3.04 Gravimetric Methods – Superconducting Gravity Meters

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84 Superconducting Gravity Meters (a) Earthquake data (5 Dec.1997) 140 120 nm s –2 100 JH JPB DC 2 80 SR 312.0 312.5 313.0 Time (days since 1 Jan. 1997) 313.5 (b) 100 Spikes or transient disturbance without offset raw res JPB nm s –2 80 DC 2 JH SR 215.0 215.5 Time (days since 1 Jan. 1997) 216.0 (c) Offset problem due to helium refill (25 Jun. 1997) 35 JPB DC 2 nm s –2 30 raw res 25 SR JH 176.0 176.5 177.0 Time (days since 1 Jan. 1997) Figure 9 Examples of the data corrections done independently by different operators (identified by initials) for the removal/ interpolation of (a) an earthquake, (b) disturbances without offsets, and (c) an offset caused by a helium refill. From Hinderer J, Rosat S, Crossley D, Amalvict M, Boy J-P, and Gegout P (2002a) Influence of different processing methods on the retrieval of gravity signals from GGP data. Bulletin d9Informations des Marées Terrestres 123: 9278–9301. Long gaps in a record make determination of offsets difficult, if not impossible. There are two saving strategies – either to rely on the co-location of AG measurement to anchor the SG record at the ends of a long gap, or possibly use a fit to the polar motion when there is a suspected large offset within the
Superconducting Gravity Meters 85 (a) 10 4 Residual gravity PSD 10 3 (nm s –2 ) 2 Hz –1 10 2 10 1 10 0 10 –1 DC 1 JH SR DC 2 JPB NLNM 10 –2 10 –5 10 –4 10 –3 10 –2 Frequency (Hz) (b) 140 120 100 Test period: Mar.–Dec. 1997 nm s –2 80 60 40 20 0 JH SR DC 2 JPB 100 150 200 250 300 350 Time (days since 1 Jan. 1997) Figure 10 The cumulative effect of data corrections for a test period of 6 months of data from different treatments (a) in the time domain, showing a 1.5 mGal offset between treatments and (b) in the frequency domain – NLNM is the new low noise model of Hinderer et al. (2002a). 400 downtime of the gravimeter. Long gaps in the record are generally due to serious problems in the instrument or data acquisition system, so an offset is certainly possible. Unfortunately, other instruments (e.g., spring gravimeters), even when available, have too much drift to provide a good interpolation signal to fill in an SG record for more than a few hours. contain the more complex effects such as hydrology. In all other respects, however, the data in the good portions are identical to the observations, and the fixed gravity can now be analyzed for the components in eqn [1]. By subtracting g (fixed) from g (observed), it is always possible to see exactly what has been removed during this stage of the processing. 3.04.2.1.8 Restoring the signal When all problems have been removed from g (temp), it is easy to recover the observed gravity from [2], so that g ðfixedÞ ¼g ðtempÞþg ðremovedÞ is now the observed gravity with the problem segments repaired. Referring to eqn [1], we note that the fixed data have a signal in the gaps that do not ½3Š 3.04.2.2 Solid Earth and Ocean Tides 3.04.2.2.1 Tide-generating potential The basis for computing a theoretical gravity tide is the tide-generating potential (TGP). Using the orbital and rotational data for the Earth as forced by the Sun and Moon, this potential was first given as a catalog by Doodson (1921), who included only terms of degree 1–3 (i.e., 24, 12, and 8 h periods). The tides occur in the
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3.04 Gravimetric Methods – Superconducting Gravity Meters

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