mise-à-la-masse survey in the hydco niche 2011 · the mise-à-la-masse technique is an electrical...

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Turo Ahokas

Eero He ikk inen

Eeme l i Hurmer in ta

December 2011

Work ing Repor t 2011 -89

Mise-à-la-Masse Surveyin the HYDCO Niche 2011

December 2011

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.

Turo Ahokas

Eero He ikk inen

Eemel i Hurmer in ta

Pöyry F in l and Oy

Work ing Report 2011 -89

Mise-à-la-Masse Surveyin the HYDCO Niche 2011

Mise-à-la-MASSE SURVEY in the HYDCO Niche 2011 ABSTRACT Posiva Oy carries out a programme for spent nuclear fuel disposal. Subsurface characterization work is underway at the ONKALO tunnel, in Olkiluoto. Detailed hydrological conductivity properties in an averagely fractured rock mass are studied in the HYDCO niche. Crosshole mise-à-la-masse survey was carried out between the two 23-24 m long horizontal drillholes, ONK-PP262 and ONK-PP274 that are 3 m apart from each other, and between the drillhole and the access tunnel. The field work was carried out in April-May 2011. The survey was aimed to define continuity of the fractures between the drillholes. Nine current earthing stations in drillhole ONK-PP262 and 13 stations in ONK-PP274 were used. Earthings were placed at fractures or conductive layers detected in core logging, assessed with geophysical logging and imaging, and with Posiva flow log measurement. Earthing used a 0.5 m long electrode assembly operated with a wireline. The potential profile was measured in the adjacent drillhole at 0.1 m spacing, using a similar electrode assembly and wireline. Potential was measured also on 130 m long profile along the tunnel wall from the earthings in drillhole ONK-PP262. The remote voltage reference and the current earthing were placed offset from the survey area. The equipment used in survey consists of ABEM Terrameter resistivity tool, as well as software, cable winch and downhole probes belonging to Posiva Flow Log tool. The high resistivity bedrock, and electrically fairly weakly conductive fractures residing in the rock mass provide rather small conductivity contrast. Nevertheless there are connections between several fractures or groups of fractures between the drillholes. Several fractures merge together in the electrical potential response. Interpretation defined six zones between the drillholes where fractures indicate continuity, and may serve as hydraulic interference pathway. One of the zones is extending also to the tunnel wall. Measurements and interpretation served as a tool in definition of the local geological model. Information of the fractures, their orientations and continuity is useful in designing and assessing the hydrological investigations in the drillholes. Keywords: Mise-à-la-masse, electrical survey, fracture continuity, hydraulic conduc-tivity, spent nuclear fuel disposal.

Latauspotentiaalitutkimus HYDCO-kuprikassa 2011 TIIVISTELMÄ Posiva Oy huolehtii käytetyn ydinpolttoaineen loppusijoitukseen liittyvistä tutkimuk-sista. Maanalaiset tutkimustyöt ovat käynnissä ONKALO-tutkimustiloissa Olkiluodos-sa. Yksityiskohtaisia vedenjohtavuuden ominaisuuksia keskimääräisesti rakoilleessa kalliossa tutkitaan HYDCO-kuprikassa. Latauspotentiaalitutkimus suoritettiin kahden vaakasuoran, n. 23-24 m pitkän ja noin kolmen metrin päässä toisistaan sijaitsevan kairanreiän ONK-PP262 ja ONK-PP274 välillä, sekä reiän ONK-PP262 ja ajotunnelin seinän välillä. Kenttätyöt tehtiin huhti-toukokuussa 2011. Tutkimuksen tarkoitus oli selvittää rakojen jatkuvuutta kairanreikien välillä. Reiässä ONK-PP262 käytettiin yhdeksää virtamaadoitusta ja reiässä ONK-PP274 maadoituksia oli 13. Maadoitusten paikat oli sijoitettu reikien näytekartoituksen, geo-fysiikan mittausten, kuvausten ja virtausmittausten perusteella rakojen ja johdekerrosten kohdalle. Maadoituksessa käytettiin 0.5 m pituista reikäanturia. Potentiaaliprofiili mitat-tiin viereisessä kairareiässä 0.1 m pistevälein käyttämällä samanlaista reikäanturia ja kaapelia. Potentiaaliprofiili mitattiin myös 1 m välein pitkin ajotunnelin seinän profiilia käyttäen reiän ONK-PP262 maadoituksia. Jännitteen ja virransyötön kiinteät maadoi-tukset asennettiin erilleen tutkimuskohteesta. Mittauksessa käytettiin kahteen perä-vaunuun asennettua kaapelivinssiä, Posiva Flow Login antureita ja ABEM Terrameter –sähköistä mittalaitetta. Kallioperässä, jossa ympäristön ominaisvastus on korkea, sähköä vain heikosti johta-vista raoista syntyy pieni sähköinen kontrasti. Siitä huolimatta reikien välillä havaittiin useita rakojen tai rakoryhmien yhteyksiä. Useat raoista yhdistyvät sähköisessä vastees-sa. Tulkinnassa määritettiin yhteensä kuusi vyöhykettä, joilla on jatkuvuutta kairan-reiästä toiseen. Yksi vyöhykkeistä jatkuu myös ajotunnelin seinään. Mittaukset ja tulkinnat toimivat lähtötietoina laadittaessa paikallista geologista mallia. Raoista, niiden suunnasta ja jatkuvuudesta saatavat tiedot ovat hyödyllisiä suunni-teltaessa ja arvioitaessa kairanreikien hydrologisia tutkimuksia. Avainsanat: Latauspotentiaali, sähköinen tutkimus, rakojatkuvuus, vedenjohtavuus, käytetyn ydinpolttoaineen loppusijoitus.

