  Several works have reported changes of the Sun's subsurface stratification
inferred from f-mode or p-mode observations. Recently a non-homologous
variation of the subsurface layers with depth and time has been deduced from
f-modes. Progress on this important transition zone between the solar interior
and the external part supposes a good understanding of the interplay between
the different processes which contribute to this variation. This paper is the
first of a series where we aim to study these layers from the theoretical point
of view. For this first paper, we use solar models obtained with the CESAM
code, in its classical form, and analyze the properties of the computed
theoretical f-modes. We examine how a pure variation in the calibrated radius
influences the subsurface structure and we show also the impact of an
additional change of composition on the same layers. Then we use an inversion
procedure to quantify the corresponding f-mode variation and their capacity to
infer the radius variation. We deduce an estimate of the amplitude of the
11-year cyclic photospheric radius variation.
