Fróðskaparrit - 01.01.1975, Síða 35

Earth Tides
43
Centre of Earth Tides located at Brussels. This Centre was
founded in 1958 under the direction of P. Melchior.
New instruments (quartz horizontal pendulums VM), cali-
bration devices, methods of computation, computer pro-
grams have been developed at the Centre.
Presently Bruxelles Centre is a fundamental world point
for calibration of instruments (tiltmeters and gravimeters).
The new problem arising now is the determination of radial
tidal deformation which is necessary to correct laser di-
stance measurements. This deformation is described by the
first Love number called h. It can not be determined di-
rectly but current earth tide measurements give two simple
arithmetic combinations containing h:
the ratio of observed to theoretical tidal amplitude measured
with a gravimeter:
<5 == 1 + h — 3h k (vertical component)
and the similar ratio measured with a tiltmeter:
y = 1 + k — h (horizontal component)
Thus, for h determination, it should be necessary to install
not only gravimeters but also horizontal pendulums or
vertical pendulutns.
Horizontal pendulums are cheap and of easy maintenance
(once a week) but one needs a 50 meters deep place to
install them. If one can find a mine in the vicinity of laser
stations, it could be done.
Vertical pendulums are very expensive and one has to drill
a borehole 50 meters deep to have a good installation. The
borehole is also expensive but not so much as a special
galery. But the azimut of the instrument must be controlled
at the moment of the installation at such a depth. Until
now nothing has yet been published or available to have
a clear idea of the performances of this instrument (calibra-
tion, phase lag etc....).
From present experience, european stations equipped with