Air-Coupled Seismic Waves at Long Range from Apollo Launchings |
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| Authors: | William L Donn Ilmars Dalins Vincent McCarty Maurice Ewing George Kaschak |
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| Institution: | Lament-Doherty Geological Observatory, Columbia University, Palisades, N.Y. 10964.;Marshall Space Flight Center, NASA, Huntsville, Alabama.;The University of Alabama, Huntsville, Alabama.;U.S. Army Electronics Command, Army Signal Corps, Fort Monmouth, New Jersey. |
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| Abstract: | Summary Microphones and seismographs were co-located in arrays on Skidaway Island, Georgia, for the launchings of Apollo 13 and 14, 374 km to the south. Simultaneous acoustic and seismic waves were recorded for both events at times appropriate to the arrival of the acoustic waves from the source. Significant comparisons of the true signals are (1) the acoustic signal is relatively broadband compared to the nearly monochromatic seismic signal; (2) the seismic signal is much more continuous than the more pulse-like acoustic signal; (3) ground loading from the pressure variations of the acoustic waves is shown to be too small to account for the seismic waves; (4) the measured phase velocities of both acoustic and seismic waves across the local instrument arrays differ by less than 6 per cent and possibly 3 per cent if experimental error is included. It is concluded that the seismic waves are generated by resonant coupling to the acoustic waves along some 10 km of path on Skidaway Island. The thickness of unconsolidated sediment on the island is appropriate to a resonant ground wave frequency of 3.5 to 4 Hz, as observed. Under appropriate conditions, ground wave observations may prove more effective means of detecting certain aspects of acoustic signals in view of the filtering of wind noise and amplification through resonance. |
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