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Munoz G.,German Research Center for Geosciences
Surveys in Geophysics | Year: 2014

Electrical conductivity of the subsurface is known to be a crucial parameter for the characterization of geothermal settings. Geothermal systems, composed by a system of faults and/or fractures filled with conducting geothermal fluids and altered rocks, are ideal targets for electromagnetic (EM) methods, which have become the industry standard for exploration of geothermal systems. This review paper presents an update of the state-of-the-art geothermal exploration using EM methods. Several examples of high-enthalpy geothermal systems as well as non-volcanic systems are presented showing the successful application of EM for geothermal exploration but at the same time highlighting the importance of the development of conceptual models in order to avoid falling into interpretation pitfalls. The integration of independent data is key in order to obtain a better understanding of the geothermal system as a whole, which is the ultimate goal of exploration. © 2013 Springer Science+Business Media Dordrecht.


Davidsen J.,University of Calgary | Kwiatek G.,German Research Center for Geosciences
Physical Review Letters | Year: 2013

We examine the temporal statistics of micro-, nano-, and picoseismicity induced by mining as well as by long-term fluid injection. Specifically, we analyze catalogs of seismic events recorded at the Mponeng deep gold mine, South Africa, and at the German deep drilling site. We show that the distribution of time intervals between successive earthquakes is form invariant between the different catalogs. In particular, the distribution can be described by the same scaling function recently established for tectonic seismicity and acoustic emissions from laboratory rock fracture. Thus, our findings bridge the energy gap between those two cases and provide clear evidence that these temporal features of seismicity are independent of the energy scales of the events and whether they are of tectonic or induced origin. © 2013 American Physical Society.


Jahn S.,German Research Center for Geosciences
Acta Crystallographica Section A: Foundations of Crystallography | Year: 2010

A combination of electronic structure calculations, classical molecular dynamics simulations and metadynamics is proposed to study the phase behavior of complex crystals. While the former provide accurate energetics for thermodynamic properties, molecular dynamics and metadynamics simulations may reveal new metastable phases and provide insight into mechanisms and kinetics of the respective structural transformations. Here, different simulation methods are used to investigate the polymorphism of MgSiO3 pyroxenes (enstatites) up to high pressures and temperatures. A number of displacive phase transitions are observed within the three basic structure types clino-, ortho- and proto-enstatite using classical molecular dynamics simulations. Transitions between these types require a change of stacking order, which is modeled using a combination of molecular dynamics and metadynamics. © 2010 International Union of Crystallography Printed in Singapore - all rights reserved.


Kroner U.,TU Bergakademie Freiberg | Romer R.L.,German Research Center for Geosciences
Gondwana Research | Year: 2013

