rock mass seismic imaging around the onkalo tunnel
TRANSCRIPT
Olk i luo to
Te l +358-2-8372 31
Mi rcea Cozma
November 2008
Rock Mass Seismic Imaging Around the ONKALO Tunnel, Olkiluoto 2007
November 2008
or pending completion.
coincide with those of Posiva.
Ca l in Cosma
V ib romet r i c Oy
Work ing Report 2008 -64
Rock Mass Seismic Imaging Around the ONKALO Tunnel, Olkiluoto 2007
Rock mass seismic imaging around the ONKALO tunnel, Olkiluoto 2007
ABSTRACT
Posiva Oy prepares for disposal of spent nuclear fuel in bedrock focusing in Olkiluoto, Eurajoki. This is in accordance of the application filed in 1999, the Decision-in- Principle of the State Council in 2000, and ratification by the Parliament in 2001.
Vibrometric Oy has performed a tunnel seismic survey in ONKALO access tunnel on a 100 m line in December 2007. Tunnel length (chainage) was 1720 - 1820 m (vertical depth 170 – 180 m).
Measurement applied 120 source positions at 1 m spacing, and on the both ends at 4 m spacing. Electromechanical Vibsist-20 tool was used as the source. Hammer produced 15.36 s sweeps. Signal was recorded with 2-component geophone assemblies, installed in 400 mm long, 45 mm drillholes in the tunnel wall. Sweeps were recorded with Summit II seismograph and decoded to seismic traces.
Also percussion drill rig, which is used in drilling the blasting holes in tunnel excavation, was tested from a 100-m distance as a seismic source. Signal is equally good as from actual seismic source, and may be applied later on for production.
Obtained seismic results were processed with tomographic reconstruction of the first arrivals to P and S wave refraction tomograms, and to tomograms of Young’s modulus and Shear Modulus. The obtained values correspond the typical levels known from Olkiluoto. There are indications of lower velocity near tunnel wall, but resolution is not adequate for further interpretation. Some variation of velocity is detected in the rock mass.
Seismic data was also processed with normal reflection profile interpretation and migrated. As a result there was obtained reflection images to a 100-m distance from the tunnel. Several reflecting events were observed in the rock mass. Features making an angle of 30° or more with tunnel axis can be imaged from distances of tens of metres. Vertical fractures perpendicular to tunnel can be imaged only near the tunnel. Gently dipping features can be imaged below and above. Images are 2D, i.e. have cylindrical symmetry, meaning the actual location of these reflecting events cannot be obtained without external information. Match was found to previous 3D VSP survey results in 2005.
The test indicated that tunnel seismic survey can be used in rock mass characterization. Operations in tunnel conditions and obtaining high quality results would require some attention in design of work flow.
Kalliotilavuuden seisminen tutkimus ONKALOssa vuonna 2007
TIIVISTELMÄ
Vibrometric Oy testasi tunneliseismisiä mittauksia ONKALOn ajotunnelissa 100 m profiililla joulukuussa 2007. Tunnelin paaluluku oli 1720 - 1820 m (vertikaalisyvyys 170 – 180 m).
Mittauksessa käytettiin 120 lähdepistettä 1 m välein, ja linjan jatkeilla 4 m välein. Sähkömekaanista Vibsist-20 lähdettä käytettiin 15,36 s mittaisiin iskusarjoihin. Signaali rekisteröitiin 2-komponenttisilla geofoniasemilla, jotka oli asennettu 400 mm syviin, halkaisijaltaan 45 mm kairanreikiin tunnelin seinässä. Tulokset tallennettiin Summit II seismografilla ja muunnettiin seismisiksi rekisteröintijäljiksi.
Iskuporakonetta (porausjumbo) jota käytetään louhinnan panosreikien poraukseen, testattiin 100 m etäisyydeltä seismisenä lähteenä. Tuloksen laatu on vastaava kuin varsinaisella seismisellä lähteellä saatu. Tekniikkaa voitaneen käyttää myöhemmin tuotantokäytössä.
Saadut seismiset tulokset prosessoitiin tomografisen inversion avulla P ja S aaltojen refraktiotomogrammeiksi. Myös Youngin modulista ja leikkausmodulista laskettiin tomogrammit. Tulosten taso vastaa kallioperän arvoja Olkiluodossa. Tunnelin seinän lähellä havaitaan viitteitä matalan nopeuden vyöhykkeestä. Tutkimuksen erotuskyky ei riitä tämän piirteen tarkempaan tulkintaan. Kalliossa esiintyy jonkin verran nopeuden vaihtelua.
Tulokset prosessointiin myös tavanomaisen heijastustulkinnan avulla ja migratoitiin. Tuloksena on saatu heijastuskartat noin 100 m etäisyydelle tunnelista. Useita heijastuspiirteitä voidaan havaita kalliotilavuudesta. Heijastajat joiden kulma on suurempi kuin 30° tunnelin akselista, havaitaan kymmenien metrien etäisyyksiltä. Tunnelin suuntaiset vertikaaliheijastajat nähdään vain läheltä tunnelia. Loivakaateiset heijastajat voidaan havaita mittaustason ala- ja yläpuolelta. Tulokset ovat kaksiulotteisia, eli sylinterisymmetrisiä, joten näiden heijastajien tarkkaa sijaintia ei tunneta ilman lisätietoja. Havaittiin että vuoden 2005 VSP-tuloksissa on yhtäläisyyksiä tämän tutkimuksen tuloksiin.
Testin tuloksena todettiin että tunneliseismistä tekniikkaa voidaan käyttää kallion tutkimuksessa. Työn suoritus tunnelityömaalla sekä korkealaatuisen tuloksen tuottaminen vaatii jonkin verran suunnittelua.
1
2. FIELD WORK ........................................................................................................ 3
2.1 General ....................................................................................................... 3 2.2 Scope of work ............................................................................................. 4 2.3 Equipment ................................................................................................... 6
2.3.1 Receivers and Recorder ................................................................... 6 2.3.2 The VIBSIST-20 seismic source ....................................................... 8 2.3.3 The detectors and acquisition system ............................................... 9
3. DATA PROCESSING .......................................................................................... 10
3.1 General ..................................................................................................... 10 3.2 Standard 2D reflection profiling ................................................................. 15 3.3 Tomographic reconstruction ...................................................................... 16 3.4 Transmission and reflection imaging along the tunnel ............................... 21 3.5 Reflection imaging and interpretation around the tunnel ............................ 22
4. CONCLUSIONS .................................................................................................. 28
1. INTRODUCTION
Posiva Oy prepares for disposal of spent nuclear fuel in bedrock focusing in Olkiluoto, Eurajoki. This is in accordance of the application filed in 1999, the Decision-in- Principle of the State Council in 2000, and ratification by the Parliament in 2001.
