Аннотация:Ultra low frequency perturbations of free mass gravitational antenna produced by surrounding can be considered from the two point of view: a possibility of it utilization for to estimete some characteristics of the antenna and a possibility of it measuring to extract a useful geophysical information as by-product in respect of the main goal of the setup. In particulary in the low frequency region 10^{-4} - 10^{-3} Hz a long base gravitational interferometer can be presented as a differential tiltmeter insensitive to perturbations of a local normal to the land. The effect of variations of reciprocal deflection of two pendulum mirrors is estimated for three conceivable scenarios of the earth core movements. the possibility of measurement of this effect is discussed according to the noise background estimate typical for the modern gravitational atenna projects.