Working Report 2003-37

Seismic Network at the Olkiluoto Site

Jouni Saari

Enprima Oy

December 2003

Base maps : ©Nat i onal Land Survey , permiss i on 41 /MYY/03

Working Reports contain information on work in progress

or pend ing completion.

The conclusions and viewpoints presented in the report

are those of author(s) and do not necessarily

coincide with those of Posiva.

Teki j aorgani saati o:

Tilaaja:

Tilausnumero:

Tilaajan yhdyshenkilo:

Konsultin yhdyshenkilo:

Tekija:

Tarkastaja:

Enprima Engineering Oy Rajatorpantie 8, Vantaa PL 61, FIN-01601 Vantaa

Posiva Oy 27160 OLKILUOTO FINLAND

9503/03/HH

H:~ -~~~~~t!va Oy

Jouni Saari Enprima Engineering Oy

Working report

SEISMIC NETWORK AT THE OLKILUOTO SITE

)t---r-_s-~,. FT, Jouni Saari

Anna-Kaisa Airaksinen

SEISMIC NETWORK AT THE OLKILUOTO SITE J ouni Saari, En prima Oy

Abstract

In February 2002, Posiva Oy established a local seismic network of six stations on the island of Olkiluoto. In the beginning, the network monitors tectonic earthquakes in order to characterise the undisturbed baseline of seismicity of the Olkiluoto bedrock. When the excavation of the underground characterisation facility (the ONKALO) starts, the system will monitor also excavation induced seismicity. The purpose of the microearthquake measurements at Olkiluoto is to improve understanding of the structure, behaviour and long term stability of the bedrock.

The system is manufactured and installed by ISS International Limited. The main target of the monitoring is the underground rock characterisation facility and its surrounding, i.e. an area of about 1 km2 down to the depth of 500 - 1000 m. Within that volume of rockmass the designed resolution threshold of the network is ML = -2 (ML = magnitude in local Richter's scale). The high frequency sensors are at the surface of the island surrounding the preliminary location of the final disposal facility.

This report describes the technical features of the microearthquake monitoring system. In the beginning of the report, a general description of the seismic monitoring system is presented. The next chapter deals with instrumentation and data acquisition. The last chapter presents data processing and interpretation methods available.

Keywords: microearhquake, se1sm1c network, instrumentation, data processing, interpretation, visualisation.

OLKILUODON SEISMINEN ASEMA VERKKO J ouni Saari, En prima Oy

Tiivistelma

Vuoden 2002 helmikuussa Posiva Oy perusti Olkiluodon saarelle kuudesta seismisesta asemasta koostuvan paikallisen asemaverkon. Aluksi asemaverkko monitoroi tektonisia maanjaristyksia. Tarkoituksena on saada kasitys hairiintymattoman kallioperan seismisyyden perustilasta. Kun maanalaisen tutkimustilan (ONKALO) louhinta alkaa, systeemi monitoroi myos ONKALOn louhinnan indusoimaa seismisyytta. Mikromaan­jaristysmittausten avulla pyritaan lisaamaan tietoa Olkiluodon kallioperan rakenteesta, liikkeista j a stabiilisuudesta.

Laitteisto valmistuksesta ja asennuksesta vastasi ISS International Limited. Monitoroita­va kohde on maanalainen tutkimustila ja sita ymparoiva kalliomassa eli noin yhden ne­liokilometrin alue 500 - 1000 km syvyyteen asti. Asemaverkko suunniteltiin si ten etta tutkimuksen kohteena olevan kalliomassan sisalla havaitaan mikromaanjaristykset, joi­den magnitudi Richterin paikallisella asteikolla (ML) on -2 tai sita suurempi. Korkeataa­juiset seismiset mitta-anturit asennettiin maan pinnalle siten etta ne ymparoivat loppu­sijoitustilan arvioitua sijaintipaikkaa.

Tassa raportissa kuvataan mikromaanjaristysten monitorointijarjestelman tekniset omi­naisuudet. Aluksi annetaan systeemin yleispiirteinen kuvaus. Seuraava kappale kasittelee instrumentointia ja tiedonkeruujarjestelmaa. Viimeisen kappaleen aiheena ovat systeemin tarjoamat tiedon kasittely- ja tulkintamenetelmat.

A vainsanat: mikromaanjaristys, seisminen asemaverko, mittalaitteisto, tiedon kasittely, tulkinta, havainnollistaminen.

1

Abstract

Tiivistelma

TABLE OF CONTENTS

Page

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

1.1 Background of monitoring .............. .. .. .. ........ .. ................................ .. .... .... .................... 2 1.2 Geological and seismological framework .................................... .......... ....................... 3

2 GENERAL DESCRIPTION ................................................................................................... 4

3 INSTRUMENTATION AND DATA ACQUISITION .............................................................. 6

3.1 Selection of locations for seismic stations ....... .............. ... .... ... ... ..... ... ..... ... ................. 6 3.2 Seismic stations ....... ... ..... ......... ....................... ...... ..... .. .... ....... .... ................................ 6

3.2.1 Sensors .................................................................. .............. ........ .................................... 6 3.2.2 Seismometer vault ..... .. .................................................................. .................................. 8 3.2.3 Stand alone data acquisition units (SAQS) .......................................... .. ...... , .. ............... 10

3.3 Central site computer ...... ........ .. .. ...... .. .. ...... .. ......... .... ........................... .................... . 12 3.4 Data management. .... ......................................... ............ ........... ......... ... .... .. ..... .......... 12

4 DATA PROCESSING AND INTERPRETATION ............................................................... 15

4.1 General. ... ..... ..... ........ ... .. ..... .... ..... ..... ... .. ..... ..... ........ ...................................... ............ 15 4.1.1 Office computer ........................................................................................................ ...... 15 4.1.2 Data used in demonstrations .................. .... .................... .......... ........ ........ .............. .. ..... 15

4.2 Data processing ........ ... ...................................... ...................... .............. ............ ........ 15 4.3 Interpretation and visualisation .. .. .............................................................................. 21

5 REPORTS AND DATA ARCHIVES ................................................................................... 28

5.1 Reporting for construction .. ................ .. .. .... .. ...... .. .... .... ................ .. .. .. ........................ 28 5.2 Reporting for rockmass modelling ........ .. .. .......... .. ........ .. ...... .. .. .............. .... .. .............. 29 5.3 Archiving of data ...... ... .. ....... ..... .. ... ................... .. ............ ... ... .... ......... ..... ....... ...... ... ... 30

ACKNOWLEDGEMENT ............................................................................................................. 30

REFERENCES ............................................................................................................................ 31

APPENDIX A- SENSOR SPECIFICATIONS ............................................................................. 32

APPENDIX B- STAND ALONE QS SPECIFICATIONS ............................................................ 37

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1 INTRODUCTION

1.1 Background of monitoring

The island of Olkiluoto in Eurajoki municipality, on the western coast of Finland, has been selected as the site for the final disposal facility of spent nuclear fuel. In 2004, Posiva will start to construct an underground characterisation facility at Olkiluoto. This facility, called the ONKALO, will be used to acquire detailed information about the bedrock at Olkiluoto, to be utilised in the planning of the final disposal facility. The construction of the actual final disposal facility is scheduled to start in the 201 Os, and the final disposal of spent nuclear fuel can be started in 2020.

