KTB Site Research Articles

Modelling discontinuous well log signal to identify lithological boundaries via wavelet analysis: An example from KTB borehole data

Identification of sharp and discontinuous lithological boundaries from well log signal stemming from heterogeneous subsurface structures assumes a special significance in geo-exploration studies. Well log data acquired from various geological settings generally display nonstationary/nonlinear characteristics with varying wavelengths and frequencies. Modelling of such complex well-log signals using the conventional signal processing techniques either fails to catch-up abrupt boundaries or at the best, do not provide precise information on insidious lithological discontinuities. In this paper, we have proposed a new wavelet transform-based algorithm to model the abrupt discontinuous changes from well log data by taking care of nonstationary characteristics of the signal. Prior to applying the algorithm on the geophysical well data, we analyzed the distribution of wavelet coefficients using synthetic signal generated by the first order nonstationary auto-regressive model and then applied the method on actual well log dataset obtained from the KTB bore hole, Germany. Besides identifying the formation of layered boundaries, the underlying method also maps some additional formation boundaries, which were hitherto undetected at the KTB site. The results match well with known geological lithostratigraphy and will be useful for constraining the future model of KTB bore hole data.

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.

Geothermal modelling of faulted metamorphic crystalline crust: a new model of the Continental Deep Drilling Site KTB (Germany)

The area of the 9.1-km-deep Continental Deep Drillhole (KTB) in Germany is used as a case study for a geothermal reservoir situated in folded and faulted metamorphic crystalline crust. The presented approach is based on the analysis of 3-D seismic reflection data combined with borehole data and hydrothermal numerical modelling. The KTB location exemplarily contains all elements that make seismic prospecting in crystalline environment often more difficult than in sedimentary units, basically complicated tectonics and fracturing and low-coherent strata. In a first step major rock units including two known nearly parallel fault zones are identified down to a depth of 12 km. These units form the basis of a gridded 3-D numerical model for investigating temperature and fluid flow. Conductive and advective heat transport takes place mainly in a metamorphic block composed of gneisses and metabasites that show considerable differences in thermal conductivity and heat production. Therefore, in a second step, the structure of this unit is investigated by seismic waveform modelling. The third step of interpretation consists of applying wavenumber filtering and log-Gabor-filtering for locating fractures. Since fracture networks are the major fluid pathways in the crystalline, we associate the fracture density distribution with distributions of relative porosity and permeability that can be calibrated by logging data and forward modelling of the temperature field. The resulting permeability distribution shows values between 10-16 and 10-19 m(-2) and does not correlate with particular rock units. Once thermohydraulic rock properties are attributed to the numerical model, the differential equations for heat and fluid transport in porous media are solved numerically based on a finite difference approach. The hydraulic potential caused by topography and a heat flux of 54 mW m(-2) were applied as boundary conditions at the top and bottom of the model. Fluid flow is generally slow and mainly occurring within the two fault zones. Thus, our model confirms the previous finding that diffusive heat transport is the dominant process at the KTB site. Fitting the observed temperature-depth profile requires a correction for palaeoclimate of about 4 K at 1 km depth. Modelled and observed temperature data fit well within 0.2 degrees C bounds. Whereas thermal conditions are suitable for geothermal energy production, hydraulic conditions are unfavourable without engineered stimulation.

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

AbstractThis 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 toph, extension in deptha, amplitude coefficientk, and amount and direction of dipθ) 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), andθ) 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.

