Eddy current method and apparatus for inspecting ferromagnetic tubular members

Apparatus for locating and evaluating defects in ferromagnetic casing comprising:

• an elongated body adapted for suspension in said casing from a well-logging cable;
• at least one coaxial transmitter coil adapted to be positioned within said casing on said body, for generating an alternating magnetic field along a portion of the longitudinal axis thereby inducing circumferential currents in said casing;
• and
• at least one array of differential receiver coils located on said body and adapted to be disposed within said casing in spaced axial relationship to said coaxial transmitter coil, for generating a signal in response to perturbations of said circumferential currents in said casing, wherein each of said differential receiver coils includes:
• a carrying member;
• a first coil having N number of windings supported by said carrying member;
• a second coil disposed adjacent said first coil having 2N number of windings serially opposed to said windings of said first coil;
• and
• a third coil disposed adjacent said second coil having N number of windings serially opposed to said windings of said second coil.

Apparatus for locating and evaluating defects in ferromagnetic oil well bore casing comprising:

• an elongated body adapted for suspension in said casing from a well-logging cable;
• at least one coaxial transmitter coil located on said body and adapted to be positioned within said casing, driven by an alternating current source, for generating an alternating magnetic field, said magnetic field inducing circumferential currents in said casing;
• and
• first and second arrays of differential receiver coils located on said body and adapted to be disposed within said casing in spaced axial relationship to said coaxial transmitter coil, for generating at least one signal in response to perturbations in said circumferential currents, each of said differential receiver coils are enclosed in a receiver pad and include:
• a carrying member;
• a first coil having N number of windings disposed within said carrying member;
• a second coil disposed adjacent said first coil having 2N number of windings serially opposed to said windings of said first coil;
• and
• a third coil disposed adjacent said second coil having N number of windings serially opposed to said windings of said second coil.

A method of distinguishing actual defects in ferromagnetic casing from magnetic anomalies comprising the steps of:

• generating a first alternating magnetic field in the casing thereby inducing circumferential currents in said casing;
• receiving a measured phase signal from a differential receiver coil arrangement in close proximity to said casing wall, said differential receiver coil arrangement being spaced axially from the source of said first alternating magnetic field;
• receiving a phase reference signal from a reference coaxial coil disposed axially from said source of first alternating magnetic field;
• generating a phase difference signal between said measured phase signal and said reference phase signal;
• and
• identifying actual defects from the shape of said phase difference signal, where a phase change from substantially 0.degree. toward -90.degree. and then through 0.degree. toward +90.degree. and then through 0.degree. degrees toward -90.degree. and back to substantially 0.degree. indicates a real defect, and where the phase difference signal changes from substantially 0.degree. degrees toward -90.degree. and then back toward substantially 0.degree. indicates a magnetic anomaly.

A receiver coil arrangement for detecting defects in ferromagnetic oil well casings comprising:

• a non-conductive carrying member;
• a first coil having N number of windings supported by said carrying member;
• a second coil disposed adjacent said first coil and having 2N number of windings serially opposed to the windings of said first coil;
• and
• a third coil disposed adjacent said second coil and having N number of windings serially opposed to the windings of said second coil.

A method of distinguishing actual defects in ferromagnetic casing from magnetic anomalies comprising the steps of:

• generating a first alternating magnetic field in the casing thereby inducing circumferential currents in said casing;
• receiving a measured phase signal from a differential receiver coil arrangement in close proximity to said casing wall, said differential receiver coil arrangement being spaced axially from the source of said first alternating magnetic field;
• receiving a phase reference signal from a reference coaxial coil disposed axially from said source of first alternating magnetic field;
• generating a phase difference signal between said measured phase signal and said reference phase signal;
• and
• identifying actual defects from the shape of said phase difference signal, where one unidirectional phase pulse indicates a magnetic anomaly, and where at least two pulses of opposite polarity indicates a real defect.

A method of distinguishing actual defects in ferromagnetic casing from magnetic anomalies comprising the steps of:

• generating a first alternating magnetic field in the casing thereby inducing circumferential currents in said casing;
• receiving a measured phase signal from a differential receiver coil arrangement in close proximity to said casing wall, said differential receiver coil arrangement being spaced axially from the source of said first alternating magnetic field;
• receiving a phase reference signal from a reference coaxial coil disposed axially from said source of first alternating magnetic field;
• generating a phase difference signal between said measured phase signal and said reference phase signal;
• receiving a measured amplitude signal from said differential receiver coil arrangement;
• multiplying said amplitude signal by said phase difference signal to generate an identification signal;
• and
• identifying actual defects from the shape of said phase difference signal, where a phase change from substantially 0.degree. toward -90.degree. and then through 0.degree. toward +90.degree. and then through 0.degree. toward -90.degree. and back to substantially 0.degree. indicates a real defect, and where the phase difference signal changes from substantially 0.degree. toward -90.degree. and then back toward substantially 0.degree. indicates a magnetic anomaly.