Variability of geophysical log data and the signature of crustal heterogeneities at the KTB

Abstract

Small-scale variations in geophysical borehole data reflect geological inhomogeneities in the Earth's crust; that is, its petrological-mineralogical and structural organization. In this paper, comprehensive geophysical log data from the boreholes of the German Continental Deep Drilling Program (KTB) are analysed. They include electric, radio-genic, magnetic, seismic and bulk properties of rocks. The logs were recorded in crystalline basement down to 9 km depth with a sampling interval of 0.15 m. We describe the vertical variability within the data series in order to relate their statistics to the small-scale heterogeneities of the upper crust. For each physical property, spectral exponents b and Hurst coefficients H were calculated. Power spectra indicate a scaling behaviour, with b values mostly falling in the range 0.5≤b≤1.5, typical of 1 / f noise; however, no particular values of b characterize the log data. Rescaled range analyses show long-range correlations with persistence of the signals from metres to thousands of metres. Values of H, ranging between 0.6 and 0.8, reveal the self-affine character of the data series. All the log data present similar statistics, suggesting a fractal variability of the physical properties of the Earth's crust. The inferred scale invariance helps to quantify the complexity of crustal heterogeneities.