Note: Descriptions are shown in the official language in which they were submitted.
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METHOD OF Ai~lD APPAiRATUS FOR MARINE SEISMIC
SURVEYI NG
The present invention relates to a method of and apparatus for marine
seismic surveying.
There are two distinct surveying operations that are generally performed.
The first is a relatively deep exploration of the geology of an area. This
is referred to hereinafter as a "deep" survey. Current techniques allow
this survey to be performed as a 3D survey. Thus a single pass of a
survey vessel or vessels working together surveys a corridor of the sea
floor rather than a line as in 2D surveying. Figure 1 of the
accompanying drawings is a schematic illustration of the streamer
arrangement of a conventional 3D survey. A seismic survey vessel 2
tows a plurality of "long" streamers 4 and seismic sources 6. The
streamers, each of which is typically at least 2Km long, carry
hydrophones along their length. The hydrophones are arranged into
groups and the length of each group is known as a "group interval". The
sources 6 typically comprise two or three sub arrays, each comprising
six to ten airguns. The sources typically produce a peak pressure of 30-
100 bar at one metre with a 8 to 70Hz frequency range. Each source is
fired in sequence and a shot is taken every 18 to 25 metres of boat
travel.
After the raw seismic data have been acquired, the reflected signais
(known as traces) received by each group of hydrophones from each
actuation of a seismic energy source are processed to produce a
subsurface image. The processing includes the steps of transforming (or
-
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"migrating") the signals to their actual sub-surface location. The traces
may be corrected to account for the separation (also known as offset)
between the source and the hydrophone or hydrophones. A first
correction accounts for the fact that the velocity of sound within the
earth tends to increase with depth as the earth layers become more
compacted. The correction is derived empirically from the data itself
and is known as normal moveout correction. To successfully make this
correction, data from a large range of offsets are required. A further
correction is made to account for the inclination (or dip) of the reflecting
surfaces or interfaces within the earth. The area being surveyed may be
notionally divided into an array of cells (or bins). All the traces which
have been assigned to a bin are then summed (stacked) to obtain a single
trace for each bin. The stacked trace has an improved signal to noise
ratio compared to the individual traces as the signal tends to add
constructively whereas the noise is generally incoherent and does not
add constructively. A more detailed description of the data processing of
traces can be found in GB 2347751.
The arrangement of sources and hydrophones defines the maximum
resolution available. The bin size can be defined arbitrarily but in
practice is normally a multiple of the smallest definable feature. On this
basis, the smallest bin size is:
parallel to vessel movement - half the group interval, and
transverse to vessel movement - half of the transverse separation
between each streamer divided by the number of energy sources used.
Thus a four streamer arrangement cooperating with three sources having
a streamer separation of 150m and a group interval of 12.5 m gives a bin
size of 6.25 m "in line" with the travel of the survey vessel and 25m
cross-line (transverse).
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The in-line and cross-line resolutions are different because it is relatively
easy and inexpensive to divide the streamer into many short groups, but
it is expensive and difficult to deploy more streamers.
The second type of marine seismic survey is a relatively shallow
exploration of the geology of an area. This is referred to hereinafter as a
"shallow" survey. A shallow survey may comprise a high resolution
survey, an example of which is commonly known as a site survey (all
such high resolution surveys are hereinafter collectively referred to as site
surveys). Here it is required to derive a lot of information about a
relatively thin portion of the earth adjacent and including the sea floor.
The site survey is used to assess the risk to equipment and personnel that
may be involved in drilling into a given region of the earth. Hazards
include pockets of gas and an unstable sea floor.
Conventionally a site survey is performed in a 2D surveying mode using
a specially constructed streamer having a reduced group interval of
typically six to 10 metres and a modified seismic energy source, such as
an airgun or a "sparker", ~or producing higher frequencies. The source
typically produces a peak pressure of less than 15 bar at one metre with
a bandwidth of 20 to more than 150Hz. It is thus possible to provide a
survey with greater resolution than is provided by a conventional deep
survey. A region of the sea bed is subjected to a site survey after a
possible drilling position has been identified from a conventional survey.
If the site survey reveals reasons why a particular location is not suitable,
there may be insufficient site survey coverage to identify an alternative
site. Thus a further survey vessel may need to be deployed at a later
date to survey a different area.
