  Realistic applications in metal detection involve multiple inhomogeneous
conducting permeable objects and the aim of this paper is to characterise such
objects by polarizability tensors. We show that, for the eddy current model,
the leading order terms for the perturbation in the magnetic field, due to the
presence of $N$ small conducting permeable homogeneous inclusions, comprises of
a sum of $N$ terms with each containing a complex symmetric rank 2
polarizability tensor. Each tensor contains information about the shape and
material properties of one of the objects and is independent of its position.
The asymptotic expansion we obtain extends a previously known result for a
single isolated object and applies in situations where the object sizes are
small and the objects are sufficiently well separated. We also obtain a second
expansion that describes the perturbed magnetic field for inhomogeneous and
closely spaced objects, which again characterises the objects by a complex
symmetric rank 2 tensor. The tensor's coefficients can be computed by solving a
vector valued transmission problem and we include numerical examples to
illustrate the agreement between the asymptotic formula describing the
perturbed fields and the numerical prediction. We also include algorithms for
the localisation and identification of multiple inhomogeneous objects.
