"Every target has a method!"
When we "define" the target, we also define the scope of possible methods to detect it. This target seems to fit the definition of an "electrical conductor" so the method of choice is "electrical conductivity".
Electrical conductivity can be measured directly using conventional electrical resistivity(inverse of conductivity) where a current transmitter applies current through stakes in the ground and potential is measured at specified distances along a line. or a second method; Using Transient EM methods see examples at Zonge geophysics and Fugro Airborne, EMpulse Geophysics Ltd., of similar systems.
To answer the question of size; a geophysical system is scaled proportional to the depth of the target. If using an array of electrical resistivity (direct contact) electrodes, the array should be configured for electrode spacings of 10 feet over a spread of 100 feet. This will provide resolution of a target on the scale of 10-15 feet and a depth/pseudosection of 100 feet.
If the method of choice is electromagnetic/TEM, the target may be resolved using a transmitter coil (loop) having a diameter of 100 feet, transmitting current of about 10 amps into 3-10 turns. The dipole moment is calculated by multiplying the loop area in square meters x turns x current. The greater the dipole moment, the greater the depth and signal to noise ratio. These systems can be home made, bought off the shelf or rented.
Keep in mind depth and scale. The big exploration companies that explore for massive sulfide mineralization on the scale of 1km at up to 1km depth use systems on the same scale. The loops can be 1km on a side and transmit thousands of watts of energy.
Cool, fun stuff!! Good luck finding that Gold!!!
Geowizard