MM43 wrote:
The BEA have said that should debris not be located in this zone by 12 May, the search area will be extended. Should that occur, then the ducting theory will have been correct, but the path loss is such that the search zone would normally have taken that into account.
If the sound does become ducted, then from that point on, spherical spreading no longer applies, so the amplitude would not drop as much with increasing range. The closest model may be circular spreading (if the sound is trapped in planar layers), or some other depending on the boundary constraints. If this happens, the detection range, as measured along the ray path can be extended. The attenuation loss continues to apply, but is a function of temperature, salinity and pressure, and thus will be somewhat different for a near-surface path than for the deep ocean path. Within a duct, there are usually multi-paths, and these interfere in ways that can decrease or increase the sound level at various points in the duct. This scenario assumes the receiver is also in the duct or is at a point where previously ducted sound is escaping the duct.. I have no idea if any such duct existed at the place and depth that Emeraude recorded the signals. A local sound speed profile would shed light on that, but one likely does not exist close enough to place and time.
I'm not sure what conditions have to exist for sound from a deep water source to enter a duct. It seems like the angle of incidence would have to be far from vertical, which would increase the path length (slant range) making for an already weak signal at entry. I was assuming previously that useful ducting was unlikely on that basis, but who knows.