CA1222814A

CA1222814A - Method of seismic exploration

Info

Publication number: CA1222814A
Application number: CA000463936A
Authority: CA
Inventors: Jack L. Wilson
Original assignee: Mobil Oil Corp
Current assignee: ExxonMobil Oil Corp
Priority date: 1983-10-03
Filing date: 1984-09-25
Publication date: 1987-06-09
Also published as: NO843769L; GB8424185D0; NL8402953A; GB2149110A

Abstract

METHOD OF SEISMIC EXPLORATION

Abstract In a method for acquiring simultaneously two profile lines of seismic data in a marine environment, two acoustic point sources are fired alternately for detection by a single line of acoustic pulse detectors to produce on a single pass an accurate subsurface profile for three dimensional surveys.

Description

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M OD OF SEISMIC EXPLORATION

This invention relates to a method of seismic exploration and more particularly to a method of acquiring seismic data having a density of control suitable for obtaining three dimensional information concerning subsurface formations.
In conventional marine seismic exploration techniques, a line of acoustic pulse detectors is towed by a marine vessel behind a line of acoustic pulse generators. The line of acoustic pulse detectors may be many kilometres in length and contain 240 channels of information associated with it. Efforts have been made to increase that number of channels in order to increase the efficiency of the seismic data acquisition system in view of the expense involved in the operation of marine exploration vessels. The line of acoustic pulse generators normally compriSes a plurality of air guns which are configured to be fired simultaneously to give the effect of a point source, or one very large air gun. After one line of data has been "shot" or acquired, the marine vessel turns around to acquire Further parallel lines of seismic data. Present day navigational aids permit relatively accurate positioning of the marine vessel, although the calculated position of the marine vessel may be as much as thirty metres from its actual position.
The seismic data acquired by means of such a method provides suitable data for a two dimensional analysis of the subsurface formations. Recently, however, the desirability of three dimensional analysis of seismic data has become important, but three dimensional analysis of seismic data is only reliable when the density of control is such that the two parallel lines along which the data is acquired are no more than thirty to sixty metres apart. Acquiring two such parallel lines of data is difficult when it is accomplished by two passes of the marine vessel. As stated previously, although modern day navigational aids are accurateS they nevertheless may have an error as high as thirty metres and the cost involved in shooting two F-2448-L ~2-lines of data is essentially double the cost of shooting one line.
Furthermore, the constant surface action of a marine environment can cause frequent intersection of the two lines of data essentially nullifying the desired parallel aspect. Although a certain degree of intersection may be compensated for in processing the data, expense is added and accuracy is not guaranteed.
The present invention provides a method for acquiring seismic data from two parallel lines that has density of control to allow three dimensional models to be formed accurately with reduced acquisition expense and without navigation dependence.
Qccording to the invention, there is provided a method of marine seismic exploation in which two lines of spaced seismic data are acquired simultaneously, comprising the steps of towing behind a marine vessel two spaced parallel lines of acoustic pulse generators, the pulse generators in each line being arranged for simultaneous firing and to act as a point source;
towing a line of acoustic pulse detectors behind the marine vessel behind the lines of acoustic pulse generators;
firing the lines of acoustic pulse generators alternately; and receiving reflected acoustic pulses at the acoustic pulse detectors.
The invention thus provides a method For obtaining two profile lines of seismic data simultaneously. This in turn provides a significant saving in terms of both time and expense in addition to permitting procurement of accurate profile seismic data.
The method of the invention will now be described in greater detail by way of example only with reference to the accompanying drawing which is a schematic top plan view of a marine seismic exploration system.
Referring to the drawing, a marine vessel 12 tows two lines of seismic pulse genrrators adapted to act as point sources and illustrated as pulse generators 14 and 16, and a line of acoustic pulse receivers illustrated as receiver 18. The pulse generators 14 ~;2Z81~

and 16 are spaced at lateral distances Sl and 52 on opposite sides of a line 20 being the line of travel of the vessel 12, and the receiver 18 is to the rear of the generators 14 and 16 on that line 20.
The distances 51 and S2 may be varied according to requirements, but may be maintained substantially constant in any particular case by use of any of the known types of directional flotation devices. The distances 51 and S2 are suitably equal but may differ according to circumstances; however, the total distance between generators 14 and 16 ~Sl and S2) is preferably about 100 metres.
Each of the lines of seismic pulse generators represented by the generators 14 and 16 suitably comprises a plurality of air gun pulse generators, the air guns in each line being synchronized to fire simultaneously, and adapted to act as a point source. One type of air gun array that acts in this manner is described in British Patent

2,0927750.
When using a plurality of air guns, the air gun in each line closest to the marine vessel 12 is preferably spaced at a distance of approximately 45 metres from the vessel, with the most distant air gun in each line preferably being spaced at a distance of approximately 100 metres from the vessel.
When the system is in operation, the pulse generators 14 and 16 are fired alternately, suitably at lo to 15 seconds' intervals, although any time interval can be used depending upon the conditions and lithography being explored. When the generator 14 is fired, an acoustic pulse travels downwardly through the water and submarine strata and is reflected upwardly by the strata interfaces and marine bottom to be detected by the receiver 18. The points of re~lection will, in general terms, be located vertically below a point 30~ being the mid~point along a line 26 bet~een the generator 14 and receiver 18. A similar refle!otion point is located vertically below the mid-point 32 of a line 28 between the generator 16 and receiver 18.

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As the vessel 12 moves Forward with generators 14 and 16 firing alternately, so points 30 and 32 - and the reflection points vertically below them - move forward also to generate lines 22 and 24 which are paral].el to line 20 and spaced apart by a distance (Sl and S2)/2. ~t is these lines 22 and 24 that represent the parallel lines of simultaneously-acquired data generated according to the method of the invention.
Using the method of the invention, therefore, two parallel lines of seismic data can be acquired simultaneously and without reliance upon navigational accuracy, and with sufficiently close spacing to permit such data manipulation techniques as migration to be performed on the data to yield useful results.

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Claims

CLAIM

1. A method of marine seismic exploration in which two lines of spaced seismic data are acquired simultaneously, comprising the steps of towing behind a marine vessel two spaced parallel lines of acoustic pulse generators, the pulse generators in each line being arranged for simultaneous firing and to act as a point source;
towing a line of acoustic pulse detectors behind the marine vessel behind the lines of acoustic pulse generators;
firing the lines of acoustic pulse generators alternately; and receiving reflected acoustic pulses at the acoustic pulse detectors.