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The geophone arrays can be summed to cancel traffic noise, beam forming to receive signals from the air guns. Recorded data are sorted into 3 northern geophone gathers, and 3 southern geophone gathers using the BEXT program (N000.seg, N001.seg, N002.seg, S001.seg, S002.seg, S003.seg). BREF is run with the following command:
bref 001 6 10. 110. 0 1 N000.seg N001.seg N002.seg S000.seg S001.seg S002.seg
The BREF program produces output files G001, D001, and E001 as in the normal shooting example (8.4.6.3 ). Some editing is required. The BREF code detected one constraint equation on this run. Further, a need for 2 more constraints was found so that all delaytimes were made equal at stations 90, 95, 100. These constraints were strongly weighted (factor of 50), and the G001 matrix was edited as shown below (relevant tail of the matrix):
0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 45.363 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 40.364 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 35.382 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 30.329 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 20.336 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 15.075 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 10.564 0 0 0 50 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 -50 0 0 0.000 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 -50 50 0 0.000 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 -50 0 50 0.000
The first 6 columns are for the 6 geophone gathers. The D001 matrix tail is shown here:
0.03300 55 0.03120 60 0.02840 65 0.02660 70 0.02390 80 0.01880 85 0.01720 90 0.00000 0 0.00000 0 0.00000 0
One extra file is required when shooting this way. One needs a water depth file which gives the depth to the river bottom below the surface shots. This is a two column ASCII text file (depth, shot#). One starts an octave session and then runs delaytmR.m:
delaytmR GUI enter names of the files G001, D001, E001 and wds.data GUI number of geophones on shore, 6 GUI smoothness weight, 0.1 a good value GUI displays refractor velocity (2216 m/s) and geophone delay times below geophones (7.8, 9.9, 11.0, 2.5, 8.7, 8.6 ms), OK PLOT shows delay times under the trans-river sources GUI enter an overburden velocity in m/s, for example 1500 m/s PLOT showing structure if overburden velocity is as assumed (ground or water surface, bottom of river, refractor structure) GUI enter alternative limiting case of constant depth refractor PLOT of alternative, constant depth refractor, V1 varies GUI enter number of constraint equations, 3 OK PLOT of the observed data and fit to the equations.
The resulting structural plot is shown in Figure 43 and the arrival time fit in Figure 44. Delay times, , are related to the distance from the surface to the refractor, by the critical angle, :
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