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

More documents

Recommendations

Info

28Figure 24. Final stacked section from the line S342 down to 300 ms using a wiggletrace display. CDP number on X-axis and two way travel time in seconds on Y-axis.
294 INTERPRETATIONAfter the seismic processing has been performed for the selected set of refractionseismic survey lines, data was interpreted. First step was to examine two dimensionalstacked sections, which can be found from the APPENDIX 1, and to locate possiblereflectors. After this, two-dimensional stacked sections were imported into thevisualization environment OpendTect (OpendTect 2006) and reflectors were picked in3D.4.1 Two dimensional reflectorsTwo dimensional seismic data from the survey lines shown in Figure 1, was processedusing the processing parameters in Table 2. The result was a set of stacked seismicsections which can bee seen in APPENDIX 1 using variable density display. Thesetwo-dimensional sections can be used to locate the reflectors but the actual picking isdone in 3D. Taking a closer look at the reflector below 130 ms in lines S28, S29, S30and S31, it can be seen continuing from line to line. Very clear reflectors can be seen inseveral lines but for example in line S47 only few weak ones are seen. Reflectors aremostly found from the upper parts of the seismic sections. This is due to the lowersignal-to-noise ratio (see shot gathers, Figure 5) in the later times of the data (greaterdepths). This can also be seen from the stacked sections (see APPENDIX 1). Twodimensionalreflectors are shown in APPENDIX 1.Some reflectors may not appear to be very clear in first place, and may be hard todetect from individual lines. Different image colour settings, aspect ratios and scaleswere applied. Picking in 3D allows locating reflectors on adjacent and crossing lines,which helps to recognise continuous reflecting events. In Figure 25 (Lines 48, 50 and362) the reflectors are marked with red arrows.In APPENDIX 1 scale is 1:5000 in order to display data more clear. Picking was easierwith stretched X dimension in 3D.Depth of the reflectors is determined using the approximate velocity of 5000 m/s. Thevelocities used in static correction and NMO give a generic idea on the real velocitiesin bedrock. However the true velocity has to be confirmed using velocity fitting ondiffractions seen on seismic sections, from external sources (VSP data and 3Dreflection data) and with using a borehole control. Processing has applied a phaseconsistent (zero phase) work flow. Correctness of depth axis will rely on recognition ofcorrect phase of the reflected arrival, e.g. ¼ - ½ wavelength late picks will meanapprox. 5-10 m too deep depth level or the reflection. Selecting the velocity e.g. either5000 m/s, or 5500 m/s, will lead to approx. 10% error in the depth 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 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: 27Figure 23. Stacked section withou
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
Page 83 and 84:
79Figure 35. Stacked section from t
Page 85 and 86:
Page 87 and 88:
Page 89 and 90:
Figure 41. Stacked section from the
Page 91 and 92:
Page 93 and 94:
Figure 45. Stacked section from the
Page 95 and 96:
Page 97 and 98:
show all

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

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?