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

Haak, V.; Stoll, Johannes; Winter, Helmuth

doi:10.1016/0031-9201(91)90100-v

Cited by 58 publications

(12 citation statements)

References 12 publications

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“…Taking the magmatic body to be of Permian age (Thybo 2001) it means that the conductor could represent the remnants of a major shear zone that was active in Palaeozoic times or earlier. In order to be conductive the shear zone would have to be associated with mineralization of graphites or sulphides related to metamorphic processes (Haak et al 1991a,b) that transported conductive material to its present position (Ritter et al 1999, 2005). The moderately conductive feature reaching the surface at distance 245 km could possible represent another such shear zone that partly survived the emplacement of the magmatic intrusion.…”

Section: Discussionmentioning

confidence: 99%

Magnetotelluric measurements across the Sorgenfrei-Tornquist Zone in southern Sweden and Denmark

Smirnov

Pedersen

2009

Geophysical Journal International

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S U M M A R YThe results of a magnetotelluric experiment across the Sorgenfrei-Tornquist-Zone (STZ) in the southwestern part of the Baltic Shield are presented. The STZ marks the border between the Proterozoic intact part of the shield in Sweden to the north and the reactivated part including the Danish basin and the Ringköbing-Fyn High (RFH) to the south.We deployed 69 magnetotelluric sites along two profiles, each 350 km long. High quality data were collected in the period range 0.003-20 000 s. Data were processed using a robust multiremote-reference technique. Strike and dimensionality analysis as well as directions of induction arrows support the general 2-D character of the conductivity distribution in the area, allowing us to model the data in 2-D. Satisfactory agreement can be obtained between the measured and calculated transfer functions projected onto the profile direction. In order to ensure that the final model is stable we have performed a sensitivity analysis using synthetic models with different hypotheses about mantle conductivity and including a priori models for the sedimentary basin.The crustal part of the model compares well with that of Thybo which is derived from seismic, gravity and magnetic data. In the Danish basin we resolve a thick sedimentary cover that extends to the southwest. At the STZ a resistive body surrounded by more conductive material coincides with a zone of high seismic velocities and densities may be interpreted as a large magmatic intrusion. The STZ manifests itself electrically very clearly in the lower crust and upper lithospheric mantle as a narrow zone of high conductivity. The thickness of the electric lithosphere shows a gradual thinning from The Baltic Shield in Sweden (about 300 km) across the STZ into the Danish basin (about 100 km).

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Section: Discussionmentioning

confidence: 99%

Magnetotelluric measurements across the Sorgenfrei-Tornquist Zone in southern Sweden and Denmark

Smirnov

Pedersen

2009

Geophysical Journal International

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“…Within a few hundred metres of the pilot hole there are outcrops of graphitic material that coincide in location with the minima of anomalies of electrical resistivity (10 52 m) and self potential (-600 mV). These are interpreted as manifestations of deep-reaching zones enriched in dispersed graphite (Haak, Stoll & Winter 1991). At these locations a pronounced induced polarization effect likewise suggests the existence of either graphite or pyrite (Haak et al 1991).…”

Section: Graphite In the Pilot Hole And In Surrounding Boreholesmentioning

confidence: 93%

Heat advection versus conduction at the KTB: possible reasons for vertical variations in heat-flow density

