Method for selecting seismic traces for higher efficiency of pre-stack two dimensional or three dimensional depth migration
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention pertains to seismic data processing and more particularly to efficient seismic data processing whereby portions of the total received seismic data are selected to provide an accurate representation of the entire set of seismic data. 
     2. Related Prior Art 
     Prior art has provided many ways to shorten processing time in the field of processing seismic data. These methods have occurred mainly in the form of data compression, where the total amount of seismic data is taken and selected bits of digital data is eliminated as inconsequential or contains nothing meaningful that cannot be surmised when the retained data is evaluated. 
     U.S. Pat. No. 4,467,460, &#34;Seismic Data Acquisition Method&#34;, (Philip W. Johnson), relates to the field locations of seismic shot points, chosen to produce partial multi-fold data. The static correction equations of this partial multi-fold data are at least partially coupled. The seismic cross sections resulting from this procedure are improved. 
     U.S. Pat. No. 4,561,074, &#34;Computationally Efficient Weighting and Vertical Stacking Methods and Apparatus for Improving the Signal-to-Noise Ratio of Seismic Data&#34;, (Ralph E. Warmack), relates to improved methods and apparatus for vertically stacking seismic trace data collected by a digital field recorder during seismic prospecting utilizing plural initiations of a low energy surface seismic source. Seismic trace data generated by consecutive seismic source initiations and acquired at a common detector location is weighted and summed together, and the sum is normalized prior to being recorded. The signal to noise ratio of the seismic trace data is improved. The methods and apparatus by which the weighting values are calculated and applied provides computational simplifications which reduce the program storage requirements and increase the computation speed of the microcomputer circuit. 
     U.S. Pat. No. 4,661,934, &#34;Ground Position Controller and Method for Automatically Indicating Parameters that Spatially Cross-Reference the Locations of seismic Spread and Source Arrays During Exploration for Hydrocarbons and the Like&#34;, (H.T. Caruth, Jr.), relates to a ground position controller utilizing a microcomputer system interconnected to a digital field system (DFS) via a system bus, for generating, formatting and displaying information under a variety of shooting and collecting conditions. 
     U.S. Pat. No. 4,665,510, &#34;Method for Attenuating Multiples in CDP Data Gathers&#34;, (R.K. Foster, et al.), relates to a method for attenuating multiples in common depth point data whereby common depth point data is received and spike deconvolution may be performed on the data. The data is constant velocity stacked and gap deconvolution is performed. The primary stack velocity is determined and all data having a velocity other than the primary stack velocity is muted. All scans within the constant velocity stack are composited. These composites are arranged in proper order and displayed. 
     U.S. Pat. No. 4,675,851, &#34;Method for Seismic Exploration&#34;, (C.H. Savit, et al.), relates to sweeps of seismic signals consisting of pulse trains having a predetermined number of pulses. In these pulse trains the periods or durations of the pulses are randomized. In addition, the wave shape and relative time displacements of the pulses in different trains provides substantially constant spectral level over a frequency range containing several octaves. 
     U.S. Pat. No. 4,779,238, &#34;Method for Fast Slant Stack of Seismic Data&#34;, (Michael S. Howard), relates to a method for processing seismic data in the x-t domain to derive a slant stack in the p-tau domain. Partial slant stacks are computed over a plurality of small groups of traces which are iteratively merged using linear interpolation into successively larger groups of traces to exhibit the p-tau data characteristics. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method for obtaining a complete seismic section in either two dimensions or three dimensions through pre-stack depth migration while requiring only a portion of the computer time previously required. All the shots are separated into several series, such that within each series the receiver locations are repeating. One or more series of shots is selected for migrating. With each series, receivers are selected from each shot such that their locations are repeating. This allows maximum re-use of the travel time tables and results in the best efficiency of the depth migration. The same principal can be applied to three dimensional prestack depth migration to improve its efficiency. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a flow chart of the method of the present invention illustrated in block form. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Pre-stack depth migration is an excellent way for imaging complex structures. Its weakness is that a lot of computer time is needed to process the large number of pre-stack seismic traces. This problem is much more severe in three dimensions. There have been several reports lately that five hundred to one thousand hours on a CRAY computer are needed for a single three dimensional pre-stack depth migration. 
     The method of the present invention outlines a way to select a subset of the input seismic traces so that the computer time needed is much less. It consists basically of two steps that can be implemented in sequence. 
     The first step is to use only part of the input traces. For example, if two hundred forty offsets are contained in a shot, and sixty are to be used for migration, only the first sixty or every fourth might be used. Since the far offsets are needed for judging the correctness of the interval velocities, the second way (selecting every fourth) is obviously superior to the first. 
     The second step is to depth migrate the selected traces and to use the time tables to obtain a seismogram of the received data with much reduced processing time. 
     Referring now to FIG. 1, a block diagram of a flow chart of the present invention is illustrated. 
     At block 14 seismic data is received. This data may take the form of any unmigrated seismic data such as that currently obtained in the art. The data used may be data received from separate seismic surveys that cover the same surface area, as long as common reference points have been established. 
     At block 15 the first shot is selected and its receiver location is noted. This shot is the beginning shot of a new series. 
     At block 16 the receiver locations of the next shot is examined. If all its receiver locations are different it is skipped, otherwise it is selected. (The shots in the series have some receiver locations in common, while shots not in the series have no common receiver location with any shots in the series. See examples provided below.) 
     At block 17 whether there is another shot not yet examined is determined. If there are more shots, the program returns to block 16. If there are no more shots the program proceeds to block 18 where a determination of whether all shots have been selected is made. If all shots have not been selected, the program returns to block 15 and a new series is started, selecting the first shot from the remaining shots. 
     For example, selection of the first sixty offsets or every fourth offset as described above can be the initial guidelines. Also as described above, the far offsets are needed for judging the correctness of the interval velocities, selecting every fourth offset is superior to selecting the first sixty. 
     Routine use of the 4, 8, 12, 16, . . . , the fourth offset from every shot gather can reduce the computer time. But this is not the best way. We should select these so that the receiver positions are repeating. Because for every new receiver position we need to compute a new travel time table, whereas for a repeating receiver location the same travel time table can be read from disk and re-used. The present invention describes a method for selecting the input traces. Test of field data indicated that it can reduce the computer time by a factor of two. 
     At block 20 one or more series of shots are selected. From blocks 22 to 24 the receivers in these series are analyzed and the repeating ones are selected. 
     At block 22 the repeating receivers are determined to further reduce the required processing time. The repeating receivers are selected at block 24. 
     For example, assume that data for a seismic line, where the near offset distance is 250 m, offset interval is 20 m is received. Further assume that the first shot point is at 0 m, and the shot interval 20 m. The following is a list of the actual shot and receiver locations for the first ten shots: 
     