The microearthquake measurements at Olkiluoto aim to improve understanding of the structure, behaviour and long term stability of the bedrock. Initially, the network monitors tectonic earthquakes in order to characterise the undisturbed baseline of seismicity of the Olkiluoto bedrock. Once excavation starts, the system will monitor excavation induced seismicity. The seismic network will continue to monitor the final repository, when, in addition to the above mentioned tasks, also the safeguarding of the repository is contributed. The results of the local geodetic network of ten GPS stations (Ollikainen & Kakkuri 2001) give extra information that may complete the interpretation of the bedrock deformation at the surface of the island of Olkiluoto.

The subsurface construction works at Olkiluoto will change the characteristics (e.g. the stress field) of the virgin bedrock. Those changes will produce microearthquakes, which can be observed by the established seismic network. These observations give an opportunity to approximate in what extent and where the bedrock is disturbed, the stability of the rock cavern and the adjustment processes going on in the surrounding rock mass. A further task is mapping of the disturbed weakness zones in the rockmass surrounding the excavated structure.

Establishing baseline conditions and monitoring during the construction of the access ramp of the ONKALO was examined by Miller et al. (2002). The report identified 54 processes which could be induced in the subsurface as a result of ramp construction. These processes were categorised according to the expected spatial and temporal scales of their impacts, as well as for their significance. Of these processes, 24 were considered to be of high significance for the site understanding but only 12 are considered to be of high significance for repository performance. Microseismic array was the most often mentioned method when the geophysical monitoring techniques applicable to those 12 processes were indicated. The processes with high significance for repository performance that are amenable to micro seismic monitoring are: 1) Development of excavation damaged zone, 2) reactivation of excising fractures in the rock mass, 3) generation of new fractures in the rock mass, 4) evolution of hydraulic network and 5) evolution of hydraulic heads (Miller et al. 2002). In addition to excavation induced seismicity, microseismic array is capable to observe tectonic seismicity of the Olkiluoto area.

Seismic monitoring has also an essential role in Posiva' s monitoring programme (Posiva 2003b ). The baseline conditions are described in Posiva 2003a.

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1.2 Geological and seismological framework

The bedrock of Olkiluoto consists of Precambrian Svecofennian rocks, 1850-1900 million years of age. The nearby long (length>> 10 km) fracture zones are bordering the 1.6 Ga old Laitila rapakivi massif. The closest one of them lies at the NW -SE oriented contact of the rapakivi massif and the 1.3-1.4 Ga old Satakunta Sandstone, approximately 15 km NW from the site (Anttila et al. 1999).

According to the earthquake catalogue for Northern Europe (Ahjos & Uski 1992) seismic events in the Olkiluoto area are generally small and sparse. The area within a distance of 100 km from Olkiluoto includes 6 events with a magnitude of M= 2.5- 3.1. The nearest events occurred about 35 km south (1926, M=3.1) and about 40 km north (1804, M=2.9) from the site. Only one of those (1971, ML = 2.5) is located by seismic instruments. It occurred 77 km north of Olkiluoto, in Siikainen. Due to the insufficient location accuracy of mainly macroseismically located earthquakes, it was not possible to associate the observed epicenters with individual zones of weakness. However, some potentially active faults was presented in a preliminary seismotectonic interpretation (Saari 1997).

The Institute of Seismology, Helsinki University, maintains the regional seismic station network in Finland. The nearest seismic stations are about 200 km from Olkiluoto: three SE, three east and one north from it. At the same distance, are also the nearest Swedish station, at the western coast of the Bothnian Sea. The current detection threshold of the Fennoscandian seismic stations is of the order of ML = 1.5, in the Olkiluoto area. The threshold will be lowered in 2004, when the seismic station in Laitila, about 40 km of Olkiluoto, starts to operate.

In addition to the permanent seismic network, the Institute of Seismology runs an array of four short period seismic stations, in 2003-2004. One of the stations is at Olkiluoto and three of them are within a distance of 10 km from it. The monitoring aims to characterise the geophysical baseline at Olkiluoto. The study is part of the safeguards project of Radiation and Nuclear Safety Authority of Finland.

Owing to the long recurrence interval, the reliability of seismotectonic interpretation will be improved rather slowly, if the current regional earthquake monitoring network is used. In addition, the location accuracy is not adequate enough to link the earthquakes to the correct weakness zones. The need to improve the quality and quantity of the earthquake data for detailed seismotectonic interpretation at the local level is obvious (Saari 1999).

A sensitive microearthquake monitoring is a powerful tool for investigating the bedrock in great detail and in relatively short time. In Finland, this has been demonstrated in the Loviisa area, where both tectonic and excavation induced seismicity has been investigated (Saari 1996 & 1998). In February 2002, Posiva established a seismic network of six seismic stations in the island of Olkiluoto. The expected seismic events can be divided into tectonic and excavation induced seismicity.

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2 GENERAL DESCRIPTION

The network consists six field stations, central site computer for data acquisition and office computer for analysis. The system is manufactured and installed by ISS International Limited (http:/ /www.issi.co.za). The operating system of the computers is Linux. The resolution target of the monitoring system is set to ML = -2, within the network and in its vicinity.

The central site computer at the Posiva office at Olkiluoto collects the seismic events from the field stations and transfers them via email to the office station in V antaa (close to Helsinki), where analysis of the data is carried out. The whole chain from the field stations to the office is operated automatically, but the data transfer, processing and analysis can also be done interactively.

At the moment the network operates in a way which aims to most accurate data collection. All detected events are stored in the fields stations until they are safely transmitted to the field central site computer and/or to the office computer in Vantaa. Another mode is automatic real time analysis, which is more prone to data loss if failures in the data transmission process occur.

There is a two-way internet connection between the computers at Olkiluoto and Vantaa. This offers a flexible way to control the operation of the network, i.e. adjustment of the settings of the field computer and seismic stations can be done from the V antaa office computer.

The high frequency sensors are at the surface of the island surrounding the preliminary location of the final disposal facility. During the first few years the main target of the monitoring is the underground rock characterisation facility and its surrounding, i.e. an area of about 1 km2 down to the depth of 500 - 1000 m. Later the focus is on the earthquakes occurring during the construction and operation of the final disposal of spent nuclear fuel.

The main interest of the investigation is in the vicinity of the disposal, i.e. mainly to induced seismicity, but potential tectonic earthquakes within a distance of 10 km from the repository might also have influence on the safety of the cavern at Olkiluoto (Lapointe & Hermanson 2002). The present network is not able to monitor seismicity within that wide area.

The current network (with a diameter of about 1 km) can be extended both in horizontal and vertical direction. Additional seismic stations within a distance from five to ten kilometres from Olkiluoto might improve the understanding of the general seismotectonic behaviour of the area. Additional sensors below the earth's surface, for example in the ONKALO and in its access tunnel, would improve the location accuracy in the vicinity of the underground facilities. That would increase the quality of the analysis when the microearthquakes related to the construction of the final disposal facility are investigated.

The field stations monitor continuously, but only signals, which can be related to a seismic events are sent to the central site computer. The recordings which are related to the same seismic event can be associated either interactively or automatically. Automatic association of seismic events can be done either in the real time association

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mode or in the post association mode. Automatic association provides a fully automated procedure for high quality event detection, source location and estimation of the spectral parameters of local earthquakes.

