A gravimeter, a gravimeter system, and a method for measuring gravitational acceleration within a borehole are described herein. The gravimeter includes a proof mass that is constrained by springs and an optical interferometer for measuring displacement of the proof mass. The optical interferometer generates a light path from a light source to a reflective surface on the proof mass. Spatial displacement of the proof mass from a reference position to a position of gravitational equilibrium is determined by measuring a change in length of the light path. In turn, gravitational acceleration can be determined from the spatial displacement of the proof mass. A number of such gravimeters can be used in a gravimeter system to make measurements of gravitational acceleration in variety of different directions.