Underground characterization will be carried out in ONKALO premises. The ONKALO access tunnel has been excavated since 2004. Currently excavation has progressed to vertical depth of 300 m, and tunnel length 3100 m.
As part of characterization programme and methodological development, seismic surveys were carried out in the tunnel. Placement of the survey was on the right hand wall, at chainage 1720 – 1820 m. The main objectives were the following:
- Image long vertical fault nearly perpendicular to tunnel
- Image long gently dipping fractures at the side and/or below
- Draw refraction velocity profile along the wall.
Initial idea was to perform refraction and reflection seismic sounding in ONKALO tunnel. A design component was included to the work. In order to obtain high quality of results and to avoid tunnel wave, the survey technique was optimized. Selection was done between installations on the wall or opened floor.
Work was initiated by Jussi Mattila, Posiva Oy. Test location in tunnel was selected near to known subvertical brittle fault location and above an extensive gently dipping brittle fault, on a straight part of tunnel.
Contact persons on Posiva side were Mari Lahti, Antti Mustonen (planning of project) and Janne Laihonen, who also organised the access to the site and preparations of geophone and shot stations, drilled by SK-Kaivin Oy. Staking of the stations was arranged by Prismarit Oy.
Design and field work were carried out by Vibrometric Oy. The field work was planned and the project managed by Calin Cosma and acquisition was carried out by Mircea Cozma. They took care of processing together with Lucian Balu. Client contact person on Vibrometric Oy side was Veli-Pekka Kantanen. Nicoleta Enescu carried out interpretation and wrote the report.
Chapter 2 below describes the field work setting and the equipment. Chapter 3 describes the processing and results.
3
2.1 General
The field work was carried out in December 2007 (Table 1). Field work extended
during a two-week time on weekdays.
Table 1. Field work time schedule.
No. Day No. of shots Remarks
1 10.12.07 - Training
3 12.12.07 - Setup & calibration
5 14.12.07 16 Tunnel closed at 18:00
6 18.12.07 - Back to ONKALO; Setting the line
7 19.12.07 50 2D production
8 20.12.07 95 2D production
9 21.12.07 26 Morning: Tamrock records; Afternoon: 2D production;
Evening: Demobilization
Preparations required drilling of 400 mm long and 45 mm in diameter drillholes for
geophones on c. 1,5 m level above the tunnel floor. Shot points were marked on the wall
between the geophone stations.
The acquisition was run during the excavation work, which caused frequent
interruptions to the production. The setup required cable connections for triggering
which had to be pulled over the tunnel. The acquisition system was moved away from
tunnel each time interruption occurred, and then reinstalled. Disconnecting electrical
mains delayed the work.
The field personnel and their responsibilities are presented in Table 2.
Table 2. Field personnel and their responsibilities.
Person Responsibility Authority
processing, Reporting
Project manager
Pierre Lyons Data Acquisition
Alexadru Prisaciuc Operation
2.2 Scope of work
Purpose was to produce seismic velocity and reflection information using a tunnel based survey layout. Target was to develop field acquisition and processing technique, and to demonstrate the method capability in characterization of faults or long fractures in different positions.
Reflection imaging ahead and aside the tunnel was attempted over 100 stations (sources and receivers) at 1 m intervals along the tunnel wall. For each source, 50 receiver positions at 2 m spacing were recorded simultaneously. Firstly the odd receiver positions were instrumented and shots were recorded every meter, then the receivers were moved to the even positions, the shot sequence being thereafter repeated.
For refraction analysis two groups of 10 shots at 4 m spacing were recorded termed A to J & U to K. The sources along the reflection array were used as well. The 100 m long even receiver position array was used to record the refraction shots. The layout is presented in Figure 1. As seen from Figure 2, the refraction shots could not be set further from the ends of the receiver array due to the bending of the tunnel.
Figure 1. Location of the seismic survey in the ONKALO Tunnel.
5
Figure 2. Survey layout: a 100 sources (blue squares) and receiver positions (magenta squares) used for reflection analysis. Refraction shots are shown as blue and yellow triangles.
The same 100 m long receiver array was used to record the drilling signature of a
percussion rig placed 800 – 1000 m further down the tunnel (at a 100 m distance from
receivers).
6
2.3.1 Receivers and Recorder
The recording system was a DMT Summit II distributed digital seismograph with 100
channels. A 100 approximately 40 cm deep drillholes of 45 mm diameter were drilled
prior to the survey on the right-hand wall at 1.5 m level above the tunnel floor.
Each drillhole was instrumented with two geophones of type Oyo Geospace SMC1850-
30 Hz or I/O-Sensor SM45. Geophones were assembled in 50 molds of Ø 44 mm x L
100 mm. Molds were made of 2-component Epoxy/ Hot Melt (polyethylene).
Geophones were placed orthogonally with the X component horizontal along the tunnel
axis, and the Y component vertical, as shown in Figure 3.
Figure 3. On the left, receiver drillholes at 1.5 m from the floor, where two components receivers have been mounted in the wall. Top on the right the geophone orientation, view from above, tunnel axis across the page. Bottom on the right: two geophones assembly mould.
7
Figure 4. Summit II receiver units along the ONKALO tunnel.
Figure 5. Vibsist-20 used for the ONKALO measurements.
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2.3.2 The VIBSIST-20 seismic source
The seismic source used for the ONKALO measurements was the Vibsist-20 (see
Figure 5 on previous page, and Table 3) with the following characteristics:
Energy: 20-25 J/impact
Trigger by geophone (Trigger geophones Geo-Space GS14-L3) placed on the
impact head. The trigger signal has been conveyed by cable to the recording
station.
The VIBSIST-20 is based on the Swept Impact Seismic Technique (SIST), for which
the seismic signals are produced as a series of pulses, according to a specific pre-
programmed sequence. The use of the monotonous variation of the impact rate controls
effectively the non-repeatability of the impact intervals and achieves a wide bandwidth
even when the coupling to the rock or ground is relatively poor.
