Franconian Lineament Research Articles

Depth-Recursive Tomography Along the Eger Rift Using the S01 Profile Refraction Data: Tested at the KTB Super Drilling Hole, Structural Interpretation Supported by Magnetic, Gravity and Petrophysical Data

The refraction data from the SUDETES 2003 experiment were used for high-resolution tomography along the profile S01. The S01 profile crosses the zone Erbendorf-Vohenstrauss (ZEV) near the KTB site, then follows the SW–NE oriented Eger Rift in the middle part and continues toward the NE across the Elbe zone and the Sudetic structures as far as the Trans-European Suture Zone. To get the best resolution in the velocity image only the first arrivals of Pg waves with minimum picking errors were used. The previous depth-recursive tomographic method, based on Claerbout’s imaging principle, has been adapted to perform the linearized inversions in iterative mode. This innovative DRTG method (Depth-Recursive Tomography on Grid) uses a regular system of refraction rays covering uniformly the mapped domain. The DRTG iterations yielded a fine-grid velocity model with a required level of RMS travel-time fit and the model roughness. The travel-time residuals, assessed at single depth levels, were used to derive the statistical lateral resolution of “lens-shaped” velocity anomalies. Thus, for the 95% confidence level and 5% anomalies, one can resolve their lateral sizes from 15 to 40 km at the depths from 0 to 20 km. The DRTG tomography succeeded in resolving a significant low-velocity zone (LVZ) bound to the Franconian lineament nearby the KTB site. It is shown that the next optimization of the model best updated during the DRTG iterations tends to a minimum-feature model with sweeping out any LVZs. The velocities derived by the depth-recursive tomography relate to the horizontal directions of wave propagation rather than to the vertical. This was proved at the KTB site where pronounced anisotropic behavior of a steeply tilted metamorphic rock complex of the ZEV unit has been previously determined. Involving a ~7% anisotropy observed for the “slow” axis of symmetry oriented coincidentally in the horizontal SW–NE direction of the S01 profile, the DRTG velocity model agrees fairly well with the log velocities at the KTB site. Comparison with the reflectivity map obtained on the reflection seismic profile KTB8502 confirmed the validity of DRTG velocity model at maximum depths of ~16 km. The DRTG tomography enabled us to follow the relationship of major geological units of Bohemian Massif as they manifested in the obtained P-wave velocity image down to 15 km. Although the contact of Saxothuringian and the Tepla-Barrandian Unit (TBU) is collateral with the S01 profile direction, several major tectonic zones are rather perpendicular to the Variscan strike and so fairly imaged in the S01 cross-section. They exhibit a weak velocity gradient of sub-horizontal directions within the middle crust. In particular, the Moldanubian and TBU contact beneath the Western Krusne hory/Erzgebirge Pluton, the buried contact of the Lusatia unit and the TBU within the Elbe fault zone were identified. The maxima on the 6,100 ms−1 isovelocity in the middle crust delimitated the known ultrabasic Erbendorf complex and implied also two next ultrabasic massifs beneath the Doupovske hory and the Ceske středohoři volcanic complexes. The intermediate mid-crustal P-wave velocity lows are interpreted as granitic bodies. The presented geological model is suggested in agreement with available gravity, aeromagnetic and petrophysical data.

The upper continental crust, an aquifer and its fluid: hydaulic and chemical data from 4 km depth in fractured crystalline basement rocks at the KTB test site

