The ability of the ground to pass current will vary locally. Clearly, wet soil is a better conductor than dry sand, and the resulting capacitance effects will vary the apparent conductivity of the conductor. The effect of high ground conductivity is to make it easier to induce current flow and therefore a signal in a buried conductor, because of the good return path. At the same time, the easy return means that the signal becomes lost along a short length of conductor.
Conversely, low ground conductivity requires more energy to induce signal, but it will then be detectable along a greater length of the conductor.
If a buried conductor is in direct contact with the ground, e.g. where a pipeline wrapping has been penetrated, the signal current will leak away by direct conduction.