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The Infrasound Network (Isnet): Background, Design Details, and Display Capability As an 88D Adjunct Tornado Detection Tool

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The Infrasound Network (Isnet): Background, Design Details, and Display Capability As an 88D Adjunct Tornado Detection Tool 1.1 THE INFRASOUND NETWORK (ISNET): BACKGROUND, DESIGN DETAILS, AND DISPLAY CAPABILITY AS AN 88D ADJUNCT TORNADO DETECTION TOOL 1 A. J. Bedard, Jr.*, B. W. Bartram, A. N. Keane, D. C. Welsh, R. T. Nishiyama National Oceanic and Atmospheric Administration, Environmental Technology Laboratory, Boulder, Colorado 1Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado 1. INTRODUCTION A frequency of 1 Hz is eight octaves below middle C. The lowest frequency on a piano keyboard Results from a series of field experiments is A at 27.5 Hz, consistent with being near the lowest indicated that detection of infrasound from tornadic limits of a typical person’s hearing. Acousticians have storms might have potential for tornado detection and adopted the convention of defining sound levels warning. A demonstration infrasonic network was relative to the threshold of human hearing. However, designed and deployed during the spring and infrasonic signals can be quite valuable and provide summer of 2003. This paper reviews the system information on a range of geophysical processes. capabilities and design elements, as well as a new More background on infrasonics may be found at the “eddy fence” that permits detection of low-level web site http://www.etl.noaa.gov/et1/infrasound/. infrasonic signals in the presence of high winds. A critical need was to present the acoustic data in 140 -102 essentially real time in forms readily interpreted and THRESHOLD OF PAIN 120 compared with radar data. The web displays -101 developed are described using data obtained during JET ENGINE 100 ROCK MUSIC the 2003 demonstration project. -100 INFRASOUND VACUUM CLEANER This background paper describes infrasonic 80 -1 T -10 H detection of tornadoes, tornadic storms, and funnels RESHOL Pa 60 SPEECH D -10-2 under various conditions and summarizes warning GEOPHYSICAL SOURCES OF HE times for several case studies. The near-infrasound Sound Pressure,dB A 40 RIN SURF -3 systems applied in this research provide a new way G -10 TYPICAL LIMIT OF PRESSURE of “hearing” low-frequency sounds in the atmosphere 20 and focus on a different, higher frequency window SENSOR SENSITIVITY -10-4 than historical infrasonic research dating to the 0 1970’s. The purposes of this paper are to provide 0.11 10 100 1000 Frequency, Hz background for the displays provided for visualizing FIG. 1. Sound pressure amplitude as a function of frequency Infrasonic Network (ISNet) data and to describe the compared with the threshold of human hearing at lower parameters measured. frequencies. (Bedard and Georges, 2000) 2. WHAT IS INFRASOUND? 3. WHAT SENSORS AND TECHNIQUES ARE REQUIRED FOR THE DETECTION OF THESE Infrasound is the range of acoustic frequencies LOW-LEVEL, LOW FREQUENCY SOUNDS? below the audible (Bedard and Georges, 2000). For a typical person, this is at frequencies below about 20 A critical need was an effective method for Hz, which is where the threshold of human hearing reducing noise from large but highly spatially and feeling crossover. There is a rational analog incoherent pressure fluctuations, while still detecting between infrasound, sound and the relationship of infrasonic signals. Daniels (1959) made the critical infrared to visible light. Thus, one can call the breakthrough with the development of a noise frequency range 1 to 20 Hz near-infrasound and the reducing line microphone. His concept was to match range from about 0.05 to 1 Hz infrasound. Below the impedance along a pneumatic transmission line about 0.05 Hz where gravity becomes important for using distributed input ports and pipe size changes to propagation, atmospheric waves are usually called minimize attenuation. This innovation exploited the acoustic/gravity waves. Figure 1 indicates the sound high spatial coherence of infrasound and the small pressure levels as a function of frequency with the spatial coherence of pressure changes related to threshold of human hearing as a reference. turbulence, and provided signal-to-noise ratio improvements on the order of 20 dB. Variations of his concept are currently in use worldwide. Figure 2 is a * Corresponding author address: Alfred J. Bedard, Jr. National photo of a spatial filter using twelve radial arms with Oceanic and Atmospheric Administration, Environmental ports at one-foot intervals and covering a diameter of Technology Laboratory, Boulder, CO 80305-3328; e-mail: fifty feet. We have adapted porous irrigation hose for [email protected]. use as a distributed pressure signal transmission line shows a typical array layout. Usually, an infrasonic (an infrasonic noise reducer), saving considerable observatory consists of four sensors equipped with costs in fabrication and maintenance. The present spatial filters in a roughly square configuration. The wind noise reducers can look a lot like an octopus twelve porous hoses are often 50-feet in length and (with four extra arms) with black porous hoses bent back toward the center so that the complete radiating outward from a central sensor. filter covers a diameter of about 50 feet. These noise reducers can operate in rain and snow without affecting infrasonic signals. Vegetation can reduce wind in the lower boundary layer and further improve wind noise reduction. Array Geometry 25 feet 25 feet 25 feet 25 feet FIG. 2. Photograph of a spatial filter for reducing wind-induced pressure fluctuations in the infrasonic frequency range. 