the permission to work in the test sites (No. 1/2005; 5/2005). REFERENCES 1. Münzer, U., Scharrer, K. & Weber-Diefenbach, K.;

Gudmundsson, Á (2005). Integration of ENVISAT-ASAR Data in a Hazard-Monitoring-GIS. ENVISAT Project [ID 142]. In Proc. of the 2004 ENVISAT & ERS Symposium (Eds. H. Lacoste & L. Ouwehand), ESA SP-5726-10, ESA Publications Division, European Space Agency, Noordwijk, The Netherlands.

2. ESA (ed.) (1998). Envisat Mission - Opportunities for Science and Applications. ESA SP-1218, Noordwijk.

3. ESA (ed.) (2002). ASAR Product Handbook, Barcelona.

4. Björnsson, H. & Einarsson, P. (1990). Volcanoes beneath Vatnajökull, Iceland: Evidence from radio echo-sounding, earthquakes and jökulhlaups. Jökull 40, 147-169.

5. Thordarson, T. and G. Larsen (2007). Volcanism in Iceland in historical time: Volcano types, eruption styles and eruptive history, Journal of Geodynamics 43, 118-152.

6. Gudmundsson, M.T. & Björnsson, H. (1991). Eruptions in Grímsvötn, Vatnajökull Iceland 1934-1991. Jökull 41, 21-45.

7. Schmid-Tannwald, K. (1954). Der untereisische Ausbruch des Vulkanes Grímsvötn 1934 im Vatnajökull auf Island. Naturwiss. Monatszeitschrift des deutschen Naturkunde-vereins 7(8), 153-159.

8. Thórarínsson, S. (1953): The Grímsvötn Expedition June-July 1953. Jökull 3, 6-22.

9. Snorrason, Á., Jónsson, P., Pálsson, S., Árnason, S., Sigurdsson, O., Víkingsson, S., Sigurdsson, Á. & Zóphoniasson, S. (1997). Hlaupið á Skeiðarásandi Haustið 1996. Útbreiðsla, Rennsli og Aurburður. In Vatnajökull. Gos og Hlaup 1996 (Eds. Vegagerdin), Reykjavík, Iceland, pp79-137.

10. Münzer, U., Bahr, T. & Weber-Diefenbach, K. (2000). Katastrophen-Monitoring am Beispiel Islands. Schlußbericht Förderkennzeichen 50 EE 9706, 62 p., Munich.

11. Hjaltadóttir S., Geirsson H. & Skaftadóttir, Þ. (2005): Seismic activity in Iceland during 2004. Jökull 55, 107-119.

12. Vogfjörd, K.S., et al. (2005). Forecasting and monitoring a subglacial eruption in Iceland, Eos Trans. AGU, 86(26), 245-248.

13. Sturkell, E., Einarsson, P., Sigmundsson, F., Geirsson, H., Olafsson, H., Pedersen, R., de Zeeuw-van Dalfsen, E., Linde, A. L., Sacks, I. S. & Stefansson, R. 2005. Volcano geodesy and magma dynamics in Iceland. J. Volc. Geotherm. Res. 150(1-3), 14-34.

14. Scharrer, K., Mayer, C., Nagler, T., Münzer, U. & Gudmundsson, Á. (2007). Effects of ash-layers of the 2004 Grímsvötn eruption on SAR backscatter in the accumulation area of Vatnajökull. Annals of Glaciology 45, (in press).

15. Gutjahr, K., Scharrer, K. & Münzer, U. (2006). Utilizing the CR-Network in Iceland for an automated interferometric processing chain – case study with ERS-Tandem data. In Proc. of Fringe 2005 Workshop (Ed. H. Lacoste), ESA SP-610 (CD-ROM), ESA Publications Division, European Space Agency, Noordwijk, The Netherlands.