A METHOD FOR ACQUISITION OF SEISMIC DATA AT SEA

Abstract: During the acquisition of seismic data from an area which is to be surveyed, especially acquisition of seismic data from a marine area, at least two seismic energy sources (S) and at least two seismic receivers (C) are used. The energy sources (S) and the receiver equipment (C), which can be seismic streamer cables (C), are arranged in groups in order to achieve an optimum coverage of recording lines.

Description: A METHOD FOR ACQUISITION OF SEISMIC DATA AT SEA The invention concerns a method for acquisition of seismic data from an area which is to be surveyed by using at least two seismic energy sources and at least two seismic receivers, and the invention particularly concerns■ the acquisition of seismic data from a marine area where several seismic energy sources together with one or more receiver cables, so-called streamers, are towed through the area. Seismic surveys are conducted according to the principle that a seismic signal is transmitted from an energy source, the signal is reflected from geological strata and thn reflection recorded in a receiver which is preferably placed at a distance from the transmitter. In the case of symmetrical reflection the reflection point will lie midway between the source and the receiver and this point is called the CDP (Common Depth Point) . In marine seismic surveys a survey is made of a particular area of the sea, in that the seismic vessel tows behind it a set of seismic energy sources together with receiver equipment which is placed in a so-called streamer cable. In principle, from several seismic energy sources pulses are transmitted, which are reflected from the seabed and the reflected signals recorded by equipment designed for this purpose fitted in the streamer, from which the signals are transferred on board the vessel for storage or processing. The vessel surveys the area by sailing back and forward along lines until a complete picture has been obtained of the geophysical conditions in the area. The principle of a survey of this nature is illustrated in the attached fig. 1. Surveys of land areas can be conducted according to the same principle, but the following description refers exclusively to marine surveys, by means of which the principle of the invention can most easily be demonstrated. In order to rationalise such a seismic survey of an area, and to ensure that the survey is conducted as quickly and efficiently as possible in order to obtain a correct picture of the area, a number of different methods have been developed. For this purpose a method has been used of towing behind a ship an array, in which the use of several seismic sources and several streamers is combined. The individual cables in which _ the receiver elements are provided, will be positioned in a distance from each other in order to cover the widest possible area during each pass along a line. In order to further rationalise and increase the efficiency of the seismic, data acquisition, a method has also been utilised where two ships sail alongside each other as illustrated in the attached fig. 2. With such a constellation of sources and recording equipment, i.e. streamers, the seismic energy sources have in principle been placed on the inside of the array and the streamers on the outside. It has thereby been possible to receive signals from the sources SI, S2, S3, S4 in each of the streamers Cl, C2, C3 and C4 and thus obtain a pattern of up to twelve CDP-lines for seismic recordings, thereby achieving coverage of a relatively wide line area with one pass. The arrays which have been used, such as that illustrated in fig. 2, have provided a relatively wide coverage area of CDP- lines, but optimum coverage has not been achieved since overlaps will occur as illustrated in fig. 2, or the CDP-lines can be left with different spacings, which can to some extent cause problems when processing the acquired data. Problems have also arisen if seismic energy sources or one of the streamer cables fails during the recording, causing gaps to occur in the outlined line pattern, i.e. one or more of the said CDP-lines are missing resulting in an incomplete picture of the desired area. This has resulted in the same area having to be covered again in order to obtain the required line density, an operation which is both time consuming and expensive. Both of these factors have thus generated a need to find a solution where sources and streamer cables are arranged in such a pattern in relation to one another that the CDP-lines provide an optimum coverage area or width, thus enabling the coverage of the area to be recorded to be conducted as quickly as possible. Should faults occur in the equipment during the survey, due to the increased coverage width there will still be advantages over the previously used methods since this method is based on a greater coverage width and it will normally only be necessary to repeat the recording in a part of the previous area. Thus an object of the present invention is to provide a method by which seismic energy sources and seismic receiver equipment can be arranged in such a relationship to one another that a maximum coverage width is obtained during the recording. A further partial object is to design the positional constellation of seismic energy sources and streamers for a marine survey in such a way that in the event of a fault a reorganization can- be performed in a simple and time-saving manner, thus enabling full coverage to be maintained of an albeit narrower area or the missing CDP-lines to be left in such a way that on a return pass full coverage can be obtained while at the same time maintaining a maximum width of