  We study the characteristic size and shape of idealized blazar-induced
cascade halos in the $1-100 \, {\rm GeV}$ energy range assuming various
non-helical and helical configurations for the intergalactic magnetic field
(IGMF). While the magnetic field creates an extended halo, the helicity
provides the halo with a twist. Under simplifying assumptions, we assess the
parameter regimes for which it is possible to measure the size and shape of the
halo from a single source and then to deduce properties of the IGMF. We find
that blazar halo measurements with an experiment similar to Fermi-LAT are best
suited to probe a helical magnetic field with strength and coherence length
today in the ranges $10^{-17} \lesssim B_{0} / {\rm Gauss} \lesssim 10^{-13}$
and $10 \, {\rm Mpc} \lesssim \lambda \lesssim 10 \, {\rm Gpc}$ where
$\mathcal{H} \sim B_0^2 / \lambda$ is the magnetic helicity density. Stronger
magnetic fields or smaller coherence scales can still potentially be
investigated, but the connection between the halo morphology and the magnetic
field properties is more involved. Weaker magnetic fields or longer coherence
scales require high photon statistics or superior angular resolution.
