  Asteroseismic forward modelling techniques are being used to determine
fundamental properties (e.g. mass, radius, and age) of solar-type stars. The
need to take into account all possible sources of error is of paramount
importance towards a robust determination of stellar properties. We present a
study of 34 solar-type stars for which high signal-to-noise asteroseismic data
is available from multi-year Kepler photometry. We explore the internal
systematics on the stellar properties, that is, associated with the uncertainty
in the input physics used to construct the stellar models. In particular, we
explore the systematics arising from: (i) the inclusion of the diffusion of
helium and heavy elements; and (ii) the uncertainty in solar metallicity
mixture. We also assess the systematics arising from (iii) different surface
correction methods used in optimisation/fitting procedures. The systematics
arising from comparing results of models with and without diffusion are found
to be 0.5%, 0.8%, 2.1%, and 16% in mean density, radius, mass, and age,
respectively. The internal systematics in age are significantly larger than the
statistical uncertainties. We find the internal systematics resulting from the
uncertainty in solar metallicity mixture to be 0.7% in mean density, 0.5% in
radius, 1.4% in mass, and 6.7% in age. The surface correction method by Sonoi
et al. and Ball & Gizon's two-term correction produce the lowest internal
systematics among the different correction methods, namely, ~1%, ~1%, ~2%, and
~8% in mean density, radius, mass, and age, respectively. Stellar masses
obtained using the surface correction methods by Kjeldsen et al. and Ball &
Gizon's one-term correction are systematically higher than those obtained using
frequency ratios.