Table 3. Technical specifications of Vibsist-20 source.
PARAMETER VALUE
Maximum current consumption: 14 A
Impact repetition rate: programmable between 10 to 30 impacts per second
Impact energy: approximately 20 J / impact for a typical 1.5 kW
impact hammer
Programmed sweep characteristics: computer controlled (any shape) or preset (linear)
sweep.
Impact rate ratio: minimum 1.5 / 1.
Sweep time adjustment: 2.5 to 30 seconds (this is restricted by the number of
samples available per channel).
2 kg
2.3.3 The detectors and acquisition system
A Summit II Plus 24-bit seismic data recording system with 100 channels was used for data acquisition (Figure 4). Table 4 displays the technical parameters of the Summit II system.
Table 4. Technical specifications of the Summit II Plus system.
Main registration parameters in ONKALO survey Value Sample rate (ms) 0.125
Trace length (ms) 15360
3.1 General
The reflection data were processed as a standard 2D seismic line. First arrivals were
defined for each refraction source and tomographic reconstruction was performed.
Seismic images from two components were processed with 3-D migration. Signal
quality was generally high (Figure 6).
Figure 6. Shot gather from the beginning, middle and the end of the source array.
11
The seismic wave direct arrivals and reflections in a different way from the upper end
(Figure 7) and lower end (Figure 8) of the array. The tunnel is bending backwards right
off the ends of the array, making it impossible to use longer offset sources in refraction.
Figure 7. Refraction sources 1, 5 and 10 of profile U - K recorded down-tunnel.
Horizontal component (X) on top, vertical component (Y) on bottom.
12
Figure 8. Refraction sources 1, 5 and 10 of profile A – J recorded up-tunnel. Horizontal
component (X) on top, vertical component (Y) on bottom.
13
Figure 9. Measured data from the Tamrock drill bit. Sources 1, 4 & 8 of the percussion
rig location (Figure 10). Horizontal component (X) on the top, vertical component (Y)
on the bottom.
14
Testing of Tamrock rig drill bit signal of percussion drill rig placed at chainage 2555 –
2558 m indicate signal quality well comparable to the one obtained with Vibsist-20
(Figure 9). Measurement arrangement for the drill bit recording included triggering of
pilot signal with two geophones, connected via cable link to the recording station 700 m
up in a tunnel. Eight percussion drillholes were prepared for the test (Figure 10). The
results can be used for localization of the drill bit, for seismic transmission tomography,
and for seismic reflection profiling.
Figure 10. Source array and pilot signal arrangement in Tamrock percussion rig signal
test. Acquisition station at chainage 1835 m, tests sources at 2555 – 2558 m.
The raw stacked data and processed data were documented and stored on DVDs. Copies
are to be forwarded to POSIVA OY for safekeeping. Results included the migrated
reflection images projected on coordinates, to be displayed in 3D.
The interpreted results will be checked by Posiva Oy for consistence against existing
realizations of the site model. The images were delivered to Posiva Oy with their
projected coordinates to accommodate in 3D modeling system.
15
3.2 Standard 2D reflection profiling
The 100 receiver and 100 sources recordings were sorted onto seismic reflection profile.
Processing steps included sorting of traces, band-pass filtering, spectral equalization,
static correction, dip-move out, and stacking of the signal.
Figure 11. The source 1 measurement down-tunnel, band pass filtered onto band 300 –
600 Hz (top), 300 – 900 Hz (middle) and 300 – 1500 Hz (bottom).
Transmitted and reflected signal contains high frequency band, up to 1500 Hz. Highest
frequency band 300 – 1500 Hz was applied. From the processed traces the direct P and
S waves can be seen clearly. Also reflections are detected already on the raw profiles.
16
3.3 Tomographic reconstruction
The obtained P and S arrival picks were combined with coordinate file. Results were
inverted through tomographic reconstruction to obtain depth sections off the tunnel
wall. Both P and S wave tomograms were computed. The results were also computed to
tomogram of Poisson’s ratio ν, Shear modulus μ and Young’s modulus E using (1), (2)
.
)1(2 vE (3)
The refraction images were produced to 8 – 10 m maximum depth from the tunnel wall.
There is a limitation caused by the short offset and high velocity.
The tomograms (Figures 12 and 13) indicate that the velocity is slightly lower in a thin,
less than 1 m layer near the tunnel wall. The resolution is in the order of 0.5 m, so the
actual thickness of the low velocity zone cannot be defined. There is some velocity
variation in the rock mass, which may originate from lithological variation and from
deformation zones.
The computed dynamic rock mechanical sections suggest there may be some local
variation of elastic properties in the rock mass, especially near the tunnel wall.
The velocity and elastic moduli value ranges match quite well to the previously known
in situ information from pilot drillhole geophysical logging (Öhberg et al. 2007a, b), as
shown in Figures 14 and 15. The P velocity ranges at 5000 – 5800 m/s and S velocity at
3000 – 3400 m/s in logging. In refraction profile the rock mass velocities away from
tunnel are on the same order, but the velocities in dry and possibly deformed rock mass
near tunnel wall are clearly lower. Shear modulus ranges at 25 – 33 GPa and Young’s
modulus at 65 – 80 GPa in logging.
The pilot drillholes ONK-PH06 and ONK-PH07 were located in ONKALO 170…310
m above and 60…160 m below the seismic section. Lithological variation of the values
is distinct.
17
Figure 12. P wave (left) and S wave (right) velocity tomographic sections.
18
Figure 13. Young’s modulus (left) and Shear modulus (right) tomographic section.
19
Figure 14. Medians (1 m) of drillhole acoustic logging velocity (top) and dynamic
elastic modulus (bottom) in all pilot drillholes. DGN = diatexitic gneiss,
KFP=potassium feldspar porphyrite, MFGN=mafic gneiss, PGR= granite pegmatite,
QGN=quartz gneiss and VGN=veined gneiss.
20
Figure 15. Medians (1 m) of drillhole acoustic logging velocity (top) and dynamic
elastic modulus (bottom) in pilot drillholes near tunnel seismic test (closer than 300 m
along tunnel or 30 m vertically). DGN = diatexitic gneiss, KFP=potassium feldspar
porphyrite, MFGN=mafic gneiss, PGR= granite pegmatite, QGN=quartz gneiss and
VGN=veined gneiss.
