KTB Drill Site Research Articles

Modeling shallow self-potential anomalies over electronic conductors

Field measurements of self-potential indicate that anomalies may be divided in a shallow anomaly and a deep anomaly. This paper presents a new understanding of the current flow causing SP-anomalies, and how this can be modeled by using the analogy between magnetic flux density lines and electric current lines. Mathematical models are presented for shallow SP-anomalies. Several older models may be applied for the deep part of the anomaly. The new model is applied for interpretation of an SP-anomaly from the KTB drilling site in Oberpfalz in Germany. The deep part was interpreted using a plate model which has been known for years. The final curves of the measured anomaly and the calculated curves for the shallow part and the deep part of the anomaly are presented.

Shallow crustal discontinuities inferred from waveforms of microearthquakes: Method and application to KTB Drill Site and West Bohemia Swarm Area

AbstractThe waveforms of microearthquakes are of high frequency and complicated. They contain many phases secondarily generated at crustal interfaces and at small‐scale inhomogeneities. They are highly sensitive to focal mechanisms and thus very different for each station of local networks. However, with a large number of microearthquakes, the scattered waves present in the waveforms can serve for identifying the prominent crustal discontinuities and for determining their depth. In this paper, we develop a new approach for extracting information on crustal structure from such waveforms and apply it for determining depth and lateral variations of crustal discontinuities. We show that strong dependence of microseismic waveforms on radiation pattern requires good station coverage and knowledge of focal mechanisms of the microearthquakes. Analysis of real observations is supported by waveform modeling and by analysis of radiation patterns of scattered waves. The robustness of the inversion for depth of crustal interfaces is achieved by stacking of a large number of waveforms and by applying a grid search algorithm. The method is demonstrated on two microseismic data sets of different origin: microseismicity induced during the Continental Super‐Deep Drilling Project (KTB) 2000 fluid injection experiment and natural seismicity in the West Bohemia swarm region. High‐frequency conversions at the KTB site indicate a prominent interface at depths of 2.3–4.1 km consistent with previous interpretations. Geologically, it may represent the contact of granitoids with much faster metabasites underneath. Seismicity in West Bohemia indicates a strong‐contrast interface at depths of 3.5–6.0 km. This interface is in agreement with previous profiling and might be related to trapping of fluid emanations ascending from the mantle.

Mineralogy and magnetic behavior of pyrrhotite from a 260 °C section at the KTB drilling site, Germany

The ultradeep bore hole of the German Continental Deep Drilling Program (KTB) reached a depth of 9100 m and in situ temperatures of about 260 °C, offering an unique opportunity to study natural pyrrhotite. An integrative approach using optical methods, electron microprobe analysis, X-ray diffraction, transmission electron microscopy (see Posfai et al. 2000), and temperature-dependent magnetic susceptibility measurements were used to characterize pyrrhotite types as a function of lithology and depth. We found a lithology-controlled distribution of pyrrhotite types to a depth of 8080 m, with ferrimagnetic, monoclinic 4C pyrrhotite (metal content 46.0 to 47.2 at%) as the dominant magnetic phase in gneisses and metabasic rocks. In the gneisses, a second pyrrhotite type with higher metal concentrations (46.9 to 48.2 at%) and antiferromagnetic behavior also occurs. At depths greater than 8080 m (in situ temperature > 230 °C) antiferromagnetic pyrrhotite, predominates in all lithologies. That 4C pyrrhotite does not occur below 8080 m, suggests that 4C is unstable above 230 °C in these rocks. Instead of 4C, a 5C type with a ferrimagnetic structure occurs below 8080 m. Thermomagnetic experiments indicate that the metal-poor Weiss-type pyrrhotite is stabilized by oxygen that causes the formation of magnetite during heating. From our observations on natural pyrrhotites we suggest that the magnetic λ-transition is related to the growth of ordered nA pyrrhotite domains to single domain size.

Focusing in Prestack Isochrone Migration Using Instantaneous Slowness Information

—Prestack migration finds increasing application in processing crustal seismic data. However, less effort has been made to incorporate slowness information in the imaging process. The combination of slowness information with migration leads to an improved image in the depth domain, especially by reducing migration artefacts and noise. A slowness-driven isochrone migration scheme is introduced for migration of 2-D seismic data. Instantaneous slowness information p(x, t) is extracted from the data using correlation analysis in moving time and space windows. Slowness values resulting from spatial coherent energy (signal) and incoherent background noise are distinguished by the simultaneous evaluation of an instantaneous coherence criterion g(x, t). In slowness-driven isochrone migration this information is used for locally weighting the amplitude A(x, t) smearing on the isochrone surface. In particular, slowness p and coherence criterion g determine position and sharpness of a Gaussian weighting function. The method is demonstrated using two synthetic data examples and is subsequently applied to two deep crustal data sets, one wide-angle (along DEKORP4) and one steep-angle reflection seismic observation (KTB8506). Both data sets were collected in the surroundings of the KTB drill site, Oberpfalz, as part of the German DEKORP project.

