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

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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  1. 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

  2. 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.

  3. 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.

  4. Samples • Outokumpu Deep Drilling Project (2516 m)

  5. 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

  6. Possible measurement setups • Uniaxial • Multianvil • Hydrostatic pressure

  7. 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

  8. 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

  9. 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

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

  11. Device: pressure • Problem with sample durability • Solved with a brass jacket • Splitting sample holder allows sample removal after compression

  12. 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

  13. 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

  14. 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

  15. Device • Transducers • Delay lines • Heating element and sample • Thermocouple • Load cell • Water cooling tubes • Jack

  16. 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

  17. 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

  18. 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

  19. 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)

  20. Damping test - ok • Reduced ringing time and increased bandwidth

  21. Transducers without backing

  22. 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

  23. 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

  24. New transducer drawings

  25. New transducer

  26. 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

  27. 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

  28. 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.

  29. Other updates • PC controlled pressure generation • Separate heating of samples to increase the throughput rate

  30. New frame • Compressed air controlled one way hydraulic cylinder replaced with electric motor controlled two way hydraulic cylinder

  31. 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

  32. Testing new hydraulics • Pressure increase at 0,1 s intervals • OK for loads over 3000 kg

  33. Testing of new hydraulics • Pressure decrease at 0,1 s intervals • No control of outcome when decreasing pressure

  34. More control needed • Manual shut off valve, needle type control • Slows down the flow of the hydraulic oil

  35. Control achieved • Needle valve can be adjusted to allow precise control of the load

  36. Measurement diagram

  37. 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

  38. 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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