A search for the Slichter triplet in superconducting gravimeter data

Abstract

There have been several investigations in the past for weak harmonic signals in the residual spectrum of superconducting gravimeter (SG) data. Recently, a stack of four European SG records led to the claim of the detection of the Slichter mode of translation of the inner core (Smylie, Science, 1992, 255: 1678–1682). We attempt to verify the presence of the Slichter triplet by two methods. We re-analyse the four European SG records with a spectral method differing from that used by Smylie et al. (Phys. Earth Planet. Inter., 1993, 80: 135–157) and compare the results to two homogeneous 2 year data sets from the French and Canadian SG. We find three weak peaks in the European stack coincident with the claimed triplet using a product spectrum, but these disappear when a cross spectrum is used which takes into account phase information. Further we find no evidence of the triplet in the 2 year data sets, where the noise level in the core-mode band is one order of magnitude lower than in the European stack. Moreover various synthetic tests (e.g. pure harmonics in random white noise; damped harmonics in brown noise) lead to the conclusion that a simulated Slichter triplet with characteristics similar to that claimed, would be easily detectable in our less noisy data, especially in cross spectral estimates.