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

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40Figure 33. Line number S46 and boreholes KR4 (on the left) and KR7 with P-wavevelocity (blue) and density (green), a view from the North. The reflector at 130 – 150ms time is clearly seen in the image, and will match also the borehole geophysicalvelocity minima and fracturing indications (borehole length 220 m in KR7 and 320 min KR4). Borehole trace is in yellow; please note the boreholes are off-plane.
415 CONCLUSIONSAim of this work was to process two-dimensional refraction seismic data collectedfrom the ONKALO area using reflection seismic processing methods and to locategently dipping reflectors from the processed survey lines. This was a demanding taskbecause of a small fold, approximately only four. Survey lines S28-S35, S43-S48,S282, S302, S312, S322, S342, S362, S372, S432, S442, S443, S50 measured in 2002were selected for the seismic processing. After the processing the two dimensionalreflectors were picked in OpendTect visualization environment and combined intothree-dimensional surfaces. Special attention was given for the detection of faults anddiscontinuities. The surfaces were transferred to 3D presentation utility and comparedto available geological 3D information (Paulamäki et al. 2006).Processing was done mainly using free seismic data processing package Seismic Unix(SU). The most important processing steps were the refraction statics anddeconvolution. Without these steps and careful selections of parameters it was notpossible to see any clear reflectors in stacked sections (Figure 20 a) and Figure 21 b)).Same processing parameters were applied to the whole seismic data set but refractionand residual statics needed to be computed separately for each survey line. Residualstatics was the most time-consuming processing step.Result of this work was a set of two-dimensional seismic sections and threedimensionalsurfaces picked from these sections presented in 3D presentation utility.Quality of the seismic data was quite good down to the 200 ms (400 m) but after thatsignal-to-noise ratio decreases, which can be seen in the seismic sections (APPENDIX1). Therefore reflectors are mainly found from the upper parts of the data. If the shotsare located near the geophones, airwave covers most of the data down to 200 ms (400m). Processing was completed successfully, as clear two-dimensional reflectors weredetected from the data. In 3D these reflectors continue from line to line and constructsurfaces which can be compared to borehole observations.Refraction statics were calculated quite roughly using picked first breaks andreplacement velocity of 5000 m/s in order to get the best possible coverage of theONKALO area within the available schedule. In practise the applied procedure led toadequate accuracy of the results. Another option could have been to use for example amethod called seismic travel time tomography (ray tracing) to determine staticcorrections and near-surface velocity variations (Bergman et al. 2004).Fold was quite small, which has an effect on stacked sections. With a bigger foldreflectors would have been clearer and have a bigger amplitude, which would havemade the interpretation easier. From the stacked sections (Figure 21 a) and Figure 24)we can see that reflectors are mainly seen because of their phase coherence.Dips and locations of three-dimensional surfaces were corrected using the procedurewhich corresponds roughly a 3D migration (Prissang et al. 2004). Migrated threedimensionalsurfaces were transported to AutoDesk environment, where they weredisplayed with other data. Depth of the reflectors was determined using theapproximate velocity of 5000 m/s. Selecting the velocity e.g. either 5000 m/s, or 5500m/s, will lead to approx. 10% error in the depth axis.Because of an airwave, that covers most of the reflectors in the upper parts of the inlineshots, one option would have been to remove all the traces with offsets bigger than
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 and 18: 13Figure 7. Original spectrum from
Page 19 and 20: 15new datum and elevation differenc
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: 39Figure 32. Matching events in the
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
Page 89 and 90: Figure 41. Stacked section from the
Page 93 and 94: Figure 45. Stacked section from the
Page 95 and 96:
91Figure 47. Stacked section from t
Page 97 and 98:
93Figure 49. Stacked section from t
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