1. Field of the Invention
The invention relates to seismic source arrays.
2. Description of the Related Art
Seismic data are usually acquired using arrays of seismic sources. In a source array, individual seismic sources are arranged in a certain spatial pattern. The most common marine seismic sources are airguns but also vibrators, waterguns and steam-injection guns are in use. The most common land seismic sources are vibrators and dynamite charges. Seismic source arrays are usually made up of one type of source. The sizes and strengths of the individual sources within the array may be different. In addition, the individual sources can be made to fire or start emitting at the same time or with small time delays between them.
In marine seismic surveys, the source array is usually towed by a vessel. A typical configuration is shown in
An individual source has three spatial positions: in-line, cross-line and depth. In the marine example in
The design of a source array amounts to the selection of the number of individual sources, their strengths, their signatures, their positions (in-line, cross-line and depth) and their firing/emission delays. The design criteria are based on the desired strength and frequency content at the geological) target depth and a desire to radiate energy principally downward.
The source arrays that are commonly used, exhibit source array directivity. This means that they do not emit the same seismic signal in all directions. The emitted signal can vary with azimuth (angle) and take-off angle. The concepts of azimuth and take-off angle are explained in
The presence of azimuthal directivity in the seismic data is undesirable. During seismic data processing seismic data traces from different azimuths are combined to give the final image. Azimuthal directivity will have a detrimental effect: it results in a loss of resolution and a reduction of the signal-to-noise ratio.
A distinction can be made between two types of marine seismic acquisition:
In both types of acquisition the sources are usually located at or near the sea-surface. The source array in
In both types of acquisition the source vessel, which might be the same vessel that is towing the receiver cable in sea-surface acquisition, sails through the survey area and activates the source at regular intervals. In 2D acquisition a single cable (called a streamer) is towed behind the vessel, while in 3D acquisition, an array of parallel streamers, normally equally spaced apart, is towed behind the vessel.
In 3D sea-floor acquisition, the receiver cables 6 (see
In 3D sea-surface acquisition the receiver cable 8 is usually towed behind the source vessel 2; a technique called end-on acquisition (see
The source arrays that are used in sea-floor acquisition are the same as the ones used in sea-surface acquisition. These were originally designed for 2D towed-streamer acquisition in which data are only acquired straight behind the vessel at a single azimuth of 180°. The directivity in azimuth was therefore of no concern. As discussed, 3D sea-surface seismic data contain a fan of azimuths and 3D sea-floor seismic data contain all azimuths. The azimuthal directivity of the source array will therefore be present in the data.
In land seismic acquisition, source arrays are usually formed by placing a number of land seismic vibrators in a spatial pattern. The acquisition geometry of a 3D land survey is similar to the sea-floor acquisition geometry as shown in
In borehole seismic acquisition, a tool 10 with receivers is located deep (e.g. 1 km) down a drilled well 12 below a rig 14 (see
U.S. Pat. No. 5,142,498 seeks to construct arrays where the phase spectrum for all take-off angles of interest will match the phase spectrum of the vertically downgoing pulse. This is referred to as phase control. Phase control is achieved by symmetrically arranging identical source elements about the array's geometric centroid. The geometric centroid is the centre line in the source array about which the identical source elements are symmetrically arranged. This is the line where phase control is achieved. If all elements are equal, phase control is achieved in all azimuths for a range of take-off angles limited by geometry. However, phase control is only achieved within a limited range of take-off angles, and although the beam pattern is identical within the limited range of take-off angles where phase control is achieved, the beam pattern is not identical outside this limit.
The invention seeks to provide a seismic source array which is azimuth-invariant, in the sense that it emits a seismic wavefield whose change over a selected range azimuths is zero or negligible. Such a source array can then be used in multi-azimuth seismic acquisition.
According to the invention there is provided a seismic source array as set out in the accompanying claims.
The design of the source array involves the selection of the number of individual sources, their strengths, their signatures, their positions and their firing/emission delays such that the emitted seismic wavefield does not change or changes unperceivably over a selected range of azimuths. The design preferably fulfills geophysical criteria such as desired frequency content and signal strength in the downward direction of the geological target, and operational criteria such as deployability.
