1.2 The MFL technique: A schematic diagram of an MFL detector is shown in Figure 1. High strength Nd-Fe-B permanent magnets produce a magnetic field in the pipe wall. When a defect such as a pit is present , some of the flux is forced (or "leaks") into the surrounding air. As the tool moves along the pipe, the sensor (typically a coil or hall probe) passes over this leakage flux region and an MFL signal is registered.