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

With the cooperation and collaboration of Courtier (Cantley), Ducarne (Brussels), Goodkind (Pinon Flats), Hinderer (Strasbourg), Imanishi and Seama (Kakioka), and Sun (Wuhan), a very large data set of global superconducting gravimeter 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 tidal signals, and barometric effects were fitted and corrected off. An examination of the resulting residuals typically shows spikes and offsets (glitches) which cannot be true gravity observations and which raise the background noise level dramatically. In the case of the Cantley record, 1-min samples were available and a spline differentiator was used as an automated glitch detector. Fifteen previously undetected glitches were found and repaired by spline interpolation. Each has been associated with a real physical event by comparison with daily log files from the site and from the data acquisition system. The repaired record is very low noise, comparable to the first long record taken at Pinon Flats. The other stations an in less favourable sites but still provide long continuous records with the exception of Kakioka which has many short gaps. Both the Kakioka and Wuhan instruments have subsequently been moved to better sites. In the absence of derailed station logs as were available for Cantley, a more arbitrary record repair procedure was developed with hour-to-hour changes of more than 1 mu gal being replaced by gaps of constant values, and the previous and subsequent record levels 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 Parzen window with 75% overlap. Product Spectra for data entirely outside Europe confirm the presence of the three resonances found earlier in data from Europe alone [Smylie, D.E., Hinderer, J., Richter, B., Ducarme, B., 1993. The 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 triplet and the density near Earth's center. Science, 255, 1678-1682.]. The observed periods of the equatorial translational modes provides a very precise measure of viscosity near the ICE [Smylie, D.E., McMillan, D.G., 1998. The Inner Core as a Dynamic Viscometer. Phys. Earth Planet. Inter., this volume.]. The gravity signals from the equatorial translational modes travel in longitude at fixed rates in the opposite sense, while that from the axial mode is axisymmetric and does not move in longitude. The signals also have different dependencies on latitude. We have developed a method of exploiting these differences for simultaneous data from a distributed network of stations and tested it with synthetic data. Initial results from a test on real data using the simultaneous portions of the Brussels, Cantley, Kakioka, Strasbourg and Wuhan records are presented. (C) 2000 Elsevier Science B.V. All rights reserved.