Working Report 2015-02

Seismic Monitoring Experiment of Raise Boring in 2014 Jouni Saari Marianne Malm

January 2015 POSIVA OY Olkiluoto FI-27160 EURAJOKI, FINLAND Phone (02) 8372 31 (nat.), (+358-2-) 8372 31 (int.) Fax (02) 8372 3809 (nat.), (+358-2-) 8372 3809 (int.)

Working Report 2015-02

Seismic Monitoring Experiment of Raise Boring in 2014

Jouni Saari, Marianne Malm ÅF-Consult Oy

January 2015

Working Reports contain information on work in progress or pending completion.

ABSTRACT In Olkiluoto, Posiva Oy has operated a local seismic network since February 2002. The purpose of the microearthquake measurements at Olkiluoto is to improve understanding of the structure, behaviour and long term stability of the bedrock. The studies include both tectonic and excavation-induced microearthquakes. An additional task of monitoring is related to safeguarding of ONKALO. The possibility to excavate an illegal access to ONKALO has been concerned when the safeguards are discussed. Therefore all recorded explosions in the Olkiluoto area and in ONKALO are located. If a concentration of explosions is observed, the origin of that is found out. Also a concept of hidden illegal explosions, detonated at the same time as the real excavation blasts, has been examined. According to the experience gained in Olkiluoto, it can be concluded that, as long the seismic network is in operation and the results are analysed by a skilled person, it is practically impossible to do illegal undetected excavation by blasting within the Olkiluoto seismic network area. In this report a possibility of seismic monitoring of undeclared excavation done by tunnel boring machine (TBM) has been investigated. In the earlier investigations the instruments were at the ground surface and the sensors were triaxial short period (1 Hz) geophones or broadband geophones. The characteristics (frequency content, polarity and amplitude) of the continuous seismic vibration generated by TMB were studied. The onset time of the seismic signal were not distinguished. Altogether 16 new 10 kHz accelerometers were installed in boreholes inside ONKALO March 2012. The sensors comprised a new subnetwork that monitored the raise boring of two shafts done 2014, from the level -455 m to the level -290 m. The aim was to record the seismic signal generated when the drill bit hits the rock at the moment the tunnel boring begins. Altogether 113 seismic signals generated by the drill bit were located during the experiment. The location accuracy was from few meters to few tens of meters in horizontal direction. In vertical direction the location accuracy was not as good and depended strongly on the vertical distribution of recordings. When the TBM is running continuous strong vibration is generated. This is usually observed by the Posiva’s permanent seismic network. This study presents a method that can be used, if the location of the TBM or tunnel cannot be verified by the documentation of construction works or by field studies. Posiva’s permanent seismic network can be designed so that it takes the possible undeclared TBM into account. The sensor types used in monitoring of the ONKALO’s excavation induced seismicity are also suitable for monitoring the hits of the TBM’s drill bit. If necessary, the network can be tuned to operate temporarily in a more sensitive mode for that purpose. Keywords: Seismic monitoring, raise boring, safeguards, ONKALO.

