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. Dierks and Neumeyer (2002) compared all three programs using both synthetic data and a 1 year observed SG data set from station Sutherland (SA). Comparisons exist between VAV and ETERNA but the former has not been as widely adopted by the SG community as the other programs. 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.
Tidal wave series#
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.Ī third program is available, called VAV, which was described most recently by Venedikov and Viera (2004). 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). The second approach is a program called BAYTAP-G that was developed by Japanese geophysicists during the 1980s ( Tamura et al., 1991). The program is well documented and available through ICET. It includes polar motion and different assumptions regarding the instrument drift function. The user has the choice of simultaneously fitting an admittance function to pressure or other auxiliary data. The program can handle different sub-blocks of data if there are gaps. ETERNA is a harmonic method that does a direct least-squares fitting of the (δ, κ) factors for various wave groups to gravity data sampled at fixed intervals, for example, 1 min or 1 h. 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. The most widespread program is ETERNA, developed over many years by Wenzel (1996b) and now in its final form (version 3.3). Three programs are used within the SG community for tidal analysis. Ocean tide variability is reflected in the time dependence of the gravimetric tidal factors at a particular 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. 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. Tidal analysis consists of determining the amplitudes and phases of tidal waves of specific frequencies from observational data.
Warburton, in Treatise on Geophysics, 2007 3.04.2.2.4 Tidal analysis
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