P O S I V A O Y

O l k i l u o t o

F I -27160 EURAJOKI , F INLAND

Te l +358-2-8372 31

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I lmo Kukkonen , Markku Paananen ,

Seppo E lo , Seppo Pau lamäk i , Jukka La i t i nen ,

H IRE Work ing Group

of the Geo log i ca l Survey o f F i n l and ,

Pekka He ikk inen , Suv i He inonen

August 2010

Work ing Repor t 2010 -57

HIRE Seismic Reflection Surveyin the Olkiluoto Area

August 2010

Base maps: ©National Land Survey, permission 41/MML/10

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 lmo Kukkonen , Markku Paananen ,

Seppo E lo , Seppo Pau lamäk i , Jukka La i t i nen ,

H IRE Work ing Group

of the Geo log ica l Survey of F in land

Geo log ica l Su rvey o f F in l and

Pekka He ikk inen , Suv i He inonen

I ns t i tu te o f Se i smo logy , Un ive rs i t y o f He l s ink i

Work ing Report 2010 -57

HIRE Seismic Reflection Surveyin the Olkiluoto Area

ABSTRACT

A seismic reflection survey comprising three vibroseismic lines (total length of 31.1

km) was carried out in the Olkiluoto area, western Finland, in July, 2008. The survey is

a part of the project HIRE (High Resolution Reflection Seismics for Ore Exploration

2007-2010) of the Geological Survey of Finland (GTK). The Olkiluoto survey was done

in co-operation with Posiva Oy.

The HIRE seismic reflection survey in the Olkiluoto area revealed numerous previously

unknown structures in the upper crust of the area.

The most prominent structures observed are the subhorizontal strong reflectors which

very probably represent Postjotnian diabase sills intruding both the Svecofennian

gneisses as well as the rapakivi granites. These reflectors can be associated with the

similar seismic structures recorded in marine seismic transects in the Bothnian Sea, and

thus they represent a large-scale structure.

The Mesoproterozoic rapakivi granites can be distinguished as homogeneous,

seismically transparent domains which extend to a depth of at least 4 km. The

interpreted rapakivi structures are in a good agreement with gravity modellings.

On the Olkiluoto Island, reflectors could be correlated with drillhole based data on

lithology and brittle fault zones. The main brittle fault zones detected in drillholes are

represented as reflectors in the seismic sections, and several new structures have been

interpreted. A synthetic seismogram constructed for a 1 km deep hole in the Olkiluoto

Island using down-hole logs of density and P-wave velocity, suggests that the main

brittle fault zones generate strong reflectors. Pegmatitic granite sometimes has a weak

reflection contrast with the surrounding gneisses, and sometimes has no contrast. There

seems to be a simultaneous correlation of reflectivity with fracture zones at the location

of the pegmatitic granite occurrences.

The HIRE 2D seismic reflection data agrees well with the earlier 3D reflection surveys

in the Olkiluoto Island, and most of the reflectors detected in HIRE surveys are present

also in the 3D results. The 2D data can be used to extend the interpretation of the 3D

results in the Olkiluoto Island.

Where the seismic lines cross the lineaments limiting the Olkiluoto bedrock block, the

seismic data can be used to provide estimates of the dip angles of the lineaments. The

results suggest that the lineament separating the Olkiluoto Island from the continent

would be dipping about 38º to SE, and one of the lineaments limiting the NE side of the

block would be dipping 18º to NE, respectively.

Keywords: Seismic reflection surveys, site surveys, spent nuclear fuel, Fennoscandian

Shield, crystalline rocks, Olkiluoto, Finland

HIRE-HANKKEEN SEISMINEN HEIJASTUSLUOTAUS OLKILUODON ALUEELLA LÄNSI-SUOMESSA TIIVISTELMÄ

Olkiluodon alueella Länsi-Suomessa tehtiin heinäkuussa 2008 seisminen heijastus-

luotausmittaus, joka koostui kolmesta vibroseismisellä menetelmällä mitatusta luotaus-

linjasta (yhteispituus 31,1 km). Luotaus kuuluu osana Geologian tutkimuskeskuksen

(GTK) HIRE-hankkeeseen (High Resolution Reflection Seismics for Ore Exploration

2007-2010). Olkiluodon alueen mittaukset tehtiin yhteistyössä Posiva Oy:n kanssa.

HIRE-luotausten perusteella Olkiluodon kallioperästä tuli esiin lukuisia ennen tunte-

mattomia yläkuoren rakenteita. Merkittävimmät havaitut rakenteet ovat vaaka- tai loiva-

asentoisia voimakkaita heijastajia, jotka todennäköisesti edustavat postjotunisia dia-

baasisillejä. Diabaaseiksi tulkitut heijastajat näyttävät leikaavan sekä svekofennialaisia

gneissejä että rapakivigraniitteja. Nämä heijastajat voidaan yhdistää samankaltaisiin

meriseismisissä luotauksissa havaittuihin heijastajiin Selkämerellä, ja siten ne toden-

näköisesti edustavat suuremman mittakaavan kallioperärakenteita.

Mesoproterotsooiset rapakivigraniitit ovat seismisillä profiileilla homogeenisiä, huo-

nosti heijastavia alueita, jotka ulottuvat ainakin 4 km:n syvyyteen. Tulkitut rapakivi-

rakenteet ovat hyvässä sopusoinnussa painovoimamittausten mallitustulosten kanssa.

Olkiluodon saarella heijastajia voitiin korreloida kairausten perusteella selvitettyyn

kivilajijakaumaan ja hauraisiin siirrosvyöhykkeisiin. Tärkeimmät kairauksissa todetut

hauraat siirrosvyöhykkeet havaittiin heijastajina heijastusprofiileilla ja myös useita

ennen tuntemattomia siirrosrakenteita voitiin tulkita. Syvän (n.1 km) kairareiän tiheyden

ja seismisen P-aallon nopeuden reikämittausten perusteella laskettu akustinen impe-

danssi ja synteettinen seismogrammi osoittavat, että hauraat siirrosvyöhykkeet synnyt-

tävät voimakkaita heijastajia. Pegmatiittisella graniitilla on paikoin vain heikko tai ei

lainkaan heijastuseroa ympäröiviin gneisseihin nähden, mutta heijastajat esiintyvät

usein paikoissa, joissa on sekä pegmatiittista graniittia että hauras siirrosvyöhyke.

HIRE-hankkeen 2-dimensionaaliset luotaukset käyvät hyvin yksiin aiemmin Olkiluo-

dossa tehtyjen 3-dimensionaalisten seismisten heijastusluotausten kanssa ja useimmat

HIRE-aineiston heijastajat voidaan havaita myös 3D-aineistosta. 2D-tuloksia voidaan

käyttää laajentamaan 3D-tulosten tulkintaa Olkiluodossa.

Siellä, missä seismiset mittauslinjat leikkaavat Olkiluodon kalliolohkoa rajaavia linea-

menttejä, heijastusluotausten perusteella voidaan tulkita näitä vastaavien kallioraken-

teiden kaadekulmia. Tulkinnan perusteella lineamentti, joka erottaa Olkiluodon saaren

kalliolohkon mantereesta kaatuu noin 38° kaakkoon, kun taas kalliolohkoa koillis-

puolelta rajaava lineamentti kaatuu noin 18° koilliseen.

Avainsanat: Seisminen heijastusluotaus, käytetty ydinpolttoaine, paikkatutkimukset,

kiteiset kivilajit, Fennoskandian kilpi, Olkiluoto, Suomi

1

TABLE OF CONTENTS

ABSTRACT

TIIVISTELMÄ

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

2 SHORT GEOLOGICAL DESCRIPTION OF THE AREA........................................ 5

3 SURVEY METHOD ............................................................................................. 11

4 DATA PROCESSING .......................................................................................... 13

5 RESULTS AND INTERPRETATION ................................................................... 21

5.1 2D and 3D representation of the results .................................................... 21

5.2 Reflection properties of the upper crust in the Olkiluoto area ..................... 27

5.3 Synthetic seismogram and reflection properties of the Olkiluoto Island ...... 29

6 COMPARISON OF REFLECTION DATA WITH LITHOLOGICAL AND FRACTURE ZONE MODELS IN OLKILUOTO .................................................... 33

7 COMPARISON OF HIRE RESULTS WITH PREVIOUS 3D SEISMIC REFLECTION SURVEYS IN OLKILUOTO .......................................................... 41

8 HIRE RESULTS AND GRAVITY SURVEYS ....................................................... 51

9 DISCUSSION ...................................................................................................... 55

10 CONCLUSIONS .................................................................................................. 59

REFERENCES ........................................................................................................... 61

2

3

1 INTRODUCTION

A seismic reflection survey comprising three vibroseismic lines (total length of 31.1

km) was carried out in the Olkiluoto area, western Finland, in July, 2008. The survey is

a part of the project HIRE (High Resolution Reflection Seismics for Ore Exploration

2007-2010) of the Geological Survey of Finland (GTK). The Olkiluoto survey was done

in co-operation with Posiva Oy.

