A seismic function whose frequency content is restricted by a maximum frequency F_m must be sampled with a sampling frequency F_e that is greater than 2 F_m. This condition is called the "Shannon condition". In effect, if a function, sampled at a frequency Fe, has a frequency content restricted by Fm, it can be sub-sampled by a factor n (n < F_e/2F_m). A sub-sampling of order n (2 for example) only conserves every 1 point out of n (1 out of 2). The new sampling frequency is now F_e/n. The modulus of the FT of the sub-sampled signal is present between 0 and F_e/2n, which is the new Shannon limit.