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Whitaker Infrasound JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, D08117, doi:10.1029/2007JD008852, 2008 Click Here for Full Article A comparison of infrasound signals refracted from stratospheric and thermospheric altitudes Rodney W. Whitaker1 and J. Paul Mutschlecner2 Received 19 April 2007; revised 24 October 2007; accepted 16 December 2007; published 29 April 2008. [1] Over the last several years, a large number of ground-based infrasound arrays have been established for explosion monitoring as part of the International Monitoring System of the Comprehensive Nuclear Test Ban Treaty Organization. Results from these arrays have become valuable in understanding long-range infrasound propagation in the atmosphere and complement earlier data. Two types of signals are often observed for a given source: those refracted at stratospheric heights and at thermospheric heights. In this contribution we compare several characteristics of these two signal types including observed pressure amplitude, average travel velocity, bearing, and signal duration. For this study we use archival data from atmospheric nuclear explosions, high explosive chemical explosions on the surface, earthquakes with previously detected stratospheric signals, and earthquakes for which stratospheric signals were not detected. We show that the combination of stratospheric and thermospheric signals may provide independent estimates of wind propagation conditions and source characteristics. The observations of both signal types may increase the confidence level of an infrasound signal detection. Citation: Whitaker, R. W., and J. P. Mutschlecner (2008), A comparison of infrasound signals refracted from stratospheric and thermospheric altitudes, J. Geophys. Res., 113, D08117, doi:10.1029/2007JD008852. 1. Introduction [4] In order to identify and interpret infrasound signals it is necessary to differentiate between S and T signals by [2] Infrasound signal detection and interpretation is a understanding their characteristics. This paper provides an significant field of research providing information on source effort to define and to compare those characteristics. As this identification and characteristics. Sources of infrasound work shows, atmospheric winds produce a very strong include earthquakes, explosions, bolides, avalanches, influence on the S signal amplitudes while the T signals weather disturbances and others. Currently the International are relatively independent of the wind structure. This allows Monitoring System of the Comprehensive Nuclear Test Ban for a potential determination of a stratospheric wind param- Treaty Organization operates about 40 infrasound arrays eter by use of S and T signal amplitudes. globally as part of an eventual 60 station network. Others [5] Figure 2 presents an example of an important differ- arrays are operated by various organizations for specific ence between the S and T signals. Figure 2 shows infra- research purposes. Such arrays, of low-frequency micro- sonic pressure amplitudes from Nevada Test Site (NTS) phones, detect the small pressure amplitude signals from atmospheric nuclear explosions observed at Bishop, CA, at distant sources and standard array processing tools provide approximately one infrasonic bounce distance nearly di- data on source direction (bearing or back azimuth), frequency rectly to the west. The observed thermospheric and strato- content, trace velocity, and correlation among others. spheric amplitudes, AT and AS respectively, have been [3] Infrasonic signals are acoustic waves propagated normalized to a nuclear yield of 1 kiloton HE equivalent through the atmosphere and can be detected at long ranges by the use of from the source. The temperature structure and atmospheric winds determine the propagation of the infrasonic signals 0 0:456 and the refraction heights from which energy is turned. Two ATorS ¼ ATorS Â ðÞW=500 ð1Þ of the most common types of returns are from the strato- 0 0 sphere, near 50 km altitude and referred to here as S signals, where A T and A S are the yield scaled amplitudes, and W is the event yield in kilotons. A kiloton of nuclear yield is and from the thermosphere, around 110 km altitude and 12 referred to here as T signals. Figure 1a illustrates these two equivalent to 4.18 Â 10 joules of energy. The scaling law paths in an acoustic ray trace model. is taken from Mutschlecner et al. [1999] (hereinafter referred to as MWA). ATorSand other amplitudes discussed throughout are peak-to-peak pressures in mbars. The source of these data is described in section 2. 1 Los Alamos National Laboratory, Los Alamos, New Mexico, USA. [6] The significant difference between the two types of 2 Comforce, Inc., Los Alamos, New Mexico, USA. signals is apparent: the S signals have a very strong seasonal variation with an amplitude swing of nearly two decades Copyright 2008 by the American Geophysical Union. 