AbstractDetailed information on the hydrogeologic and hydraulic properties of the deeper parts of the upper continental crust is scarce. The pilot hole of the deep research drillhole (KTB) in crystalline basement of central Germany provided access to the crust for an exceptional pumping experiment of 1‐year duration. The hydraulic properties of fractured crystalline rocks at 4 km depth were derived from the well test and a total of 23100 m3 of saline fluid was pumped from the crustal reservoir. The experiment shows that the water‐saturated fracture pore space of the brittle upper crust is highly connected, hence, the continental upper crust is an aquifer. The pressure–time data from the well tests showed three distinct flow periods: the first period relates to wellbore storage and skin effects, the second flow period shows the typical characteristics of the homogeneous isotropic basement rock aquifer and the third flow period relates to the influence of a distant hydraulic border, probably an effect of the Franconian lineament, a steep dipping major thrust fault known from surface geology. The data analysis provided a transmissivity of the pumped aquifer T = 6.1 × 10−6 m2 sec−1, the corresponding hydraulic conductivity (permeability) is K = 4.07 × 10−8 m sec−1 and the computed storage coefficient (storativity) of the aquifer of about S = 5 × 10−6. This unexpected high permeability of the continental upper crust is well within the conditions of possible advective flow. The average flow porosity of the fractured basement aquifer is 0.6–0.7% and this range can be taken as a representative and characteristic values for the continental upper crust in general. The chemical composition of the pumped fluid was nearly constant during the 1‐year test. The total of dissolved solids amounts to 62 g l−1 and comprise mainly a mixture of CaCl2 and NaCl; all other dissolved components amount to about 2 g l−1. The cation proportions of the fluid (XCa approximately 0.6) reflects the mineralogical composition of the reservoir rock and the high salinity results from desiccation (H2O‐loss) due to the formation of abundant hydrate minerals during water–rock interaction. The constant fluid composition suggests that the fluid has been pumped from a rather homogeneous reservoir lithology dominated by metagabbros and amphibolites containing abundant Ca‐rich plagioclase.

Seismic reflections from the crystalline crust below the Continental Deep Drilling Site KTB: Modeling and inference on reflector properties

The three‐dimensional (3‐D) reflection‐seismic data set ISO‐89 3D was recorded near the deep borehole KTB in southeastern Germany. Reflections from the SE1 reflector and from the top of the Erbendorf body (EB) in the upper crystalline crust can be identified in 5–10% of the single‐shot sections. The reflectors have been first identified in previous studies of stacked and migrated seismic data. In this paper the velocity and density variations of these two structures are estimated in a new way using true amplitude single‐shot (vibroseis) data. The method uses the direct wave Pg as a reference phase and models the amplitude ratios of the SE1 and EB reflections to Pg. Modeling in this paper uses a combination of ray theory and the reflectivity method, and the SE1 and the top of the EB are assumed to be obliquely oriented 1‐D structures. Pg modeling shows that a depth‐dependent velocity function within the uppermost crystalline basement explains the amplitudes and travel times of this phase with sufficient accuracy. The largest observed amplitude ratios SE1/Pg and EB/Pg are explained by laminated models with strong velocity contrasts and with reflection coefficients of magnitude 0.1–0.2 (SE1) and 0.05–0.15 (EB). The total thickness of the reflecting zones is less than ∼300 m. Pg amplitude modeling requires low Qp factors (<100) to a depth of ∼1 km, whereas at larger depths, values of several hundred are necessary to keep the SE1 and EB velocity contrasts in realistic ranges. Both reflectors can be interpreted as cataclastic zones. For the SE1 this interpretation agrees with the view that it is a steeply dipping thrust fault which continues the tectonic Franconian Lineament into the upper crust. We assume that the EB is the fractured top of a high‐velocity zone at depths below ∼10 km, known from earlier wide‐angle measurements. Both reflectors have large weakly reflecting or nonreflecting parts. The SE1 is nonreflecting at the intersection with the KTB borehole.