NOUSUPORAUKSEN SEISMINEN MONITOROINTIKOE VUONNA 2014 TIIVISTELMÄ Posiva Oy:n paikallinen seisminen asemaverkko aloitti toimintansa vuoden 2002 helmikuussa. Mikromaanjäristysmittausten avulla pyritään lisäämään tietoa Olkiluodon kallioperän rakenteesta, liikkeistä ja stabiilisuudesta. Tutkimuksen kohteena ovat tektoniset ja louhinnan indusoimat mikromaanjäristykset. Mittaukset ovat myös osa ONKALOn ydinsulkuvalvontaa. Ydinsulkuvalvontaan liittyen Olkiluodon alueella ja ONKALOssa tehdyt räjäytykset on paikallistettu. Mikäli samalta paikalta on havaittu useita räjäytyksiä, on niiden alkuperä selvitetty. Myös ajatus, että laiton räjäytys voitaisiin piilottaa laukaisemalla se samanaikaisesti ONKALOn louhintaräjäytyksen kanssa, on tutkittu. Olkiluodon mittauksista saadun kokemuksen perusteella voidaan todeta, että jos seisminen asemaverkko on toiminnassa ja sen havainnot analysoi ammattitaitoinen henkilö, laitonta louhintaa räjäyttämällä ei ole käytännössä mahdollista tehdä huomaamatta Olkiluodon seismisen asemaverkon alueella. Tässä raportissa tutkitaan uusia mahdollisuuksia monitoroida seismisesti laitonta louhintaa, joka tehdään tunnelikairauslaitteen avulla. Aikaisemmissa tutkimuksissa instrumentit olivat maan päälle sijoitettuja kolmikomponenttisia lyhytperiodisia (1 Hz) geofoneja tai laajakaista-geofoneja (BP). Tutkimuksissa kohteena olivat tunnelikairauslaitteen synnyttämän jatkuvan tärinän ominaisuudet (taajuussisältö, polariteetti ja amplitudi). Signaalin alkuhetkeä ei havaittu. Maaliskuussa 2012 ONKALOssa asenettiin porareikiin 16 uutta 10 kHz:n kiihtyvyysanturia. Anturit muodostivat Posivan seismisen asemaverkon aliverkon. Sen avulla monitoroitiin vuonna 2014 tehtyä kahden kuilun nousuporausta tasolta -455 m tasolle 290 m. Tarkoituksena oli rekisteröidä seisminen signaali, joka syntyy kun poran kärki iskee kallioon tunnelikairauskoneen käynnistyessä. Monitorointikokeen aikana paikallistettiin yhteensä 113 poran kärjen synnyttämää seismistä signaalia. Poran kärjen paikallistustarkkuus oli horisontaalisuunnassa muutamasta metristä muutamaan kymmeneen metriin. Vertikaalisuunnassa paikallistarkkuus ei ollut yhtä hyvä ja riippui selvästi saatujen rekisteröintien jakautumasta pystysuunnassa. Tunnelikairaus tuottaa voimakasta jatkuvaa tärinää. Yleensä se havaitaan Posivan pysyvän seismisen asemaverkon avulla. Tässä työssä esitetään menetelmä, jota voidaan käyttää jos tunnelikairauslaitteen tai itse tunnelin sijaintia ei pystytä varmistamaan rakennustöiden dokumenttien tai kenttätutkimusten avulla. Posivan kiinteä seisminen asemaverkko voidaan suunnitella niin, että myös ilmoittamattoman tunnelikairauksen mahdollisuus otetaan huomioon. ONKALOn louhinnan indusoiman seismisyyden monitoroinnissa käytettävien antureiden ominaisuudet soveltuvat myös monitoroimaan tunnelinkairauslaitteen terän iskuja kalliota vasten. Jos se katsotaan tarpeelliseksi, asemaverkko voidaan säätää toimimaan tilapäisesti normaalia herkemmällä tasolla, tätä tehtävää varten. Avainsanat: Seisminen monitorointi, nousuporaus, ydinsulkuvalvonta, ONKALO.

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TABLE OF CONTENTS ABSTRACT TIIVISTELMÄ 1 

INTRODUCTION .................................................................................................... 2 

2 

INSTRUMENTATION ............................................................................................. 4 

3 

OBSERVATIONS.................................................................................................... 8 

4 

3.1 

Location of the head of the raise boring machine ............................................ 8 

3.2 

Possible induced events................................................................................. 15

CONCLUSIONS.................................................................................................... 19 

REFERENCES ............................................................................................................. 21 

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1

INTRODUCTION

The possibility to excavate an illegal space close to ONKALO or an illegal access to it, is a concern from the perspective of safeguards. Normal explosions and excavation blasts in the Olkiluoto area are well detected and located by Posiva’s seismic network. The expected sensitivity of the network is ML = -2.5 … -2.0 (ML = magnitude in local Richter scale) near ONKALO. Also a concept of hidden illegal explosions, detonated at the same time as the real excavation blasts, has been presented. According to the experience gained in Olkiluoto, it can be concluded that, as long the seismic events are analysed by a skilled person, it is practically impossible to do so. In Olkiluoto, there are several examples of identified legal explosions performed close to each other in time and space (Saari 2006). The origins of such kind of the events are always checked from the daily blast record delivered by the contractor of the ONKALO project. Lately, a possibility to use tunnel boring machines (TBM) in purpose to build illegal constructions has been presented. The seismic signal generated by the raise boring machine is quite different to the signals generated by seismic events. The signal can be described as a continuous vibration that seems to start slowly. In traditional seismic monitoring that kind of seismic signal is called seismic noise. The current seismic network is designed to reject any kind of seismic noise. It can be temporary (generated by traffic, sea waves, wind, human beings or animals close to sensor, etc.) or more or less continuous (generated by turbines, generators, large pumps, electric power lines, TBM, drilling, etc.). Seismic signal generated by raise boring machines was monitored in Olkiluoto in 2006 2008. The preliminary report of the 2006 recordings (Saari & Lakio 2007) defined the guidelines of the measurements and studies performed in 2007 and 2008. The report by Saari & Lakio (2009) summarizes measurements done in 2007 and 2008 and gives some technical proposals for seismic monitoring of TBM activity in Olkiluoto. Different types of continuously recording triaxial seismic stations were operating in the Olkiluoto area in 2006 - 2008. Seismograms of those stations were analysed in order to characterise the signal generated by TBM. The results of this study indicate that seismic signal generated by TBM can be detected and the location of the source of the signal can be approximated in more or less accurate level. The seismic signal generated by raise boring machine was clearly distinguishable at the distance of less than about 750 m, but not at the distances larger than 2.5 km. Within the distance range from 1 to 1.5 km the signal is distinguishable, but not strong enough that it could be used accurately in routine monitoring of TBM. In the earlier measurements the instruments were at the ground surface. The sensors used in 2006 were low period triaxial (1 Hz) velocity sensors. Broadband sensors were used in the experiments of 2007 and 2008 in order to record frequencies below 1 Hz. The characteristics (frequency content, polarity and amplitude) of the continuous seismic vibration generated by TMB were studied. The onset times of the seismic signals were not distinguished.

