Method of seismic exploration

Claim 1

A method of improving the signal-to-noise ratio of a final seismogram in seismic exploration of a medium to be explored having different reflectors, said method comprising the steps of: (a) determining a maximum frequency to be emitted in a frequency spectrum for the medium to be explored; (b) determining a minimum frequency to be emitted in said frequency spectrum for the medium to be explored; (c) emitting, in said medium from at least one emission source, vibrational acoustic signals of long time-duration, the frequencies of said emitted signals being within said frequency spectrum, said frequency spectrum divided into a number of separate, contiguous, non-overlapping bands, each band having a different bandwidth than immediately adjacent bands, each bandwidth a function of the ratio V S /CV B , where said number of bands is at least equal to a whole number which is immediately below and closest to the value of the ratio ##EQU2## where F max is the highest frequency of said desired spectrum, F min is the lowest frequency of said desired spectrum, C is a coefficient higher than 1 and dependent upon the cutoff of a wave-number filter, V B is the apparent velocity of sound within said medium, and V S is the lowest apparent velocity of waves refelected within said medium; (d) receiving, in at least on receiver, the waves reflected by the different reflectors of said medium, said receiver defining with said source emission, said wave number filter; (e) processing the received signals in order to determine the detected reflectors by (1) forming the complex spectrum for each band of frequencies of the received signal corresponding to the emitted signal, (2) forming the complex spectrum for each band of frequencies corresponding to a reference of the emitted signal, (3) dividing the complex spectrum of the received signal by the complex spectrum of the reference of the emitted signal to obtain an elementary complex spectrum for each band of frequencies, and (4) setting the amplitude of at least one portion of the elementary complex spectrum to zero if that portion is external to the band; and (f) producing a seismogram from said received processed signals in which, for each of said frequency bands, an improved signal-to-noise ratio is obtained, whereby said final seismogram has an improved signal-to-noise ratio.

Claim 2

A method of improving the signal-to-noise ratio of a final seismogram in seismic exploration of a medium to be explored having different reflectors, said method comprising the steps of: (a) determining a maximum frequency to be emitted in a frequency spectrum for the medium to be explored; (b) determining a minimum frequency to be emitted in said frequency spectrum for the medium to be explored; (c) emitting, in said medium from at least one emission source, vibrational acoustic signals of long time-duration, the frequencies of said emitted signals being within said frequency spectrum, said frequency spectrum divided into a number of separate, contiguous, non-overlapping bands, each band having a different bandwidth than immediately adjacent bands, each bandwidth a function of the ratio V S /CV B , where said number of bands is at least equal to a whole number which is immediately below and closest to the value of the ratio ##EQU2## where F max is the highest frequency of said desired spectrum, F min is the lowest frequency of said desired spectrum, C is a coefficient higher than 1 and dependent upon the cutoff of a wave-number filter, V B is the apparent velocity of sound within said medium, and V S is the lowest apparent velocity of waves refelected within said medium; (d) receiving, in at least on receiver, the waves reflected by the different reflectors of said medium, said receiver defining with said source emission, said wave number filter; (e) processing the received signals in order to determine the detected reflectors by (1) forming the complex spectrum for each band of frequencies of the received signal corresponding to the emitted signal, (2) forming the complex spectrum for each band of frequencies corresponding to a reference of the emitted signal, (3) dividing the complex spectrum of the received signal by the complex spectrum of the reference of the emitted signal to obtain an elementary complex spectrum for each band of frequencies, and (4) setting the amplitude of at least one portion of the elementary complex spectrum to zero if that portion is external to the band; and (f) producing a seismogram from said received processed signals in which, for each of said frequency bands, an improved signal-to-noise ratio is obtained, whereby said final seismogram has an improved signal-to-noise ratio.