Patent ID: 6975121

Claim:
An apparatus for measuring formation resistivity through a conductive pipe in a wellbore, comprising: a sonde adapted to be moved through the wellbore; a plurality of voltage measurement electrodes disposed on the sonde at spaced apart locations, the voltage electrodes adapted to make electrical contact with the pipe; at least one current source electrode disposed on the sonde, the current source electrode adapted to make contact with the pipe; a digital voltage measuring circuit controllably coupled to selected ones of the voltage measurement electrodes; and a current return electrode coupled to the pipe proximate the Earth's surface and a current return electrode disposed proximate the Earth's surface at a selected lateral distance from the current return electrode coupled to the pipe, and a switch to select a return path for measuring current from the current source electrode to the selected one of the electrode coupled to the top of the pipe and the electrode disposed at the selected lateral distance from the pipe.