1. Sebastian Böser sboeser@ifh.de Development of glaciophones and acoustic transmitters for ice 1 st International ARENA Workshop Zeuthen May 2005

2. Acoustic sensors and transmitters – 2 sboeser@ifh.de Overview Motivation Thermoacoustic model Target material properties Sensors Principle and design Calibration Piezoceramics Sensors Transmitters Transmitter design HV signal generators

3. Acoustic sensors and transmitters – 3 sboeser@ifh.de Thermoacoustic modell

4. Acoustic sensors and transmitters – 4 sboeser@ifh.de Signal amplitudes Wasser (20 ℃) Ice (-50 ℃) Density ρ 10.92 Energy deposition L ≈ 5 md ≈ 10 cm Velocity of sound vsvs 14803900 Peak frequency f peak 7.419.5 Expansion coefficient α 200 · 10 -6 150 · 10 -6 Heat capacity CpCp 0.9990.5 Peak pressure amplitude P max 0.22 · 10 -3 2.2 · 10 -3

5. Acoustic sensors and transmitters – 5 sboeser@ifh.de Sensor design Requirements: sensitive to mPa pressures all-φ sensitivity / radial symmetry (directional information) Environmental: deployment in hot-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 Simplicity vs. Suitability

6. Acoustic sensors and transmitters – 6 sboeser@ifh.de Piezoelectric ceramics material: lead zirkonium titanate (PXE5 = PZT) pervoskit structure polycrystalline poling:  heat above T curie ≈ 300 ˚C  cool in strong E-Field (E ≈ 2 MV/m)  reorientation of polarization domains sensitivity: d 33 ≈ 500pC/N typical signal: 0.1  V @ 1 mPa T > T curie T < T curie shapes: tubes plates cylinders resonances: mode frequency

7. Acoustic sensors and transmitters – 7 sboeser@ifh.de Sensor design: schematic signal:U ∝ Δl ∝ ma  mass/spring load amplifier: three stages ( +80 dB ) low noise ( ≈ 8  V ) housing: high pressure  thickness impedance matching  resonances Z  (25 kHz) ice3.59*10 6 15.6 cm brass28.5*10 6 13.6 cm PXE514.2*10 6 7.4 cm piezoceramics housing amplifier (brass) head

8. Acoustic sensors and transmitters – 8 sboeser@ifh.de Sensors

9. Acoustic sensors and transmitters – 9 sboeser@ifh.de Lab measurements 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

10. Acoustic sensors and transmitters – 10 sboeser@ifh.de Calibration of piezoceramics stability: stable with temperature, time, … manufacturing variations problem: input impedance of voltmeter  decharge = RC ≈ 3  s  charge integration

11. Acoustic sensors and transmitters – 11 sboeser@ifh.de Calibration of sensors Problem interesting frequency ≈ 20 kHz  λ water = 7.5 cm  λ ice = 20 cm “Ringing” signal  reflections distort signal  need container with x cont » 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. Acoustic sensors and transmitters – 12 sboeser@ifh.de Sensitivity: Method Method transmit same signal to  reference  sensor to calibrate compare response  relative calibration Transmitted signals gated burst  precisely measure single frequency  limited by system relaxation time reflections pulse  in one shot measure full spectrum  limited by noise level

13. Acoustic sensors and transmitters – 13 sboeser@ifh.de Sensitivity: Gated burst Time window start: after initial excitation stop: before 1 st reflection Fit A(t) = A 0 sin(2πf·t + φ) + bt +c free phase and amplitude fixed frequency linear offset term  very good χ 2 But: low-f and DC background  large error for small signals  probably overerstimated

14. Acoustic sensors and transmitters – 14 sboeser@ifh.de Sensitivity: pulse method Transmitted signal P ∞ ∂ 2 U in / ∂t 2  “soft” step function Received signal Fourier transform  compare spectral components Errors and noise A(t) = Σ f s(f)e i (2πft + φ s ) + n(f)e i (2πft + φ n ) coherent signal: φ s (f) = const random noise: φ s (f) = random Noise spectrum from average  fourier transform fourier transform  average  define signal dominated freq. ranges

15. Acoustic sensors and transmitters – 15 sboeser@ifh.de Comparison of methods Results high sensitivity and S/N Glass ball: factor ≈ 20 Iron ball: factor ≈ 50 very good agreement strongly structured  many different resonance modes only valid for water

16. Acoustic sensors and transmitters – 16 sboeser@ifh.de 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 Equivalent Noise Level [mPa] Frequency range [kHz] 5 - 1205 - 65 Hydrophone50.1± 0.740.3 ± 8.3 Glass Ball17.1 ± 1.715.9 ± 1.7 Iron Ball6.6 ± 0.64.7 ± 0.7

17. Acoustic sensors and transmitters – 17 sboeser@ifh.de 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. Acoustic sensors and transmitters – 18 sboeser@ifh.de 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. Acoustic sensors and transmitters – 19 sboeser@ifh.de 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. Acoustic sensors and transmitters – 20 sboeser@ifh.de HV signal generation Problem build a HV generator for arbitrary signals I max = 2πf C tot U max C ring = 16 nF f = 100 kHz U max = 1kV k 33 = 0.34 I max = 16 A, P ≈ 5.4 kW  too large Solution large capacity at low duty cycles 100 cycle burst  1ms  16 W large inductivity  discharge via capacitance  shortcut after N cycles

21. Acoustic sensors and transmitters – 21 sboeser@ifh.de 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