mise-à-la-masse survey in the demo area 2010 · mise-à-la-masse survey in the demo area 2010 ......

Mise-à-la-Masse Survey in the Demo Area 2010

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POSIVA OY

Olki luoto

FI-27160 EURAJOKI, F INLAND

Phone (02) 8372 31 (nat. ) , (+358-2-) 8372 31 ( int. )

Fax (02) 8372 3809 (nat. ) , (+358-2-) 8372 3809 ( int. )

August 2015

Working Report 2015-32

Turo Ahokas

Eero Heikkinen, Petr i Heikkinen

August 2015

Working Reports contain information on work in progress

or pending completion.

Turo Ahokas

Astrock Oy

Eero Heikkinen, Petr i Heikkinen

Pöyry Finland Oy

Working Report 2015-32

Mise-à-la-Masse Survey in the Demo Area 2010

MISE-À-LA-MASSE SURVEY IN THE DEMO AREA 2010

ABSTRACT

A detailed Mise-à-la-masse (MAM) survey was carried out in 2010 in the ONKALO tunnel as a part of the programme for final disposal of spent nuclear fuel. The aim of the survey was to define connections between some electrically conductive fractures and fractured zones intersected by two pilot holes drilled in the demo area or detected on the tunnel walls and between the pilot holes and the ONKALO tunnel. Earthings were placed both in drillholes ONK-PH16 and ONK-PH17 and on the tunnel walls at the detected fractures. The potential profile was measured in the drillholes and along the tunnel wall. The observed data was modelled and the results were compared with the geological model. The electrically conductive zones modelled from the surveyed data mainly coincide with the brittle deformation zones or single fractures. According to the modelling results a detailed MAM survey seems to be a useful tool in definition of the orientations and continuity of fractures and fractured zones. Keywords: Mise-à-la-masse, electrical survey, fracture continuity, spent nuclear fuel disposal.

LATAUSPOTENTIAALITUTKIMUS DEMO-ALUEELLA 2010

TIIVISTELMÄ

Yksityiskohtainen latauspotentiaalitutkimus (MAM) suoritettiin vuonna 2010 ONKALO-tunnelissa osana käytetyn ydinpolttoaineen loppusijoitusohjelmaa. Tutkimuksen tarkoituksena oli määrittää kahden demoalueelle kairatun pilottireiän lävistämien tai tunnelin seiniltä havaittujen sähköisesti johtavien rakojen ja rakovyöhykkeiden yhteyksiä reikien välillä sekä reikien ja ONKALO-tunnelin välillä. Maadoitukset asetettiin havaittuihin rakoihin sekä kairanrei’íssä ONK-PH16 ja ONK-PH17 että tunnelin seinissä. Potentiaaliprofiilit mitattiin kairanrei’issä ja pitkin tunnelin seinää. Havaittu data mallinnettiin ja tuloksia verrattiin geologiseen malliin. Mittausdatasta mallinnetut sähköiset vyöhykkeet osuvat pääosin hyvin yksiin hauraiden vyöhykkeiden tai yksittäisten rakojen kanssa. Mallinnustulosten perusteella yksityiskohtainen MAM-tutkimus näyttäisi olevan käyttökelpoinen työkalu rakojen ja rakovyöhykkeiden suuntien ja jatkuvuuksien määrittelyssä. Avainsanat: Latauspotentiaali, sähköinen tutkimus, rakojatkuvuus, käytetyn ydin-polttoaineen loppusijoitus.

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TABLE OF CONTENTS

ABSTRACT

TIIVISTELMÄ

1 INTRODUCTION .................................................................................................... 2

2 SURVEY ................................................................................................................. 3

2.1 Used equipment and survey details ............................................................... 3

2.2 Survey plan .................................................................................................... 9

2.3 Survey results .............................................................................................. 11

3 MODELLING OF THE DATA ................................................................................ 14

3.1 MAM_zone1 ................................................................................................. 14

3.2 MAM_zone2 ................................................................................................. 16

3.3 MAM_zone3 ................................................................................................. 17

3.4 MAM_zone4 ................................................................................................. 18

3.5 MAM_zone5 ................................................................................................. 19

3.6 MAM_zone6 ................................................................................................. 21

3.7 MAM_zone7 ................................................................................................. 22

3.8 MAM_zone8 ................................................................................................. 23

4 SUMMARY ........................................................................................................... 25

5 REFERENCES ..................................................................................................... 26

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1 INTRODUCTION

A Mise-à-la-masse (MAM) survey was carried out in the demo area, pilot holes and tunnels, by PRG-Tec Oy / Pöyry Finland Oy during December 2010 to model the conductive features detected in the drillholes and in the tunnel (Figure 1-1).

