A Hartley transform is a variation of the Fourier transform with an integration kernal cos(omega)t + sin(omega)t. Most of its properties are analogous to those of the Fourier transform. Computationally useful properties include that the forward and inverse transforms are identical and the transform of real numbers remain real numbers. Solutions to the wave equation can be derived in Hartley transform coordinates and used to image seismic data. These are cheaper to compute than in the Fourier domain.
STANFORD