Abstract:
A method for acquiring three-dimensional seismic data for sub-surface geologic features wherein a seismic source array is moved along a survey pattern having a plurality of source lines of unequal lengths that are substantially parallel to each other and intersect a receiver line. The survey pattern can be repeated in an overlapping and interleaved fashion to survey a larger area.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates generally to marine seismic surveying. In particular aspects, the invention relates to systems and methods for conducting a reflection seismic survey. 
         [0003]    2. Description of the Related Art 
         [0004]    Seismic exploration is used to survey subterranean geological formations to determine the location of hydrocarbon formations within the earth. Reflection seismology is used to estimate the properties of the subsurface from reflected seismic waves. In reflection seismology, generated acoustic waves (i.e., shots) are propagated down through subterranean strata and reflect from acoustic impedance differences at the interfaces between various subterranean strata. Because many commercially viable hydrocarbon formations are located beneath bodies of water, marine seismic surveys have been developed. Marine seismic survey systems have been described in, for example, U.S. Pat. No. 6,026,059 issued to Starr. 
         [0005]    The presence of background noise and multiple seismic signals tends to cover up the desired signals (traces) which reveal actual subsurface geological structure. As a result, it has become conventional to enhance the desired traces by collecting multiple signals having the same common mid-point (CMP). “Fold” quantifies the number of seismic traces that are recorded at a given CMP. Higher fold generally improves data quality as the traces are summed together such that the primary signal is enhanced by in-phase addition while ambient noise and interference are reduced. In 3-dimensional surveys, data is gathered by taking all seismic traces from an area around each CMP and assigning these traces to a “bin,” which is a discrete rectangular area of the surface area being surveyed. A 3-dimensional “image” of the subterranean structure can then be modeled from the bin data. 
         [0006]    In a typical ocean bottom seismic survey system, a source vessel tows a source array through a body of water. The source array contains a number of seismic sources, such as air guns, which can create a seismic signal as known in the art. The source array produces seismic signals (shots) that are propagated down through the water and into the strata beneath the sea floor. As the seismic signals encounter the various subterranean strata, they are reflected back and are detected by one or more seismic receiver devices which record the signals and permit them to be analyzed. In an ocean bottom 3D seismic survey, it is typical to have a plurality of seismic recorders incorporated into an ocean bottom cable that is disposed in a linear fashion along the sea floor. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention provides an ocean bottom seismic 3D survey system which includes a plurality of seismic signal receivers that are disposed in one or more linear arrays upon the sea floor. A source vessel tows a single or plurality of seismic source arrays in the water above the receivers. The source array is towed in a pattern of substantially parallel source lines that intersect the arrays of receivers. In preferred embodiments, the source lines are substantially perpendicular to the linear receiver arrays. Further, in preferred embodiments, the source lines of the pattern have variable, unequal lengths. At least one of the source lines has a first length, and at least one of another of the source lines has a second length that is less than the first length. 
         [0008]    In a currently preferred embodiment, the invention features a survey pattern made up of a series of source lines having three unequal lengths: long, intermediate, and short. In a further aspect of the present invention, it is preferred that the source lines in the pattern occur in a particular sequence. A currently preferred sequence for the source lines is long-intermediate-short-intermediate. In a further currently preferred embodiment, the long source line is four units in length, the intermediate source line is three units in length, and the short source line is two units in length. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    For detailed understanding of the invention, reference is made to the following detailed description of the preferred embodiments, taken in conjunction with the accompanying drawings in which reference characters designate like or similar elements throughout the several figures of the drawings. 
           [0010]      FIG. 1  illustrates an exemplary marine survey system. 
           [0011]      FIG. 2  is a representation of an exemplary prior art survey source line pattern. 
           [0012]      FIG. 3  illustrates the use of seismic sources and a receiver to image a relatively shallow target point. 
           [0013]      FIG. 4  illustrates the use of seismic sources and a receiver to image a relatively deep target point. 
           [0014]      FIG. 5  illustrates an exemplary survey source line pattern for a single swath of data acquisition in accordance with the present invention. 
           [0015]      FIG. 6  depicts an exemplary cross-spread plot for a single fold of data. 
           [0016]      FIG. 7  is a graph depicting the offset distribution of a single cross-spread. 
           [0017]      FIG. 8  is a graph depicting traces obtained and the offset distances of those traces according to four different exemplary source line survey patterns. 
           [0018]      FIG. 9  depicts the use of two swaths of the survey pattern depicted in  FIG. 5  in a rollover used to image an adjacent target area. 