acquisition. In any case it is an essential main object of the invention that in addition to the maximum possible coverage width for the acquisition a recording of data should be obtained in CDP-lines which lie equally spaced from one another and with the desired density. These objects are achieved with a method of the kind described in the introduction, which is characterized by the features in the claims presented. By following the method in accordance with the invention an array of streamers and energy sources can be provided which supplies an absolutely even line pattern with parallel CDP- lines and where those lines which are generated by the interaction between the respective sources and receivers give an optimum number of lines. By means of the method according to the invention, it will therefore be possible to achieve a marked increase in capacity with the same technical equipment > which was used previously and by using larger or more towing vessels a substantial increase in width can be achieved for the survey. As an example, reference can be made to the constellation illustrated as prior art in fig. 2. ith, the equipment shown there, by proceeding in accordance with the invention, it will be possible to record 16 lines of data alongside one another, while to date it has only been possible to record a maximum of 12 such lines. This represents a clear increase in capacity. In the event of faults in the equipment, for special combinations, and particularly for the embodiment described in claim 6 in the event of faults in the equipment, it will be possible by means of a simple transfer, i.e. alteration of the distance between sources or sources and streamers, or streamers, to obtain a distribution which enables recording to continue, albeit with fewer lines, but with closely spaced lines, thus avoiding the necessity of making an accurate repeat pass of the same area at a later date. In the case of the normally occurring faults, i.e. failure either of a streamer or an energy source, it will still be possible to obtain up to 12 recording lines, i.e. a result will be obtained which is at least as good as that achieved by the previously used technique. It can be mentioned that previous attempts have also been made to make such measurements more efficient. Thus in US 4868793, a method is described where seismic recording elements or individual sources are used in each area (array)/point source as elements in adjacent sources, so that the geometrical point for the source can be varied sequentially. However, this is a relatively complicated method and does not provide the same width coverage as the invention. In UK 1193507 a vertical positioning of receivers is described, but the same problems arise here in the case of faulty equipment as in the recognized technique. In UK 1285769 an acquisition technique is described where two or more boats are used. In this case, however, the operation is conducted with a separate boat for the seismic energy sources and a separate boat for streamers and recording. With this kind of constellation it will not be possible to achieve the advantages in accordanace with the invention. In US 3921124 a combination is described of two sources and one streamer, but here too no attempt has been made to solve those problems which form the basis of the present invention. In UK 1580835 the use of several sources and several receivers is described. Here a correlation programme is used in order to distinguish hich data samples/traces are to be compared. However, no description is given here of any geometrical arrangement combining several energy sources and receivers in the same way as in the invention. A more detailed description of the invention will be presented in the following section by means of embodiments illustrated in the drawings. The drawings illustrate: Fig. 1 the principle of acquisition of seismic data which forms the basis of both recognized technique and the present inventio . Fig. 2 illustrates recognized technique for seismic surveys using two vessels and for recording of twelve lines of seismic data with one pass of a marine area. Figs. 3a and 3b are two embodiments which illustrate the principle of the method according to the invention with two energy sources and two receiver cables, with a symmetrical and an asymmetrical positioning respectively, fig. 4a is an illustration of positioning possibilities for sources/receivers using the method according to the invention, with sources and cables exchanged in the lower part of the illustration, fig. 4b an embodiment achieved by the method according to the invention, figs. 4c-4j are various examples which illustrate the positioning of groups, based on the illustration in fig. 4a. Fig. 5 illustrates an embodiment for the invention, fig. 6 shows the embodiment in fig. 3, but with one streamer out of operation, and fig. 7 shows the embodiment in fig. 3, but with one of the seismic energy sources out of operation, this figure also illustrating the pass of the marine area in two directions. As already described above, fig. 1 illustrates the basic principle of acquisition of seismic ' data from the seabed, with the wave propagation path shown. For reasons of simplicity only the seabed and reflection together with a flat seabed are illustrated here. A vessel tows a seismic energy source K which is illustrated as a point source which at a given point in time will transmit a seismic pulse which is reflected in the points Ml'-M4' . From these points signals are reflected up to the seismic streamer which records reflections in four receiver channels M1-M4. Fig. 2 illustrates the method used until now where two ships sail alongside each other and where the signals