The seismic tomographic sections and migrated reflection images were layered together
(Figure 16 and Figure 17). Migrated reflection images can be presented with good
confidence to a limited distance from the tunnel, as the profile is comparably short, a
100 m.
Tomograms are limited in extent but they can suggest some velocity variation. The
reflection images of different components indicate in the near field some clear
reflections. Some of these may be correlated to geological features met in tunnel
mapping. The parallel to tunnel oriented reflecting events can be seen only from close to
the tunnel.
Figure 16. Horizontal component migrated profile, with P wave tomographic section.
22
Figure 17. Vertical component migrated profile, with S wave tomographic section.
3.5 Reflection imaging and interpretation around the tunnel
The seismic images from two components were processed separately and migrated in 3-
D. Results are presenting reflecting events arriving from different directions.
Subvertical reflecting events can be detected well in the horizontal component migrated
profile, when the reflector’s angle with the tunnel axis is more than 30°. Subhorizontal
reflecting events can be detected in the vertical component reflection image. The events
can be located either below or above the measurement level, but it cannot be resolved
without external information in 2-component survey, where exactly the events are
located.
The seismic images can be oriented in a display in ONKALO near volume, placing the
tunnel as one of image axis, and defining the image rotation angle according to the
component: for vertical component upwards or downwards, and for axial component
near horizontally (+/- 25°) off the tunnel (to the NW). Three-dimensional interpretation
was carried out using reflecting events interpreted from 2003 and 2005 VSP reflection
campaign. Some of the reflections match well to the interpreted lines of modeled
reflecting planes (Figure 18).
23
Figure 18. Reflector 23 in 2003 VSP survey of OL-KR04 (Enescu et al. 2004), 140°/60°
imaged on horizontal component migrated profile.
24
The reflector nr. 23 of 2003 re-interpretation of VSP survey of OL-KR04 data (Enescu
et al. 2004) has been projected to depth 140 m in OL-KR04, and is oriented 140°/60°.
Reflector is seen at 80 m direct distance from measurement array to the NW.
Figure 19. Migrated profiles with reflector 2 on tunnel survey. Horizontal component
(X) on the top, vertical component (Y) on the bottom.
25
The interpretations can be layered on the migrated profiles (Figures 19 and 21).
Reflecting events which are not previously interpreted from other seismic
investigations, cannot be determined with their orientation on basis of tunnel seismic
data alone (Figure 20).
Figure 20. A 3D view from NW. Reflector 2 (dip c. 45°) imaged on the horizontal and
vertical component migrated profiles. The orientation cannot be determined based only
on the tunnel 2D seismic data.
Previously interpreted subvertical reflectors can be observed near tunnel. These have
not been intersected the drillholes OL-KR08, OL-KR27, OL-KR29 and OL-KR38 from
where the interpretations were made. Some of the events can be checked from tunnel for
their correspondence.
26
Figure 21. Reflection interpretations from VSP survey of 2003 and 2005 (Enescu et al.
2004 and 2007) on lines, imaged on migrated seismic profiles. Horizontal component
on the top, vertical component on the bottom.
27
Figure 22. 3D view from NW. Reflectors of 2005 (Enescu et al. 2007) imaged on the
horizontal and vertical components migrated profiles.
Reflector numbers correspond to the ones given in the 2005 report (Enescu et al. 2007) like presented in Table 5.
Table 5. Reflector correspondence in Tunnel Seismic work and drillhole VSP survey (Enescu et al 2007) from OL-KR08, KR27, KR29 and KR38.
Drillhole VSP reflector Orientation
# 23 dip 60º, reflection arrival azimuth 320º (dip dir 140 º)
# 24 dip 83º, reflection arrival azimuth 300º (dip dir 120 º)
# 25 dip 73º, reflection arrival azimuth 353º (dip dir 173 º)
# 26 dip 83º, reflection arrival azimuth 305º (dip dir 125 º)
# 27 dip 85º, reflection arrival azimuth 317º (dip dir 137 º)
# 28 dip 86º, reflection arrival azimuth 335º (dip dir 155 º)
# 29 dip 83º, reflection arrival azimuth 300º (dip dir 120 º)
28
4. CONCLUSIONS
The tunnel seismic field work was accommodated in the course of excavation work in ONKALO. The task included design of the work flow to perform with acquisition and achieving high quality of results.
Some directions for further development were recognised:
- Use a source powered by batteries and inverter to avoid delays from mains
disconnecting
- Use radio transmission for the pilot signal to avoid cable connections;
- Mount the geophone groups in metallic casing – easy to install and orient
- Apply 3-component geophone groups to be able to resolve the true position
of reflecting events from different directions
- Use longer profiles to penetrate deeper to bedrock with refraction and to
have deeper reliable reflection interpretation
- Drill bit signal can be used as a source
- Drill bit measurements may be done without triggering/ pilot signal cables,
as decoding of the pilot signal on a normal geophone is feasible.
The quality of signal is good and frequency band is rather high. Velocity variation near
tunnel wall can be observed, which may be due to dry volume or deformation due to
excavation. The resolution is not high enough to measure the thickness of the zone.
Otherwise the interpreted velocities match well with previous information on seismic
velocities in the site.
There are also observed several reflecting events in the near volume of tunnel, up to a 100 m distance. Events are seen on the same side (right-hand) where the sources and receivers have been placed.
Fractures making an angle of 30° or more with the tunnel axis can be imaged several tens of meters in depth. Vertical fractures perpendicular to tunnel can be imaged only in the vicinity of the tunnel.
Gently dipping fractures are imaged below and above. With one line on one wall, one may not be able to resolve between locations above/below.
The survey demonstrated that investigations can be carried out in tunnel conditions. It is possible to obtain high resolution and bring the characterization closer to the target than before. The tunnel working conditions need to be considered carefully in design of possible later surveys.
29
REFERENCES
Enescu, N., Cosma, C., Balu, L. 2004. Reflection Seismics Using Boreholes at Olkiluoto in 2003 – from investigation design to result validation, Volume 1. Posiva Working Report 2004-62, 167 p.
Enescu, N., Cosma, C. & Balu, L. 2007. Seismic VSP investigations at Olkiluoto, 2005. Posiva Working Report 2007-72, 147 p.
Öhberg, A. Hirvonen, H., Kemppainen, K., Niemonen, J., Nordbäck, N., Pöllänen, J., Rautio, T., Rouhiainen, P., and Tarvainen, A-M. 2007a. Drilling and the associated drillhole measurements of the pilot hole ONK-PH06, Posiva Working report 2007-68, 177 p.
