M. Freybourger et al., COMPARATIVE-STUDY OF SUPERCONDUCTING GRAVIMETERS AND BROAD-BAND SEISMOMETERS STS-1 Z IN SEISMIC AND SUBSEISMIC FREQUENCY BANDS/, Physics of the earth and planetary interiors, 101(3-4), 1997, pp. 203-217
Superconducting gravimeters and broadband seismometers (vertical compo
nent) both measure gravity, but whereas the former are most sensitive
to very long period signals (gravity tides with periods longer than 6
h), the fatter are designed for recording the seismic band (elastic no
rmal modes with periods shorter than 1 h). We investigate here the beh
aviour of each type of instrument in the spectral band where it is not
generally used. More precisely, we compare the French superconducting
gravimeter, located at Station J9 near Strasbourg, and the vertical c
omponent of an STS-1 seismometer located in a mine at Echery (ECH) in
the Vosges, about 70 km away. Two different frequency bands are consid
ered: the seismic band (frequencies between 0.2 and 1.667 mHz), for th
e study of normal modes after the Bolivian earthquake of 9 June 1994,
and the subseismic band (frequencies lower than 0.2 mHz), including th
e study of gravity tides. The analysis of Fourier amplitude spectra an
d power spectral densities shows the obvious result that the broadband
seismometer is more sensitive than the superconducting gravimeter in
the seismic band because of a lower noise level, whereas the reverse i
s true in the subseismic band. The poorer quality of the gravimeter re
cord in the seismic band is probably due to site effects (sediments vs
. bedrock) rather than of instrumental origin. In contrast, the higher
noise level of the seismometer in the subseismic band is probably due
to the temperature response of the instrument. It is expected that op
erating the STS-1 isothermally, or recording on-site temperature chang
es for correction will considerably improve its signal-to-noise ratio
in the subseismic band. In comparison with recent mean models of high
and low seismic background noise levels, both instruments nevertheless
indicate low noise levels at all frequencies. (C) 1997 Elsevier Scien
ce B.V.