Magnetic focusing scheme for flux leakage measurement

A system for detecting discontinuities on or below a surface of a magnetizable material, comprising:

• a magnetic flux measurement apparatus, comprising:
• a first magnet carried on a support for generating a first magnetic field that extends radially outward into the material,
• a second magnet carried on the support adjacent a first end of the first magnet,
• a third magnet carried on the support adjacent a second end of the first magnet, the second end being opposite the first end, and
• a sensor, carried on the support, that detects magnetic flux leakage of the first magnetic field from the material and produce a corresponding output signal;
• and
• a computing device that receives the output signal from the sensor and identifies defects in a wellbore casing based on the output signal.

A method for detecting discontinuities on or below a surface of a magnetizable material, comprising:

• lowering a magnetic flux leakage (“MFL”) tool into a wellbore, the MFL tool comprising:
• at least one source magnet flanked on opposite sides by first and second focusing magnets, respectively;
• a different magnetic pole piece between each of the at least one source magnet and first and second focusing magnets;
• and
• a sensor array including sensors on pads;
• positioning at least some of the pads against an inner wall of a first casing in the wellbore, wherein the sensors produce output signals that represent a measurement of at least one component of a magnetic flux leakage of the at least one source magnet;
• and
• receiving the output signals
• at a computing device.