Nature and origin of upper crustal seismic velocity fluctuations and associated scaling properties: Combined stochastic analyses of KTB velocity and lithology logs

The main borehole of the German Continental Deep Drilling Program (KTB) ext ends over 9000 m into a crystalline upper crust consisting primarily of int erlayered gneiss and metabasite. We present a joint analysis of the velocit y and lithology logs in an effort to extract the lithology component of the velocity log. Covariance analysis of lithology log, approximated as a bina ry series, indicates that it may originate from the superposition of two Br ownian stochastic processes (fractal dimension 1.5) with characteristic sca les of similar to 2800 m and similar to 150 m, respectively. Covariance ana lysis of the velocity fluctuations provides evidence for the superposition of four stochastic process with distinct characteristic scales. The largest two scales are identical to those derived from the lithology, confirming t hat these scales of:velocity heterogeneity are caused by lithology variatio ns. The third characteristic scale, similar to 20 m, also a Brownian proces s, is probably related to fracturing based on correlation with the resistiv ity log. The superposition of these three Brownian processes closely mimics the commonly observed 1/k decay (fractal dimension 2.0) of the velocity po wer spectrum. The smallest scale process (characteristic scale similar to 1 .7 m) requires a low fractal dimension, similar to 1.0, and accounts for si milar to 60% of the total rms velocity variation. A comparison of successiv e logs from 6900-7140 m depth indicates that such variations are not repeat able and thus probably do not represent true velocity variations in the cru st. The results of this study resolve disparity between the differing publi shed estimates of seismic heterogeneity based on the KTB sonic logs, and br idge the gap between estimates of crustal heterogeneity from geologic maps and borehole logs.