Figure 1-1. The location of the Mise-à-la-masse survey area in 2010. The locations of the earthings used in the survey were selected according to the known fractures or fracture zones both in the drillholes and on the tunnel walls. The technical part of the report was compiled by Eero Heikkinen and Petri Heikkinen (Pöyry Finland Oy) and the analysis and modelling of the survey data were carried out by Turo Ahokas.

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2 SURVEY

2.1 Used equipment and survey details

The drillhole electrodes in conventional mise à-la-masse method are steel or copper rods where current can flow along the drillhole when feeding current in measured fracture. Posiva has developed its own electrodes (Figure 2-1). Rubber disks at both end of electrode are used to prevent current leakage along the drillhole. Therefore current is focused directly to the specific fracture. Also in potential side results are more detailed measuring between the rubber disks.

Figure 2-1. The Posiva’s mise à-la-masse electrodes. Resistivity is measured using Terrameter SAS 1000 equipment (Figure 2-2). The purpose of the equipment is to transmit a current with varying polarity and simultaneously measure the corresponding voltage. The equipment measures voltage responses created by the transmitter current while rejecting both DC voltage and noise. The ratio U/I is automatically calculated for ohms.

Figure 2-2. Terrameter SAS 1000. Potential is measured when the tool is at rest. After transfer to a new position measurement starts immediately. Measurement cycle time depends on acquisition time and acquisition delay. Typically it is approximately 2.6 s (delay 0.2 sec and acquisition time 0.3 sec), then moving automatically to next position. Recording interval was 0.1 m in a drillhole and 5 m on the wall.

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The survey was operated with tools installed on a trailer, which was hauled to the measurement position at ONK-PH16 and PH17 collar position. Before the survey there was carried out flow logging, imaging and geophysical drillhole logging, and core logging, which were used as starting values for positioning of the earthing stations. Drillhole measurements were performed using same winches and logging computers as in flow logging. This method needs two winches, one for each electrode. Data communication between a logging computer and Terrameter SAS 1000 goes via serial communication RS-232. The winch system is equipped with tension gauge and procedure for automated depth matching which was calibrated in position during startup. For the earthings and measurements on the tunnel wall a handheld steel rod (Figure 2-3) was used equipped with moist sponge to keep good contact against the tunnel wall. Saline water leaking from drillhole ONK-PH17 was soaked into a sponge, which was tied tightly with gargo straps on bolts attached on the wall and secured. Before the field survey the Demonstration Area starter niche (PL4350) was levelled to allow normal working elevation with collar at 1,5 m from tunnel ground level. Working platforms were constructed by Telinekataja Oy.

Figure 2-3. Steel rod earthing with sponge in MAM6. The survey preparations consisted firstly of placement of trailers and setup of tools and earthings in their position. Earthings were pulled on the wall using bolts attached on 8 mm drilled holes. Cables were lifted over the tunnel and tied on the installations. The measurement setup was checked before production and primary results evaluated on site for sampling interval, earthing positions, earthing contacts, wall measurement technique, and other. Connections of Terrameter SAS1000 were maintained in same position throughout the work. Best drillhole earthings enabled as high as 1 Ampere current feed. Some dry and least fractured wall connections provided 20 or 50 mA current, which was still found adequate for measurements. A highest current which

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allowed continuing measurement without changes, was selected for each earthing during startup. Works were carried out in the pace of excavation, during normal working hours between 06:00 – 18:00. After blasting there was one hour ventilation, after which the work could start. Measurement was running also during loading of crush rock. Mapping, drilling and explosive loading took variable time each day. Survey was ended well in time before next blasting. The survey arrangement after primary tests consisted of 1) Placing the current electrode on the wall on its indicated position, testing and

securing. 2) Placing the downhole potential electrode at a drillhole to the lowermost position

using push rods; installation and removal of the pushrod applied electronic wrench to speed up (30-45 minutes)

3) Survey in the 85 m long drillhole at 10 cm station interval, c. 2 h measurement time. During the automated measurement process the tool was left unattended. Electrodes were removed from the drillholes during blasting. This step was repeated for all earthings on the wall. Maximum three drillhole runs were possible during one day, but due to interference it could be only one.