The general aims of the HIRE project are (1) to introduce reflection surveys as an

exploration tool for the Precambrian crystalline bedrock of Finland, (2) to apply 3D

visualization and modelling techniques in interpretation, and (3) to improve the

structural data base on the most important mineral resource provinces in Finland. The

HIRE targets comprise exploration and mining camps in very diverse geological

environments. Targets include Cu, Ni, Fe, Cr, PGE, Zn, and Au deposits, most of them

economic, as well as the Finnish site for nuclear waste disposal (Olkiluoto).

Olkiluoto Island has been chosen for the disposal site of spent nuclear fuel of the power

companies Teollisuuden Voima Oyj and Fortum Power and Heat Oy. The aims of the

HIRE survey in the Olkiluoto area was to reveal the upper crustal structures in an area

much wider than the Olkiluoto Island, and to explore the structures in detail and much

deeper than is possible with other geophysical techniques.

The survey was carried out by the Geological Survey of Finland using SFUE

Vniigeofizika, Moscow, Russia, as the seismic contractor. The HIRE project is partly

funded from the debt conversion agreement between Finland and Russia. The Olkiluoto

survey was agreed between GTK and Posiva Oy based on the offer by GTK dated

February 28, 2008.

The survey comprised three vibroseismic soundings along roads. The field work was

carried out during July 1-16, 2008.

4

5

2 SHORT GEOLOGICAL DESCRIPTION OF THE AREA

The Palaeoproterozoic supracrustal rocks of the HIRE survey area (Fig. 5-1) are

metasedimentary, which is demonstrated by the quartz-feldspar-rich and biotite-rich

layers within the mica gneisses, representing psammitic and pelitic layers, respectively

(Suominen et al. 1997). Almost all the mica gneisses are variably migmatised, and the

type and the intensity of the migmatisation vary considerably. The older component of

the migmatite, or palaeosome, is mainly composed of quartz, plagioclase, biotite and

often also potassium feldspar. It contains macroscopic garnet and cordierite

porphyroblasts, as well as microscopic sillimanite inclusions in the biotite and cordierite

grains. The younger component of the migmatite, or neosome, mainly occurs as coarse-

grained and pegmatitic granite veins parallel to the foliation and banding. Thus, the

migmatites are usually veined gneisses in their migmatite type (Fig. 2-1A). The main

minerals of the neosome are quartz, potassium feldspar and plagioclase. Occasionally it

contains the same porphyroblasts as the palaeosome.

The largest Palaeoproterozoic plutonic intrusions of the Rauma map sheet area are

composed of trondhjemites, tonalites and granodiorites (Suominen et al. 1997), but in

the HIRE survey area they are scarce. The contacts between the different rock types are

gradual. The most felsic of the above rock types, trondhjemites, are medium-grained

and massive or weakly foliated. This group of plutonic rocks also includes gneissose

granodiorites, which are fine- or medium-grained, almost always hornblende-bearing,

and include a few centimetres wide light- and dark-coloured streaks (Suominen et al.

1997). There are indications that they may be paragneisses in origin.

The Palaeoproterozoic granites, porphyritic granites and pegmatites of the HIRE survey

area are very heterogeneous, containing numerous inclusions and granitised restites of

the mica gneiss. They contain garnet and sometimes also cordierite from the assimilated

mica gneisses. The chemical composition of the granites resembles the so-called S-type

granites, which derive from partial melting of the sedimentary rocks (Suominen et al.

1997).

The Palaeoproterozoic rocks are intruded by Mesoproterozoic rapakivi granites. In the

survey area they belong to the Eurajoki rapakivi stock (ca. 70 km2), which is a satellite

massif to much larger (ca. 1400 km2) Laitila rapakivi batholith, located east of the

Eurajoki stock and dated at 1583±3 Ma (Vaasjoki 1996). The Eurajoki stock is

composed of two main rapakivi types: older hornblende-biotite granite, or the Tarkki

granite, dated at 1571±3 Ma, and younger, topaz-bearing microcline-albite granite, or

the Väkkärä granite, dated at 1548±3 Ma (Haapala 1977; Vaasjoki 1996; Suominen et

al. 1997). The contact between the marginal Tarkki granite and the surrounding

Palaeoproterozoic migmatites is sharp. Also the contact between the Tarkki granite and

the central Väkkärä granite is sharp, the Väkkärä granite dipping gently outwards

(Haapala 1977; Elo 2001). The gravity interpretations (Elo 2001) suggest that the

Eurajoki rapakivi granite extends downwards and outwards in every direction, and its

depth exceeds 5 km.

6

The Tarkki granite is homogenous, medium- and even-grained rock, which contains

potassium feldspar ovoids, 3 - 6 cm in diameter, mantled with plagioclase (Haapala

1977) (Fig. X1C). The ovoids are sparsely distributed, the distance between individual

ovoids being often several metres. The Tarkki granite is occasionally cut by topaz-

bearing quartz porphyre dykes, which mineralogically and chemically resemble the

Väkkärä granite, indicating a close genetic connection between the two (Haapala 1977).

The Väkkärä granite is composed of several texturally different types, the contacts of

which are either sharp or gradual (Haapala 1977). The porphyritic (Fig. 2-1c) and

coarse-grained granites are topaz-bearing, the phenocrysts being potassium feldspar and

quartz. Accessory minerals include, in addition to topaz, fluorine-rich siderophyllite,

monazite, ilmenite, Nb- and Ta-rich cassiterite, columbite and bastnaesite. Miarolitic

cavities are common in the porphyritic granites (Haapala 1977). The topaz-bearing

Väkkärä granite resembles typical tin granites in chemical composition. It is enriched in

fluorine, lithium, gallium, rubidium, tin and niobium, and depleted in strontium, barium

and zircon.

The even-grained Väkkärä granite is mainly composed of potassium feldspar, quartz

and biotite, accessory minerals being zircon, ilmenite, anatase, monatzite, as well as

topaz and cassiterite as secondary minerals. The contact type of Väkkärä granite against

the Tarkki granite has potassium feldspar, plagioclase and quartz phenocrysts, 1 - 10

mm in diameter, in a fine-grained matrix. The contact type contains fragments of Tarkki

granite and sends apophyses into it (Haapala 1977).

E-W-trending greisen veins and associated quartz veins occur within the Tarkki granite.

The width of the veins varies from a few centimetres up to two metres, and the drillings

have shown that at least some of the veins and vein swarms extend over 100 m in depth

and over 300 m in a horizontal direction (Haapala 1977). In the Väkkärä granite the

veins are more randomly oriented, and also lensoid, rounded and irregular greisen

bodies occur. The main minerals of the greisen veins are quartz, micas and iron-rich

chlorite. Often they also contain abundant topaz, fluorine, garnet, beryl, genthelvite and

bertnandite (Haapala 1977). The most common ore minerals include sphalerite,

cassiterite, chalcopyrite, wolframite, gahnite, molybdenite, rutile, secondary iron oxide,

pyrite, pyrrhotite, arsenopyrite and galena. The greisen veins were caused by hot,

hydrous fluids, migrating in interstices and fractures of the rapakivi granites (Haapala

1977). The greisen veins both in the Tarkki and Väkkärä granite were formed by fluids

emanated from the Väkkärä granite.

