grizzled
So, if no seismic event was recorded within the appropriate time frames for AF447, can one conclude that:
1 The aircraft did not impact the water with sufficient force to leave such a signature (which would tend to eliminate a scenario of an "uncontrolled plunge")? Or,
2 There may not be sufficiently sensitive seismic information available for that area? (I have no idea the practical science / instrumentation behind this, but I do know the USGS, for instance, receives / records even very small seismic events from all over the globe.)
I am assuming that no such event was recorded because, if it were, the impact location would be known.
Way back in the mid 60s when I was a student at the University of Michigan I worked for one of their geophysical research labs, a seismology department. One experiment was field work recording blasts that took place off Delaware in the ocean using small surface seismometers in various places around the Northeast and Midwest as far east as Western Indiana. The blasts were something like 100# charges dropped into the ocean. They were visibly detectable in raw data into Northern Michigan from well sited instruments.
At the time of the experiments conditions were ideal. The ocean was as calm as it ever gets off Delaware. And there were no storms in between. That is one data point.
The second experiment I aided with was a precursor to something I kibitzed professionally 5 years later - using seismometers for intrusion detection with the intention of implementing it in Vietnam. Footsteps were very plain and very hard to disguise with shuffling and such. DC3s were uncannily obvious in the seismometer tape recordings WAY before the voice track could discern the plane's propellers. And there was a way to mask footsteps. Heavy winds made it difficult, and a DC3 going overhead also made it difficult.
That's a second data point.
If Swiss Air 111 hit the water within about 125 miles of Nova Scotia then the coupling for the event into the ground would be pretty good. (The Delaware tests were in comparatively shallow water.) The coupling for AF447 would be bad, it would have to hit with enough energy that it left a signature easily distinguishable from storm events, and you'd have had to have sensors closer to the event than the nearest real shoreline. Or such is my guess from limited but quite pertinent experience.
Hydrophones might have had a better chance. And with vagaries of sound transmission in the ocean I am not convinced they'd do a whole lot better at locating the crash. They might be able to detect the crash event, though.
Your idea was a good one had the water been substantially shallower and the crash been closer to land. As it is I'd consider it a bad bet. But at this juncture a bad bet may be better than none at all, such as after they've gone through mm43's magic triangle.
{^_^}