possible iocg systems, page-20

Transmitter and Reciever are down the hole in downhole EM. The area covered is a sphere around the hole where the survey is taken. The size of the sphere is relative to the energy output, but a rule of thumb would be between 50 to 150 m for most units. It is wise to take numerous surveys down the hole to get away from masking properties of conductive rocks such as pyritic shales.

It is best to be somewhat up the hole, away from the conductor as this gives the sphere a better chance to form, if you are on the conductor you won't see much apart from a strong response all around.

Still, luck plays an imporant role - you need to have the hole near the conductor in order to get a resoponse. What downhole EM does in effect, is conceptually broaden the hole to the size of the EM sphere, as opposed to a thin 10-15cm wide RC/DD hole. And if you do have conductors within the sphere, you know where to target a followup hole!