4) Placing the downhole current earthing on a planned measurement position in ONK-PH16 using pushrods. Removal of pushrods.

5) Survey on the wall at 5 m interval with manual launch of measurement. Walkie-talkies were used to signal move to next position (2 hours each wall profile). Measurement was carried out using ABEM standard connection cables.

6) Installation of potential electrode on second trailer into bottom of ONK-PH17. Removal of push rods. Survey on ONK-PH17 from bottom to top at 10 cm station interval.

7) Moving the current earthing to next planned position in ONK-PH16 and repeating the steps 5 and 6. Maximum one drillhole earthing and both wall and drillhole potential profiles during a day, but quite often much less due to different kind of interferences and damages causing delay.

The field work was carried out between 9.11.2010 and 8.12.2010. The activity schedule for the mise-à-la-masse measurements is presented in Table 2-1. Tool and earthing installations and measurement ran as planned. Some minor problems occurred with a measurement software and SAS 1000, but those were solved during the measurements. There was some electrical noise encountered in the recording. For this reason a checking procedure was added to the measurement. In case a difference of more than 5% from previous recording occurred, the measurement was repeated twice, and median of the results was selected for the final data. All readings including the repeats were stored. The biggest headache was the excavation. It delayed the measurements all the time because the end of the tunnel was close and for the safety reasons the electrodes had to be removed from the drillholes for the duration of the detonation. Also earthing cables were several times broken because of the detonation and some other installation work

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on the tunnel wall. Largest delay was caused due to de-installation of the measurement system and removing the trailers from the niche for excavation work. The breaking of the electrical mains as well as the collapse of a drillhole and subsequent cleaning of it also delayed the work considerably. Table 2-1. Activity schedule for mise-à-la-masse survey.

Started Finished Drillhole/

Wall Activity

9.11.2010 16:59

9.11.2010 18:26

PL4150-PL4445 (Floor)

Earthing in drillhole ONK-PH16 at the depth 30.75-31.25 m. (L = 0.5 m, dL = 0.1 m).

10.11.2010 8:41

10.11.2010 11:01

PL4100-PL4443 (Wall)


10.11.2010 12:15

10.11.2010 16:23

ONK-PH16 MAM8, tunnel earthing

11.11.2010 9:19

11.11.2010 12:57


11.11.2010 13:31

11.11.2010 15:30


17.11.2010 8:51

17.11.2010 11:53


17.11.2010 12:35

17.11.2010 15:40


17.11.2010 16:13

17.11.2010 18:03


18.11.2010 8:31

18.11.2010 10:11


18.11.2010 12:58

18.11.2010 15:04


18.11.2010 15:28

18.11.2010 17:36

ONK-PH16 SPR measurement. (L = 0.5 m, dL = 0.1 m).

30.11.2010 14:54

30.11.2010 17:02



01.12.2010 11:41

01.12.2010 13:43

ONK-PH17 Earthing in drillhole ONK-PH16 at the depth 21.57-22.07 m. (L = 0.5 m, dL = 0.1 m).

01.12.2010 15:49

1.12.2008 17:47

ONK-PH17 SPR measurement. (L = 0.5 m, dL = 0.1 m).

02.12.2010 10:34

02.12.2010 11:52



02.12.2010 14:26

02.12.2010 17:03


03.12.2010 8:50

03.12.2010 9:47



07.12.2010 15:19

07.12.2010 17:30


08.12.2010 9:05

08.12.2010 10:25



08.12.2010 11:19

08.12.2010 13:56


08.12.2010 18:23

08.12.2010 20:35


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The remote current electrode (four electrode group) was placed in the tunnel some hundred meters from the survey area (Figure 2.4). The coordinates of the electrodes are presented in Table 2-2.

Figure 2-4. Fixed current electrode. Table 2-2. The location of the remote current electrode.

The location of the reference potential electrode (fixed electrode, group of three electrodes) was on the tunnel wall close to the survey area (Table 2-3 and Figure 2-5). Table 2-3. The location of the reference potential electrode.

The locations of the surveyed tunnel line, remote current electrode and the reference potential electrode are presented also in Figure 2-6.

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The measured stations on the tunnel wall were marked with red paint and their location was later measured with tachymeter.

Figure 2-5. Fixed potential electrode.

Figure 2-6. Location map of the surveyed tunnel line (green), remote current electrode (red) and the fixed potential electrode (blue). View from above.

