GNGTS 2015 - Atti del 34° Convegno Nazionale

Method and data acquisition. Gravity prospecting is a geophysical method used to infer the subsurface density distribution measuring changes, of the order of a few parts per million or lower, in the Earth’s gravitational field caused by densities’ lateral variations. Agravity survey was carried out on a rectangular area (6 x 28 km) that runs between Traghetto (near Molinella) and Formignana, in the eastern Ferrara Province (Fig. 1b); ��������� �� ����� according to local topography and roads’ pattern, the average spacing between contiguous measurement points was roughly 800 m. �� ����� ��� ��� ������� ������ � ��������������� ���� �� �������� To carry out the gravity survey a LaCoste&Romberg mod. D, equipped with a ZLS feedback, whose range is about 10 mGal, was used and a whole of 274 stations, as much as homogeneously distributed, were acquired. Since the gravity meters measure gravity differences from place to place, a First Order Gravity Net (FOGN) has been established in the surveyed area; and to define the gravity datum, 2 stations belonging to FOGN have been linked to an eccentric point of the absolute station situated at the Radio Astronomical Station of Medicina (Bo) (Cerruti et al. , 1992). The FOGN gravity data have been adjusted by means of the least square method after removing the instrumental drift whose systematic variations, which are common for all measured stations, have been modelled by means of a third order polynomial curve. The rms of the FOGN gravity station values has been ±0.0025 mGal. Starting from FOGN’s stations, 24 gravity loops have been organized according to a sequence data acquisition allowing the instrumental drift checking and loops’s closure error detection. The loops’ gravity data have been adjusted in two steps: firstly, the instrumental drift has been linearly distributed, in function of the time, among the loop’s stations, then the error closure has been distributed in function of the number of ties of the loop. The instrumental drift spans between -0.004 and 0.003 mGal/h and the closure errors between -0.003 and 0.005 mGal. The elevation and position of the measuring points were estimated from the CTR of the Regione Emilia Romagna at scale 1:5000; the estimated altimetric and planimetric errors were ±0.15 m and ±5.00 m, respectively. The observed gravity values, g obs , are then compared with the theoretical values g th , computed on the a homogeneous earth at the ellipsoid surface at the coordinates of the measuring point, corrected also for the height, Faye (CF) and Bouguer (CB) corrections, and terrain effects (CT). Therefore, the so called Bouguer anomaly (g geol ) could be written as: g geol g obs – (g th – CF CB) CT. It means that the Bouguer anomaly is the difference between the gravity acceleration measured, on the true Earth, and the theoretical gravity acceleration at the same point computed on a homogeneous Earth, whose density could change vertically but not horizontally. Therefore, the pattern of the Bouguer anomalies reflect horizontal variations of densities inside the Earth. In the data processing, the theoretical gravity values (g th ) were computed according to the GRS80 ellipsoid formula (Moritz, 1980); the Faye correction (CF), according to the GRS80 ellipsoid and considering also the second order term of the height (Hinze et al. , 2005); the Bouguer correction (CB) considering also the Earth’s curvature, i.e. Bouguer slab and Bullard B term, which reduces the infinite Bouguer slab to that of a spherical cap, (LaFehr, 1991a, 1991b). The terrain effects (CT) were computed in two different steps: from 0.001 up to 0.010 km by means of the sloping wedge technique (Olivier et al. , 1981; Barrows et al. , 1991), due to the mostly flat surveyed area this technique was applied only in three points; from 0.010 to 15,000 km by means of an algorithm based on vertical right parallelepiped (Banerjee et al. , 1977), in particular from 0.010 to 1,000 km it was utilized the Digital Elevation Model of the Regione Emilia-Romagna, opportunely resampled with a grid resolution of 0.005 km, instead of the original 0.010 km; and from 1,000 to 15,000 km the SRTM database resampled with a grid resolution of 0.050 km, instead of the original 3”x3”, roughly 0.090x0.065 km. Starting from 10 km it was also considered the Earth’s curvature; it is worth notice that the spherical approach in CT calculation introduces negative contribution due the masses located above the GNGTS 2015 S essione 1.2 129