Motion of sea waves through it may look random it is actually a combination of number of SHMs of different frequency & amplitude.
These waves have been studied characterized by their power spectnal density (PSD) as a function of frequency.
The PSD is nothing but square of max acceleration divided by a small frequency sample in which acceleration is assumed to be constant.
This is a meat means of representing such seemingly random phenomenon.
This technique is used to describe wind velocities which are gusty, the vibrations experienced in a motor car, aeroplane, etc.
