The magnetic fields produced by defects in magnetic flux leakage (MFL) enable the non-contact and nondestructive detection and characterisation of defects in magnetic structures for health and condition monitoring. The frequency response and bandwidth of the leakage magnetic fields have not been derived from physical field descriptions, which is fundamental for accurate sizing of defects from measured signals and for the appropriate design of MFL detection channels. In this letter, the Fourier transforms of the leakage fields from a two-dimensional surface defect are evaluated to produce analytical expressions for the frequency response of magnetic flux leakage (MFL) signals for flux sensitive elements. The derived expressions explicitly show the correlation between the spectral response of the leakage fields and defect dimensions and sensing element lift-off spacing, in the form of a product of frequency dependent defect width loss function, spacing loss function, and thickness enhancement function. The lower and upper bandedges of the band-limited leakage magnetic fields are theoretically identified. A spectral based method for sizing of defects is also proposed based on the frequency response derivation.

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A high-sensitivity, low-power and portable coiled-tubing (CT) inspection tool is developed based on magnetic flux leakage (MFL) technology. The tool provides enhanced real-time integrity monitoring of CT operations to minimize the risks of unexpected failures and enable efficient management of CT operations. This paper discusses practical design and engineering considerations to enhance the sensitivity of the magnetic inspection head, including magnetic characterization of the CT material, pole-piece separation, parametric calculations of the gap field, eddy currents, and MFL signal bandwidth. Experimental measurements illustrate the capability of detecting defects down to 1 mm in diameter and depth in a 1.5" CT pipe.