SPWLA Glossary

Uses a method of relating the effects of eddy currents on a magnetic field to casing wall thickness. The instrument consists of two radial coils – an exciter and a pickup coil. The exciter coil is fed from an AC voltage source at the surface, in turn producing a magnetic field downhole. This field sets up eddy currents in the casing wall. These currents cause the magnetic field to be attenuated and shifted in phase. The resulting magnetic field is detected by the pickup coil and transmitted to the surface. The magnetic field as detected by the pickup coil is then compared with the original field generated by the exciter coil, and the resulting phase shift in the magnetic field that has occurred is recorded. The phase shift can indicate casing wall thickness, splits, and holes.