In a very recent biography of Nikola Tesla by Bernard Carlson, the author explains that Tesla measured the resonant frequency of the earth using his invention - the amplifying transmitter. Tesla came up with a number ~ 8.5 Hz. His belief was that this represented a standing wave that traveled between his point of excitation through the ground to the opposite side of the globe and back again.
It appears that Tesla was not measuring resonance within the earth but rather the Schuman Resonance, which is an atmospheric resonance; an ELF oscillation of the ionosphere.
I would suppose that Tesla was not actually exciting the ionosphere, but rather passively monitoring this resonance with the coils of his invention.
My question gets back to Tesla's original quest:
Is there an internal resonance of the earth? Or is the structure just so complex/diverse that energy on any achievable global scale just cannot propagate interfering waves that lead to a standing wave?