Hydraulic pathways in the crystalline rock of the KTB

SUMMARY Fracture systems and fluid pathways must be analysed in order to understand the dynamical processes in the upper crust. Various deterministic as well as stochastic fracture networks in the depth section of the Franconian Lineament (6900 to 7140 m), which appears as a brittle ductile shear zone and prominent seismic reflector, were modelled to simulate the hydraulic situation at the two boreholes of the Continental Deep Drilling Program (KTB). They led to estimations of the hydraulic permeability in crystalline rock. The geometrical parameters of the fractures, such as fracture locations and orientations, were determined from structural borehole measurements, which create an image of the borehole wall. The selection of potentially open fractures was decided according to the stress field. Only fractures with the dip direction (azimuth) of the fracture plane perpendicular to the maximum horizontal stress field were assumed to be open. The motivation for this assumption is the fact that the maximum horizontal stress is higher than the vertical stress from the formation, indicating that the state of stress is a strike-slip faulting. Therefore, the probability of open fractures due to this particular stress field at the KTB sites is enhanced. Length scales for fracture apertures and extensions were stochastically varied and calibrated by hydraulic experiments. The mean fracture aperture was estimated to be 25 mm, assuming an exponential distribution, with corresponding permeability in the range of 10’16 m2. Similar results were also obtained for log-normal and normal distributions, with a variation of permeability of the order of a factor of 2. The influence of the fracture length on permeability of the stochastic networks was also studied. Decreasing the fracture length beyond a specific threshold of 10 m led to networks with vanishing connectivity and hence vanishing permeability. Therefore, we assume a mean fracture length exceeding the threshold of 10 m as a necessary assumption for a macroscopic hydraulically active fracture system at the KTB site. The calculated porosity due to the fracture network is of the order of 10’3 per cent, which at first sight contradicts the estimated matrix porosity of 1 to 2 per cent from borehole measurements and core measurements. It can be concluded from these results, however, that if the fluid transport is due to a macroscopic fracture system, only very low porosity is needed for hydraulic flow with permeabilities up to several 10’16 m2, and hence the contribution of matrix porosity to the hydraulic transport is of a subordinate nature.

3-D Prestack Kirchhoff Migration of the ISO89-3D Data Set

— This paper presents an overview of the results obtained from a 3-D prestack depth migration of the ISO89-3D data set. The algorithm is implemented as a Kirchhoff-type migration, in which the migrated image is generated by weighted summation along diffraction surfaces through the shot record section. The diffraction surfaces are computed by a 3-D finite difference solution of the eikonal equation. A 3-D macro-velocity model derived mainly from wide-angle tomographic inversion served as input for the travel-time calculations. The results of the migration are presented as slices through a volume covering an area of 21 km × 21 km in the horizontal and 15 km in the vertical direction, centered around the KTB drill hole. In these slices the continuation of the Franconian Lineament or SE1 reflector, respectively, can be identified over most of the survey area as a northeast dipping reflector plane. Its signature appears partly curved and discontinuous and with different strength of reflection down to a maximum depth of 9 km. About 5 km to the south-southeast of the KTB drill hole the uppermost top reflection of the Erbendorf body (EB) can be recognized at approximately the same depth. The slices clearly show its complicated internal structure consisting of several apparently separated reflective parts. Moreover, the geometry and the shape of a few other subsurface structures are described.

Three-dimensional pre-stack Kirchhoff migration of deep seismic reflection data

This paper describes the implementation of a 3-D pre-stack depth migration and its application to deep seismic reflection data. Based on the Kirchhoff migration approach, the migrated image is generated by weighted integration along diffraction surfaces through the shot record sections. A finite-difference solution of the eikonal equation is used to compute the diffraction surfaces for a given 3-D velocity model. A test on synthetic data and an application to a 3-D deep seismic reflection data set (ISO89-3D), recorded in the vicinity of the German continental deep drill hole (KTB), are detailed. Because of strong lateral velocity contrasts in some parts of this area, the migration was performed in two steps. First, parts of the shot records were migrated using a constant velocity in order to obtain a general overview of the subsurface structure. Afterwards, the whole data set was migrated using a 3-D velocity model derived mainly from wide-angle measurements. Slices through the migrated volume covering an area of about 21 km × 21 km in the horizontal and 15 km in the vertical direction were compared with results from a 2-D survey (KTB8502) carried out within the same region. Over nearly the whole survey area, the SE1 reflector shows up as the continuation of the Franconian Lineament with varying strength of reflection. Its signature, which deviates partly from that of a simple plane reflector, can be followed through the whole upper crust down to about 9 km depth. There, the Erbendorf body appears as a complicated, strongly reflective bowl-shaped pattern with its uppermost top reflection shifted about 5 km to the south-southeast of the KTB drill hole. The results make it clear that the lateral extent and the true depth of these main geological structures (SE1, Erbendorf body) can be interpreted correctly only from the 3-D migration result. Finally, the parallel implementation procedure is described, both for constant- and variable-velocity models.

