GNGTS 2015 - Atti del 34° Convegno Nazionale

The horizontal gravity gradient map, whose purpose is to delineate the location of lateral discontinuities corresponding to the maxima gravity gradient, shows a strong horizontal gradient (roughly 5 mGal km-1) whose maxima is in the area between Ospital Monacale/ Consandolo and Molinella, whose geological meaning is briefly explained below. The vertical gravity gradient map, whose aim is to sharpen anomalies, roughly shows the same features of the least-square residual anomaly separation that will be described below. To enhance the shallowest bodies whose presence affects the data, a residual map has been obtained by means of least-square residual anomaly separation where the regional field has been fitted by means of a second-order surface (Agocs, 1951). Gravity anomalies and data modelling. In Fig. 2a, the negative Bouguer anomalies span between -66 and -23 mGal, steeply decreasing northwards from a minimum near Molinella to the central part (Voghiera), then the anomaly pattern rotates towards NW. The profile shown in Fig. 2b represents the Bouguer anomaly along white trace in map, which corresponds to the same profile investigated in the companion paper (see Mantovani et al. , 2015). The residual anomaly map, i.e. the difference between the Bouguer anomaly map and the best fit surface, is shown in Fig. 2c, where it is possible to observe a sequence of minima and maxima (see also the profile in Fig. 2d). From SSW to NNE, a strong maximum is located below the villages of Ospital Monacale and Consandolo ( 3.4 mGal), while two less strong maxima appear between Voghiera and Masi Torello ( 2.1 mGal) and below Formignana ( 2.9 mGal) The minima are located to the north of Molinella (-4.0 mGal), between Quartiere and Gambulaga (-1.5 mGal) and between Masi Torello and Sabbioncello San Vittore (-1.5 mGal). Both Bouguer and residual anomaly profiles, shown in the Figs. 2b and 2d were modelled in order to reconstruct the density distribution in the subsurface, but here we shown only the reconstructued density model distribution (Fig. 3b) based on the residual anomaly (Fig. 3a). The Talwani’s et al. (1959) method, further developed by Won et al. (1987) was used and a “trial- and-error” approach was applied, bearing in mind the subsurface structure known by available geological information, like seismic reflection profiles and wells for hydrocarbon exploration (Boccaletti et al. , 2004; Fantoni and Franciosi, 2008, 2010). In particular, �� ��������� ��� we correlate the southern residual positive anomaly of the gravity field to the Argenta anticline, the northern one to the Ferrara anticline and the anomaly in the middle of the investigated profile to a secondary structure, here referred as the Monestirolo structure. Fig. 3 – a) Observed residual anomaly along the investigated profile (blue dots) and modelled (red line). b) Model of the density distribution in the subsurface, up to ca. 2 km, obtained by the interpretation of the observed residual anomaly. GNGTS 2015 S essione 1.2 131