Magnitude estimation for microseismicity induced during the KTB 2004/2005 injection experiment

We determined the magnitudes of 2540 microseismic events measured at one single 3C borehole geophone at the German Deep Drilling Site (known by the German acronym, KTB) during the injection phase 2004/2005. For this task we developed a three-step approach. First, we estimated local magnitudes of 104 larger events with a standard method based on amplitude measurements at near-surface stations. Second, we investigated a series of parameters to characterize the size of these events using the seismograms of the borehole sensor, and we compared them statistically with the local magnitudes. Third, we extrapolated the regression curve to obtain the magnitudes of 2436 events that were only measured at the borehole geophone. This method improved the magnitude of completeness for the KTB data set by more than one order down to [Formula: see text]. The resulting b-value for all events was 0.78, which is similar to the [Formula: see text]-value obtained from taking only the greater events with standard local magnitude estimation from near-surface stations, [Formula: see text]. The more complete magnitude catalog was required to study the magnitude distribution with time and to characterize the seismotectonic state of the KTB injection site. The event distribution with time was consistent with prediction from theory assuming pore pressure diffusion as the underlying mechanism to trigger the events. The value we obtained for the seismogenic index of [Formula: see text] suggested that the seismic hazard potential at the KTB site is comparatively low.

Prominent occurrence of iron oxides at KTB mass extinction: a review

Out of the five major mass extinction, which have taken place in the history of life, Cretaceous–Tertiary boundary (KTB) mass extinction was the 2nd most disastrous, the most severe being that at Permian–Triassic boundary (PTB). On the basis of iridium anomaly at a number of KTB sites it has been established that the cause of extinction was impact with an extraterrestrial bollide. The iron oxide/oxyhydroxide and iron minerals such as illite, phyllosilicate, jarosite etc. present in the rock samples from KTB form a major part. One can study different phases of iron oxide/oxyhydroxide present in the KTB samples with the help of Mossbauer spectroscopy. The Mossbauer studies showed that at KTB sites nanophase goethite and/or hematite are present without coexistence of their counterpart bulk size iron oxide/oxyhydroxide minerals. The amount of nanophase iron oxides well correlate with iridium concentration showing that the nanophase iron minerals are genetically related to the events at KTB. Thus these nanophase iron compounds were formed in the geochemical conditions created by the impact, and not by slow weathering. The iron mineralogy at some other extinction sites is found to be very similar to that at KTB although there is no iridium anomaly. This raises an interesting suggestion that iron mineralogy i.e. presence of nano-sized oxide and oxhydroxide particles alone without the existence of bulk iron-oxide in sedimentary layers can be used as impact markers. Studies carried out on some off-KTB rock samples also showed iron mineral composition similar to that at KTB without having iridium anomaly. This opens up the new questions whether these layers are also result of some large meteoritic impact? Keywords : Mossbauer spectroscopy, KT boundary, nanophase iron, iron mineralogy International Journal of Engineering, Science and Technology , Vol. 2, No. 8, 2010, pp. 110-117

Pressure-dependent seismic reflection amplitude changes in crystalline crust: lessons learned at the Continental Deep Drilling Site (KTB)

SUMMARY We conducted an active seismic experiment aimed at measuring changes in seismic reflection amplitudes as a consequence of fresh water injection and corresponding pressure changes at the German Continental Deep Drilling site (KTB). The injection took place at the bottom of the 4-km-deep pilot borehole in the SE2 fault zone in crystalline rock units between the springs of 2004 and 2005. Prior to the experiment, theoretical calculations indicated a possible increase in the compressional wave reflection coefficient as a result of an injection-induced reduction of the seismic velocities within the fault zone. Despite good repeatability of the emitted source signals, the experiment suffered from missing the clear reflection signals expected from the fault zone with regard to seismic data from past experiments. Applying various data-processing steps did not enhance the signals enough to obtain clear reflections or even pressure-dependent reflection amplitude changes. The signal-to-noise ratio remains smaller than the effects under observation. Provided that reflections are present in the data, the error bar of the recorded signals is of the order of 100 per cent. Therefore, we conclude that the experiment was not successful in seismically measuring pressure variations. However, important lessons for land seismic time-lapse measurements in crystalline environments have been learned: (i) The source should be capable of emitting frequencies below 30 Hz. (ii) The detector array setup proved to be partly questionable because in a scattering environment like the crystalline rocks at the KTB site, the incidence of a plane wave precondition might be violated for high-frequency signals. (iii) Near-surface variations of elastic properties likely influence seismic monitoring. (iv) Using a step function, that is a first-order pressure discontinuity, to model the subsurface pressure build-up, is very likely too simple an approach.