GB 2 233 455, GB 2 125 550, GB 1 330 628 and GB 1 306 586
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disclose two dimensional marine seismic surveying techniques which
simultaneously survey deep and shallow targets using streamers or
streamer regions of different sizes and/or different resolutions.
GB 967 589 discloses the use of a streamer having a region of reduced
group interval to provide vertically enhanced resolution and a region of
large group interval to obtain a large spread of hydrophones so that
normal moveout correction can be applied to reveal multiple reflections.
US 4 781 140 discloses a boom arrangement for deploying multiple
sources and streamers laterally of a ship for three dimensional seismic
surveying.
According to a first aspect of the present invention, there is provided a
method of marine seismic surveying, comprising towing a first seismic
array for deep three dimensional seismic surveying, towing a second
seismic array for shallow three dimensional seismic surveying, and
performing the deep surveying and the shallow surveying concurrently,
wherein the lateral separation between streamers of the second seismic
array is less than the lateral separation between streamers of the first
seismic array.
It is thus possible to make a deep marine seismic survey in which the
geology of the area surveyed is probed to a considerable depth
concurrently with a shallow survey, such as a site survey in which a
more detailed investigation of the geology within a few hundred metres
of the sea bed is established, with greater lateral resolution.
Preferably the streamers of the first seismic array have a first group
interval and the streamers of the second seismic array have a second
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group interval smaller than the first group interval. This enables the
second array to achieve a greater longitudinal resolution than the first
array.
Advantageously some or all of the streamers of the second array may be
embodied within some or all of the streamers of the first array. Thus
some or all of the streamers of the first array may have a first region
therein having the first group interval and a second region therein having
the second group interval. The second region may act as a streamer of
the second array, whereas both the first and second regions may act as a
streamer for the first array. Advantageously the second regions are
located nearer to the seismic energy sources than the first regions. This
has the advantage that the second regions are adjacent short streamers
which may be deployed solely for the purpose of performing a site
survey. The position of the second regions with respect to the survey
vessel may also be less affected by wind, waves and tide. The control of
streamer position also has an effect upon the resolution of the survey.
Preferably the streamers of the first array are physically longer than
streamers which only belong to the second array. Typically the
minimum length of streamers of the first array is approximately equal to
the maximum depth that the survey is required to investigate.
Advantageously the first and second arrays further comprise respective
seismic sources. The or each source of the second array may be
arranged to produce a signal having a higher cut-off frequency than the
or each source of the first array. Alternatively the seismic sources may
be common to the first and second arrays.
According to a second aspect of the present invention, there is provided
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an apparatus for marine seismic surveying, comprising a first seismic
array for deep three dimensional seismic surveying and a second seismic
array for shallow three dimensional seismic surveying, the seismic arrays
being arranged to perform the deep surveying and the shallow surveying ,r
concurrently, wherein the lateral separation between streamers of the
second seismic array is less than the lateral separation between streamers
of the first seismic array.
It is thus possible to provide a shallow survey of greater lateral resolution
concurrently with a survey of the deeper geological features. This is of
particular advantage since the analysis of the deeper survey may indicate
the possibility of mineral resources such as oil or gas. Data from the
shallow survey such as site survey data may then be examined to assess
the hazards to drilling operations (such as pockets of gas or an unstable
sea floor) for reaching those mineral resources.
The present invention will further be described, by way of example, with
reference to the accompanying drawings, in which:
Figure 1 is a schematic diagram showing the streamer positions in a
conventional 3D marine seismic survey;
Figure 2 is a schematic diagram of a survey arrangement constituting a
first embodiment of the present invention;
Figure 3 is a schematic diagram of a survey arrangement constituting a
second embodiment of the present invention;
Figure 4 is a schematic diagram of a survey arrangement constituting a
third embodiment of the present invention;
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Figure 5 shows a firing sequence of the first and second embodiments;
and
Figure 6 shows a firing sequence of the third embodiment.
The seismic survey arrangement shown in Figure 2 comprises three
relativeiy long streamers 14 and two relatively short streamers 16 towed
behind a survey vessel 2. The long streamers 14 are divided into first
and second regions 18 and 20, respectively. The regions 18 and 20
belong to a first array for deep surveying, whereas the regions 20 and the
short streamers 16 belong to a second array for shallow surveying, such
as site surveying. The first regions 18 have a group interval of, for
example, between 12 and 15 meters. The second regions 20 and the
short streamers 16 may, for some applications, have the same group
spacing as the first regions 18 but, for site surveying normally have a
relatively short group interval of, for example, between 6 and 10 meters.