Te l +358-2-8372 31
Mi rcea Cozma
November 2008
Rock Mass Seismic Imaging Around the ONKALO Tunnel, Olkiluoto 2007
November 2008
or pending completion.
coincide with those of Posiva.
Ca l in Cosma
V ib romet r i c Oy
Work ing Report 2008 -64
Rock Mass Seismic Imaging Around the ONKALO Tunnel, Olkiluoto 2007
Rock mass seismic imaging around the ONKALO tunnel, Olkiluoto 2007
ABSTRACT
Posiva Oy prepares for disposal of spent nuclear fuel in bedrock focusing in Olkiluoto, Eurajoki. This is in accordance of the application filed in 1999, the Decision-in- Principle of the State Council in 2000, and ratification by the Parliament in 2001.
Vibrometric Oy has performed a tunnel seismic survey in ONKALO access tunnel on a 100 m line in December 2007. Tunnel length (chainage) was 1720 - 1820 m (vertical depth 170 – 180 m).
Measurement applied 120 source positions at 1 m spacing, and on the both ends at 4 m spacing. Electromechanical Vibsist-20 tool was used as the source. Hammer produced 15.36 s sweeps. Signal was recorded with 2-component geophone assemblies, installed in 400 mm long, 45 mm drillholes in the tunnel wall. Sweeps were recorded with Summit II seismograph and decoded to seismic traces.
Also percussion drill rig, which is used in drilling the blasting holes in tunnel excavation, was tested from a 100-m distance as a seismic source. Signal is equally good as from actual seismic source, and may be applied later on for production.
Obtained seismic results were processed with tomographic reconstruction of the first arrivals to P and S wave refraction tomograms, and to tomograms of Young’s modulus and Shear Modulus. The obtained values correspond the typical levels known from Olkiluoto. There are indications of lower velocity near tunnel wall, but resolution is not adequate for further interpretation. Some variation of velocity is detected in the rock mass.
Seismic data was also processed with normal reflection profile interpretation and migrated. As a result there was obtained reflection images to a 100-m distance from the tunnel. Several reflecting events were observed in the rock mass. Features making an angle of 30° or more with tunnel axis can be imaged from distances of tens of metres. Vertical fractures perpendicular to tunnel can be imaged only near the tunnel. Gently dipping features can be imaged below and above. Images are 2D, i.e. have cylindrical symmetry, meaning the actual location of these reflecting events cannot be obtained without external information. Match was found to previous 3D VSP survey results in 2005.
The test indicated that tunnel seismic survey can be used in rock mass characterization. Operations in tunnel conditions and obtaining high quality results would require some attention in design of work flow.
Kalliotilavuuden seisminen tutkimus ONKALOssa vuonna 2007
TIIVISTELMÄ
Vibrometric Oy testasi tunneliseismisiä mittauksia ONKALOn ajotunnelissa 100 m profiililla joulukuussa 2007. Tunnelin paaluluku oli 1720 - 1820 m (vertikaalisyvyys 170 – 180 m).
Mittauksessa käytettiin 120 lähdepistettä 1 m välein, ja linjan jatkeilla 4 m välein. Sähkömekaanista Vibsist-20 lähdettä käytettiin 15,36 s mittaisiin iskusarjoihin. Signaali rekisteröitiin 2-komponenttisilla geofoniasemilla, jotka oli asennettu 400 mm syviin, halkaisijaltaan 45 mm kairanreikiin tunnelin seinässä. Tulokset tallennettiin Summit II seismografilla ja muunnettiin seismisiksi rekisteröintijäljiksi.
Iskuporakonetta (porausjumbo) jota käytetään louhinnan panosreikien poraukseen, testattiin 100 m etäisyydeltä seismisenä lähteenä. Tuloksen laatu on vastaava kuin varsinaisella seismisellä lähteellä saatu. Tekniikkaa voitaneen käyttää myöhemmin tuotantokäytössä.
Saadut seismiset tulokset prosessoitiin tomografisen inversion avulla P ja S aaltojen refraktiotomogrammeiksi. Myös Youngin modulista ja leikkausmodulista laskettiin tomogrammit. Tulosten taso vastaa kallioperän arvoja Olkiluodossa. Tunnelin seinän lähellä havaitaan viitteitä matalan nopeuden vyöhykkeestä. Tutkimuksen erotuskyky ei riitä tämän piirteen tarkempaan tulkintaan. Kalliossa esiintyy jonkin verran nopeuden vaihtelua.
Tulokset prosessointiin myös tavanomaisen heijastustulkinnan avulla ja migratoitiin. Tuloksena on saatu heijastuskartat noin 100 m etäisyydelle tunnelista. Useita heijastuspiirteitä voidaan havaita kalliotilavuudesta. Heijastajat joiden kulma on suurempi kuin 30° tunnelin akselista, havaitaan kymmenien metrien etäisyyksiltä. Tunnelin suuntaiset vertikaaliheijastajat nähdään vain läheltä tunnelia. Loivakaateiset heijastajat voidaan havaita mittaustason ala- ja yläpuolelta. Tulokset ovat kaksiulotteisia, eli sylinterisymmetrisiä, joten näiden heijastajien tarkkaa sijaintia ei tunneta ilman lisätietoja. Havaittiin että vuoden 2005 VSP-tuloksissa on yhtäläisyyksiä tämän tutkimuksen tuloksiin.
Testin tuloksena todettiin että tunneliseismistä tekniikkaa voidaan käyttää kallion tutkimuksessa. Työn suoritus tunnelityömaalla sekä korkealaatuisen tuloksen tuottaminen vaatii jonkin verran suunnittelua.
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2. FIELD WORK ........................................................................................................ 3
2.1 General ....................................................................................................... 3 2.2 Scope of work ............................................................................................. 4 2.3 Equipment ................................................................................................... 6
2.3.1 Receivers and Recorder ................................................................... 6 2.3.2 The VIBSIST-20 seismic source ....................................................... 8 2.3.3 The detectors and acquisition system ............................................... 9
3. DATA PROCESSING .......................................................................................... 10
3.1 General ..................................................................................................... 10 3.2 Standard 2D reflection profiling ................................................................. 15 3.3 Tomographic reconstruction ...................................................................... 16 3.4 Transmission and reflection imaging along the tunnel ............................... 21 3.5 Reflection imaging and interpretation around the tunnel ............................ 22
4. CONCLUSIONS .................................................................................................. 28
1. INTRODUCTION
Posiva Oy prepares for disposal of spent nuclear fuel in bedrock focusing in Olkiluoto, Eurajoki. This is in accordance of the application filed in 1999, the Decision-in- Principle of the State Council in 2000, and ratification by the Parliament in 2001.
