A petroleum well has an electronic module and a number of sensors which communicate with the surface using the tubing string and casing as conductors. Induction chokes at the surface and downhole electrically impede AC flow through the (tubing or casing if so configured) with the resulting voltage potential...http://www.google.de/patents/US6679332?utm_source=gb-gplus-sharePatent US6679332 - Petroleum well having downhole sensors, communication and power
