  Steady meridional flow makes no first-order perturbation to the frequencies
of helioseismic normal modes. It does, however, Doppler shift the local
wavenumber, thereby distorting the eigenfunctions. For high-degree modes, whose
peaks in a power spectrum are blended into continuous ridges, the effect of the
distortion is to shift the locations of those ridges. From this blended
superposition of modes, one can isolate oppositely directed wave components
with the same local horizontal wavenumber and measure a frequency difference
which can be safely used to infer the subsurface background flow. But such a
procedure fails for the components of the more-deeply-penetrating low-degree
modes that are not blended into ridges. Instead, one must analyze the spatial
distortions explicitly. With a simple toy model, we illustrate one method by
which that might be accomplished by measuring the spatial variation of the
oscillation phase. We estimate that by this procedure it might be possible to
infer meridional flow deep in the solar convection zone.
