Very Low to Low (128Hz - 1 kHz)

Frequencies in the 1kHz and under range need good circuits. Breaks, gaskets or bad connections increase resistance to flowing current and will stop low frequency immediately as it does not flow easily into the ground and around these breaks. Very low and low frequencies do not induce well.

If you do have a good circuit these lower range frequencies will travel very long distances and will not jump onto other pipes or cables unless they are part of the same good circuit. They follow the path of least resistance so the signal stays on the target conductors that are continuously metallic and well-grounded at both the transmitter and the far end.

Medium (8 kHz)

Frequencies in the 8kHz range are a bit stronger and less susceptible to ambient noise, (e.g. other frequency sources like power lines) then the lower frequencies but they still limit the effects of signal bleed into the surrounding earth and onto adjacent utilities.

If you do not need to trace very long distances and you have a good circuit then 8kHz is a good frequency to start with when you make a direct connection. Medium frequencies will induce weakly to moderately and high induction power is best.