P- and S-wave velocities in rock as a function of pressure and temperature I. Lassila 1,T. Elbra 2,...

P- and S-wave velocities in rock as a function of pressure and temperature

I. Lassila 1,T. Elbra 2, E. Hæggström1 and L. J. Pesonen 2

V. Kananen1 and M. Perä

J. Haapalainen1 and R. Lehtiniemi 3

P. Heikkinen 4 and I. Kukkonen 5

1 Electronics Research Unit

2 Division of Geophysics

3 Nokia Research Center

4 Institute of Seismology

5 Geological Survey of Finland

7.12.2007 Ilkka Lassila

Motivation - Understanding the structure of the earth’s crust

FIRE (Finnish Reflection Experiment) - project Seismic reflection and refraction measurements (longitudinal

and shear wave modes)

Photo: Seismic signal is produced by vibrators. Courtesy Jukka Yliniemi.

Location of the FIRE reflection seismic lines.


TOF and depth

Seismic measurements give TOF data Need to know Vp and Vs to calibrate the depth

Example of FIRE results from the end of line FIRE 3A in western Finland. The reflector amplitudes of a migrated section are presented as gray tone intensities.


Samples

Outokumpu Deep Drilling Project (2516 m)


Device: requirements

Vp and Vs measurements preferably simultanously

10 m/s accuracy

Controlled pressure 0 - 300 MPa (15 ton for OKU samples)

Controlled temperature 20-300ºC

Data acquisition Preferably automatic

25 mm22

mm


Possible measurement setups

Uniaxial

Multianvil

Hydrostatic pressure


Timetable

Material considerations

Mechanical design

Ultrasonic testing and designingTransducers, pulser / signal generator, amplifiers, switches, oscilloscope

Pressure generating Pressure monitoring

Heating Temperature monitoring

Transducer cooling

Ordering parts

Planning the measurement procedure

Assembling the setup

Programming the DAQ software

Jan Feb Mar Apr May Jun Jul

Validation


Device: Vp and Vs

Pitch-catch method Two similar transducers, both comprising shear (1,1 MHz)

and longitudinal (1 MHz) piezo (Pz-27) ceramics At first only the shear crystal was in useAt first only the shear crystal was in use

Longitudinal mode well presentLongitudinal mode well present

Caused by silver epoxy?Caused by silver epoxy?

Removable delay lines Fused quartz

Brass

Water cooling No load over the piezo crystal


Device: pressure simulation

Pressure simulations by Mr. Haapalainen Device can withstand the required pressure

Fused quartz can be used as a delay line material in case of

no roughness


Device: pressure

Generating: 15 ton jack borrowed from Department of Chemistry Measuring: Sensotec Model 53 (max 23 ton) + Lebow 7528 amplifier


Device: pressure

Problem with sample durability Solved with a brass jacket Splitting sample holder allows sample removal after

compressioncompression


Device: Temperature, simulations

Thermal simulations by PhD Lehtiniemi and Mr.

Haapalainen 160 W heater is sufficient for 300ºC in case of fused quartz

delay lines

Transducer temperature stays below solder melting / epoxy Transducer temperature stays below solder melting / epoxy

softening temperaturesoftening temperature


Device: Temperature

Heating: Nozzle heater ACIM T197 (160 W / 240 Vac) Max 400ºC

Covers the sample holder

Cooling: Water cooler (Lauda WK502) Measuring: Custom AD595 based thermocouple amplifier

K-type Thermocouple inside the sample holder


Device: Data acquisition

US signals: 5072 PR, LeCroy 9410, GPIB, PC, LabVIEW, Matlab

Thermocouple and load cell: AD-conversion and transfer to PC with NI PCI-6024E


Device

Transducers Delay lines Heating element and sample Thermocouple Load cell Water cooling tubes Jack


Preliminary results

7 samples from Outokumpu Deep Drill Core T: 300ºC20ºC, Load: 7000 kg 500 kg (resembling

the conditions in the Earth’s crust) Results comparable with literature values

0 0.5 1 1.5 2

x 106

4000

4500

5000

5500

6000

Vp(

m/s

)

T*load(ºC kg)0 0.5 1 1.5 2

x 106

3000

3500

4000

Vs(

m/s

)

T*load(ºC kg)


Pressure test

The error if we don’t measure

the compression of the

sample? Compression = 0,1 mm

(Δhsample- Δhno sample)