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2.2 Survey plan

According to the original survey plan seven earthings were planned to drillhole ONK-PH16 and four to drillhole ONK-PH17. In addition to the earthings in drillholes also eight earthings were planned to the known fractures or fracture zones on the tunnel walls. The locations were advised by Posiva geologists, and they were indicated with paint marks exactly at the anticipated geological feature, like long fracture or brittle fault zone. The positions were measured with tachymeter. The original plan is presented in Table 2-4. The earthings marked by red had the highest priority. By the earthings in drillhole ONK-PH16 both drillhole ONK-PH17 and the tunnel wall were planned to be measured, by the earthings in drillhole ONK-PH17 only drillhole ONK-PH16 was planned to be measured and by the earthings on the tunnel wall drillhole ONK-PH16 was planned to be measured. Table 2-4. Original MAM survey plan. MAM plan

Drillhole / tunnelEarthing

(m / chainage) Target Surveyed hole / tunnel section NoteONK-PH16 21.9 Slickensided fracture 21.81 m ONK-PH17 and from PL4150 to the tunnel

end of the demo central tunnel30 - 32 Graphite zone (best conductor at 31-

32 m), possible TCF at 31.04 m (or 31.07) and 31.25-31.70 m

ONK-PH17 and from PL4150 to the tunnel end of the demo central tunnel

Possible ONK56 zone

54.31 - 54.36 The core of OL-BFZ045 (visible in drill core), same kind of core seen also in the drill core from ONK-PH17


63.70 - 63.80 The core of OL-BFZ084, three slickensided fractures


Possible influence-zone of ONK56

71.3 Fracture, weak hydraulical conductor ONK-PH17 and from PL4150 to the tunnel end of the demo central tunnel


73.5 (73.7) Hydraulical conductor, possible TCF at 73.62 m



75.13 Slickensided fracture, possible TCF, hydraulical conductor



ONK-PH17 52 - 54 The core of OL-BFZ084 at 54.26 m ONK-PH1664.9 Hydraulically conductive fracture ONK-PH16 (ONK56?)68 The core of OL-BFZ045 at 67.94 m ONK-PH1674 Electrical conductor ONK-PH16

Tunnel PL4260 Fracture P382 with graphite ONK-PH16 Earthing_1PL4280 Fracture 4280_1, the core of OL-

BFZ084, nearby also fracture P383ONK-PH16 Earthing_2

PL4290 Fracture P384 ONK-PH16 Earthing_3PL4295 Fracture 4290_34 ONK-PH16 Earthing_4PL4330 Fracture 4330_1, probably not

intersecting the demo tunnelsONK-PH16 Earthing_5

PL4365 Fracture 4365_1, possibly weak conductor

ONK-PH16 Earthing_6

PL4382 The core of OL-BFZ045b including at least two P-fractures

ONK-PH16 Earthing_7

Demo central tunnel

PL30-40 (PL4430)

TCFbetween OL-PH16 and OL-PH17 ONK-PH16 Earthing_8

During the survey more geological information was gathered from detailed mapping and the survey plan was modified according to the new information (Table 2-5). Unfortunately the construction work in the tunnel stopped the MAM survey several times and it was not possible to carry out the whole planned survey in the reserved time.

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Therefore three earthings from the new plan in drillhole ONK-PH16 and all earthings planned for drillhole ONK-PH17 were dropped out. The measurements carried out during this survey are marked by pink in Table 2-5. Table 2-5. Modified MAM survey plan. MAM plan, edited 1.12.2010

Drillhole / tunnelEarthing (m /

chainage) Target Surveyed hole / tunnel section NoteONK-PH16 21.8 Slickensided fracture 21.81 m ONK-PH17 and from PL4150 to the

tunnel end of the demo central tunnelCan be dropped out of the survey plan

31 Graphite zone (best conductor at 31-32 m), possible TCF at 31.04 m (or 31.07) and 31.25-31.70 m


May continue to demo central tunnel, the core of ONK-PH16_3050_3273 may continue to the fracture in OL-PH17 at 20.30m

54.31 - 54.36 The core of OL-BFZ045 (visible in drill core), same kind of core seen also in the drill core from ONK-PH17


OK

63.70 - 63.80 The core of OL-BFZ084, three slickensided fractures


OK

71.3 Fracture, weak hydraulical conductor ONK-PH17 and from PL4150 to the tunnel end of the demo central tunnel

Fracture at 71.00 m in OL-PH16 possibly continuing to the fracture at 61.58 m in OL-PH17; (uncertain from OL-PH16_71.39m to OL-PH17_64.81m