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The ventilation shaft for incoming air was constructed from 17 January 2014 to 1 February 2014 and the personnel shaft from 10 March 2014 to 2 April 2014. The construction method was raise boring, where the boring was done upwards, from the level -455 m to the level -290 m. The design of the 2014 experiment is different. The sensors are inside ONKALO not at the ground surface. Altogether 16 new sensors (12 uniaxial and 4 triaxial) and four GS units were installed in March 2012. The sensors comprised a new subnetwork that monitored the raise boring experiment in 2014. The sensors are 10 kHz accelerometers produced by Wilcoxon Research Inc. The purpose was to record as accurately as possible the signal generated when the drill bit hits the rock at the moment the tunnel boring begins. If that can be done, the location of TMB’s drill can be determined. The accelerometers were installed in boreholes and they were grouped with the Posiva’s permanent seismic network in Olkiluoto. The diameters of the holes were 64 mm at level -437 m and 56.5 mm at level -290 m (Toropainen 2014). The sensors were grouted permanently few meters away from the ONKALO tunnel. The sensor type is selected for the purposes of the raise boring test and it has not been used before in Posiva’s network. Communication between the new seismic stations and Olkiluoto server was arranged via telephone line and DSL (digital subscriber line) modems. Timing of the stations was done by the GPS-antenna connected to the Olkiluoto server. The sampling rate of the new sensors (12 000 Hz) is higher than the sampling rate (6000 Hz) applied generally in the sensors in the ONKALO block (2 km * 2 km * 2 km cube surrounding ONKALO). The higher sampling rate was chosen for the purposes of the subnetwork monitoring the raise boring experiment.

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2

INSTRUMENTATION

The triaxial sensors were installed in 5 – 10 m vertical boreholes. At the level -290 m (ONK-OS3) the direction was downwards and at the level -437 m upwards (ONK-OS4, ONK-TT1T9 and ONK-TT2T14). Sensors ONK-OS3 and ONK-OS4 remain as permanent seismic stations after the raise boring experiment. The sensor of ONK-OS3 was grouted permanently 7.8 m below the ONKALO floor and the sensor of ONK-OS4 was grouted 9.2 m above the ceiling of the ONKALO tunnel. Triaxial sensors are able to record the whole seismic vibration, but the uniaxial sensors record only on one component of the movement. Therefore they were installed in the boreholes pointing towards the arrival direction of the expected raise boring signal. The locations of the sensors are presented in Table 2-1 and Figures 2-1, 2-2, 2-3, 2-4 and 2-5. Table 2-1. Location and sensor types of the Posiva’s seismic stations installed for raise boring experiment in the Finnish KKJ coordinate system (zone 1). Elevation is determined from the sea level. Seismic station Site name ONK-OS3 ONK-OS3U1 ONK-OS3U2 ONK-OS3U4 ONK-OS4 ONK-OS4U5 ONK-OS4U6 ONK-OS4U8 ONK-TT1T9 ONK-TT1U10 ONK-TT1U11 ONK-TT1U12 ONK-TT2T14 ONK-TT2U13 ONK-TT2U15 ONK-TT2U16

N (m) 6791972 6791964 6792000 6792008 6792007 6791998 6792014 6792024 6791981 6791972 6791985 6791966 6791984 6791990 6791984 6792018

E (m) 1525919 1525957 1525946 1525914 1525897 1525902 1525896 1525903 1525917 1525914 1525906 1525926 1525921 1525935 1525933 1525922

Z (m) -293 -295 -299 -298 -420 -438 -432 -437 -418 -425 -425 -423 -417 -421 -426 -428

sensor type triaxial uniaxial uniaxial uniaxial triaxial uniaxial uniaxial uniaxial triaxial uniaxial uniaxial uniaxial triaxial uniaxial uniaxial uniaxial

