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 4.
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].