Jobmann¹,

Clauser²

1994

Geophysical Journal International

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S U M M A R Y Data from the 4 km deep KTB pilot hole (VB) show a strong vertical variation in heat-flow density (HFD) by as much as 50 per cent. This may be caused both by heat conduction, by advection, and by transient diffusion. At the moment it is not possible to quantify exactly the contribution of each of these. However, 2-D simulations help to define the parameter ranges and structural features required if these processes are to be thermally efficient. The main results are: (1) thermal conductivity contrasts combined with structural heterogeneities as seen in the drilled profile give rise to steady-state, lateral refraction of heat. 2-D simulations of heat conduction indicate that this effect alone is sufficiently strong to account for the observed variation of HFD with depth.(2) Vertical PCclet number analyses of T-logs in shallow boreholes and the KTB-VB indicate a NE-SW flow of meteoric water across the Franconian Line (FL). However, average PCclet numbers of -0.37f0.13 in the potential recharge zone east of the FL are compatible with 2-D, steady-state simulations of heat and fluid flow only up to a distance of about 10 km east of the FL, and only if a crystalline permeability k , = m2 is assumed. (3) A permeability this high, however,' is not confirmed by a comparison of temperature and HFD from numerical simulations and data from the KTB boreholes, neither for a model focusing on shallow flow systems nor a deep structural model investigating potential contributions of convection in the entire upper crust. (4) Alternatively, a joint inversion of T-logs from the same shallow holes yields a ground-temperature history (GTH) that is in remarkably good agreement with long-term meteorological records. (5) It appears, therefore, as if the thermal regime at the KTB was generally dominated by conduction, with additional advective, topography-driven contributions mainly at shallow depths. The conductive regime, however, is a complicated one, characterized by lateral heat flow due to structural heterogeneity (and possibly anisotropy), and, at least at shallower depths, by transient diffusion of paleoclimatic temperature signals into the subsurface.

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“…They concluded that the high electrical conductivities in the upper crust are primarily caused by graphite accumulations rather than by fluids and that these anomalous conductivities are related to shearing stress regimes. However, it is essential to note that for both graphite and saline fluids, it is their connectivity rather than their pure presence that is the most crucial factor controlling conductivity (ELEKTB Group 1997;Emmermann and Lauterjung 1997;Haak et al 1991;Stoll et al 1995). The ELEKTB Group (1997) also found and concluded that the original (primary) graphite existing in the gneisses cannot be interconnected.…”

Section: Introductionmentioning

confidence: 99%

Tracing of paleo-shear zones by self-potential data inversion: case studies from the KTB, Rittsteig, and Grossensees graphite-bearing fault planes

Mehanee

2015

Earth Planets Space

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This paper describes a new method for tracing paleo-shear zones of the continental crust by self-potential (SP) data inversion. The method falls within the deterministic inversion framework, and it is exclusively applicable for the interpretation of the SP anomalies measured along a profile over sheet-type structures such as conductive thin films of interconnected graphite precipitations formed on shear planes. The inverse method fits a residual SP anomaly by a single thin sheet and recovers the characteristic parameters (depth to the top h, extension in depth a, amplitude coefficient k, and amount and direction of dip θ) of the sheet. This method minimizes an objective functional in the space of the logarithmed and non-logarithmed model parameters (log(h), log(a), log(k), and θ) successively by the steepest descent (SD) and Gauss-Newton (GN) techniques in order to essentially maintain the stability and convergence of this inverse method. Prior to applying the method to real data, its accuracy, convergence, and stability are successfully verified on numerical examples with and without noise. The method is then applied to SP profiles from the German Continental Deep Drilling Program (Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschla -KTB), Rittsteig, and Grossensees sites in Germany for tracing paleo-shear planes coated with graphitic deposits. The comparisons of geologic sections constructed in this paper (based on the proposed deterministic approach) against the existing published interpretations (obtained based on trial-and-error modeling) for the SP data of the KTB and Rittsteig sites have revealed that the deterministic approach suggests some new details that are of some geological significance. The findings of the proposed inverse scheme are supported by available drilling and other geophysical data. Furthermore, the real SP data of the Grossensees site have been interpreted (apparently for the first time ever) by the deterministic inverse scheme from which interpretive geologic cross sections are suggested. The computational efficiency, analysis of the numerical examples investigated, and comparisons of the real data inverted here have demonstrated that the developed deterministic approach is advantageous to the existing interpretation methods, and it is suitable for meaningful interpretation of SP data acquired elsewhere over graphitic occurrences on fault planes.

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

Cited by 58 publications

References 12 publications

Magnetotelluric measurements across the Sorgenfrei-Tornquist Zone in southern Sweden and Denmark

Magnetotelluric measurements across the Sorgenfrei-Tornquist Zone in southern Sweden and Denmark

Heat advection versus conduction at the KTB: possible reasons for vertical variations in heat-flow density

Tracing of paleo-shear zones by self-potential data inversion: case studies from the KTB, Rittsteig, and Grossensees graphite-bearing fault planes

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