           [0019]      FIG. 10  is a plan view of a larger target area made up of a number of overlapping survey patterns. 
           [0020]      FIG. 11  is a plan view of an alternative survey pattern in accordance with the present invention. 
           [0021]      FIG. 12  is a plan view of a further alternative survey pattern in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]      FIG. 1  depicts an exemplary marine seismic survey system  10  which includes a source vessel  12  which is moving through a body of water  14  above a sea floor  16 .  FIG. 1  depicts two subterranean strata  18 ,  20  which underlie the sea floor  16 ; although those of skill in the art will recognize that typically there are a large number of such strata in any given instance. A seismic receiver cable  22  is disposed upon the sea floor  16  and, as shown in Prior Art  FIG. 2 , includes a number of individual seismic receivers  24 , such as hydrophones, geophones, or similar devices of a type known in the art, for detecting a seismic signal and thereafter recording the signal or transmitting the signal to a distal storage device. 
         [0023]    The source vessel  12  is towing a source array  26  in the water  14 . The source array  26  incorporates a number of seismic sources  28 . The seismic sources  28  are typically air guns of a type known in the art for producing a suitable acoustic signal which can propagate downwardly through the water  14  and into the strata  18 ,  20  below. In  FIG. 1 , there are nine seismic sources  28 . However, those of skill in the art will understand that this is merely by way of example, and that there may be more or fewer than nine. 
         [0024]    As the source vessel  12  moves through the water  14 , the seismic source array  26  is repeatedly actuated to create a series of acoustic signals at known intervals that propagate downwardly and are partially reflected off of the strata  18 ,  20  and the interfaces between the strata and are received at the receiver cable  22 .  FIG. 2  is a plan view depicting an exemplary cross-spread survey pattern which might be used by the survey system  10  in order to obtain data relating to the strata  18 ,  20  beneath the sea floor  16 . The dashed line  30  illustrates an exemplary path that the source vessel  12  could take above the seismic receiver cable  22  while activating the seismic sources  28  to produce seismic data relating to the formations below the sea floor  16  proximate the cable  22 . The path  30  includes a plurality of substantially parallel source lines  32  separated by turns  34 . It is noted that the source lines  32  are substantially equal in length. The source vessel  12  would travel in the direction indicated by the arrows  36 . 
         [0025]    More frequent and shorter source line lengths are useful for obtaining good imaging of relatively shallow targets.  FIG. 3  is a schematic drawing depicting the surveying of an exemplary relatively shallow target point T 1 , which is located at approximately 2000 feet below the sea floor  16 . Seismic rays  38  that originate from seismic source points S 1  and S 2  have short offset distances with respect to the receiver R and at least a portion of this seismic ray  38  will reflect back to the receiver R substantially along line  41 . Seismic rays  40  that originate from seismic source points S 3  and S 4  have longer offset distances, and, as a result, are likely to refract off the target point T 1  (as depicted by dashed line  42 ) rather than be reflected back to the receiver R. Therefore, the seismic rays  40  do not produce useable traces. The inventors have recognized, then, that for shallower targets, traces with longer offsets are less useful than those with shorter offsets and which are positioned substantially over the target point. Further, when using a longer source line length and a shallow target, most of the useful data is acquired when the seismic source points are located in relatively close proximity to a point directly above the target point T 1 . 
         [0026]      FIG. 4  illustrates the instance of imaging a deeper target (T 2 ). Target point T 2  is at a deeper location, for example, 20,000 feet below the sea floor  16 ) than target point T 1 . Seismic rays  38  originating from seismic source points S 1  and S 2  will be largely reflected back to the receiver R 1 , R 2 , respectively. Because there is a more acute angle of approach to the deeper target point T 2 , a significant portion of seismic rays  40  originating from seismic source points S 3  and S 4  will also be reflected back toward receivers R 3  and R 4  along lines  41 . Therefore, there are more usable traces for deeper targets since traces with both short and long offsets provide useful data. 
