Patent ID: 6850055

Claim:
A method for measuring how a material property that affects permeability in an object of ferromagnetic material varies with depth below the surface, the method using at least one probe, the or each probe comprising an electromagnet means, means to generate an alternating magnetic field in the electromagnet means and consequently in the object, and a magnetic sensor arranged to sense a magnetic field due to the electromagnet means; and the method comprising resolving signals from the magnetic sensor into an in-phase component and a quadrature component; mapping the in-phase and quadrature components directly into material property and lift-off components; and deducing a material property from the material property component so determined; repeating these measurements for at least five different frequencies of the alternating magnetic field; and deconvolving the measurements of material property obtained at different frequencies by assuming a functional form for the variation of material property with depth, the function having no more unknown constants than the number of different frequencies, assuming values for the unknown constants, assessing the accuracy of the values of the unknown constants in the function by comparing the observed measurements to the corresponding predicted measurements with those values of the unknown constants, and adjusting the values of the unknown constants to obtain the best fit between observed measurements and predicted measurements, so as to determine how the material property varies with depth.