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TABLE OF CONTENTS

ABSTRACT

TIIVISTELMÄ

PREFACE ....................................................................................................................... 2

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

2 MISE-À-LA-MASSE METHOD ............................................................................ 4

3 MISE-À-LA-MASSE SURVEY IN THE HYDCO NICHE ..................................... 6

3.1 Used equipment and survey details ........................................................ 6

3.2 Earthings ............................................................................................... 13

3.3 Data ....................................................................................................... 15

4 UNCERTAINTIES OF THE MODELLING ......................................................... 23

5 MODELLING OF THE DATA ............................................................................ 25

5.1 MAM_HYDCO_zone1 ........................................................................... 26

5.2 MAM_HYDCO_zone2 ........................................................................... 27

5.3 MAM_HYDCO_zone3 ........................................................................... 29

5.4 MAM_HYDCO_zone4 ........................................................................... 30

5.5 MAM_HYDCO_zone5 ........................................................................... 32

5.6 MAM_HYDCO_zone6 ........................................................................... 34

6 CONCLUSIONS ................................................................................................ 36

REFERENCES ............................................................................................................. 37

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PREFACE

This report presents the modelling results of the Mise-à-la-masse (MAM) survey data measured in the investigation niche ONK-TKU4 (HYDCO) in ONKALO in 2011.

Work is a part of Posiva Oy’s programme for spent nuclear fuel disposal. This study has been carried out by Pöyry Finland Oy and is based on contract for Posiva Oy. The contact person on Pöyry Finland Oy’s side was Turo Ahokas. Field work was carried out by Eemeli Hurmerinta. Turo Ahokas took care of management, planning and modelling as well as reporting. Eero Heikkinen participated in planning of the work and finalised the report. The contact person on Posiva Oy’s side was Mari Lahti.

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1 INTRODUCTION

Posiva Oy carries out the final disposal of spent nuclear fuel in Finland. In 2001 Olkiluoto was selected for the site of final disposal. Excavation of the ONKALO underground investigation premises commenced in 2004.

The purpose of the mise-à-la-masse survey was to give information of the continuation of some fractures between drillholes ONK-PP262 and ONK-PP274 drilled in the investigation niche 4 (ONK-TKU-4) and between the drillholes and the ONKALO tunnel. The investigation niche is excavated in hydraulically poorly conductive bedrock. These studies belong to the larger study program (HYDCO) in which the fracture network and flow properties in a volume of rock representative of the rock close to the deposition holes are being studied. The primary objective is to study the groundwater flow pattern in poorly conductive (T < 10–7 m2/s) fractures in the rock mass. A detailed description of the overall HYDCO-project is given in Aalto et al. 2009. The modelling of the survey data followed the same method as used in the Posiva Working Reports 2010-08 (Ahokas and Paananen 2010) and 2010-75 (Ahokas 2010). Due to poor conductors the potential field anomalies are weak making the modelling challenging. The short distance between the drillholes also caused difficulties in separating the anomaly due to the closeness of the current electrode in the other hole from the anomaly caused by the continuation of the examined conductor.

The planning of the survey was carried out in co-operation between geologists, hydrogeologists and geophysicists from Posiva Oy and Pöyry Finland Oy. Field work was carried out in April-May 2011. This report describes the field work and modelling.