         [0027]      FIG. 5  illustrates an exemplary survey pattern  44  for a source vessel and towed source array conducted in accordance with the present invention. It will be understood by those of skill in the art that the pattern  44  depicted in  FIG. 5  may represent a portion of a larger survey pattern having a larger number of source lines.  FIG. 5  depicts a pair of substantially parallel receiver lines  46  and  48 . The receiver lines  46 ,  48  are each made up of a plurality of individual seismic receivers  50  which are arranged in substantially linear arrays. In one embodiment of the present invention, the receivers  50  are autonomous receivers, often known as “nodal” receivers, of a type known in the art and which are operable to detect and record acoustic energy on-board. In another embodiment of the present invention, the seismic receivers  50  are receivers, of a type known in the art, that will detect acoustic energy and then transmit it to a distal, typically surface-based recorder via telemetry. The receiver lines  46 ,  48  are preferably deployed along the sea floor  16 , as described earlier. Also, the receivers  50  are preferably spaced in a substantially equidistant manner from one another along each of the receiver lines  46 ,  48 . The survey pattern  44  depicts the path to be taken by a source vessel towing an array of seismic sources, as previously described. The source vessel tows the seismic array in the direction of arrows  52  in a series of substantially parallel and linear source lines  53  which are connected by turns  54 , which are located at the linear ends of adjacent source lines  53 .  FIG. 5  depicts an exemplary pattern  44  wherein the source lines  53  are oriented in a generally orthogonal, or perpendicular, manner to the orientation of the receiver lines  46 ,  48 . However, it is also contemplated that the source lines  53  could be oriented to intersect the receiver lines  46 ,  48  at a non-perpendicular angle, such as a 45 degree angle with respect to the direction of orientation of the receiver lines  46 ,  48 . However, other angles of intersection (i.e., 30 degrees, 50 degrees, etc.) may also be used. A target zone  56  to be surveyed and imaged by the acquisition of seismic data by the survey pattern  44  is illustrated generally by boundary lines in  FIG. 5 . 
         [0028]    It can be seen from  FIG. 5  that not all of the source lines  53  are of the same length. In accordance with the present invention, at least two of the source lines  53  are of different lengths.  FIG. 5  depicts a currently preferred embodiment wherein the survey pattern  44  includes source lines  53   a  that are of a first, longer length and source lines  53   b,  which have a length that is shorter than the first length. Currently preferred embodiments also include source lines  53   c  that are of an intermediate length that is greater than the length of the shorter source line  53   b,  but shorter than the length of the longer source line  53   a.  In further preferred embodiments of the present invention, the lengths of the different source lines  53   a,    53   b,    53   c  are related as multiples of a particular unit length such that the long source line  53   a  is four units in length, the short source line  53   b  is two units in length, and the intermediate source line  53   c  is three units in length. Using a unit length of 3,000 meters as an example, the long source lines  53   a  would be 12,000 meters in length, the intermediate source lines  53   c  would be 9,000 meters in length, and the short source lines  53   b  would be 6,000 meters in length. It is noted that the source lines  53   a,    53   b,    53   c  have been intentionally positioned so that the last shot point of any given source line  53   a,    53   b  or  53   c  is at the same cross-line position (i.e., perpendicular to the receiver lines  46 ,  48 ) as the first shot point of the next source line  53   a,    53   b  or  53   c  to be recorded. This provides for a minimum distance traveled by the source vessel  12  between the end of any shot line and the beginning of the next shot line, resulting in maximum efficiency of recording operations. 
         [0029]    In another aspect of the present invention, it is currently preferred that the length of the source lines are related to the interval between the receiver lines  46 ,  48 . Interval  55 , depicted in  FIG. 5 , is the distance between the two receiver lines  46 ,  48 . In presently preferred embodiments, the length of the short source line  53   b  is a certain minimum of an even multiple of the interval  55 . In a further preferred embodiment, the short source line is at least four times the length of the interval  55 . For example, if the interval  55  were 100 meters, the short source line  53   b  would be 400 meters in length or more. Using source line lengths that are an even multiple of the number of receiver lines  46 ,  48 , multiplied by the interval  55  between the receiver lines  46 ,  48  ensures that the fold of coverage produced is consistent between adjacent swaths of coverage. 
         [0030]    It is further currently preferred that the source lines  53   a,    53   b,    53   c  occur in an order within the survey pattern which permits alternating coverage of opposite sides of a central target area by the intermediate source lines  53   c.  In a currently preferred pattern  44  illustrated in  FIG. 5 , a source vessel follows a path wherein it traverses a long source line  53   a  followed by an intermediate length source line  53   b  and then a short source line  53   b.  Next, the vessel traverses an intermediate length source line  53   b,  a long source line  53   a  and an intermediate length source line  53   b,  and then the pattern is repeated. 
