Abstract: Corrosion in well tubing strings, pipe and similar conduits may be determined by a circuit which includes a current sensitive meter and a known voltage source which is placed at selected intervals in the tubing string to make current flow measurements and voltage drop measurements between spaced apart contactors engaged with the tubing string at the selected intervals. The current flow measurements from the known source are used to determine resistance of the measured interval and voltage drop from a second source imposed on the tubing string at the surface is measured at the selected intervals to determine current flow from the second source. The differences in current flow at the selected intervals at a first measurement are compared with the differences in current flow at the same intervals at a later time to indicate any changes in current flow which may be related to corrosion in the tubing string between the intervals of measurement.

Abstract: A system for measuring earth formation resistivity while drilling a wellbore through an earth formation using a self-contained drilling assembly supported at the lower end of a casing string. The drilling assembly includes a drilling fluid driven motor driving a drill bit, where the motor and bit are electrically isolated from the casing string. Formation resistivity measuring circuitry is provided in the assembly. Data transmission mechanism is provided for encoding the resistivity data and transmitting it through the drilling fluid to the surface location of the wellbore.