ELF/VLF stands for Extremely Low Frequency and Very Low Frequency, and refers to the range 300 Hz to 30 kHz. We're talking about radio waves, like the AM/FM signals you get, just at an even lower frequency. On our planet, the most potent source of ELF/VLF waves is lightning, so a lot of what we do comes down to studying lightning and its various impacts on our Earth's environment, detectable even in Antarctica.
So many signals is generating per second by many different natural and manmade sources in the ELF/VLF band which can propagating several thousands of kilometers throw the Earth waveguide between the ground and the ionosphere and reach the antenna. Each signal has an individual characteristic at dynamic spectrum which depends on the two factors. The first factor is due to initial physical conditions of EM wave source and the second factor is due to the distance of source from the antenna.