  GRS 1915+105 has been in a bright flux state for more than 2 decades, but in
2018 a significant drop in flux was observed, partly due to changes in the
central engine along with increased X-ray absorption. The aim of this work is
to explore how X-ray spectro-polarimetry can be used to derive the basic
geometrical properties of the absorbing and reflecting matter. In particular,
the expected polarisation of the radiation reflected off the disc and the
putative outflow is calculated. We use \textit{NuSTAR} data collected after the
flux drop to derive the parameters of the system from hard X-ray spectroscopy.
The spectroscopic parameters are then used to derive the expected polarimetric
signal, using results from a MonteCarlo radiative transfer code both in the
case of neutral and fully ionised matter. From the spectral analysis, we find
that the continuum emission becomes softer with increasing flux, and that in
all flux levels the obscuring matter is highly ionised. This analysis, on the
other hand, confirms that spectroscopy alone is unable to put constraints on
the geometry of the reflectors. Simulations show that X-ray polarimetric
observations, like those that will be provided soon by the Imaging X-ray
Polarimetry Explorer (IXPE), will help to determine the geometrical parameters
which are left unconstrained by the spectroscopic analysis.