Supracrustal intraplate thickening of Variscan basement due to Alpine foreland compression: Results from the superdeep well KTB (Bohemian Massif, Germany)

Until its final depth of 9101 m the superdeep well KTB (Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland) drilled through Variscan basement rocks which suffered strong brittle‐ductile and brittle deformation during late‐ and post‐Variscan times. Investigations of faults and mineralized tension gashes revealed the following deformation stages: (1) Upper Carboniferous subvertical tension gashes, (2) Upper Carboniferous reverse faults, (3) Cretaceous subhorizontal tension gashes and reverse faults, (4) ?Neogene normal faults. Clear depth‐dependent variations of metamorphic conditions of the post‐Variscan deformation structures are obvious, whereas the Upper Carboniferous structures show almost no variations with depth. This refers to the metamorphic index minerals as well as to the deformation fabrics in quartz. We explain this considerable lack of gradients as being the result of reactivative reverse faulting that led to considerable vertical thickening of the upper crust. The fault geometry indicates an antiformal stack, the frontal ramp of which was drilled by KTB at 7000 m depth. There is strong evidence that this frontal ramp, referred to as the Franconian Lineament at the surface, rises from a subhorizontal detachment at about 10 km depth which corresponds to the brittle‐ductile boundary layer of quartz‐bearing rocks. The considerable amount of supracrustal vertical thickening above the Franconian Lineament results from repeated movements along this detachment since Upper Carboniferous times. It seems reasonable to suppose that in intraplate tectonic settings the brittle‐ductile boundary layer is prone to form a long‐lived decoupling horizon along which the upper brittle crust is detached from the viscous middle and lower crust.

Post‐Variscan thermal and tectonic evolution of the KTB site and its surroundings

The post‐Carboniferous crustal evolution of the German Continental Deep Drilling Program (KTB) area, as summarized in this paper, could not be predicted from surface observations: deep drilling was essential for its revelation. The most conspicuous and unexpected feature discovered in the drill hole is the absence of marked gradients with respect to the pre‐Carboniferous record. There are no depth‐related differences in K‐Ar cooling ages of hornblende and white mica, in petrology or in lithology. All metamorphic rocks encountered, both at the surface as well as in the drill hole down to 9100 m depth, were below 300°C from the Carboniferous onward. The late to post‐Carboniferous deformation is essentially confined to several fault zones. A major fault zone encountered in the drill hole at 7000 m depth is linked by a prominent seismic reflector to the Franconian Lineament, the surface boundary between Variscan basement and Mesozoic cover. This fault zone probably formed in the late Paleozoic and reactivated as a reverse fault in the Mesozoic. Two important episodes of NE‐SW directed shortening by movements along reverse faults took place in the early Triassic and in the late Cretaceous, as indicated by the distribution of apatite and titanite fission‐track ages, the sericite K‐Ar ages of fault rocks, and the sedimentary record in the adjacent basins. Upper crustal slices were detached at a specific level, corresponding to the approximate position of the brittle‐ductile transition in post‐Variscan times, and form an antiformal stack that was penetrated by the KTB throughout its entire depth range.

Faults and veins in the superdeep well KTB: constraints on the amount of Alpine intra-plate thrusting and stacking of Variscan basement (Bohemian Massif, Germany)

The crystalline rocks of the superdeep well KTB (9101 m depth) display abundant late- to post-Variscan brittle–ductile and brittle deformation structures. Clear depth-dependent variations in metamorphic conditions are obvious concerning Cretaceous subhorizontal tension gashes and reverse faults, whereas the Upper Carboniferous structures (subvertical tension gashes and reverse faults) show almost no variations with depth. The lack in depth-dependent gradients can be explained by strong Cretaceous reverse faulting that led to an intra-plate antiformal stack. The frontal ramp of this stack, a broad fault bundle referred to as the Franconian lineament, was drilled at 7300 m depth. It rises from a flat-lying detachment at approximately 9 km depth which corresponds to the brittle–ductile transition zone of quartz-bearing rocks. Supracrustal stacking and thickening along and above this ramp was particularly active during Alpine (Upper Cretaceous) foreland compression. Close to the Franconian lineament the Alpine impact strongly affected the Variscan crustal edifice disturbing the Variscan and late-Variscan depthdependent gradients in KTB.