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.

Non-double-couple mechanisms of microearthquakes induced during the 2000 injection experiment at the KTB site, Germany: A result of tensile faulting or anisotropy of a rock?

Moment tensors of microearthquakes induced during the 2000 injection experiment at the KTB deep drilling borehole at a depth level of 5.4 km are studied. A family of 37 most reliable moment tensors contains significant non-double-couple (non-DC) components. The DC is on average 60% and the non-DC is 40%. Fault plane solutions computed from the DC part show preferred strike-slip mechanisms with small normal or reverse components. A predominant azimuth of P and T axes is in the range of N320°–340°E and of N230°–250°E, respectively. The non-DC components contain both the isotropic (ISO) and compensated linear vector dipole (CLVD) components. The mean value of ISO is 1.5%, the mean value of CLVD is − 5.7%. The predominantly negative CLVD components are inconsistent with the concept of the non-DC mechanisms as a result of tensile faulting due to fluid injection into the rock. The main origin of the non-DC components is probably anisotropy in the focal area. The other origins are errors produced by mismodelling of the medium when calculating the Green functions, and numerical errors produced by noise and limitations of input data. Adopting four alternative models of anisotropy obtained by other seismic measurements at the KTB, we have employed the non-DC components for estimating an optimum orientation of anisotropy in the focal area. The optimum orientation of the symmetry plane of anisotropy is nearly vertical with a strike of N335°–340°E. This strike coincides well with the strike of 330° typical for many major lithological units and faults and with the orientation of the transversely isotropic model inferred by other authors. After removing the anisotropy effects from the moment tensors by calculating the source tensors, the distribution of ISO is significantly narrowed. This indicates predominantly shear, but not tensile faulting.

Near-Fault Peak Ground Velocity from Earthquake and Laboratory Data

We test the hypothesis that peak ground velocity (pgv) has an upper bound independent of earthquake magnitude and that this bound is controlled primarily by the strength of the seismogenic crust. The highest pgvs, ranging up to several meters per second, have been measured at sites within a few kilometers of the causative faults. Because the database for near-fault pgv is small, we use earthquake slip models, laboratory experiments, and evidence from a mining-induced earthquake to investigate the factors influencing near-fault pgv and the nature of its scaling. For each earthquake slip model we have calculated the peak slip rates for all subfaults and then chosen the maximum of these rates as an estimate of twice the largest near-fault pgv. Nine slip models for eight earthquakes, with magnitudes ranging from 6.5 to 7.6, yielded maximum peak slip rates ranging from 2.3 to 12 m/ sec with a median of 5.9 m/sec. By making several adjustments, pgvs for small earthquakes can be simulated from peak slip rates measured during laboratory stick- slip experiments. First, we adjust the pgv for differences in the state of stress (i.e., the difference between the laboratory loading stresses and those appropriate for faults at seismogenic depths). To do this, we multiply both the slip and the peak slip rate by the ratio of the effective normal stresses acting on fault planes measured at 6.8 km depth at the KTB site, Germany (deepest available in situ stress measurements), to those acting on the laboratory faults. We also adjust the seismic moment by replacing the laboratory fault with a buried circular shear crack whose radius is chosen to match the experimental unloading stiffness. An additional, less important adjustment is needed for experiments run in triaxial loading conditions. With these adjustments, peak slip rates for 10 stick-slip events, with scaled moment magnitudes from −2.9 to 1.0, range from 3.3 to 10.3 m/sec, with a median of 5.4 m/sec. Both the earthquake and laboratory results are consistent with typical maximum peak slip rates averaging between 5 and 6 m/sec or corresponding maximum near-fault pgvs between 2.5 and 3 m/sec at seismogenic depths, independent of magnitude. Our ability to replicate maximum slip rates in the fault zones of earthquakes by adjusting the corresponding laboratory rates using the ratio of effective normal stresses acting on the fault planes suggests that the strength of the seismogenic crust is the important factor limiting the near-fault pgv.

