Analysis of environmental effects on electromagnetic induction sensors

(eng) Electromagnetic induction sensors are widely used in a number of applications, such as mine clearance, improvised explosive detection, treasure hunting and geophysical survey. Our focus is on pulse induction metal detectors used in the scope of humanitarian demining to detect anti-personnel mines, but most developments remain valid for other applications and for other types of electromagnetic induction sensors. The detection performance of metal detectors may significantly be affected by the environment. In this thesis, we consider the effect of a magnetic soil, the effect of a water layer on the head of the detector and the effect of the electromagnetic background. The analysis is based on a detailed model of the detector, including the coil and the fast time electronics. Classically, the voltage induced in a coil is assumed to be equal to the time derivative of the linked flux. We show, by resorting to the quasi-static approximation of the reciprocity expression, that an additional contribution, related to the incident electro quasi-static field, must be taken into account. In many applications this contribution is negligible but in some cases, for example when using some metal detectors over dew grass, the additional term is required to explain the observed phenomena. Regarding the soil, a general model is developed, which is valid in the presence of inhomogeneities or soil relief and for an arbitrary head geometry. Then the volume of influence is rigorously defined and computed for typical head geometries. Regarding the effect of water, important losses of sensitivity were reported from the field when scanning over dew grass with some detectors. The problem was investigated in the nineties. The effect could be reproduced and the conditions under which it occurs were well understood but the underlying physics could not be explained. Circuit and field level models are developed to explain the various phenomena observed. We show that the loss of sensitivity is due to an electro quasi-static interaction between the water layer and the coil. Finally, we show that the electromagnetic background may affect the detector for frequencies from below 1 Hz to about 20 MHz, with a sensitivity peak around 100 kHz. For the maximum allowed background fields, the effect may be very severe, significantly lowering the sensitivity or even preventing the normal functioning of the detector.