Seismic data acquisition technique having superposed signals

Apparatus for producing separate seismic records form spaced-apart vibrator seismic sources with the seismic records having reduced harmonic distortion, comprising:

• first and second vibrator seismic sources for concurrently generating seismic energy in response to receipt of respective pilot signals,
• first and second means for generating said respective pilot signals for driving said vibrator seismic sources, said pilot signals comprising a sequence of at least four sweeps and the pilot signals for each vibrator seismic source having a plurality of initial phase angles during said sweeps,
• means for detecting a seismic signal produced during each of said sweeps of said concurrently operated vibrator seismic sources,
• means for separately correlating each of said detected seismic signals with the corresponding pilot signals which drove said vibrator seismic sources for each said sweep to produce first and second correlated records for each sweep, and
• means for summing said first correlated records from each of said sweeps to produce a first seismic record derived predominantly from said first vibrator seismic source and for summing said second correlated records from each of said sweeps to produce a second seismic record derived predominantly from said vibrator seismic source.

A method for producing separate seismic records derived respectively from a pair of concurrently operated and spaced-apart vibrator seismic sources, the seismic records having reduced harmonic distortion, comprising the steps of:

• generating for a first of said vibrator seismic sources a set of eight frequency modulated pilot signals, each pilot signal comprising a sweep and having an increasing frequency through each sweep, said first vibrator seismic source pilot signals for driving said first vibrator seismic source and said first vibrator pilot signals having respective initial phase angles of 0.degree., 45.degree., 90.degree., 135.degree., 180.degree., 225.degree., 270.degree. and 315.degree.,
• generating for a second of said vibrator seismic sources a set of eight frequency modulated pilot signals, each pilot signal comprising a sweep and having an increasing frequency through each sweep, said second vibrator seismic source pilot signals generated concurrently and respectively with said first vibrator seismic source signals wherein said pilot signals are generated concurrently in pairs to produce eight pairs of sweeps, said second vibrator seismic source pilot signals for driving said second vibrator seismic source and having respective initial phase angles of 270.degree., 225.degree., 180.degree., 135.degree., 90.degree., 45.degree., 0.degree. and 315.degree.,
• detecting a reflected seismic signal following generation of each pair of said sweeps thereby producing eight seismic signals,
• correlating each of said detected seismic signals with the respective one of said first seismic source signals which served to produce the corresponding detected seismic signal, thereby producing eight correlated records for said first seismic source,
• correlating each of said detected seismic signals with the respective one of said second seismic source signals which served to produce the corresponding detected seismic signal, thereby producing eight correlated records for said second seismic source,
• summing said eight correlated records for said first seismic source to produce a first seismic record, wherein said first seismic record is derived almost exclusively from said first vibrator seismic source, and
• summing said eight correlated records for said second seismic source to produce a second seismic record, wherein said second seismic record is derived almost exclusively from said second vibratory seismic source.

A method for producing separate seismic records derived respectively from a pair of concurrently operated and spaced-apart vibrator seismic sources, the seismic source having reduced harmonic distortion, comprising the steps of:

• generating a set of pairs of frequency modulated pilot signals, said pairs having a sequence, the first and second pilot signals in each said pair corresponding respectively to said first and second vibrator seismic sources, a first of said pairs having a predetermined initial phase angle for each of the pilot signals therein, the first pilot signal in each of the remaining pairs having an initial phase angle which is increased over the initial phase angle of the first pilot signal in the preceding pair in the sequence by a predetermined phase increment and the second pilot signal in each of the remaining pairs having an initial phase angle which is decreased from the initial phase angle of the second pilot signal in the preceding pair in the sequence by said predetermined phase increment,
• driving said first and second vibrator seismic sources concurrently with said first and second pilot signals, said driving performed for each of said pairs of pilot signals and said driving not necessarily performed in the order of said sequence,
• detecting a seismic signal following the driving of said vibrator seismic sources by each pair of said pilot signals thereby producing a sequence of seismic signals,
• correlating each of said detected seismic signals with the respective one of said first vibrator seismic source pilot signals that served to produce the corresponding detected seismic signal, thereby producing a sequence of correlated records for said first vibrator seismic source,
• correlating each of said detected seismic signals with a respective one of said second vibrator seismic source signals which served to produce the corresponding detected seismic signal, thereby producing a sequence of correlated records for said second seismic source,
• summing said sequence of correlated records for said first vibrator seismic source to produce a first vibrator seismic record, wherein said first vibrator seismic record is derived almost exclusively from said first vibrator seismic source, and
• summing said sequence of correlated records for said second vibrator seismic source to produce a second vibrator seismic record, wherein said second seismic record is derived almost exclusively from said second vibrator seismic source.