73.5 (73.7) Hydraulical conductor, possible TCF at 73.62 m


Theoretical continuation from 73.62 m in OL-PH16 to 43.36 m in OL-PH17, (the zones OL-BFZ084 and OL-BFZ045b located between)

75.13 Slickensided fracture, possible TCF, hydraulical conductor


Possible continuation 4365_1 in the access tunnel, probably not intersecting OL-PH17

76.16 (76.36) Possible TCFs One continuing to the tunnel and another to the beginning of OL-PH17

ONK-PH17 52 - 54 The core of OL-BFZ084 at 54.26 m ONK-PH16 OK

61.59 TCF ONK-PH16 Possible continuation to 71.00 m in OL-PH16

64.9 Hydraulically conductive fracture ONK-PH16 Possible continuation to 74.64 m in OL-PH17

65.41 TCF? ONK-PH16 Unlikely TCF68 The core of OL-BFZ045 at 67.94 m ONK-PH16 OK

74 Electrical conductor ONK-PH16 Unlikely TCF

Access tunnel PL4260 Fracture P382 with graphite ONK-PH16 Earthing_1PL4280 Fracture 4280_1, the core of OL-

BFZ084, nearby also fracture P383ONK-PH16 Earthing_2

PL4290 Fracture P384 ONK-PH16 Earthing_3PL4295 Fracture 4290_34 ONK-PH16 Earthing_4PL4330 Fracture 4330_1, probably not

intersecting the demo tunnelsONK-PH16 Earthing_5

PL4365 Fracture 4365_1, possibly weak conductor

ONK-PH16 Earthing_6

PL4382 The core of OL-BFZ045b including at least two P-fractures

ONK-PH16 Earthing_7

Demo central tunnel

PL30-40 (PL4430)

TCFbetween OL-PH16 and OL-PH17 ONK-PH16 Earthing_8

Although the reductions in the survey plan caused some difficulties in modelling of the continuations of different fractures or fracture zones, most of the wanted information was possible to get out of the measured data.

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2.3 Survey results

Although the location of the remote current electrode was quite close to the surveyed area it did not disturb the measured potential field values. Before starting the survey a test was carried out to find out how to measure the potential field values in the tunnel, from the tunnel floor or from the tunnel wall. The results observed by one earthing showed that both ways were usable but the measuring from the wall was showed a little better data (Figure 2-7) and was therefore used in the survey as discussed in Chapter 2.1 above.

Figure 2-7. Potential field curves measured from the tunnel floor (blue) and from the tunnel wall (orange) by the earthing at the depth of 31 m in drillhole ONK-PH16. The potential anomalies on the tunnel wall measured by different earthings show very detailed anomalies giving good possibilities to detect the connections of different conductors between the drillhole and the tunnel (Figure 2-8). A test showed that some more detail was gained with 1 m recording spacing, but as main features were clearly visible with 5 m interval, this was selected for production. When using the earthings placed on the tunnel wall and measuring the pilot holes very high potential field values were observed in the upper parts of the drillholes (Figures 2-9 and 2-10). The anomalies in the beginning of the pilot holes are due to highly conductive graphite-bearing rocks. There also seems to be galvanic connections between different earthings and these graphite-bearing rocks causing some difficulties to model the data.

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Figure 2-8. Potential field curves measured from the tunnel by the earthings in drillhole ONK-PH16 and the locations of some geological features detected in the tunnel.

Figure 2-9. Potential field curves measured from drillhole ONK-PH16 by the earthings in the tunnel. The single-point resistance (SPR) measured from the drillhole is presented as pink curve.

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Figure 2-10. Potential field curves measured from drillhole ONK-PH17 by the earthings in the tunnel. The single-point resistance (SPR) measured from the drillhole is presented as pink curve. The potential field anomalies caused by the other conductors than the graphite-bearing zone were much weaker but could be detected when looking at the data more detailed. (Figure 2-11).

Figure 2-11. Potential field curves measured from drillhole ONK-PH16 by the earthings in the tunnel (detailed view). The single-point resistance measured from the drillhole is presented as pink curve.

