I'm new here, and also not having any sonar experience, just some basics in acoustics. I overthought the may 6 - 12 "navy_search" and based on this drafted a small extended search zone, maybe someone more experienced comment on this. ( I think that the french navy's signal detection has more precise position compared to the backdrifted vertical stabilizer, just some interpretation is missing from the results ).
the following contains a lot of guesses:
- first guess ( may be ruled out by navy ): the navy used a signal processing which traded in directional sensitivity for signal to noise ratio, to rise the signal above background noise, this way the direction information was lost, they could only tell the location of Emeraude where the signal could be rised from background noise. Based on this, probably line shape, BEA made a search zone in the long axis of this position with a few nautical mile border. In this case the highest signal level probably was detected, with the Emeraude in the center of the navy_search_zone.
- the search in this zone found nothing, but if the navy could pesuade BEA to look there, then the detection had to be strong enough, but if nothing was there then the signal maybe travelled there on other ways. The attenuation of the pinger signal on this frequency doesn't allow direct detection from great distances sideways from the ocean bottom. my guess is: a natural amplifier had to amplify the signal to be detectable from greater distance.
- the vertical center of the sofar channel, based in the june 6 BEA celerity data, was in 700 - 900 m depth
- as far as I know sofar channel entered sound reaches specific depth in more or less circular patterns/spots, the attenuation in horizontal distance, and the max height reached during propagation depends on incident angle of the sound, a relatively shallow incident angle causes sound to propagate further, but not reaching high enough, a close to vertical incident angle enables reaching the surface but not propagating horizontally.
- there is an underwater mount ( red in bathymetry ) North-East from the supposed Emeraude position, if the color code is identical with the earlier published BEA bathymetry scale, then the top of the mountains are in the middle, middle-bottom of the sofar channel.
- the South-West side of this mountain is very steap, and three few nm sized parabolic like shapes may be identified on it.
- my guess is: a pinger located around the verical focal plane of these parabolas on the ocean bottom could concentrate the pinger signal into
the sofar channel, the incident angle beeing not horizontal, the signal travelled in the sofar channel with larger vertical swings, and one of these swings reached Emeraude.
Emeraude could detect it only at the place where the upper peaks in the channel were, and because swing length can be significant, maybe this was the first place from the mountain, or maybe further places can be identified with the same processing which was done with sonar data ( but if the Emeraude track distance was not enough fine, during the first search, then further refinement may not be made ).
I don't say that these shapes are perfectly formed parabolas, in the need of acoustic reflections ( if I estimate correctly then pinger wavelength is around 4 cm ) but may contain alligned surfaces wich concentrate waves.
I think I would take a look at S-W from the mountain, where the base of the perpendicular from the Emeraude highest detection position to the mountain is.
- if the wreck is there then the mountain blocked any signals into the direction of the largest part of the search zone
- there is a wide flat bottom area, between the Emeraude and the mountains, which may helped the propagation.
- if the pinger had been located on top of the mountain in 800 - 1000 m depth then it could also penetrate the sofar channel, but then it had to be heard from other positions also.
- if the pinger had been located further to the N-E direction then the mountain would block at least partially the sofar channel, preventing it to reach Emeraude in the middle of the navy_search area.
- the published diving depth for the french rubis class submarines are 300 m, and if the published rating is "low profile" then it could be in a depth of 400 - 500 m-s which made it possible to pick up the pinger signal.
Further bathimetry, west from the navy_search_zone was not done, but if similar sea mounts are present west of it but close to the navy search zone,
I would look there also.
I made this simple drawing based on the BEA bathymetry, I would search the yellow bordered area, and if I had time constraints I would take a look at least at the middle and the south-east one of the three parabolic shaped formations.