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Geophysical studies with laser-beam detectors of gravitational waves

The existing high technology laser-beam detectors of gravitational waves may find very useful applications in an unexpected area—geophysics. To make possible the detection of weak gravitational waves in the region of high frequencies of astrophysical interest, ∼30–103 Hz, control systems of laser interferometers must permanently monitor, record and compensate much larger external interventions that take place in the region of low frequencies of geophysical interest, ∼10−5–3 × 10−3 Hz. Such phenomena as tidal perturbations of land and gravity, normal mode oscillations of the Earth, oscillations of the inner core of the Earth, etc will inevitably affect the performance of the interferometers and, therefore, information about them will be stored in the data of control systems. We specifically identify the low-frequency information contained in distances between the interferometer mirrors (deformation of the Earth) and angles between the mirrors' suspensions (deviations of local gravity vectors and plumb lines). We show that access to angular information may require some modest amendments to the optical scheme of the interferometers, and we suggest ways of doing that. The detailed evaluation of environmental and instrumental noises indicates that they will not prevent, even if only marginally, the detection of interesting geophysical phenomena. Gravitational-wave instruments seem to be capable of reaching, as a by-product of their continuous operation, very ambitious geophysical goals, such as observation of the Earth's inner core oscillations.