GNGTS 2013 - Atti del 32° Convegno Nazionale

Remote sensing approach to characterize the snow wall. Measurements of radiating temperature (thermal infrared), reflectivity in the intervals of visible and near infrared and of light polarization have been performed on the vertical wall of trench, roughly 1.8 m high, facing north (on March) and, respectively, south (on May). The measurements were performed immediately after the execution of the trenches to limit as much as possible the alteration caused by excavation and by the exposure to the external environment. The field of radiating temperature has been acquired by means of a thermal camera operating in the 8÷12 μm range while a digital photocamera - provided by polarizing rotating filter - has been used for the visible (0.4÷0.7 μm) and near infrared (0.7÷0.8 μm) bands and to show the polarizing property of the snow. The thermal camera has gathered data on the radiating temperature field, function of contact temperature and of surface emissivity (ratio between the actual radiance and the one emitted at the same temperature by the ideal black body). The snow colour has also been analysed, comparing the reflectance in the region of green (high transparency) with the one in the near infrared (very low transparency). For the analysis of polarization, the polarizing property of snow has been described taking into account the intensity of the reflectance in the interval of red (0.6÷0.7 μm) for different positions of a polarizing filter put in front of the photocamera. Starting from an arbitrary initial orientation the filter was rotated by steps, 15° each, till reaching the final 135° angle. From the frames acquired, the value of maximum and minimum intensity ( max and min ) have been extracted and compared. The zones where max - min = 0 have isotropous behaviour while where max – min ≠ 0 polarization occurs. For such a reason, in the polarization analysis, was selected areas where max - min ≠ 0, corresponding to anomalous response. On the snow surface facing North, in absence of direct sun illumination (early morning), the vertical thermal profile puts in evidence an anomalous variation around the depth of 45 cm which can be explained as the resulting seepage of “greenhouse effect”. The snow cover partially transparent to sunlight radiation at some extent, is completely opaque to the re-emitted thermal infrared radiation, thus some heat remains trapped inside the snow mass with the consequence of generating a clear discontinuity around a depth of 45 cm. This effect, highlighted by the thermal profile, is confirmed by analysis of the polarization of the light too (Fig. 2). On the snow wall facing South, with direct sun illumination, data within the bands of blue, green, red and near infrared have been collected with selective filters applied on a digital open spectrum photocamera (Sony, provided by “night shot” configuration and Kodak Wratten 87 filter for visible-light rejection). The analysis of the polarization of the solar light impinging on the wall facing South confirms the presence of a discontinuity around the depth of 40 cm. The temperature trend and the polarization behaviour seem to be important for the analysis of the first package of snow, which is interesting for risk assessment. The shooting in the visible have provided data on the reflectivity in the regions of the red, green and blue, while polarization analysis gives information on the state of aggregation of the snow. Further information can be obtained from the multispectral analysis; on the wall facing South the profile of reflectance in near infrared (high opacity) contrasts with that in the green (high transparency). The operating conditions of detection of the walls facing north and south were, of course, different. The wall facing North was minimally influenced by the exposure to the external environment, at the same time, however, it was impossible to collect useful data in the near infrared for the lack of direct solar radiation. The wall facing South was strongly affected by the exposure to the external environment due to the intense radiation of the sun, that determined an external water film, altering the thermal infrared emissivity value. On the other side, the sun light allowed the acquisition of near infrared images, visible and polarization with an excellent signal to noise ratio. Electrical and seismic prospecting to characterize the first subsoil. Measurements of electrical and seismic data, in passive and active mode, were made on March along a transect crossing the frozen and snowy Monticello Lake, including banks, in order to verify if the 213 GNGTS 2013 S essione 3.3