A Seismic Survey at Adventdalen, Svalbard Islands, (Norway), for Permafrost Studies: the IMPERVIA Project G

GNGTS 2014 SESSIONE 3.1 A SEISMIC survey at ADVENTDALEN, Svalbard ISLANDS, (Norway), FOR PERMAFROST STUDIES: THE IMPERVIA PROJECT G. Rossi1, F. Accaino1, J. Boaga2, L. Petronio1, R. Romeo1, W. Wheeler3 1 OGS - Istituto Nazionale di Oceanografia e di Geofisica Sperimentale, Trieste, Italy 2 Dipartimento di Geoscienze, Università di Padova, Italy 3 Centre for Petroleum Research (CIPR), University of Bergen, Norway Introduction. Climate warming and permafrost thawing would allow the release into the atmosphere of any greenhouse gasses trapped beneath. Research to date has focussed mainly on the upper fifteen meters of the permafrost as this reacts most rapidly to changes in air temperature. Little focus is given to the deeper permafrost, which may be a good long-term climate indicator. Knowledge of the fluids (waters or gases) present within and beneath the permafrost allows evaluation of the impact of atmospheric release upon thawing. Svalbard archipelago is an ideal natural peri-arctic laboratory for such a kind of studies. The fluid circulation and permafrost characteristics are constrained by a series of pingos (periglacial mounds of Earth-covered ice), shallow (< 50 m) and a few deep (to 970 m) wells. Near Longyearbyen, in the Adventdalen (Advent Valley), where the Longyearbyen CO2Lab is located (Braathen et al., 2012), deep-target 2D reflection and borehole seismic data are available (Oye et al., 2013). However, no studies targeted full-thickness permafrost characterization, or determining its relation with regional hydrology are available. These considerations motivated the present study, done within the PNRA project IMPERVIA - Integrated Methods to study PERmafrost characteristics and Variations In an Arctic natural laboratory (Svalbard). The project is led by OGS (Istituto Nazionale di Oceanografia e di Geofisica Sperimentale, Trieste, Italy), in collaboration with CIPR (��Centre for Petroleum Research, Bergen University, Norway), UNIS (University Centre in Svalbard, Longyearbyen, Norway), and the Department of Geosciences, University of Padua, Italy.�� The aim is to combine exploration geophysical tools (2D and 3D) to image and characterize the mid- to lower permafrost, to determine the aquifer architecture (bedrock and fluvio-glacial deposits). Such information can be useful to state permafrost capability of acting as additional cap-rock to future injected CO2. Since the fluid flow from pingos indicates significant fluid circulation 114 001-258 volume 3 114 24-10-2014 16:45:09 GNGTS 2014 SESSIONE 3.1 Fig. 1 – Map of Adventdalen in Svalbard Archipelago, with location of Innerhytta Pingo (red ellipse), and the existing seismic lines (black lines) and boreholes (blue and red circles). Above: a view of the Innerhytta pingo. which likely affects permafrost thickness even away from the pingos, the experiment is done in the vicinity of the Innerhytta (or Innerhytte) Pingo, a well-known giant pingo (Fig. 1). The study area. Pingos are oval dome-shaped hills which form in permafrost areas when the hydrostatic pressure of freezing groundwater causes the raising of frozen ground. They can reach even 90 m altitude and over 800 m of diameter. In open system pingos, artesian pressure builds up under the permafrost layer, and as the water rises and pushes up the overlying material, it freezes in a lens shape. Innerhytta (Inner hut) pingo is located about 16 km east of Longyearbyen (Spitzbergen, Svalbard Islands) in the valley bottom in Adventdalen, a valley about E-W oriented, ending into Adventfjorden, nearby Longyearbyen, on the Spitzbergen Island of Svalbard Archipelago (Fig. 1). It has been first studied in detail by Piper and Porritt (1966), who produced a topographical map. The pingo is 410 m wide in E-W direction and 200 m in N-S direction. The height above the valley floor is 28 m, above the recognized maximum Holocene marine limit (Ross et al., 2005). It is developed within, and uplifts, Jurassic shales of the Agardhfjellet formation (Major et al., 2000). A mineralized spring generates a noticeable icing covering the pingo summit and southern flank in the late winter, before the active-layer summer thawing (Fig. 1). In the area, both GPR and resistivity survey were done (Ross et al., 2005; 2007), and continuous temperature monitoring is on-going (Christiansen et al., 2010). Ross et al. (2005), from GPR measurements, hypothesized that a southwards migration of the water uprising occurred, so that the actual pingo activity is limited to the area around its apex. According to them, furthermore, the NE part of the structure would be an erosional remnant of bedrock due to the fluvial incision of the overlying relief of Janssonhaugen. The results of the successive