Most people know that an electric circuit has to be completed to allow a current to flow. So how can a low powered signal source at the surface make a detectable current flow in a properly insulated buried conductor? The voltages available are obviously quite incapable of punching through insulation. The answer lies in the effect of capacitance on AC circuits.
A classic capacitor requires two conductors, but there is only one conductor, the pipe or cable. Where is the other? It is the ground. While each particle of soil, sand or rock seems to have a high resistance, there is so much of it that the ground acts as if there is a conducting layer around the conductor. So the conductor charges up relative to ground.
Because a large area of ground is needed to produce this effect, the buried conductor behaves as if it has a string of small capacitors along it. So if AC is applied to the conductor at one point, current will flow out both ways, decreasing in magnitude as more and more leaks away.
The higher the frequency the greater the current because the reactance of the capacitors drop. As the capacitance is effectively distributed along the length of the cable, the signal strength dissipates along the length cable.
Capacitance increases with conductor area, so that surface area of the conductor, and therefore the size of a pipe, affects the distance the signal will carry. The same signal strength will leak away over a much shorter distance from a large pipe than from a small one.
At the opposite end of the scale the capacitance of a small diameter cable may be so low that little or no current will flow and result in too small a signal to be detectable.
The effect of capacitance explains why the signal usually decreases and finally disappears some distance before the end of an insulated conductor that has not been grounded, such as a cable pot end.