The olivine diabase dykes and sills, dated at 1270 - 1250 Ma (Suominen 1991; Vaasjoki

1996) cut all the other rock types of the survey area. The main minerals of the diabases

are plagioclase, augite, olivine, opaques (ilmenite, magnetite) and rarely biotite

(Hämäläinen 1987: Suominen et al. 1997; Veräjämäki 1998). Usually they also contain

small amounts of quartz and potassium feldspar, which form micrographic intergrowths

(Kahma 1951). The diabases are subophitic in texture indicating a hypabyssal origin.

The diabases are tholeitic in chemical composition, and their geochemical features

suggest that they are feeder channels to continental flood basalts. No such basalts,

however, exists in the area today.

7

In the subhorizontal contact of the Sorkka diabase against the surrounding migmatitic

gneisses, the medium-grained diabase becomes fine-grained or aphanitic towards the

contact (Kahma 1951). Also at the contact between the Sorkka diabase and the Tarkki

rapakivi granite (Fig. 2-1C-D), the diabase is rather aphanitic, but becomes medium-

grained a few metres away from the contact. The diabase, in places, has palingenic

veins, which have resulted from the partial remelting of the country rocks. They are

from a few millimetres up to 10 cm wide and proceed 2 to 3 m, at the most, into the

diabase from the contact. The veins in the diabase-rapakivi granite contact mainly

consist of quartz and alkali feldspars. In addition they contain some biotite, chlorite and

especially near the diabase contact also hornblende (Kahma 1951). One analysed vein

within the Sorkka diabase in Hankkila contains potassium feldspar, plagioclase,

hornblende, muscovite, olivine or pyroxene, with accessory quartz and carbonate

(Suominen et al. 1997). In the contact between the palingenic veins and the olivine

diabase, a narrow reaction zone has been observed. In this zone the olivine and augite of

the diabase have altered into serpentine, amphibole, chlorite and an iron-bearing opaque

mineral, whereas the plagioclase has become corroded (Kahma 1951).

On the basis of outcrop observations during the mapping campaign in 2002 (Paulamäki

2007), the western contact of the Eurajoki stock (Tarkki granite) against the

Palaeoproterozoic migmatitic country rock seems to be gently dipping, the dip of the

contact being about 20° to the west/northwest. The contact is very sharp and intact. No

visible features of contact metamorphism have been observed within the migmatitic

gneiss.

The bedrock of the Olkiluoto Island, where the final disposal of spent nuclear fuel is

planned, is mostly composed of Svecofennian high-grade metamorphic supracrustal

rocks. In terms of their mineral composition, texture and migmatite structure, the rocks

of Olkiluoto can be divided into four major classes: 1) migmatitic gneisses, 2) tonalitic-

granodioritic-granitic gneisses or TGG gneisses, 3) other gneisses including mica

gneisses, quartz gneisses and mafic gneisses, and 4) pegmatitic granites (Kärki &

Paulamäki 2006). The migmatitic rocks can further be subdivided into stromatic

gneisses, veined gneisses and diatexitic gneisses on the basis of their migmatite

structures. All the rock types mentioned above are very sharply cut by ca. 1560 Ma old,

NE-SW striking narrow Sub-Jotnian diabase dykes, which dip steeply to the NW

(Mattila et al. 2008). Post-Jotnian diabases are not known in the Olkiluoto Island.

8

Figure 2-1. A) Veined gneiss, Olkiluoto. B) Homogeneous gneissose tonalite. Ilavainen.

C) Alkali feldspar ovoid with a plagioclase mantle within the medium- and even-

grained Tarkki rapakivi granite, Hankkila. D) Porphyritic Väkkärä rapakivi granite,

Hankkila. E) Subhorizontal contact between the Sorkka diabase (above) and the Tarkki

rapakivi granite (below), Hankkila quarry. F) Close-up photo of the contact in E.

Photos by Seppo Paulamäki, GTK (A-B, E-F) and Aimo Kuivamäki, GTK (C-D).

C D

A B

E F

9

On the basis of the whole-rock chemical compositions, the supracrustal rocks of

Olkiluoto can be divided into four distinct series or groups: a T series, S series, P series

and basic, volcanogenic gneisses (Kärki & Paulamäki 2006), T, S and P standing for

Turbidite, Skarn and Phosphorus, respectively. In addition, pegmatitic granites and

diabases form groups of their own that can be identified both macroscopically and

chemically.

The refolding and crosscutting relationships indicate that the metamorphic supracrustal

rocks have been subject to polyphased ductile deformation, including at least four stages

(D1-D4), the D2 being locally the most intensive phase and producing thrust-related

folding, strong migmatisation and pervasive foliation (Mattila et al. 2008). The age of

the D2 phase is estimated to be approximately at 1860 Ma based on the age of the TGG

gneisses (1863 Ma) deformed in D2. In subsequent thrust-related deformation phase D3,

the earlier deformed migmatites were zonally refolded, rotated or sheared. Zones

dominated by ductile D3 shear structures and tight F3 folds were formed, and often the

D2 structures were rotated parallel to the F3 axial surface (S3) striking to the NE and

dipping to the SE. D3 shear zones are thrust zones parallel to the F3 axial surfaces or E-

W striking oblique-slip shear zones. Simultaneously with D3 deformation new granitic

leucosomes intruded often parallel to the F3 axial surfaces and shear structures. The

approximate age of the D3 deformation at Olkiluoto is 1860-1830 Ma based on the U-Pb

(SIMS) age of one D3 related pegmatitic granite dyke cutting the TGG gneiss.

Subsequently, D3 elements and earlier structures were again redeformed in deformation

phase D4, which produced close to open F4 folds with axial planes striking to the NNE

and dipping to the ESE. In places, a few centimetres to several metres wide thrust-

related ductile D4 shear zones, subparallel to the regional direction of S4, have been

observed. The central and south-eastern parts of the Olkiluoto site, the ONKALO area

and the region ca. 500 m eastward from it, have been affected more strongly by D4. At

least a part of intrusive-like rocks named at Olkiluoto as K-feldspar porphyries seem to

be related with D4 shear zones.

10

11

3 SURVEY METHOD

The Olkiluoto reflection survey consisted of 2D lines located according to the geology,

and road conditions. The applied method is the CMP (Common Mid Point) method with

symmetrical split-spread geometry. At end of lines asymmetric shooting was applied.

The number of active channels was 402, and the channel interval was 12.5 m. The

maximum offset between source and receivers was 2,502 m in case of symmetrical

geometry and at ends of lines in asymmetric geometry up to 5,025 m.

The source point interval was 50 m, and locally, for instance in the proximity of

interesting structures, it was reduced to 25 m. Vibrators were used to generate the

seismic source signal. Three (minimum two) 15.4 ton Geosvip vibrators were used as a

group. The applied force was about 10 ton/vibrator. The sweep was a 16 s linear

upsweep with a frequency band of 30-165 Hz, and the total listening time was 22 s. The

final correlated signal length is 6 s. The number of sweeps/source point was six. The

sweeps were stacked and the stacked data were saved.

Positioning of the line layout and recording station stakes in the field were done

immediately before the acquisition with 25 m steel rope. Horizontal positioning was

done with differential GPS to an accuracy of at least ±2 m, and elevations were

determined with levelling to an accuracy of at least ± 0.5 m.

The survey parameters are shown in Table 3-1 (Zamoshnyaya 2008).

12

Table 3-1. HIRE survey parameters in Olkiluoto.

Recording I/O-4

Number of active channels 402

Sampling interval, ms 1

Record length after correlation, s 6

Preliminary gain, dB 24-36

Notch filter, Hz off

Noise suppression editor

(BURST+DIVERSITY)

on

High-pass filter, Hz off

Tape format SEG-Y

Medium type HARD - DISC

Acquisition geometry Symmetrical

split spread

Stacking fold varying

Receiver group spacing, m 12.5

Spacing of source locations, m 25 or 50

Spread length, m 5012.5

Linear geophone grouping 6 geophones on

12.5 m base

Linear SV-14/150 vibrator grouping 3 on 25 m base

Sweep frequency limits, Hz 30-165

Sweep period, s 16

Number of vibrations at a source point 6

Ground force 65%

Control system and vibrator synchronization control VIB PRO

13

4 DATA PROCESSING

Data processing was done in three main steps. First, on-site processing was done by

Vniigeofizika in the field base. The first results were used mainly for quality control.

