Peculiarities in the work of laser gravitational antennas in the infra-low frequency range

We study the methodology of measuring quasistatic perturbations of geophysical nature in laser gravitational-wave interferometers. We calculate the transfer function of such an instrument in the destructive interference mode (the “dark spot” mode) taking into account the natural asymmetry of the optical Fabry-Perot cavity lengths in its arms. Formulas have been obtained for estimating the output signal power modulation at the frequency of photon circulation in the arms due to geophysical perturbations. The estimates are compared with the results of observations at the LIGO interferometer.