passive laser sonar

Passive sonar systems have a practical difficulty: you must place many
microphones far apart and run wires to all of them (or have nearby
computers to transmit their signal over e.g. radio.)

An alternative is to use laser vibration-detection systems, such as
those used for remote snooping on speech.  These use Doppler shifts in
laser light reflected from an object (such as a window or wall) to
observe changes in the distance to an object, changes much smaller
than a light wavelength.

A set of such lasers could put many "virtual microphones" hundreds of
meters apart, with a consequent dramatic improvement in spatial
resolution, without having to distribute equipment over a large area.
I suspect that even a single laser beam scanning, say, a wall, could
improve spatial resolution significantly, analogously to synthetic
aperture radar.

Some possible uses; maybe not all of these are plausible:
- scanning three-dimensional human body shapes by their sound
  reflections, for recognition or to find concealed weapons;
- scanning environments on the other side of a wall, by the way they
  shape sound wavefronts that impinge upon that wall;
- measuring macroscopic distances and shapes such as buildings, caves,
  or terrain, for instance for building planning or inspection;
- spatially localizing sound sources in order to filter out background
  noise, for example when recording music;
- identifying, for example, vehicles by sound (having filtered out
  background noise) and measuring their position and velocity;
- characterizing sound wavefronts some distance away in order to have
  time to precompute a canceling waveform for active cancellation;