Three-Arm Caliper Probe

The three-arm caliper provides a single continuous log of borehole diameter as determined by three mechanically coupled arms in contact with the borehole walls. Both standard (38mm) and long-arm models (60mm) are available to suit a wide range of well diameters. The standard caliper range can be increased using extension arms supplied with the probe. The caliper is a useful first log to determine the borehole condition before running more costly probes or those containing radioactive sources.

PRINCIPLE OF MEASUREMENT
Opening and closing of the motor-driven caliper arms is under surface control, allowing the probe to be run into the borehole with the arms retracted. Once opened, at the start of logging, the spring-loaded arms respond to borehole diameter variations as the probe is raised up the borehole. The coupling between the arms means that the resulting log usually represents the minimum diameter in non-circular boreholes. A field calibrator is provided to allow logs to be displayed directly in linear metric or imperial units.