Abstract: A tool for monitoring corrosion levels within a wellbore particularly suited for remote monitoring over extended periods. The monitoring tool utilizes a thin wall electrical resistance type corrosion sensor to determine rate of metal loss within the wellbore. The corrosion sensor is preferably made of the same material as the production tubing or drill pipe of the well and is positioned to allows the fluid within the wellbore to flow over the sensor at the same rate as the fluid flows over the tubing. A reference element is incorporated into the corrosion sensor to provide primary temperature compensation. A temperature sensor provides temperature data and allows for secondary temperature compensation for the corrosion sensor. A central processing unit reads and stores the data from the three sensors into solid state memory. The probe records data at programmable time intervals.

Abstract: A method and apparatus for measuring the differential or gradient of an earth variable within a well bore (e.g., the spontaneous earth potential) and producing improved logs of this gradient or differential and its integral variable essentially free of any accumulated instrument and base line drift or error. The differential spontaneous potential of an earth formation traversed by a well bore is measured at repeated multiple depths by moving a pair of closely spaced electrodes through the well bore wherein each electrode is electrically insulated externally from the other and from a third downhole local ground (such as the well tool cable) to which each is internally resistively referenced.