Error Vp = 24-33 m/s Error Vs = 15-18 m/s

Pressure test without sample

48.6

48.7

48.8

48.9

49

49.1

49.2

49.3

mea

sure

d h

eig

ht

(mm

)

1000 kg

13000 kg

Pressure test with sample

70.5

70.6

70.7

70.8

70.9

71

71.1

71.2

71.3

mea

sure

d h

eig

ht

(mm

)

1000 kg

13000 kg


TOF (time of flight) through the delay lines

Pulse-echo measurement of the delay

line Subtraction of the TOF through the

delay lines from the total TOF Pressure and temperature effects to

the delay lines and transducers are

cancelled


Damping the transducers

Ringing of the piezo element makes pulse-echo (PE)

measurements difficult. Ringing can be reduced with applying attenuating,

material with acoustic impedance close to the piezo to the

back side of the transducer PE responses to water load

a) zero backing, b) backing of crown glass, c) backing of

tungsten-epoxy, d) backing of material with Z=Ztransducer

Egypt. J. Sol., Vol. (23), No. (2), (2000)


Damping test - ok

Reduced ringing time and increased bandwidth


Transducers without backing


Outcome of applying the backing

No signal Resistance between transducer electrodes ca. 5 Ω

Short-circuit

Difference between test Amount of tungsten in the mixture was higher

- In the test the resistance between the electrodes was ca.

500 Ω

This type of backing method requires isolation of the

electrodes

Instead of scraping out the backing it was decided to build

new transducers


New transducers

Increased sample size: Height 20-70 mm

Diameter 25-62 mm

Better modal purity required Mode conversion in the gap between transducer housing

and delay line

Material: stainless steel

No separate delay lines


New transducer drawings


New transducer


New thermal simulations

Stainless steel:

thermal conductivity=20 W/(m K)

Specific heat=500J/(kg K) Sample (rock):

thermal conductivity=2 W/(m K)

Specific heat=790J/(kg K)

T(t=0)=350ºC

T=10ºC

T=?

h = 20-70mm

D = 25-62 mm


Temperature as a function of time in the middle of the

sample and on the transducer inner surface where the

piezos are fixed.

Sample D = 25,5 mm, h = 24 mm Sample D = 62 mm, h = 70 mm


Temperature distribution in the sample and the upper

transducer

Sample D = 25,5 mm, h = 24 mmt = 200s.

Sample D = 62 mm, h = 70 mm,t = 400s.


Other updates

PC controlled pressure generation Separate heating of samples to increase the throughput

rate


New frame

Compressed air controlled one way

hydraulic cylinder replaced with

electric motor controlled two way

hydraulic cylinder


Modification for hydraulic control

Controls of the pump replaced with relay circuit that

is controlled from PC DAQ-card Two valves that are controlled

Valve 1 open increasing pressure

Valve 2 open decreasing pressure

Valves closed no change


Testing new hydraulics

Pressure increase at 0,1 s intervals OK for loads over 3000 kg

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

0 0,5 1 1,5 2 2,5 3

throttle time (s)

load

(kg

)


Testing of new hydraulics

-2000

0

2000

4000

6000

8000

10000

12000

14000

0 0,05 0,1 0,15 0,2 0,25

throttle time (s)

load

(kg

) Pressure decrease at 0,1 s intervals No control of outcome when decreasing pressure


More control needed

Manual shut off valve, needle type control Slows down the flow of the hydraulic oil


Control achieved

0,00E+00

1,00E+03

2,00E+03

3,00E+03

4,00E+03

5,00E+03

6,00E+03

7,00E+03

8,00E+03

loa

d (

kg

)

measuredload

targetload

Needle valve can be adjusted to allow precise control

of the load

0,00E+00

1,00E+03

2,00E+03

3,00E+03

4,00E+03

5,00E+03

6,00E+03

7,00E+03

loa

d (

kg

)measuredload

targetload


Measurement diagram


Conclusions

Device is used for measuring Vp and Vs values that are

needed to interpret seismic data Preliminary results ok At the moment system is going through some changes


Future tasks

Temperature inside the sample vs. on the sample surface Validation tests Implement a LVDT/gauge to measure the sample

thickness and thickness change inline

Licentiate thesis

P- and S-wave velocities in rock as a function of pressure and temperature I. Lassila 1,T. Elbra 2,...

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