Note: Descriptions are shown in the official language in which they were submitted.
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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
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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
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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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