0148-0227/08/2007JD008852$09.00 while the T signals have no apparent seasonal variation. The D08117 1of13 D08117 WHITAKER AND MUTSCHLECNER: THERMOSPHERIC INFRASOUND SIGNALS D08117 Figure 1. (a) A sample acoustic ray trace is shown. The rays illustrate the return of infrasound from the stratospheric region at about 50 km and from the thermospheric region at about 110 km. (b) An example of the sound velocity versus altitude in the atmosphere (no wind) is shown. Figure 2. Log of S (diamonds) and T (dots) signal amplitudes versus day of year for nuclear explosions observed at Bishop, CA. Both amplitudes are normalized to 1 kT; the T log amplitudes have been shifted down by two units to provide separation of the two sets of points. While the S signal values show a clear seasonal variation the T signals values appear to have no seasonal changes. 2of13 D08117 WHITAKER AND MUTSCHLECNER: THERMOSPHERIC INFRASOUND SIGNALS D08117 S signal variation depends upon the direction to the source. 2.2. Travel Velocity For example, observations at St. George, UT, nearly directly [12] Figure 3a shows the T signal velocity, VT, for the to the east of the NTS, have a phase about a half year stations at St. George, Bishop and China Lake. VT is the different from the Bishop signal trend. great circle distance from source to receiver divided by the [7] These differences between the S and T signal varia- signal transit time. The averages are: St. George, 228m/s; tions can be understood by examination of the height versus Bishop, 218 m/s; and China Lake, 209 m/s. There is sound velocity profile of the atmosphere. Figure 1b gives an reasonable consistency, especially for the east and west example of the profile. It can be shown in an acoustic ray stations. The values of VT cover a fairly large range from trace model that the return of a given ray will occur where about 200 to 260 m/s; there is no indication of seasonal the total sound velocity, i.e., thermal velocity and horizontal variation. By comparison S signal average velocities, VS, component of the wind velocity, matches the sound velocity were shown (MWA) to have a seasonal variation at all at the source location. In general this can occur efficiently stations which appears to be related to the stratospheric for S waves near the stratopause when there is a sufficient wind. An average value for the S signals is 294 m/s. wind velocity component added to the sound velocity. The Figure 3b shows spline fits for the S signal velocities for strength of the return is increased with the strength of the the three stations. Seasonal variations can be seen that are as wind component. Thus the S signals will vary as the winds large as about 14% in the fits. The variations are azimuth- near the stratopause change seasonally. This condition has ally dependent, and their amplitudes are maximum to the been examined empirically by MWA. east or west of the source and minimum to the north or [8] In contrast, as Figure 1b shows, in the thermosphere, south of the source. It is important to note that these results at about 110 km altitude, the increasing thermal sound are for stations at approximately one bounce distance from velocity is sufficient to produce T signals, by refraction at the source. these heights, without the necessity of a wind component. [13] Under a simplifying assumption that the S and T Thus the T signals can be present throughout the year average velocities primarily reflect the propagation path without seasonal variation. distances we can predict the velocity ratio, VS/VT,bythe [9] It must be noted in this simplified discussion that S reciprocal of the path lengths for a single bounce signal. signals from sufficiently large sources are observed even This is roughly 330 km/270 km or 1.22. For comparison during counterwind periods (that is when the wind direction the average S/T velocity ratio is about 294 m/s/220 m/s is counter to the direction of propagation), as seen in Figure or 1.34. This suggests that a substantial part of the S 2, probably as a result of partial reflections from locally and T velocity differences are produced by the path layered atmospheric structure. This has been discussed by differences. Kulichkov [2000], and Godin and Naugolnykh [2005] have discussed the effects of atmospheric diffraction in a guided 2.3. Frequency of Detection mode model, which may contribute to counter wind signals. [14] The percentage of events with T signals is only 40% Similarly, T signal arrivals are seen in zones not predicted for St. George, 57% for Bishop and 67% for China Lake. by ray trace modeling. A full understanding of S and T By contrast, S signals were detected on average for 95% of signal propagation will ultimately require the use of a full the events. At St. George there were five T signals observed wave model combined with observations. with no corresponding S signals or 5%. All of those were [10] For this investigation four data sets have been during the ‘‘summer’’ or counter-wind period for the station employed: (1) atmospheric nuclear explosions, (2) high when the S signals have a greatly reduced amplitude.
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