Underground characterization will be carried out in ONKALO premises. The ONKALO access tunnel has been excavated since 2004. Currently excavation has progressed to vertical depth of 300 m, and tunnel length 3100 m.
As part of characterization programme and methodological development, seismic surveys were carried out in the tunnel. Placement of the survey was on the right hand wall, at chainage 1720 – 1820 m. The main objectives were the following:
- Image long vertical fault nearly perpendicular to tunnel
- Image long gently dipping fractures at the side and/or below
- Draw refraction velocity profile along the wall.
Initial idea was to perform refraction and reflection seismic sounding in ONKALO tunnel. A design component was included to the work. In order to obtain high quality of results and to avoid tunnel wave, the survey technique was optimized. Selection was done between installations on the wall or opened floor.
Work was initiated by Jussi Mattila, Posiva Oy. Test location in tunnel was selected near to known subvertical brittle fault location and above an extensive gently dipping brittle fault, on a straight part of tunnel.
Contact persons on Posiva side were Mari Lahti, Antti Mustonen (planning of project) and Janne Laihonen, who also organised the access to the site and preparations of geophone and shot stations, drilled by SK-Kaivin Oy. Staking of the stations was arranged by Prismarit Oy.
Design and field work were carried out by Vibrometric Oy. The field work was planned and the project managed by Calin Cosma and acquisition was carried out by Mircea Cozma. They took care of processing together with Lucian Balu. Client contact person on Vibrometric Oy side was Veli-Pekka Kantanen. Nicoleta Enescu carried out interpretation and wrote the report.
Chapter 2 below describes the field work setting and the equipment. Chapter 3 describes the processing and results.
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2.1 General
The field work was carried out in December 2007 (Table 1). Field work extended
during a two-week time on weekdays.
Table 1. Field work time schedule.
No. Day No. of shots Remarks
1 10.12.07 - Training
3 12.12.07 - Setup & calibration
5 14.12.07 16 Tunnel closed at 18:00
6 18.12.07 - Back to ONKALO; Setting the line
7 19.12.07 50 2D production
8 20.12.07 95 2D production
9 21.12.07 26 Morning: Tamrock records; Afternoon: 2D production;
Evening: Demobilization
Preparations required drilling of 400 mm long and 45 mm in diameter drillholes for
geophones on c. 1,5 m level above the tunnel floor. Shot points were marked on the wall
between the geophone stations.
The acquisition was run during the excavation work, which caused frequent
interruptions to the production. The setup required cable connections for triggering
which had to be pulled over the tunnel. The acquisition system was moved away from
tunnel each time interruption occurred, and then reinstalled. Disconnecting electrical
mains delayed the work.
The field personnel and their responsibilities are presented in Table 2.
Table 2. Field personnel and their responsibilities.
Person Responsibility Authority
processing, Reporting
Project manager
Pierre Lyons Data Acquisition
Alexadru Prisaciuc Operation
2.2 Scope of work
Purpose was to produce seismic velocity and reflection information using a tunnel based survey layout. Target was to develop field acquisition and processing technique, and to demonstrate the method capability in characterization of faults or long fractures in different positions.
Reflection imaging ahead and aside the tunnel was attempted over 100 stations (sources and receivers) at 1 m intervals along the tunnel wall. For each source, 50 receiver positions at 2 m spacing were recorded simultaneously. Firstly the odd receiver positions were instrumented and shots were recorded every meter, then the receivers were moved to the even positions, the shot sequence being thereafter repeated.
For refraction analysis two groups of 10 shots at 4 m spacing were recorded termed A to J & U to K. The sources along the reflection array were used as well. The 100 m long even receiver position array was used to record the refraction shots. The layout is presented in Figure 1. As seen from Figure 2, the refraction shots could not be set further from the ends of the receiver array due to the bending of the tunnel.
Figure 1. Location of the seismic survey in the ONKALO Tunnel.
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Figure 2. Survey layout: a 100 sources (blue squares) and receiver positions (magenta squares) used for reflection analysis. Refraction shots are shown as blue and yellow triangles.
The same 100 m long receiver array was used to record the drilling signature of a
percussion rig placed 800 – 1000 m further down the tunnel (at a 100 m distance from
receivers).
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2.3.1 Receivers and Recorder
The recording system was a DMT Summit II distributed digital seismograph with 100
channels. A 100 approximately 40 cm deep drillholes of 45 mm diameter were drilled
prior to the survey on the right-hand wall at 1.5 m level above the tunnel floor.
Each drillhole was instrumented with two geophones of type Oyo Geospace SMC1850-
30 Hz or I/O-Sensor SM45. Geophones were assembled in 50 molds of Ø 44 mm x L
100 mm. Molds were made of 2-component Epoxy/ Hot Melt (polyethylene).
Geophones were placed orthogonally with the X component horizontal along the tunnel
axis, and the Y component vertical, as shown in Figure 3.
Figure 3. On the left, receiver drillholes at 1.5 m from the floor, where two components receivers have been mounted in the wall. Top on the right the geophone orientation, view from above, tunnel axis across the page. Bottom on the right: two geophones assembly mould.
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Figure 4. Summit II receiver units along the ONKALO tunnel.
Figure 5. Vibsist-20 used for the ONKALO measurements.
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2.3.2 The VIBSIST-20 seismic source
The seismic source used for the ONKALO measurements was the Vibsist-20 (see
Figure 5 on previous page, and Table 3) with the following characteristics:
Energy: 20-25 J/impact
Trigger by geophone (Trigger geophones Geo-Space GS14-L3) placed on the
impact head. The trigger signal has been conveyed by cable to the recording
station.
The VIBSIST-20 is based on the Swept Impact Seismic Technique (SIST), for which
the seismic signals are produced as a series of pulses, according to a specific pre-
programmed sequence. The use of the monotonous variation of the impact rate controls
effectively the non-repeatability of the impact intervals and achieves a wide bandwidth
even when the coupling to the rock or ground is relatively poor.
Table 3. Technical specifications of Vibsist-20 source.