Second, basic processing was continued from the field results in the Moscow office of

Vniigeofizika. Third, post stack processing was done by the Institute of Seismology of

the University of Helsinki (HY-Seismo), working as a contractor and research partner

for GTK.

The on-site and basic processing sequence of data processing is shown in Tables 4-1 -

4-2 (Zamoshnyaya 2008).

Post stack processing (Table 4-3) was made by HY-Seismo starting from the NMO

(normal-move-out) stacked sections by Vniigeofizika. The post stack processing

included four processing steps:

1) Whole trace amplitude equalization,

2) Stolt migration with depth dependent velocity function,

3) Spectral balancing,

4) Depth conversion.

The purpose of the first step was to eliminate the amplitude variations along the lines

caused by changes in surface conditions and possible processing artefacts. The second

step improves the migration results as the original migrations were done using the

constant velocity of 5000 m/s. As a part of the basic processing, Vniigeofizika

performed velocity analysis at every 100th CMP. From the measured values the average

(root-mean-square) Vrms-velocity was estimated and this velocity function was used in

Stolt migration, i.e. migration in frequency - wave-number domain. This takes into

account the average increase of velocity as a function of traveltime, i.e. depth. The

stacking velocities as well as the velocity function are shown in Fig. 4-1.

In the spectra of the migrated traces the amplitudes decrease as a function of frequency,

which results correspondingly in decreasing the resolution of the data. This can be

improved by spectral balancing, i.e. by increasing the contribution of higher

frequencies. Spectral balancing was done multiplying the spectra with a linearly

increasing function of the frequency. The applied value of the multiplier was 1.0 at 40

Hz and 2.0 at 160 Hz.

The migrated traces are functions of travel time and before plotting they have to be

converted to functions of depth. The conversion velocity was calculated from the

average interval velocities estimated from the average stacking velocity function. The

values of the depth conversion velocity are listed in Table 6. Between the listed values

linear interpolation was used. The difference between the stacking velocity of Fig. 4-1

and the depth conversion function is very small, less than 200 m/s. As one can see in

Fig. 4-1, the variation of the velocities is of the order of 4 % and similar uncertainties

are possible in depth conversion.

14

Figure 4-1. The stacking velocities (coloured dots) and the velocity function used for

post stack processing (black line). The colour of the dot indicates the line (V1-V2).

15

Table 4-1. On-site processing of vibroseis data.

16

Table 4-2a. Basic processing of vibroseis data.

17

Table 4-2b. Basic processing of vibroseis data (cont.)

18

Table 4-2c. Basic processing of vibroseis data (cont.)

19

Table 4-3. Steps of post-stack processing.

Final NMO stack

Migration (Stolt)

TWT (ms) Vrms (m/s)

0 5152

100 5402

200 5615

300 5751

400 5849

500 5926

600 5980

700 6041

800 6097

900 6144

1000 6184

1500 6307

2000 6369

Spectral balancing

Band pass filter

Frequency Filter amplitude

20.0 0.0 40.0 1.0

160.0 2.0

200.0 2.0 240.0 0.0

Depth conversion

Depth (m) Conversion velocity (m/s)

0 5152

270 5401

561 5611

861 5745

1168 5843

1479 5919

1791 5973

2076 5934

2432 6081

2760 6135

3093 6188

4786 6382

6487 6487

Plotting

20

21

5 RESULTS AND INTERPRETATION

5.1 2D and 3D representation of the results

The location of the survey lines is shown on a geological map in Fig. 5-1. The map

shows two lines for each section, first the receiver station line as it was located in the

field, and second, the common midpoint (CMP) line. The CMP line indicates the

surface projection of average locations of reflection points. For a deep and long section

these may differ noticeably if the survey line is curved or crooked. This issue must be

taken into account, when locating reflectors in the field. Those very close to surface

(less than ca. 200 m) are best located with the shooting line in the terrain, whereas

deeper reflectors are best located with the CMP line.

When interpreting 2D seismic sections the effects of the cross dip of reflecting

structures must be taken into account. The apparent dip of a planar reflector as seen in

the seismic section depends on the true dip and strike of the reflector. If the strike is

perpendicular to the line, the apparent dip is equal to the true dip, but the more the strike

angle deviates from perpendicular, the smaller becomes the apparent dip angle. The

relations between true and apparent dip are shown in a nomogram in Fig. 5-2. As can be

seen in the figure, subvertical structures surveyed at small strike angles are imaged by

apparent dip angles significantly smaller than the true dip.

Datum level of the reflection data is 0 metres above sea level. The uppermost layers

(Quaternary sediments, weathered bedrock) have lower velocities than the intact

crystalline rocks. Velocity and thickness variations of the surface layer generate

spatially dependent delays in the arrival times of reflections, and the data must be

corrected for these „static‟ effects. This is done by shifting the signals to a common

depth level, which is usually the highest level of topography in the survey area. The

datum level is also the level to which the upper boundary of seismic sections (depth 0

m) should be referenced.

Frequency content of the data is very good. Examples of frequency spectra are shown in

Fig. 5-3. The applied frequency band of 30-165 Hz is well covered with received data.

This predicts good resolution in the final images.

The processed sections show a wealth of reflectors. Reflectors as thin as 10 m vertically

and 200-300 m wide horizontally can be distinguished in the sections.

22

Figure 5-1. Survey lines in the Olkiluoto area. Numbers along the lines indicate

receiver station pole numbers (italics) and CMP coordinates (normal text). The

geological map is based on the geological map sheet 1:100 000 Rauma (Suominen et

al., 1997) and the digital geological data base of the Geological Survey of Finland.

23

0

10

20

30

40

50

60

70

80

90

0 10 20 30 40 50 60 70 80 90

True dip (º)

Ap

paren

t d

ip (

º)

10

20

30

40

50

60

70

90

80

Figure 5-2. Relations between true and apparent dip angles of planar reflectors. The

curve parameter is the angle between the survey line and the strike of the reflector at

surface.

The results of the survey are shown in migrated and depth converted NMO (normal-

move-out) sections in Figs. 5-4 – 5-6. The sections were converted from time sections

to depth sections using the velocity function in Table 6. In the figures the reflectors are

displayed as variable area plots of averaged instantaneous amplitude (traces). In

addition, the amplitudes were averaged in moving windows (60 m in vertical and 90 m

in horizontal directions, respectively) and displayed in dB scale in the background as a

colour-coded map. Lithological boundaries are given on the top of the 2D sections

according to the digital database on Precambrian geology (“DigiKP”) of the GTK.

To obtain three-dimensional visualization of the reflector structures, the seismic

sections were input to SURPAC 3D visualization and modelling software. The sections

were imported into SURPAC as images which were draped along vertical „curtains‟

defined by the CMP coordinates of the lines. Relevant units were interpreted first from

the sections in 2D, and the corresponding strings were digitized again in SURPAC on

the drape surfaces of the sections. As a result, the 3D correlation of the strongly

reflective units is presented with the 3D strings.

In addition to seismic reflection images, the available geological map and airborne

magnetic map were also applied to provide a means to correlate reflectors with bedrock

structures. These correlations were used in the interpretations below.

24

Figure. 5-3. Frequency spectra of the vibrator field data from V1 with two offsets from

the shot point. Upper panel: offset 150-200 m, lower panels: offset 1950-2050 m. Shot

point location (SP 1065, CMP 2130) is in Olkiluoto Island. Increasing time windows

correspond to increasing depths (Zamoshnyaya 2008).

Figure. 5-4. Migrated NMO section of line V1. Reflectors with high amplitude are automatically enhanced with red background colour.

Geological boundaries at the surface are taken from the geological map sheet 1:100 000 Rauma (Suominen et al. 1997) and the digital

geological data base of the Geological Survey of Finland. The boundaries of reflectors and their geological interpretations have been

indicated with thick lines. BFZ stands for brittle fault zone. See text for details.

25

Figure. 5-5. Migrated NMO section of line V2. Data representation and geological boundaries as in Fig. 5-4. BFZ stands for brittle fault

zone. See text for details.

