Method and apparatus for measuring the electrical resistivity of geologic formations through metal drill pipe or casing
US4837518A · kind A · utility
Assignee
Inventors
Key dates
| Filing date | Aug 18, 1987 |
| Grant date | Jun 6, 1989 |
| Priority date | — |
| Expiry date | Aug 18, 2007 |
Classification
- Technology area (CPC G)Physics
- CPC primaryG01V3/20
- WIPO fieldMeasurement
- WIPO sectorInstruments
Abstract
The resistivity of a geologic formation is measure through drillpipe or casing by applying a low frequency bipolar voltage to the casing and to a ground electrode and traversing the casing with a tool having contractors which are connected to a differential amplifier to detect a differential voltage within the casing caused by current leaving the casing 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. The casing and formation resistivities may be measured by applying a voltage along a predetermined length of casing between two contractors which are spaced apart farther and on opposite sides of the differential amplifier pickup contactors. The voltage applied across the formation may be applied to the casing at the earth's surface or downhole adjacent the points of measurement of casing resistivity and …
Source: USPTO / EPO open patent data. Objective bibliographic and citation counts.