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3 MODELLING OF THE DATA

Because there seems to exist a large conductor in the demo-tunnel area connecting almost all the other conductive zones together it was quite laborious to model the measured data and to separate different conductors from each other. But when using all the measured data at the same time quite reasonable modelling results were achieved. The modelling was carried out in the same way as used in the modelling work discussed in Posiva Working Reports 2010-08 (Ahokas and Paananen 2010) and 2010-75 (Ahokas 2010). A total of eight different conductive zones were modelled and in the following chapters these zones are described closer. The connections are associated with brittle fault zones BFZ084 and BFZ045b, tunnel crosscutting fractures Demo-keskt_tcf_PH16_3148, TCF_PH17_2030, 4365_1_TCF_PH16_7513, TCF_PH16_7464_PH17_6481 and TCF_PH16_7100_PH17_6158, and conductive body ONK56 (Kemppainen et al. 2007).

3.1 MAM_zone1

The earthing at the depth of 21.8 m in drillhole ONK-PH16 showed good connection to the central tunnel of the demo area, between drillholes ONK-PH16 and ONK-PH17 and the earthing in the central tunnel, tunnel earthing 8, showed good connection to the first part of drillhole ONK-PH16. These both connections belong possibly to the same fractured zone and are modelled as MAM_zone1. The modelled connection points are presented in Table 3-1. Table 3-1. Modelled connections from the earthing in ONK-PH16 at 21.8 m and the tunnel earthing 8 (between ONK-PH16 and ONK-PH17).

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This modelled MAM_zone1 coincides partly with one tunnel cutting fracture (TCF), Demo_keskt_tcf_PH16_3148, although according to the MAM survey there seems to be a quite thick zone including several fractures. Figures 3-1a and 3-1b show the modelled connections from both earthings and the modelled TCF.

Figure 3-1a. The MAM connections (pink lines) modelled from the earthing at the depth of 21.8 m in drillhole ONK-PH16 and the tunnel earthing 8 in the central tunnel of the demo area. Demo_keskt_tcf_PH16_3148 fracture is presented by yellow. View from above.

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Figure 3-1b. The MAM connections (pink lines) modelled from the earthing at the depth of 21.8 m in drillhole ONK-PH16 and the tunnel earthing 8 in the central tunnel of the demo area. Demo_keskt_tcf_PH16_3148 fracture is presented by yellow. View to NE.

3.2 MAM_zone2

From the earthing at 21.8 m in drillhole ONK-PH16 also moderate connection can be seen both to the ONKALO access tunnel and to drillhole ONK-PH17. The earthing in the tunnel at chainage PL4365 (tunnel earthing 6) shows quite good connection to the depth of 23 – 24 m in drillhole ONK-PH16. These two connections were modelled as MAM_zone2 (Table 3-2). Table 3-2. Modelled connections from the earthing in ONK-PH16 at 21.8 m and the tunnel earthing 6 (at chainage PL4365).

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This zone, MAM_zone2, coincides partly with the zone called as ONK56 in the report by Kemppainen et. al (2007). Because the model of ONK56 zone was based on the surveys in the deep drillholes alone the model cannot be very exact in the demo area where no deep drillholes exist. But the direction and the location of the possible hanging wall of quite thick ONK56 zone is close to the modelled conductive zone MAM_zone2 (Figure 3-2). The earthing positions on the wall are associated with slickenside fractures.

Figure 3-2. The moderate MAM connections modelled from the earthing at the depth of 21.8 m in drillhole ONK-PH16 (green), the good connection modelled from the tunnel earthing 6 at chainage PL4365 (pink) and the possible hanging wall of ONK56 zone (light brown). View from above. The possible hanging wall and footwall are the conductors modelled from earlier MAM surveys carried out by the earthing at the depth of 416 m in drillhole OL-KR7 and at the depth of 490 m in drillhole OL-KR4 (the modelling presented in the report by Ahokas and Paananen 2010). See also the modelling of MAM_zone5 later in this paper.

3.3 MAM_zone3

From the earthing at the depth of 31 m in drillhole ONK-PH16 connections both to drillhole ONK-PH17 and to the access tunnel were detected and they were modelled as MAM_zone3. The connection points are presented in Table 3-3.

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Table 3-3. Modelled connections from the earthing in ONK-PH16 at 31 m.

The modelled MAM_zone3 coincides very well with fracture TCF_PH17_2030 as Figure 3-3 shows.

Figure 3-3. The MAM connections modelled from the earthing at the depth of 31 m in drillhole ONK-PH16 (pink) and fracture TCF_PH17_2030 (yellow). View from above. It is also possible that this MAM_zone3 belongs to quite thick ONK56 zone just like MAM_zone2.

