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Agnew, DC.  2018.  An improbable observation of the diurnal core resonance. Pure and Applied Geophysics. 175:1599-1609.   10.1007/s00024-017-1522-1   AbstractWebsite

The resonance associated with the ellipticity of the core-mantle boundary is usually measured with observations of either the Earth's nutations, or of tidal gravity, strain, or tilt. But, improbably, it can also be seen in a dataset collected and processed with older and simpler technologies: the harmonic constants for the ocean tides. One effect of the resonance is to decrease the ratio of the amplitude of the constituent to the amplitude of the constituent to 0.96 of the ratio in the equilibrium tidal potential. The compilation of ocean-tide harmonic constants prepared by the International Hydrographic Bureau between 1930 and 1980 shows considerable scatter in this ratio; however, if problematic stations and regions are removed, this dataset clearly shows a decreased ratio. While these data apply only a weak constraint to the frequency of the resonance, they also show that the effect could have been observed long before it actually was.

Hart, RHG, Gladwin MT, Gwyther RL, Agnew DC, Wyatt FK.  1996.  Tidal calibration of borehole strain meters: Removing the effects of small-scale inhomogeneity. Journal of Geophysical Research-Solid Earth. 101:25553-25571.   10.1029/96jb02273   AbstractWebsite

We investigate the estimation of Earth strain from borehole strain meter data in a study of tidal calibration of the Gladwin borehole tensor strain meter (BTSM) at Pinon Flat. Small-scale geological inhomogeneity is one of several effects examined that cross couple remote areal/shear strain into measured areal/shear strain. A methodology is developed for incorporating cross coupling into the strain meter calibration. Using the measured strain tides from the colocated laser strain meter (LSM) as a reference, we show that calibration of the BTSM with cross coupling removes systematic errors of up to 30% in the borehole strain meter tides. This calibration accurately relates the BTSM measurements to strains at the scale length of the LSM, about 1 km. The calibration technique provides a solution to a major criticism of all short-baseline strain measurements, namely, that tectonic strains are not representatively sampled due to small-scale inhomogeneities. The technique removes errors potentially greater than 50% in observed strain offsets from fault slip, permitting improved constraint of slip mechanisms. We show that current theoretical estimates of strain tides in the instrument locality are not yet of sufficient accuracy for cross-coupled calibration. Comparison of theoretical tides with measurements from the LSM suggest that at least half of the error is in the ocean load tide estimates.

Cummins, P, Wahr JM, Agnew DC, Tamura Y.  1991.  Constraining Core Undertones Using Stacked IDA Gravity Records. Geophysical Journal International. 106:189-198. AbstractWebsite

A search is made for oscillatory gravity signals, possibly associated with core undertones, in the frequency range between the diurnal and semidiurnal tidal bands. IDA tidal data recorded during 6 month intervals following four large earthquakes were used, and the data from different stations were stacked according to spherical harmonic surface amplitude patterns with angular order l less-than-or-equal-to 4. The detection level of a signal in the diurnal-semidiurnal frequency band was roughly 10-20 ngal, but the only signal detected was a 9 ngal signal of 9.54 hr period whose origin remains unexplained. Two of the earthquakes were chosen because of claims that core undertones were excited following these events, and it is established that signals of the required amplitude, if they exist, can only be associated with a few spherical harmonic patterns for which the Brussels instrument is near an antinode. We also used NCAR barometric data to correct the gravity recordings for the effects of atmospheric pressure variations, which reduces the noise level in the diurnal-semidiurnal frequency band by 2-3 dB.