THE AUSTRALIAN NATIONAL UNIVERSITY Infrasound Technology Workshop, November 2007, Tokyo, Japan RECENT PROGRESS IN WIND NOISE REDUCTION AT INFRASOUND MONITORING STATIONS Douglas R. Christie Research School of Earth Sciences The Australian National University Canberra, ACT 0200 Australia

THE AUSTRALIAN NATIONAL UNIVERSITY Infrasound Technology Workshop, November 2007, Tokyo, Japan Introduction  Wind-generated background noise is probably the most important technical problem in the field of infrasound monitoring.  Any technique that will improve currently used wind-noise-reduction systems will significantly increase the reliability and performance of the global network.

THE AUSTRALIAN NATIONAL UNIVERSITY Infrasound Technology Workshop, November 2007, Tokyo, Japan Wind Noise at IMS Infrasound Monitoring Stations  Wind-generated noise is a potentially serous problem at stations in the IMS infrasound network.  Wind noise levels will be low at stations located in dense forests (~ 25% of all IMS stations).  Continental stations located in areas with little shelter shelter from the ambient wind will be subject to significant levels of wind-noise, especially during the daytime (~ 35% of all IMS stations).  Noise levels will often be high at stations located on small islands (~ 30% of all stations)  Stations located in the Arctic and Antarctic regions may be subject to high wind noise levels (10%).

THE AUSTRALIAN NATIONAL UNIVERSITY Infrasound Technology Workshop, November 2007, Tokyo, Japan Typical Background Noise Levels  IS07 Warramunga is located in a semi-desert environment with little protection from the ambient winds.  Noise levels at IS07 are high during the daytime and usually very low at night when the boundary layer winds are decoupled from the surface by a nocturnal radiation inversion.  A standard 18-m diameter 96-port CTBTO noise-reducing pipe array is installed at IS07 site H2 (illustrated).

THE AUSTRALIAN NATIONAL UNIVERSITY Infrasound Technology Workshop, November 2007, Tokyo, Japan Diurnal Variation of Wind Noise at IS07 Warramunga

THE AUSTRALIAN NATIONAL UNIVERSITY Infrasound Technology Workshop, November 2007, Tokyo, Japan Noise Reduction at Infrasonic Monitoring Stations  Traditionally, porous hose systems or pipe arrays with discrete inlet ports have been used to reduce wind noise.  The number of inlet ports and size of conventional pipe arrays have reached practical limits. Further refinements to conventional pipe arrays will not lead to significant improvements. Wind-noise-reducing pipe arrays Used at IMS infrasound stations.

THE AUSTRALIAN NATIONAL UNIVERSITY Infrasound Technology Workshop, November 2007, Tokyo, Japan Wind-Noise Reduction Using Porous Barriers and Wind Fences.  Frank Grover (1971) tested wind shields in the form of 1-m high x 1.1 m diameter perforated metal domes at the Blacknest UKAEA array. These devices provided only marginal noise reduction at high frequencies.  Ludwik Liszka (1972) pioneered the use of small-scale porous barriers to reduce high-frequency wind noise. These devices has also been studied by Doug ReVelle (2000) and Michael Hedlin and Jon Berger (2001). Hedlin and Berger showed that a wire mesh cover on the sides improves performance.  An important development is the use by Al Bedard (2003) of a larger scale porous wind fence with solid vertical corrugations along the top. This device was used to reduce higher frequency wind noise in a tornado warning system.

THE AUSTRALIAN NATIONAL UNIVERSITY Infrasound Technology Workshop, November 2007, Tokyo, Japan  Adaptive signal processing of data from a compact array with a large number of sensors has been proposed as an enhanced noise-reducing technique ( Bass and Shields, 2004).  Noise-reduction using an optical fiber infrasound sensor (OFIS) (Berger et al., 2000; Zumberge et al., 2001, 2003); Walker et al., 2004, 2005, 2006, 2007 ) is achieved by integrating the surface pressure field along the path of the fiber. This sensor provides very effective wind noise- reduction, especially at high frequencies.  This investigation is focussed on noise reduction using turbulence-reducing enclosures which : (a) degrade turbulent eddies near the surface and (b) lift the turbulent boundary layer above the sensor inlets. Other Wind-Noise-Reducing Techniques

THE AUSTRALIAN NATIONAL UNIVERSITY Infrasound Technology Workshop, November 2007, Tokyo, Japan Noise Reduction Using Turbulence-Reducing Enclosures Schematic diagram illustrating Versions 1 and 2 of the turbulence-reducing enclosures.

THE AUSTRALIAN NATIONAL UNIVERSITY Infrasound Technology Workshop, November 2007, Tokyo, Japan Performance: Version 2 Enclosure with 2.4 m High Walls  Significant noise reduction in monitoring passband.  Performance deteriorates rapidly in winds of more than 4.5 m/s.  Performance of Version 2 with 2.4 m-high walls is better than the performance of Version 1 with 1.6-m high walls.

THE AUSTRALIAN NATIONAL UNIVERSITY Infrasound Technology Workshop, November 2007, Tokyo, Japan Versions 3 and 4 of the Turbulence-Reducing Enclosure Schematic illustrations of Versions 3 and 4. Version 3 has one 3.2-m high porous wall. Version 4 has two 3.2 m high porous walls.