Currently, all detected events are emailed to the Vantaa office, where the events are analysed interactively. In the beginning of the analyses, "false detections" (caused by traffic etc.) are rejected. The way the accepted events are analysed, depends on the number of recordings related to a certain seismic event. The analysis based on one or two recordings is always interactive. If there are more recordings associated to the seismic event, the analysis can be done interactive or by means of the same procedures used in the automatic analysis.

For the current configuration, also post association mode is available in the latest version of the data processing software. The post associator operates in the office computer, in V antaa. When this option is selected, when ever a new seismic event comes to the office computer, the software looks if that could be related together with another recordings already in the data archive. For example, if one seismic station has suffered data transmission failure for several days, it is still part of automatic analysis.

The real time associator operates in the central site computer at Olkiluoto. This operation mode may be preferred during the construction of the underground facilities, because all the activities related to that phase (explosions, drilling, loading, transportation, etc) will increase the amount of earth vibrations in the area. If seismic signals of, say three or four seismic stations, which could be associated to the same event are accepted to be transferred to the central site computer, unwanted detections could be rejected automatically. The central site computer could then automatically locate seismic events in real time and alarm if a certain magnitude or number of seismic events is exceeded close to the excavated area. Real time information about the behaviour of the bedrock might be valuable for the underground employees and for the real time design of excavation.

One can also combine these two modes so that, for example, the sensor on the Olkiluoto island record in real time, but the stations outside of it, recording mainly tectonic earthquakes, are set to operate in the post association mode. Data transfer from those distant stations to the central site computer is more expensive than inside Olkiluoto, which makes that kind of data processing preferable.

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3 INSTRUMENTATION AND DATA ACQUISITION

3.1 Selection of locations for seismic stations

Based on the prevailing knowledge of the rock model and the location of ONKALO, altogether 20 bedrock outcrops were studied, in June 2001. The purpose was to find suitable locations for the six seismic stations. The study area surrounded the expected location of the disposal cavern within c. 2 km in EW -direction and c. 1 km in NS­direction. The selected sites are presented in Figure 3-1 and Table 3-1.

The main criteria of choice were good intact bedrock and suitable geometry for seismic measurements. It was aimed to find the locations of the seismic stations so that they cover and surround the main target area, i.e. the ONKALO and its surrounding, uniformly. Easy access to the site for construction and maintenance purposes, as well as availability of electricity and telephone connections were seen as advantages.

External sources of disturbance were avoided where possible. Electric power lines and switchyards are potential sources of disturbance. Different nearby artificial sources of noise like heavy traffic, generators, large pumps, rock -crushing installations and rock­waste areas are disadvantageous. Wind generates disturbances if the seismic stations are close to the shoreline or if trees are close. Generally, winds produce lower frequency vibrations than microearthquakes.

Table 3-1. Location of seismic stations in the local co-ordinate system. Sensor type ace = three-component accelerometer and geoph = three component geophones.

Station North (m) East (m) Elevation (m) Sensor type OS1 6792 814,33 1525 470,76 9,60 ace OS2 6792 368,93 1525 092,83 7,61 acc+geoph OS3 6791 997,32 1525 397,31 12,65 acc+geoph OS4 6792 851,65 1526 284,22 8,57 acc+geoph OS5 6792 405,61 1525 530,04 12,81 ace OS6 6792 157,08 1526 151,35 8,92 ace

3.2 Seismic stations

3.2.1 Sensors

Most of the significant information contained in the seismic signal is at its corner frequency (fc) and in the frequency band near it. Generally, the rule is that the lower is the magnitude of the earthquake the higher is the corner frequency. The selected resolution target of the monitoring system (ML = -2) sets a demand that the equipment should be able to analyse spectral amplitudes up to the corner frequencies 100- 150Hz or higher (Saari 1999). The resolution target is set for the six stations. For lower magnitudes the analysis can be based on recordings of a one three component station or part of the network. Of course, the quality of the results is not as good as if all stations were available.

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The amplitude of seismic signal is attenuated as a function of distance and frequency. The higher are the predominant frequency radiated from the source, the closer the sensor needs to be.

The most important parts of the seismic station are the three component sensors installed on the top of the bedrock. In microearthquake studies the sensors are either accelerometers or geophones, which record ground velocity. The most suitable sensor type can be selected when the resolution target and the distance from the source to the sensor are known (see e.g. Mendecki et al. 1999). At Olkiluoto, all the seismic stations have three component accelerometers (type 3A2300, usable frequencies: 0.2- 2300Hz). In addition, three of the furthermost seismic stations have also geophones (3030, Natural frequency: 30Hz and usable frequencies: 15 -2000Hz). Response curves and other technical specifications are presented in Appendix A.

It depends on the hypocentral distance, or to be excact, on frequency content of the seismic signal, which type of sensor would have better signal to noise ratio. Therefore, both sensor types are tested simultaneously in the most distant stations: OS2, OS3 and OS4. Later, either of these sensors can be moved, for example to the ONKALO if necessary, or the geophones can be electronically enhanced to lower frequency sensor (Natural frequency= 5.5 Hz). One electronic enhancing module was bought and testing began in OS2, in December 2002. The 5.5 Hz geophones are able to record lower frequencies (down to 3 Hz), i.e. more distant events than the 30 Hz geophones. Those may be suitable if the target are of monitoring is extended outside the Olkiluoto island. However, the low frequency background noise will be increased as well.

3.2.2 Seismometer vault

The sensors are placed at the bottom of a seismometer vault (Figs. 3-2 and 3-3). A concrete slab was cast on the bedrock outcrop on the bottom of the vault for the purpose of attaching the structural rings and to ensure a level base. The vault does not need separate heating, but it must have proper insulation. The diameter of the vault is about 1.2 m, which accommodates any necessary servicing of the sensors. The thickness of the insulating ground cover is from three to four meters (Figure 3-4). Covered drains, cable pipes and lightning protection cables are mainly inside the ground cover.

Figure 3-2. Basic structure of a seismometer vault. The objective is to provide stable conditions in respect of vibration, moisture, temperature and electrical properties.

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b)

Figure 3-3. a) A seismometer vault under construction. The concrete slab and the first concrete ring are shown. The final height of the fault is about 1.5m. b) Access to the vault (ladder), cabling, orange cabling box, floor drain and accelerometers (by the wall). In the upper corner in a detailed picture: sensor mounting block, triaxial accelerometers and the raw! bolt.

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The three sensor elements (accelerometers or geophones) are mounted onto small metal block (8*4*4 cm), and secured to the concrete slab via a rawl bolt (Fig. 3-3b, detail) . The bolts are about 13 cm long and have a diameter of 8-10 mm. In order to secure the bolt, a 4 - 5 cm deep hole (diameter 12-15 mm) was drilled in the slab. The block is adjusted so that the sensor orientations are N-S, E-W and up-down.

- -- ~

Figure 3-4. Seismometer vault and the equipment cabin (OS6). GPS antenna is in the left corner of the cabin.

3.2.3 Stand alone data acquisition units (SAQS)

The digital stand alone data acquisition unit (SAQS) is in a heated glass fibre cabin 10 -15 m away from the vault (Figure 3-4). The cabin is connected to the electrical network and to the local telephone network. The hut is fitted with SAQS and its peripheral devices: modem for data transfer, AC/DC converter to produce 12 V DC power for SAQS and GPS antenna for timing.

