Seismic 2D Reflection Processing and Interpretation of ... - Posiva

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14Figure 8. Shot gather from the line S28 after the preliminary filtering with automaticgain control. First nine shots. Trace number on X-axis and two way travel time inseconds on Y-axis.Refraction static corrections (Table 2, item 6) were calculated using Microsoft Exceland were applied to data after the preliminary filtering. Values of static correctionsmust be integers in milliseconds. The purpose of the refraction static corrections was toeliminate the effect of the weathering layer and the surface elevation variations (seeFigure 9). Refraction statics calculation was based on a first P-wave direct arrivaltimes (first breaks, see Figure 3). Refracted first breaks represent the base of aweathering layer.SHOTRECEIVERSURFACEWEATHERING LAYERv = 5000 m/sFigure 9. Refraction static corrections eliminate the effect of the weathering layer andthe surface elevation variations. Arrows represent the value of correction applied totrace located between the shot and the receiver.The static error was removed from each trace by computing the theoretical first breaksusing the replacement velocity of 5000 m/s and trace offset. Correction was obtainedby subtracting theoretical first break from observed first break. After applyingrefraction static corrections to seismic data, the base of weathering layer becomes a
15new datum and elevation differences between the shot and the geophone and betweenthe geophones are eliminated (Figure 9). Because depth of the real weathering layermay vary from trace to trace, some differences between the shots might still remain.We can estimate that these variations are approximately +/- 2 m based on a previousinterpretations (Ihalainen 2003, Lehtimäki 2003), which leads to +/- 0,8 ms error intwo way travel times using the replacement velocity of 5000 m/s. This is acceptable,because the average amplitude among the traces is many times greater than this errorand because error is smaller than the 1 ms resolution of refraction static corrections.Another problem caused by this approach to refraction statics, is unknown datum,because there is no information about the thickness of the weathering layer or lowvelocity volumes near surface. These may be defined using ray-tracing inversion offirst breaks. In practice the applied procedure led to adequate accuracy of the result.Comparison between stacked sections with and without refraction static corrections(Figure 20 a) and Figure 20 b) and Figure 22 and Figure 24) indicates that theapproach is easy and fast and therefore adequate and reasonable to perform. Exampleof a shot gather before and after the refraction statics is shown in Figure 10 andFigure 11.Figure 10. First three shots with AGC from the line S28 after the preliminary filteringbut without refraction statics. Trace number on X-axis and two way travel time inseconds on Y-axis.
Page 1 and 2: Working Report 2006-114Seismic 2D R
Page 3 and 4: Seismic 2D Reflection Processing an
Page 5: 1TABLE OF CONTENTSABSTRACTTIIVISTEL
Page 9 and 10: 52 SOURCE DATA2.1 Surface seismic d
Page 11 and 12: 7Figure 2. Survey array. At the lin
Page 13 and 14: 93 DATA PROCESSINGThe aim was to pe
Page 15 and 16: 11Processing was performed using fr
Page 17: 13Figure 7. Original spectrum from
Page 21 and 22: 1750-200 mslower cutoff frequency:
Page 23 and 24: 19Original first breaks were picked
Page 25 and 26: 21Figure 17. Shot gather from the l
Page 27 and 28: 23a)CDP position b)CDP numberFigure
Page 29 and 30: 25a) b)Figure 21. a) Final stacked
Page 31 and 32: 27Figure 23. Stacked section withou
Page 33 and 34: 294 INTERPRETATIONAfter the seismic
Page 35 and 36: 31events) indicates presence of off
Page 37 and 38: 33Table 4. Positions and dips for m
Page 39 and 40: 35In Figure 28 are shown the picked
Page 41 and 42: 37Figure 30. 3D view (from the sout
Page 43 and 44: 39Figure 32. Matching events in the
Page 45 and 46: 415 CONCLUSIONSAim of this work was
Page 47 and 48: 43REFERENCESAhokas, H. & Vaittinen,
Page 49 and 50: 45APPENDIX 1. STACKED SEISMIC SECTI
Page 51 and 52: 47Figure 3. Stacked section from th
Page 59 and 60: 55Figure 11. Stacked section from t
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65Figure 21. Stacked section from t
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Figure 41. Stacked section from the
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Figure 45. Stacked section from the
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Seismic 2D Reflection Processing and Interpretation of ... - Posiva

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