Effect of Very Low Frequency Ground Motion on the LHC

The power spectrum of ground motion noise is known to increase
extremely fast with decreasing frequency. The wavelength of groundwaves
will eventually become larger than the machine dimensions. Ideally the
effect should disappear for these long waves since all the elements of
the machine are supposed to move in the same way. In fact this is not
the case since these long powerfull waves loose coherence. Therefore
relatively slow orbit drifts are to be expected in the LHC. The beams
will tend to separate in the collision points if no counter measures
are taken. A model is presented based on geophysical arguments and it
is confronted with observations concerning slow orbit changes in large
existing accelerators.