4.1 Seismic noise

Seismic noise at a reasonably quiet site on the earth follows a spectrum in all three dimensions close to 10–7f–2 m(Hz)–1/2 (where here and elsewhere we measure f in Hz) and thus if the disturbance to each test mass must be less than 3 × 10–20 m(Hz)–1/2 at, for example, 30 Hz then the reduction of seismic noise required at that frequency in the horizontal direction is greater than 109. Since there is liable to be some coupling of vertical noise through to the horizontal axis, along which the gravitational wave induced strains are to be sensed, a significant level of isolation has to be provided in the vertical direction also. Isolation can be provided in a relatively simple way by making use of the fact that, for a simple pendulum system, the transfer function to the pendulum mass of the horizontal motion of the suspension point falls off as 1/(frequency)2 above the pendulum resonance. In a similar way isolation can be achieved in the vertical direction by suspending a mass on a spring. In the case of the VIRGO detector system the design allows operation to below 10 Hz and here a seven stage horizontal pendulum arrangement is adopted with six of the upper stages being suspended with cantilever springs to provide vertical isolation [9]. Similar systems are being developed in Australia [49] and at Caltech [21]. For the GEO 600 detector, where operation down to 50 Hz is sought, a triple pendulum system is used with the first two stages being hung from cantilever springs to provide the vertical isolation necessary to achieve the desired performance. This arrangement is then hung from a plate mounted on passive ‘rubber’ isolation mounts and on an active (electro-mechanical) anti-vibration system [72][98] as shown in Figure 4View Image.
View Image

Figure 4: Schematic view of one suspension system for use in the GEO 600 interferometer.

In order to cut down motions at the pendulum frequencies, active damping of the pendulum modes has to be incorporated, and to reduce excess motions at low frequencies around the micro-seismic peak, low frequency isolators have to be incorporated. These low frequency isolators can take different forms – tall inverted pendulums in the horizontal direction and cantilever springs whose stiffness is reduced by means of attractive forces between magnets for the vertical direction in the case of the VIRGO system [59], Scott-Russell mechanical linkages in the horizontal and torsion bar arrangements in the vertical for an Australian design [110], and a seismometer/actuator system for GEO 600 [71].

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