         __________________________________________________________________________
   shot                                                                   
shot                                                                      
   location                                                               
        receiver location                                                 
__________________________________________________________________________
1   0   250                                                               
           270                                                            
              290                                                         
                 310                                                      
                    330                                                   
                       350                                                
                          370                                             
                             390                                          
                                410                                       
                                   430                                    
                                      450                                 
2   20  270                                                               
           290                                                            
              310                                                         
                 330                                                      
                    350                                                   
                       370                                                
                          390                                             
                             410                                          
                                430                                       
                                   450                                    
                                      470                                 
3   40  290                                                               
           310                                                            
              330                                                         
                 350                                                      
                    370                                                   
                       390                                                
                          410                                             
                             430                                          
                                450                                       
                                   470                                    
                                      490                                 
4   60  310                                                               
           330                                                            
              350                                                         
                 370                                                      
                    390                                                   
                       410                                                
                          430                                             
                             450                                          
                                470                                       
                                   490                                    
                                      510                                 
5   80  330                                                               
           350                                                            
              370                                                         
                 390                                                      
                    410                                                   
                       430                                                
                          450                                             
                             470                                          
                                490                                       
                                   510                                    
                                      530                                 
6  100  350                                                               
           370                                                            
              390                                                         
                 410                                                      
                    430                                                   
                       450                                                
                          470                                             
                             490                                          
                                510                                       
                                   530                                    
                                      550                                 
7  120  370                                                               
           390                                                            
              410                                                         
                 430                                                      
                    450                                                   
                       470                                                
                          490                                             
                             510                                          
                                530                                       
                                   550                                    
                                      570                                 
8  140  390                                                               
           410                                                            
              430                                                         
                 450                                                      
                    470                                                   
                       490                                                
                          510                                             
                             530                                          
                                550                                       
                                   570                                    
                                      590                                 
9  160  410                                                               
           430                                                            
              450                                                         
                 470                                                      
                    490                                                   
                       510                                                
                          530                                             
                             550                                          
                                570                                       
                                   590                                    
                                      610                                 
10 180  430                                                               
           450                                                            
              470                                                         
                 490                                                      
                    510                                                   
                       530                                                
                          550                                             
                             570                                          
                                590                                       
                                   610                                    
                                      630                                 
__________________________________________________________________________
 
    
     Selection of the 4th, 8th, 12th receiver from every shot would result in having selected the following receivers: 
     
