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

Small-scale variations in geophysical borehole data reflect geological inho mogeneities in the Earth's crust; that is, its petrological-mineralogical a nd structural organization. In this paper, comprehensive geophysical log da ta from the boreholes of the German Continental Deep Drilling Program (KTB) are analysed. They include electric, radiogenic, magnetic, seismic and bul k 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 t he small-scale heterogeneities of the upper crust. For each physical proper ty, spectral exponents b and Hurst coefficients H were calculated. Power sp ectra indicate a scaling behaviour, with b values mostly falling in the ran ge 0.5 less than or equal to b less than or equal to 1.5, typical of 1/f no ise; however, no particular values of b characterize-the log data. Rescaled range analyses show long-range correlations with persistence of the signal s 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 physica l properties of the Earth's crust. The inferred scale invariance helps to q uantify the complexity of crustal heterogeneities.