Bayesian geological and geophysical data fusion for the construction and uncertainty quantification of 3D geological models

Traditional approaches to develop 3D geological models employ a mix of quantitative and qualitative scientific techniques,which do not fully provide quantification of uncertainty in the constructed models and fail to optimally weight geological field observations against constraints from geophysical data.Here,using the Bayesian Obsidian software package,we develop a methodology to fuse lithostratigraphic field observations with aeromagnetic and gravity data to build a 3D model in a small(13.5 km×13.5 km)region of the Gascoyne Province,Western Australia.Our approach is validated by comparing 3D model results to independently-constrained geological maps and cross-sections produced by the Geological Survey of Western Australia.By fusing geological field data with aeromagnetic and gravity surveys,we show that 89%of the modelled region has>95%certainty for a particular geological unit for the given model and data.The boundaries between geological units are characterized by narrow regions with<95%certainty,which are typically 400-1000 m wide at the Earth's surface and 500-2000 m wide at depth.Beyond~4 km depth,the model requires geophysical survey data with longer wavelengths(e.g.,active seismic)to constrain the deeper subsurface.Although Obsidian was originally built for sedimentary basin problems,there is reasonable applicability to deformed terranes such as the Gascoyne Province.Ultimately,modification of the Bayesian engine to incorporate structural data will aid in developing more robust 3D models.Nevertheless,our results show that surface geological observations fused with geophysical survey data can yield reasonable 3D geological models with narrow uncertainty regions at the surface and shallow subsurface,which will be especially valuable for mineral exploration and the development of 3D geological models under cover.     

The Erbisberg drilling 2011: Implications for the structure and postimpact evolution of the inner ring of the Ries impact crater

The 26 km diameter Nördlinger Ries is a complex impact structure with a ring structure that resembles a peak ring. A first research drilling through this “inner crystalline ring” of the Ries was performed at the Erbisberg hill (SW Ries) to better understand the internal structure and lithology of this feature, and possibly reveal impact‐induced hydrothermal alteration. The drill core intersected the slope of a 22 m thick postimpact travertine mound, before entering 42 m of blocks and breccias of crystalline rocks excavated from the Variscan basement at >500 m depth. Weakly shocked gneiss blocks that show that shock pressure did not exceed 5 GPa occur above polymict lithic breccias of shock stage Ia (10–20 GPa), with planar fractures and planar deformation features (PDFs) in quartz. Only a narrow zone at 49.20–50.00 m core depth exhibits strong mosaicism in feldspar and {102} PDFs in quartz, which are indicative of shock stage Ib (20–35 GPa). Finally, 2 m of brecciated Keuper sediments at the base of the section point to an inverse layering of strata. While reverse grading of clast sizes in lithic breccias and gneiss blocks is consistent with lateral transport, the absence of diaplectic glass and melt products argues against dynamic overthrusting of material from a collapsing central peak, as seen in the much larger Chicxulub structure. Indeed, weakly shocked gneiss blocks are rather of local provenance (i.e., the transient crater wall), whereas moderately shocked polymict lithic breccias with geochemical composition and ⁸⁷Sr/⁸⁶Sr signature similar to Ries suevite were derived from a position closer to the impact center. Thus, the inner ring of the Ries is formed by moderately shocked polymict lithic breccias likely injected into the transient crater wall during the excavation stage and weakly shocked gneiss blocks of the collapsing transient crater wall that were emplaced during the modification stage. While the presence of an overturned flap is not evident from the Erbisberg drilling, a survey of all drillings at or near the inner ring point to inverted strata throughout its outer limb. Whether the central ring of the Ries represents remains of a collapsed central peak remains to be shown. Postimpact hydrothermal alteration along the Erbisberg section comprises chloritization, sulfide veinlets, and strong carbonatization. In addition, a narrow zone in the lower parts of the polymict lithic breccia sequence shows a positive Eu anomaly in its carbonate phase. The surface expression of this hydrothermal activity, i.e., the travertine mound, comprises subaerial as well as subaquatic growth phases. Intercalated lake sediments equivalent to the early parts of the evolution of the central crater basin succession confirm a persistent impact‐generated hydrothermal activity, although for less time than previously suggested.     

