GNGTS 2013 - Atti del 32° Convegno Nazionale

analysis on seismic sections will also provide information about the interaction between the sedimentation rate and the glacier evolution. Alluvial fan environments, often present significant challenges for high-resolution seismic exploration. The main factors hindering seismic imaging are: 1) strong lateral velocity contrasts in heterogeneous sediments of the shallow portion of subsurface; 2) unfavorable topographic conditions along the profile and 3) presence of dipping reflectors representing the bedrock. A dense wide-aperture acquisition geometry, allowing nonstandard processing and a meaningful interaction and comparison between refraction and reflection data has been proven capable of overcome most of the above-mentioned limiting factors (Improta and Bruno, 2007; Bruno et al. , 2010, 2011, 2012). A dense wide-aperture acquisition geometry differs from typical common-midpoint small aperture reflection because it allows recording both multi-fold reflection data spanning a large range of offsets (from small-offset near-vertical reflections to large-offset large amplitude post-critical reflections) and deep penetrating refracted waves, which are suitable for first-arrival travel-time tomography. Tomography not only contributes information about the subsurface structure but also provides a good control on the near-surface velocity structure that is crucial for improving the quality of static corrections and, ultimately, the stacking of shallow reflections. Local setting. Val Venosta is a major glacial trough of the east-central Italian Alps. Its lower part trends west-east for 42 km, decreasing its main altitude from 900 to 500 m above sea level moving toward East end above Meran and it is obstructed by a large fan complex into the Adige gorge. At its West end, the Upper Val Venosta ascends north into the lowest gap in the main Alpine divide west of Brenner, the Reschen Pass (1500 m). The terrain character falls into three elevation ranges, reflecting Quaternary glacial impact over an orogenic landscape that had become generally ‘mature’ and well adjusted to late Tertiary base levels. Below 2400 m altitude, steep slopes fall to the Venosta glacial trough, without high rock-walls; tributary valleys are more V-shaped than U-shaped because of gravitational and fluvial overprinting. The geology is predominantly represented by Austro-Alpine metamorphic units (Agliardi et al. , 2009b), comprising metapelites and metapsammites, with subordinate orthogneiss, metabasites and calcschists. slices of sedimentary Upper Palaeozoic-Mesozoic cover occur around the Upper Val Venosta. The entire stack is cut by north, east-, NE-and SW-trending fractures, strongly constraining the drainage pattern and the strength of rock masses. (Jarman et al. , 2011) The valley bottom hosts the biggest group of anomalously large fans within the European Alps, the origin of which is still matter of debate. If on one hand Jarman et al. (2011) hypothesize a catastrophic formation of the fans modulated by large, rapid slope failures, on the other hand, a mechanism of paraglacial progradation, mainly mediated by debris flows, represents the alternate theory. The latter interpretation would be sustained by the high contemporary rates of debris-flow transport recorded in basins covered by glacial and glacio-fluvial deposits across the entire AltoAdige/Sudtirol region (Brardinoni et al. , 2012). Data acquisition and processing. We acquired four seismic profiles, with a total profile length of 4 km. Three profiles (Lasa_1, Lasa_2, Lasa_3) are longitudinal to the valley, and one profile (Lasa_4) cuts the alluvial fan as a transverse section, crossing profile Lasa_1 (Fig. 1). The logistic of line acquisition was very hard on profiles Lasa_2 and Lasa_3, because these profiles come across the residential area of the village of Laas. P-wave seismic reflection data were collected using a single 6382-kg IVI-Minivib® truck. At each vibration point we stacked 2, 15 s long, 10–200 Hz sweeps generating a 1s correlated record with 1ms sample interval. Source move-up was 5 m. Single 10-Hz vertical geophones were also placed at 5 m intervals. Tight spacing of both geophone and vibration points ensured a very regular and dense subsurface coverage, with common midpoints spaced of only 2.5 m. The acquisition parameters were expressly designed to keep maximum data redundancy. The identification of the optimal processing sequence depends on the quality and characteristics of the data recorded. The reflection processingwas performed through Landmark 52 GNGTS 2013 S essione 3.1