A series of medium‐size explosions are monitored by a variety of ground sensors. A preliminary study of one typical explosion at 250 meters above the ocean surface has revealed many interesting results; from the acoustical signal the effects of the atmosphere on infrasonic waves generated as a pulse have been identified both as a function of frequency and as a function of distance. VLF and ELF electromagnetic signals have been observed by stations more than 100 kilometers from the explosion center. The signals are described fully, and the experimental objectives are summarized.
A system is discussed for the instrumentation of infrasonic and hydromagnetic waves traveling through the atmosphere and the ionosphere. Two large missiles were used as the sources of energy. The advantages of the instrumentation system are: a minimum of manpower and time is required to calibrate and place the system into operation; the system is easily transported; and the system is very stable over long periods of time. A description of the missile engines as a moving source of energy in the infrasonic-frequency spectrum (20–0.025 cps) was obtained. It is concluded that the instrumentation system has the capability of providing reliable data on long period waves and that the equipment is stable.
Large quantities of ultra low-frequency acoustic data accumulated over long periods of time can be analyzed by several methods. Preliminary analysis of the analog waveform determines specific areas of primary interest prior to digital-computer analysis. Data of no consequence can be discarded; or, the worth of such data may be graded, to scale their importance for computer processing. This technique also provides a means of expediting initial findings, and primary conclusions can be deduced. This study was conducted by the University of California, Los Angeles, and entitled, “Ionospheric Disturbance Experiment.” The raw tape-recorded data are played back through an electronic system. Some important parts of this system are narrow bandpass filters and a Missilyzer (sound spectrograph). Some sample results are summarized.
The source characteristics of the Saturn F-1 engine in the infrasonic frequency range of 0.1–20 Hz and the seismic and magnetic fluctuations from 4 to 16 Hz have been ascertained. The acoustic signals have shown a characteristic ignition and cutoff pulse. The ignition signal shows a typical explosion signature (N-type wave), while the cutoff signal produces a bubble pulse-type effect, which might have resulted from an implosion structure. The engine run produces a steady state signal. However, its acoustic energy peak in the ultralow frequency range is a function of the engine-run duration. The power spectrum shows energy distribution in two regions, with a maxima at 4–8 Hz and at approximately 1.4 Hz, with energy extending to about 0.1 Hz. The lower frequency peak becomes more significant with the increase in the engine-run duration. The magnetic signals have shown a narrow-band signal between 8 and 12 Hz during the engine run and a high-intensity cutoff signal of a wider band. The cutoff signal resembles a pulse. The downward exhaust of the static running of a rocket motor is a source for ultralow frequency acoustic and magnetic waves. Similar conclusions are drawn for the behavior of an Atlas rocket during the first few kilometers of flight.
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