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WIND NOISE REDUCTION AT IMS INFRASOUND STATIONS Douglas R. Christie

WIND NOISE REDUCTION AT IMS INFRASOUND STATIONS Douglas R. Christie Research School of Earth Sciences The Australian National University Canberra, ACT 0200 Australia. Outline. Summary of background noise at infrasound stations . Wind-noise and wind-noise-reducing techniques.

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WIND NOISE REDUCTION AT IMS INFRASOUND STATIONS Douglas R. Christie

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  1. WIND NOISE REDUCTION AT IMS INFRASOUND STATIONS Douglas R. Christie Research School of Earth Sciences The Australian National University Canberra, ACT 0200 Australia

  2. Outline • Summary of background noise at infrasound stations. • Wind-noise and wind-noise-reducing techniques. • Brief review of earlier work on turbulence-reducing enclosures. • Recent work on turbulence-reducing enclosures. • Comparison of wind-noise-reducing enclosures with IMS infrasound pipe arrays. • Suggestions for use of turbulence-reducing enclosures with IMS infrasound pipe arrays.

  3. Noise at IMS Infrasound Stations • Wind-generated turbulence (all frequencies). • Microbarom infrasonic waves (~0.1 to 0.4 Hz). • Surf-generated noise (usually > 1 Hz). • Auroral-generated infrasound (usually < 0.1 Hz). • Noise generated by volcanic eruptions, waterfalls and forest fires (usually > 1 Hz). • Noise generated by highways, trains, airports, hydroelectric stations, gas flares, industry etc. (usually > 1 Hz) • Mountain-generated infrasound (< 0.1 Hz). • Noise associated with trapped gravity waves (long period). • Micropressure fluctuations generated by shear instabilities in the upper tropospheric jet streams (long period).

  4. Wind Noise • More than 50% of the stations in the IMS infrasound network are subject at times to undesirably high levels of wind-generated noise. • Wind noise reduction continues to be the most important technical problem in the field of infrasound monitoring. • Any improvement in wind-noise-reduction systems at IMS infrasound stations will significantly increase the reliability and performance of the global network.

  5. Diurnal Variation of Wind Noise at IMS Infrasound Monitoring Stations • Amplitude of wind-generated noise at exposed infrasound stations may exceed 10 Pa during the daytime. • Stations located in dense forests usually have acceptable wind-noise levels at all times of day. • Background noise levels should be less than 0.1 Pa Diurnal variation of filtered and unfiltered background noise at a typical IMS infrasound station located in a exposed semi-desert environment.

  6. Typical Background Noise Levels at IMS infrasound stations • Noise levels increase rapidly with increasing wind speed. • Detection capability may be marginal when surface winds (at 2 m) exceed a few m/s. • Signals from distant explosions can be detected reliably when noise levels at 1 Hz are less than 5x10-5 Pa2/Hz. Standard noise-reducing pipe arrays are installed at IS07.

  7. Wind-Noise-Reducing Techniques • Traditionally, pipe arrays (or porous hoses) have been used to reduce wind noise by averaging pressure variations over a limited area around a microbarometer sensor. • Pipe arrays are used at all IMS stations. • Signals may be attenuated or distorted at high frequencies. • May need impedance matching capillaries to suppress high frequency resonances (Hedlin, 2001). • Design of pipe arrays has reached practical limits. • Effective wind-noise-reduction can be achieved using an optical fiber infrasound sensor (OFIS) to integrate surface pressure along the path of the fiber (Berger et al., 2000; Zumberge et al., 2001, 2003); Walker et al., 2004, 2005, 2006, 2007).

  8. Other Wind-Noise-Reducing Techniques • Wind noise can be reduced using adaptive signal processing of data from a compact arraywith a large number of sensors (Tamadgee et al., 2001; Bass and Shields, 2004; Shields, 2005). • High frequency wind noise can be reduced by burying the sensor in a porous medium (the ‘sandbox’ approach (Herrin et al. 2001). • Wind shields and wind barriers: • Grover (1971). Small perforated domes. Marginal performance. • Liszka (1972) pioneered use of relatively small porous wind barriers. Also studied by ReVelle (2000) and Hedlin and Berger (2001). Hedlin and Berger showed that a wire screen on the sides improves performance. Good performance at high frequencies. • Bedard (2003) used a larger scale porous wind fence with solid vertical corrugations along the top to effectively reduce higher frequency wind noise in a tornado warning system.

