Development of glaciophones and acoustic transmitters for ice

1. Development of glaciophones and acoustic transmitters for ice 1st International ARENA Workshop Zeuthen May 2005

2. Overview • Motivation • Thermoacoustic model • Target material properties • Sensors • Principle and design • Calibration • Piezoceramics • Sensors • Transmitters • Transmitter design • HV signal generators

3. Thermoacoustic modell

4. Signal amplitudes

5. Requirements: sensitive to mPa pressures all-φ sensitivity / radial symmetry (directional information) Environmental: deployment inhot-water drilled holes  Water tight  temperature: -30℃ to -55℃  Refreezing: pressures up to 200 bar Electrical: very small signals  high gain  shielded against EM noise Piezoelectric ceramics:  well understood  cheap Housings:  thick walls or  solid (cast out) Amplifiers:  custom build Sensor design Simplicity vs. Suitability

6. Piezoelectric ceramics • material: • lead zirkonium titanate (PXE5 = PZT) • pervoskit structure • polycrystalline • poling: • heat above Tcurie ≈ 300 ˚C • cool in strong E-Field (E ≈ 2 MV/m)  reorientation of polarization domains • sensitivity: d33≈ 500pC/N • typical signal: • 0.1 mV @ 1 mPa T > Tcurie T < Tcurie • shapes: • tubes • plates • cylinders • resonances: • mode • frequency

7. housing amplifier piezoceramics (brass) head Sensor design: schematic • signal: U ∝Δl ∝ ma  mass/spring load • amplifier: • three stages ( +80 dB ) • low noise ( ≈ 8mV ) • housing: • high pressure  thickness • impedance matching  resonances

8. Sensors

9. Medium: ice water Linearity: all sensors nicely linear absolute values  calibration Self noise: power supply temperature Temperature: increasing with lower temp  not understood Pressure: no results (yet) Frequency response: need larger volume than in lab  calibration Excitation: piezoceramics laser proton beam Lab measurements

10. Calibration of piezoceramics • stability: • stable with temperature, time, … • manufacturing variations • problem: • input impedance of voltmeter tdecharge= R•C ≈ 3 ms • charge integration

11. Calibration of sensors • Problem • interesting frequency ≈ 20 kHz λwater = 7.5 cm λice = 20 cm • “Ringing” signal reflections distort signal need container with xcont» λ • Setup at HSVA • water tank 12m × 3m × 70m • deep section 12m × 5m × 10m • Sensors • Reference Hydrophone Sensortech SA03 163.3±0.3 dB re 1 V/µPa ( 5 to 65 kHz) • Glass Ball, Iron Ball • Transmitter • piezoceramic in epoxy arbitrary signal generator

12. Sensitivity: Method • Method • transmit same signal to reference  sensor to calibrate • compare response relative calibration • Transmitted signals • gated burst precisely measuresingle frequency limited by • system relaxation time • reflections • pulse in one shot measurefull spectrum limited by • noise level

13. Sensitivity: Gated burst • Time window • start: after initial excitation • stop: before 1st reflection • Fit • A(t) = A0sin(2πf·t + φ) + bt +c • free phase and amplitude • fixed frequency • linear offset term •  very good χ2 • But: low-f and DC background •  large error for small signals •  probably overerstimated

14. Sensitivity: pulse method • Transmitted signal • P ∞∂2Uin/ ∂t2 “soft” step function • Received signal • Fourier transform compare spectral components • Errors and noise • A(t) = Σf s(f)ei (2πft + φs) + n(f)ei (2πft + φn) • coherent signal: φs(f)= const • random noise: φs(f)= random • Noise spectrum from • average  fourier transform • fourier transform  average •  define signal dominated freq. ranges

15. Comparison of methods • very good agreement • strongly structured many different resonance modes • only valid for water • Results • high sensitivity and S/N • Glass ball: factor ≈ 20 • Iron ball: factor ≈ 50

16. Equivalent noise level • Method • fourier transform  scaling, frequency range  inverse transform • Problem • noise recording from water tank • lab self noise higher due to EM coupling

17. How to do it for ice ? • Theoretical • use formula for transmission • Problem • temperature dependance  resonance modes  amplifier gain× bandwidth • solid state vs. liquid • Practical • use large ice volume (glacier, pole) • use small ice block with changing boundary conditions(e.g. air, water)  determine reflections from comparison

18. Transmitters • Large absorption length  Need high power transmitter • Piezoceramics • can be driven with kV signals • easy to handle • cheap • well understood • Ring-shaped piezoceramic • azimuthal symmetry • larger signals than cylinders • more expensive

19. Ring vs. cylinder • Linearity • tested from 100 mV to 300 Vperfect linearity • Frequency response • three resonance modes width, thickness and diameter  wide resonance at lower frequencies • Testing • frequency sweep  dominated by reflections resonance modes of container • white noise signal  reflections not in phase resonance modes of transmitter

20. HV signal generation • Problem • build a HV generator forarbitrary signals • Imax = 2πf Ctot Umax • Cring = 16 nF • f = 100 kHz • Umax = 1kV • k33 = 0.34 • Imax = 16 A, P ≈ 5.4 kW  too large • Solution • large capacity at low duty cycles100 cycle burst  1ms  16 W • large inductivity discharge via capacitance  shortcut after N cycles

21. Summary • Developed sensors are cheap and sensitive • Developed transmitters are powerful  Problem: HV signal generation • Properties of both need to be better understood  Testing in ice limited by limited volume and freezing time • With only two years R&D,glaciophones are already quite successful