Remote seismic sensing

Methods and apparatus are provided for remotely sensing motions of the earth from the Doppler shift of reflected electromagnetic waves. The apparatus is based on a heterodyne continuous wave or pulsed laser system. The airborne laser system described herein is intended to remotely sense seismic motion. Motion of the earth's surface, when coupled to a reflector, causes a continuously shifting frequency (Doppler) of the reflected sensing laser beam, the Doppler frequency being proportional to the particle velocity of the ground. The method uses electromagnetic waves reflected from reflectors, some of which are coupled to the earth's surface. Similarly, "inertial" reflective surfaces (i.e., vibrationally isolated from any earth motions) are also located at the surface of the earth and employed by the apparatus and methods of the present invention. Both an inertial reflector and an earth-coupled reflector are positioned at each location for which seismic signals are desired.