resistivity tomographic experiment, however, were not resolutive. They were, in fact, compatible both with the hypothesis of the existence of a ground-ice body at depth (Piper and Porritt, 1966), as well as with the bedrock remnant, without massive ice-body (Ross et al., 2007). The 11 001-258 volume 3 115 24-10-2014 16:45:14 GNGTS 2014 SESSIONE 3.1 TSP NORWAY IPY Innerhytta borehole stratigraphy provided information on the presence of ice and of its characteristics in the first 20 meters below the topographic surface (Juliussen et al., 2010), not sufficient to clarify the doubts about the deeper internal structure. IMPERVIA project experiment, hence, was designed to try to add seismic information to the other existing ones, so to contribute to distinguish between massive-ice and iced-bedrock response, by applying near-surface seismic methods, surface wave information, and tomographic inversion of direct, reflected and refracted arrivals. The combination of different arrivals in a tomographic approach can be successful in overcoming the difficulties of building a near- surface velocity model in permafrost regions, due to near surface high velocity, strong lateral velocity variations and negative velocity gradients present in these regions. The spring 2014 seismic experiment. The experiment of spring 2014 was aimed to verify the capability of a low-environmental impact(?) near-surface seismic survey in the arctic environment to image the permafrost architecture and characteristics, completing the available information from GPR and electric resistivity measurements. The Svalbard Environmental Protection Act, aimed to safeguard virtually untouched area in Svalbard, regulate the research activities in this delicate environment. In particular, authorities strongly discourage research activities that may set marks in the terrain. Geophysical surveys, therefore, are generally done in late-winter to early-spring times, when work can be carried out on snowy, frozen ground, limiting the damages. IMPERVIA survey took place between April 29 and May 6, 2014. Apart the above mentioned environmental protection issues, this period was chosen since the use of sledges and snowmobiles facilitated equipment transport from Longyearbyen, reducing logistical efforts and costs. However, in order to ensure a good geophone and sources ground coupling, we ought to dig snow pits through the snow coverage, which on the leeward side of pingos reached the two meters. The seismic equipment was composed of conventional vertical geophones, 4.5 Hz and 14 Hz, and a 24 channel gimballed 14 Hz mini snow-streamer, 115 m long with 5 m takeout spacing. Five 24 channel seismographs (Geode, Geometrics) enabled the recording of the signals. To reduce the possible noise due to the frequent strong wind, we buried the geophones and covered with snow the snow- streamer sensors and connecting cables. Also for the choice of the sources, the low-impact on the environment is an issue. We used sledge hammer on a steel plate, seisgun with 12-gauge shotgun shell, firecrackers in shallow drilled boreholes. Firecrackers are allowed as self-protection devices against polar bears. The list was completed by two different weight drops, falling from a tripod from an approximate height of 2.5 m. The latter source was aimed to provide low-frequency source to record surface waves, to complement the higher frequency 2D lines (e.g., Fig. 2 – a) Seisgun acquisition; b) one of the weight drops used for et al. the surface wave experiments, suspended at the tripod; c) map of the Boaga , 2011). In Fig. 2 the acquired seismic lines (orange), the snowstreamer (orange-yellow seisgun and one of the weight drops lines) and of the shots positions, overimposed to an orthophoto of are shown (a, b), together with a the Inerhytta pingo (from toposvalbard.npolar.no). map of the seismic survey pattern, 11 001-258 volume 3 116 24-10-2014 16:45:20 GNGTS 2014 SESSIONE 3.1 overlapped on an orthophoto of Innerhytta pingo (c). The source and the receiver points were positioned with DGPS. We acquired three lines for reflection/refraction seismic in a tomographic inversion perspective. We used five 24-channels Geode seismographs, sampling rate 0.250 ms, for a total recording window of 2 s. The vertical 14 Hz geophones were listening, together with the snow streamer. Seisgun, sledgehammer and firecrackers were used as sources. To increase the coverage of the tomographic experiment, while acquiring the 2D lines, we recorded also cross- line, using both the snow-streamer and the geophones, or shooting offline. The design of the seismic lines was done taking into account the needs of illuminating the different parts of the pingo area, reducing the possible effects of the rough topography and optimizing acquisition costs

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