The need for inspecting conductive materials using nondestructive (NDT) methods faster and more efficiently has always been a challenge. This work presents a method to evaluate sub-surface defects in non-ferromagnetic materials using the diffusion of motion induced eddy currents. A probe, including a permanent magnet and a magnetic field sensor, is moved with a constant speed in the vicinity of an aluminium plate with sub-surface defects machined at different depths. The constant magnetic field due to the magnet, which moves relative to the plate, induces eddy currents that diffuse inside the material. At higher speeds the eddy current diffusion creates a tilted transition zone. The location of the interaction between the frontier of this transition zone with the defect shall be used to determine the depth of the sub-surface defect. The study was carried out using a numerical model to verify the progression of the eddy currents along the depth of the conductive plate as it moves relatively to the probe and obtain the magnetic field perturbation due to the presence of the sub-surface defects. The validation of the model is done comparing the results obtained experimentally with those obtained by simulation for identical situations at lower speeds.