The Variscides of Europe and N-Africa are the result of the convergence of the plates of Gondwana and Laurussia in the Paleozoic. This orogen is characterized by the juxtaposition of blocks of continental crust that are little affected by the Variscan orogeny. These low strain domains principally consist of Neoproterozoic/Cambrian Cadomian basement overlain by volcano-sedimentary successions of an extended peri-Gondwana shelf. These Cadomian blocks are separated by high strain zones containing the record of subduction-related processes. Traditionally the high strain zones are interpreted as sutures between one or more postulated lithospheric microplates sandwiched between the two major plates. Paleobio-geographic constraints in combination with geochemical and isotopic fingerprints of the protoliths, however, imply that the Variscides are the result of the exclusive interaction of the two plates of Gondwana and Laurussia. Here we explain the Variscan orogen in a two plate scenario, reasoning that the complexity of the Variscan orogen (multitude of high-grade metamorphic belts, compositional diversity of coeval magmatism, and arrangement of foreland basins) is the result of the distribution of crustal domains of contrasting rheological properties. Post-Cadomian rifting along the Cadomian-Avalonian belt, which culminated in the opening of the Rheic Ocean, resulted in vast coeval intracontinental extension and the formation of extended peri-Gondwana shelf areas, namely the Avalonian shelf and the Armorican Spur to the north and south of the evolving Rheic Ocean, respectively. Both shelf areas affected by heterogeneous extension consist of stable continental blocks separated by zones of thinner continental crust. During Variscan collisional tectonics the continental blocks behave as unsubductable crust, whereas the thinner continental crust was subductable and came to constitute the high strain domains of the orogen. The variable interplay between both crustal types in space and time is seen as the principal cause for the observed sequence of orogenic processes. The first collisional contact along the convergent Gondwana-Laurussia plate boundary occurred between Brittany and the Midland microcraton causing the early Devonian deformation along the Anglo-Brabant Fold Belt. This process is coeval with the initiation of continental subduction along the Armorican Spur of the Gondwana plate and the formation of back arc and transtensional basins to both sides of the Armorican Spur (e.g., Lizard, Rheno-Hercynian, Careón, Sleza) on the Laurussia plate. As further subduction along this collision zone is blocked, the plate boundary zone between the Gondwana and Laurentia plates is reorganized, leading to a flip of the subduction polarity and a subduction zone jump outboard of the already accreted blocks. The following Devonian-Early Carboniferous subduction accretion process is responsible for the juxtaposition of additional Cadomian blocks against Laurussia and a second suite of high-pressure rocks. The final collision between Gondwana and Laurussia is marked by an intracontinental subduction event affecting the entire internal zone of the orogen. Subduction stopped at 340. Ma and the following isothermal exhumation of the deeply subducted continental crust is primarily responsible for Late Variscan high-temperature metamorphism and cogenetic voluminous granitic magmatism. During this final transpressional stage the irregular shape of the Variscan orogen was established by the highly oblique motion of the decoupled lithospheric blocks (e.g. Iberia and Saxo-Thuringia). Rapid overfilling of synorogenic marine basins in the foreland and subsequent folding of these deposits along vast external fold and thrust belts finally shaped the Variscides, feigning a relatively simple architecture.In terms of plate tectonics, the model places the opening of the Paleotethys in the Devonian with a rotational axis of the spreading center just east of the Variscan orogen. The movement of Gondwana relative to Laurussia follows small circle paths about this axis from 370 to 300. Ma. As a consequence of the incomplete closure of the Rheic Ocean after the termination of the Variscan orogeny, Gondwana decoupled from the European Variscides along the dextral Gibraltar Fault Zone. The relative motion between Gondwana and Laurussia after 300. Ma is associated with a shift of the rotational axis to a position close to the Oslo Rift, and is related to the opening of the Neotethys and the evolution of the Central European Extensional Province. The Permian convergence of Gondwana and Laurussia led to the final Permian collisional tectonics along the Mauritanides/Alleghanides. The assembly of the "Wegenerian" Pangea is complete by the end of the Paleozoic. © 2013 International Association for Gondwana Research.


Shearer [2012] finds three differences of the seismicity clustering in southern California compared to self-similar triggering models: (i) a significantly lower b-value for the aftershocks, (ii) a too large aftershock number, and (iii) a too large foreshock-aftershock ratio to be consistent with the Båth law. Based on these observations, the author concluded that the observed seismicity is not in agreement with self-similarity triggering and/or the observed clustering is not primarily caused by earthquake-to-earthquake triggering. However, I show that the observed lower b-value is likely related to incomplete recordings after mainshocks and that the apparently too large aftershock number does not disprove the self-similarity. Thus, only the enhanced foreshock-to-aftershock ratio seems to indicate some discrepancy to self-similar triggering. ©2013. American Geophysical Union. All Rights Reserved.


Mueller H.J.,German Research Center for Geosciences
Journal of Geodynamics | Year: 2013

The direct access to the interior of our planet is very limited. Scientific drilling can reach about 15. km depth. Natural exhumation processes in conjunction with orogeny bring massive rock packages from up to 100. km depth back to surface. Explosion breccia and kimberlite pipes can carry small rock and mineral fragments as xenoliths from up to 250. km depth. But all the detailed knowledge we achieved about Earth's deep interior structures and dynamics, especially during the last two decades is based on highly resolved seismic data, in particular seismic tomography. That means it is a three-dimensional distribution of elastic and inelastic data with the maximum resolution of the seismic wavelength, i.e. at great depth several kilometres in principle. Consequently any material information is a matter of interpretation. Thus a detailed knowledge about the elastic properties of rocks in dependence on pressure, temperature, mineral content, grain size, deformation, crack distribution, etc., is crucial for this interpretation. © 2013 Elsevier Ltd.


