WELL LOG-DERIVED ESTIMATES OF THERMAL-CONDUCTIVITY IN CRYSTALLINE ROCKS PENETRATED BY THE 4-KM DEEP KTB VORBOHRUNG

Well log measurements of compressional and shear velocity (V(p), V(s)) , density, and temperature from the 4 km-deep KTB Vorbohrung (pilot ho le) were applied in a phonon conduction model for the thermal conducti vity of a crystalline solid. The resulting conductivity estimates were compared with conductivities (k(LAB)) measured on the nearly continuo us (91% recovery) core. Previous studies have shown the log-derived co nductivity (K(LOG)) to be within +/- 15 % of k(LAB) in isotropic or fl at-lying anisotropic crystalline rocks. The section penetrated by the KTB pilot hole includes both predominantly isotropic metabasites and h ighly anisotropic gneisses with foliation dips ranging from horizontal to vertical. The predictions of the phonon model were accurate within +/- 4% in the metabasites but inaccurate by as much as 23% in the gne isses. The accuracy of the model in the metabasites confirms the utili ty of the phonon conduction approach in isotropic or weakly anisotropi c rocks, but the discrepancies in the anisotropic gneisses remain unex plained. These relatively large discrepancies between k(LOG) and k(LAB ) correspond to depths at which laboratory measurements of V(s) under in situ conditions deviate from the sonic log V(s). This suggests that sonic log determinations of V(s) may not be reliable in dipping, anis otropic rocks. Alternatively, the laboratory V(s) measurements may not constitute a representative sample, or there may be errors in the pho non conduction model. If the discrepancies can be tied to errors in so nic log V(s) measurements, the phonon conduction model may provide a t ool for deriving thermal conductivity profiles of the Earth's crust fr om seismic studies of V(p) and V(s).