subsurface waves in an ocean are created by the turbulence in a submarine &# 39 ; s wake . these waves can be remotely detected by a search submarine by monitoring subsurface water temperatures using a laser . a pulsed laser beam is directed into the water to at least the depth of the thermocline and an analysis is made of the resultant brillouin and rayleigh backscatter components . wavelength shifted brillouin scatter is mixed with the unshifted rayleigh scatter in a self - heterodyne manner for each volume element of illuminated water , and the frequency of the heterodyne signal is measured and converted into equivalent temperature values . this produces the desired temperature - depth profile of the water enabling detection of the first submarine by tracking the internal waves at or near the ocean thermocline .