Method and apparatus for removal of the double indication of defects in remote eddy current inspection of pipes

An apparatus for measuring defects in a well bore pipe using remote field eddy current measurement comprising:

• a transmitter coil and a plurality of receiver coils in spaced axial relationship from each other, wherein the plurality of receiver coils comprises two coil pairs, each of the two coil pairs comprising a closer receiver coil and a farther receiver coil, wherein the closer receiver coil is spaced from the transmitter coil a distance of L1=k1×dz, and the farther receiver coil is spaced from the transmitter coil a distance of L2=k2×dz, where k1 and k2 have no common divisor and dz is an incremental length along the longitudinal axis of the apparatus;
• a circuit generating a transmitter current and sensing a receiver voltage at each of the plurality of receiver coils collecting a convoluted signal proportional to the thickness of the pipe adjacent each receiver coil;
• and,
• a data analysis circuit which deconvolutes each sensed signal to remove ghost-image signals from such receiver signal.

An apparatus for measuring defects in a pipe using remote field eddy current measurement comprising:

• a transmitter providing a selectable low frequency electromotive force;
• a pair of receiver coils spaced longitudinally on a first side of the transmitter and a pair of receiver coils spaced longitudinally on a second side of the transmitter each pair of coils symmetrically matched with the similarly situated receiver coil on the opposing side of the transmitter, wherein the pair of receiver coils on the first sided comprises a first coil and a second coil, where the first coil is spaced at a distance of L1=k1×dz from the transmitter and the second coil is spaced at a distance of L2=k2×dz from the transmitter, where k1 and k2 have no common divisor and dz is an incremental length along the longitudinal axis of the apparatus;
• a circuit for correlating the transmitter current and the receiver voltages, as the apparatus is moved through the pipe;
• thereby permitting said circuit to measure phase deviations of a transimpedance as the apparatus moves past the same point in the pipe allowing the duplicate image to be eliminated and the impulses transmitted and received by the transmitter to be averaged lowering a signal to noise ratio for the measurement of receiver voltage.

A method of removing ghost defects from a remote field eddy current sensing device comprising:

• generating from a transmitter an eddy current on a well bore tubular outer surface;
• detecting the eddy current signal with more than one remote receiver, wherein the more than one remote receiver comprises a closer receiver coil spaced a distance of L1=k1×dz from the transmitter and a farther receiver coil spaced a distance of L2=k2×dz from the transmitter, where k1 and k2 have no common divisor and dz is an incremental length along the longitudinal axis of the apparatus;
• generating an eddy current signal on a well bore tubular outer surface from an incrementally different position;
• detecting the eddy current with the more than one remote receiver from the incrementally different position;
• determining a linear combination of eddy current signals detected by the more than one remote receiver to eliminate ghost defects in such received signals.

A method of removing ghost defects from a remote field eddy current sensing device providing a transmitting coil and a plurality of symmetrically placed receiving coils on a longitudinal axis of the device comprising:

• exciting the transmitting coil in a well bore tubular with a low frequency current to induce an eddy current in the well bore tubular;
• sensing an induced electromotive force at the plurality of receiving coils in a spaced relation with the transmitting coil at a first location, wherein the plurality of receiving coils in a spaced relation comprises a closer receiver coil spaced a distance of L1=k1×dz from the transmitter coil and a farther receiver coil spaced a distance of L2=k2×dz from the transmitter coil, where k1 and k2 have no common divisor and dz is an incremental length along the longitudinal axis of the apparatus;
• storing a sensed signal from each receiving coil at said first location;
• repeatedly moving the transmitting coil in the well bore tubular to a new discrete location and sensing the induced electromotive force at the plurality of receiving coils for each movement of the transmitting coil;
• saving each sensed signal from each receiving coil at said plurality of locations;
• and,
• manipulating the saved signal from each sensed coil to eliminate the duplicate readings.