Parameter identification and uncertainty analysis for heat transfer at the KTB drill site using a 2-D inverse method

The German continental deep-drilling program (KTB) provided a unique opportunity for studying heat transfer processes in deep continental crust. We used an inversion technique based on the Bayesian parameter estimation to constrain parameters involved in crustal heat transfer at the KTB site and their standard deviations. Measurements in the two deep KTB boreholes and on rock material recovered from these holes were used to compile a set of a priori information for a 2-D cross-section of 40 km × 30 km. Thermal and hydraulic parameters were determined in a steady-state inverse finite-element model for a simplified geological structure. The inversion algorithm yielded a posteriori information for thermal conductivity, permeability, and heat production rate as well as for temperature and hydraulic head. Because of the large temperature range (up to 800°C), we introduced the variation of thermal conductivity with temperature into the numerical algorithm. While thermal conductivity could be well resolved, the distribution of heat production rate was relatively poorly resolved. The uncertainties for the parameters varied depending on the number of free parameters. If heat production rate was constrained tightly, the resolution of the thermal conductivity was improved. A zone of high heat production rate between 3.8 km and 10 km combined with a reduced thermal conductivity above it provided the best fit to the measured temperatures in the borehole. The steady-state inversion yielded a better solution when paleoclimatic temperature perturbations, such as those caused by the last ice age, were removed from the temperature data.

Three‐dimensional interpretation of the KTB gravity and magnetic anomalies

A combination of a closely spaced surface gravity survey with a high‐resolution helicopter aeromagnetic survey as well as borehole gravity and magnetometer measurements allowed a detailed three‐dimensional (3‐D) modeling of the anomalies at the KTB drill site. The models could be constrained by new evidence from a 3‐D seismic survey and by structural geology and petrophysical data from drill cores and cuttings. The source body for the positive gravity anomaly consist of high‐density metabasite. The vertical derivative of the Bouguer anomaly does not resemble the aeromagnetic anomaly in all areas, indicating that parts of the metabasites are more or less nonmagnetic. Surprisingly and confirming the observation in other deep drill holes into continental crystalline basement rocks, pyrrhotite is the dominant magnetic mineral below a depth of about 300 m. Magnetite mainly occurs in the depth intervals 360–520 m and 7300–7900 m. The lower interval causes the anomalous vertical gradient of 60 nT/km for the geomagnetic field. The occurrence of strongly magnetic minerals in the borehole down to about 3000 m correlates with the lithology, while in the deeper parts it is more related to fissures and fault zones where chemical processes (reduction/oxidation) are active.

Crustal evolution of the KTB drill site: From oldest relics to the Late Hercynian granites

The crustal unit penetrated during the German Continental Deep Drilling Program (KTB), the Zone of Erbendorf‐Vohenstrauss (ZEV), comprises an association of metabasites and paragneisses and orthogneisses that underwent high‐ and medium‐pressure metamorphic cycles during the early Palaeozoic. In this summary of the structural, petrological, geochemical and geochronological information from borehole and surface rocks, we show that geological models proposed prior to drilling have had to be significantly modified. Features of the ZEV, such as the dominant NW‐SE trending structures, Devonian (>370 Ma) medium‐pressure, amphibolite facies, metamorphism (0.6–0.8 GPa, ∼720°C), and earlier eclogite stage, are directly comparable with those of the nearby Bohemicum unit in western Bohemia. Intervening units, in contrast, exhibit NE‐SW trending structures and Carboniferous (315–325 Ma), low‐pressure, metamorphism: all units are cut by predominantly posttectonic granites (mostly <320 Ma). Earlier models to explain this contrast by means of a nappe emplacement of the ZEV, in the narrow time interval between the younger regional metamorphism and granite intrusion, could not be substantiated because, contrary to predictions, the same rock association was present in the whole of the drilled 9.1 km and therefore no nappe boundary was drilled. Instead, it is interpreted that shallow crustal levels reached by the ZEV, subsequent to the Devonian event, shielded most of it from the effects of the Carboniferous metamorphism. A lack of expected thermal and baric gradients within the drilled metamorphic pile is a result of repeated stacking of an original ∼3 km depth section in Mesozoic times. An important result of the geological investigations of the KTB project is the recognition that the present‐day geometry of the ZEV is a result of cold, shallow‐level, thin‐skinned tectonics that, in the absence of the view into the third dimension offered by the drill hole, could not have been accurately predicted from the fossil, high‐grade assemblages and structures seen at the surface.