THE AUSTRALIAN NATIONAL UNIVERSITY Infrasound Technology Workshop, November 2007, Tokyo, Japan Performance of Versions 3 and 4 of the Turbulence-Reducing Enclosure  The performance of Versions 3 (one 3.2-m high wall) and 4 (two 3.2-m high walls) is only marginally better than the performance of Version 2 with a single 2.4-m high wall.  The performance of Versions 3 and 4 deteriorates rapidly in winds of more than about 5 m/s.  Increasing wall height results in only slight improvement.  Turbulence generated by the interaction of the serrations at the top of the walls with the higher winds at 3.2 m is mixed to lower levels inside the open structure.  In order to reduce turbulence inside the structure, Version 4 was modified to include baffles, multiple interior enclosures surrounding a single-port system and a porous roof.

THE AUSTRALIAN NATIONAL UNIVERSITY Infrasound Technology Workshop, November 2007, Tokyo, Japan Version 4B of the Turbulence-Reducing Enclosure Version 4B includes a) radial baffles, b) interior screened chambers around a single-port system, c) a porous roof over the inner structure and d) a 6-port pipe array.

THE AUSTRALIAN NATIONAL UNIVERSITY Infrasound Technology Workshop, November 2007, Tokyo, Japan Performance of Version 4B of the Turbulence-Reducing Enclosure Performance of Ver. 4B is significantly better than Ver. 3 and 4 Single-port at center is better than 6-port array at high frequencies.

THE AUSTRALIAN NATIONAL UNIVERSITY Infrasound Technology Workshop, November 2007, Tokyo, Japan Design Criteria for Version 5 of the Turbulence-Reducing Enclosure  Turbulence generation by the upper edge of the 3.2-m high walls in Version 4B is significant.  The installation of a porous roof over the interior chamber in Version 4B was found to be very beneficial.  In order to eliminate turbulence generated by the flow over the walls, Version 5 was designed as a closed enclosure with: A relatively low profile (maximum height 2.0 m); Screened closed chambers around the single-port system; A fairly rigid screened roof over the entire structure; Outward-facing horizontal serrations and larger downward-inclined outward-facing serrations, both of which are fastened to the upper edge of the outer wall.

THE AUSTRALIAN NATIONAL UNIVERSITY Infrasound Technology Workshop, November 2007, Tokyo, Japan Version 5: Turbulence-Reducing Enclosure Version 5 effectively eliminates turbulence created in the flow over the walls. This is accomplished by adding a screened roof over the entire structure and rows of horizontal and downward-inclined serrations fastened to the top edge of the outer wall.

THE AUSTRALIAN NATIONAL UNIVERSITY Infrasound Technology Workshop, November 2007, Tokyo, Japan Version 5 of the Turbulence-Reducing Enclosure Views of Version 5 of the turbulence-reducing enclosure with a rigid screened roof and screened interior chambers. This version of the wind-noise- reducing enclosure is very effective.

THE AUSTRALIAN NATIONAL UNIVERSITY Infrasound Technology Workshop, November 2007, Tokyo, Japan Performance: Version 5 of the Turbulence-Reducing Enclosure Single-inlet port system is more efficient at high frequencies. 6-port array is better at low frequencies. Noise is reduced by 4 orders of magnitude at high frequencies.

THE AUSTRALIAN NATIONAL UNIVERSITY Infrasound Technology Workshop, November 2007, Tokyo, Japan Signals: Version 5 of the Turbulence-Reducing Enclosure Comparison of explosion- generated signals recorded simultaneously on a single port system and a 6-port pipe array located inside Version 5 of the enclosure with the signal recorded on a single-port system located outside the enclosure. Signals are not attenuated or distorted by the enclosure.

THE AUSTRALIAN NATIONAL UNIVERSITY Infrasound Technology Workshop, November 2007, Tokyo, Japan Wind-Noise-Reduction Version 5 Turbulence-Reducing Enclosure Comparison of background noise in the monitoring passband recorded simultaneously on a single inlet port system (green) and a 6-port pipe array system (blue) located inside the enclosure with background noise recorded on a single reference port (red) located outside the enclosure

THE AUSTRALIAN NATIONAL UNIVERSITY Infrasound Technology Workshop, November 2007, Tokyo, Japan Summary: Wind Noise Reduction  Wind-generated noise is attenuated by up to 4 orders of magnitude by Version 5 of the enclosure.  Noise reduction provided by Version 5 of the enclosure when used with only a single inlet port system is more than one order of magnitude better than the noise reduction provided by a conventional 18-m diameter 96-port pipe array at higher frequencies.  The performance of a turbulence-reducing enclosure at low frequencies can be improved by increasing the diameter of the enclosure.

THE AUSTRALIAN NATIONAL UNIVERSITY Infrasound Technology Workshop, November 2007, Tokyo, Japan Conclusions  Turbulence-reducing enclosures can be used to effectively reduce wind-generated noise at infrasound stations in the global network.  The new system does not necessarily require a pipe array. However, the performance of existing pipe arrays can be enhanced considerably if the pipe array is placed inside a turbulence-reducing enclosure.  It is therefore recommended that existing pipe arrays located in high wind environments should be used in conjunction with turbulence-reducing enclosures.

THE AUSTRALIAN NATIONAL UNIVERSITY Infrasound Technology Workshop, November 2007, Tokyo, Japan