63 2. Surface geophysical methods 2.2.1 Gravity method Gravity method concerns the study of the gravity field. The variations in gravity over the earth’s surface have become a powerful technique in the investigation of geological structures at various depths. The variations in gravity reflect the inhomogeneous distribution of the densities in the ground. The density of the rocks (Figure 2.6) depends mainly on the rock composition and its porosity. Figure 2.6 Density versus rock type. After D. Chapellier (2001a). The purpose of a relative gravity survey is to directly map the structure of the subsurface. Gravity is the attractive force between two or more bodies of mass. The force, given by Newton’s law, is proportional to the mass m of the object and decreases with distance R: F = G M m/R2 (2.8) G = 6.674×10–11 N·m2·kg2 is the universal gravitational constant, R = 6371 km radius of the earth, Mearth = 5977×10 24 kg. The force exerted on a body at the earth’s surface is due to the attraction of the earth. The gravitational acceleration g (referred as gravity) may be considered as the force exerted by the earth on a unit mass: g = F/m. The SI unit for the gravity, g, is m/s2. In geophysics this unit is referred to as the Gal (in honor of Galileo). One-tenth of a miligal is called a gravity unit (g.u.), which is used more commonly in exploration work. 1 g.u. = 0.1 mGal.
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