Bernard S.,CNRS Institute of Mineralogy, Materials Physics and Cosmochemistry | Horsfield B.,German Research Center for Geosciences
Annual Review of Earth and Planetary Sciences | Year: 2014

Shale gas systems serve as sources, reservoirs, and seals for unconventional natural gas accumulations. These reservoirs bring numerous challenges to geologists and petroleum engineers in reservoir characterization, most notably because of their heterogeneous character due to depositional and diagenetic processes but also because of their constituent rocks' fine-grained nature and small pore size-much smaller than in conventional sandstone and carbonate reservoirs. Significant advances have recently been achieved in unraveling the gaseous hydrocarbon generation and retention processes that occur within these complex systems. In addition, cutting-edge characterization technologies have allowed precise documentation of the spatial variability in chemistry and structure of thermally mature organic-rich shales at the submicrometer scale, revealing the presence of geochemical heterogeneities within overmature gas shale samples and, notably, the presence of nanoporous pyrobitumen. Such research advances will undoubtedly lead to improved performance, producibility, and modeling of such strategic resources at the reservoir scale. © 2014 by Annual Reviews. All rights reserved.


Willenbring J.K.,German Research Center for Geosciences | von Blanckenburg F.,German Research Center for Geosciences
Earth-Science Reviews | Year: 2010

Rainfall scavenges meteoric cosmogenic 10Be from the atmosphere. 10Be falls to the Earth's surface, where it binds tightly to sediment particles in non-acidic soils over the life-span of those soils. As such, meteoric 10Be has the potential to be an excellent geochemical tracer of erosion and stability of surfaces in a diverse range of natural settings. Meteoric 10Be has great potential as a recorder of first-order erosion rates and soil residence times. Even though this tracer was first developed in the late 1980s and showed great promise as a geomorphic tool, it was sidelined in the past two decades with the rise of the "sister nuclide", in situ10Be, which is produced at a known rate inside quartz minerals. Since these early days, substantial progress has been made in several areas that now shed new light on the applicability of the meteoric variety of this cosmogenic nuclide. Here, we revisit the potential of this tracer and we summarize the progress: (1) the atmospheric production and fallout is now described by numeric models, and agrees with present-day measurements and paleo-archives such as from rain and ice cores; (2) short-term fluctuations in solar modulation of cosmic rays or in the delivery of 10Be are averaged out over the time scale soils accumulate; (3) in many cases, the delivery of 10Be is not dependent on the amount of precipitation; (4) we explore where 10Be is retained in soils and sediment; (5) we suggest a law to account for the strong grain-size dependence that controls adsorption and the measured nuclide concentrations; and (6) we present a set of algebraic expressions that allows calculation of both soil or sediment ages and erosion rates from the inventory of meteoric 10Be distributed through a vertical soil column. The mathematical description is greatly simplified if the accumulation of 10Be is at a steady state with its export through erosion. In this case, a surface sample allows for the calculation of an erosion rate. Explored further, this approach allows calculation of catchment-wide erosion rates from river sediment, similar to the approach using 10Be produced in situ. In contrast to the in situ10Be approach, however, these analyses can be performed on any sample of fine-grained material, even where no quartz minerals are present. Therefore, this technique may serve as a tool to date sediment where no other chronometer is available, to track particle sources and to measure Earth-surface process rates in soil, suspended river sediment, and fine-grained sedimentary deposits. © 2009 Elsevier B.V. All rights reserved.


