Sorting to Common Receiver Gathers

In this example, we have 21 shot records taken with shot positions moving across the river. Each shot record contains two blocks of channels with a total of 64 channels (some of which were disconnected since the cable take outs would have been on the bridge). One block is from the North bank, the other from the South bank of the river. A partial listing of the header dump shows the south block of channels for the northern most shot, 1001. To save space, not all channels are shown. This type of survey can be very challenging, since good record keeping is essential. Here, a significant number of cable take outs are left unconnected. Getting off by even one channel can lead to chaos. The actual shots were in the river, suspended from the bridge deck by the compressed air hose. The coordinates in the headers are for the airgun position in the river, NOT the suspension point on the bridge. The airguns drift somewhat west of the bridge due to the current.

{listdump} |-------------------------------| | PARTIAL SEGY HEADER DUMP | | 1001.seg | |-------------------------------| ----------------------------------------------------------------------------------- Length = 4000 samples | Shot Elevation = 789.3 Sample Interval = 0.00025 sec. | Shot Depth = 0.0 Delay Time = 0 msec. | Up Hole Time = 0 msec Low Cut Filter = 10 Hz. | Shot X-COORD = 1052.01 High Cut Filter = 1000 Hz. | Shot Y-COORD = 1101.10 Line ID: | Shot Date (year.day) = 0.0000 Shot Orientation: | Shot Time (hr:min) = 00:00 Azimuth= 0 Deg. Vertical=180 Deg.| Charge Size (grams)= 0 ----------------------------------------------------------------------------------- TRACE|SHOT| STATION | OFFSET| RECEIVER |VERT|1STBRK|K-GAIN|AZI|VER| # |REC.|SHOT REC| | ELEV. X-COORD Y-COORD|FOLD|(SEC.)| (dB) | | | -----|----|---------|-------|----------------------------|--|------|------|---|---| 1 |1001| 00 00|1715.28| 0.00 0.00 0.00| 7|0.0000| 24 | 0| 0| 2 |1001| 00 00|1715.28| 0.00 0.00 0.00| 7|0.0000| 24 | 0| 0| 3 |1001| 00 00|1715.28| 0.00 0.00 0.00| 7|0.0000| 24 | 0| 0| 4 |1001| 00 00|1715.28| 0.00 0.00 0.00| 7|0.0000| 24 | 0| 0| 5 |1001| 00 00|1715.28| 0.00 0.00 0.00| 7|0.0000| 24 | 0| 0| 6 |1001| 00 00|1715.28| 0.00 0.00 0.00| 7|0.0000| 24 | 0| 0| 7 |1001| 00 00|1715.28| 0.00 0.00 0.00| 7|0.0000| 24 | 0| 0| 8 |1001| 00 00|1715.28| 0.00 0.00 0.00| 7|0.0000| 24 | 0| 0| 9 |1001| 00 00|1715.28| 0.00 0.00 0.00| 7|0.0000| 24 | 0| 0| 10 |1001| 00 30| 90.56| 789.53 1010.76 1020.48| 7|0.0000| 24 | 0| 0| 11 |1001| 00 29| 90.62| 789.48 1009.83 1020.89| 7|0.0000| 24 | 0| 0| 12 |1001| 00 28| 90.54| 789.48 1008.99 1021.43| 7|0.0000| 24 | 0| 0| 13 |1001| 00 27| 90.56| 789.44 1008.22 1021.83| 7|0.0000| 24 | 0| 0| 14 |1001| 00 26| 90.58| 789.37 1007.26 1022.35| 7|0.0000| 24 | 0| 0| 15 |1001| 00 25| 95.28| 789.86 1005.26 1018.09| 7|0.0000| 24 | 0| 0| 16 |1001| 00 24| 95.34| 789.90 1006.10 1017.55| 7|0.0000| 24 | 0| 0| 17 |1001| 00 23| 95.32| 789.90 1006.98 1017.09| 7|0.0000| 24 | 0| 0| 18 |1001| 00 22| 95.41| 789.93 1007.81 1016.55| 7|0.0000| 24 | 0| 0| 19 |1001| 00 21| 95.51| 789.98 1008.65 1016.01| 7|0.0000| 24 | 0| 0| 20 |1001| 00 20| 100.45| 790.21 1006.44 1011.59| 7|0.0000| 24 | 0| 0| 21 |1001| 00 19| 100.38| 790.23 1005.57 1012.11| 7|0.0000| 24 | 0| 0| 22 |1001| 00 18| 100.31| 790.22 1004.69 1012.65| 7|0.0000| 24 | 0| 0| 23 |1001| 00 17| 100.29| 790.23 1003.86 1013.13| 7|0.0000| 24 | 0| 0| 24 |1001| 00 16| 100.25| 790.28 1003.02 1013.64| 7|0.0000| 24 | 0| 0| 25 |1001| 00 00|1715.28| 0.00 0.00 0.00| 7|0.0000| 24 | 0| 0| 26 |1001| 00 00|1715.28| 0.00 0.00 0.00| 7|0.0000| 24 | 0| 0| 27 |1001| 00 00|1715.28| 0.00 0.00 0.00| 7|0.0000| 24 | 0| 0| 28 |1001| 00 00|1715.28| 0.00 0.00 0.00| 7|0.0000| 24 | 0| 0| . . . continues for total of 64 channels . . .

Note that channels 1-9 are disconnected from any geophones, as are 25-28. The live channels in the portion of the dump shown are 10-24. We are particularly interested in the “STATION REC” label, as this will be used to select a single geophone signal in our gather. We begin by concatenating all the shot records into a single file using simple Unix commands in a bash script.