Carboniferous tectonics of the Variscan basement collage in eastern Bavaria and western Bohemia

The tectonometamorphic units in the Variscan basement of eastern Bavaria and western Bohemia have a long and complex record of large-scale continental deformation, metamorphism, and syn- and posttectonic magmatism. Although there is a large database describing each of the above-mentioned phenomena, an integrated tectonic synthesis is hard to achieve. In this paper we review some of the prominent effects in the Carboniferous tectonic evolution, which is accountable for much of the deformation, regionally extensive low-pressure/high-temperature metamorphism, and abnormally intense partial anatexis and plutonism. Deformation was multistage and its kinematics were dominated by lateral compression, transpression, and strike-slip shearing. Tectonic units were mostly deformed “en masse”, without operation of large, discrete shear zones. An exception were the boundary zones of those units that had already cooled to sub-greenschist-facies temperatures before the onset of Carboniferous deformation. These boundaries were operated as normal or transtensive faults. The large present vertical thickness of the Zone of Erbendorf-Vohenstrauss at the KTB drill site is most probably due to a combination of Carboniferous block tilting related to the intrusion of the Falkenberg granite, and Cretaceous distributed reverse faulting and imbrication. Late-orogenic plutonism in the Bohemian massif led to the emplacement of at least 176 000 km3 of granitoids. Melt flux through the Moldanubian unit may have been as high as 0.2 km/m.y., allowing the estimated volume of granitoids to be segregated by partial anatexis of the continental crust within a time span of approximately 8.8 million years.

Neogene fault activity and morphogenesis in the basement area north of the KTB drill site (Fichtelgebirge and Steinwald)

The Fichtelgebirge and Steinwald areas are situated at the western end of the Cenozoic Eger graben. Repeated Neogene fault activity and polyphase morphogenesis created a complex morphotectonic structure. Based on morphotectonic criteria three stages of tectonic activity can be determined: (a) late Oligocene to early Miocene; (b) middle-late Miocene; and (c) late Pliocene to early Pleistocene. Due to the Neogene tectonic movements, the pre-basaltic (preOligocene to Miocene) landscape has been differentially uplifted/downthrown and intensely modified by post-basaltic surface planation (HF). Broad paleovalleys (AT) of late Miocene to early Pliocene age and a system of Pliocene and Quaternary fluvial terraces and pediments (BT and ET) have been incised into the widespread HF relief elements. Due to repeated faulting and erosion, the stratigraphic record of Neogene sediments within the studied area is fragmentary.

The Present State of Art of Long Period Magnetotellurics in the Western Part of the Bohemian Massif

Research into the complex geological structure of the Middle European Variscides in the western part of the Bohemian Massif has largely intensified in connection with the KTB deep borehole project in the Oberpfalz, Germany. First results of long period magnetotelluric studies along an about 100 km long profile in SW Bohemia, close to the KTB drilling site, are presented here. They represent a partial step within a broader project of constructing a 3-D geoelectrical model of the western margin of the Bohemian Massif and resolving the relations of anomalous geoelectrical features to the geological structures of the region. The data roughly fit a composite model of a 2-D regional structure, striking E-W, overlaid by a layer of highly distorting near-surface inhomogeneities. The distorting layer displays large anisotropy with preferred conductivity in NW-SE to NNW-SSE direction. Attempt is made to estimate the absolute static shift of the MT curves. Several variants of 2-D models for the deep regional structure are discussed, including an anisotropic model which can qualitatively explain the discrepancy of the principal directions indicated by the MT and GDS data. The relation of the results obtained along the particular profile to the regional geoelectrical background is studied.

