Abstract:
A seismic source array  15  comprises a plurality of seismic source  26  arranged about a central point of the source array  15  in such a way that an imaginary circle drawn with said central point at its center, and containing all of said seismic sources  26 , can be divided into at least three whole sectors each of which contains a substantially identical arrangement of seismic sources  26.

Description:
BACKGROUND OF THE INVENTION 
     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  FIG. 1 , in which a vessel  2  tows an airgun source array  4 . In land seismic surveys, a vibrator is mounted on a truck; a dynamite charge is placed in a drilled hole. 
     An individual source has three spatial positions: in-line, cross-line and depth. In the marine example in  FIG. 1 , the cross-line separation of the airguns is 8 m, the in-line separation is 3 m and their depth is 6 m. 
     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  FIG. 2 , in which the vessel  2  and source array  4  are again shown. The present specification is only concerned with azimuth. 
       FIG. 3  shows the directivity pattern of the source array in  FIG. 1 . At frequencies 90 Hz and 130 Hz the directivity of the source array is clearly varying with azimuth. The directivity in azimuth decreases for lower frequencies as is shown at frequencies 60 Hz and 20 Hz.  FIGS. 4   a, b  and  c  show the seismic signal and its amplitude and phase spectrum emitted at a take-off angle of 30° and at a range of azimuths. The change in the signal shape, its amplitude spectrum and its phase spectrum is significant. 
     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:
     (a) Sea-surface acquisition, in which a vessel tows one or more cables with built-in receivers. The receiver cables are usually towed at a depth between 3 m and 12 m. This is the most common type of acquisition and is usually referred to as towed-streamer acquisition.   (b) Sea-floor acquisition, in which the receivers are planted at the sea floor or built into a receiver cable, which is laid at the sea floor. This type of acquisition is a relatively recent development.   

     In both types of acquisition the sources are usually located at or near the sea-surface. The source array in  FIG. 1  would be typical for both sea-surface and sea-floor acquisition. 
     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  FIG. 5 ) are laid out in an area over which the source vessel  2  sails a 3D pattern. Thus, seismic data are recorded in all directions from the source  4  (see  FIG. 5 ), that is for a full circle of azimuths: 0°-360°. 
     In 3D sea-surface acquisition the receiver cable  8  is usually towed behind the source vessel  2 ; a technique called end-on acquisition (see  FIG. 6 ). Thus, during one sail line, the seismic data are recorded for a half-circle of azimuths −90° to +90°. In fact, because the streamer is longer (typically, 4 km to 8 km) than the cross-line offset of the outer streamers (typically, 200 m to (500 m), much of the data have an azimuth fairly close to 0°. Occasionally, receiver cables are towed both in-front-of and behind the source vessel; a technique called split-spread acquisition. Then, the seismic data are recorded for an entire circle of azimuths although much of the data have an azimuth close to either 0°, for the receiver cable behind the source vessel, or 180°, for the receiver cable ahead of the source vessel. 
     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  FIG. 5 , but with the receiver cables at the earth&#39;s surface. Thus, 3D land seismic data are acquired for all azimuths and the azimuthal directivity of the source array is present in the seismic data. 
     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  FIGS. 7   a  and  b ). The source  4  is located at the surface. Borehole seismic acquisition can be done either at sea or on land. The employed source arrays are usually smaller than in the previously mentioned types of seismic acquisition. A borehole seismic survey is usually called a Vertical Seismic Profile (VSP). An acquisition geometry of a 3D VSP in a vertical well at sea is shown in  FIGS. 7   a  and  b . It can be seen that the seismic data are acquired for all azimuths and the azimuthal directivity of the source array  4  will be present in the data. To a lesser degree it can also be present in a 2D VSP. 
     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&#39;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. 
     SUMMARY OF THE INVENTION 
     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: 
     
         
         
           
             wide-tow streamer acquisition; 
             end-on and split-spread acquisition;
 
3. 3D and 2D land seismic acquisition; and
 
4. borehole seismic acquisition, both marine and land including:
 
             2D and 3D walkaway VSP; 
             offset VSP. 
           
