Sam
Sounds about right, however, you assume detonation at the surface (?). If that weapon was a delayed charge that detonated some 30 metres underground then it would have minimal impact within 10 Km (dirty fallout & wind direction ignored). If I wanted to launch a nuclear bomb on Israel I would ensure that it was on Tel Aviv with an atmospheric detonator to explode at 5-10 thousand feet. No "dirty radioactive" dustcloud, but maximum effect to destroy most of Israel instantly.
Depending upon what the local earth/rock is made of, getting the bomb about 30M into the ground before detonating is no small trick.
Can it be done? No comment. Maybe, maybe not.
The idea of the conventional bomb isn't that it waits until 200 feet deep to detonate, it's that its energy is sufficient to reach deep into the substrate once it penetrates as far as it can.
@ Willard: I agree that dropping bombs on Iran doesn't seem to resolve the underlying political problem very much. It may allow blowing up those buildings in the nuclear program that the Iranians are't blowing up themselves. Last I read, the 22 Nov event was a controlled demolition of some buildings on that site, done by the Iranian authorities for their own reasons. (Covering tracks? Contamination? Who knows?)
As to today's blast ... which occurred but then didn't occur ... who knows?