  This investigation is concerned with the 2D acoustic scattering problem of a
plane wave propagating in a non-lossy fluid host and soliciting a linear,
isotropic, macroscopically-homogeneous, lossy, flat-plane layer in which the
mass density and wavespeed are different from those of the host. The focus is
on the inverse problem of the retrieval of either the layer mass density or the
real part of the layer wavespeed. The data is the transmitted pressure field,
obtained by simulation (resolution of the forward problem) in exact, explicit
form via separation of variables. Another form of this solution, which is exact
and more explicit in terms of the mass-density contrast (between the host and
layer), is obtained by a domain-integral method. A perturbation technique
enables this solution to be cast as a series of powers of the mass density
contrast, the first three terms of which are employed as the trial models in
the treatment of the inverse problem. The aptitude of these models to retrieve
the mass density contrast and real part of the layer wavespeed is demonstrated
both theoretically and numerically.
  Keywords: 2D acoustics, forward scattering, inverse scattering, domain
integral equation, constant mass density assumption, small density contrast,
retrieval accuracy
