I did some last test of submerged-submerged situations. I have finally cleared all my doubts and I updated (for the last time I hope) the first post once again.
It showed that in the end in DW the sound transmission is symmetric. It means when A hears B, B will hear A at exactly same quality. So if surface listener hears sub bellow the layer with some transmission loss, the sub will hear the surface boat with same loss. If sub bellow the layer is in the shadow zone, surface boat will not be able to hear it and also the sub will not be able to hear the surface boat. Surface won't be able to ping the sub: sub won't hear the pings, and there will be no returns from the sub. This all is correct and realistic.
Please note that this can seem to be broken sometimes, since for example FFG uses long towed array. So it can seem to the array that sub is lost, but for FFG it is not in shadow zone yet and it still can ping it. That's what fooled me few times.
So what we already now, but inverted: sub above the layer can be heard normally. Sub bellow the layer can be heard somewhat worse (30 SNR to 20 SNR, 3 lines to 1 line). Active sonar is affected in similar way. The 'dot' is smaller and you can hear the return. Sub in the shadow zone can't be heard nor pinged.
Now what we didn't know. When the listener is under the layer too, it can hear the sub-layer target even in the shadow zone and the detection range is limited only by distance and gradual signal weakening. That was to be expected. But what is little surprise, the signal too will be weaker then if both target and listener were above the layer. Simply bellow the layer there are worse conditions.
Check the first post for final conclusions.
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