1. A method of processing seismic images of the subsurface, comprising the steps of: analyzing a seismic image to estimate seismic horizons in an area of the subsurface; calculating an accumulation value associated with each pixel of the seismic image by accumulation along a set of estimated seismic horizons to form a synthesized image composed of accumulation values, wherein the calculating an accumulation value comprises accumulating signed quantities respectively associated with horizons of said set passing through said pixel, the signed quantity associated with a horizon depending on the sign of an amplitude of a starting pixel of said horizon in the seismic image; and transforming the synthesized image to obtain a seismic representation in which signed amplitudes are allocated to the pixels of the synthesized image, wherein the transforming the synthesized image comprises a spatial convolution of the synthesized image with a convolution kernel.