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2 MISE-À-LA-MASSE METHOD

The mise-à-la-masse technique is an electrical resistivity method that has been conventionally used in ore prospecting for delineating electrically conductive subsurface ore bodies. In this application, a current electrode is placed in the conductive body at a surface exposure, in a drillhole or in a tunnel, with a second current electrode in the ground at effective infinity (in practice at the distance of 2 – 3 km at minimum, depends on the dimensions of the body). Electrical potential is measured between a moving and a fixed electrode (pole-pole configuration) or by a moving potential dipole (pole-dipole configuration).

Equipotential surfaces caused by a point current source are spherical in the case of a homogeneous medium (Figure 2-1). A grounded conductive body, surrounded by a resistive medium, deforms the potential field, resulting in equipotential surfaces subparallel to the conductor (Figure 2-2).

In the case of an ideal conductor, equipotential conditions prevail within the conductive body. Accordingly, the conductor is detected as a potential maximum with a potential flat at any location (Figure 2-3). It has been noted, however, that in practice the equipotential conditions are attained already with the resistivity contrast of c. 100 (e.g. Eloranta 1984).

Although the mise-à-la-masse method has been applied mostly in studying ore bodies, it has proved to be a feasible method also in different kinds of environmental investigations. In these applications, the conductors may be significantly weaker than typical ore bodies, resulting in complexity in the interpretation.

Figure 2-1. Equipotential lines caused by a point current source in homogeneous half-space.

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Figure 2-2. Equipotential lines caused by a point current source in a conductive body embedded in resistive surroundings.

Figure 2-3. Hypothetical potential distribution along a survey line related to a grounded conductor. The conductor is observed as a flat potential maximum. Arrow shows the location of the conductor.

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3 MISE-À-LA-MASSE SURVEY IN THE HYDCO NICHE

A total of 22 earthings were placed in drillholes ONK-PP262 and ONK-PP274. The potential field values were measured from both drillholes and from the tunnel wall near the HYDCO niche.

The earthing locations were selected in co-operation with geologists, hydrogeologists and geophysicists. The most interesting zones, fractures or conductors were chosen for the survey.

3.1 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 Oy has developed its own electrodes (Figure 3-1). 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 obtained potential results are more detailed when measured between the rubber disks. Resistivity is measured using Terrameter SAS 1000 equipment (Figure 3-2). The tool transmits a current with varying polarity and simultaneously measures the corresponding voltage. The equipment measures voltage responses created by the transmitter current while rejecting both DC voltage level and noise. The ratio U/I is automatically calculated for ohms.

Figure 3-1. The Posiva’s mise à-la-masse electrodes.

Figure 3-2. Terrameter SAS 1000.

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Potential is measured when the downhole probe is in position. 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.

The survey was operated with tools installed on a trailer, which was hauled to the measurement position at ONK-PP262 and PP274 collar position. Before the survey the flow logging, drillhole imaging and geophysical logging together with the geological core logging were carried out, and those were used as background information for positioning the earthing stations.

Drillhole measurements were performed using same winches and logging computers than 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.

The measurements on the tunnel wall were carried out using a handheld steel rod (Figure 3-3) that was equipped with moist sponge to maintain a good contact against the tunnel wall.

Figure 3-3. Steel rod earthing with sponge (Ahokas 2010). 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. Drillhole earthings provided 10 to 50 mA current feed,

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which was found adequate for the measurements. A highest current which 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 crushed 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 (10 – 15 minutes)

3) Survey in the drillhole at 10 cm station interval, c. 25 – 35 minutes measurement time. During the automated measurement process the tool was left unattended. This step was repeated for all earthings on the wall. Maximum of nine drillhole runs were achievable during a working day. Lowest number in practise due to various interferences was only one drillhole run.

4) Placing the downhole current earthing on a planned measurement position in ONK-PP262 using pushing rods. Removal of the rods.

5) Survey on the wall at 1 m interval with manual launch of measurement. Walkie-talkies were used to signal move to next position (each profile of 130 m took 1 hour on wall). Measurement was carried out using ABEM standard connection cables.

6) Installation of potential electrode on second trailer into bottom of ONK-PP274. Removal of pushing rods. Survey on ONK-PP274 from bottom to top at 10 cm station interval.

7) Moving the current earthing to next planned position in ONK-PP262 and repeating the steps 5 and 6.

The field work was carried out between 28 April 2011 and 11 May 2011. The activity schedule for the mise-à-la-masse measurements is presented in Table 3-1.

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Table 3-1. Activity schedule for the mise-à-la-masse survey.