         
       
    
     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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Specific embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  shows a prior art airgun source array comprising three sub-arrays each comprising five airguns; 
         FIG. 2  illustrates the azimuth and take-off angles in a marine source array; 
         FIG. 3  shows energy directivity diagrams at 20 Hz, 60 Hz, 90 Hz and 130 Hz for the source array of  FIG. 1 , in which each circle represents a different take-off angle; 
         FIGS. 4   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  FIG. 1 ; 
         FIG. 5  is a schematic illustration of sea-floor acquisition, in which a vessel tows a source array over receiver cables spread out on the sea floor; 
         FIG. 6  is a schematic illustration of towed-streamer sea-surface acquisition, in which a vessel tows both a source array and receiver cables close to the sea surface; 
         FIG. 7   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; 
         FIG. 7   b  is a schematic illustration of the rig of  FIG. 7   a , in which a tool with receivers is suspended from the rig down a well; 
         FIG. 8  shows a source geometry according to the invention, using fixed azimuth sampling; 
         FIG. 9  shows four energy directivity diagrams at 20, 60, 90 and 130 Hz for the source geometry of  FIG. 8 ; 
         FIGS. 10   a, b  and  c  show respectively seismic signatures amplitude spectra and phase spectra for  FIG. 8 , and illustrate that the seismic signal is substantially the same at all of the displayed azimuths; 
         FIG. 11  shows a source geometry in accordance with the invention, using hexagonal sampling; 
         FIG. 12  shows four energy directivity diagrams at 20, 60, 90 and 130 Hz for the source geometry of  FIG. 11 ; 
         FIGS. 13   a, b , and  c  show respectively seismic signatures, amplitude spectra and phase spectra for the source array of  FIG. 11 , and show that the seismic signal is the same at all of the displayed azimuths up to 180 Hz; 
         FIG. 14  shows four energy directivity diagrams at 20, 60, 90 and 130 Hz for a source geometry based on  FIG. 11 , but using only three sub-arrays; 
         FIGS. 15   a, b  and  c  show seismic signatures amplitude spectra and phase spectra for the geometry used for  FIG. 14 , and show that the seismic signal is the same at all of the displayed azimuths up to 180 Hz; 
         FIG. 16  shows a source geometry according to the invention, with equal spatial sampling in-line and cross-line; 
         FIG. 17  shows four energy directivity diagrams at 20, 60, 90 and 130 Hz for the source geometry of  FIG. 16 ; 
         FIGS. 18   a, b  and  c  show seismic signatures, amplitude spectra and phase spectra for the source array of  FIG. 16 , and show that the seismic signal is the same at all of the displayed azimuths up to 160 Hz; 
         FIG. 19  shows a source geometry according to the invention using six vibrator trucks distributed uniformly on a circle; and 
         FIG. 20  shows energy directivity diagrams at 20, 40, 60 and 90 Hz for the source geometry of  FIG. 19 , and show the change in azimuth for a fixed apparent velocity (ν app =ν/sin φ), where ν is the propagation velocity and φ is the take-off angle. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1 to 7  have already been described above in relation to the background to the invention. 
       FIG. 8  shows an example of an azimuth-invariant source geometry for a source array  15 . The vessel direction is indicated by arrow  16 . Eight sources  18  of type 1 are spaced equally on an outer circle labelled “a”, eight sources  20  of type 2 are spaced equally around an inner circle labelled “b”, and a single source  22  of type 3 is located at the centre of the array  15 . The geometry of the array is the result of a design procedure consisting of the following steps:
     1. Select the radius of the outer circle. The outer dimension of the array generally determines the width of spatial mainlobe.   2. Select the angular sampling interval, Δθ=360°/N, where N is the number of elements on each circle. E.g. in  FIG. 8 , Δθ=45°. A dense angular sampling will give small variations with azimuth angle.   3. Distribute equal elements regularly over the circle at the same depth.   4. Draw lines through each element parallel to a fixed direction.   5. Determine the number of different source element types, which is less than or equal to the number of different circles. Different source elements may be necessary in order to fulfil spectral constraints on the composite wavefield and/or constraints on the composite signatures.   6. The relationship between the M circles is defined such that for every circle m=1: M−1 do:
       (a) The next circle, #m+1, is defined such that the line next to the outermost line of the current circle, #m, is the tangent of the next circle (#m+1). E.g., line #2 in  FIG. 8  is a tangent to circle b and is the line immediately next to the outermost line of circle a.   (b) Distribute the N elements over circle #m+1 such that one of the elements is placed on the line of circle #m. E.g., one of the elements of circle b is placed on line #2 in  FIG. 8 .   (c) Draw lines through each element parallel to the lines in Step 4.   
       7. An element of the last element type is placed at the centre.   

