Thanks for the new comments, Papabravo. We do only have a surface and downhole system, so no intermediate sensors, etc. For the resistance, the cable provides abut 280 ohms for 3000 metres, but that is with the cable wound on the drum, so the armour is shorted out. As you run into the hole, the resistance increases. I was thinking I can measure the voltage arriving at the tool, and send this back up the telemetry to surface, so we can control the surface voltage to keep the downhole voltage correct. We have several ADCs available on our downhole processor board.