Day Time Earthing Profile File28.4.2011 10:37 – 10:41 - no ONK-PP262 SPR

(8 – 10 m) O3P262RE00M005D205.CSV

28.4.2011 9:10 - 9:38 - no ONK-PP262 SPR O3P262RE00M005D105.CSV

2.5.2011 13:36 – 13:50 - no ONK-PP262 SPR (10-12 m)

O3P262RE00M005D305.CSV

28.4.2011 11:04 - 15:49 ONK-PP262, 8 m Wall MAM O3WALLRE08Z000D105. CSV

2.5.2011 14:19 – 15:29 ONK-PP262, 10 m Wall MAM O3WALLRE10Z000D205.CSV

29.4.2011 8:30 – 8.56 - ONK-PP274 SPR O3P274RE00M005D105.CSV

28.4.2011 17:21 – 17:42 (ended 8:03 – 8:05 on 29.4.2011)

ONK-PP262, 8 m ONK-PP274 MAM

O3P274RE08Z005D105.CSV

3.5.2011 11:23 – 11:26 no ONK-PP262 SPR (20 – 22 m)


3.5.2011 16:59 – 17:02 no ONK-PP262 SPR (17 – 19 m)



3.5.2011 9:47 – 10:38 ONK-PP262, 10 m ONK-PP274 MAM


3.5.2011 11:51 – 15:16 ONK-PP262, 20 m ONK-PP274 MAM


4.5.2011 14:01 - 14:39 ONK-PP262, 14.7 m

Wall MAM O3WALLRE14Z000D605.CSV

4.5.2011 13:10 – 13:43 ONK-PP262, 16.4 m



5.5.2011 9:40 – 10:10 ONK-PP262, 14 m ONK-PP274 MAM


5.5.2011 10:31 – 11:07 ONK-PP262, 13 m ONK-PP274 MAM


5.5.2011 13:58 – 14:09 no ONK-PP262 single hole 18 – 20 m


5.5.2011 15:28 – 16:16 ONK-PP262, 17m ONK-PP274 MAM


5.5.2011 16:38 – 17:16 ONK-PP262, 17m ONK-PP274 MAM


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5.5.2011 14:22 – 15:02 ONK-PP262, 17m Wall MAM O3WALLRE17Z000D805.CSV

5.5.2011 11:32 – 12:17 ONK-PP262, 13.15 m


5.5.2011 17.37 – 18.03 (ended 8:10 on 6.5.2011)

ONK-PP262, 16 m ONK-PP274 MAM

O3P274RE16Z005D505.CS

6.5.2011 9:13 – 9:40 ONK-PP262, 23.78 m

ONK-PP274 MAM



9.5.2011 13:51 – 14:17 ONK-PP274, 21 m ONK-PP262 MAM


9.5.2011 14:38 – 15:11 ONK-PP274, 20 m ONK-PP262 MAM


10.5.2011 8:34 – 9:00 ONK-PP274, 19 m ONK-PP262 MAM


10.5.2011 9:17 – 9:40 ONK-PP274, 17 m ONK-PP262 MAM


10.5.2011 9.53 – 10:18 ONK-PP274, 16 m ONK-PP262 MAM


10.5.2011 10.30 – 10:55 ONK-PP274, 15 m ONK-PP262 MAM


10.5.2011 12:20 – 12:43 ONK-PP274, 14 m ONK-PP262 MAM


10.5.2011 12:54 – 13:21 ONK-PP274, 13 m ONK-PP262 MAM


10.5.2011 13:33 – 13:56 ONK-PP274, 12 m ONK-PP262 MAM


10.5.2011 14:08 – 14:34 ONK-PP274, 11 m ONK-PP262 MAM


10.5.2011 14:44 – 15:11 ONK-PP274, 10 m ONK-PP262 MAM


11.5.2011 8:11 – 8:37 ONK-PP274, 8 m ONK-PP262 MAM


11.5.2011 8:48 – 9:13 ONK-PP274, 7 m ONK-PP262 MAM


The tool and earthing installations and the measurement ran as planned. Some minor problems occurred with a measurement software and SAS 1000, but those were solved during the measurements. The grounding in trailer and the cable reel affected the results a lot, and the trailer had to be insulated electrically from the tunnel floor.

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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 survey was stopped for the excavation blasts. The probes were kept in the drillhole, because the excavation was running on a considerable distance from the survey location.

The remote current electrode (four electrode group) was placed in the tunnel some hundred meters from the survey area (Figure 3-4). The coordinates of the electrodes are presented in Table 3-2.

