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. Please note, the physics doesn't say nothing gets through the reflective layers. It would be a lot simpler if that were just a 1 or a zero, but it's in-between. In daytime it doesn't reflect very well, and at night reflects and refracts somewhat somewhat better. Nobody said nothing gets through, however. So that mix complicates your situation; it's time varying and weather dependent. Finally, if you say you'll only look at one mode, then the problem of time-gating a 1 KHz carrier which is hard-put to effectively modulate at rates above 10 Hz, what happens to your time-gating resolution? 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. To top it all, your Rx noise is lightning from all over the world. The background noise level isn't so great.