[1] [2] In the study of supersonic flight, Schlieren photography observes the density gradient of air as it interacts with aircraft.
[4] In the life sciences, a special technique called density gradient separation is used for isolating and purifying cells, viruses and subcellular particles.
A blood donation technique called Pheresis involves density gradient separation.
The existence of a fast breeder nuclear reactor system at the core of the earth is one theory by reason of density gradient.
Additional information is to be found in the frequencies of the graver normal modes of the Earth which are sensitive to density through self-gravitation effects induced in deformation".