Figure 3-4. Fixed current electrode.

Table 3-2. The location of the remote current electrode.

Position Northing, m Easting, m Elevation, m Current reference 6792154.661 1525696.580 -317.672 The location of the reference potential electrode (fixed electrode, group of three electrodes) was on the tunnel wall close to the survey area (Table 3-3 and Figure 3-5).

Table 3-3. The location of the reference potential electrode.

Position Northing, m Easting, m Elevation, m Voltage reference 6792324.971 1525378.257 -353.977

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The locations of the surveyed drillholes, surveyed tunnel line, remote current electrode and the reference potential electrode are presented in Figure 3-6.

The measured stations on the tunnel wall were marked with red paint and their locations were later measured with tacheometer.

Figure 3-5. Fixed potential electrode.

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Figure 3-6. Location map of the surveyed tunnel line (orange), remote current electrode (red) and the fixed potential electrode (blue).

3.2 Earthings

Altogether nine earthings were planned for the drillhole ONK-PP262. Because it was not technically possible to place the earthing to the deepest planned location at the depth of 24.59 m (too close to the bottom of the hole) the location was moved to the depth of 23.78 m.

The earthings planned for the surveyed drillhole ONK-PP262 are presented in Table 3-4 together with some geological, hydrogeological and geophysical information from the drillings. Figure 3-7 shows the single-point resistance and flow measurement data from drillhole ONK-PP262 and the locations of the planned earthings. A total of thirteen earthings were placed in drillhole ONK-PP274 (Table 3-5 and Figure 3-8).

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Table 3-4 Earthings used in drillhole ONK-PP262.

Figure 3-7. The single-point resistance (blue), flow measurement (purple) curves and the locations of the planned earthings (yellow) for the drillhole ONK-PP262.

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Table 3-5. Earthings used in drillhole ONK-PP274.

Figure 3-8. The single-point resistance (blue), flow measurement (purple) curves and the locations of the planned earthings (yellow) for the drillhole ONK-PP274.

3.3 Data

The quality of the data is quite good but due to weak conductivity values of the selected earthings in the drillholes the potential field values caused by the earthings are mostly low reflecting mostly the resistivity variation of the bedrock. Therefore the anomalies from the continuations of the examined fractures are occasionally difficult to detect from the data. It is important to note that the contrast between the examined conductor

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and the host rock should normally be more than 100 as discussed in Chapter 2 above, but in this case the contrast was only 10 or lower.

Figures 3-9 and 3-10 show the measured data from the drillholes ONK-PP262 and ONK-PP274. The data measured from the tunnel wall by the earthings in drillhole ONK-PP262 is quite smooth and the potential field is clearly weaker than observed in the drillholes. This may be due to the poor conductors and at least partly due to limited continuations of the examined fractures. Smoothing out is also possible due to the electrically conductive shotcrete layer on the tunnel wall. The tunnel data is presented in Figures 3-11 – 3-20.

A distinct potential maximum is encountered at top of both the drillholes at tunnel stations 43 - 55. Possibly the drillhole water and the steel armoured cable are bringing the potential close to tunnel wall. When comparing the profiles from different earthings the variation between the connections is visible. The weak connections can be detected when comparing the results with the curve of the inverse distance to the earthing.

The potential profiles in the tunnel fall into two categories: profiles where earthing location is producing maximum to the geometrically shortest distance (no or weak conductor connection, earthing at 8 m and 14.74 m in ONK-PP262) and profiles where a maximum or change in profile trend is observed at a location differing from the geometrically shortest distance.

Figure 3-9. The potential field data from drillhole ONK-PP262 measured by the earthings in drillhole ONK-PP274.

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Figure 3-10. The potential field data from drillhole ONK-PP274 measured by the earthings in drillhole ONK-PP262.

Figure 3-11. The potential field data from tunnel wall measured by the earthings in drillhole ONK-PP262.

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Figure 3-12. The potential field data from tunnel wall measured by the earthing at 8 m in drillhole ONK-PP262.

Figure 3-13. The potential field data from the tunnel wall measured by the earthing at 10.11 m in drillhole ONK-PP262.

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4 UNCERTAINTIES OF THE MODELLING

Due to the weak conductivity values of the examined fractures the potential field mostly followed the resistivity variation in the bedrock and the weak potential anomalies caused by different earthings were quite difficult to detect. Also the short distance between the drillhole where the earthings were installed and the surveyed drillhole caused difficulties to separate the anomaly caused by the earthing from the anomaly due to the distance from the earthing. These conditions induce some uncertainty in the modelling results.