Investigation of coupled hydraulic–geomechanical processes at the KTB site: pressure‐dependent characteristics of a long‐term pump test and elastic interpretation using a geomechanical facies model

AbstractThe German Continental Deep Drilling Program comprising a pilot borehole down to 4000 m and a main borehole down to 9101 m in southeast Germany (KTB) is continuing to provide a unique opportunity for the identification of important factors and processes in deep‐seated fluid and energy transfer. In situ stress conditions significantly impact flow, transport and exchange characteristics of fracture networks that dominate the permeability of crystalline reservoir rocks. In this paper, several scales of information are combined to present a fully three‐dimensional hydraulic finite element model of the principal KTB fault zones, and linked to a geomechanical model describing the alteration of the hydraulic parameters with stress changes caused by fluid extraction. The concept of geomechanical facies is introduced to define and characterize architectural elements in the subsurface system. Evaluation of a long‐term pump test in the KTB pilot hole, June 2002–July 2003, coupled with a geomechanical model gives an insight into some of the elastic and nonelastic processes controlling hydraulic transport in the basement rocks. Trends in the decline of the permeability and the degree of storage in the system could only partially be explained by elastic processes, clearly indicating the importance of nonelastic processes. A number of inelastic processes are suggested as areas for further research.

Fluid induced seismicity guided by a continental fault: Injection experiment of 2004/2005 at the German Deep Drilling Site (KTB)

Recent hydraulic experiments at the KTB site have shown that seismicity induced by long‐term fluid injection directly into a continental crustal fault remains guided by this fault. The seismicity is triggered by pressure perturbations as low as 0.01–1 bars at the hypocenters. A combination of sequential one‐year fluid extraction (2002/2003) and one‐year fluid injection (2004/2005) experiments has shown that only positive pore pressure perturbation (i.e., injections) was able to induce the seismicity. Moreover, the onset of seismicity roughly coincides with the time of compensation of the extracted fluid volume by the following injection. This confirms that the pressure diffusion is a dominant mechanism of seismicity triggering by fluid injections. The probed fault shows a significant anisotropy and non‐linearity of its hydraulic behaviour. Its hydraulic diffusivity is up to one order of magnitude larger than that of surrounding rocks.

Fluid injection and surface deformation at the KTB location: modelling of expected tilt effects

AbstractThis investigation is indented to explore the relationship between changes in pore fluid pressure and deformation of the land surface induced by a large‐scale injection experiment at the KTB site. Deformation will be monitored by ASKANIA borehole tiltmeters at five locations. During the year 2003, a network of borehole tiltmeters was installed, data transmission links established and tested, and recording of tilt data started. Our first main interest was to receive data sets of all stations well before the injection experiment to start in May 2004, to be able to evaluate local site effects. Thus, the separation of injection‐induced effects will be more reliable. Principal 3D numerical modelling (poro‐elastic modelling and investigations, using the finite element method, FEM) of poro‐elastic behaviour showed that significant tilt amplitudes can be expected during controlled fluid injection. Observed deformation will be investigated within the framework of the fluid flow behaviour and resulting deformation. Two models have been used: a coupled hydro geomechanical finite element model (abaqus) and, as a first step, also a multi‐layered poro‐elastic crust (poel). With the numerical model two effects can be quantified: (i) the deformation of the upper crust (tilt measurements) and (ii) the spatial distribution and the changes of material properties in the KTB area. The main aim of the project is to improve the knowledge of coupled geomechanic–hydraulic processes and to quantify important parameters. Thus, the understanding of fracture‐dominated changes of the hydrogeological parameters will be enhanced, geomechanical parameter changes and the heterogeneity of the parameter field quantified. In addition, the induced stress field variation can be explained, which is believed to be mainly responsible for the increase of local seismic activity. Here, we introduce the tiltmeter array at the KTB site, the modelling for a poro‐elastic crust and the preliminary FEM modelling.