The basic task of SAQS is data acquisition (Fig. 3-5). It has a 24-bit digitizer with a dynamic range of over 110 dB. The software selectable sampling rates of the signal are from 50 to 24 000 Hz. At Olkiluoto, the selected sampling rate is 6000 Hz. At the moment the network operates in post association mode without real time communication with the site central computer. A detailed technical description of the SAQS unit is in Appendix Boron the manufactures web site (http://www.issi.co.za).

Due to the high sampling rate needed in microearthquake studies automatic detection of the brief seismic signals from the continuos data flow of noise is desirable. The SAQS unit controls the continuous data flow and when a pre-set trigger value is exceeded a

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potential seismic signal is recorded to a hard disk drive. The central site computer is set to phone every hour to each of the stations to collect any stored triggered events to it (Table 3-2).

Table 3-2. Current dial up times and telephone numbers for the Olkiluoto network. Every 10 minutes the central cite computer makes a phone call (*=every hour). For example, OSJ is dialled ten past one, ten past two, etc.

Station id OS1 OS2 OS3 OS4 OS5 OS6

Telephone number 3891 3892 3893 3896 3894 3895

Dial Time (h:min) *:10 *:20 *:30 *:40 *:50 *:00

Figure 3-5. Equipment inside the cabin from left to right: A CIDC converter, modem and SAQS.

The user of the office computer or the central site computer can change the trigger level i. e the sensitivity of the network. Each station can have its own trigger value, which can be changed whenever it is necessary. For example, when some field studies are done close to a seismic station, a higher trigger level can be set for that time period, in order to avoid an increase in unnecessary triggering.

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The SAQS units are able to record six sensor channels (two triaxial sensors, or six uniaxial, or any combination in between). In addition to the seismic channels (up to 6) the SAQS units can support extra non-seismic channels (up to 16), which can be connected to strain meters, tilt meters or other geotechnical sensors. This enables simultaneous interpretation of various field information. The extended and centralized data acquisition requires installation of an additional interface for non-seismic data.

The event detector of the SAQS unit compares the short term average (ST A) of the amplitudes to the long term average (LTA) of the amplitudes. The event detector starts recording data when the ST AIL TA ratio exceeds the pre-set trigger value. Different triggering values from four to eight has been tested. Currently, if the local noise level is normal, the trigger value is set to six at each of the stations. Usually, the present setting gives detections of few tens to few hundreds per day.

3.3 Central site computer

The central site computer is a link between the office computer and the seismic stations. In relation to the SAQS units, it is a device, which acquires recorded seismic events and controls their operation. In relation to the office computer, it is a device, which sends the acquired data and offers an opportunity to control SAQS units via the ssh­connection.

The central site computer supports the run time system (RTS) program, which continually acquires, processes, analyses and archives seismic data. In addition, tools for automatic alarm, event locations and reporting are available. The peripherals of the simple desk PC are GPS antenna for timing, CD-ROM for software installation and a modem for data communication between the central site computer and the six SAQS units. The operating system is Linux (Redhat 7.2).

R TS consists of a number of co-operating software modules that manage a number of logical communication links, each being connected by means of the communications subsystem to data acquisition devices. Those modules are used also to set or change parameters in the SAQS units.

The automatic processes can be controlled by an operator interface. There are number of parameters that provide information to the RTS. Some of them are set when the system is installed. For example, RTS identifies each seismic sensor with a site id, which combines the sensor properties and the recorded signal. Some parameters (like trigger levels) are often changed by the user.

Monitor displays included in the RTS provide operators with a continuous view of system activity and alarms are used to bring exceptional conditions to the operator's attention. System status can be displayed in a variety of forms either locally or remotely. Waveforms of the recorded signals can be inspected and online event locations can be displayed.

3.4 Data management

The network time is synchronised to Universal Time Co-ordinated (UTC) time by means of a GPS timing unit. This enables recorded seismic data that is to be compared

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with other seismic or geotechnical data that is GPS stamped. The technical guidelines of the Olkiluoto network are presented in Figure 3-6.

samplig rate: 6000 Hz

Seismic network Olkiluoto site

telepha1e wires

Figure 3-6. Operation chart of the seismic network at the 0/kiluoto site.

GPS

Special attention has been paid to reliable data recording. All seismic events are stored on the hard disk drive of SAQS unit before it is transferred to the central site computer. This kind of arrangement guarantees that any failure in the local communication (telephone line or modems) or in the central site computer does not cause loss of seismic events.

The benefits of current data management system were demonstrated in summer 2002, when there was a physical failure of the hard disk drive in the central cite computer, at Olkiluoto. Finding and analysing the failure, deciding the method to solve the problem and to rescue the collected data to a new computer could be done carefully. The six stand alone stations were recording ground vibrations all the time. After over two months, Linux, software and data were installed in the new computer and the recorded data (over 15 000 triggered events) was loaded from the hard disk drives of the six SAQS units. If the break had been longer, the removable hard disk drives of the SAQS units could have been sent to V antaa, where the data could have been loaded in the computer and analysed with the usual way.

Between Olkiluoto to Vantaa the data management is based on internet technology, which is an economical and reliable way to transmit data (Fig. 3-7). Email server keeps the seismic data until the office computer has received the data. The operation and settings of SAQS units and site central computer at Olkiluoto can be monitored and controlled from Vantaa or from the ISS head office in South Africa via the internet connection (ssh). The whole chain of data management is checked every day by a calibration signal. The signal controls the prevailing status of the seismic sensors and the data flow from a single station to the office computer in Vantaa.

At the moment, all triggered signals are emailed to Vantaa, because some microearthquakes might be recorded just in one seismic station. Also those events can be located and utilised in the interpretation, although the quality of interpretation is lower than if recordings of several stations are available. In a real time association mode, one could also send just events recorded by a predetermined number of seismic

14

stations. In that case, the triggering level could be lowered and the full benefit of automatic processing could be utilised. On the other hand, then some small but valuable indications of bedrock movements could be lost.

Operation via Internet

ssh-connection

Figure 3-7. Operation chart of the seismic network via Internet. Ssh (Secure Shell) is a program to log into another computer over a network. In this system, only two computers are allowed to log into the Central Site computer in Olkiluoto. The observed Seismic events are sent via emailfrom Olkiluoto to the Office computer in Vantaa.

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4 DATA PROCESSING AND INTERPRETATION

4.1 General

In the following sections, the general description of the data processing and interpretation procedures is based on the presentation on the web page of ISS International Limited (http:/ /www.issi.co.za). References to the methodology and terminology used in ISS system is available in the literature (Mendecki 1977, Mendecki et al. 1999 and Saari 1999). The above mentioned four "events" are used when examples and background of the application at Olkiluoto is presented.

4.1.1 Office computer

The basic technical requirements for the data processing and interpretation are a PC with Linux operating system and intemet facilities. In this case a desktop PC in the V antaa office is used. The power is connected via UPS unit. The peripherals of the computer are a CD station (read/write), a DAT tape recorder, printer and a plotter. The CD station is used for updating the software and for some data archiving purposes. The tape recorded is used as a backup unit.

Internet facilities are utilised to two-way communication between the office computer and the central site computer. Seismic events are received by email and the central site computer and the SAQS units are controlled by ssh-connection.

