KTB Borehole Research Articles

Compressive and tensile failure of boreholes arbitrarily-inclined to principal stress axes: Application to the KTB boreholes, Germany

Compressive and tensile failure of boreholes arbitrarily-inclined to principal stress axes: Application to the KTB boreholes, Germany

Upper-crustal strength inferred from stress measurements to 6 km depth in the KTB borehole

IT has been suggested1–6that in many cases the average strength of the continental crust is quite low (tens of megapascals), so that the crust has little effect on the large-scale deformation of the lithosphere. But laboratory friction studies7,8, combined with simple faulting theory9,10 (as well as extrapolation ofin situ stress measurements from the upper 3 km of the crust11), imply that if pore pressure is approximately hydrostatic at mid-crustal depth, crustal strength is appreciable (hundreds of megapascals) and would markedly constrain the nature of lithospheric deformation12–15. Here we report estimates of the magnitude of in situstresses to 6 km depth in the KTB borehole in southern Germany. Our results indicate a high-strength upper crust, in which the state of stress is in equilibrium with its frictional strength. We suggest that plate-driving forces in the continental lithosphere in this part of western Europe are transmitted principally through the upper crust, and that this may also be the case in other continental areas of moderate to elevated heat flow.

Electromagnetic measurements in the vicinity of the KTB drill site. Part II. Magnetotelluric results

In addition to the magnetovariational measurements across an array in Western Bohemia, close to the KTB ultradeep borehole (Germany), discussed in part I of this paper [1], magnetotelluric results from pivot sounding point Ostrůvek within the array are presented here. Good quality of long-period magnetotelluric data (period range from 30 s to about 1 hour) allowed structural dimensionality of the medium to be analysed in detail. The geoelectrical structure was identified as a slightly distorted two-dimensional regional substratum, with dominating E - W strike, overlaid by a heterogeneous subsurface layer with extremely strong and anisotropic galvanic distortion effect on the magnetotelluric data. Estimating the total static shift distortion tensor by fitting the local magnetotelluric curve to the curve of the global magnetovariational soundings (for the European continent), the static distortions were identified as of generally multidirectional origin. The resulting telluric ellipse is, however, strongly anisotropic, indicating an approximately SW - NE apparent local strike, which is in the approximate agreement with remote reference magnetovariational results. Finally, the magnetotelluric results from the station Ostrůvek are compared with long-period data from the immediate neighbourhood of the KTB borehole on the German territory.

Lithological and fracture response of common logs in crystalline rocks

Abstract Log responses in crystalline rocks are a function of the lithology (rock matrix), macro- and microfractures. Basically, the porespace-geometry dependent petrophysical parameters (porosity, internal surface area, permeability, formation factor) are similarly interrelated as in sediments, but mineralogy and fractures exert greater control than in sediments. A qualitative log interpretation method was developed and tested. Fractured and otherwise deteriorated zones with anomalous properties are identified, using a suitable combination of five standard logs. Before a quantitative well-log analysis of petrophysical parameters is possible, the logs are corrected for lithology effects, using a quick-look lithology identification program. For lithology identification seven logs are considered sufficient. Their mean lithology-dependent response in the German KTB borehole is reported.

Laboratory measurements of electrical conductivity of biotites

For the sake of helping to clarify the anisotropic behaviour of KTB borehole rock samples, AC (103 Hz) and DC electrical conductivities of powdered biotites and flogopites (sheet silicates) were investigated in various atmospheres (air, argon) at temperatures of 20–1200°C.

Why is the electrical resistivity around the KTB hole so low?

The low-resistivity anomaly close to the KTB borehole coincides with both a self-potential and a static magnetic anomaly. If this coincidence is not accidental, it may yield information about the conditions required for the existence of low-resistivity anomalies in the deeper crust. A possible answer to the question in the title of this paper is that the oxygen fugacity in the crust around the KTB hole must be low enough to stabilize graphite on a grain boundary scale. This could partially explain the extremely low resistivities of the graphitized cataclastic zones, the formation of magnetic pyrrhotite from ∼ 250 to 4000 m deep and the large self-potential anomaly. The latter requires a large vertical gradient of the oxygen fugacity and a continuous graphitic conductor through this gradient zone.