         [0031]    As the pattern  44  is traversed by a source vessel  12 , the seismic source array  26  is activated to create a series of acoustic energy shots in accordance with a predetermined scheme. This scheme may be based upon timed actuation of the seismic source array  26 , GPS-determined location of the vessel  12 , speed of the vessel  12 , or in accordance with other suitable actuation schemes known in the art. Reflected acoustic energy from the target zone  56  is then detected by the seismic receivers  50  of the receiver lines  46 ,  48 . 
         [0032]      FIG. 6  illustrates a graph that is representative of a single cross-spread fold of data between the seismic source line  32 / 53  and a single receiver line  22  or  46 . The area of common midpoint coverage is indicated by the square  60 . The square  60  is made up of a number of smaller squares, commonly known as bins,  62 , as is known in the art. The intersection  64  between the source and receiver lines  32 / 53 ,  22 / 46  represents the location of the shortest offset distance between a seismic source  28  and receiver  24 . Therefore, bins  62   a  in that area will be those with the shortest offset distance. On the other hand, those bins, such as  62   b,  proximate the outer periphery of the square  60  will have the longest offset distances. 
         [0033]    One consequence of a conventional cross-spread geometry for obtaining seismic data is that many more long offsets are recorded in each full fold bin as compared to short offsets.  FIG. 7  depicts the offset distribution of a single cross-spread, by percentage, as taken along a long source line, such as source line  53   a.  As can be seen, the majority of traces have an offset that is 4000 meters or more in length. The inventors have recognized that longer offsets, while useful in providing geological data relating to deeper and intermediate depth targets, are less effective in providing useful data for shallower targets. 
         [0034]      FIG. 8  is a graph which illustrates the number of traces obtained and the offset distances of these traces according to four different exemplary survey patterns. The first survey pattern is depicted by the dashed line  66  is a pattern which employs a source line length of 12,000 meters. This pattern requires a time of 6.0 hours to complete surveying of a hypothetical target zone. The second survey pattern is depicted by the dashed and dotted line  68  in  FIG. 8 . The second survey pattern utilizes a source line length of 9,000 meters. The second survey pattern  68  requires a survey time of 4.7 hours for the same target zone. The second pattern  68  requires a shorter survey time than the first survey pattern  66 , which is desirable. However, the second survey pattern  68  excludes traces having longer offset distances. There are no traces with offset distances longer than around 13,500 meters. As a result, imaging for some deeper targets is sacrificed. 
         [0035]    A third exemplary survey pattern is depicted by the dotted line  70  in  FIG. 8 . The third survey pattern uses a survey source line length of 6,000 meters. The third pattern requires a survey time of only 3.4 hours. However, the medium and long offset lengths are missing, with the longest offset lengths being slightly over 11,000 meters. 
         [0036]    The fourth survey pattern is an exemplary survey pattern which is currently preferred in accordance with the present invention, and it is depicted by the solid line  44  in  FIG. 8 . The survey pattern  44  is also depicted in  FIG. 5 . The source line lengths in the fourth survey pattern  44  are in a sequence of 12,000 meters followed by a source line of 9,000 meters, then a source line of 6,000 meters, and then a source line of 9,000 meters. These sequences then repeat for the balance of the survey being conducted. This survey pattern requires a survey time of 4.7 hours to conduct. All of the patterns depicted in  FIG. 8  provide a significant number of traces with a relatively short offset distance (i.e, less than 6,000 meters). The inventors consider the survey pattern  44  to be preferred since it provides a relatively short survey time (4.7 hours) while providing a mixture of traces having short and long offset lengths without excluding long offset length traces. In addition, the flatness of the curve for survey pattern  44  shows that a more even distribution of offset lengths (as among short, intermediate and long) is provided. 
         [0037]    The order of the source lines in the preferred survey pattern  44  is advantageous since it provides relatively balanced lateral coverage of the survey area. Referring once again to  FIG. 5 , it can be seen that an equivalent amount of intermediate source line offset length is provided on each lateral side portion  72 ,  74  of the area defined between the receiver lines  46 ,  48  and the outer boundaries of the target area  56 . There are portions of the source lines  53  and three turns  54  located in the lateral side portion  72  in  FIG. 5 , and there are portions of source lines  53  and three turns  54  located in the opposite lateral side portion  74 . There is also substantially equivalent coverage of the target area  56  by longer offset distance traces on each of the distal lateral side portions  76 ,  78  which lie outside of the target zone  56 . There are portions of source lines  53   a  and  53   c  in each of these side portions as well as three turns  54  in each. 
