Detection of clandestine tunnels and underground facilities is of continuing interest to DoD and Customs and Border Protection. Ongoing research to find functional and reliable sensing methods include seismic methods, since dynamic activity in tunnels emits mechanical energy that propagates in seismic waves. Resulting ground vibrations can be measured at offset distances and these signals can be used in sensing algorithms for detection, location, and discrimination of the activity. Current detection methods rely largely on emplaced sensors, such as geophones. Space-based or airborne surveillance can drastically improve standoff and surveillance coverage. To address this critical need, IAI and its collaborator, University of Maryland (UMD), have been awarded a new contract entitled, “Standoff Coherent Optical Detection of Acoustic Signals (SCODAS).” A vibrometry technique will be developed for long-range detection of very small vibrations induced in passive objects exposed to acoustic disturbances. SCODAS will leverage the vibrometry experience of UMD, and signal processing and hardware expertise of IAI. A powerful, eye-safe, infrared laser is aimed at the ground or an airborne target from an airborne platform or satellite. The possible vibrations of an object causes scattered or reflected light from the laser to return to the transmitter with a superimposed phase modulation. SCODAS measures the amplitude and phase change of the probe beam simultaneously, and the measured phase will directly translate into displacement of the sensed area or object. The distinctive frequency spectrum and other signal features will be exploited for both interference due to turbulence, and the target signatures of vehicles and underground activities. The standoff distance will be extended by trading resolution for SNR, and the effects of atmosphere induced phase fluctuations will be corrected and compensated for using a reference beam.