The seismic source array of the invention can be used for many applications, including the following:
1. 3D and 2D marine sea-floor acquisition;
2. 3D and 2D marine sea-surface acquisition, including:
It should be appreciated that all of the seismic source elements do not necessarily have to be positioned at the same depth. Where the seismic sources are arranged in a number of concentric circles, this can be achieved by putting the circles at different depths. Concentric circles of sources may also be placed directly above or below each other. Where the sources are arranged in concentric circles, each circle preferably contains identical source elements.
To fulfil geophysical criteria on the spectral contents of the total emitted wavefield, it may be necessary to use array elements with different spectral output. It may also be necessary to assign different firing/emission delays to the array elements, particularly if elements are placed at different depths.
The invention is not limited to the specific embodiments described hereinafter. In particular, the invention recognises that perturbations to the symmetry of the geometry of the elements and/or perturbations to the symmetry of the output of the elements can also give all azimuth-invariant source array, provided the perturbations are small.
Specific embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:
a, b and c show seismic signatures, amplitude spectra and phase spectra respectively at a take-off angle of 30° for a range of azimuths (0°, 45°, 90°, 135°, 180°) for the source array in
a is a schematic illustration of marine borehole seismic acquisition, in which a vessel tows a source array in the survey area around a rig;
b is a schematic illustration of the rig of
a, b and c show respectively seismic signatures amplitude spectra and phase spectra for
a, b, and c show respectively seismic signatures, amplitude spectra and phase spectra for the source array of
a, b and c show seismic signatures amplitude spectra and phase spectra for the geometry used for
a, b and c show seismic signatures, amplitude spectra and phase spectra for the source array of
This embodiment is particularly suited for marine acquisition since imaginary parallel lines 24 in
The example in
a, b and c show respectively the seismic signal, its amplitude spectrum and phase spectrum emitted at a takeoff angle of 30° and at a range of azimuths. It can be seen that the seismic signal is the same for all the azimuths.
The farfield beam pattern of a realisation of this array 28 is given in
a, b and c show respectively the seismic signal, its amplitude and phase spectrum emitted at a take-off angle of 30° and at a range of azimuths. It can be seen that the seismic signal is the same for all the azimuths for frequencies up to 180 Hz.
Typically, a source array consists of three subarrays. The geometry in
The farfield beam pattern of a realisation of such a 7 element array is given in
a, b and c show the seismic signal, its amplitude and phase spectrum emitted at a take-off angle of 30° and at a range of azimuths for such a 7 element array. It can be seen that the seismic signal is the same for all the azimuths for frequencies up to 180 Hz.
The geometry described here and the geometries of
The farfield beam pattern of a realisation of the array 34 of
a, b and c show respectively the seismic signal, its amplitude and phase spectrum emitted at a take-off angle of 30° and at a range of azimuths. It can be seen that the seismic signal is the same for all the azimuths for frequencies up to 160 Hz.
The hexagonal embodiment of
On land, a seismic source generates elastic waves with different propagation velocities. These propagation velocities can be very different from one survey location to another. The farfield beam pattern is therefore not expressed in terms of azimuth and take-off angle but in terms of azimuth and apparent velocity.
where ν is the propagation velocity and φ is the take-off angle.
The equivalent of the farfield beam patterns in the previous sections is to use the approximate propagation velocity of sound in water: ν=1500 m/s. In land acquisition, useful reflection data can have apparent velocities from ∞ down to about 1500 m/s. Strong coherent noise, known as groundroll, is commonly present. Groundroll travels along the earth's surface so it has a take-off angle of ±90°. Its propagation velocity is low: usually between 1000 m/s and 100 m/s. Groundroll is low frequent; its bandwidth does usually not extend beyond 40 Hz.
The farfield beam pattern of the array in
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0007034.2 | Mar 2000 | GB | national |
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PCT/IB01/00370 | 3/9/2001 | WO | 00 | 2/19/2003 |
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WO01/71385 | 9/27/2001 | WO | A |
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