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

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Working Report 2006-114Seismic 2D Reflection Processing andInterpretation of Shallow Refraction DataIda ÖhmanEero HeikkinenTomas LehtimäkiPöyry Environment OyDecember 2006Working Reports contain information on work in progressor pending completion.The conclusions and viewpoints presented in the reportare those of author(s) and do not necessarilycoincide with those of Posiva.
Seismic 2D Reflection Processing and Interpretation ofShallow Refraction DataABSTRACTPosiva Oy takes care of the final disposal of spent nuclear fuel in Finland. In year 2001Olkiluoto was selected for the site of final disposal. Currently construction of theunderground research facility, ONKALO, is going on at the Olkiluoto site.The aim of this work was to use two-dimensional reflection seismic processing methodsto refraction seismic data collected from the ONKALO area in year 2002, and to locategently dipping reflectors from the stacked sections.Processing was done using mainly open source software Seismic Unix. After theprocessing, the most distinct two-dimensional reflectors were picked from seismicsections using visualization environment OpendTect. After picking the features fromcrossing lines were combined into three-dimensional surfaces. Special attention wasgiven for the detection of possible faults and discontinuities. The surfaces were givencoordinates and their orientation was adjusted using a geometric procedure, whichcorresponds roughly a 3D migration, transferred to 3D presentation utility andcompared to available geological information.The advantage of this work is to be able to get three-dimensional reflection seismicresults from existing data set at only processing costs. Survey lines are also partlylocated in ONKALO area where extensive surface seismic surveys may not be possibleto perform.The applied processing method was successful in detecting the reflectors. Mostsignificant steps were the refraction and residual statics, and deconvolution. Somedistinct reflectors can be seen at times 20-200 ms (vertical depths 50...500 m). Thesignal gets noisier below 200 ms. Reflectors are best visible as coherent phase betweenthe adjacent traces, but do not raise much above the surrounding noise level. Higheramount of traces to be stacked would emphasis the reflections and their continuitymore.Reflectors picked on crossing lines match well to borehole observations (KR4, KR7,KR24 and KR38) of fracture zones, and get support from geological and hydrologicalmodels of the site. The observed reflections coincide with fracturing intensity and P-wave velocity minima from boreholes. Reflections coincide also rather well to theseparate 3D seismic results from overlapping area.The results demonstrate that seismic measurements intended for refraction interpretationcan also be successfully processed using reflection seismic processing methods.Increasing number of active geophones and shots, and line density, would enhancereliability of the reflections.Keywords: Seismic, reflection, refraction, processing, interpretation, crystalline,bedrock, migmatite, fracture zone, spent nuclear fuel, geological disposal
Page 1: Working Report 2006-114Seismic 2D R
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 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
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49Figure 5. Stacked section from th
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55Figure 11. 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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