Analysis of Environmental Effects on Electromagnetic Induction Sensors

Abstract

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.

Date of Award	14 Oct 2011
Original language	English
Supervisor	Philippe Lataire (Jury), Christophe Craeye (Promotor), Marc Acheroy (Promotor), Danielle Vanhoenacker (Jury), Xavier Neyt (Jury) & Yogadish Das (Jury)