4.1.2 Data used in demonstrations

The network, surrounding the approximated location of the repository, is able to record earthquakes with magnitude ML>-2, within the network and close to it. During the first year of monitoring, there have not been indications of seismic movements in the island of Olkiluoto. However, there are four events in the record of accepted seismic events that are of uncertain origin (see event information in Fig. 4-4).

The events used to demonstrate the processes related to data processing and visualialisation are not typical examples of microearthquakes, but they are not similar to the typical artificial seismic signals either. One of them occurred during the period when there were thousands of signals generated by the switchyard of Olkiluoto. That event belongs likely to this group, but not with full certainty. For example, they are located to a reasonable depth. The other three are single station recordings of OS 1 and OS3 that look quite much like signals generated by vehicles. However, there are signals that could be interpreted as S-phases. Those phases makes single station location possible, but they can be something else. For example, they may be generated by another vehicle running nearby.

4.2 Data processing

The current data processing program is called XMTS. The program operates only in Linux environment. It will be replaced by the Java based version of data processing package JMTS which is able to operate both in Linux and Windows machines.

16

Otherwise, the new version has the same features as XMTS. After early 2004, when all the tests of JMTS are carried out, there will be a transition period when both packages are available. After that only JMTS is supported and developed.

Data processing occurs both automatic and interactive. The incoming data is archived automatically in the office computers directories according to the logic the ISS data processing and interpretation softwares are using.

XMTS is able to process three- or one-component waveforms recorded either by geophones or accelerometers. Source parameters produced automatically by the Run Time System, may be modified after interactive reprocessing. The package has interactive functions for phase picking, location of local earthquakes, magnitude calculation, spectral parameter estimation as well as moment tensor-, focal mechanism-, and source time function calculation.

Figure 4-1 shows an example of a XMTS screen dump. The recorded signals in are most likely related to electronic pulses of the nearby switchyard, 24.5.2002. On the left and right side are shown the menu buttons for different functions available in data processing. The components of the seismograms are presented by different colours: red is N-S, green is E-W and blue vertical. Particle movement is shown as positive in directions South, West and down. Station indexes are: OS4-g is recorded by the geophones in the station OS4 and OS4-a by the accelerometers, etc .. The following parameters are calculated automatically: onset time, location, location error, scalar seismic moment, local magnitude, radiated energy of P and S, corner frequencies, source radius, static stress drop, apparent stress drop and apparent volume (see yellow printing in Fig. 4-1 ).

The raw waveforms are pre-processed by applying baseline corrections. Instrument correction for geophones and accelerometers has to be applied to remove the effects of frequency dependent instrument response to a waveform. An industrial environment may produce a ground loop which could inject a 50/60Hz hum into the signal which is significantly reduced if not eliminated by the software. Precursory artefacts generated by anti-alias filters are suppressed by applying several low pass filters.

XMTS is able to display waveforms, rotated components, polar diagrams, signal energy, P- and S-wave spectra and the displacement stacked spectrum which is corrected for attenuation. The user has the option to filter waveforms according to the upper and lower cut-off frequencies. Several ground motion parameters are displayed as well.

The P and S Pick functions allow the user to manually pick the P and S wave arrival position on a selected seismogram. The auto picking function will automatically pick the arrival time (menu buttons on the right in Fig. 4-1 ). The automatic determination of P-wave arrival is based on multi- component autoregressive techniques (Wiener filter), which is supplemented by detection algorithms. The S-wave arrival is estimated using a polarisation filter and characteristic function. The implemented procedures for phase detection utilise travel time information.

Preliminary event location is based on assumption of constant P- (Vp=5600 m/s) and S­wave velocities (Vs= 4350 m/s) within the target area. The velocities applied are approximated according to the several seismic studies conducted in the study area (e.g.

17

Front et al. 2001 and Cosma et al. 1996). These velocities can be changed for each of the seismic station if anomalous velocities are recognised).

Figure 4-1. Screen dump of recordings and automatically produced source parameters (yellow printing in the box in the lower right corner of the screen).

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The event location calculation is performed automatically when the event is accepted. Numerous reliable methods have been developed to automatically estimate the source location using P and S wave arrivals and/or S-P differences that are supplemented by directions towards source and azimuth. Single station location is provided for three component data. The required accuracy for location is approximately 3% of the average hypocenter distance from the source to the stations used in the location procedure for a reasonable configuration of stations in relation to an event. A higher precision location is usually achieved with relative location procedures. To use it several well located seismic events or blasts have to be selected as master events.

The quality of the interpretation can be verified in different ways. Usually, if the approximated location error is small also other indicators of quality are good. In addition, the program makes a quality control (Fig. 4-2, lower right corner) and gives the time difference between the selected and theoretical onset times for the estimated hypocenter.

If the result of the initial location needs improvement, one usually changes the estimated onset times. The incorrect onset times can be identified by means of the above mentioned quality analysis and by means of the analysis of the recorded particle motions. Each P- and S-onset pick has an indicator above the seismograms. For example, the probabilities that the P- and S-onsets of the recording of the OS3 are correctly estimated are 0.58 and 0.68 (value behind the letter "w" in the upper left corner of Fig. 4-1). By sliding the cursor, one can find the most probable onset times and relocate the seismic event.

There are numerous tools, which can be applied when the onset times are set interactively. For example, one can zoom the selected seismogram, display the component separately and filter them (Fig. 4-2, upper left corner), display the energy curve, square the amplitude (Fig. 4-2, second line of display) or display the sum of squares of all three components (Fig. 4-2, lower left corner).

Spectral analysis is a standard technique used in studies of local earthquakes. Most seismic theories predict that the far field di~placement spectrum remains constant at low frequencies and decays at higher frequencies. Source parameters are calculated from P­and S-wave spectra. Multitaper spectral estimation techniques are used to minimise the effect of data windowing. Noise and site effect caused by local geology are reduced by stacking individual spectra. The spectra of individual seismograms and the stacked spectrum are corrected for the attenuation effect. The attenuation parameters are calculated for each event independently; however it is recommended the option of estimating attenuation using a group of events be chosen. The following parameters are calculated: scalar seismic moment, local magnitude, radiated energy of P and S, corner frequencies, source radius, static stress drop, apparent stress drop and apparent volume.

Comparison of the observed and theoretical spectra can be used when the quality of the estimated source parameters is assessed. Spectral analysis is also an useful tool for event identification. Spectral characteristics earthquakes and explosions are quite different. Another powerful tool for event identification is rotation of seismograms (upper right corner in Fig. 4-2 and Fig. 4-3). The P-wave should be seen in red, SH­wave in green and SV -wave in blue. The lack of S-wave recording is one of the reasons

19

that the event occurred in 24.5 .2002, is not interpreted as an earthquake. A similar conclusion was reached after comparison of original and rotated components of the 8.5.2002 event (Fig 4-3).

Figure 4-2. Screen dump presenting some options useful in the interactive analysis.

20

1 06.30 OS3-a 00:1 0:04.207 P• 690 W•0.56 S• 765 W•0.66 f1

Figure 4-3. Example of a recording and analysis of the event recorded by a single seismic station (Event number 3 in the event info box Fig. 4-4).

Example of a single station location is shown in Figure 4-3. The analysis gives always two hypocenters in opposite sides of the seismic stations (lower right corner, Fig. 4-3). When the sensors are at the surface, the correct solution is the one that is located below the ground surface. The final result of the analysis is shown the "Display text" box in Figure 4-3 .