PARAMETER VALUE
Maximum current consumption: 14 A
Impact repetition rate: programmable between 10 to 30 impacts per second
Impact energy: approximately 20 J / impact for a typical 1.5 kW
impact hammer
Programmed sweep characteristics: computer controlled (any shape) or preset (linear)
sweep.
Impact rate ratio: minimum 1.5 / 1.
Sweep time adjustment: 2.5 to 30 seconds (this is restricted by the number of
samples available per channel).
2 kg
2.3.3 The detectors and acquisition system
A Summit II Plus 24-bit seismic data recording system with 100 channels was used for data acquisition (Figure 4). Table 4 displays the technical parameters of the Summit II system.
Table 4. Technical specifications of the Summit II Plus system.
Main registration parameters in ONKALO survey Value Sample rate (ms) 0.125
Trace length (ms) 15360
3.1 General
The reflection data were processed as a standard 2D seismic line. First arrivals were
defined for each refraction source and tomographic reconstruction was performed.
Seismic images from two components were processed with 3-D migration. Signal
quality was generally high (Figure 6).
Figure 6. Shot gather from the beginning, middle and the end of the source array.
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The seismic wave direct arrivals and reflections in a different way from the upper end
(Figure 7) and lower end (Figure 8) of the array. The tunnel is bending backwards right
off the ends of the array, making it impossible to use longer offset sources in refraction.
Figure 7. Refraction sources 1, 5 and 10 of profile U - K recorded down-tunnel.
Horizontal component (X) on top, vertical component (Y) on bottom.
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Figure 8. Refraction sources 1, 5 and 10 of profile A – J recorded up-tunnel. Horizontal
component (X) on top, vertical component (Y) on bottom.
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Figure 9. Measured data from the Tamrock drill bit. Sources 1, 4 & 8 of the percussion
rig location (Figure 10). Horizontal component (X) on the top, vertical component (Y)
on the bottom.
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Testing of Tamrock rig drill bit signal of percussion drill rig placed at chainage 2555 –
2558 m indicate signal quality well comparable to the one obtained with Vibsist-20
(Figure 9). Measurement arrangement for the drill bit recording included triggering of
pilot signal with two geophones, connected via cable link to the recording station 700 m
up in a tunnel. Eight percussion drillholes were prepared for the test (Figure 10). The
results can be used for localization of the drill bit, for seismic transmission tomography,
and for seismic reflection profiling.
Figure 10. Source array and pilot signal arrangement in Tamrock percussion rig signal
test. Acquisition station at chainage 1835 m, tests sources at 2555 – 2558 m.
The raw stacked data and processed data were documented and stored on DVDs. Copies
are to be forwarded to POSIVA OY for safekeeping. Results included the migrated
reflection images projected on coordinates, to be displayed in 3D.
The interpreted results will be checked by Posiva Oy for consistence against existing
realizations of the site model. The images were delivered to Posiva Oy with their
projected coordinates to accommodate in 3D modeling system.
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3.2 Standard 2D reflection profiling
The 100 receiver and 100 sources recordings were sorted onto seismic reflection profile.
Processing steps included sorting of traces, band-pass filtering, spectral equalization,
static correction, dip-move out, and stacking of the signal.
Figure 11. The source 1 measurement down-tunnel, band pass filtered onto band 300 –
600 Hz (top), 300 – 900 Hz (middle) and 300 – 1500 Hz (bottom).
Transmitted and reflected signal contains high frequency band, up to 1500 Hz. Highest
frequency band 300 – 1500 Hz was applied. From the processed traces the direct P and
S waves can be seen clearly. Also reflections are detected already on the raw profiles.
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3.3 Tomographic reconstruction
The obtained P and S arrival picks were combined with coordinate file. Results were
inverted through tomographic reconstruction to obtain depth sections off the tunnel
wall. Both P and S wave tomograms were computed. The results were also computed to
tomogram of Poisson’s ratio ν, Shear modulus μ and Young’s modulus E using (1), (2)
.
)1(2 vE (3)
The refraction images were produced to 8 – 10 m maximum depth from the tunnel wall.
There is a limitation caused by the short offset and high velocity.
The tomograms (Figures 12 and 13) indicate that the velocity is slightly lower in a thin,
less than 1 m layer near the tunnel wall. The resolution is in the order of 0.5 m, so the
actual thickness of the low velocity zone cannot be defined. There is some velocity
variation in the rock mass, which may originate from lithological variation and from
deformation zones.
The computed dynamic rock mechanical sections suggest there may be some local
variation of elastic properties in the rock mass, especially near the tunnel wall.
The velocity and elastic moduli value ranges match quite well to the previously known
in situ information from pilot drillhole geophysical logging (Öhberg et al. 2007a, b), as
shown in Figures 14 and 15. The P velocity ranges at 5000 – 5800 m/s and S velocity at
3000 – 3400 m/s in logging. In refraction profile the rock mass velocities away from
tunnel are on the same order, but the velocities in dry and possibly deformed rock mass
near tunnel wall are clearly lower. Shear modulus ranges at 25 – 33 GPa and Young’s
modulus at 65 – 80 GPa in logging.
The pilot drillholes ONK-PH06 and ONK-PH07 were located in ONKALO 170…310
m above and 60…160 m below the seismic section. Lithological variation of the values
is distinct.
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Figure 12. P wave (left) and S wave (right) velocity tomographic sections.
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Figure 13. Young’s modulus (left) and Shear modulus (right) tomographic section.
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Figure 14. Medians (1 m) of drillhole acoustic logging velocity (top) and dynamic
elastic modulus (bottom) in all pilot drillholes. DGN = diatexitic gneiss,
KFP=potassium feldspar porphyrite, MFGN=mafic gneiss, PGR= granite pegmatite,
QGN=quartz gneiss and VGN=veined gneiss.
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Figure 15. Medians (1 m) of drillhole acoustic logging velocity (top) and dynamic
elastic modulus (bottom) in pilot drillholes near tunnel seismic test (closer than 300 m
along tunnel or 30 m vertically). DGN = diatexitic gneiss, KFP=potassium feldspar
porphyrite, MFGN=mafic gneiss, PGR= granite pegmatite, QGN=quartz gneiss and
VGN=veined gneiss.
The seismic tomographic sections and migrated reflection images were layered together
(Figure 16 and Figure 17). Migrated reflection images can be presented with good
confidence to a limited distance from the tunnel, as the profile is comparably short, a
100 m.