Thermal and hydraulic aspects of the KTB drill site

SUMMARY The extensive data sets obtained by the KTB drilling project (lithological and structural information, BHT values, temperature logs, rock thermal properties) provide a unique opportunity to construct realistic thermal models and thus lo shed light on thermal conditions in the upper crust. Our numerical simulation study, a Swiss contribution to the German KTB drilling project, aims to understand the steady-state thermal and hydraulic field in the surroundings of the KTB. The simulations consider state-of-the-art petrophysical aspects relevant to deep, pressurized, high-temperature structures and were performed on discretized 2-D/3-D finite-element meshes that contain topography, geological structures and hydrogeological features. Our analysis of the KTB temperature field suggests three zones of particular geothermal settings: a low-heat-flow zone in the uppermost layers with a transition to high heat flow at 500 m depth; the underlying region accessed by the borehole with its characteristic uniform gradient; and the mid-lower crust that must be responsible for the high-heat-flow regime at the KTB site. The two first zones are treated in the present paper. A 3-D thermo-hydraulic model was set up in order to evaluate the first 2000 m. including the uppermost 500 m low-heat-flow zone. This model incorporates the complex geological information from the KTB pilot hole and topography-driven fluid flow. The lateral boundaries of the model were carefully chosen by analysing the flow pattern within a large, regional 3-D domain. The drilled section is analysed by a 2-D model using the available structural information. Due to dominating refraction effects, a careful temperature gradient analysis has to be carried out for such steeply dipping, anisotropic structures. Both models indicate a thermal regime dominated by diffusive heal transfer. Hydraulic flow seems to be important only for the uppermost (∼400 m) part of the drilled depth section; our simulations do not support significant fluid circulation at greater depths. In the drilled section the rather uniform gradient and the pronounced vertical heat-flow variations can now be explained. Finally, the potential and the limitation of the analysis of heat flows and temperature gradients are demonstrated. Heat-flow interpretations are conclusive only for nearly horizontally layered, isotropic geological units, [n steeply dipping and anisotropic formations the heat-flow field is perturbed over a large distance (>1 km) around the point of interest. In such geological units only the temperature gradient interpretation can provide reliable information on the surrounding material.

The effect of a deviatoric stress on physical rock properties. An experimental study simulating the in-situ stress field at the KTB drilling site, Germany: H. Kern, T. Popp & R. Schmidt, Surveysin Geophysics, 15(5), 1994, pp 467–479

The effect of a deviatoric stress on physical rock properties. An experimental study simulating the in-situ stress field at the KTB drilling site, Germany: H. Kern, T. Popp & R. Schmidt, Surveysin Geophysics, 15(5), 1994, pp 467–479

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.

Electromagnetic measurements in the vicinity of the KTB drill site. Part I: The MV results across a 2-D array

We have analysed magnetovariational data of ten temporary field stations deployed as a 2-D array covering about 50 km square of a CSFR/FRG border area adjacent to the KTB superdeep drill site. Single- and inter-station induction vectors were computed by frequency and time domain analyses. The distribution of induction vector characteristics was analysed, interpreted in terms of the electrical conductivity distribution across the array, and correlated with geological phenomena.

Velocity and density structure of the 4000m crustal segment at the KTB drilling site and their relationship to lithological and microstructural characteristics of the rocks: an experimental approach : Kern, H; Schmidt, R; Popp, T Sci Drilling V2, N2–3, 1991, P130–145

Velocity and density structure of the 4000m crustal segment at the KTB drilling site and their relationship to lithological and microstructural characteristics of the rocks: an experimental approach : Kern, H; Schmidt, R; Popp, T Sci Drilling V2, N2–3, 1991, P130–145

Textures of Paragneisses From the KTB Drilling Site, Ne Bavaria (FRG)

Textures of Paragneisses From the KTB Drilling Site, Ne Bavaria (FRG)

ODP‐KTB Petrophysical Workshop: A blueprint for interprogram collaboration

The international Ocean Drilling Program (ODP) and the Continental Deep Drilling Program of the Federal Republic of Germany (KTB) are large exploration projects in which core‐calibrated borehole measurements play an important scientific role. In the organizations that run both projects, separate working groups are responsible for planning the borehole logging program, supervising the downhole operations, stimulating the research and development of interpretation methods, and initiating new tool designs and developments.These groups came together September 13–15, 1989, for a meeting at the KTB drill site laboratory in Windischeschenbach, Oberpfalz, FRG. Conveners were Ralph Hänel (NLfB, Hannover, FRG) and Paul F. Worthington (BP Research Centre, Sunbury‐on‐Thames, U.K.) About 50 scientists attended. They agreed that integrated working groups be established, through which the two programs could collaborate even more closely in borehole logging.