Off-shore seismic prospection method using a coded vibratory signal and a device for implementing this method

• US 4,780,856 A
• Filed: 10/30/1986
• Issued: 10/25/1988
• Est. Priority Date: 10/30/1985
• Status: Expired due to Fees

• Current Assignee: Institut Francais Du Petrole (IFP Energies Nouvelles)
• Original Assignee: Institut Francais Du Petrole (IFP Energies Nouvelles)
• Inventors: Becquey, Marc
• Primary Examiner(s): Moskowitz, Nelson
• Application Number:
  US06/924,734
• Time in Patent Office: 726 Days
• Field of Search: 367/23, 367/39, 367/41, 342/145, 375/1
• US Class Current: 367/23
• CPC Class Codes: G01V 1/005 with exploration systems emitting special signals, e.g. frequency swept signals, pulse sequences or slip sweep arrangements
• CPC Class Codes: G01V 1/3808 Seismic data acquisition, e.g. survey design

Abstract

An offshore reflection seismic prospection method is provided including a vibratory signal coded in accordance with a pseudo-random code.

Correlation of the signals received by the seismic sensors by means of the cycle of the coded signal transmitted allows seismic plots to be obtained similar to what a pulsed source would provide firing at time intervals equal to the duration of the cycle of the coded signal. Correlation with time shifted cycles reduces the distance between plots recorded on the same sensor. The transmissions coming from two or more vibrators, transmitting the same coded signal with shifted cycles, may be separated by correlation with one of the cycles used.

Claims

1. A marine seismic prospection method including transmission, by at least one vibratory source towed by a ship moving continuously in a profile plane to be studied, of repetitive sequences of acoustic vibrations modulated non-repetitively during each of said sequences

reception of the acoustic waves reflected back by the reflectors, recording of acoustic signals corresponding to the received acoustic waves received and processing said signals so as to determine the position of said reflectors, wherein the transmission includes a succession of linked sequences each formed of a periodic carrier signal phase modulated by a pseudo-random binary coded signal and the processing of the acoustic signals received includes correlation of the signals received during a time equal to that of several successive sequence time periods with a single sequence of coded signals, so as to obtain correlation peaks at time intervals less than or equal to the repetition period of the successive transmission sequences.

2. The method as claimed in claim 1, wherein the two sequences used for the correlation are shifted with respect to each other by a time interval less than the maximum duration of propagation of the acoustic waves in the zone to be explored.

3. The method as claimed in claim 1, wherein the time shift between the two sequences used for the correlation is equal to half the repetition period of the sequences of signals transmitted.

5. A marine seismic prospection method including transmission, by a single vibratory source towed by a ship moving continuously in a profile plane to be studied, of repetitive sequences of acoustic vibrations modulated non-repetitively during each of said sequences, reception of the acoustic waves reflected back by the reflectors, recording of acoustic signals corresponding to the received acoustic waves and processing said signals so as to determine the position of said reflectors, wherein the transmission includes a succession of linked sequences each formed of a periodic carrier signal phase modulated by a pseudo-random binary coded signal and the processing of the acoustic signals received includes correlation of the signals received during a time equal to that of several successive sequence time periods alternately with a single sequence of coded signals and with the same sequence shifted in time by an interval less than the repetition period of the transmitted sequence, so as to obtain correlation peaks at time intervals less than or equal to the repetition period of the successive transmission sequences.

6. A marine seismic prospection method including simultaneously transmitting, by at least two vibratory sources towed by a ship moving continuously in a profile plane to be studied, of repetitive sequences of acoustic vibrations modulated non-repetitively during each of said sequences, reception of the acoustic waves reflected back by the reflectors, recording of acoustic signals corresponding to the received acoustic waves and processing said signals so as to determine the position of said reflectors, wherein the transmission includes a succession of linked sequences each formed of a periodic carrier signal phase modulated by a pseudo-random binary coded signal the succession, of sequences of signals transmitted by one of said vibratory sources being the same as that transmitted by the other one, but shifted in time, and said processing comprising correlation between the received signals resulting from the simultaneous transmission of the two sources and the sequence of coded signals transmitted by one or other of the two vibratory sources so as to obtain alternately correlation peaks corresponding to the first and second vibratory sources.

7. An offshore seismic prospection method including simultaneously transmitting, by at least two vibratory sources towed by a ship moving continuously in a profile plane to be studied, of repetitive sequences of acoustic vibrations modulated non-repetitively during each of said sequences, reception of the acoustic waves reflected back by the reflectors, recording of the acoustic signals received and processing thereof so as to determine the position of said reflectors, wherein the transmission includes a succession of linked sequences each formed of a periodic carrier signal phase modulated by a pseudo-random binary coded signal, the succession of sequences of signals transmitted by one of said vibratory sources being the same as that transmitted by the other one but shifted in time, and said processing comprising correlation between the received signals resulting from the simultaneous transmission of the two sources and, alternately, the sequence of coded signals transmitted by one or other of the two vibratory sources, so as to obtain alternately correlation peaks corresponding to the first and to the second vibratory sources.

4. The device as claimed in claim 7, wherein the vibration generating means includes two vibrators offset laterally with respect to the path of the ship.

8. A marine seismic prospection device including vibrating means towed by a ship for generating vibrations in water, a central control and recording system, a seismic streamer also towed by said ship and provided with an assembly of seismic receivers and a multiplexed transmission system for transmission of signals received by said seismic receivers to said central control and recording system which comprises a generator for generating a carrier wave repetitively phase modulated by sequences of a binary pseudo-random coded signal and applying said phase modulated carrier wave to said vibrating means, and two recording means for recording each of the received signals corresponding to a predetermined number of said sequences, a fraction of said received signals being recorded at the same time on said two recording means.