Spectral analysis of the KTB sonic and density logs using robust nonparametric
Alan G Jones & Klaus Holliger
We use robust techniques to estimate power spectra, coherences and
transfer functions of the KTB sonic and density logs, and
lithologicallydefined subsets thereof. Our results confirm the
overall 1/wavelengthdecay of the power spectra inferred by parametric
analyses, but provide superior resolution and nonparametric estimates
of errors and statistical significance. We demonstrate the absence of
any statisticallymeaningful coherence between the velocity logs from
the main and pilot holes, suggesting a spatially quasiisotropic upper
crustal velocity structure. Also, there is little coherence between
the physical and caliper logs, indicating that disturbances introduced
by breakouts and uneven relief of the borehole wall mostly contribute
to the uncorrelated portions of the velocity logs. Coherence between
the gamma and physical logs is weak to absent, indicating that the
observed velocity and density fluctuations are dominated by the physical
state of the rocks rather than by their petrological composition.
Attempts to derive Poisson s ratio, and its variation with wavelength,
from the relationship between the shear andcompressional velocity logs
met with limited success, but imply that caution should be exercised
when comparing Poisson's ratio derived from laboratory studies on
samples representative of a region to crustalscale seismic determinations.
Our preferred interpretation is that fluctuations in the physical logs
in the intermediate wavelength range (~10150 m) are dominated by cracks
and their level of fluidsaturation. At larger wavelengths (>50150 m)
the effects of the petrology becomes more significant as shown by changes
in slope of the power spectra and the emerging coherence between the Vp
and the gamma logs.
J. Geophys. Res., 102, 18,391-18,403, 1997.
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Alan G Jones / 10 June 2004 /