JD-EE, That's where the significance of the air percussion guns come into play. The ELF (low audio spectrum) pulse is used because of its ability to penetrate the inversion layers as well as deep into the terra firma beneath the seabed. Lower frequency means "better" in this type of seismic operation, and the resolution is improved by the difference signal from a tandem operation.
- from Woods Hole Oceanographic Institution ....
The air guns are relatively deep penetration sources, operating at 100Hz to about 1200 Hz, to identify subsurface geologic layers and define the subsurface structure. In studies that require less resolution but substantial penetration, the air gun is usually preferable as compared to a water gun, because it is far more efficient at producing low frequency energy.
For those not familiar with the ULB pinger issue, it is the subsurface inversion layers that have been fingered for the non detection of the AF447 pinger, and hence the BEA recommendation that the frequency be lowered from
37.5kHz to a nominal
9kHz.
But back to seismic detection of aircraft crashes. It should be remembered that the WTC Twin Towers aircraft impacts were recorded, i.e. the buildings moved, but no meaningful amplitudes were measured over 200 miles miles away. Likewise, it is debatable that the UA93 impact at Shanksville, PA, was actually recorded due to the 3 minutes difference between DFDR/ATC and seismometer times. Perhaps the difference could mean that the vertical percussion wave generated travelled some distance into the earth before being reflected, as signal amplitudes just above the background noise were recorded and triangulation pointed to the crash site.
mm43