{listmerge} #!/bin/bash # script to concatenate all shot records into file 0000.seg F="1021.seg 1020.seg 1019.seg 1018.seg 1017.seg 1016.seg 1015.seg 1014.seg 1013.seg 1012.seg 1011.seg 1010.seg 1009.seg 1008.seg 1007.seg 1006.seg 1005.seg 1004.seg 1003.seg 1002.seg 1001.seg" for f in $F; do cat $f >> 0000.seg done

We then use program bext to extract those signals who have a “STATION REC” header of 26.

The result can be viewed by examining the header dump for file VP26.seg. Because the shots were concatenated in order from largest file number to smallest, our extraction of geophone 26 signals retains that ordering. Here is the partial header dump of the new file. Note that the first trace corresponds to SHOT RECORD, 1021.seg. This is labeled as SP100 on the map (SP=shot point). The last trace corresponds to the Northern most shot, SP00 on the map. All the traces are for geophone STATION REC = 26. We named the file VP26.seg following the convention of VP=voltage point (a historic reference to the take-out where a geophone is plugged in). On Figure 38, VP26 is labeled V26 to save space, and the SP is dropped for the similar reason, to avoid clutter.

{list26} |-------------------------------| | PARTIAL SEGY HEADER DUMP | | VP26.seg | |-------------------------------| ----------------------------------------------------------------------------------- Length = 4000 samples | Shot Elevation = 789.9 Sample Interval = 0.00025 sec. | Shot Depth = 0.0 Delay Time = 0 msec. | Up Hole Time = 0 msec Low Cut Filter = 10 Hz. | Shot X-COORD = 1006.80 High Cut Filter = 1000 Hz. | Shot Y-COORD = 1012.04 Line ID: | Shot Date (year.day) = 0.0000 Shot Orientation: | Shot Time (hr:min) = 00:00 Azimuth= 0 Deg. Vertical=180 Deg.| Charge Size (grams)= 0 ----------------------------------------------------------------------------------- TRACE|SHOT| STATION | OFFSET| RECEIVER |VERT|1STBRK|K-GAIN|AZI|VER| # |REC.|SHOT REC| | ELEV. X-COORD Y-COORD|FOLD|(SEC.)| (dB) | | | -----|----|---------|-------|----------------------------|--|------|------|---|---| 1 |1021| 100 26| 10.33| 789.37 1007.26 1022.35| 1|0.0000| 24 | 0| 0| 2 |1020| 95 26| 6.30| 789.37 1007.26 1022.35| 1|0.0000| 24 | 0| 0| 3 |1019| 90 26| 4.33| 789.37 1007.26 1022.35| 1|0.0000| 24 | 0| 0| 4 |1018| 85 26| 6.51| 789.37 1007.26 1022.35| 1|0.0000| 24 | 0| 0| 5 |1017| 80 26| 10.92| 789.37 1007.26 1022.35| 1|0.0000| 24 | 0| 0| 6 |1016| 75 26| 15.79| 789.37 1007.26 1022.35| 1|0.0000| 24 | 0| 0| 7 |1015| 70 26| 20.72| 789.37 1007.26 1022.35| 1|0.0000| 24 | 0| 0| 8 |1014| 65 26| 25.83| 789.37 1007.26 1022.35| 1|0.0000| 24 | 0| 0| 9 |1013| 60 26| 30.77| 789.37 1007.26 1022.35| 1|0.0000| 24 | 0| 0| 10 |1012| 55 26| 35.74| 789.37 1007.26 1022.35| 1|0.0000| 24 | 0| 0| 11 |1011| 50 26| 41.20| 789.37 1007.26 1022.35| 1|0.0000| 24 | 0| 0| 12 |1010| 45 26| 45.68| 789.37 1007.26 1022.35| 1|0.0000| 24 | 0| 0| 13 |1009| 40 26| 50.67| 789.37 1007.26 1022.35| 1|0.0000| 24 | 0| 0| 14 |1008| 35 26| 55.63| 789.37 1007.26 1022.35| 1|0.0000| 24 | 0| 0| 15 |1007| 30 26| 60.64| 789.37 1007.26 1022.35| 1|0.0000| 24 | 0| 0| 16 |1006| 25 26| 65.61| 789.37 1007.26 1022.35| 1|0.0000| 24 | 0| 0| 17 |1005| 20 26| 70.62| 789.37 1007.26 1022.35| 1|0.0000| 24 | 0| 0| 18 |1004| 00 26| 90.58| 789.37 1007.26 1022.35| 9|0.0000| 24 | 0| 0| 19 |1003| 15 26| 75.61| 789.37 1007.26 1022.35| 1|0.0000| 24 | 0| 0| 20 |1002| 10 26| 80.62| 789.37 1007.26 1022.35| 1|0.0000| 24 | 0| 0| 21 |1001| 00 26| 90.58| 789.37 1007.26 1022.35| 7|0.0000| 24 | 0| 0|

A single shot record collected with an airgun in the river can produce a large amplitude surface wave train along the river bottom. That was the case with this survey, and so the P-wave refractions required enhancement to be picked. This included array forming by summation of geophone signals from geophones in the grid, followed by filtering to enhance the higher frequency content. Figure 39 illustrates the benefit of additional processing. The enhancement processing with BSU included:

**Figure 39:** Array forming and filtering to enhance higher frequencies were needed to pick refractions. (A) shows an array formed record with strong Rayleigh and SV wave content. (B) is a blowup of the shallow data enhanced for P-waves by filtering.
$\includegraphics[scale=.7]{FigureT}$