Olkiluoto servers’ need of memory capacity has increased in March 2012 after the installation of the 16 new seismic stations for the raise boring. Before the actual raise boring, after summer 2012 the Olkiluoto server reached few times the level of 90 % of the disk space allocated for seismograms. In order to secure continuous operation of the network, the redundant memory capacity of the Olkiluoto server was increased by 2 * 72 GB (RAID 1, redundant array of independent disks) on 7 November 2012. An event is sent to the Olkiluoto server, when a predetermined number of seismic stations detect earth vibrations that exceed the trigger value within a certain time

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window. In the group called RaiseB an event is recorded if 8 of the 16 new sensors detect a seismic event. The trigger level of the raise boring network was tuned interactively in order to avoid loss of memory capacity due to unwanted recordings, without losing the hits of the drill bit of the raise boring machine. The number of unwanted recordings could be thousands in a day and they were related to raise boring and other human activities in ONKALO. Interactive tuning of triggering level was done also during the raise boring period in order to find the right level for the purpose of the monitoring experiment.

ONK-OS3, ONKOS3U1, ONK-OS3U2, ONK-OS3U4 ONK-OS4, ONK-OS4U5, ONKOS4U6, ONK-OS4U8, ONK-TT1T9, ONK-TT1-U10, ONK-TT1U11, ONK-TT1U12, ONK-TT2T14, ONKTT2U13, ONK-TT2U15, ONKTT2U16

Figure 2-1. Locations of the seismic sensors. View from south.

Figure 2-2. Locations of the seismic sensors on level -290 m. View from south.

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exhaust air shaft

personnel shaft

inlet air shaft

Figure 2-3. Locations of the shafts and seismic sensors on level -290 m. View from above.

Figure 2-4. Locations of the seismic sensors on level -437 m. View from south.

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The subnetwork is designed to monitor very small events, with good location accuracy and within a small rock volume. According to the simulations, the detection threshold around the shafts, at depths between -290 m and -437 m, is about ML = - 3.0 and the location error from 10 to 15 m.

exhaust air shaft

personnel shaft

inlet air shaft

Figure 2-5. Locations of the seismic sensors on level -437 m. View from above.

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3 3.1

OBSERVATIONS Location of the head of the raise boring machine

Altogether 113 events related to raise boring were located during the monitoring experiment. 45 of these were from the inlet air shaft between 17 January – 1 February 2014 and 68 from the personnel shaft between 10 March – 2 April 2014. Around 0.3 % of the seismic signals generated by the tunnel boring machine were located. Very small amount of the recordings had a clear signal with sufficient amount of sensors for locating (example in Figure 3-1). Mostly the recordings were pure noise where the hit of the drill bit could not be distinguished (Figure 3-2). Locations of the events are presented in Figures 3-3, 3-4, 3-5, 3-6, 3-7, 3-8 and Tables 3-1 and 3-2. Over 35 000 seismic signals were rejected in total. The locations of the events were compared with the drilling reports by Bergteamet who was the raise boring contractor in ONKALO. Bergteamet’s reports showed the time when the boring was taking place and the drilled length. The approximate vertical error between the located events and the actual location of the drill bit is mainly 10 – 15 m while the raise boring machine was working between -455 … -400 m. When the drill moves upwards from there, the location error begins to increase significantly. The reason for this is that only 15 of the located events (marked with grey in Tables 3-1 and 3-2) are using sensors from the -290 m level (see Figure 2-1). Those events have much better location accuracy in the vertical direction while the rest of the events tend to locate between -455 … -400 m depths. The location accuracy of the events is quite good in the horizontal direction, mainly 10 – 15 m.

Figure 3-1. Example of located raise boring event with clear signal from station ONKTT1T9 (14 March 2014 18:26:30 UTC). Length of the sample is 0.417 seconds.

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Figure 3-2. Example of raise boring signal noise from station ONK-TT1T9 (20 January 2014 02:03:16 UTC). Length of the sample is 6.265 seconds.

Figure 3-3. Located events (45) in the vicinity of the inlet air shaft perimeter during the raise boring between 17 January – 1 February 2014. View from above. The distance between the grid lines is 100 m. Size of the sphere is relative to magnitude.

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Figure 3-4. Located events (45) in the vicinity of the inlet air shaft perimeter during the raise boring between 17 January – 1 February 2014. View from south. Size of the sphere is relative to magnitude. The distance between the grid lines is 100 m.

Figure 3-5. Located events (45) in the vicinity of the inlet air shaft perimeter during the raise boring between 17 January – 1 February 2014. View from west. Size of the sphere is relative to magnitude. The distance between the grid lines is 100 m.

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Figure 3-6. Located events (68) in the vicinity of the personnel shaft perimeter during the raise boring between 10 March – 2 April 2014. View from above. Size of the sphere is relative to magnitude. The distance between the grid lines is 100 m.

Figure 3-7. Located events (68) in the vicinity of the personnel shaft perimeter during the raise boring between 10 March – 2 April 2014. View from south. Size of the sphere is relative to magnitude. The distance between the grid lines is 100 m.