which are transmitted from four energy sources S1-S4 are recorded by hydrophones in four streamers C1-C4. Due to the mutual positioning, data will be recorded along twelve lines where data, e.g. from source SI is recorded in streamer C1-C4, and further that signals which are transmitted from source S2 and which are reflected from the seabed are also recorded in streamer Cl etc. so that a line pattern is created which is illustrated at the bottom of the figure. It can be seen that in the central area there will be an overlap of four recording lines which will thus cover the same line two by two. This is not an optimum utilization of expensive technology and will only offer security to the data recorded in the central area if any of the sources should fail. It has been discovered, therefore, that by reorganizing the streamers and seismic energy sources a marked improvement in efficiency of acquisition can be obtained. Fig. 3a illustrates the principle which is applied in the method according to the invention. Behind a ship (not shown) are towed two energy sources which can be single sources or groups of streamers arranged in so-called "arrays" and which are indicated by SI and S2 (S = Source) . These are arranged at a relative distance from each other equal to x. In addition to these sources SI and S2, behind the vessel are also towed two streamer cables Cl and C2 which have a relative distance from each other equal to 2x. The distance between the centre of the sources and the centre of the cables is indicated by y. In the symmetrical arrangement this distance between the source centre and the cable centre will be 0. The ' principle is illustrated with two elements in each group, but there can also be 3, 4, etc. According to the principle for the method in accordance with the invention, sources and cables can also be arranged alongside one another as illustrated in fig. 3b. The same spacing conditions as described in connection with fig. 3a also apply here. The groups of sources or cables correspond to the minimum number of elements usable in the groups indicated by A and B in the claims. In the drawing the sources are indicated by *, while cables are indicated by a circle. It is completely within the scope of the invention to exchange cables and sources. The invention does however not include the embodiment illustrated on fig. 3 as this figure only is ment to illustrate the principle. If the number of sources/cables in fig. 3 is raised to three or more, this will be an embodiment within the scope of invention. Fig. 4 illustrates how, by means of the method according to the invention, the principle which is indicated schematically in fig. 3 can be applied to create a more wide-ranging or broader acquisition system for seismic data. The different groups are indicated by A, B, C, D in the same way as in the patent claims and illustrate a build-up originating from the smallest possible grouping, consisting of two sources in fig. 4a (see the upper part of the drawing) and two cables (shown in the lower part of the drawing) . Group B shows two such cabies (upper part, fig. 4a) and two sources (lower part, fig. b) , the distance between the elements in B being twice the distance in group A, (where two elements are used in the group A) . Had three elements been used in group A the distance in group B would have been three times the distance in group A. This system is further developed in group C, the groups A being applied as units and deployed as two group elements, since four elements are now used so that the distance between the groups will now be four times the distance between the elements in group A. The distance is measured between median lines or centre lines. In the same way a further development is made from group B to group D where the distance consequently is 8x, if the distance between the elements in group A is x. This principle can be continued for an arbitrary length. Thereby some of the groups may be deleted, without departing from the scope of invention. As an example of a practical embodiment for the method according to the invention sources and cables can be deployed in accordance with the combination of group C and group D in fig. 4b. Those CDP-lines which are recorded will be symmetrical to the tow/constellation since sources and cables are arranged symmetrically. Fig. 4c illustrates an asymmetrical positioning of sources and cables, where the same combination of groups C and D is used as described above, but in this case the groups are positioned displaced laterally to each other. This will give exactly the same pattern of CDP-lines as in the symmetrical arrangement. The lines, i.e. the CDP-lines, will be placed symmetrically in relation to the midpoint of the distance between the centre lines in the source arrangement and the cable arrangement. Should a displacement occur in the towing constellation, this will mean a displacement of the line positioning since the centering line for the CDP-line will be displaced, but : the measurements will not be altered. In this case ' it is important to ensure that the spacing between sources and between cables is maintained, i.e. the distance x and multiplied distances of x must be kept constant, i.e. these values must not be altered. Fig. 4d illustrates an embodiment of the method using four sources with spacing x as group A combined with four cables in group B which will- then have the spacing 4x. Fig. 4e illustrates the corresponding structure, where sources and cables have changed places in relation to fig. 4d. Otherwise the same pattern will be obtained in both cases for the CDP- coverage, as illustrated at the bottom of the figures. Fig. 4f illustrates an arrangement