  Determining magnetic field properties in different environments of the cosmic
large-scale structure as well as their evolution over redshift is a fundamental
step toward uncovering the origin of cosmic magnetic fields. Radio observations
permit the study of extragalactic magnetic fields via measurements of the
Faraday depth of extragalactic radio sources. Our aim is to investigate how
much different extragalactic environments contribute to the Faraday depth
variance of these sources. We develop a Bayesian algorithm to distinguish
statistically Faraday depth variance contributions intrinsic to the source from
those due to the medium between the source and the observer. In our algorithm
the Galactic foreground and the measurement noise are taken into account as the
uncertainty correlations of the galactic model. Additionally, our algorithm
allows for the investigation of possible redshift evolution of the
extragalactic contribution. This work presents the derivation of the algorithm
and tests performed on mock observations. With cosmic magnetism being one of
the key science projects of the new generation of radio interferometers we have
made predictions for the algorithm's performance on data from the next
generation of radio interferometers. Applications to real data are left for
future work.