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Figure 3-8. Located events (68) in the vicinity of the personnel shaft perimeter during the raise boring between 10 March – 2 April 2014. Size of the sphere is relative to magnitude. The distance between the grid lines is 100 m. Table 3-1. Located events during the raise boring of the inlet air shaft (17 January – 1 February 2014). Events that used stations from -290 m level are marked with grey.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Date and time (UTC) 18.1.2014 11:08:05 18.1.2014 11:13:49 18.1.2014 11:14:34 18.1.2014 11:15:12 18.1.2014 20:09:29 19.1.2014 15:47:38 20.1.2014 2:02:34 20.1.2014 6:14:50 20.1.2014 9:07:35 20.1.2014 13:21:47 21.1.2014 12:05:46 22.1.2014 4:28:07 23.1.2014 8:28:23 23.1.2014 17:16:07 24.1.2014 1:49:32

X 6791986.50 6792000.50 6791994.10 6791986.80 6791978.10 6791978.30 6791983.30 6791975.80 6791987.30 6791977.30 6791990.50 6791975.60 6791965.10 6791978.70 6791980.30

Y 1525930.40 1525914.80 1525914.90 1525914.50 1525913.80 1525913.60 1525920.40 1525916.80 1525925.30 1525924.50 1525938.40 1525924.30 1525918.00 1525920.90 1525919.00

Z -424.0 -428.2 -428.1 -425.1 -412.3 -434.2 -416.7 -417.5 -419.0 -412.7 -411.8 -409.7 -438.4 -444.6 -363.8

Local magnitude -2.8 -2.1 -1.5 -2.6 -2.5 -3.8 0.0 -4.3 -3.5 -3.5 -3.1 -3.4 -3.2 -3.5 -2.7

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16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45

Date and time (UTC) 24.1.2014 4:11:58 24.1.2014 6:00:20 28.1.2014 7:51:28 28.1.2014 7:55:10 28.1.2014 8:08:55 28.1.2014 8:17:58 30.1.2014 12:11:19 30.1.2014 18:38:29 30.1.2014 18:56:46 30.1.2014 19:08:25 30.1.2014 19:17:27 30.1.2014 19:43:26 30.1.2014 19:53:14 30.1.2014 19:56:03 30.1.2014 20:08:06 30.1.2014 20:52:44 30.1.2014 21:46:11 30.1.2014 22:58:56 31.1.2014 6:20:57 31.1.2014 11:16:17 31.1.2014 11:22:07 31.1.2014 20:54:02 1.2.2014 6:26:44 1.2.2014 6:30:51 1.2.2014 7:08:42 1.2.2014 9:36:50 1.2.2014 10:35:59 1.2.2014 11:35:46 1.2.2014 13:43:08 1.2.2014 13:58:35

X 6792002.80 6791972.20 6791987.70 6791972.50 6791978.60 6791972.10 6791985.20 6791987.90 6791982.20 6791987.80 6791980.30 6791985.40 6791972.10 6791986.90 6791979.40 6791978.70 6791977.60 6791987.40 6791986.60 6791985.20 6791987.50 6791976.80 6791990.80 6791980.30 6791981.30 6791979.30 6791978.20 6791983.00 6791985.90 6791976.90

Y 1525917.80 1525929.00 1525926.50 1525919.00 1525917.70 1525942.10 1525902.00 1525916.30 1525935.40 1525917.70 1525924.50 1525928.10 1525925.50 1525923.90 1525929.00 1525932.00 1525928.70 1525916.80 1525924.50 1525933.70 1525930.10 1525936.80 1525922.50 1525934.30 1525930.10 1525925.90 1525926.20 1525929.00 1525926.70 1525929.60

Z -389.6 -393.8 -430.6 -292.7 -432.3 -428.7 -338.8 -427.3 -408.9 -425.8 -425.8 -407.4 -316.7 -425.2 -408.2 -432.6 -410.5 -429.7 -428.1 -409.3 -408.8 -396.3 -433.9 -400.7 -405.6 -394.8 -296.8 -398.2 -404.8 -404.0

Local magnitude -3.2 -3.3 -3.2 -2.0 -2.9 -1.9 -2.0 -3.4 -2.8 -3.0 -3.5 -2.9 -1.9 -3.1 -2.6 -2.8 -2.4 -2.8 -3.0 -2.6 -2.3 -2.6 -2.4 -2.6 -2.9 -2.4 -1.7 -2.8 -2.7 -2.5

Table 3-2. Located events during the raise boring of the personnel shaft (10 March – 2 April 2014). Events that used stations from -290 m level are marked with grey.