The modelling method used in this work also involves difficulties in the complicated cases where conductors create a network or where several parallel conductors exist close to each other.

Figure 4-1 shows an example of the potential field curves measured from drillhole ONK-PP262 using two earthings in drillhole ONK-PP274. The potential curve shape (potential field values multiplied by 50 in Figure 4-1) mostly follow the single-point resistance curve. Only at such points where the active current electrode (the earthing) in drillhole ONK-PP274 was close or the examined fracture was continuing to drillhole ONK-PP262 the potential curve and the resistance curve separate, depending on the conductivity and extent of the fracture. In complicated cases the information from several different earthings was used together to help the modelling.

Figure 4-1. The single-point resistance (blue curve) measured in drillhole ONK-PP262 and two potential field curve forms (purple and yellow) measured by the earthings in drillhole ONK-PP274 at the depth of 13.44 m and 15.1 m.

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Figure 4-2 presents an example of the modelling difficulties caused by the adjacent current earthing (earthing in ONK-PP274 at 15.82 m). When looking at the whole drillhole data (Figure 4-2) from drillhole ONK-PP262 the form of the potential field curve (yellow) differs from the resistance curve (dark blue) in the area where the earthing is closest to the surveyed drillhole. In more detailed examination the weak potential maximum values just outside the maximum of the distance-from-the-earthing curve (light blue) are detected. The resistance curve has local minimums at the same locations. It is possible that these weak potential maximums are due to the continuation of the examined fracture. This result fits also the fracture orientation information mapped in both drillholes.

Figure 4-2. The single-point resistance (dark blue), flow measurement (purple) curves, potential field curve measured by the earthing in ONK-PP274 at 15.82 m (yellow) and the closeness of the earthing (light blue). The arrows show the location of one station with local low resistance and local high potential.

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5 MODELLING OF THE DATA

After careful modelling and taking into account the uncertainties discussed in previous chapter the connections between the drillholes were detected as presented below.

The modelling results can be divided into six areas or zones with one or more fractures showing almost the same continuity and direction (Figure 5-1). It is also possible that almost all examined fractures belong to the same thick zone and the fractures are representing a fracture network inside this zone. Weak connections were detected between the above mentioned zones or in the directions diverging clearly from the directions of the zones.

No connections were detected from the earthing at the depth of 24.59 m in drillhole ONK-PP262 to drillhole ONK-PP274. Weak indications suggest connection to the tunnel wall at 88 m. Also some weak continuations from some earthings to the tunnel were detected but due to poor conductors these connections are not reliable. All connections merge practically to the same zone at 88-89 m, which is the location of a tunnel crosscutting fracture P355.

Figure 5-1. The modelled conductors between drillholes ONK-PP262 and ONK-PP274 (purple = connection from ONK-PP274, blue = connection from ONK-PP262, white and yellow lines = weak connections). The disks show the fracture orientations mapped from the drill cores (red = slickensided, green = grain filled, yellow = open and grey = clay filled). Numbers show the modelled conductive zones. View to the north.

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5.1 MAM_HYDCO_zone1

The earthings in drillhole ONK-PP262 at the depth of 8 m and in ONK-PP274 at the depth of 8.55 m show parallel conductors between the holes (Table 5-1 and Figure 5-2). These conductors may belong to the same conductive fractured zone.

From the earthing in drillhole ONK-PP274 at the depth of 7.64 m there are weak connections detected to drillhole ONK-PP262 as Figure 5-2 shows (yellow lines) but it is possible that the weak conductor where the earthing was placed is not continuing to ONK-PP262.

Also a weak connection was detected from the earthing in ONK-PP274 at 8.55 m to the depth of 5.19 m in ONK-PP262. Although the direction of this modelled connection is quite close to the direction of some fractures in the drillholes this connection is not very reliable. There is no connection to the tunnel. Table 5-1. The detected electrical connections from the earthings in ONK-PP262 at 8 m and in ONK-PP274 at 8.55 m. The modelled connections are marked by crosses and the circles show the earthing locations.

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Figure 5-2. The detected electrical connections from the earthings in ONK-PP262 at 8 m (blue line) and in ONK-PP274 at 8.55 m (purple line). Also the weak connections from the earthings at the depths of 7.64 m and 8.55 m in ONK-PP274 (yellow lines) are presented. View to the north.