The moment tensor solution for a seismic event can be obtained automatically, if there are enough recordings available. The inversion is performed in the frequency and/or

21

time domains. For the best double-couple solutions, parameters of nodal planes are calculated. The user can modify the location of the seismic event. Time domain inversion additionally provides the temporary evolution of the source which is a reflection of the source complexity.

The program has also different options for reporting. It allows the user to print short and full reports concerning the selected event. The user also has the option to print the seismograms and/or moment tensor as displayed on the screen. Further option is the screen dump, which allows to combine different kind of information in the same figure. Seismograms and screen dumps can be emailed to other users.

4.3 Interpretation and visualisation

Two basically similar programs (Xdi and Jdi) are used for visualisation and interpretation of the Olkiluoto data. Xdi operates in Linux machines and it has been the main tool so far. In the future, the development will occur in Java based Jdi program which does not need a certain system to operate. At the moment Xdi has certain benefits compared to Jdi and vice versa. Further developments of these packages are focused on Jdi (see details in http://www.issi.co.za).

Xdi is the primary ISS visualisation and interpretation software package. The seismic data that is visualised and interpreted consists of the event parameters extracted from seismograms or XMTS (seismological processing software). Seismic events are displayed on a three-dimensional map with symbol sizes and colours adding information. Xdi has the option to connect to the RTS (runtime system), where events are displayed live as they are collected by the system.

The data for the three-dimensional map is imported to the ISS in ASCII format. The map can be produced manually or it can be imported from other digital maps. The way the map is generated depends on what kind of data is already available. For example, the Olkiluoto map is produced manually, converted from the existing digital maps made by the program Mapinfo and imported from the AutoCAD rock model. ISS data can also be imported to mine CAD systems (such as Surpac, Map3D, etc.).

The microearthqueke network at Olkiluoto monitors seismic response of the rockmass and engineering structures to natural and/or induced forces. A common approach in seismology has been to relate an earthquake to a certain point (hypocenter) or to a fault plane. However, an earthquake is not only an event, which takes place in a point or plane, it has an influence to the rock mass surrounding the hypocenter and the fault plane. These different approaches (see details in Saari 1999) are utilised when the observations at Olkiluoto are visualised and interpreted.

Various parameters (like radiated seismic energy, seismic moment, magnitude, event time etc.) can be presented by different symbol types, sizes and colours. Seismic events can be filtered by time period, ranges of seismic parameter values and spatially by filtering on polygons, mine boundaries, sites etc. Settings are saved to file and can be accessed by selecting a project. There are 50 different options which can be combined when the filter is created. For example, by filtering the ratio of radiated P- and S-wave energy (Es/Ep) one can display and investigate events which represent either typical tensile cracking or shear failure (see Ming et al. 1998).

22

Figure 4-4 shows some elements of the three dimensional model of Olkiluoto. Shorelines are shown by green lines, seismic stations by red triangles, boreholes by yellow bars and roads by brown lines. Faults are excluded in this figure. Alands archipelago and about 100 km of the shore line North and South of Olkiluoto are outlined more generally in the model. Basically, the model helps the interpreter to associate the hypocenters geographically.

Filtering applied in Fig. 4-4 is simple: all accepted events that have occurred 1.3.2002-31.3.2003 at the depth of less than 10 km are plotted. Time period including the events is presented by colours. The selected event parameter is magnitude, which is presented by 3D spheres with different sizes. Event summary at the lower right corner is produced automatically, but the more detailed event info is one of the interactive options.

Users can display or print event information by selecting Event Info from the pull down menu. Users can select the content of the columns from 78 fields. In this case (Fig. 4-4) the columns are: Event numbers, File name, Onset time, co-ordinate (X,Y,Z), Magnitude, Logarithms of Energy (Log(E)) and Seismic Moment (Log(M)), Apparent Volume, Apparent stress, Energy Index and ratio of S- and P-wave energies (Es/Ep). The selected parameters from Mag. to Es/Ep are the same that can be displayed by symbols or colours in the three dimensional map. Time is an additional option for colour display. The equation for these parameters are presented in (Mendecki 1997 and Mendecki et al. 1999).

Different parameters give different kinds of information and view to the seismic response of the rock mass, as can be seen, for example, when sizes of the symbols in Fig. 4-4 (magnitude) and Fig. 4-5 (apparent volume) are compared. Figure 4-5 also illustrates how a single event can be reprocessed: When the event number 4 is clicked by the cursor, it opens a box that displays detailed information about that event. After that one can accept the interpretation (OK), look what seismic stations have recorded the event (show sites) or reprocess (call xmts) the data. The latest option actually opens the view shown in Fig. 4-1.

Identification of active fracture zones is an essential element in a comprehensive study of potential hazards related to the spent nuclear fuel. The zones of weakness adjust releasing stresses and strains of the rock mass as well as they are the main paths of hydraulic flow in the bedrock. The movements occurring on these zones accumulate during the lifespan of the repository and possibly cause changes in the stability, stress field and groundwater conditions of the rock mass. When the fracture zone model is presented together with the observed seismic events, active or unstable zones can be identified. Another option is that, instead of the whole rock model, a certain interesting structure can be studied in relation to the nearby seismicity (see Fig. 4-6).

Seismic data can be utilised to describe the fault geometry, block movements and stress field related to an earthquake. When the number of observations related to the same structure increases different statistical methods can be applied in order to improve the quality of the interpretation. In addition, group location of events significantly improves the location accuracy and can bring out the geometry of the active structure.

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25

Figure 4-6. View from the direction SWW-NEE. Presentation of Fig. 4-6 is rotated and zoomed to the repository site. Hydrologica/ly significant fracture zone R21 (see Anttila et al., 1999) is added to the interpretation and shown as a gray surface below the events 1-3. Event 4 is far below the strucuture.

When the number of recorded events is large enough it is possible to characterise the coseismic and seismic processes within the rock volume in time and space. For contouring and time histograms the investigated area is delimited by a polygon. Contouring is possible for various parameters like Logarithm of Radiated Energy, Logarithm of Seismic Moment, Apparent Stress, Apparent Volume, Energy Index, Logarithm of Energy Index, Viscosity, Seismic Stiffness Modulus, etc.. In this way regions of high seismic stress, etc. can easily be identified.

Time history plots display the time behavior of seismicity in given polygons and have proved very useful in trend detection and indicating areas of potential instability. A nearly same group of variables as in the spatial analysis can be plotted. Different additional features, like averaging procedures, indicator that shows time of large events and error bars can be included in the time histograms.

The magnitude-frequency distribution of earthquakes indicate the mean recurrence times of seismic events above certain magnitudes, seismic network sensitivities, etc. and plot the results in concise graphical formats. Empirical probabilities that an event of a certain magnitude will occur within a certain time period based on collected seismic data can be calculated.

With the current set of observed events it is not possible or reasonable to give examples of contouring or time histograms. At Olkiluoto, those applications are likely applied

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26

later in the analysis of induced seism1c1ty or when the tectonic earthquakes are concerned, after longer period of observations with a larger network. So far, the best examples are found in the references (http://www.issi.co.za, Mendecki 1997 and Mendecki et al. 1999).

The above mentioned description and examples of visualisation and interpretation are related to the program Xdi. Quite many of those features are included already to the current version of Jdi. The same structures and events of Olkiluoto are imported into Jdi and the example figures could be presented with that program as well. Figure 4-7 gives an example of the slightly different visual layout and structure of the operation menu available in Jdi.

