Click here to load reader

Seismic 2D Reflection Processing and Interpretation of Shallow

  • View
    234

  • Download
    9

Embed Size (px)

Text of Seismic 2D Reflection Processing and Interpretation of Shallow

  • P O S I V A O Y

    FI -27160 OLKILUOTO, F INLAND

    Tel +358-2-8372 31

    Fax +358-2-8372 3709

    I da hman

    Eero He ikk inen

    Tomas Leht imk i

    December 2006

    Work ing Repor t 2006 -114

    Seismic 2D Reflection Processing andInterpretation of Shallow Refraction Data

  • December 2006

    Working Reports contain information on work in progress

    or pending completion.

    The conclusions and viewpoints presented in the report

    are those of author(s) and do not necessarily

    coincide with those of Posiva.

    I da hman

    Eero He ikk inen

    Tomas Leht imk i

    Pyry Env i ronment Oy

    Work ing Repor t 2006 -114

    Seismic 2D Reflection Processing andInterpretation of Shallow Refraction Data

  • Seismic 2D Reflection Processing and Interpretation of Shallow Refraction Data

    ABSTRACT

    Posiva Oy takes care of the final disposal of spent nuclear fuel in Finland. In year 2001 Olkiluoto was selected for the site of final disposal. Currently construction of the underground research facility, ONKALO, is going on at the Olkiluoto site.

    The aim of this work was to use two-dimensional reflection seismic processing methods to refraction seismic data collected from the ONKALO area in year 2002, and to locate gently dipping reflectors from the stacked sections.

    Processing was done using mainly open source software Seismic Unix. After the processing, the most distinct two-dimensional reflectors were picked from seismic sections using visualization environment OpendTect. After picking the features from crossing lines were combined into three-dimensional surfaces. Special attention was given for the detection of possible faults and discontinuities. The surfaces were given coordinates and their orientation was adjusted using a geometric procedure, which corresponds roughly a 3D migration, transferred to 3D presentation utility and compared to available geological information.

    The advantage of this work is to be able to get three-dimensional reflection seismic results from existing data set at only processing costs. Survey lines are also partly located in ONKALO area where extensive surface seismic surveys may not be possible to perform.

    The applied processing method was successful in detecting the reflectors. Most significant steps were the refraction and residual statics, and deconvolution. Some distinct reflectors can be seen at times 20-200 ms (vertical depths 50...500 m). The signal gets noisier below 200 ms. Reflectors are best visible as coherent phase between the adjacent traces, but do not raise much above the surrounding noise level. Higher amount of traces to be stacked would emphasis the reflections and their continuity more.

    Reflectors picked on crossing lines match well to borehole observations (KR4, KR7, KR24 and KR38) of fracture zones, and get support from geological and hydrological models of the site. The observed reflections coincide with fracturing intensity and P-wave velocity minima from boreholes. Reflections coincide also rather well to the separate 3D seismic results from overlapping area.

    The results demonstrate that seismic measurements intended for refraction interpretation can also be successfully processed using reflection seismic processing methods. Increasing number of active geophones and shots, and line density, would enhance reliability of the reflections.

    Keywords: Seismic, reflection, refraction, processing, interpretation, crystalline, bedrock, migmatite, fracture zone, spent nuclear fuel, geological disposal

  • Seismisen refraktiomittauksen 2D reflektioprosessointi ja tulkinta

    TIIVISTELM

    Posiva Oy vastaa radioaktiivisen ydinjtteen loppusijoituksesta Suomessa. Vuonna 2001 Olkiluoto valittiin ydinjtteiden loppusijoituspaikaksi. Parhaillaan Olkiluotoon raken-netaan ONKALOA, maanalaista tutkimustilaa.

    Tmn tyn tarkoituksena on prosessoida ONKALO-alueelta vuonna 2002 kertty refraktioseismist dataa kytten heijastusseismiikkaa varten kehitettyj prosessointi-menetelmi, sek tulkita pinotuista sektioista loivakaateisia heijastajia.

    Prosessointi suoritettiin kytten pasiassa vapaan lhdekoodin ohjelmaa Seismic Unix. Prosessoinnin jlkeen selvimmin erottuvat kaksiulotteiset heijastajat poimittiin seismisist vertikaalileikkauksista OpendTect-ohjelmalla. Poiminnan jlkeen ristevien linjojen heijastuspiirteit yhdistettiin kolmiulotteisiksi pinnoiksi. Poiminnassa kiinni-tettiin erityisesti huomiota mahdollisiin siirroksiin ja epjatkuvuuksiin. Pinnoille laskettiin koordinaatit ja tehtiin asentokorjaus kytten menetelm, joka vastaa kar-keasti 3D-migraatiota (Paulamki et al. 2006). Migratoidut pinnat siirrettiin ohjelmaan, jossa ne voitiin esitt kolmiulotteisesti, ja niit verrattiin olemassa olevaan geologiseen informaatioon.