3.4 MAM_zone4

According to the MAM measurements by the earthing at the depth of 54 m in drillhole ONK-PH16 and at chainage PL4382 (tunnel earthing 7) in the access tunnel there seems to be a conductive zone (MAM_zone4) coinciding quite well with the brittle fracture zone OL-BFZ045b as Table 3-4 and Figure 3-4 show.

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Table 3-4. Modelled connections from the earthing in ONK-PH16 at 54 m and the tunnel earthing 7 (at chainage PL4382).

Figure 3-4. The MAM connections modelled from the earthing at the depth of 54 m in drillhole ONK-PH16 (pink) and from the tunnel earthing 7 at chainage PL4382 (green). The core of the brittle deformation zone OL-BFZ045b is marked by yellow. View from above. From the measurements it is not possible to model exactly the location of the core of this brittle zone because the conductive zone is thicker presenting more the location of the influence zone than the core of this brittle zone.

3.5 MAM_zone5

There are also some moderate connections to drillhole ONK-PH17 detected from the earthing at the depth of 54 m in drillhole ONK-PH16. Possibly the weak connections from the tunnel earthing 5 at chainage PL4330 is the same conductive zone although the

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connections to ONK-PH16 do not fit exactly to the earthing point at 54 m in the drillhole. The connections modelled from these two earthings are presented in Table 3-5. Table 3-5. Modelled connections from the earthing in ONK-PH16 at 54 m and the tunnel earthing 5 (at chainage PL4330).

The modelled MAM_zone5 may be the same as the possible footwall of ONK56 zone as Figure 3-5 shows.

Figure 3-5. The moderate MAM connections modelled from the earthing at the depth of 54 m in drillhole ONK-PH16 (green), the weak connections modelled from the tunnel earthing 5 at chainage PL4330 (yellow) and the possible footwall of ONK56 zone (light brown). View from above.

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3.6 MAM_zone6

Good connections were detected from the earthing at the depth of 64 m in drillhole ONK-PH16 both to drillhole ONK-PH17 and to the access tunnel. From three earthings placed on the tunnel wall (at chainages PL4280, 4290 and 4295) connections were detected to drillhole ONK-PH16 (good connections from PL4280 and 4295 and weak connection from PL4290). The connections are listed in Table 3-6. Table 3-6. Modelled connections from the earthing in ONK-PH16 at 64 m and the tunnel earthings 2 - 4 (at chainages PL4280, PL4290 and PL4295).

These connections form a conductive zone MAM_zone6, which coincides quite well with the core of the brittle deformation zone OL-BFZ084 (Figure 3-6a) but even better with the influence zone of OL-BFZ084 (Figure 3-6b).

Figure 3-6a. The MAM connections modelled from the earthings at the depth of 64 m in drillhole ONK-PH16 (pink) and from the tunnel earthings 2 - 4 at chainages PL4280, PL4290 and PL4295 (pink = good connection and yellow = weak connection). The core of the brittle deformation zone OL-BFZ084 is marked by blue. View from above.

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Figure 3-6b. The MAM connections modelled from the earthings at the depth of 64 m in drillhole ONK-PH16 (pink) and from the tunnel earthings 2 – 4 at chainages PL4280, PL4290 and PL4295 (pink = good connection and yellow = weak connection). The influence zone of the brittle deformation zone OL-BFZ084 is marked by green. View from above.

3.7 MAM_zone7

Moderate connections were detected from the earthing at the depth of 75 m in drillhole ONK-PH16 to the access tunnel around chainage PL4365 and also a moderate connection from this chainage (tunnel earthing 6) was detected to drillhole ONK-PH16 (Table 3-7). The modelled zone coincides quite well with one tunnel cutting fracture, 4365_1_TCF_PH16_7513, although the conductive zone seems to include several conductors. One of these conductors fits very well the TCF but it is not possible to say from the MAM data alone which one is the same as the fracture with the earthing in ONK-PH16. This zone cannot be connected to drillhole PH17 for geometrical reasons; and it is not seen in the data either. The modelled connections and the tunnel cutting fracture are presented in Figure 3-7.

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Table 3-7. Modelled connections from the earthing in ONK-PH16 at 75 m and the tunnel earthings 6 (at chainage PL4365).