26

27

Figure. 5-6. Migrated NMO section of line V2. Data representation and geological

boundaries as in Fig. 5-4. BFZ stands for brittle fault zone. See text for details.

5.2 Reflection properties of the upper crust in the Olkiluoto area

The general observation of the HIRE results in the Olkiluoto area is the very strong

subhorizontal or gently dipping reflectors at the depth of 2-3 km. These reflectors are

typically 500-1000 m thick and very continuous laterally (Figs. 5-4 and 5-5). One of

these reflectors is at the surface on line V2, where it is associated with the Sorkka

diabase sill. Although this reflector is thinner and weaker than those at deeper levels, we

expand this correlation and attribute the strong reflectors to diabase sills. The

interpretation is supported on one hand by the gravity modelling of the Olkiluoto area

(see chapter 8) which is in good agreement with interpreted diabase reflectors at surface.

On the other hand, very extensive subhorizontal reflectors in the uppermost 5-14 km are

known in the Bothnian Sea (BABEL Working Group, 1993). They can be correlated

with Postjotnian on-shore sills in Finland and Sweden, and in one location the reflector

28

reaching the sea bottom is associated with an off-shore diabase outcrop (Korja et al.

2001).

In the Olkiluoto area the subhorizontal strong reflectors consist of numerous reflective

elements at a few tens of metres‟ vertical distances from each other. Due to their

internal reflectivity they cannot represent only homogeneous slabs of diabase. The

internal impedance contrasts may be attributed to shearing or compositional variations,

i.e., more felsic interlayers. Thus, the reflectors may rather represent dyke swarms than

single diabase sills.

Areas of Mesoproterozoic rapakivi granites are characterized by transparent, poorly

reflective domains in the sections (Figs. 5-4 and 5-5). This is attributed to the

homogeneous composition of the granites. There seems to be only weak reflection

differences between the rapakivi granites and the gneisses, which makes it difficult to

exactly locate their contacts at depth. The rapakivi granite is cut by the diabase dykes

(the strong subhorizontal reflectors) in several places. No difference can be found

between the Tarkki and Väkkärä types of rapakivi. Generally the rapakivi granite

extends at least to about 4 km depth in the SE part of line V1 and NE part of line V2.

In areas where the Palaeoprotezoic gneisses dominate the surface geology, such as the

Olkiluoto Island, the reflectors are relatively discontinuous and short, forming weakly

reflective packages. In the Olkiluoto Island, where abundant drilling data exist, the

major shear and fault zones seem to be well observable in the seismic section of V1

(Fig. 5-4). On the seismic section, the reflectors are dipping at an apparent angle of

about 25º, and assuming a strike of about 160º they would have a true dip of about 42º

to SE (Fig. 5-2). The reflectors undulate and they are disrupted at many places but

nevertheless they can be followed for distances of over 1 km. The reflectors are thicker

than the brittle fault zones detected in drillholes, and locally seem to correlate also with

pegmatitic granite (see also discussion in chapter 5.3).

Figure. 5-7. Stacking velocities of line V1. Interpreted geology is taken from Fig. 5-4.

The vertical scale is the two-way-travel time. To obtain an approximate depth estimate

in metres, multiply milliseconds by three.

29

Absolute velocities of the reflectors, if available, are very important information in

interpreting the geological character of reflectors. Seismic reflection data provide only

very coarse velocity information, i.e., the velocities used in compiling the CMP stacks.

These data have very poor resolution but can be applied for general comparison with

reflectors. As an example the stacking velocities of the line V1 are plotted at horizontal

intervals of 100 CMP points (625 m) together with interpreted geology of the reflectors

in Fig. 5-7. The results indicate that high stacking velocities (> 6 km/s) and velocity

gradients are generally associated with the strong subhorizontal reflectors at depths of 1-

4 km. It implies that the strong reflectors are indeed faster than their host rocks, which

means that they represent mafic rocks in felsic surroundings.

5.3 Synthetic seismogram and reflection properties of the Olkiluoto Island

In order to provide geological and petrophysical control to the interpretation of the

reflectors in the Olkiluoto Island, a synthetic seismogram was constructed from logging

data on drillhole OL-KR1 which is one of the deepest holes on the Island. The down-

hole density (gamma-gamma) and P-wave velocity (full wave sonic) logs were provided

for our use by the Client.

Seismic reflectivity is generated by seismic impedance contrasts in the rock. The

impedance, which is the product of velocity and density, was calculated from the depth

matched density and velocity logs. The impedance curve was convolved into a synthetic

seismogram, which can be directly compared with the traces of the measured seismic

section. Results are shown in Fig. 5-8. The projection of the hole on the seismic section

V1 is shown in Fig 5-9.

30

Figure 5-8. Drillhole logs of density and P-wave velocity, together with calculated

seismic impedance and synthetic seismogram of drillhole OL-KR1. Short normal

galvanic log is included as an indicator of fracture zones. Legend for the lithology

column is given in Fig. 5-9.

Lithological boundaries seem to generate only weak if any reflectivity in the OL-KR1

data (Fig. 5-8). An exception is the thin (< 5 m) mafic vein at 221 m, which has a

narrow spike on the impedance curve and relatively strong synthetic reflector at 212-

233 m. In contrast to this, the thick pegmatitic granite at 918-980 m does not generate

reflections, although there are considerable variations in density and velocity in

comparison to the hosting gneiss. Unfortunately, the variations act in opposite directions

and cancel out each other, and the resulting impedance profile and synthetic

seismogram do not show any anomalies. On the other hand, pegmatitic granite layers at

420-490 m associate with weak reflectors on the synthetic seismogram.

31

Figure 5-9. Projection of the drillhole OL-KR1 on the seismic profile V1. View is from

NE.

The synthetic seismogram predicts strong reflectivity to be related to fracture zones at

518-540 m and 609-621 m (depths taken from the galvanic log, Fig. 5-8). The fracture

zones are distinctly represented in the density and velocity variations, but the fracture

zones themselves are thinner than the seismic reflectors. Impedance contrast at the

depth of the fracture zones is of the order 40 %, and the corresponding reflection

coefficients are as high as 0.25. It is much above the detection limit of 0.06 typically

considered representative for seismic reflection data. The widening of the reflections in

comparison to the fracture zones can be attributed also to the applied frequency band of

the seismic signal, which is not sufficient to image accurately structures with

thicknesses in the same order of magnitude or smaller than the wave length of seismic

waves. The wave length is about 33 m assuming a frequency of 165 Hz and a velocity

of 5500 m/s. The Rayleigh criterion for thinnest detectable layer is one quarter of the

wavelength, i.e. about 8 m.

The weak correlation of pegmatitic granite and reflectors in Olkiluoto is probably due to

a small impedance contrast between the gneisses and pegmatite but also to brittle fault

zones that seem to be associated with pegmatite and surround the pegmatite granite

layers (see lithology and galvanic log in Fig. 5-8). In Figs. 5-4 – 5-6 we have interpreted

the uppermost (< 1 km) reflectors mostly as brittle fault zones, but these may include

also pegmatitic granite. A more detailed comparison with drillhole based 3D lithology

models is given in chapter 6.

32

33

6 COMPARISON OF REFLECTION DATA WITH LITHOLOGICAL AND FRACTURE ZONE MODELS IN OLKILUOTO

The comparison of the reflection data with lithological and fracture zone models refers

to Geological Site Model Version 1.1, published in the Site Description Report 2008

(Posiva Oy 2009). The model is an update of previously released model version 1.0

(Mattila et al. 2008). In the model, the geometry and properties of 152 lithological units

and 132 brittle deformation zones has been determined. The depth extent of the model is

1000 m, and it is laterally limited to major lineaments surrounding the Olkiluoto Island

(Fig. 6-1). The model is based on large amounts of data gathered during over 20 years,

including 48 deep and 34 shallow drillholes, over 450 observation points at bedrock

outcrops and 14 investigation trends as well as observations from the ONKALO tunnel.

In addition to geological observations, extensive geophysical surveying has been done

using a wide range of different methods, including airborne, ground and drillhole

measurements. Also a number of seismic surveys with different geometries have been

done, including two campaigns of 3D seismic surveys, VSP surveys from 14 drillholes

and seismic refraction on the ground surface. References to the original survey reports

can be found in the Site Description Report (Posiva Oy 2009).

