GNGTS 2014 - Atti del 33° Convegno Nazionale

116 GNGTS 2014 S essione 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 seismicmethods, 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., Boaga et al. , 2011). In Fig. 2 the seisgun and one of the weight drops are shown (a, b), together with a map of the seismic survey pattern, Fig. 2 – a) Seisgun acquisition; b) one of the weight drops used for the surface wave experiments, suspended at the tripod; c) map of the acquired seismic lines (orange), the snowstreamer (orange-yellow lines) and of the shots positions, overimposed to an orthophoto of the Inerhytta pingo (from toposvalbard.npolar.no) .