50 Hz coil to locate power cables?

I want to construct a coil of enamelled copper wire to use as an "antenna" to pick up the 50 Hz radiation from 220 volt power cables buried under 2 inches of plaster and close to steel reinforcing rods in concrete.

With so much metal around I can't use the usual heterodyne methods so I plan to keep it simple and just listen for the 50 Hz hum. (Actually, I might pass it through a frequency doubler and square it off to make it easier to hear.)

The part I'm stuck with is the coil. Is a 6 inch diameter coil going to be more effective than a small diameter coil? How many turns should I need and what capacitance to get resonance at 50 Hz?

Suggestions welcomed.

Martin

Making the coil too large will give less accurate localization. I don't think that you need resonance, just a large number of turns and sufficient amplification. I have previously used a standard telephone recording pickup coil with a sensitive microphone amplifier. It's important to have a load connected to the circuit of interest.

Thanks and apologies for omitting to mention that I don't need accuracy. Also, I'm trying not to spend any money if I can avoid it, so I'd like to use what I have available (enamelled copper wire, wood, nails and plenty of general purpose electronic components). I'd like to achieve resonance if it's feasible to do so but I accept that it probably isn't necessary.

Good point about the load.