G - Physics – 01 – V
Patent
G - Physics
01
V
324/10
G01V 3/18 (2006.01) G01V 3/20 (2006.01)
Patent
CA 1278826
ABSTRACT OF THE DISCLOSURE A method and apparatus for measuring the resis- tivity of a geologic formation through a metal conduit such as drillpipe or well casing comprising applying a relatively low frequency bipolar potential to the metal conduit and to a ground electrode and traversing the conduit with a tool which includes contactors which are connected to a differential amplifier to detect a differential voltage within the conduit caused by current leaving the conduit into the formation. The differential amplifier is connected to a unity gain inverter and a switching arrangement for alternately reversing the polarity of the connection of input voltages to the differential amplifier. A switching network alternately connects the outputs of the differential amplifier and the output of the inverter to the input of a low pass filter, and a clock is provided for driving the network in synchronization. Alternate measurements of the conduit resistivity and formation resistivity are carried out by applying a voltage along a predetermined length of casing between two contactors which are spaced apart farther and on opposite sides of the differential amplifier pickup contactors. The potential applied across the formation may be applied at the surface of the metal conduit or down- hole adjacent the points of measurement of conduit resistivity and formation resistivity.
573595
Gard Michael F.
Kingman John E. E.
Klein James D.
Atlantic Richfield Company
Osler Hoskin & Harcourt Llp
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