An apparatus and a method for measuring the gravitational acceleration on the ground, for example adapted to geophysical explorations applied to mining, including a base frame, a resonator having a first disc of sapphire integral to the base frame, and a second disc of sapphire arranged at a minimum distance from the first mass and aligned vertically with respect to the first mass such that the first and the second disc form a resonator for electromagnetic signals; a cantilever arm connected to the base frame and holding resiliently the second disc in order to provide a steady elastic modulus; a magnetic probe for applying an electromagnetic signal to the resonator arranged according to a plane existing between the two discs; a linear electric probe, arranged under the fixed disc aligned vertically with respect to its side edge, for measuring a resonance frequency of the resonator responsive to the electromagnetic signal, device for correlating the variation of the resonance frequency to a variation of distance between the discs, and a device for correlating the variation of distance to local gravitational acceleration.