Figure 6-1 shows that from the three HIRE survey lines, V3 and the NW end of line V1

are located at Olkiluoto Island, enabling the detailed comparison of the HIRE data with

the existing geological model and other (especially seismic) data at these lines.

Figure 6-1. Surface map showing the location of the Olkiluoto site area (white colour)

bound by regional lineaments shown as solid black lines. The locations of HIRE survey

lines V1, V2 and V3 (blue lines), ONKALO facility (red line) and 3D seismic survey

sites are also shown (black rectangles).

V1

V2

V3

34

Figures 6-1 and 6-2 show a plan view and a vertical section of the lithological and

brittle deformation models. The dips of the modelled lithological units are typically

moderate or gentle to SSE – S, following the general orientation of foliation. The only

exception is a number of narrow diabase dykes, dipping steeply to NW. Also most of

the brittle features follow the direction of foliation, but there are also steeply dipping

zones, trending N-S, NNE-SSW and NE-SW.

Figure 6-3 presents the results from the NW end of HIRE line V1 with lithology. It can

be seen, that several reflectors within the depth of 0 – 1000 m correlate rather well with

the lithological features, especially pegmatitic granite veins, giving also a clear picture

of the foliation system of the area. No clear seismic indications are related to diatexitic

or TGG gneisses. The strong reflection at the depth of 1000 – 1700 m seems to be an

extension of two major pegmatitic granite veins, located at the left margin of Fig. 6-4c.

According to drillhole data, P-wave velocity of these granites is relatively high (e.g.

5913 m/s, average for section 815 – 865 m in OL-KR47) compared to the surrounding

gneisses (e.g. 5703 m/s, average for section 740 – 785 m in OL-KR47). The synthetic

seismogram suggests only weak or no reflectivity at the pegmatite-gneiss contacts (Fig.

5-7). Apparently the impedance contrast is variable and may be affected by the gneiss

composition and the directions of schistosity and layering.

35

Figure 6-2. Site Geological Model of Olkiluoto, lithology and brittle deformation. Z = 0

m.a.s.l.

Figure 6-3. Site Geological Model of Olkiluoto, lithology and brittle deformation. N-S

trending vertical section at X = 1526000. BFZ (with numbering) denotes brittle fault

zone. Arrows in Fig. 6-2 show the location of the section.

36

Figure 6-4. NW end of HIRE profile V1 a) with the 3D lithological model, b) as such, c)

with a vertical section of lithology (red = PGR, orange = TGG, light blue = DGN, dark

blue = MGN, green = DB). View to NNE.

a

b

c

37

Figure 6-5 shows the results from the NW end of HIRE line V1 with the modelled

features of the brittle deformation model. Two rather discontinuous lines of reflectors

above the depth of 1500 m nicely correlate with gently dipping fault zones OL-BFZ098,

OL-BFZ080, OL-BFZ099 and OL-BFZ002, forming two separate pairs of fault zones.

The core zone thicknesses of these features are only some meters at maximum, but their

influence zones, characterized by increased fracturing, slickensided fractures, alteration

and elevated hydraulic conductivity, are several tens of meters thick. In some places, the

reflectors are clearly cut, displaced or bent (e.g. locations 1, 2, and 3 in Fig. 6-4a). At

location 1, there is a dense group of steeply dipping fault zones, explaining the

discontinuity of the reflectors. At locations 4, 5 and 6, the reflectors end up or they are

significantly displaced, possibly suggesting the existence of a major steeply dipping

fracture zone related to a lineament, bounding the whole site (it location is shown by the

left arrow in Fig. 6-5a).

Despite of the nice correlation with some features, all the details in the seismic results

can not be explained with the existing brittle deformation model. On the other hand,

there are numerous modelled brittle deformation zones that do not give any seismic

indications, probably due to unfavourable geometry (e.g. steep dips) or limitations in

resolution (small dimensions of the features or insignificant contrasts in acoustic

impedance).

The seismic results indicate that the extents of fault zone pairs OL-BFZ098 & 080 and

OL-BFZ099 & 002 are rather well defined, however, they may be more displaced or

bending in some places. A clear reflection between locations 1 and 3 in Fig. 6-5a is

totally a new indication. On the ground surface, it coincides with the Liikla shear zone,

a major ductile zone, trending ENE – WSW. Its dip is not well known, however.

According to drillhole observations, also brittle deformation is related to this zone. In

GSM V. 1.1, the brittle fault zone OL-BFZ146 has been modelled within the Liikla

shear zone, having an orientation of 155-165/60. The seismic data, however, indicates

that the shear zone and the probable overprinting brittle deformation zone might be

more gently dipping. This is supported by the observations from drillholes OL-KR49

and OL-KR50.

On the short HIRE line V3 (see the location in Fig. 6-1), the same main reflectors can

be observed as on line V1. However, due to the shortness and the sharp turn of the line,

the data are poorer than on line V1 and fewer details can be seen. In Figure 6-6, the

results from line V3 with the main gently dipping brittle fault zones can be seen.

Especially the lowest fault pair (OL-BFZ002 & 099) induces clear seismic reflections,

whereas the indications related to upper OL-BFZ080 & 098 and OL-BFZ056 (&018)

are less clear but still observable.

38

Figure 6-5. NW end of HIRE profile V1 a) as such, b) with some gently dipping main

features c) with a vertical section of the brittle model. View to NNE.

1

2

3

Locations of bounding lineaments

5

4

6

a

b

c

39

Figure 6-6. HIRE profile V3 with the main gently dipping fault zones OL-BFZ056, OL-

BFZ098, OL-BFZ080, OL-BFZ099 and OL-BFZ002. Behind profile V3, also a portion

of profile V1 is shown. View to SW.

V3 V1

BFZ056

BFZ098 & 080

BFZ099& 002

V1

40

41

7 COMPARISON OF HIRE RESULTS WITH PREVIOUS 3D SEISMIC REFLECTION SURVEYS IN OLKILUOTO

Two 3D seismic surveys have been done in Olkiluoto as separate campaigns. The first

survey was done as a pilot study in 2006 (Juhlin & Cosma 2007). The survey covered

an area of about 650 m x 600 m, located slightly NW of ONKALO facility. As a follow-

up to the work carried out in 2006, a new 3D seismic survey was done in 2007, focusing

on the eastern part of the site (Cosma et al. 2008). The survey covered an area of about

1150 m x 1050 m. The locations of the both survey areas with HIRE lines V1 and V3

are shown in Fig. 7-1. HIRE line V1 passes through only the SW margin of 2007 survey

area (with poorer data than in the central area), but crosses the 2006 survey area more

centrally. HIRE line V3 traverses rather nicely the 2007 survey area. However, the

crooked geometry complicates the interpretation, and the SW-NE strike of the brittle

fault zones runs more parallel to the line V3 than V1.

Figure 7-1. Locations of the seismic 3D surveys (purple rectangles) and HIRE profiles

V1 and V3 (green lines).

42

In the processing phase, the data from 2006 have been binned into 12 x 12 m and that

from 2007 into 8.33 m x 12.5 m CDP bins. Fold is highly variable in both surveys

(Figures 7-2 and 7-3). For 3D migration and depth conversion of the migrated volume,

DMO (dip-move-out) velocities between 5300 - 6000 m/s have been used in 2006

survey. In 2007 survey, the applied velocity has been 5750 m/s (Cosma et al. 2008). The

results of the 3D surveys have been available as depth migrated volumes, slices (inline,

crossline and horizontal) (Fig. 7-4) as well as amplitude block models (Fig. 7-5).

In seismic surveying it is not possible to give any exact detectability limit for the

thickness of geological structures, since it depends on so many factors (contrast in

elastic parameters and dimensions of the reflector). The contrasts in Olkiluoto are

typically rather small, and according to wave theory thicknesses down to ¼ of

wavelength are considered adequate for detection, when the reflection coefficient is

above 0.06. The upper limit of frequency range achieved in the 3D surveys of Juhlin

and Cosma (2007) and Cosma et al. (2008) is limited to 200…300 Hz. Using P-wave

velocities 5000 – 6000 m/s this corresponds to thicknesses between 4 and 7.5 m.

