Physical meaning: the self-correlation function and the energy spectrum density function compose a Fourier transform pair.

By using the Fourier transform, the problem can be solved with a pair of dual integral equations in which the unknown variable is the jump of displacements across the crack surfaces.

Relationship between the target scattering function and the echo signal is deduced to hold as a Fourier transform pair by analyzing the texture of echo signal for a passive radar imaging system;