Radio in the Tunnel

[Robert S Chase]

I accept all of that but have a further question, the submarines use a very 
low frequency and very high power system to receive their orders and other 
essential communications under water. Given the AM ground wave attenuates 
and the FM is actually line of sight leaking in, how come the submarine 
communications at VVLF works and just as matter physics, which has more 
capability of penetrating the ground with signal, the AM or higher frequency 
FM?
>
> AM signals are more susceptible to tunnel signal attenuation than FM
> signals are, but parts of the Big Dig are 120ft below street level
> (that's the area under Dewey Square)...unless you've got a very powerful
> signal hitting the street level to begin with...that 120ft is going to
> provide some serious shielding/attenuation.  Ditto for the Ted Williams
> which is roughly 90ft below sea level.