Abstract: An interferometric geophone includes a freely-falling reference mass and an interferometric arrangement to measure motion of a surface to which the geophone is attached. The mass may fall through an evacuated chamber. A carriage arrangement may reposition the reference mass for another fall. The reference mass has top and bottom retro-reflectors. The interferometric arrangement includes a laser and a beam-splitter that divides the laser beam into two beams directed toward the top and bottom of the reference mass by partially reflecting surfaces on prisms or mirrors. When the beam encounters the first face of each prism or a first mirror it is split again. One of the split beams is directed toward the falling retro-reflector; the other beam passes through the prism or mirror and encounters the second face. The retro-reflector returns the first split-beam to the second face via a lens arrangement.