1. It is less efficient because there is no metal to metal contact to transfer the current onto the line. A smaller portion of the transmitter’s signal makes it onto the target conductor, and that limits the distance you can trace the line.

2. Induction using just the transmitters coils does not discriminate between utilities above ground and utilities below ground. The higher the frequency, the easier it is to induce a current onto all of the sorrounding utilities.

3. When using the transmitter coils Air Coupling needs to be tested for and avoided.

4. Induction is less effective on deeper conductors.

Requirements

To create a circuit inductively, the signal must be able to travel in a both directions on a conductor. In other words, the conductor needs to be grounded at both ends. Some conductors do this better than others. As a rule of thumb, lines that require electrical continuity in order to move their product (electric, cable, phone) are better at this than those that do not (gas, water).

Remember that you need a good circuit for current to flow. Higher frequencies used while inducing may help but Induction in practice will not make current flow in a bad (high resistant) circuit.

Induction also requires a higher frequency than direct connect. Nearly all transmitters work better at a frequency of 33kHz or higher for induction. (The SeekTech ST-33Q “Q-ring” is an exception. In many cases it can produce a locatable signal at 8kHz on highly conductive utilities.)