40 to 80 meter typical spacings In addition, the development of an “eddy fence” Fig. 4. View of a typical infrasonic observatory array (Figure 3) provides essentially all-weather detection configuration. The exact positioning of the twelve porous capability. For example, we estimate that in the irrigation hoses radiating outward from each of the four sensors -1 presence of winds of 30 ms , signals from distant is not critical. (>200 km range) tornadic storms will be detectable. The fence raises the atmospheric boundary layer and Conventional audio microphones do not respond the corrugations at the top break up the wind shear to infrasonic frequencies. In addition to the fact that layer into small, random eddies. there was no practical need to detect inaudible sounds, extended low frequency sensitivity could cause undesirable dynamic response limitations. Thus, a microphone having a diaphragm deflecting in response to sound waves typically has a leakage path to a reference volume behind the membrane. This permits frequencies below the cutoff of such a high pass filter to appear on both sides of the sensor, canceling response. We use sensitive differential pressure sensors FIG. 3. Photograph of the 50 foot diameter eddy fence. The integrated with a large, well-defined reference lower portion covered with a porous green fencing material is 6 volume and calibrated flow resistor providing a stable feet high. high-pass filter time constant. The volume is A reasonable assumption, for sound waves from insulated to create a stable temperature environment. point sources after traveling paths of tens or These sensors are rugged, relatively small (less than hundreds of wavelengths, is that the wave fronts are 2×2×2 feet), and relatively light (about 22 pounds). planar. This model is usually used in processing data They typically operate for years with few problems. from infrasonic observing systems and was the basis Because arrays of sensors are applied to detect and for the design of beam steering algorithms developed process signals, there is a need to match all of the to determine correlation coefficient, azimuth, and microphone sensitivities and phase characteristics. horizontal phase speed. Effective separations for This required the creation of a family of static and microphones in arrays are usually about one-fourth of dynamic pressure calibration techniques, as well as the primary acoustic wavelength to be detected. In methods for measuring flow resistance. The addition, the fact that infrasonic signals show little infrasonic sensors are carefully matched and change with distance is the basis of the method for interchangeable. Table 1 summarizes the parameters the reduction of unwanted pressure noise. Figure 4 measured by infrasonic observing systems. Parameter Significance Correlation coefficient, a measure Index of signal quality and a measure of confidence in the accuracy of other of signal-to-noise ratio (S/N) parameters Azimuth Critical measure of the direction from which sound is originating Phase speed Angle-of-arrival can indicate the location of source regions aloft (Indicating the elevation angle) Spectral content The dominant frequency (frequencies) can indicate important characteristics of the sources Sound pressure level The amplitude, although a measure of source strength, is greatly affected by propagation Duration Typical vortex-related signals continue for tens of minutes or more. Signals of very short durations (e.g. ~one minute) are unlikely to be related to a coherent vortex Persistence Even at low S/Ns, confidence can be obtained in the characterization of signal sources if they persist for significant periods of time. Histograms of parameters over intervals can quantify distributions and, for example, identify source direction even for weak signals TABLE 1. Parameters measured by infrasonic observatories and their significance. 4. AT WHAT DISTANCES FROM A SOURCE CAN 1980’s as a part of a global observing network and INFRASOUND BE DETECTED? the higher frequency near-infrasound systems now in use. Several features of the global observing system Cook (1962) has shown how unimportant (including the array dimensions, the spatial filter, and molecular attenuation is to infrasonic propagation -8 the sensor itself) combined to limit the high frequency (5×10 dB/km), whereas sound at 2 kHz will response to below 0.5 Hz. The severe weather- attenuate at 5 dB/km. Since infrasound below 1 Hz is related infrasound reported in the literature from the virtually unattenuated by atmospheric absorption, it is historic global infrasonic network involves sound two detectable at distances of thousands of kilometers orders in magnitude longer in wavelength (λ) than the from the source.
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