First-iteration adjoint-state sensitivity maps are examined for qualitative ultrasonic imaging of defects in orthotropic plates under a single pitch–catch illumination. The five signed kernels of the classical least-squares misfit are displayed as absolute-value normalised images, not as material updates of an iterative FWI reconstruction.We form a reflection–transmission residual-wave interaction indicator, correlating two fields back-propagated from residuals on opposed apertures. It is bilinear in the two residual families, where every single-residual construction is linear in one; since the degree of homogeneity in the data is invariant under linear data-space operators, no reweighting of the misfit produces it. It is not the propagator term of the first variation of any standard single-state constrained functional, both fields sharing the same time orientation. It does, however, admit an exact variational status in the perturbation variable, and a gradient in the discrete space where the maps are computed.The set-up being mirror-symmetric about the axis through the inclusion, all four C66 constructions vanish quadratically along it; reversing the source polarisation moves that zero to C11, C22 and C12, as predicted: a causal test. For the density and the three normal-block coefficients the classical maps localise the inclusion to within two tenths of a defect radius, which the indicator does not improve. Its benefit is specific to C66, whose multipolar map keeps a fitted centre of symmetry within half a defect radius of the inclusion under both apertures, against classical displacements of 0.83 and at least 1.5 radii.
