Casing Collar Locators (CCL) respond to changes in metal volume, such as pipe collars and perforations. As the tool passes a collar, or changes in metal volume, lines of magnetic flux between two opposing permanent magnets are disturbed.

This causes a low frequency voltage or Electro Magnetic Field (EMF) to be induced in a coil, mounted between the permanent magnets.

Finally, the signal is converted by a voltage-controlled oscillator for recording in the Sondex Ultrawire™ production logging section of the tool.