  Fluctuations of the earth's gravity field are a major noise source for
ground-based experiments investigating general relativity phenomena such as
Gravitational Waves (GWs). Mass density variations caused by local seismic or
atmospheric perturbations determine spurious differential displacements of the
free falling test masses, what is called Gravity Gradient Noise (GGN); it
mimics GW effects. This GGN is expected to become dominant in the infrasound
domain and must be tackled for the future realization of observatories
exploring GWs at low frequency. GGN will be studied with the MIGA experiment, a
demonstrator for low frequency GW detection based on atom interferometry - now
in construction at the low noise underground laboratory LSBB in France. MIGA
will provide precise measurements of local gravity, probed by a network of
three free-falling atom test masses separated up to 150~m. We model the effect
of GGN for MIGA and use seismic and atmospheric data recorded at LSBB to
characterize their impact on the future measurements. We show that the antenna
will be able to characterize GGN using dedicated data analysis methods.
