  Unveiling the intergalactic magnetic field (IGMF) in filaments of galaxies is
a very important and challenging subject in modern astronomy. In order to probe
the IGMF from rotation measures (RMs) of extragalactic radio sources, we need
to separate RMs due to other origins such as the source, intervening galaxies,
and our Galaxy. In this paper, we discuss observational strategies for the
separation by means of Faraday tomography (Faraday RM Synthesis). We consider
an observation of a single radio source such as a radio galaxy or a quasar
viewed through the Galaxy and the cosmic web. We then compare the observation
with another observation of a neighbor source with a small angular separation.
Our simulations with simple models of the sources suggest that it would be not
easy to detect the RM due to the IGMF of order ~ 1 rad/m/m, an expected value
for the IGMF through a single filament. Contrary to it, we find that the RM of
at least ~10 rad/m/m could be detected with the SKA or its
pathfinders/precursors, if we achieve selections of ideal sources. These
results would be improved if we incorporate decomposition techniques such as
RMCLEAN and QU-fitting. We discuss feasibility of the strategies for cases with
complex Galactic emissions as well as with effects of observational noise and
radio frequency interferences.