Figure 3-7. The moderate MAM connections modelled from the earthings at the depth of 75 m in drillhole ONK-PH16 and from the tunnel earthing 6 at chainage PL4365 (green lines). The tunnel cross cutting fracture 4365_1_TCF_PH16_7513 is marked by yellow. View from above.

3.8 MAM_zone8

From the earthing at the depth of 75 m in drillhole ONK-PH16 also weak connections were detected to drillhole ONK-PH17 and to the access tunnel outside MAM_zone7 discussed above. Also from the tunnel earthing 1 at chainage PL4260 a weak connection was detected to drillhole ONK-PH16. These connections form the modelled weakly conductive zone MAM_zone8 (Table 3-8).

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Table 3-8. Modelled connections from the earthing in ONK-PH16 at 75 m and the tunnel earthing 1 (at chainage PL4260).

MAM-zone8 shows the same direction as two possibly tunnel cutting fractures detected in drillholes ONK-PH16 and ONK-PH17 (Figure 3-8). The connections to drillhole ONK-PH17 are quite uncertain due to the closeness of the earthing in drillhole ONK-PH16. But although the galvanic connections of this MAM zone are weak and slightly uncertain it is possible that the detected fractures continue to the access tunnel like the MAM survey suggests.

Figure 3-8. The weak MAM connections modelled from the earthings at the depth of 75 m in drillhole ONK-PH16 and from the tunnel earthing 1 at chainage PL4260 (yellow lines). The tunnel cutting fractures TCF_PH16_7464_PH17_6481 and TCF_PH16_7100_PH17_6158 detected in drillholes ONK-PH16 and ONK-PH17 are marked by light green colour. View from above.

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4 SUMMARY

The main task of the MAM survey carried out in the ONKALO demo area during December 2010 was to find out the continuities of some fractures and fracture zones detected either in the tunnel wall or in drillholes ONK-PH16 and ONK-PH17. Purpose of the survey was to demonstrate and confirm the possibilities of MAM method in detailed scale investigation of rock mass near the spent fuel disposal area, in terms of Rock Suitability Criteria (RSC). Targets of such survey are detection or disclosure of extensive fractures and otherwise avoidable structures in the rock volume, as well as their orientation and continuity. In spite of a thick and very conductive area (graphite-bearing rocks) intersected in the upper parts of drillholes ONK-PH16 and ONK-PH17 causing very high potential anomalies it was possible to detect several conductive zones in the surveyed area. These conductive zones fit mostly quite well to the known brittle fracture zones or the tunnel cutting fractures giving so more information for geological modelling. Although the work was planned carefully in advance together with the geologists, the work needed guidance all the time. Checking the results and interpretation is useful to carry out daily as the work progresses. The geological information increased during the survey when the detailed mapping was going on at the same time and therefore also the need of measurements increased or changed. Also the construction work caused a lot of changes to the survey schedule and led finally to cutting down of the survey plan (several earthings were dropped out of the original plan). Some of the reciprocal measurements from ONK-PH17 back to PH16 could be left out as the initial survey already confirmed the connection. The survey procedure proved feasible. Work may be speeded up using switchbox, which would enable drillhole measurement of several tunnel wall earthings in one drillhole run. Installing more than one drillhole earthing at a time is not feasible as the installation would become a very heavy work due to weight of tool and hydraulic pressure behind the probe. Anyway the survey fulfilled the purpose and proved once again the usefulness of the Mise-à-la-masse method in following fractures and fracture zones also very detailed in ONKALO circumstances (see also the report by Heikkinen et al. 2010).

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5 REFERENCES

Ahokas, T. 2010. Preliminary modelling of the 2010 MAM survey data. Posiva Oy. Working report 2010-75. Ahokas, T. and Paananen, M., 2010. Updated and integrated modelling of the 1995 – 2008 Mise-à-la masse survey data in Olkiluoto. Posiva Oy. Working report 2010-08. Heikkinen, E., Ahokas, T., Heikkinen, P., Kristiansson, S., Tiensuu, K. and Pere, T. 2010. Summary of investigations in personnel shaft pre-grouting drillholes ONK-PP131, -PP134 and -PP137 in Olkiluoto, 2008. Posiva Oy. Working Report 2010-34. Kemppainen, K., Ahokas, T., Ahokas, H., Paulamäki, S., Paananen, M., Gehör, S. and Front, K., 2007. The ONKALO area model, Version 1. Posiva Working Report 2007-71, 141 p.

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