     This embodiment is particularly suited for marine acquisition since imaginary parallel lines  24  in  FIG. 8  can be defined as subarrays, which makes the array  15  easy to tow. The number of elements per subarray is maximised by Step 6. However, for other applications the design procedure could be more general by omitting this step. The array  15  has rotational symmetry about the centre of the array. 
     The example in  FIG. 8  has 7 subarrays with a total of 17 guns  26  distributed over three types of source elements. The beam pattern of an array with this geometry is shown in  FIG. 9 . The radius of the outer circle (a) is here 6 m and element type 1 is Bolt 1900LLX 54 in 3  airgun, element type 2 is Bolt 1900LLX 3×54 in 3  airgun cluster, and element type 3 is Bolt 1500LL 3×235 in 3  airgun cluster. 
       FIGS. 10   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. 
       FIG. 11  shows a further array  28  formed from 19 elements  30  of three types. For arrays with a large aperture it might not be desirable to sample each radius by the same angular step size, which was the case for the embodiment of  FIG. 8 . By placing the array elements  30  on a hexagonal grid, as shown in  FIG. 11 , one obtains an array configuration that samples a large radius denser than a small radius. Here, the unique geometry is defined within a sector of 60°. In addition the array elements line up (see imaginary parallel lines  32 ), which makes the array easy to tow in marine acquisition. 
     The farfield beam pattern of a realisation of this array  28  is given in  FIG. 12 , where the sides in each of the hexagons are 2 m. Element type 1 is Bolt 1900LLX 2×54 in 3  airgun cluster, element type 2 is Bolt 1900LLX 54 in 3  airgun and element type 3 is Bolt 1900LLX 30 in 3  airgun. The resulting beam pattern is azimuth-invariant in the seismic frequency range. 
       FIGS. 13   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  FIG. 11 , with the 12 outer elements of type 1 removed, only needs three subarrays (7 elements in total) and is therefore a particularly practical embodiment. Element type 2 is now Bolt 1500LL 3×235 in 3  airgun cluster and element type 3 is now Bolt 1900LLX 3×125 in 3  airgun cluster. 
     The farfield beam pattern of a realisation of such a 7 element array is given in  FIG. 14 , where the sides in each of the hexagons are 3.5 m. The resulting beam pattern is azimuth-invariant for frequencies up to 130 Hz and for take-off angles up to 60°. 
       FIGS. 15   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. 
       FIG. 16  shows a perturbation of an azimuth-invariant geometry that still renders an azimuth-invariant source within the definition of the invention. An array  34  comprises 13 elements (guns)  36  of four different types. The number of guns in the array equals the number of grid nodes inside the circle of the outermost elements. The unique geometry is defined by an octant of the circular disk, such that the other positions are given by symmetry. 
     The geometry described here and the geometries of  FIGS. 8 and 11  are different in the way they approximate a circular disk. In  FIG. 8  the disk was sampled regularly in azimuth and irregularly in the radial direction. With the hexagonal geometry of  FIG. 11  the disk was sampled irregularly both in azimuth and in the radial direction. The geometry of  FIG. 16  also samples the disk irregularly both in azimuth and in the radial direction. 
     The farfield beam pattern of a realisation of the array  34  of  FIG. 16  is given in  FIG. 17 , where the element separation is 3 m both in-line and cross-line. Element type 1 is Bolt 1500LL 195 in 3  airgun, element type 2 is Bolt 1500LL 2×1 55 in 3  airgun cluster, element type 3 is Bolt 1500LL 3×235 in 3  airgun cluster and element type 4 is Bolt 1900LLX 125 in 3  airgun. This beam pattern is azimuth-invariant for all take-off angles up to 100 Hz. 
       FIGS. 18   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  FIG. 11  can be applied for vibrator arrays in land seismic acquisition. A number of vibrators  38 , in this case six, are distributed uniformly on a circle  39  as shown in  FIG. 19 . The lower bound for the radius of the circle is determined by the outer dimensions of the vibrator trucks  40 . In  FIG. 19  the outer dimensions of the trucks  40  are width 3 m by length 10 m. The radius of the circle is 7 m. 
     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. 
               v   app     =     v     sin   ⁢           ⁢   ϕ             
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&#39;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  FIG. 19  is given in  FIG. 20 . All vibrators  38  generate the same 6-90 Hz sweep. The circles in the diagram show the change in azimuth for a fixed apparent velocity. It can be seen that for reflection data, with apparent velocities that are higher than 1500 m/s, the source array is azimuth-invariant down to 200 m/s at 20 Hz. At 40 Hz, the source array is azimuth-invariant for groundroll down to 500 m/s.