1 2 3 4

Date and time (UTC) 10.3.2014 12:52:33 10.3.2014 22:35:37 12.3.2014 9:06:59 12.3.2014 9:13:19

X 6791998.03 6791996.53 6791992.23 6791994.41

Y 1525925.44 1525910.94 1525920.78 1525922.24

Z -438.5 -438.7 -437.5 -452.4

Local magnitude -3.5 -2.9 -1.9 -2.2

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5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44

Date and time (UTC) 12.3.2014 10:11:58 12.3.2014 11:21:00 13.3.2014 6:47:04 13.3.2014 11:09:58 13.3.2014 15:48:16 14.3.2014 4:53:32 14.3.2014 5:04:33 14.3.2014 5:46:27 14.3.2014 5:54:49 14.3.2014 6:05:44 14.3.2014 6:19:18 14.3.2014 7:37:59 14.3.2014 10:42:56 14.3.2014 18:26:30 14.3.2014 19:39:44 16.3.2014 1:00:06 16.3.2014 4:22:09 16.3.2014 4:40:45 16.3.2014 14:37:05 16.3.2014 15:14:20 16.3.2014 23:35:18 17.3.2014 1:13:49 17.3.2014 2:54:39 17.3.2014 21:58:08 19.3.2014 2:48:51 19.3.2014 6:42:23 19.3.2014 6:49:18 19.3.2014 14:28:48 19.3.2014 21:03:40 20.3.2014 5:58:39 20.3.2014 22:08:20 21.3.2014 14:33:14 22.3.2014 15:15:53 23.3.2014 21:51:04 23.3.2014 22:10:14 30.3.2014 0:25:38 30.3.2014 3:19:55 30.3.2014 15:45:34 30.3.2014 23:38:51 31.3.2014 3:05:56

X 6791998.68 6792001.48 6791997.63 6791996.92 6791997.10 6791998.90 6791989.60 6791987.27 6792000.51 6791995.63 6791995.81 6791999.17 6791985.10 6791998.46 6791987.66 6791997.62 6791996.64 6791996.10 6791990.91 6791992.50 6791986.67 6791988.07 6791995.56 6791987.64 6791996.41 6791996.33 6791992.48 6791988.47 6791993.16 6791978.03 6791989.59 6791992.37 6791989.74 6791992.03 6791981.42 6791988.62 6791992.80 6792000.81 6791988.69 6791998.58

Y 1525924.23 1525928.31 1525920.81 1525923.09 1525917.87 1525926.63 1525920.21 1525924.47 1525928.35 1525922.65 1525913.69 1525924.96 1525921.37 1525927.02 1525927.32 1525935.87 1525920.48 1525937.68 1525922.24 1525922.29 1525919.49 1525919.51 1525921.53 1525920.01 1525920.76 1525921.41 1525921.88 1525923.44 1525923.34 1525920.23 1525916.10 1525921.34 1525920.76 1525918.70 1525925.64 1525920.21 1525918.21 1525923.62 1525919.44 1525922.74

Z -433.9 -431.8 -436.7 -436.0 -438.0 -443.0 -441.4 -435.5 -430.3 -442.7 -455.2 -431.7 -440.0 -436.3 -439.0 -428.7 -428.1 -416.1 -430.8 -429.9 -429.5 -439.5 -429.3 -430.3 -426.5 -427.9 -431.3 -439.8 -429.9 -429.4 -434.2 -400.5 -434.6 -425.4 -356.5 -432.5 -424.9 -436.6 -425.3 -409.2

Local magnitude -1.7 -2.1 -2.8 -3.1 -2.7 -2.9 -2.1 -2.2 -1.3 -2.4 -1.7 -2.9 -2.4 -2.6 -1.8 -3.3 -2.4 -1.9 -3.1 -2.9 -3.5 -3 -3 -3 -3 -3.2 -2.2 -2.5 -2.9 -3.8 -3.8 -3.1 -3.2 -2.8 -2.6 -3.1 -2.6 -2.4 -2.8 -2.6

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45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68

3.2

Date and time (UTC) 31.3.2014 3:07:52 31.3.2014 8:20:47 31.3.2014 9:11:18 1.4.2014 0:25:15 1.4.2014 1:21:41 1.4.2014 11:30:20 1.4.2014 11:44:46 1.4.2014 13:14:38 1.4.2014 13:39:36 1.4.2014 15:34:23 1.4.2014 16:08:18 2.4.2014 1:24:37 2.4.2014 3:10:36 2.4.2014 5:01:59 2.4.2014 6:20:01 2.4.2014 8:47:48 2.4.2014 9:11:45 2.4.2014 10:39:06 2.4.2014 11:18:22 2.4.2014 11:49:56 2.4.2014 11:56:34 2.4.2014 11:58:53 2.4.2014 12:18:56 2.4.2014 12:23:00

