A caliper log is a set of measurements of the size and shape of a bore hole commonly made when drilling oil and gas wells. This can be an important indicator of cave ins or shale swelling in the bore hole.

The caliper tool measures the variation in bore hole diameter as it is withdrawn from the bottom of the hole. It is constructed with two or more articulated arms that push against the bore hole wall to take measurements. The arms show variable movements of the cursor by measuring electrical resistance, creating electrical variation. The variation in output is translated into changes of diameter after a simple calibration. The caliper log is printed as a continuous series of values of hole diameter with depth.

Known challenges with caliper logging include borehole spiralling. The position of the drill bit

Drill bit

Drill bits are cutting tools used to create cylindrical holes. Bits are held in a tool called a drill, which rotates them and provides torque and axial force to create the hole. Specialized bits are also available for non-cylindrical-shaped holes....

 may precess as it drills, leading to spiraling shapes in the wellbore wall, as if the hole had been drilled by a screw

Screw

A screw, or bolt, is a type of fastener characterized by a helical ridge, known as an external thread or just thread, wrapped around a cylinder. Some screw threads are designed to mate with a complementary thread, known as an internal thread, often in the form of a nut or an object that has the...

. If the arms of the caliper log follow the grooves of the spiral, it will report too high an average diameter. Moving in and out of the grooves, the caliper will give erratic or periodically varying readings.