5.2 MAM_HYDCO_zone2

The MAM survey suggests a zone consisting of several conductive fractures between 10 m and 12 m in drillholes ONK-PP262 and ONK-PP274. One earthing was placed in drillhole ONK-PP262 at the depth of 10 m and three earthings in ONK-PP274 at the depths of 10.7 m, 11.4 m and 12.1 m. All these earthings have connections also between each other forming a network of conductors. Weak connection to the tunnel at 80 m is likely.

The modelled connections from each earthing are presented in Table 5-2 and in Figure 5-3.

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Table 5-2. The detected electrical connections from the earthings in ONK-PP262 at 10.11 m, ONK-PP274 at 10.7 m, ONK-PP274 at 11.4 m and in ONK-PP274 at 12.1 m. The modelled connections are marked by crosses and the circles show the earthing locations.

Figure 5-3. The detected electrical connections from the earthings in ONK-PP262 at 10.11 m (blue lines) and in ONK-PP274 at 10.7 m, 11.4 m and 12.1 m (purple lines). View to the north.

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5.3 MAM_HYDCO_zone3

From the earthing at the depth of 13.15 m in drillhole ONK-PP262 connections were detected at the depths of 13.51 m and 13.71 m in drillhole ONK-PP274. These coincide well with the fracture information in both drillholes. Also the earthings at the depth of 13.44 m in drillhole ONK-PP274 show connections between these two holes.

In addition to the connections presented above, two weak connections can be detected from the earthing in ONK-PP262. Probably these weak features do not continue to drillhole ONK-PP274 but they may continue close to this hole. There is a weak connection indicated in the tunnel to station 84 m.

The modelled connections are presented in Table 5-3 and in Figure 5-4. Table 5-3. The detected electrical connections from the earthings in ONK-PP262 at 13.45 m and in ONK-PP274 at 13.44 m. The modelled connections are marked by crosses and the circles show the earthing locations. Two connections are quite weak.

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Figure 5-4. The detected electrical connections from the earthings in ONK-PP262 at 13.15 m (blue lines) and in ONK-PP274 at 13.44 m (purple lines). Two weak connections from the earthing in ONK-PP262 at 13.15 m are presented as white lines. View to the north.

5.4 MAM_HYDCO_zone4

The connections detected from the MAM data measured by the earthings at the depth of 14.74 m in drillhole ONK-PP262 and at the depths of 15.1 m and 15.82 m in drillhole ONK-PP274 show two parallel conductors. These conductors are modelled as a zone, MAM_HYDCO_zone4. There is no clear connection to the tunnel. Possible indication is found at 88 m.

The modelled connections are presented in Table 5-4 and in Figure 5-5. In Figure 5-5 the weak connection from the earthing in ONK-PP262 at the depth of 13.15 m (white line) seems to continue to this MAM_HYDCO_zone4 but as discussed above it is probable that it is only a local feature around drillhole ONK-PP262.

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Table 5-4. The detected electrical connections from the earthings in ONK-PP262 at 14.74 m and in ONK-PP274 at 15.1 m and 15.82 m. The modelled connections are marked by crosses and the circles show the earthing locations.

Figure 5-5. The detected electrical connections from the earthings in ONK-PP262 at 14.74 m (blue line) and in ONK-PP274 at 15.1 m and 15.82 m (purple lines). View to the north.

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5.5 MAM_HYDCO_zone5

The modelled connections from the earthings in drillhole ONK-PP262 at the depth of 16.54 m, 17.07 m and 17.91 m and in drillhole ONK-PP274 at the depth of 16.82 m and 17.4 m form a group of parallel conductors. Because these conductors also have connections between each other they seem to belong to a thicker zone. There is a likely connection to tunnel at 88-89 m, indicated by more distinct potential maximum than from other earthings.

The modelled connections are presented in Table 5-5 and in Figure 5-6. Two of the earthings in drillhole ONK-PP262, at the depth of 16.54 m and 17.07 m, were placed at weakly hydraulically conductive fractures and according to the modelling they seem to continue to drillhole ONK-PP274. The weak hydraulically conductive fractures are presented as blue disks in Figure 5-6. This modelled MAM_HYDCO_zone5 may be the continuation of the tunnel crosscutting fracture P355 (Figure 5-7). Table 5-5. The detected electrical connections from the earthings in ONK-PP262 at 16.54 m, 17.07 m and 17.91 m and in ONK-PP274 at 16.82 m and 17.4 m. The modelled connections are marked by crosses and the circles show the earthing locations. One connection is very weak.