X 6791987.65 6792000.59 6792001.55 6791996.12 6791998.95 6791997.74 6792000.56 6791998.91 6791988.71 6791997.10 6791994.12 6791994.25 6791990.77 6791993.93 6791995.09 6791991.46 6791998.28 6791995.29 6791993.98 6791984.20 6791984.48 6791993.92 6791992.13 6791991.99

Y 1525925.86 1525923.59 1525930.54 1525922.90 1525917.30 1525922.30 1525923.08 1525922.35 1525920.42 1525924.55 1525919.71 1525926.46 1525918.88 1525921.15 1525918.31 1525921.02 1525923.01 1525920.01 1525919.80 1525914.34 1525926.08 1525919.53 1525923.74 1525921.01

Z -437.1 -436.3 -428.1 -432.0 -427.3 -422.2 -433.1 -429.6 -426.4 -398.8 -425.9 -429.9 -426.3 -430.6 -412.4 -425.9 -437.9 -424.3 -416.7 -362.8 -441.5 -421.0 -434.6 -431.9

Local magnitude -2.6 -2.1 -2.6 -2.9 -3 -2.3 -2.2 -2.5 -2.8 -2.7 -2.6 -2.7 -3 -2.9 -2.9 -3.2 -3 -2.5 -2.9 -2.5 -3.2 -3.2 -2.7 -3.3

Possible induced events

Only five possible raise boring induced seismic events were recorded in ONKALO during the raise boring experiment on 17 January – 2 April 2014. This result was expected, because even the bigger explosions related to the excavation work of ONKALO have only rarely induced seismic events. The events listed in Tables 3-3 and 3-4 have happened during the breaks of raise boring informed by the raise boring contractor Bergteamet. Especially the events listed in Table 3-3 were clearly separate in time from other raise boring related recordings. It is highly unlikely that these two events are related to moving of the raise boring machinery or similar activities. Both of these events have been located in the shafts where the raise boring was going on at that time: on 22 January in the inlet air shaft and on 18 March in the personnel shaft (Figures 3-9, 3-10, 3-11 and 3-12). The raise boring was approximately at level -400 m on 22 January (inlet air shaft) and on 18 March (personnel shaft) at the time of the events.

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Table 3-3. Possible induced seismic events during the breaks of raise boring (17 January – 2 April 2014). The raise boring was approximately at level -400 m at the time of the events. Date

Time

22.1.2014

18:05:00 (UTC)

18.3.2014

18:19:45 (UTC)

X

Y

Z

Local magnitude

location

6791972.0 1525913.7

-401.6

-3.5

near the inlet air shaft

6791992.7 1525918.9

-431.3

-3.4

near the personnel shaft

Table 3-4. Possible induced seismic events, which had too few stations for location, during the breaks of raise boring in the personnel shaft (10 March – 2 April 2014). Date

Time

Comment

14.3.2014

12:45:09 (UTC)

one event

14.3.2014

12:47:49 (UTC)

two events

The two events listed in Table 3-3 have been recorded with the temporary seismic network installed for monitoring of the raise boring experiment. This network consisted of 16 accelerometers installed on levels -290 m (4 sensors) and -437 m (12 sensors). The depth location accuracy is uncertain for both of the events (22 January and 18 March), because they have been recorded only with the sensors on level -437 m (8 seismograms in January event and 9 seismograms in March event). Seismic analysis software Trace could not calculate reliable fault plane solutions for either of the events. It is also possible and likely that more induced events have occurred during the actual raise boring. However it is impossible to distinguish those among the raise boring signal (see Figure 3-2).

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Figure 3-9. Possible induced seismic events during the breaks of raise boring (17 January – 2 April 2014). View from south. Blue sphere: 22 January 2014 at 18:05:00 (UTC), raise boring in the inlet air shaft. Red sphere: 18 March 2014 at 18:19:45 (UTC), raise boring in the personnel shaft.

Figure 3-10. Possible induced seismic events during the breaks of raise boring (17 January – 2 April 2014). View from west. Blue sphere: 22 January 2014 at 18:05:00 (UTC), raise boring in the inlet air shaft. Red sphere: 18 March 2014 at 18:19:45 (UTC), raise boring in the personnel shaft.

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Figure 3-11. Seismogram of the possible induced seismic event from the seismic station ONK-TT1T9 on 22 January 2014 at 18:05:00 (UTC).

Figure 3-12. Seismogram of the possible induced seismic event from the seismic station ONK-TT1T9 on 18 March 2014 at 18:19:45 (UTC).

