GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 3.1 85 by the use of compressional waves. In this regards, SH-waves can be used to overcome this limitation (Deidda and Balia, 2001; Guy, 2006; Pugin et al. , 2006), but they require a specifically- designed energy source for waves generation, ��������� ��������� ���������� ���������� geophones measuring horizontal components of particles motion ��� �� �������� ������ �� ����������� ����������� �� ��� ��������� ��� �� and an accurate choice of acquisition parameters. On the contrary, due to attenuation, the depth of investigation for SH-waves can be lower than for P-waves (Pugin et al. , 2006). Therefore the geological understanding of a mass movement can take advantage of a combined use of both these geophysical methodologies. This is the case of the Patigno landslide (Federici et al. , 2000), a great landslide located in the upper basin of Magra River, in the Northern Appennines, Italy (Fig. 1), where a P-wave study carried out in the last years (Stucchi et al. , 2014) was able to image the deepest discontinuity of the landslide body at around 40-50 m depth, but no description of the shallower layers can be inferred. Because these surface layers are the slip surfaces of quick reactivation movements of the landslide, an SH high-resolution reflection seismic survey was planned along the previous P-wave profile (Fig. 1). This new survey associated to the P-wave investigation allows a more robust description of the landslide body, from the deepest discontinuity up to the very shallow portions of the landslide. This work describes the planning, acquisition and processing of the SH reflection seismic survey, and also gives a possible combined interpretation of both P and SH seismic images. Acquisition and processing. The SH seismic survey was acquired using 48 10-Hz horizontal component geophones and a source specifically designed and built for this work, which consisted of a swinging weight of 16 kg striking a baseplate firmly anchored on the ground. The total length of the seismic line was 85 m in NW-SE direction, overlapping part of the previously acquired P-waves seismic profile (Fig. 1). To obtain an accurate data record it is important to pay attention at the correct positioning of equipment because to generate SH wave is required to energize the ground in transversal direction respect to line acquisition direction; ���� ��� ����������� ����������� �� ��� ��������� also the appropriate positioning of the geophones along the seismic line is of fundamental importance; in fact, S-waves generated by the source are divided into a vertical (SV) and a horizontal component (SH). In order to record only the SH waves is therefore necessary to exclude from registration SV waves, positioning geophones with the direction of oscillation perpendicular to the acquisition line. The eventuality to also record events related to P converted waves is at least theoretically excluded a priori, since the geophones do not records vertical components. A geometry acquisition was planned to obtain the best advantage from the survey, with the purpose to better manage the equipment supplied in function of the complex geological context to investigate. ���� ��� ��� �� ��������� With the aim of obtaining a sufficiently high fold coverage and a good lateral continuity as required in a landslide context, receiver interval was set at 0.75 m and source interval at 1.5 m. Fig. 1 – a) Location of the P-wave reflection seismic line R1 (from Stucchi et al. , 2014 ) with overlapped the SH-wave reflection seismic profile in yellow. S 3 and S 4 : boreholes; Q 1 : ERT line; P 5 and P 6 refraction seismic lines; b) close-up showing the SH profile.