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 At and near the surface, assuming the Tx antenna is at the surface, you have to deal with both groundwave and skywave in several modes. The groundwave amplitude depends on surface conductivity and dielectric constant, which varies wildly from ice to seawater and everything in-between. That would have to be experimentally mapped. The skywaves depend on time of day and ionospheric effects.
This suggests that the real issue will be an accurate map of the various losses. So, it all comes down to how much money and time you are willing to invest.

 Why is the BW so bad? Antennas. You have chosen a wavelength for which there is literally no such thing as an reasonable effective broadband radiator. 300 miles of buried cables in Michigan does not count as practical.
If we forget about Tx modulation for a moment and solely focus on the presence of a signal at a given frequency our Rx antenna can be something like a ball antenna. Any antenna will receive on all frequencies, so its really a DSP issue to detect the presence of a signal on a discreet frequency.

 To top it all, your Rx noise is lightning from all over the world. The background noise level isn't so great.
Dealing with RX noise should be pretty straightforward. There will, of course, be certain amount of bad RX readings that would need to be discarded. The other way to deal with such noise is to discard just the data that is corrupted and utilize the rest of the information. A third way, would be to monitor the lightening through a separate system and use a mathematical model to remove the interference from the data you recorded.

As a result, I have been examining Schumann resonances and how this may serve to amplify radio waves in the 3-69Hz spectrum. This would obviously throw off any power calculations and potentially even mask a highly discreet frequency. Again, we have a choice to either discard <70Hz or mathematically account for its effects using readings from a separate system.

Any other factors that need to be considered at low frequency?