Tomograms are limited in extent but they can suggest some velocity variation. The
reflection images of different components indicate in the near field some clear
reflections. Some of these may be correlated to geological features met in tunnel
mapping. The parallel to tunnel oriented reflecting events can be seen only from close to
the tunnel.
Figure 16. Horizontal component migrated profile, with P wave tomographic section.
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Figure 17. Vertical component migrated profile, with S wave tomographic section.
3.5 Reflection imaging and interpretation around the tunnel
The seismic images from two components were processed separately and migrated in 3-
D. Results are presenting reflecting events arriving from different directions.
Subvertical reflecting events can be detected well in the horizontal component migrated
profile, when the reflector’s angle with the tunnel axis is more than 30°. Subhorizontal
reflecting events can be detected in the vertical component reflection image. The events
can be located either below or above the measurement level, but it cannot be resolved
without external information in 2-component survey, where exactly the events are
located.
The seismic images can be oriented in a display in ONKALO near volume, placing the
tunnel as one of image axis, and defining the image rotation angle according to the
component: for vertical component upwards or downwards, and for axial component
near horizontally (+/- 25°) off the tunnel (to the NW). Three-dimensional interpretation
was carried out using reflecting events interpreted from 2003 and 2005 VSP reflection
campaign. Some of the reflections match well to the interpreted lines of modeled
reflecting planes (Figure 18).
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Figure 18. Reflector 23 in 2003 VSP survey of OL-KR04 (Enescu et al. 2004), 140°/60°
imaged on horizontal component migrated profile.
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The reflector nr. 23 of 2003 re-interpretation of VSP survey of OL-KR04 data (Enescu
et al. 2004) has been projected to depth 140 m in OL-KR04, and is oriented 140°/60°.
Reflector is seen at 80 m direct distance from measurement array to the NW.
Figure 19. Migrated profiles with reflector 2 on tunnel survey. Horizontal component
(X) on the top, vertical component (Y) on the bottom.
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The interpretations can be layered on the migrated profiles (Figures 19 and 21).
Reflecting events which are not previously interpreted from other seismic
investigations, cannot be determined with their orientation on basis of tunnel seismic
data alone (Figure 20).
Figure 20. A 3D view from NW. Reflector 2 (dip c. 45°) imaged on the horizontal and
vertical component migrated profiles. The orientation cannot be determined based only
on the tunnel 2D seismic data.
Previously interpreted subvertical reflectors can be observed near tunnel. These have
not been intersected the drillholes OL-KR08, OL-KR27, OL-KR29 and OL-KR38 from
where the interpretations were made. Some of the events can be checked from tunnel for
their correspondence.
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Figure 21. Reflection interpretations from VSP survey of 2003 and 2005 (Enescu et al.
2004 and 2007) on lines, imaged on migrated seismic profiles. Horizontal component
on the top, vertical component on the bottom.
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Figure 22. 3D view from NW. Reflectors of 2005 (Enescu et al. 2007) imaged on the
horizontal and vertical components migrated profiles.
Reflector numbers correspond to the ones given in the 2005 report (Enescu et al. 2007) like presented in Table 5.
Table 5. Reflector correspondence in Tunnel Seismic work and drillhole VSP survey (Enescu et al 2007) from OL-KR08, KR27, KR29 and KR38.
Drillhole VSP reflector Orientation
# 23 dip 60º, reflection arrival azimuth 320º (dip dir 140 º)
# 24 dip 83º, reflection arrival azimuth 300º (dip dir 120 º)
# 25 dip 73º, reflection arrival azimuth 353º (dip dir 173 º)
# 26 dip 83º, reflection arrival azimuth 305º (dip dir 125 º)
# 27 dip 85º, reflection arrival azimuth 317º (dip dir 137 º)
# 28 dip 86º, reflection arrival azimuth 335º (dip dir 155 º)
# 29 dip 83º, reflection arrival azimuth 300º (dip dir 120 º)
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4. CONCLUSIONS
The tunnel seismic field work was accommodated in the course of excavation work in ONKALO. The task included design of the work flow to perform with acquisition and achieving high quality of results.
Some directions for further development were recognised:
- Use a source powered by batteries and inverter to avoid delays from mains
disconnecting
- Use radio transmission for the pilot signal to avoid cable connections;
- Mount the geophone groups in metallic casing – easy to install and orient
- Apply 3-component geophone groups to be able to resolve the true position
of reflecting events from different directions
- Use longer profiles to penetrate deeper to bedrock with refraction and to
have deeper reliable reflection interpretation
- Drill bit signal can be used as a source
- Drill bit measurements may be done without triggering/ pilot signal cables,
as decoding of the pilot signal on a normal geophone is feasible.
The quality of signal is good and frequency band is rather high. Velocity variation near
tunnel wall can be observed, which may be due to dry volume or deformation due to
excavation. The resolution is not high enough to measure the thickness of the zone.
Otherwise the interpreted velocities match well with previous information on seismic
velocities in the site.
There are also observed several reflecting events in the near volume of tunnel, up to a 100 m distance. Events are seen on the same side (right-hand) where the sources and receivers have been placed.
Fractures making an angle of 30° or more with the tunnel axis can be imaged several tens of meters in depth. Vertical fractures perpendicular to tunnel can be imaged only in the vicinity of the tunnel.
Gently dipping fractures are imaged below and above. With one line on one wall, one may not be able to resolve between locations above/below.
The survey demonstrated that investigations can be carried out in tunnel conditions. It is possible to obtain high resolution and bring the characterization closer to the target than before. The tunnel working conditions need to be considered carefully in design of possible later surveys.
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REFERENCES
Enescu, N., Cosma, C., Balu, L. 2004. Reflection Seismics Using Boreholes at Olkiluoto in 2003 – from investigation design to result validation, Volume 1. Posiva Working Report 2004-62, 167 p.
Enescu, N., Cosma, C. & Balu, L. 2007. Seismic VSP investigations at Olkiluoto, 2005. Posiva Working Report 2007-72, 147 p.
Öhberg, A. Hirvonen, H., Kemppainen, K., Niemonen, J., Nordbäck, N., Pöllänen, J., Rautio, T., Rouhiainen, P., and Tarvainen, A-M. 2007a. Drilling and the associated drillhole measurements of the pilot hole ONK-PH06, Posiva Working report 2007-68, 177 p.