  We demonstrate a proof-of-principle of direct Earth gravity gradient
measurement with an atom interferometer-based gravity gradiomter using a single
proof mass of cold 87 rubidium atoms. The atomic gradiometer is implemented in
the so-called double-loop configuration, hence providing a direct gravity
gradient dependent phase shift insensitive do DC acceleration and constant
rotation rate. The atom interferometer (AI) can be either operated as a
gravimeter or a gradiomter by simply adding an extra Raman $\pi$-pulse. We
demonstrate gravity gradient measurements first using a vibration isolation
platform and second without seismic isolation using the correlation between the
AI signal and the vibration signal measured by an auxilliary classical
accelerometer. The simplicity of the experimental setup (a single atomic source
and unique detection) and the immunity of the AI to rotation-induced contrast
loss, make it a good candidate for onboard gravity gradient measurements.