Grant
Agency: GTR | Branch: NERC | Program: | Phase: Research Grant | Award Amount: 1.47M | Year: 2015

Concerns are growing about how much melting occurs on the surface of the Greenland Ice Sheet (GrIS), and how much this melting will contribute to sea level rise (1). It seems that the amount of melting is accelerating and that the impact on sea level rise is over 1 mm each year (2). This information is of concern to governmental policy makers around the world because of the risk to viability of populated coastal and low-lying areas. There is currently a great scientific need to predict the amount of melting that will occur on the surface of the GrIS over the coming decades (3), since the uncertainties are high. The current models which are used to predict the amount of melting in a warmer climate rely heavily on determining the albedo, the ratio of how reflective the snow cover and the ice surface are to incoming solar energy. Surfaces which are whiter are said to have higher albedo, reflect more sunlight and melt less. Surfaces which are darker adsorb more sunlight and so melt more. Just how the albedo varies over time depends on a number of factors, including how wet the snow and ice is. One important factor that has been missed to date is bio-albedo. Each drop of water in wet snow and ice contains thousands of tiny microorganisms, mostly algae and cyanobacteria, which are pigmented - they have a built in sunblock - to protect them from sunlight. These algae and cyanobacteria have a large impact on the albedo, lowering it significantly. They also glue together dust particles that are swept out of the air by the falling snow. These dust particles also contain soot from industrial activity and forest fires, and so the mix of pigmented microbes and dark dust at the surface produces a darker ice sheet. We urgently need to know more about the factors that lead to and limit the growth of the pigmented microbes. Recent work by our group in the darkest zone of the ice sheet surface in the SW of Greenland shows that the darkest areas have the highest numbers of cells. Were these algae to grow equally well in other areas of the ice sheet surface, then the rate of melting of the whole ice sheet would increase very quickly. A major concern is that there will be more wet ice surfaces for these microorganisms to grow in, and for longer, during a period of climate warming, and so the microorganisms will grow in greater numbers and over a larger area, lowering the albedo and increasing the amount of melt that occurs each year. The nutrient - plant food - that the microorganisms need comes from the ice crystals and dust on the ice sheet surface, and there are fears that increased N levels in snow and ice may contribute to the growth of the microorganisms. This project aims to be the first to examine the growth and spread of the microorganisms in a warming climate, and to incorporate biological darkening into models that predict the future melting of the GrIS. References 1. Sasgen I and 8 others. Timing and origin of recent regional ice-mass loss in Greenland. Earth and Planetary Science Letters, 333-334, 293-303(2012). 2. Rignot, E., Velicogna, I., van den Broeke, M. R., Monaghan, A. & Lenaerts, J. Acceleration of the contribution of the Greenland and Antarctic ice sheets to sea level rise. Geophys. Res. Lett. 38, L05503, doi:10.1029/2011gl046583 (2011). 3. Milne, G. A., Gehrels, W. R., Hughes, C. W. & Tamisiea, M. E. Identifying the causes of sea-level change. Nature Geosci 2, 471-478 (2009).


Grant
Agency: GTR | Branch: NERC | Program: | Phase: Research Grant | Award Amount: 384.68K | Year: 2014

The polar ionosphere has a range of effects on technological systems produced by the coupling of the solar wind with the magnetosphere, and the resulting electrodynamic interaction between magnetosphere, ionosphere and atmosphere. These effects include the degradation of trans-ionospheric satellite communications and point-to-point radio communications, clutter effects in over-the-horizon radars, and increased levels (and decreased predictability) of satellite drag. Such effects are known to be strongly influenced by the activity level of the sun, and hence the phase of the 11-year solar activity cycle. Recent changes in solar activity have taken the scientific community by surprise, in that the recent solar minimum was both extended in time, and much lower in activity than predicted. As we now approach solar maximum we see an increasing solar activity, but this rise in activity is also less than expected. We propose an extensive statistical investigation of the profound changes imposed on a number of fundamental ionospheric characteristics by the changing solar cycle, focussing on the inter-cycle differences between solar cycle 23, a standard solar cycle, and the most recent, unusual solar cycle 24. Furthermore, we will extend this analysis of solar cycle dependence and inter-cycle differences to a number of ionospheric and atmospheric characteristics which have a direct effect on the operational characteristics of key technologies such as trans-ionospheric communication, satellite navigation and radar systems. This major step forward in defining and understanding these effects and their dependence on the level of solar activity will allow a prediction of their consequences for the polar ionosphere and atmosphere, and for the technological systems we operate in the polar regions.

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