Jdi has some advantages which are not included in Xdi. The special features of Jdi are not yet utilised, since the data have not yet required it. In the following the benefits of J di are summarised shortly.

• In the case seismicity is related to 3D structures (faults and dykes), plan view alone is inadequate. Interpretation in 2D blur or lose valuable information, also events and plans at different depths obscure each other. Jdi provides full 3D interaction, which remedies this situation. The body is created from a surface structure, which can have an arbitrary shape. The thickness of that surface determines the 3D structure and the volume included in the analysis.

• Integrating of modelled and seismic data is advanced. The first step in integration is to view contours of your modelled data with your seismicity in order to gauge correlation. J di facilitates this and more advanced integration techniques, such as Map3Di and Differential Maps.

• The observed events can be filtered by event parameters, planes and polygons. It is also possible to view multiple filters at the same time and export filtered events into event catalogue available to other packages. This combined with flexible contouring facilities and time history analysis provides a powerful platform for advanced analysis.

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5 REPORTS AND DATA ARCHIVES

5.1 Reporting for construction

During the excavation, reports of induced seismicity can be utilised to support the design and construction of the ONKALO. These reports can be produced automatically and interactively. The reports are based on the results generated by the interpretation and visualisation packages available in the Vantaa office computer.

Reporting functions of Xdi enable the user to pre-define routine report formats and automatically generate HTML reports. The text and graphical reports that are generated can be viewed and printed from Linux, Microsoft Windows or IRIX computers using an HTML browser (e.g. Netscape or Internet Explorer). Reports can be displayed on the intranet, allowing the user who has access to view any report at any time on his desktop PC. When a report is created, a corresponding pdf file is also created. This file can bee­mailed and contains the graphical information pertaining to the report. Each HTML report also has a link to the pdf report.

Automatic reports can be emailed to selected group of project members (project manager, etc .. ) daily, twice a day or as often as required. If the reports are send once or twice in a day, the current set-up of monitoring is as good as the real time association mode. However, during the excavation activity in the area (traffic, blasts, etc.) increases so much that change to real time operation mode is reasonable. In that case, the PC in Olkiluoto has an option to send only events that trigger a certain pre-set number of the sensors to V antaa, where the automatic report is generated. That would decrease the number of emails from Olkiluoto to Vantaa and allows to lower the detection threshold ofthe sensors.

The content of the report is controlled by different filters. For example, only events occurred within the last 24 hours, with location accuracy less than 1 0 meters and that are re~orded at least with five sensors could be accepted. Also filters related to magnitude, energy, stress drop, etc. can be defined. If the quarrying blast has always a predefined time it can filtered out.

The report based on the automatic analysis could be grouped according to different depth intervals (e.g. first 50 meter, 50 - 100 m, etc ), which can be easily associated with a certain section of the ONKALO. The report can include a table descriptive parameters (location, onset time, magnitude, ... ) and a map of recent observations for each of the depth interval. By means of a geometry filter events occurred within any predefined boundaries can be selected.

The purpose is that the result of the automatic analysis is as complete as possible. However, the results must always be checked interactively: Some disregarded events may belong to the group of real seismic events and vice versa. Also some results of automatic analysis (location, magnitude etc .. ) may need improvement. The results of the interactive analysis could be reported with longer time intervals, say weekly, twice in a week, monthly or what ever is seen possible and necessary. The interactive report could be published in Microsoft Word or Power Point format and emailed to a group of predetermined people. An interactive report is released more delayed but it is more reliable than a report produced automatically.

29

In practice, the amount of induced seismicity and needs of the project group define how often automatic and interactive reports are generated. For example, the seismic network in the Pyhasalmi mine records few or tens of seismic events daily. They stopped daily automatic reporting after realising that it is not essential for everyday operation in the mine. Nowadays they rely on weekly interactive reports.

It is expected that excavations close to the ground surface do not induce many seismic events. Before having any experience about the excavation induced seismic activity in Olkiluoto, it could be reasonable to start with monthly interactive reports. Later, the release interval of the reports can be changed, if it is seen necessary. Instead of the automatic reporting, the interactive reporting could be supported by an alarm system.

Generally, the only time immediate reporting is useful is in the case where the location of a single event is important - for example for rescue efforts after a large event. For that purpose the alarm system could be more useful than a daily automatic report. The alarm function allows a notification to be sent to specific people by several different media (email, mobile text message, etc.). Each person can have a personal id. The idea is that with that id it is possible to select those alarms she or he wishes to receive and the medium of distribution. For example, at the moment, if the seismic software in Olkiluoto is failed, a notification is emailed to the Vantaa office. That kind of alarm is not important to many other people.

Automatic alarm can be based on the same filters as automatic reporting. Of course, when alarm is concerned, the criteria must be strict. The difficulty will be to eliminate alarms caused by blasts, if they are not exploded at certain time of day. Real time alarm requires a change to the real time operation mode of the seismic network.

5.2 Reporting for rockmass modelling

Results of the interactive analyses are planned to be summarised annually. The main purpose of the annual report is to support modelling of the rockmass surrounding the ONKALO. If possible, interpretation of the observed induced seismicity related to certain areas or weakness zones of the rock mass is presented. Fault plane solutions and interpretation of movements can be included in the report. Annual reports can be utilised as a source material in further going analysis seismic interpretation as well as in other geophysical studies.

The annual report will include also descriptions of technical events, like changes in the configuration of the seismic network, technical failures occurred during the previous year, etc .. The report will be published in series of Posiva's Working reports.

Due to better statistical data and methods available, a summary report of few years seismic observations would be more reliable and comprehensive than a view based on individual annual reports. The amount of seismic information will increase during the course of the ONKALO's construction. This enables to utilise various methods, which are not so effectively used in annual reports: Group location can be used to improve location accuracy, statistical analysis methods can be applied, as well as time histograms and contours of various variables can be presented.

30

5.3 Archiving of data

The development of seismic interpretation methods during the lifespan of the final disposal of spent nuclear fuel will quite likely be remarkable. Therefore, a well­documented and comprehensive database for the purposes of the future scientists and interpretation methods is an essential part of the project.

When the seismic events have been received to the central site computer they are immediately emailed to the office computer. All detected events are stored for three months in both of the computers. After that only the events accepted by the analyst remain in the computers directories. The accepted events are classified either as seismic events or blasts. The accepted events and the results of the analysis are also stored in the backup tapes of the DAT recorder, in Vantaa. The same backup tapes include also the a copy of the software applied at that date. Backup is done every 1-3 month.

Real blasts can be utilised as master events when the velocity model is modified. Also other events with some special interest are accepted in the group of blasts. These are mainly recordings with unusual but interesting wave form. The purpose is to store them longer than three months. These events have often an unknown origin, which might be solved later. For example, numerous signals, which appeared to be generated by the switchyard have been classified as blasts. Indications of possible malfunctions are named "blasts", as well. Likewise, a calibration signal of each sensor is stored every month, because a recent verification of the quality of recordings is valuable for the interpretation.

In addition to the above mentioned archiving, the recordings of induced seismicity will be annually saved to Posiva's investigation data archive in an appropriate format.

ACKNOWLEDGEMENT

Special thanks go to Errol de Chock, Manager System Engineering, Technology Division (ISST), ISS International Ltd for overseeing the technical description of the network.

