3.04 Gravimetric Methods – Superconducting Gravity Meters

88 Superconducting Gravity Meters
Ocean loading corrections are frequently geared
to the need for precise geodetic information in particular
areas such as Antarctica (Bos et al., 2000) or the
Pacific Northwest (Lambert et al., 2003).
3.04.2.2.4 Tidal analysis
Tidal analysis consists of determining the amplitudes
and phases of tidal waves of specific frequencies from
observational data. How many waves can be determined
and to what accuracy depends on the length of
the record used and on the noise characteristics of the
site. In most approaches, the ocean tides are grouped
in with the solid tides of the same frequency, and
they cannot be separated by a simple fit of data to
known tidal frequencies. Ocean tide variability is
reflected in the time dependence of the gravimetric
tidal factors at a particular site.
Three programs are used within the SG community
for tidal analysis. The most widespread program
is ETERNA, developed over many years by Wenzel
(1996b) and now in its final form (version 3.3). It
consists of a suite of several programs that deal with
all the common aspects of processing gravimeter data
and it can be adapted to a variety of different data
sets; it can also be used for analysis of strain and tilt
data as well as gravity. ETERNA is a harmonic
method that does a direct least-squares fitting of the
(, ) factors for various wave groups to gravity data
sampled at fixed intervals, for example, 1 min or 1 h.
The program can handle different sub-blocks of data
if there are gaps. The user has the choice of simultaneously
fitting an admittance function to pressure or
other auxiliary data. It includes polar motion and
different assumptions regarding the instrument drift
function. The program is well documented and available
through ICET.
The second approach is a program called
BAYTAP-G that was developed by Japanese geophysicists
during the 1980s (Tamura et al., 1991).
The approach is a hybrid method using a combination
of harmonic series and the response method
(Lambert, 1974) to estimate the various components
of a gravity record (i.e., the tidal parameters, a pressure
perturbation effect, a ‘drift’ function, and
irregular noise). One of the distinguishing aspects of
the program is a focus on a statistical description of
the ‘drift’, which in this context means the entire
long-term gravity signal and not just the instrument
drift, and of the random noise Using Bayesian estimates,
the procedure involves the nonlinear
estimation of a tradeoff parameter between the tidal
harmonic series and the residual gravity.
A third program is available, called VAV, which
was described most recently by Venedikov and Viera
(2004). The origins of this approach go back 50 years
or more when gravimeter records were processed by
the application of suitable filters of various lengths to
account for the tides. Comparisons exist between
VAV and ETERNA but the former has not been as
widely adopted by the SG community as the other
programs. Dierks and Neumeyer (2002) compared all
three programs using both synthetic data and a 1 year
observed SG data set from station Sutherland (SA).
They found the performance of the three programs
to be similar, but with different treatments of the
statistics between signals (tides, pressure, and drift)
and residual gravity. The spectra of the final residuals
were noticeably different between the programs, and
the reader should consult their paper for details (also
for valuable tips on how to set some of the parameters
in the program inputs).
A few words should be said about the response
method in SG tidal analysis, as discussed by Merriam
(2000). Rather than allocating for all waves within a
group the same gravimetric factor, the response
method selectively interpolates the gravimetric factors
using only waves that seem unperturbed by noise
(i.e., that are close to their theoretical expectations).
Waves that seem anomalous are thus treated independently,
requiring a more hands-on treatment, but
one that has some practical advantages. The additional
flexibility allows a greater reduction in tidal
residuals, because it can accommodate the more variable
ocean loading.
3.04.2.3 Atmospheric Pressure Effects
The atmosphere provides a significant gravity effect
(up to 10% of the tidal signal) with a transfer function
(or admittance factor) that approximates –0.3 mGal
hPa 1 for a typical continental station. The effect is a
combination of gravitational attraction by atmospheric
density anomalies with a loading that
vertically deforms the crust and mantle. For a positive
atmospheric density anomaly, simple theory
gives about –0.4 mGal hPa 1 for the upward attraction
and þ0.1 mGal hPa 1 for the loading. A number
of well-studied empirical and physical methods exist
for making a pressure correction to the gravity data,
but even with the most sophisticated treatments it is
not possible to completely remove the atmospheric
pressure effect.