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

31 INTRODUCTIONPosiva 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 anunderground research facility, ONKALO, is going on at the Olkiluoto site. SuomenMalmi Oy has carried out shallow refraction seismic surveys in 2001 and 2002 in orderto determine the overburden thickness and to study seismic P-velocity on bedrocksurface and to locate e.g. possible fracture zones (Ihalainen 2003). In year 2001 surveylines from S1 to S27 were measured (15,4 km in total) and in year 2002 survey linesfrom S27 to S69 (17,57 km in total).The aim of this work was to use two-dimensional reflection seismic processingmethods to refraction seismic data collected from the ONKALO area, and to locategently dipping reflectors from the processed survey lines.Processing applied open source software Seismic Unix (SU) (Stockwell & Cohen2002). Processing included reading and re-arranging the data set, removal of noisytraces, geometric spreading correction, bandpass filtering, refraction and residualstatics, deconvolution, airwave muting, automatic gain control, normal moveout andstacking. The goal was to find the best possible processing parameters for the seismicdata set. The refraction measurement array is not optimized for reflection processingwhich leads to quite small 2D fold, approximately only 4, which makes the processinga demanding task.After the processing two-dimensional reflectors will be picked in 3D visualizationenvironment and combined into three-dimensional surfaces. Special attention will begiven for the detection of possible faults and discontinuities.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 site where extensive surface seismic surveys may not be possibleto perform and therefore the effective exploration of existing data is important.The work was conducted on Posiva’s commission, order number 9742/06/TUAH.Contact person on Posiva side was Mr. Turo Ahokas. Processing and reporting wascarried out by Ms. Ida Öhman of Pöyry Environment. Techniques were discussedthrough with and advised by Dr. Christopher Juhlin of Uppsala University, Dr. CalinCosma and Mr. Mircea Cozma of Vibrometric and Mr. Tomas Lehtimäki and Mr. EeroHeikkinen of Pöyry Environment. Mr. Lehtimäki organised the raw data and computedthe geometric conversion of picked surfaces. Mr. Jorma Nummela of PöyryEnvironment converted the surfaces into AutoDesk and Surpac Vision 3D visualisationsoftware.