         [0038]    Following the completion of the survey pattern  44 , the receiver lines  46 ,  48  are relocated to a second, adjacent location on the ocean bottom  16 . Thereafter, the survey pattern  44  is conducted again in this adjacent location.  FIG. 9  depicts an exemplary swath rolling technique wherein the receiver lines  46 ,  48  are moved from their initial locations (designated by  46 ,  48  in  FIG. 9 ) to locations  46   a,    48   a,  respectively, after the initial survey pattern  44  has been completed. After movement of the receiver lines  46 ,  48  to the new locations  46   a,    48   a,  a second survey pattern  44   a  is then conducted. The new locations  46   a,    48   a  are preferably based upon the interval distance  55  between the two receiver lines  46 ,  48 . The distance that the receiver lines  46 ,  48  are moved (the “rollover distance”  57 ) is preferably twice the interval distance  55 . For example, if the interval  55  is 100 meters, then the swath rolling distance  57  should be 200 meters. This results in a uniform layout of receiver lines. 
         [0039]    The second survey pattern  44   a  is preferably identical to the first survey pattern  44 . As can be seen from  FIG. 9 , the second survey pattern  44   a  overlaps and is interleaved with the first survey pattern  44 . The patterns  44 ,  44   a  are overlapping since the source lines  53   a,    53   b  and  53   c  of the second pattern  44   a  extend onto and overlap the first pattern  44 . The patterns  44 ,  44   a  are also interleaved since the source lines  53   a,    53   b  and  53   c  of the second pattern  44   a  are located laterally between the source lines  53   a,    53   b  and  53   c  of the first pattern. As a result, coverage is optimized. 
         [0040]    As  FIG. 10  illustrates, the survey pattern and swath rolling process is repeated with the receiver lines  46 ,  48  being moved in both the cross-line direction  82  and in-line direction  80 .  FIG. 10  depicts further subsequent locations  46   a,    48   a,    46   b,    48   b  and  46   c,    48   c  for the receiver lines  46 ,  48 . Locations  46   a,    48   a  and  46   b,    48   b  are subsequent locations in the cross-line direction  82  while the locations  46   c,    48   c  are exemplary subsequent locations in the in-line direction  80 . In  FIG. 10 , a larger scale survey area is shown which is made up of a number of individual survey patterns  44 ,  44   a,    44   b  and  44   c  which are double overlapped by virtue of being overlapping and interleaved. It will be appreciated that having a number of contiguous survey areas will result in imaging of a larger area. 
         [0041]      FIGS. 11 and 12  depict two alternative survey patterns in accordance with the present invention.  FIG. 11  illustrates an exemplary survey pattern  90  which includes first, long source lines  92 , intermediate length source lines  94  and short source lines  96 . In a preferred embodiment, the long source line  92  is three units in length, the intermediate length source line  94  is two units in length, and the short source line  96  is one unit in length. As an example, if the unit length is 2000 meters, the long source line  92  would be 6000 meters, the intermediate length source line  94  would be 4000 meters in length, and the short source line would be 2000 meters in length. A currently preferred order for the sequence of the source lines  92 ,  94 ,  96  in the survey pattern  90  is: a long source line  92 , intermediate length source line  94 , short source line  96 , and then an intermediate source line  94 . However, there are also other sequences for the source lines  92 ,  94 ,  96  which would also be effective and yield good imaging results. For example, the source lines could be sequenced as: long  92 , long  92 , intermediate  94 , short  96 , intermediate  94 , long  92 , long  92 , intermediate  94 , short  96 , intermediate  94 , and long  92 . In another example, the source lines could be sequenced as: long  92 , intermediate  94 , short  96 , short  96 , short  96 , intermediate  94 , and long  92 . 
         [0042]      FIG. 12  illustrates a further exemplary survey pattern  100  in accordance with the present invention. The survey pattern  100  includes first, long source lines  102 , intermediate length source lines  104 , and short source lines  106 . In a currently preferred embodiment, the long source line  102  is eight units in length, the intermediate length source line  104  is five units in length, and the short source line  106  is two units in length. A currently preferred order for the sequence of the source lines  102 ,  104 ,  106  is: a long source line  102 , three short source lines  106 , and an intermediate length source line  104 . Thereafter, the sequence is repeated. There are other sequences for the source lines  102 ,  104  and  106  which could also be used to yield good imaging results. For example, the source lines  102 ,  104 ,  106  could be sequenced as long  106 , intermediate  104 , short  102 , intermediate  104 , and long  102 . 
         [0043]    Those of skill in the art will recognize that numerous modifications and changes may be made to the exemplary designs and embodiments described herein and that the invention is limited only by the claims that follow and any equivalents thereof.