400 km are required to resolve postglacial rebound and large-scale, plate-tectonic
interactions. Networks with station spacing of 100 km and 20 km would be required to
resolve megaearthquake and intermediate earthquake processes, respectively.
CAPABILITY OF ABSOLUTE GRAVIMETRY
Absolute gravimeters measure the acceleration of a mass (corner cube) in free fall (or rise and
fall) in a vacuum using a laser wavelength standard (ReNe polarization stabilized or Iodine
stabilized) and an atomic (Rubidium) frequency standard. A gravity value is obtained by
averaging on the order of one thousand drops over a period of about one day. The instrument
most commonly used today (Faller et al., 1983) compares the acceleration of the falling mass
in a Michelson interferometer to the acceleration of a reference corner cube isolated from the
accelerations of the floor. A number of these instruments were built by the Joint Institute of
Laboratory Astrophysics (JILA), Boulder, Colorado and are still in use. A more recent
version of this instrument, the FG5 series, was originally produced by the AXIS Instrument
Company, Boulder, Colorado and featured a number of design improvements (Carter et al.,
1994~ Sasagawa et al., 1995). Other designs of absolute gravimeter are in active use (see
Boulanger et aI., 1991).
Another instrument available for making terrestrial measurements of the temporal variations
in the gravity field is the superconducting gravimeter (Goodkind, 1991) manufactured by
GWR Instruments, San Diego. Superconducting gravimeters are site-fixed and are designed
to provide a stable and continuous record of relative gravity variations over many years.
Absolute gravimeters are transportable and are designed to provide stable and accurate
gravity values over the long term. For practical reason, however, they cannot be operated
continuously for long periods of time and gravity cannot be sampled more frequently than
once every 15 seconds. Comparison of the nominal power density spectra for typical
superconducting and absolute gravimeters (Lambert et aI., 1995) shows that in the period
range from 30 seconds to one day noise levels are expected to be lower for superconducting
gravimeters than for absolute gravimeters. However, below a period of a few days the
absolute gravimeter arguably provides superior performance. Consequently, the transportability and stability of absolute gravimeters make them the instrument of choice for the
measurement of temporal gravity variations. Ideally, absolute gravimeters and superconducting gravimeters would be combined at any given station to achieve optimum noise
characteristics over a large frequency range.
Absolute gravity observations using the JILA series of instruments have been carried out
since the mid to late 1980's. Repeat measurements at some stations (peter et aI., 1992~
Lambert et al., 1994) and international intercomparisons (Boulanger et aI., 1991) indicated
that repeatabilities of about 2-4 IlGai and accuracies of about 7 IlGal are typical for the JILA
instruments. However, offsets of the order of 10-15 IlGal as a result of malfunctions were
also found (Lambert et aI., 1994). Comparisons between JILA-2 and superconducting
gravimeter GWR-12 over a period of three years (1990-1993) at the Canadian Absolute
Gravity Site showed a common response to seasonal variations with a correlation coefficient
of 0.6, provided the daily to weekly variations were filtered out. The standard deviation of
the difference between the filtered absolute and superconducting data was 4.3 IlGaI and has
been taken as the error in field measurements made using JILA-2 during the same period.
Improvements in the wavelength standard (conversion to I-stabilized laser) and in the fringe
detection and counting system of ID.-A-2 since 1993 have improved its performance to a level
approaching that of the FG5 series instruments.
The FG5 series of absolute gravimeter was designed to have an a.ccuracy of 2 IlGal.
24
interactions. Networks with station spacing of 100 km and 20 km would be required to
resolve megaearthquake and intermediate earthquake processes, respectively.
CAPABILITY OF ABSOLUTE GRAVIMETRY
Absolute gravimeters measure the acceleration of a mass (corner cube) in free fall (or rise and
fall) in a vacuum using a laser wavelength standard (ReNe polarization stabilized or Iodine
stabilized) and an atomic (Rubidium) frequency standard. A gravity value is obtained by
averaging on the order of one thousand drops over a period of about one day. The instrument
most commonly used today (Faller et al., 1983) compares the acceleration of the falling mass
in a Michelson interferometer to the acceleration of a reference corner cube isolated from the
accelerations of the floor. A number of these instruments were built by the Joint Institute of
Laboratory Astrophysics (JILA), Boulder, Colorado and are still in use. A more recent
version of this instrument, the FG5 series, was originally produced by the AXIS Instrument
Company, Boulder, Colorado and featured a number of design improvements (Carter et al.,
1994~ Sasagawa et al., 1995). Other designs of absolute gravimeter are in active use (see
Boulanger et aI., 1991).
Another instrument available for making terrestrial measurements of the temporal variations
in the gravity field is the superconducting gravimeter (Goodkind, 1991) manufactured by
GWR Instruments, San Diego. Superconducting gravimeters are site-fixed and are designed
to provide a stable and continuous record of relative gravity variations over many years.
Absolute gravimeters are transportable and are designed to provide stable and accurate
gravity values over the long term. For practical reason, however, they cannot be operated
continuously for long periods of time and gravity cannot be sampled more frequently than
once every 15 seconds. Comparison of the nominal power density spectra for typical
superconducting and absolute gravimeters (Lambert et aI., 1995) shows that in the period
range from 30 seconds to one day noise levels are expected to be lower for superconducting
gravimeters than for absolute gravimeters. However, below a period of a few days the
absolute gravimeter arguably provides superior performance. Consequently, the transportability and stability of absolute gravimeters make them the instrument of choice for the
measurement of temporal gravity variations. Ideally, absolute gravimeters and superconducting gravimeters would be combined at any given station to achieve optimum noise
characteristics over a large frequency range.
Absolute gravity observations using the JILA series of instruments have been carried out
since the mid to late 1980's. Repeat measurements at some stations (peter et aI., 1992~
Lambert et al., 1994) and international intercomparisons (Boulanger et aI., 1991) indicated
that repeatabilities of about 2-4 IlGai and accuracies of about 7 IlGal are typical for the JILA
instruments. However, offsets of the order of 10-15 IlGal as a result of malfunctions were
also found (Lambert et aI., 1994). Comparisons between JILA-2 and superconducting
gravimeter GWR-12 over a period of three years (1990-1993) at the Canadian Absolute
Gravity Site showed a common response to seasonal variations with a correlation coefficient
of 0.6, provided the daily to weekly variations were filtered out. The standard deviation of
the difference between the filtered absolute and superconducting data was 4.3 IlGaI and has
been taken as the error in field measurements made using JILA-2 during the same period.
Improvements in the wavelength standard (conversion to I-stabilized laser) and in the fringe
detection and counting system of ID.-A-2 since 1993 have improved its performance to a level
approaching that of the FG5 series instruments.
The FG5 series of absolute gravimeter was designed to have an a.ccuracy of 2 IlGal.
24