I wondered if other passive methods (radiography) would be applicable but they all seem to rely upon having a sensor behind the subject. I suppose your suggestion would be something like sonar, however I cannot find any clear references to the difficulties of the applications of such techniques through solids. Did find this interesting tidbit, though: http://en.wikipedia.org/wiki/Anti-frogman_techniques#Ultraso...
Good links. I think I'd seen one of them before but still very interesting in aggregate. Perhaps time to take to the off-grid lifestyle, or perhaps the oceans?
I wondered if other passive methods (radiography) would be applicable but they all seem to rely upon having a sensor behind the subject. I suppose your suggestion would be something like sonar, however I cannot find any clear references to the difficulties of the applications of such techniques through solids. Did find this interesting tidbit, though: http://en.wikipedia.org/wiki/Anti-frogman_techniques#Ultraso...