Superdeep vertical seismic profiling at the KTB deep drill hole (Germany): Seismic close‐up view of a major thrust zone down to 8.5 km depth

The lowermost section of the continental superdeep drill hole German Continental Deep Drilling Program (KTB) (south Germany) has been investigated for the first time by vertical seismic profiling (VSP). The new VSP samples the still accessible range of 6–8.5 km depth. Between 7 and 8.5 km depth, the drill hole intersects a major cataclastic fault zone which can be traced back to the Earth's surface where it forms a lineament of regional importance, the Franconian line. To determine the seismic properties of the crust in situ, in particular within and around this deep fault zone, was one of the major goals of the VSP. For the measurements a newly developed high‐pressure/high‐temperature borehole geophone was used that was capable of withstanding temperatures and pressures up to 260°C and 140 MPa, respectively. The velocity‐depth profiles and reflection images resulting from the VSP are of high spatial resolution due to a small geophone spacing of 12.5 m and a broad seismic signal spectrum. Compared to the upper part of the borehole, we found more than 10% decrease of the P wave velocity in the deep, fractured metamorphic rock formations. P wave velocity is ∼5.5 km/s at 8.5 km depth compared to 6.0–6.5 km/s at more shallow levels above 7 km. In addition, seismic anisotropy was observed to increase significantly within the deep fracture zone showing more than 10% shear wave splitting and azimuthal variation of S wave polarization. In order to quantify the effect of fractures on the seismic velocity in situ we compared lithologically identical rock units at shallow and large depths: Combining seismic velocity and structural logs, we could determine the elastic tensors for three gneiss sections. The analysis of these tensors showed that we need fracture porosity in the percent range in order to explain seismic velocity and anisotropy observed within the fault zone. The opening of significant pore space around 8 km depth can only be maintained by differential tectonic stress combined with intense macroscopic fracturing. VSP reflection imaging based on PP and PS converted reflected waves showed that the major fault system at the KTB site is wider and more complex than previously known. The so‐called SE1 reflection previously found in two‐ and three‐dimensional surface seismic surveys corresponds to the top of an ∼1 km wide fault system. Its lower portion was not illuminated by surface seismic acquisition geometry. VSP imaging shows that the fault zone comprises two major and a number of smaller SE dipping fault planes and several conjugate fracture planes. The previously recognized upper fault plane is not associated with a strong velocity anomaly but indicates the depth below which the dramatic velocity decrease starts. Regarding the complexly faulted crustal section of the KTB site as a whole, we found that fluctuation spectra of rock composition and seismic velocity show similar patterns. We could verify that a significant amount of P wave energy is continuously converted into shear energy by forward scattering and that multipathing plays an important role in signal formation. The media behaves effectively smoothly only at wavelength larger than 150 m. It was shown by moving source profiling that the media is orthorhombic on a regional scale. The tilt of the symmetry axes of anisotropy varies with depth following the dip of the geological structure.

Mutual relationship between microseismicity and seismic reflectivity: Case study at the German Continental Deep Drilling Site (KTB)

Two fluid injection experiments were conducted at the German Continental Deep Drilling Site (KTB) in 1994 and 2000. Microseismicity occurred at different depth intervals. Hypocenter locations were precisely localized. Here, microseismicity is analyzed in terms of its spatio‐temporal evolution characteristics. An approach is applied which assumes microseismicity to be triggered by a diffusive process of pore pressure relaxation. The method yields estimates of hydraulic parameters of rocks on large spatial scales. At the KTB site the method enables us to study hydraulic diffusivity at two different depth intervals. We observe significant variations in the evolution characteristics of the seismic activity at different depths. Estimates of hydraulic diffusivity for shallower parts of the crust seem to be much smaller than for deeper regions. To understand reasons for this, we have analyzed spatial relations of hypocenter locations to the distribution of intensities of seismic reflections. Low values of hydraulic diffusivity correlate with low reflection intensities and high diffusivities with large intensities, respectively. The analysis confirms the hypothesis that the process of pore pressure relaxation along pre‐existing and critically stressed natural fractures is an important triggering factor for induced microseismicity.

