the number m of vibrators vibrating simultaneously by the listening time i e t s u2267 mt e this sequence can be split up into m parts of length s 1 s 2 s n greater than or equal to the listening time correlation of the record of a reflected signal starting to arrive at the time t by correlation signal s n s 1 s 2 s n s 1 gives a peak at the time t and a very weak correlation noise everywhere else between 0 and t c correlation of the record between 0 and t e by any one of the sequences deduced from the first sequence by circular permutation with a time lag such as a multiple of t e s 2 s 3 s n s 1 s 3 s n s 1 s 2 s n s 1 s 2 s n u2212 1 will only give a minimum correlation noise if a second vibrator vibrates at the same time as the first one with a sequence modified by a time lag and completed by circular permutation s u2032 s 2 s 3 s n correlation by s gives only a minimum correlation noise only correlation by s u2032 will lead to a peak at the time t the record obtained by simultaneously recording several vibrators can thus be split up into components connected to each vibrator by successive correlations with sequences.