The signals received by the regions 20 and the short streamers 16 are
processed to provide site survey information. The short streamers 16 are
positioned intermediate the regions 20 of the long streamers 14 thereby
reducing the cross-line separation of the streamers participating in the site
survey compared to the cross-line separation of the streamers 14
participating in the conventional survey.
Data collected by the hydrophones in the second regions 20 of the
streamers 14 can be combined with data collected by the hydrophones
in the first regions 18 to produce the conventional survey.
J
The seismic sources used for a conventional deep survey are spread out
in an area of typically 20 x 20 meters. Such an arrangement provides
some directivity to the energy and focuses it downwards. The frequency
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range of such a source can be limited as the energy reflected from
several kilometres into the earth is only ever of a low frequency. The "
peak energy level provided by such sources is high.
i~
The seismic source requirements for site surveys are opposite to those for
deep surveys. Low frequency energy is not normally important, whilst
significant energy in the range of 50 to 150 Hz is necessary to provide
the vertical resolution needed. Furthermore, the towing depth of the
source needs to be different since reflection from the water-air interface
can interfere destructively with the down going energy. High resolution
sources for site surveying are typically towed at depths of less than 4
meters, whereas conventional sources are typically towed at depths of
greater than 6 meters.
The conventional source used in a deep survey may be adapted to
provide high frequency components. Towing the source at a shallower
depth also enhances the high frequency components produced by the
source. As noted hereinabove, each source comprises an array of sub-
elements. Different sub-elements may be towed at different depths so as
to provide both the high frequency components used for site surveying
and the low frequency components required for the conventional deep
seismic surveying. Alternatively, the sources 6 may include high
frequency generating components, within the subarrays.
When separate sources for the deep and shallow surveys are provided,
the source providing the shot energy for the shallow survey is fired
between the source actuations of the sources for the deep survey. The
energy used for the shallow survey is given sufficient time to decay away
before the actuation of the sources for the deep survey so as to avoid
interference between the surveys.
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The time required to record a shallow survey is typically two seconds or
less. The length of recording for the deep survey is usually between five
and seven seconds with the streamers being digitally sampled every two
or four milliseconds. 1-hus the high resolution source for use with the
site survey is fired a couple of seconds before the sources of the deep
survey.
In the arrangement shown in Figure 2, the high resolution sources are
attached to the port and starboard conventional sources 6. The firing
pattern of the sources for such an arrangement is shown in Figure 5. H
represents the firing of the high resolution source, whereas CS and CP
represent the firings of the conventional starboard and port sources,
respectively.
The embodiment shown in Figure 3 is a variation of the embodiment
shown in Figure 2. Additional short streamers 16 are provided
intermediate the long streamers 14 so as to further increase the resolution
of the shallow survey.
The embodiment shown in Figure 4 has a centrally disposed high
resolution source 26 separate from the port and starboard conventional
sources 28. The firing pattern for such an arrangement is illustrated in
Figure 6. The terms H, CS and CP are as defined hereinabove.
Following or during collection of the seismic survey data, seismic
processing of the conventional and site surveys can proceed. The bin
~ sizes used for the site survey can be smaller than those used for the deep
survey given that the resolution available from the site survey array is
better than that from the deep survey array. The site survey and deep
survey data may be merged during the processing so as to give enhanced
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data quality for shallow formations.
The sample rates used for the site survey and the deep survey may differ
since the reflected energy in a site survey may have a higher cut-off
frequency. If recording equipment is shared by the first and second
arrays, the equipment may require the ability to change the effective
group interval between shots, to record different shots for different
durations, to change sample rates and to send different data to different
recording devices.
The control system for the sources may also require the ability to firedifferent sources or different elements (airguns and/or sparkers) at
different times and/or with different peak powers.
The collection of the site survey data at the same time as the
conventional survey data removes the need to perform a separate site
survey and also ensures that adequate site survey coverage is always
available in respect of an area which has been subjected to the
conventional survey.