Platinum-group elements (PGE) and rhenium in marine sediments across the Cretaceous–Tertiary boundary: constraints on Re-PGE transport in the marine environment

The nature of Re–platinum-group element (PGE; Pt, Pd, Ir, Os, Ru) transport in the marine environment was investigated by means of marine sediments at and across the Cretaceous–Tertiary boundary (KTB) at two hemipelagic sites in Europe and two pelagic sites in the North and South Pacific. A traverse across the KTB in the South Pacific pelagic clay core found elevated levels of Re, Pt, Ir, Os, and Ru, each of which is approximately symmetrically distributed over a distance of ∼1.8 m across the KTB. The Re-PGE abundance patterns are fractionated from chondritic relative abundances: Ru, Pt, Pd, and Re contents are slightly subchondritic relative to Ir, and Os is depleted by ∼95% relative to chondritic Ir proportions. A similar depletion in Os (∼90%) was found in a sample of the pelagic KTB in the North Pacific, but it is enriched in Ru, Pt, Pd, and Re relative to Ir. The two hemipelagic KTB clays have near-chondritic abundance patterns. The ∼1.8-m-wide Re-PGE peak in the pelagic South Pacific section cannot be reconciled with the fallout of a single impactor, indicating that postdepositional redistribution has occurred. The elemental profiles appear to fit diffusion profiles, although bioturbation could have also played a role. If diffusion had occurred over ∼65 Ma, the effective diffusivities are ∼10 −13 cm 2/s, much smaller than that of soluble cations in pore waters (∼10 −6 cm 2/s). The coupling of Re and the PGEs during redistribution indicates that postdepositional processes did not significantly fractionate their relative abundances. If redistribution was caused by diffusion, then the effective diffusivities are the same. Fractionation of Os from Ir during the KTB interval must therefore have occurred during aqueous transport in the marine environment. Distinctly subchondritic Os/Ir ratios throughout the Cenozoic in the South Pacific core further suggest that fractionation of Os from Ir in the marine environment is a general process throughout geologic time because most of the inputs of Os and Ir into the ocean have Os/Ir ratios ≥1. Mass balance calculations show that Os and Re burial fluxes in pelagic sediments account for only a small fraction of the riverine Os (<10%) and Re (<0.1%) inputs into the oceans. In contrast, burial of Ir in pelagic sediments is similar to the riverine Ir input, indicating that pelagic sediments are a much larger repository for Ir than for Os and Re. If all of the missing Os and Re is assumed to reside in anoxic sediments in oceanic margins, the calculated burial fluxes in anoxic sediments are similar to observed burial fluxes. However, putting all of the missing Os and Re into estuarine sediments would require high concentrations to balance the riverine input and would also fail to explain the depletion of Os at pelagic KTB sites, where at most ∼25% of the K-T impactor’s Os could have passed through estuaries. If Os is preferentially sequestered in anoxic marine environments, it follows that the Os/Ir ratio of pelagic sediments should be sensitive to changes in the rates of anoxic sediment deposition. There is thus a clear fractionation of Os and Re from Ir in precipitation out of sea water in pelagic sections. Accordingly, it is inferred here that Re and Os are removed from sea water in anoxic marine depositional regimes.

Investigation of the Clay Fraction (&lt;2 µm) of the Clay Minerals Society Reference Clays