         ______________________________________                                    
shot     receiver location                                                
______________________________________                                    
1        310          390    470 . . .                                    
2        330          410    490 . . .                                    
3        350          430    510 . . .                                    
4        370          450    530 . . .                                    
5        390          470    550 . . .                                    
6        410          490    570 . . .                                    
7        430          510    590 . . .                                    
8        450          530    610 . . .                                    
9        470          550    630 . . .                                    
10       490          570    650 . . .                                    
______________________________________                                    
 
    
     Thus we would need to generate travel time tables at 310, 330, 350, 370, 390, . . . , which requires a significant amount of computer time and storage locations to retain the travel times. 
     A better way to select receiver locations may be chosen as follows: (For this data all the shots belong in a single series) 
     
         ______________________________________                                    
      shot     receiver                                                   
shot  location location                                                   
______________________________________                                    
1      0       310     390  470   (the 4, 8, 12 th . . . )                
2      20      310     390  470   (the 3, 7, 11 th . . . )                
3      40      310     390  470   (the 2, 6, 10 th. . . )                 
4      60      310     390  470   (the 1, 5, 9 th . . . )                 
5      80      390     470  550   (the 4, 8, 12 th . . . )                
6     100      390     470  550   (the 3, 7, 11 th . . . )                
7     120      390     470  550   (the 2, 6, 10 th . . . )                
8     140      390     470  550   (the 1, 5, 9 th . . . )                 
9     160      470     550  630   (the 4, 8, 12 th . . . )                
10    180      470     550  630   (the 3, 7, 11 th . . .                  
______________________________________                                    
                                  )                                       
 
    
     This way only travel time tables at 310, 390, 470, 550, . . . need to be generated which reduces the total number of travel time tables by a factor of four. 
     There are other shot and receiver patterns, for example, the shot interval could be thirty and the receiver interval twenty: 
     
         __________________________________________________________________________
   shot                                                                   
shot                                                                      
   location                                                               
        receiver location                                                 
__________________________________________________________________________
1   0   250                                                               
           270                                                            
              290                                                         
                 310                                                      
                    330                                                   
                       350                                                
                          370                                             
                             390                                          
                                410                                       
                                   430                                    
                                      450                                 
2   30  280                                                               
           300                                                            
              320                                                         
                 340                                                      
                    360                                                   
                       380                                                
                          400                                             
                             420                                          
                                440                                       
                                   460                                    
                                      480                                 
3   60  310                                                               
           330                                                            
              350                                                         
                 370                                                      
                    390                                                   
                       410                                                
                          430                                             
                             450                                          
                                470                                       
                                   490                                    
                                      510                                 
4   90  340                                                               
           360                                                            
              380                                                         
                 400                                                      
                    420                                                   
                       440                                                
                          460                                             
                             480                                          
                                500                                       
                                   520                                    
                                      540                                 
5  120  370                                                               
           390                                                            
              410                                                         
                 430                                                      
                    450                                                   
                       470                                                
                          490                                             
                             510                                          
                                530                                       
                                   550                                    
                                      570                                 
6  150  400                                                               
           420                                                            
              440                                                         
                 460                                                      
                    480                                                   
                       500                                                
                          520                                             
                             540                                          
                                560                                       
                                   580                                    
                                      600                                 
7  180  430                                                               
           450                                                            
              470                                                         
                 490                                                      
                    510                                                   
                       530                                                
                          550                                             
                             570                                          
                                590                                       
                                   610                                    
                                      630                                 
8  210  460                                                               
           480                                                            
              500                                                         
                 520                                                      
                    540                                                   
                       560                                                
                          580                                             
                             600                                          
                                620                                       
                                   640                                    
                                      660                                 
9  240  490                                                               
           510                                                            
              530                                                         
                 550                                                      
                    570                                                   
                       590                                                
                          610                                             
                             630                                          
                                650                                       
                                   670                                    
                                      690                                 
10 270  520                                                               
           540                                                            
              560                                                         
                 580                                                      
                    600                                                   
                       620                                                
                          640                                             
                             660                                          
                                680                                       
                                   700                                    
                                      720                                 
__________________________________________________________________________
 
    
     This can effectively be treated as two series of shots, the even numbered shots and the odd numbered shots: 
     
