P and s wave velocities in rock as a function of pressure and temperature
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P- and S-wave velocities in rock as a function of pressure and temperature. I. Lassila 1 ,T. Elbra 2 , E. H æggström 1 and L. J. Pesonen 2 V. Kananen 1 and M. Perä J. Haapalainen 1 and R. Lehtiniemi 3 P. Heikkinen 4 and I. Kukkonen 5. 1 Electronics Research Unit

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

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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


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

22 mm

25 mm


Possible measurement setups

  • Uniaxial

  • Multianvil

  • Hydrostatic pressure


Timetable

Jan Feb Mar Apr May Jun Jul

Material considerations

Mechanical design

Ultrasonic testing and designing

Transducers, 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

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 use

    • Longitudinal mode well present

    • 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 compression


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 softening 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: 300ºC20ºC, Load: 7000 kg  500 kg (resembling the conditions in the Earth’s crust)

  • Results comparable with literature values


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


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=10ºC

h = 20-70mm

T(t=0)=350ºC

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 mm

t = 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


Testing of new hydraulics

  • 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

  • Needle valve can be adjusted to allow precise control of the load


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


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