similar to the two previous figures, but with a relative displacement of the groups in relation to each other, although not displaced the complete distance. Also in this embodiment the CDP-lines will be placed as described earlier, i.e. symmetrically in relation to the distance between the centre lines of the respective groups. Fig. 4g illustrates a method using two elements of group A, four elements/units from group B, and two elements consisting of groups A as group C, after which the created groups B and C are combined. Fig. 4h shows the same example with exchange of sources and cables and this figure also illustrates the CDP- line coverage obtained with 16 lines. Fig. 4i and fig. 4j illustrate the use of the method in connection with an uneven number of sources/receivers in the groups. Fig. 4i is based on the use of three sources in group A and five receiver units in group B, which will be placed in a distance 3x from each other. When combining these groups A and B 15, CDP lines are achieved as shown in the figure. In fig. 4j there has been used two elements in group A, three elements in group B (distance 2x) and three elements in group C (distance 2x*3=6x). The number of CDP-lines will be 18. It should be evident from these illustrations that it is possible to gradually achieve the potential widths which can still be obtained in practice with the aid of towing vessels at sea or where the limits are set by what can be processed by computer. As mentioned earlier, in all the illustrated embodiments and also the other possible combinations which can be obtained by the method, an exchange of sources and cables can be performed, the vital factor being that this should be done completely i.e. by exchanging all sources with cables and vice versa. The principles can also be applied on dry land, even though the major benefits are particularly marked in connection with seismic surveys where desired routes can more easily be followed by boat. In the following figures an embodiment is illustrated, which shows how to alter the towing configuration in order to compensate for any faults that may occur. Thus fig. 5 illustrates an array with two boats alongside each other as was also illustrated in fig.2, with the seismic energy sources being placed on the outside of the array, while the streamers are placed in the centre, i.e. in accordance with the invention (and also as shown on fig. 4b) . This reorganization means that there is no longer any overlapping of recording lines as in the embodiment in fig. 2 and a recording coverage 16 lines wide is obtained. Should any faults occur in the equipment during the survey, this can be compensated for as illustrated in the following figures. Fig. 6 shows the same arrangement as in fig. 5, but in this case a fault has arisen in the streamer which was called C3, so that it no longer functions. The streamer can then be taken on board the ship and repaired or replaced with another streamer. In the meantime the survey can continue, since all that is required is to transfer the remaining streamer C4 from one of the vessels together with the two energy sources, so that they are positioned as illustrated in fig. 6. Streamer C4 has thus been transferred to the position previously held by streamer C3. The energy sources S3 and S4 have been moved so that the distance between them remains x with 2x corresponding to the spacing between two streamer cables as illustrated between streamer C2 and streamer C4. The distance between source S3 which lies closest to the streamer and the streamer C4 is set at x/2. If a line diagram is now drawn at the bottom in the same way as in fig. 3, twelve lines will be shown, which corresponds to the previously used technique, but with these twelve lines at an equal and even distance from one another, so that a complete picture of this strip is obtained. In other words, even after a fault has arisen a complete coverage has been obtained of a relatively wide area. When the ship returns and surveys the next line width of the marine area, it can sail into this area with twelve lines, thus achieving maximum utilization of the ship. Fig. 7 illustrates the case where one of the seismic energy sources has failed. As illustrated in the upper part of the figure, in this case too it will be possible to record twelve lines, but three of these lines will have double the required spacing. This pattern is achieved by, e.g., transferring energy source S4 to the position of energy source S3 if it was energy source S3 which failed. What has been achieved in this case is to concentrate the lower degree of coverage on a peripheral area of the survey, thereby easily compensating for this greater width when making recordings in the opposite direction by making the boat sail in the manner illustrated in the lower part of the figure. In this way it can be seen that "gaps" in the recording lines can be covered, thus obtaining a complete picture with dense and even coverage all the way, thereby avoiding the necessity of making a pass of exactly the same area twice. Should the fault arise on the wrong side of the ship in relation to the survey direction, the ships can change places or the sources can be re-deployed. Thus it can be seen that by the special positioning of the energy source/streamer in accordance with the invention clear advantages can be achieved in relation to the method previously used, and a rational and efficient as well as a complete acquisition of seismic data can be achieved for a desired marine area. Many modifications for positioning of sources and streamers in relation to each other will be possible on the basis of the special basic constellation in accordance with the invention.