43

Figure 7-2. Fold for the 2006 pilot 3D survey. Fold is for 12 m x 12 m square CDP bins

(Juhlin & Cosma 2007).

44

Figure 7-3. Fold for the 2007 3D survey. Fold is for 8.33 m x 12.5 m rectangular CDP

bins (Cosma et al. 2008).

The comparison of HIRE and 3D seismic results (Fig. 7-6) shows that within the

investigation depth of 1000 m for the 2006 and 2000 m for the 2007 survey, there are a

number of common features. Most of the distinct reflectors observed by HIRE have

been detected also with the 3D seismics. Also the depth conversions are well in

concordance. However, there are some clear features in HIRE data at shallow depths

(<300 m) that have not been detected by 3D surveys. The data from the uppermost 300

m in the 3D surveys are incoherent due to the sparse receiver line spacing and

comparably long receiver-transmitter offsets applied in the primary processing.

In Figure 7-7, a part of HIRE line V1 is presented with the data from the two 3D

surveys. The clear reflectors at the depth of c. 500 – 800 m observed in the 2006 survey

(area 1 in Fig. 7-7) are almost identical in the HIRE data. These reflectors are related to

brittle fault zone OL-BFZ099 and 002, forming a pair of gently/moderately dipping pair

45

of thrust fault zones. Furthermore, at location 2 (Fig. 7-7) there are rather weak

reflectors in both datasets, indicating another pair of gently dipping faults, OL-BFZ098

and 098. Due to the low fold, the corresponding data from 2007 survey is poorer at the

location of V1 HIRE line, but there is some correlation deeper down at the depth of c.

1700 m (location 3 in Fig. 7-7) probably related to massive pegmatitic granite veins. In

more central locations, the data from 2007 survey indicates several clear reflectors than

can be correlated to HIRE results and 2006 data.

In Figure 7-8, HIRE profile V3 (with V1 in the background) with 3D data is presented.

The slice of 2007 data at the location of V3 shows only rather weak reflections

compared to the HIRE data (Figure 7-8b). However, the most significant reflectors from

the whole 3D volumes correlate rather nicely with HIRE data (Figure 7-8c) also on V3.

In spite of this, compared to HIRE line V1 the correlation is poorer on line V3 probably

due to the shortness and sharp bending of the line.

Figure 7-4. 3D depth migrated data cubes, 2006 survey (right) and 2007 survey (left)

(Cosma et al. 2008). View from N.

46

Figure 7-5. Seismic 3D data as amplitude block models, 2006 and 2007 surveys. Only

the blocks with highest (> 0.5 for 2006 survey and > 1.8 for 2007 survey) amplitudes

are shown. View from SW.

The most important advantage of HIRE data compared to the 3D surveys is the

considerably greater investigation depth, exceeding 5000 meters. It gives totally new

information on the probable subhorizontal diabase system located at the depth of c.

2500 – 3000 m below Olkiluoto. Due to the high quality of the data, the HIRE survey

supplements the 3D seismic results also at the shallower depths. It confirms the

interpretation of the main gently dipping fault zones and suggests some new features not

observed in the existing 3D seismic data. In addition, the bigger offsets allowed steeper

structures to be observed.

47

Figure 7-6. NW part of HIRE profile V1 a) as such, b) with the most distinct reflectors

from 2006 and 2007 3D surveys (blue colour). View to S.

a

b

48

Figure 7-7. NW part of HIRE profile V1 a) as such, b) with the data from 2006 and

2007 3D surveys.

1

1

2

3

V1

V1 2006 3D data 2007 3D data

a

b

49

Figure 7-8. HIRE profile V3 (with V1 behind) a ) as such, b) with the data from 2007

3D survey, c) with most distinct reflectors from 2006 and 2007 surveys (blue colour).

V3 V1 V1

V1 V1

V3

2007 3D data

a

b

c

V3

50

51

8 HIRE RESULTS AND GRAVITY SURVEYS

Geological Survey of Finland has measured altogether 1994 gravity stations, shown in

Figure 8-1, in the vicinity of the HIRE survey lines. The Common Depth Points (CDP)

of the HIRE survey have been plotted on the corresponding Bouguer anomaly map

shown in Figure 8-2. The major anomaly is the negative gravity anomaly of about -10

mGal due to the Väkkärä rapakivi granite intrusion. There are positive residual gravity

anomalies of +1.0 mGal to +2.4 mGal to the SE and to the NNE from the Väkkärä

intrusion. The gravity anomalies due to the Väkkärä rapakivi granite and the diabase

sills have been interpreted and discussed earlier by Elo (2001). Here we present results

of gravity modelling of the HIRE lines V1 and V2 based on new gravity measurements

along the roads.

Gravity anomalies were modelled using 3D frustum and 2.5D polygonal bodies with the

ModelVision V9.0 program. Some distortion is inevitable due to the measurements

along meandering roads. The models are shown in Figures 8-3 and 8-4. The thickness of

the Väkkärä intrusion is about 3500 m and the thickness of the Sorkka diabase is about

140 m – 170 m. The contacts and depth extents are realistic but the shape of the bottom

Väkkärä intrusion is indeterminate.

Figure 8-1. The location of the GTK gravity stations. New observations along the HIRE

lines are shown in red.

52

Figure 8-2. The location of Common Depth Points (CDP) on a Bouguer anomaly map.

Figure 8-3. The gravity model of the line V1 along the road with projected locations of

Pole locations.

53

Figure 8-4. The gravity model of the line V2 along the road with projected locations of

Pole locations.

54

55

9 DISCUSSION

The HIRE seismic reflection survey resulted in a new deep image of the bedrock

structures in the Olkiluoto area. Numerous previously unknown structures are revealed

in the seismic sections.

The geological correlation of the reflectors indicates that the strong subhorizontal

reflectors are mostly Postjotnian diabase sills, or swarms of sills which form strong 500-

1000 m thick packages of reflectors. The data implies that there is considerable

variation in the internal impedance properties of the reflectors. The outcropping diabase

reflectors in the Olkiluoto area and in western Finland in general do not give a direct

explanation to the implied heterogeneity. The outcropping diabases are dominantly

olivine diabase, but locally also other diabase types are known. In the island of Säppi,

which provides one of the largest surface outcrops of Postjotnian diabase, about 25 km

to NNW of Olkiluoto, the diabase types present are olivine diabase, pyroxene diabase,

megaophitic diabase and diabase pegmatite (Inkinen 1963; Suominen 1991). Magmatic

layering is also observed in Säppi.

The seismic velocities and densities of different diabase varieties in Säppi were

calculated with the code of Hacker and Abers (2004) using the modal composition data

by Inkinen (1963). The results imply that the compositional variations of diabase

generate reflection coefficients in the range of 0.01-0.04 which is too small to generate

the observed internal reflectivity of the major subhorizontal reflectors. Therefore the

internal reflectivity the strong reflectors should probably be attributed to shearing, or

alternating layers of diabase and a more felsic rock type.

Fracture zones are clearly reflective on the Olkiluoto Island. Many of these reflectors

correlate also with pegmatitic granite, which seems to have variable reflection contrasts

with the surrounding gneisses. Therefore, the reflectors interpreted as brittle fault zones

(Fig. 5-4) may also be partly due to pegmatite. A more thorough discussion on this

problem would require a detailed comparison of drillhole data, reflectors as well as

synthetic seismograms calculated for other holes in the Olkiluoto Island. Moreover, we

pay attention to the fact that the research tunnel ONKALO runs across one of the

reflectors interpreted as a brittle fault zone (Fig. 9-1). Therefore the structure could be

directly correlated with geological observations in the tunnel.

56

Figure 9-1. A detailed perspective view of the seismic line V1 from NE together with the

research tunnel “ONKALO” (yellow lines). The tunnel is cutting the brittle fault zone

OL-BFZ098 (pink surface) modelled from earlier drillhole data. BFZ: Brittle fault zone.

The seismic sections provide many reflectors potentially representing the lineaments

constraining the Olkiluoto bedrock block (Fig. 6-1). On line V1, the interpreted brittle

fault zone reaching the surface at CMP 2450 is a candidate for the SW-NE lineament

running along the strait between the island and the continent (Fig. 5-4 and 6-1). The

apparent dip of the reflector on the seismic section of V1 (33º) and the angle between

the lineament and the seismic line (60º) indicate a true dip of about 38º in the azimuth of

246º (Fig. 5-2).

