By Nhan Nguyen
The caliper log is one of many logging tools used for measuring the diameter of a borehole. It also is able to measure the shape of the borehole. It is a rod with 2 or more spring driven arms that can extend and retract. The arms move in and out along the borehole walls as the caliper is pulled out of the borehole. The in and out movement of the caliper arms are then converted into an electrical signal used to produce a borehole geometry log.
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caliper_1.jpg
A Caliper Log Tool with multiple arms. (http://www.bakerhughes.com/products-and-services/evaluation/cased-hole-wireline-systems/well-integrity/imaging-caliper-log-icl)

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The caliper log is one of many logging tools used for measuring the diameter of a borehole. It also is able to measure the shape of the borehole. It is a rod with 2 or more spring driven arms that can extend and retract. The arms move in a
nd out along the borehole walls as the caliper is pulled out of the borehole. The in and out movement of the caliper arms are then converted into an electrical signal used to produce a borehole geometry log.
There are calipers with 30 arms arranged on and all around the tool to give a very accurate and detailed shape of the borehole.
Some other calipers have sensor pads that are pressed against the borehole and is another form of a caliper and works in the same way.

Caliper_Log_Example.jpg
An Example of a Caliper Log (http://www.dot.ca.gov/hq/esc/geotech/gg/caliper_log.htm)