Global superconducting gravimeter observations and the search for the translational modes of the inner core

With the cooperation and collaboration of Courtier (Cantley), Ducarne (Brus sels), Goodkind (Pinon Flats), Hinderer (Strasbourg), Imanishi and Seama (K akioka), and Sun (Wuhan), a very large data set of global superconducting g ravimeter observations (294,106 h) has been assembled for analysis. Except for the Brussels record, for which residuals were provided, synthetic tides for each station generated by Merriam's G-wave code were used to remove ti dal signals, and barometric effects were fitted and corrected off. An exami nation of the resulting residuals typically shows spikes and offsets (glitc hes) which cannot be true gravity observations and which raise the backgrou nd noise level dramatically. In the case of the Cantley record, 1-min sampl es were available and a spline differentiator was used as an automated glit ch detector. Fifteen previously undetected glitches were found and repaired by spline interpolation. Each has been associated with a real physical eve nt by comparison with daily log files from the site and from the data acqui sition system. The repaired record is very low noise, comparable to the fir st long record taken at Pinon Flats. The other stations an in less favourab le sites but still provide long continuous records with the exception of Ka kioka which has many short gaps. Both the Kakioka and Wuhan instruments hav e subsequently been moved to better sites. In the absence of derailed stati on logs as were available for Cantley, a more arbitrary record repair proce dure was developed with hour-to-hour changes of more than 1 mu gal being re placed by gaps of constant values, and the previous and subsequent record l evels adjusted to eliminate discontinuities at the end points of all gaps. This procedure was applied to both the Strasbourg and Kakioka series. Power spectra for each residual series were then estimated using a 12,000-h Parz en window with 75% overlap. Product Spectra for data entirely outside Europ e confirm the presence of the three resonances found earlier in data from E urope alone [Smylie, D.E., Hinderer, J., Richter, B., Ducarme, B., 1993. Th e product spectra of gravity and barometric pressure in Europe. Phys. Earth Planet. Inter., 80, 135-137.], and associated with the three translational motions of the inner core by the strict adherence of their central periods to splitting laws [Smylie, D.E., 1992. The inner core translational triple t and the density near Earth's center. Science, 255, 1678-1682.]. The obser ved periods of the equatorial translational modes provides a very precise m easure of viscosity near the ICE [Smylie, D.E., McMillan, D.G., 1998. The I nner Core as a Dynamic Viscometer. Phys. Earth Planet. Inter., this volume. ]. The gravity signals from the equatorial translational modes travel in lo ngitude at fixed rates in the opposite sense, while that from the axial mod e is axisymmetric and does not move in longitude. The signals also have dif ferent dependencies on latitude. We have developed a method of exploiting t hese differences for simultaneous data from a distributed network of statio ns and tested it with synthetic data. Initial results from a test on real d ata using the simultaneous portions of the Brussels, Cantley, Kakioka, Stra sbourg and Wuhan records are presented. (C) 2000 Elsevier Science B.V. All rights reserved.