Claims: PATENT CLAIMS 1. A method for acquisition of seismic data from an area which is to be surveyed using at least two seismic energy sources and at least two seismic receivers, and especially acquisition of seismic data from a marine area where several seismic energy sources together with one or more receiver cables, so-called streamers are towed through the area, characterized in that energy sources and receiver equipment or receiver cables are arranged in accordance with one or more of the following steps: I two or more group elements in the form of energy sources (S) or streamer cables (C) are deployed with a spacing (x) to form a first group (A) , two or more group elements of the opposite kind to that used in the first group, i.e. either cables or sources are deployed to form a second group (B) , the spacing between the elements in group (B) being equal to x multiplied by the number of elements in group (A) , II a group (C) is formed of two or more elements consisting of units of the type group (A) and with a spacing between the elements in group (C) equal to the distance between two elements in group (B) multiplied by the number of elements/units in group (B) , the distance referring to the midpoint of a group, III a group (D) is formed of two or more elements of the type group (B) and with a spacing between the elements in group (D) equal to the distance between two elements in group (C) , calculated from the midpoints for the groups, multiplied by the number of elements in the subgroup (C) in group (D) , IV etc., until the required CDP-line number is obtained, and that by deploying sources/cables exclusively according to step I, without any combination with groups from other steps, at least three element are used in each group. SUBSTITUTESHEET 2. A method in accordance with claim 1, characterized in that a combination is made of two or more successive groups, such as (A and B) , (B and C) , (C and D) , etc. 3. A method in accordance with claim 1, characterized in that the elements in the groups are arranged symmetrically. 4. A method in accordance with claim 1, characterized in that the groups which are combined are arranged asymmetrically alongside each other. 5. A method for acquisition of seismic data from an area which is to be surveyed using at least two seismic energy sources and at least two seismic receivers, particularly acquisition of seismic data from a marine area where several seismic energy sources together with one or more receiver cables, so-called streamers, are towed through the area, characterized in that all sources are towed at the outside of the total seismic array which is created by the seismic equipment which is towed behind the vessels and that the streamers are towed in parallel tracks in the middle of the seismic array, while an equal spacing is maintained between the cables. 6. A method in accordance with claim 5, characterized in that the seismic energy source which lies closest to the streamers on each side has a distance from the nearest streamer equal to 1/4 of the spacing between the streamer cables. 7. A method in accordance with claim 6, characterized in that the next sources, if there are several energy sources on one side or the other of the array, are towed out to the side of the innermost source in a parallel track or tracks if there are several energy sources and that the SUBSTITUTESHEET distance to the nearest source is maintained at equal to half of the spacing between the streamer cables. 8. A method in accordance with claim 7, characterized in that three vessels are used and that the middle vessel only tows streamers and that each of those vessels which are on the outside tow either an array of sources or an array of sources and streamers and that in the case where streamers are also towed, these are placed in parallel tracks on the side behind the vessel which is nearest the vessel which only tows streamers so that in the total array the sources will be positioned in the outermost, parallel tracks. 9. A method in accordance with claim 8, characterized in that that source which lies closest to the streamer on both sides of the streamer array is kept at a distance from the nearest streamer equal to 1/4 of the spacing between the streamers and in those cases where there are several sources on each side a distance is maintained which is equal to 1/2 of the distance of the streamers to the nearest source. 10. A method in accordance with one of the claims 1-4, characterized in that all sources are positioned where the placing of cables is indicated, and vice versa. SUBSTITUTESHEET

Inventors: BALE RICHARD, MCMILLAN IAN, BALE, RICHARD, MCMILLAN, IAN
Applicants: GECO AS, GECO A.S
Application year: 1991
Main classification: G01V1
IPC codes: G01V1