  We present the results of Monte-Carlo simulations of three-dimensional
electromagnetic cascade initiated by interactions of the multi-TeV gamma-rays
with the cosmological infrared/optical photon background in the intergalactic
medium. Secondary electrons in the cascade are deflected by the intergalactic
magnetic fields before they scatter on CMB photons. This leads to extended
0.1-10 degree scale emission at multi-GeV and TeV energies around extragalactic
sources of very-high-energy gamma-rays. The morphology of the extended emission
depends, in general, on the properties of magnetic fields in the intergalactic
medium. Using Monte-Carlo simulated data sets, we demonstrate that the decrease
of the size of extended source with the increase of energy allows to measure
weak magnetic fields with magnitudes in the range from < 1e-16 G to 1e-12 G if
they exist in the voids of the Large Scale Structure.