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Figure 5-6. The detected electrical connections from the earthings in ONK-PP262 at 16.54 m and 17.07 m (blue lines) and in ONK-PP274 at 15.1 m and 15.82 m (purple lines). Weak connections from the earthings in ONK-PP262 at the depth of 17.07 m and 17.91 m are presented by white lines. The blue disks show fractures with detected weak hydraulical conductivity. View to the north.

Figure 5-7. The location of the modelled conductive zone MAM_HYDCO_zone5 and the tunnel crosscutting fracture p355. View to the north.

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Figure 5-8. The likely electrical connections from earthings in ONK-PP262 (10.11 m, 13.15 m, and 16.54 – 17.91 m) to the tunnel stations 80, 84 and 88-89 m, respectively. View to the north.

5.6 MAM_HYDCO_zone6

According to the MAM survey the connections from the earthings at the depth of 20.04 m in drillhole ONK-PP262 and at the depth of 19.32 m and 19.72 m in drillhole ONK-PP274 are weak. The connection to drillhole ONK-PP262 from the earthing at 21.5 m in drillhole ONK-PP274 is quite close to the potential maximum caused by the closeness of the earthing but the anomaly is quite sharp and therefore probably due to the continuation of the conductor. Although the modelling results from these earthings are presented as a zone in this report it is difficult to say whether they belong together or not. But they seem to have some connections between each other. There is a weak indication of connection to tunnel at station 89 m.

The modelled connections are presented in Table 5-6 and in Figure 5-9. The weak connection from the earthing ONK-PP274_19.32m coincides well with the direction of one weak hydraulically conductive fracture (hydraulically conductive fractures presented as blue disks in Figure 5-9). This fracture was not detected in drillhole ONK-PP262 and it is possible that the fracture is local.

The connection modelled from the earthing at the depth of 21.5 m in drillhole ONK-PP274 coincides well with the directions of the fractures mapped in both drillholes (Figure 5-9).

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Table 5-6. The detected electrical connections from the earthings in ONK-PP262 at 20.04 m and in ONK-PP274 at 19.32 m, 19.72 m and 21.5 m. One modelled connection is marked by a cross and the circles show the earthing locations. All the other connections are quite weak.

Figure 5-9. The detected electrical connections from the earthings in ONK-PP262 at 20.04 m (white line) and in ONK-PP274 at 19.32 m, 19.72 m (yellow lines) and 21.5 m (purple lines). The blue disks show fractures with detected weak hydraulical conductivity. View to the north.

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6 CONCLUSIONS

The purpose of the MAM survey carried out in the HYDCO niche was to find out the continuities of some fractures and fractured zones detected in the drillholes ONK-PP262 and ONK-PP274.

The starting point for the mise-à-la-masse survey in these drillholes was not promising because the resistivity values of the fractures selected for the earthings were high and the contrast between the “conductors” and the host rock was small. In addition to the difficulties caused by the weak potential field anomalies also the complexity of the network of conductors hampered partly the modelling. Also the small distance between the earthings to the surveyed drillhole caused troubles with the anomalies due to the survey geometry alone. In some cases it was difficult to separate this anomaly from the anomaly caused by the connection from the examined fracture.

In spite of quite poor starting point the data was reasonable and gave useful data for modelling of the connections between the drillholes. The equipment used in the survey (electrodes and survey system developed by PRG-Tec) proves to work well also in difficult conditions. Integrated modelling of geological, hydrogeological and geophysical data reduce some uncertainties included now in the modelling results.

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REFERENCES

Aalto, P., Aaltonen, I., Ahokas, H., Andersson, J., Hakala, M., Hellä, P., Hudson, J., Johansson, E., Kemppainen, K., Koskinen, L., Laaksoharju, M., Lahti, M., Lindgren, S., Mustonen, M., Pedersen, K., Pitkänen, P., Poteri, A., Snellman, M. & Ylä-Mella, M. 2009. Programme for Repository Host Rock Characterisation in the ONKALO (ReRoC). Posiva Oy. Working report 2009-31. 64 p. Ahokas, T. 2010. Preliminary modelling of the 2010 MAM survey data. Posiva Oy. Working report 2010-75, 72 p.

Ahokas, T. and Paananen, M. 2010. Updated and integrated modelling of the 1995 – 2008 Mise-à-la-masse survey data in Olkiluoto. Posiva Working Report 2010-08, 266 p.

Eloranta, E., 1984. A method for calculating mise-à-la-masse anomalies in the case of high conductivity contrast by the integral equation technique. Geoexploration 23, pp. 471-481.

mise-à-la-masse survey in the hydco niche 2011 · the mise-à-la-masse technique is an electrical...

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