On the other hand the brittle fault zone interpreted on V2 at CMP 2900 (Fig. 5-5) is

located on the extension of one of the lineaments constraining the NE border of the

Olkiluoto block (Fig. 6-1). Here the apparent dip of the structure is 15º NE, and the

angle between the lineament and the seismic line is about 60º. From Fig. 5-2 we obtain

a true dip of about 18º to azimuth 75º (NE). Assuming that the correlation of the

structure on the seismic section with the lineament is correct would suggest quite

different dip direction for the lineaments on the NE side of the Olkiluoto block, than

suggested in earlier studies. The result requires, however, more detailed analysis. An

interesting option would be to combine statistical fracture orientation analysis of

fractures on outcrops in the vicinity of the crossing points of lineaments and seismic

lines. The fracturing is expected to indicate the orientation of the lineaments (strike and

dip) which could then be correlated with the seismic reflection data.

The structures interpreted as rapakivi granite are characterized by low-amplitude

reflectivity, and often seismically transparent areas. It is in contrast with the gneisses

which show commonly internal small-scale reflectivity, which can be attributed to the

migmatitic and other compositional variations. However, the difference between

reflection properties of rapakivi granite and gneisses is small and gradual, and therefore

their contact zones are not obvious in the sections. Here we have applied the gravity

57

models by Elo (2001) as guides to find the most probable structures representing the

rapakivi-gneiss contacts. A comparison with the seismic models and gravity models is

shown in Fig. 9-2.

The HIRE 2D seismic reflection data agrees well with the earlier 3D surveys (Juhlin and

Cosma 2007; Cosma et al. 2008) in the Olkiluoto Island, and most of the reflectors

detected in HIRE surveys are present also in the 3D results. The 2D data can be used to

extend the interpretation of the 3D results in the Olkiluoto Island.

Figure 9-2. Perspective view of seismic section V1 and the gravity profiles 1 and 2 of

Elo (2001). View is from SW. In the gravity models the black and white areas refer to

the modelled boundaries of the Väkkärä rapakivi granite. The Tarkki granite cannot be

modelled with gravity because it has practically no density contrast with the

surrounding gneisses. RKGR: rapakivi granite, DB: diabase.

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59

10 CONCLUSIONS

The HIRE seismic reflection survey in the Olkiluoto area revealed numerous previously

unknown structures in upper crust of the area.

The most prominent structures observed are the subhorizontal strong reflectors which

very probably represent Postjotnian diabase sills intruding both the Svecofennian

gneisses as well as the rapakivi granites. These reflectors can be associated with the

similar seismic structures recorded in marine seismic transects in the Bothnian Sea, and

thus they are parts of large-scale crustal structures.

The Mesoproterozoic rapakivi granites can be distinguished as homogeneous,

seismically transparent domains which extend to a depth of at least 4 km. The

interpreted rapakivi structures are in a good agreement with earlier gravity modellings.

On the Olkiluoto Island, reflectors can be correlated with drillhole based data on

lithology and brittle fault zones. The main brittle fault zones detected in drillholes are

represented as reflectors in the seismic sections, and several new structures have been

interpreted. A synthetic seismogram constructed for a deep hole in the Olkiluoto Island

using down-hole logs of density and P-wave velocity, suggests that the main brittle fault

zones are able to generate strong reflections. Pegmatitic granite sometimes has a weak

reflection contrast with the surrounding gneisses, but sometimes has no contrast. There

seems to be a common, although not systematic correlation of reflectivity, fracture

zones and locations of the pegmatitic granite occurrences.

The HIRE 2D seismic reflection data agrees well with the earlier 3D surveys in the

Olkiluoto Island, and most of the reflectors detected in HIRE surveys are present also in

the 3D results. The 2D data can be used to extend the interpretation of the 3D results in

the Olkiluoto Island.

Where the seismic lines cross the lineaments limiting the Olkiluoto bedrock block, the

seismic data can be used to provide estimates of the dip angles of the lineaments. The

results suggest that the lineament separating the Olkiluoto Island from the continent

would be dipping about 38º to SE, and one of the lineaments limiting the NE side of the

block would be dipping 18º to NE, respectively.

60

61

REFERENCES

Cosma, C., Cosma, M, Enescu, N. & Juhlin, C., 2008. 3D seismic investigations at

Olkiluoto: Factual report. Posiva Oy, Working Report 2008-43, 49 p.

Elo, S. 2001. Gravimetric studies at Eurajoki and Olkiluoto in 2000. Working report

2001-05. Posiva Oy, Helsinki. 5 p.

Haapala, I. 1977. Petrography and geochemistry of the Eurajoki stock: a rapakivi-

granite complex with greisen-type mineralization in southwestern Finland. Espoo:

Geological Survey of Finland. Bulletin 286, 128 p.

Hacker, B.R. and Abers, G.A., 2004. Subduction factory 3. An Excel worksheet and

macro for calculating the densities, seismic wave speeds, and H2O contents of minerals

and rocks at pressure and temperature. Geochem. Geophys. Geosyst. G3, 5(1), Q01005,

doi: 10.1029/2003GC000614.

Inkinen, O., 1963. Säpin diabaasi. Laudatur-tutkielma, Turun yliopisto, geologian ja

mineralogian laitos, 78 p. (unpublished Master‟s thesis, in Finnish).

Juhlin, C. & Cosma, C., 2007. A 3D surface seismic pilot study at Olkiluoto, Finland:

Acquisition and processing report. Posiva Oy, Working Report 2007-65, 47 p.

Kahma, A. 1951. On contact phenomena of the Satakunta diabase. Bulletin de la

Commission géologique de Finlande n:o 152, pp. 1-84.

Kärki, A. & Paulamäki, S. 2006. Petrology of Olkiluoto. Posiva report 2006-02. Posiva

Oy, Eurajoki. 77 p.

Mattila, J., Aaltonen, I., Kemppainen, K., Wikström, L., Ahokas, T, Paulamäki, S.,

Paananen, M., Gehör, S., Kärki, A., Front, K. 2008. Geological model of the Olkiluoto

Site, Version 1. Posiva Oy, Eurajoki, Working Report 2007-92, 510 p.

Mattila, J., Aaltonen, I., Kemppainen, K., Wikström, L., Paananen, M., Paulamäki, S.,

Front, K., Gehör, S., Kärki, A. & Ahokas, T., 2008. Geological Model of the Olkiluoto

Site, Version 1.0. Posiva Oy, Working Report 2007-92, 509 p.

Paulamäki, S., 2007. Geological mapping of the region surrounding the Olkiluoto site.

Working Report 2007-30. Posiva Oy, Eurajoki. 90 p.

Posiva Oy., 2009. Olkiluoto Site Description 2008. Posiva Report 2009-01, Eurajoki.

714 p.

Suominen, V. 1991. The chronostratigraphy of south-western Finland with special

reference to Postjotnian and Subjotnian diabases. Espoo: Geological Survey of Finland.

Bulletin 356, 100 p.

62

Suominen, V., Fagerström, P. & Torssonen, M. 1997. Pre-Quaternary rocks of the

Rauma mapsheet area (in Finnish with an English summary). Geological Map of

Finland 1 : 100 000. Explanation to the maps of Pre-Quaternary rocks, Sheet 1132, 54 p.

Vaasjoki, M. 1996a. The Laitila rapakivi batholith revisited: new, more precise

radiometric ages. In: Haapala, I., Rämö, O. T. & Kosunen, P. (eds.) The Seventh

International Symposium on Rapakivi Granites and Related Rocks, Wednesday 24 -

Friday 26 July, 1996, University of Helsinki, Helsinki - Finland: abstract volume.

Helsinki: University Press, p. 82.

Veräjämäki, A. 1998. Pre-Quaternary rocks of the Kokemäki map-sheet area (in Finnish

with an English abstract). Espoo: Geological Survey of Finland. Geological map of

Finland 1:100 000, explanation to the maps of pre-Quaternary rocks sheet 1134

Kokemäki, 51 p.