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4

CONCLUSIONS

The earlier raise boring studies (Saari & Lakio 2007 and 2009) indicate that the seismic signal generated by raise boring machine is clearly distinguishable at the distance of less than about 750 m, but not at the distances larger than 2.5 km. The seismic signal generated by TBM can be detected, but the location of the source of the signal is not quite accurate. Usually, when raise boring is observed, the exact location of the tunnel and/or TBM is verified by field studies or by the declared documentation of the excavation. Altogether 16 new 10 kHz accelerometers were installed in boreholes inside ONKALO in March 2012. The sensors comprised a new subnetwork that monitored the raise boring of two shafts in 2014, from the level -455 m to the level -290 m. The purpose was to record as accurately as possible the signal generated when the drill bit hits the rock at the moment the tunnel boring begins. Over 35 000 seismic signals were located during the experiment. The total numbers of located events were 45 in the inlet air shaft (17 January – 1 February 2014) and 68 in the personnel shaft (10 March – 2 April 2014). This study shows that it is possible to locate the tunnel boring machine by sensitive seismic monitoring, within distances from few tens to about 100 m from the sensors. The location accuracy of the drill bit study performed in 2014 was from few meters to few tens of meters in horizontal direction. In vertical direction the location accuracy depended strongly on the recordings from the level -290 m. If they were available in the analysis, the location accuracy was good. If all the recordings were from about the same level (circa - 437 m) the sensor geometry was not suitable for accurate locations in vertical direction. Only minority (5 events from the inlet air shaft and 10 events from the personnel shaft) of the located events were using sensors at the level -290 m. One reason for that is the better sensitivity of the network at the lower level: there were 12 sensors at the level 437 m and only four at the level -290 m. The network was built this way, because there were more suitable tunnels and space to install the seismometers at the lower level. The other main reason was the very strong background noise level close to the four sensors at the upper level, which made the picking of the seismic signal arrival very difficult. The noise was mainly caused by the running engine of the TBM and the crew operating the machine. Five possible raise boring induced seismic events were recorded in ONKALO during the raise boring experiment. Two of them were located, but the depths of both of the events are uncertain, because they have been recorded only with the sensors of the lower level (-437 m). The events occurred during the relatively short breaks of raise boring between the longer periods of the actual drilling. It is assumed that more induced events have occurred during boring, because the tremors caused by the TBM have likely triggered rockbursts and rock falls.

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When the TBM is running continuous strong noise is generated. This is normally observed by the Posiva’s permanent seismic network. This study presents a method that can be used, if the location of the TBM or tunnel cannot be verified by the documentation of construction works or by field studies. Usually, when the drill bit hits the rock at the moment the machine starts to rotate, the background noise is low and generated signal can be detected. That requires very sensitive measurements, lot of disk space and time to reject manually unwanted signals, which are not reasonable during basic seismic monitoring. However, a set of sensors in Posiva’s permanent seismic network can be tuned to operate temporarily in a more sensitive mode. When a sufficient number of hits of the drill bit are recorded and located, the normal settings of the seismic network can be restored. In the future, the Posiva’s seismic network expands in the repository level. The network will be denser and it will cover much wider area than today. It is likely that there will be enough sensors in quiet places, if the drill bit needs to be located. However, the network geometry is designed to monitor the excavation induced seismicity. When the network is expanded, it would be useful to take the needs of the safeguards into account. The location of the new sensors can be suitable for both tasks, but sometimes the accurate safeguards might require additional seismometer or seismometers. The sensor types used in the monitoring of the ONKALO’s excavation induced seismicity are also suitable for monitoring the hits of the TBM’s drill bit. TBM generates continuous vibration that can be recorded in Posiva’s permanent seismic stations. When that kind of vibration is observed, the following step-by-step safeguards procedure could be applied. 1. Check all possible documents and contact persons in order to verify, if the origin of the vibration relates to the declared activity in ONKALO.   2. Generally, the source of vibration is solved after step one. If that is not

successful treatment, field studies are needed. The target area of the field studies can be approximated on the basis of the locations of recording seismic stations and the amplitudes of recorded seismic signal.  3. Finally, if the origin of the vibration is still unknown after steps 1 and 2, tune a

set of seismic stations temporarily in a more sensitive mode in order to locate the signal generated by the hit of the drill bit. When the potential location of the drill bit is estimated, verify the location by field studies.   

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REFERENCES Saari, J. & Lakio, A. 2007. Raise Boring of the Ventilation Shaft in Olkiluoto, 17.23.52006- Preliminary Analysis of Seismic Signal. Posiva Oy, 15 p. Working report 2007-03. Saari, J. & Lakio, A. 2009. Feasibility Study and Technical Proposal for Seismic Monitoring of Tunnel Boring Machine in Olkiluoto. Posiva Oy, 15 p. Working report 2009-03. Toropainen, V. 2014. Posiva Microseismic Network – Core drilling of Drillholes ONKPP348...351 in ONKALO at Olkiluoto 2012. Posiva Oy, 40 p. Working report 2014-10.