31

REFERENCES

Ahjos, T. & Uski, M. 1992. Earthquakes in northern Europe 1n 1375-1989. Tectonophysics. Vol. 207, pp. 1-23.

Anttila, P., Ahokas, H., Front, K., Hinkkanen, H., Johansson, E., Paulamaki, S., Riekkola, R., Saari, J., Saksa, P., Snellman, M., Wikstrom, L. & Ohberg A. 1999. Final disposal of spent nuclear fuel in Finnish bedrock - Olkiluoto site report. Posiva Oy. 206 p. POSIVA-99-10.

Cosma, C., Heikkinen, P., Honkanen S. and Keskinen, J. 1996. VSP-survey at Olkiluoto in Eurajoki, borehole Ol-KR8 and extended parts of boreholes OL-KR2 and OL-KR4. Posiva Oy. Work report PATU-96-11e.

Front, K., Okko, 0. and Hassinen, P. 2001. Interpretation of geophysical logging of borehole OL-KR12, the Olkiluoto site at Eurajoki. Posiva Oy. Working report 2001-03.

La Pointe, P. & Hermanson, J. 2002. Estimation of rock movements due to future earthquakes at four candidate sites for a spent fuel repository in Finland. Posiva Oy 89 p. POSIV A 2002-02.

Mendecki, A.J. 1997. Seismic Monitoring in Mines. Chapman & Hall.

Mendecki, A.J., van Aswegen, G. & Mountfort, P. I. 1999. A guide to routine seismic monitoring in mines. To be published in Handbook on Rock Engineering Practice for Tabular Hard Rock Mines. Jager, A.J. and Ryder J.A. (ed). Creda Press, Cape Town.

Miller, B., Arthur, J., Bruno, J., Hooker, P., Richardson, P., Robinson, C., Arcos, D. and West J. 2002. Establishing baseline conditions and monitoring during construction of the Olkiluoto URCF access ramp. Posiva Oy, 109 p. POSIVA-2002-07.

Ming, C., Kaiser, P.K., & Martin, C.D. 1998. A tensile model for the Interpretation of Microseismic events near Underground Openings. Pure appl. geophys. 153, pp. 67-92.

Posiva 2003a. Baseline Conditions at Olkiluoto. Posiva Oy. POSIV A 2003-02, Posiva Oy, Helsinki, Finland.

Posiva 2003b. Programme of Monitoring at Olkiluoto During Construction and Operation of the ONKALO. POSIVA 2003-05, Posiva Oy, Helsinki, Finland.

Ollikainen, M. & Kakkuri, J. 2001. GPS operations at Olkiluoto, Kivetty and Romuvaara for 2000. Posiva Oy, Working Report 2001-16.

Saari, J. 1996. Seismic emissions induced by the excavations of the rock repository in Loviisa. Posiva Oy, 19 p. POSIVA-96-05.

Saari, J. 1997. Seismicity in the Olkiluoto area (in Finnish with an English abstract). Posiva Oy, 37 p. Working Report 97-61.

Saari, J. 199 8. Regional and local seismotectonic characteristics of the area surrounding the Loviisa nuclear power plant in SE Finland. Institute of Seismology, University of Helsinki. 248 p. Report S-36.

Saari, J. 1999. An overview of possible applications of microearthquake monitoring at the repository site of spent nuclear fuel in Finland. Posiva Oy, 36 p. Working Report 99-64.

I

32

APPENDIX A- SENSOR SPECIFICATIONS

- SM-11 Geophone (Natural frequency= 30Hz)

- SM-11 Geophone response curve (Electronically enhanced to natural frequency = 5. 5 Hz)

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37

APPENDIX 8- STAND ALONE QS SPECIFICATIONS

38

StandAlo.ne QS

ISS International Limited South Africa

email: info@issi.co.za internet: www.issi.co.za

1 Description

·The QS is the fourth generation1 of digital seismometer forming part of the Integrated Seismic System (ISS).

The StandAlone QS is designed to be used in applications where the con­nection to the Central Computer is not permanent, thus requiring that the QS is able to determine its. timing. (via a GPS2) and parameters (from flash) without reference to the Central Computer. Data is normally stored on the internal HDD3.

In applications where a connection the the Central Computer is available but some form of data logging to HDD is required, the StandAlone QS can be used with a permanent connection to the Central ·Computer.

The StandAlone QS is available with different types ofcligitizer, including:

Type Sensor type

G Geophone, Short period, Force Balance Accelerometer H Broadband A . Piezoelectric Accelerometer

1Intelligent Seismometer (IS), Processing Seismometer (PS), Multi Seismometer (M;S) · 2Global Positioning System 3Hard Disk Drive

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39

The main features of the StandAlone QS are as follows:

• Small, light and rugged for rapid field deployment

• Software selectable sampling rates from 50Hz to 24000 Hz

• Suitable for geophone, FBA, broadband sensors or piezoelectric ac­celerometers

• 3, 6 channels ( 12 channels optional) with 24 bit .digitizing

• Continuous and/ or triggered: recording

• Multiple logical channels

· • Logging to removable HDD and/or telemetry to central site

• LCD for status information

• Calibration functions built-in (except piezoelectric accelerometers)

• Power management functions built-in

• RS232 ports for setup and telemetry

• Built-in GPS

• Seismological processing via ISS packages ( xmts) or SEISAN

The StandAlone QS features multiple logical channels per physical channel. The logical channels can be used ·to record

• standard triggered or continuous waveform

• average parameters of the ground motion (average amplitude, max­imum amplitude and assoCiated frequency) recorded continuously in each predetermined time interval

• basic waveform parameters for each waveform triggered at a second (lower than standard) trigger level

40

2 Technical Specification

CHARACTERISTICS Feature Specification

Timing Internal GPS module Recording Modes triggered and/ or continuous

multiple logical channels Trigger mode STA/LTA· · Calibration Built-in function Data Storage Internal HDD (> 10GB) Logging modes Linear buffer (stop when full}

Circular buffer (wrap when full} Status Display on LCD screen Power management CPU, GPS and HDD controlled

A/D CHARACTERISTICS Specification Types G .and A TypeH

Seismic Channels 6 3 Sampling Rate software selectable software selectable

50, 100, 125, 200, 250, 50, 100, 200, 400, 800; 400, 500, 1000, 2000, 1600 3000, 6000, 12000, 16000, 24000

Bandwidth up to 10 000 Hz up to 600Hz Dynamic Range (dB) at 50 sps > 110 127 Signal to Distortion (dB). > 100 120

EXTERNAL INTERFACES serial port (RS232) for dial-in modem serial port (RS232) for setup and debug. (via lap top) Firewire (IEEE 1394) for additional HDD GPS antenna DC Power (12 V) Sensor (geophone, Force Balance or Piezoelectric Accelerometer, Broadband)

41

POWER CONSUMPTION Power Used

A/D Type TypeG TypeH Type A Number of channels running 6 3 6 Minimum 3.3W· 2.4 w 3.7W Normal monitoring <4W <3W <4W (average typical) Maximum < 12W < 12W < 12W (with GPS running and HDD spin-up)

POWER SUPPLY 12 V DC (externally supplied)

PHYSICAL Mass 3 kg Length 270mm Width 250mm Height 125 mm Environment IP67 Temperature Operating {0 0) 5 to 55 Storage {0 0) -40 to 65

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