Caliper log

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A Caliper log tool measures the size and shape of a hole. The simple caliper measures a continuous vertical profile of the hole's diameter. The mechanical caliper measures the variation in bore hole diameter as it is withdrawn vertically from the bottom. It is constructed with two 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 value of hole diameter with depth.

Known challenges with caliper logging include borehole spiralling. The position of the drill bit may precess as it drills, leading to spiraling shapes in the wellbore wall, as if the hole had been drilled by a screw. 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.