         __________________________________________________________________________
   shot                                                                   
shot                                                                      
   location                                                               
        receiver location                                                 
__________________________________________________________________________
1   0   250                                                               
           270                                                            
              290                                                         
                 310                                                      
                    330                                                   
                       350                                                
                          370                                             
                             390                                          
                                410                                       
                                   430                                    
                                      450                                 
3   60  310                                                               
           330                                                            
              350                                                         
                 370                                                      
                    390                                                   
                       410                                                
                          430                                             
                             450                                          
                                470                                       
                                   490                                    
                                      510                                 
5  120  370                                                               
           390                                                            
              410                                                         
                 430                                                      
                    450                                                   
                       470                                                
                          490                                             
                             510                                          
                                530                                       
                                   550                                    
                                      570                                 
7  180  430                                                               
           450                                                            
              470                                                         
                 490                                                      
                    510                                                   
                       530                                                
                          550                                             
                             570                                          
                                590                                       
                                   610                                    
                                      630                                 
9  240  490                                                               
           510                                                            
              530                                                         
                 550                                                      
                    570                                                   
                       590                                                
                          610                                             
                             630                                          
                                650                                       
                                   670                                    
                                      690                                 
2   30  280                                                               
           300                                                            
              320                                                         
                 340                                                      
                    360                                                   
                       380                                                
                          400                                             
                             420                                          
                                440                                       
                                   460                                    
                                      480                                 
4   90  340                                                               
           360                                                            
              380                                                         
                 400                                                      
                    420                                                   
                       440                                                
                          460                                             
                             480                                          
                                500                                       
                                   520                                    
                                      540                                 
6  150  400                                                               
           420                                                            
              440                                                         
                 460                                                      
                    480                                                   
                       500                                                
                          520                                             
                             540                                          
                                560                                       
                                   580                                    
                                      600                                 
8  210  460                                                               
           480                                                            
              500                                                         
                 520                                                      
                    540                                                   
                       560                                                
                          580                                             
                             600                                          
                                620                                       
                                   640                                    
                                      660                                 
10 270  520                                                               
           540                                                            
              560                                                         
                 580                                                      
                    600                                                   
                       620                                                
                          640                                             
                             660                                          
                                680                                       
                                   700                                    
                                      720                                 
__________________________________________________________________________
 
    
     It appears that selection should be made from the series of odd numbered shots as follows: 
     
         ______________________________________                                    
      shot     receiver                                                   
shot  location location                                                   
______________________________________                                    
1      0       310     390  470   (the 4, 8, 12 th . . . )                
3      60      310     390  470   (the 1, 5, 9 th . . . )                 
5     120      390     470  550   (the 2, 6, 10 th . . . )                
7     180      470     550  630   (the 3, 7, 11 th . . . )                
9     240      550     630  710   (the 4, 8, 12 th . . .                  
______________________________________                                    
                                  )                                       
 
    
     The selection pattern is the same for the even shots: 
     
         ______________________________________                                    
      shot     receiver                                                   
shot  location location                                                   
______________________________________                                    
2      30      340     420  500   (the 4, 8, 12 th . . . )                
4      90      340     420  500   (the 1, 5, 9 th . . . )                 
6     150      420     500  580   (the 2, 6, 10 th . . . )                
8     210      500     580  660   (the 3, 7, 11 th . . . )                
10    270      580     660  740   (the 4, 8, 12 th . . .                  
______________________________________                                    
                                  )                                       
 
    
     Usually, if a third of the traces are selected, the 3rd, 6th, 9th . . . receiver from all the shots would be selected. In such case, travel time tables for 290, 320, 350, 380, . . . (every other receiver) would be needed, even though only a third of the traces are selected. With the new method, the odd shots would be used and the receivers 310, 390, 470 . . . are selected or the even shots would be used and receivers 340, 420, 500, 580, 600 . . . are selected. 
     At block 26, the selected receiver data is depth migrated. 
     For the line of the example, the best way is to migrate either the odd shots or the even shots and select as described above, which would result in a very large saving in computer time. 
     However, the shot locations should also be examined to determine whether any of the locations can be re-used for receivers. This is because travel time table for each shot that is migrated always have to be generated. In the above example, the even shot points can only be re-used for odd receivers, and vice versa. Thus if only the odd shots or the even shots are used, the shot tables can not be re-used. Thus, there is no need to store them. 
     A simple computer program can be written to compute the percentage of travel time tables that can be re-used for different selection method. It will point to the most efficient selection quickly. 
     Finally, at block 28, the depth migrated portions of the data which has been selected can be displayed. 
     To summarize the method of the present invention, all the shots are separated into several series, such that within each series the receiver locations are repeating. One or more series of shots are selected for migrating. With each series, receivers are selected from each shot such that their locations are repeating. This allows maximum re-use of the travel time tables and results in the best efficiency of the depth migration. 
     The same principal can be applied to three dimensional prestack depth migration to improve its efficiency. 
     While there has been illustrated and described a particular embodiment of the present invention, it will be appreciated that numerous changes and modifications will occur to those skilled in the art, and it is intended in the appended claims to cover all those changes and modifications which fall within the true spirit and scope of the present invention.