  Magnification and de-magnification due to gravitational lensing will
contribute to the brightness scatter of Type Ia supernovae (SNe Ia). The
purpose of this paper is to investigate the possibility to decrease this
scatter by correcting individual SNe Ia using observations of galaxies in the
foreground, without introducing any extra bias. We simulate a large number of
SN Ia lines of sight populated by galaxies. For each line of sight the true
magnification factor and an estimate thereof are calculated. The estimated
magnification factor corresponds to what an observer would infer from a survey
like SNLS. Using the simulated data we investigate the possibility to estimate
the magnification of individual supernovae with enough precision to be able to
correct their brightness for gravitational lensing with negligible bias. Our
simulations show that the bias arising from gravitational lensing corrections
of individual SNe Ia is negligible for current and next generation surveys and
that the scatter from lensing can be reduced by approximately a factor 2. The
total scatter in the SN Ia magnitudes could be reduced by 4% for an intrinsic
dispersion of 0.13 mag. For an intrinsic dispersion of 0.09 mag, which might
not be to unrealistic for future surveys, the total scatter could be reduced by
6%. This will reduce the errors on cosmological parameters derived from
supernova data by 4-8%. The prospect of correcting for lensing is thus very
good.