    Tyn etuna on se, ett heijastusseismisi 3D-tuloksia saadaan olemassa olevasta refraktiodatasta pelkin prosessointikustannuksin. Mittauslinjat sijaitsevat osittain alueel-la, jolla laajoja seismisi mittauksia ei en ole mahdollista suorittaa, joten jo olemassa olevan datan hydyntminen on trke.

    Kytetyll prosessointimenetelmll onnistuttiin tulkitsemaan heijastajat. Trkeimmt tyvaiheet olivat refraktio- ja residuaalinen staattinen korjaus sek dekonvoluutio. Joita-kin selvi heijastuksia nhdn 20 200 ms aikavlill (50 500 m vertikaalisyvyys). Signaalin kohina voimistuu 200 ms jlkeen. Heijastukset havaitaan parhaiten koherentin vaiheen perusteella vierekkisten kuvaajien vlill, koska amplitudi ei kohoa paljoa ympristn kohinatason ylpuolelle. Suurempi pinoamisen kertaluku vahvistaisi heijas-tuksia ja niiden jatkuvuutta.

    Ristevilt linjoilta poimitut heijastajat osuvat hyvin yhteen kairanreikhavaintojen (KR4, KR7, KR24 ja KR38) rikkonaisuusvyhykkeiden kanssa, ja saavat tukea geologisista ja hydrologisista malleista. Heijastukset korreloivat kasvaneen, reiist havaitun rakotiheyden ja seismisen P-aallon minimien kanssa. Samoin yhteensopivuus on hyv erillisten 3D-heijastustulosten kanssa pllekkiselt alueelta.

    Tulokset osoittavat ett refraktiotulkintaa varten tehtv seisminen mittaus kannattaa suunnitella siten, ett mys reflektioprosessointi on mahdollinen. Lismll aktiivisten geofonien ja lhdepisteiden mr sek linjatiheytt heijastukset nhdn luotetta-vammin.

    Avainsanat: Seisminen, heijastus, taittuminen, ksittely, tulkinta, kiteinen peruskallio, migmatiitti, rikkonaisuusvyhyke, kytetty ydinpolttoaine, geologinen loppusijoitus

  • 1

    TABLE OF CONTENTS

    ABSTRACT

    TIIVISTELM

    1 INTRODUCTION .............................................................................................. 3

    2 SOURCE DATA................................................................................................ 5

    2.1 Surface seismic data............................................................................. 5

    2.2 Background information ........................................................................ 8

    3 DATA PROCESSING ....................................................................................... 9

    4 INTERPRETATION......................................................................................... 29

    4.1 Two dimensional reflectors ................................................................. 29

    4.2 Three dimensional surfaces ................................................................ 30

    4.3 Comparison to borehole data.............................................................. 33

    5 CONCLUSIONS.............................................................................................. 41

    REFERENCES ........................................................................................................... 43

    APPENDIX 1. STACKED SEISMIC SECTIONS AND PICKED REFLECTORS. ....... 45

  • 2

  • 3

    1 INTRODUCTION

    Posiva Oy takes care of the final disposal of spent nuclear fuel in Finland. In year 2001 Olkiluoto was selected for the site of final disposal. Currently construction of an underground research facility, ONKALO, is going on at the Olkiluoto site. Suomen Malmi Oy has carried out shallow refraction seismic surveys in 2001 and 2002 in order to determine the overburden thickness and to study seismic P-velocity on bedrock surface and to locate e.g. possible fracture zones (Ihalainen 2003). In year 2001 survey lines from S1 to S27 were measured (15,4 km in total) and in year 2002 survey lines from S27 to S69 (17,57 km in total).

    The aim of this work was to use two-dimensional reflection seismic processing methods to refraction seismic data collected from the ONKALO area, and to locate gently dipping reflectors from the processed survey lines.

    Processing applied open source software Seismic Unix (SU) (Stockwell & Cohen 2002). Processing included reading and re-arranging the data set, removal of noisy traces, geometric spreading correction, bandpass filtering, refraction and residual statics, deconvolution, airwave muting, automatic gain control, normal moveout and stacking. The goal was to find the best possible processing parameters for the seismic data set. The refraction measurement array is not optimized for reflection processing which leads to quite small 2D fold, approximately only 4, which makes the processing a demanding task.

    After the processing two-dimensional reflectors will be picked in 3D visualization environment and combined into three-dimensional surfaces. Special attention will be given for the detection of possible faults and discontinuities.

    The advantage of this work is to be able to get three-dimensional reflection seismic results from existing data set at only processing costs. Survey lines are also partly located in ONKALO site where extensive surface seis

Search related