The KTB superdeep borehole: petrography and structure of a 9-km-deep crustal section

The KTB boreholes that were drilled from 1989 to 1994 near Windischeschenbach, NE Bavaria, penetrated rocks of the Zone Erbendorf-Vohenstrauss (ZEV). The upper and lower sections of the 9101m-deep profile are mainly composed of gneiss units (former greywacke sediments) and variegated units (alternating gneisses and metabasites, probably of volcano-sedimentary origin). Metabasic units (amphibolites, metagabbros) constitute mainly the middle section. Geochemical data suggest their derivation from oceanic crust. All rocks are probably pre- to early Ordovician in protolith age. They underwent early Devonian MP metamorphism. Relics of preceeding HP metamorphism are preserved in metagabbros. Felsic to mafic dykes (dominantly Upper Carboniferous lamprophyres) crosscut the metamorphic rocks. The ZEV units are steeply inclined down to the final depth. The main structures (foliation, folds, faults) trend NW–SE. Semibrittle to brittle deformation plays an unexpected important role. In contrast to the conditions of the Mesozoic (mainly Cretaceous/Tertiary) faulting the P–T conditions of MP metamorphism, the cooling ages and the late-Variscan graphite-bearing prehnite–actinolite facies mineralization show no significant depth-dependent gradients. This can be explained by Mesozoic supracrustal stacking. A bundle of reverse faults between 6850 and 7300 m belongs to the Franconian lineament and can be correlated with the most prominent seismic reflector (SE1) in the area.

Nature of seismic reflections and velocities from VSP-experiments and borehole measurements at the KTB deep drilling site in southeast Germany

The 1989 VSP program at the KTB pilot hole was complemented in 1992 by a standard VSP in the KTB super-deep borehole from 3000 to 6013 m. P-wave velocities oscillate around 5.8 km/s in the upper 3150 m in accordance with sonic log velocities and correlate with paragneisses which prevail in this depth range. At about 3150 m depth, velocities increase to 6.4 km/s correlating with metabasites which dominate in the depth range 3150 to 7500 m. Laboratory measurements and petrophysical modelling provide evidence that the intrinsic velocities were reduced by 5–10% by fracture density and porosity at all depth ranges. Subvertical dip (50–70°) in structures and textures prevail, causing about 10% S-wave anisotropy, indicated by direct observations of S-wave splitting. Pronounced P-wave reflections, accompanied by P- to S-wave conversions and a lack of S-wave reflections, occur in the lower depth range only (3000–6000 m) and correlate with fluid-filled fracture systems. Lithological contrasts (gneiss-amphibolite) play a minor role in generating reflections. The most prominent reflecting elements known from surface profiling and 3D-surveys (i.e. the ‘Franconian Lineament’ reflector at about 7 km, and P-wave reflections at 8.3 km with notably absent associated S-wave reflections) coincide with pronounced anomalies observed in the logging data indicating the presence of major fracture zones.

Pre-stack migration and true-amplitude processing of DEKORP near-normal incidence and wide-angle reflection measurements

An alternative to CMP-processing is presented which seems to be more appropriate to crustal seismics. It has been developed for wide-angle seismics but it can also be applied to near-normal incidence data. The essentials of this processing sequence are shot domain processing, retention of true amplitudes and pre-stack migration. True-amplitude processing of land data requires very careful editing. In addition, the ground coupling of individual sources and receivers must be taken into account by applying surface-consistent amplitude corrections. Subsequent pre-stack migration is very time-consuming. Although the isochron migration applied is based on kinematic principles, the amplitude distribution may be regarded qualitatively as an image of the reflectivity. The method can resolve steeply dipping reflectors and works with physically meaningful (dip-independent) migration velocities. The migration velocities are computed prior to migration by ray tracing using a macro velocity model. Robustness of the method can be increased by forming envelopes after single shot migration and stacking them to obtain a final image. When applied to DEKORP-data from near the German Continental Deep Drilling site (KTB), the Franconian Lineament fault zone (dipping at ∼55°) has been well imaged, in addition to many other reflectors throughout the crust. Furthermore, areas with high reflectivity can be distinguished from areas with low reflectivity due to the true-amplitude processing.