  9. Wind Noise Reduction Using Turbulence-Reducing Enclosures Objective: To improve wind noise reduction at 1 Hz at IMS infrasound stations by at least two orders of magnitude. • Turbulence-reducing enclosures are designed to: • degrade turbulent eddies near the surface and • lift the turbulent boundary layer above the sensor inlets. • Enclosures are constructed from a series of screens which mechanically extract energy from turbulent wind-generated eddies in the atmospheric boundary layer. • A number of turbulence-reducing enclosures were described at the 2006 and 2007 Infrasound Technology Workshops.

  10. Examples: Turbulence-Reducing Enclosures • Versions 1 and 2 • Multiple-walled open enclosures with outward-facing inclined serrations. • Versions 3 and 4a (not shown) • Similar to Version 2, but with higher walls. • Version 4b • includes radial baffles, interior screened chambers and a porous roof over the inner structure.

  11. Performance: Comparison of Version 2 with Version 4B Version 2 • Multiple walls with serrations. • 2.4-m high, open • Porous hose array used for evaluation. Version 4B • Multiple walls with serrations • 3.2-m high, roof over interior • Multiple chambers and baffles • Single port system and 6-port array used for evaluation.

  12. Version 5:Turbulence-Reducing Enclosure Version 5: Lower profile (2.0-m high) enclosure. Includes screened roof over entire structure, horizontal and larger scale downward-inclined serrations fastened to the top edge of the outer wall,screened closed chambers around a single-port system, a 6-port pipe array and radial baffles.

  13. Performance: Version 5 of the Turbulence-Reducing Enclosure • Version 5 is effective even in relatively high winds. • Single-inlet port system appears to be more efficient at high frequencies, but 6-port array has high-frequency resonance. • 6-port array is better at low frequencies. • Noise is reduced by 4 orders of magnitude at high frequencies.

  14. Performance of Version 5 of the EnclosureWith and Without Serrations Horizontal and downward-inclined outward-facing serrations attached to the upper edge of the outer wall effectively reduce wind noise at the center of Version 5 of the enclosure.

  15. Version 6:Turbulence-Reducing Enclosure Version 6: Larger scale, low-profile enclosure. • Attempt to improve performance at longer periods. • Similar to Version 5, except larger down-ward inclined serrations have been replaced by new inclined outer screened wall that extends to surface. V6 also has horizontal serrations.

  16. Performance: Version 6 of the Turbulence-Reducing Enclosure Comparison of Versions 5 and 6 of the wind-noise-reducing enclosure. • Performance of Version 6 is almost the same as that of Version 5. • Version 5 with single port is slightly better at lower frequencies. • Version 6 with single port is slightly better at high frequencies. • 6-port array is better at lower frequencies in both V5 and V6.

  17. Best Practical Design:Version 5

  18. Summary: WindNoise ReductionUsingTurbulence-Reducing Enclosures • Version 5 is the best practical design for a noise-reducing enclosure. • Wind noise is attenuated by up to 4 orders of magnitude at high frequencies. • Signals are not distorted or attenuated. • A turbulence-reducing enclosure with a single inlet port at the center can be used in some cases as a stand-alone noise-reducing system. • Further noise reduction can be achieved by combining turbulence-reducing enclosures with pipe arrays.

  19. Comparison: Version 5 Enclosure with a 18-m Diameter IMS Pipe Array The performance of Version 5 of the wind-noise-reducing enclosure with a single inlet port at the center is significantly better than the performance of a 96-port 18-m diameter rosette pipe array at higher frequencies.

  20. Wind-Noise Reduction at IMS ArraysLocated in Low Wind Environments Recommended for use in low wind environments (wind speeds up to ~2.5 m/s measured at a height of 2.0 m).

  21. Wind-Noise Reduction at IMS ArraysLocated in Medium Wind Environments Recommended for use in medium wind environments (wind speeds up to ~4.5 m/s measured at a height of 2.0 m).

  22. Wind-Noise Reduction at IMS ArraysLocated in High Wind Environments Recommended for use in high wind environments (wind speeds > 4.5 m/s measured at a height of 2.0 m).

  23. Conclusions • Turbulence-reducing enclosures can be used to effectively reduce wind-generated noise at infrasound monitoring stations. • The detection capability at more than 50% of all stations in the IMS infrasound network can be improved significantly by using both turbulence-reducing enclosures and existing wind-noise-reducing pipe arrays. • Any improvement in wind-noise-reduction at IMS infrasound stations would increase the reliability and performance of the global network.

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