  Application of the large scale gravitational wave interferometers for
measurement of geophysical signals at very low frequencies is considered.
Analysis is concentrated on the mechanism of penetration of quasistatic
geophysical perturbation through the main interferometer output. It is shown
that it has a parametrical nature resulted in slow variations of the optical
transfer function of the interferometer. Geophysical modulation index is
calculated for any harmonical component of the output spectrum, but mainly for
a photon circulation frequency appeared in the case of stochastic illumination
of modes neighbour to the central resonance. Value of the effect is estimated
for different operational regimes of the device. For improvement of geophysical
signal readout a modernization of the instrument with using of two component
resonance optical pump is proposed and a correspondent calculation is carried
out. Numerical estimations for different regimes of the setup are given
together with discussion of possible application for measuring some weak
gravitational effects.