AbstractWe studied a set of 15 reference clays from The Clay Minerals Society (CMS) Source Clays repository. Our aim was to use them as reference materials in our version of the QUAX mineral database. The QUAX software (Quantitative Phase-Analysis with X-ray Powder Diffraction) has been used successfully at the KTB site (German Continental Deep Drillling) to determine mineral assemblages quickly, in an automatic fashion, on a large number of samples (∼40,000). It was also applied to Quaternary marine sediments of the Japan Sea. Our current research focuses on marine and lacrustrine sediments from the Arctic Ocean and Siberia.QUAX is a full-pattern method using a reference materials database. The quality of a particular quantification depends on the availability of the relevant mineral phases in the database. Our aim is to extend and improve the database continuously with new data from our current projects, particularly from clay and feldspar minerals.A reference material in the QUAX software must be monomineralic. Before X-ray diffraction (XRD) patterns of CMS clays could be added to the database, quantification of any impurities was necessary. After measuring the bulk material by XRD, the &lt;2 µm fraction was separated because we assumed it would contain the smallest amount of impurities. Here we present grain-size data, XRD data and X-ray fluorescence (XRF) data for this clay-sized fraction. The results of chemical and mineralogical preparation techniques and (elemental) analysis methods were combined. For XRD, random and oriented clay-aggregate samples as well as pressed pellets for QUAX analysis were prepared. Semi-quantitative clay mineral determinations were run for comparison.

断層間での破壊の乗り移り 応力が深さに依存する場合についての考察

Numerical simulations have revealed that a rupture jumps from one fault to another near the Earth's surface. We show that a rupture jump occurs at a deep portion under depthdependent stress. We investigated rupture behaviour at a gap region between two faults, by calculating spontaneous rupture processes on two parallel strike-slip faults in a 3-D elastic medium. Depth-dependent stresses introduced from the results of hydraulic fracturing experiments at the KTB site are assumed. The differential stress is assumed to be proportional to depth. The rupture processes under depth-dependent and uniform stress conditions are compared : similar rupture processes on the first fault, but different ones on the second fault are obtained. Under the depth-dependent stress condition, an upward rupture on the first fault can trigger a rupture at a deep portion of the second fault, because strength is reduced at shallower depths. Near the Earth's surface, a rupture on the second fault can be triggered by only a P-wave radiated from the first fault. Strength and stress drop heterogeneities on faults cause a rupture jump at a deep portion and the complexity of the rupture process. Our results suggest that the stress increase with depth is an important factor.

Seismic scattering attenuation estimates for the German KTB Area derived from well‐log statistics

We applied a recently developed scattering attenuation theory to model Q‐values measured at the German KTB site and found that scattering attenuation plays an important role in the upper crust in this area. This does not conform with previous studies that suggest absorption mechanisms as the main reason for the attenuation. The scattering attenuation model used is based on the wave propagation theory in random media and allows quantification of scattering attenuation in complex geological regions using simple statistical estimates from well‐log data. For our estimates of scattering Q we used statistical models of heterogeneities obtained from KTB logs. A good agreement between the magnitude of scattering attenuation and the measured Q‐values shows the necessity of evaluating the role of scattering in seismic attenuation phenomena.

Boundary conditions in thermal models: An application to the KTB site, Germany

Las estimaciones de la temperatura a profundidad son de suma importancia para los estudios reológicos de la corteza y también para una planeación correcta de la perforación de pozos profundos. La modelación térmica requiere de condiciones de frontera reales, para así poder obtener valores confiables de la temperatura a profundidad. Las condiciones de frontera necesarias para los modelos térmicos pueden ser inferidas a partir de parámetros geoquímicos y geofísicos medidos en el campo. Se pueden obtener soluciones numéricas de las ecuaciones de transferencia de calor a través de modelos de elementos finitos o de diferencias finitas para una, dos o tres dimensiones, tomando en cuenta varias suposiciones en relación con los parámetros del medio, tales como homogeneidad y anisotropía de la conductividad térmica. En este trabajo presentamos modelos uni- y bidimensionales y resaltamos las diferencias entre ambos. El área de Oberpfalz fue seleccionada para probar el modelo y las condiciones de frontera debido a que los datos geoquímicos y geofísicos se encontraban disponibles para estimar las condiciones de frontera, ya que las temperaturas a profundidad habían sido medidas en el pozo KTB. Se encontró concordancia entre las temperaturas medidas y las calculadas para el modelo bidimensional que incluía condiciones de frontera inferidas a partir de los datos geoquímicos y geofísicos.