Backscatter
Backscatter turns up in 2 fields: fiber optics, hydrography. Here is what it means to the people in each one.
Backscatter in fiber optics
The small fraction of light scattered in the fiber that travels back toward the source, which OTDRs measure.
Light scattered by Rayleigh scattering in the glass that happens to be captured by the core heading backward. It is weak, on the order of minus 80 dB relative to a 1 ns pulse, but continuous along the whole fiber, so its slow decline traces the fiber loss. The backscatter level depends on the fiber's composition and mode field diameter, which is why dissimilar fibers produce gainers.
"Backscatter's down in the noise past 60 km at that pulse width, go to a longer pulse to see the far end."
Backscatter in hydrography
The strength of sound returned from the seabed, used to map bottom type alongside depth.
The intensity of the echo returned toward the sonar, logged by multibeam and side-scan systems along with the travel times used for depth. Hard, rough bottoms such as gravel and rock return strong backscatter, soft mud returns weak backscatter, and incidence angle changes the level a great deal. Once corrected for gain, power, beam pattern and grazing angle, it is mosaicked into an image used for habitat maps, sediment classes and spotting debris.
"Backscatter jumps right at the old disposal site boundary, that's coarse dredge spoil over mud."