  The general problem of characterizing gas source parameters based on
concentration measurements is known to be a difficult task. As many inverse
problems, one of the main obstacles for accurate estimation is the
non-uniqueness of solution, induced by the lack of sufficient information. As
the number of detectors is lowered, which is more than a plausible scenario for
many practical situations, the number of possible solutions that can
characterize the source increases dramatically, leading to severe errors. In
this paper, a Lagrangian stochastic based method for identifying these
suspected points, which will be referred to as 'degenerate space', is
formulated and analysed. Then, a new procedure for quantitative prediction of
the effect of deploying a new detector in space is used to design an adaptive
scheme for source term estimation. This scheme has been tested for several
scenarios, differing by the location of the initial detectors, and is shown to
reduce dramatically the degeneracy formed by insufficient information. The
combined formulation of degenerate space with the new adaptive scheme is shown
to give improved accuracy, and in particular for a relatively small number of
detectors.
