OK sure, you know the thing exploded because the ground shook. That doesn’t say a whole lot. Are there seismic measurements or calculations on the displacement of earth that tell you it blew up with the expected destructive power and that everything else went as planned? Would you do the measurements differently if it was beneath you rather than buried in a mountain tunnel?
Suppose the pop was less than expected or didn’t go off at all? It’s not like you can dig it up and see what failed.
Presumably there are more tools to assist the evaluation than there were in the Fifties. Does the US share the improved methods they developed or does everyone have to start from scratch to figure out to detonate one of these buggers?
IIRC, they did things like drive long lateral shafts to the actual bomb itself (before detonation) to measure various parameters.
So if you had a fizzle (the technical term believe it or not), they’d have basically watched it happen.
All the non-obvious stuff is still super-secret and probably only shared with the UK, if at all. Why would we want to publish it and make it any easier for N. Korea, Iran, Pakistan or India to get or improve their bombs?
The amount of ground-shaking is mathematically related to the yield of the weapon, so you can get very precise information about the explosion. Seismic sensors (like would be used for earthquakes) are part of the sensors used.
It was brought up in a recent thread about supercomputing that predicting the yield of nuclear weapons was very difficult back in the day, so your only good way to evaluate a particular configuration was to set it off and measure what happened.
The sites for testing were chosen based on population and risk of contamination. Deserts were good for the US because they had few people and little water, but many tests were carried out elsewhere, including Pacific islands/atolls.
bump has it right. The detonation takes place in a chamber, but the observations are from other chambers down long, long tunnels and tubes (sometimes a kilometer or more). Sometimes the tubes remain open and the blast destroys whatever’s at the other end - but not before it can transmit its information elsewhere - and sometimes there are explosive-driven shutters that slam shut, containing the blast wave before it gets to the test equipment, which have already recorded speed-of-light (or nearly-so) events.