An apparatus and a method for measuring the 
gravitational acceleration on the ground, for example adapted to geophysical explorations applied to mining, comprising a base frame (31), a 
resonator (10) having a first disc of 
sapphire (2) integral to said base frame (31), an a second disc of 
sapphire (1) arranged at a 
minimum distance from said first 
mass (2) and aligned vertically with respect to said first 
mass (2) such that said first and said second disc (1 and 2) form a 
resonator for electromagnetic signals (10); a 
cantilever arm (41) connected to said base frame (31) and holding resiliently the second disc (1) in order to provide a steady 
elastic modulus; a 
magnetic probe (11) for applying an 
electromagnetic signal to said 
resonator (10) arranged according to a plane existing between the two discs (1 and 2); a linear 
electric probe (12), arranged under the fixed disc (2) aligned vertically with respect to its side edge, for measuring a 
resonance frequency of said resonator responsive to said 
electromagnetic signal, means for correlating the variation of said 
resonance frequency to a variation of distance between said discs (1 and 2), and means for correlating the variation of distance to local 
gravitational acceleration. The support (7) has 
variable length for adjusting the unexcited distance d between the discs (1 and 2). Preferably, a first (10) and a second resonator are provided as above defined coupled and mounted on two respective base frames integral to each other for measuring the 
gravitational acceleration gradient in a vertical direction. Furthermore, the resonators are contained in a closed box in which vacuum conditions are created.