An evaluation of the CMIP3 and CMIP5 simulations in their skill of simulating the spatial structure of SST variability

The natural sea surface temperature (SST) variability in the global oceans is evaluated in simulations of the Climate Model Intercomparison Project Phase 3 (CMIP3) and CMIP5 models. In this evaluation, we examine how well the spatial structure of the SST variability matches between the observations and simulations on the basis of their leading empirical orthogonal functions-modes. Here we focus on the high-pass filter monthly mean time scales and the longer 5 years running mean time scales. We will compare the models and observations against simple null hypotheses, such as isotropic diffusion (red noise) or a slab ocean model, to illustrate the models skill in simulating realistic patterns of variability. Some models show good skill in simulating the observed spatial structure of the SST variability in the tropical domains and less so in the extra-tropical domains. However, most models show substantial deviations from the observations and from each other in most domains and particularly in the North Atlantic and Southern Ocean on the longer (5 years running mean) time scale. In many cases the simple spatial red noise null hypothesis is closer to the observed structure than most models, despite the fact that the observed SST variability shows significant deviations from this simple spatial red noise null hypothesis. The CMIP models tend to largely overestimate the effective spatial number degrees of freedom and simulate too strongly localized patterns of SST variability at the wrong locations with structures that are different from the observed. However, the CMIP5 ensemble shows some improvement over the CMIP3 ensemble, mostly in the tropical domains. Further, the spatial structure of the SST modes of the CMIP3 and CMIP5 super ensemble is more realistic than any single model, if the relative explained variances of these modes are scaled by the observed eigenvalues.     

Oxygen isotope systematics of gem corundum deposits in Madagascar: relevance for their geological origin

The oxygen isotopic composition of gem corundum was measured from 22 deposits and occurrences in Madagascar to provide a gemstone geological identification and characterization. Primary corundum deposits in Madagascar are hosted in magmatic (syenite and alkali basalt) and metamorphic rocks (gneiss, cordieritite, mafic and ultramafic rocks, marble, and calc-silicate rocks). In both domains the circulation of fluids, especially along shear zones for metamorphic deposits, provoked in situ transformation of the corundum host rocks with the formation of metasomatites such as phlogopite, sakenite, and corundumite. Secondary deposits (placers) are the most important economically and are contained in detrital basins and karsts. The oxygen isotopic ratios (¹⁸O/¹⁶O) of ruby and sapphire from primary deposits are a good indicator of their geological origin and reveal a wide range of δ¹⁸O (Vienna Standard Mean Ocean Water) between 1.3 and 15.6‰. Metamorphic rubies are defined by two groups of δ¹⁸O values in the range of 1.7 to 2.9‰ (cordieritite) and 3.8 to 6.1‰ (amphibolite). “Magmatic” rubies from pyroxenitic xenoliths contained in the alkali basalt of Soamiakatra have δ¹⁸O values ranging between 1.3 and 4.7‰. Sapphires are classified into two main groups with δ¹⁸O in the range of 4.7 to 9.0‰ (pyroxenite and feldspathic gneiss) and 10.7 to 15.6‰ (skarn in marble from Andranondambo). The δ¹⁸O values for gem corundum from secondary deposits have a wide spread between −0.3 and 16.5‰. The ruby and sapphire found in placers linked to alkali basalt environments in the northern and central regions of Madagascar have consistent δ¹⁸O values between 3.5 and 6.9‰. Ruby from the placers of Vatomandry and Andilamena has δ¹⁸O values of 5.9‰, and between 0.5 and 4.0‰, respectively. The placers of the Ilakaka area are characterized by a huge variety of colored sapphires and rubies, with δ¹⁸O values between −0.3 and 16.5‰, and their origin is debated. A comparison with oxygen isotope data obtained on gem corundum from Eastern Africa, India, and Sri Lanka is presented. Giant placer deposits from Sri Lanka, Madagascar, and Tanzania have a large variety of colored sapphires and rubies with a large variation in δ¹⁸O due to mingling of corundum of different origin: mafic and ultramafic rocks for ruby, desilicated pegmatites for blue sapphire, syenite for yellow, green, and blue sapphire, and skarn in marbles for blue sapphire.     

The Mesoproterozoic basement in the southern Baltic Sea: insights from the G 14–1 off-shore borehole

The continuation of the Mesoproterozoic basement of the southern Fennoscandian Shield is documented in the G 14–1 off-shore borehole, northeast of the island of Rügen, where crystalline rocks of monzogranitic composition occur beneath flat-lying early Palaeozoic sediments at a depth of approximately 2,000 m. The greenish-grey, or partly reddish-grey, granites show a slightly porphyritic texture marked by plagioclase crystals or aggregates in a groundmass dominated by fresh microcline. Chloritized biotite occurs as a subordinate mafic phase. Ductile and brittle deformation is indicated by a weak foliation and the occurrence of several cataclastic zones, respectively. Major and trace element geochemistry suggest that these rocks are K-rich calc-alkaline granites and represent a restite-poor melting product of a granodioritic protolith. The low MgO, Cr, Ni and Co concentrations and relatively high content of accessory phases (apatite, zircon) point to formation of water undersaturated, high temperature (>900 °C) melts at low degrees of partial melting. Although the G 14 granite lacks hornblende, in contrast to most of the granites from Bornholm, it seems geochemically related to them. Analysed samples mostly fit an intermediate position between the Rønne and Vang granitoids, which both belong to a group of Mesoproterozoic intrusions showing partial ductile deformation. (Y+Nb) vs. Rb plots suggest that all these granitic rocks were generated in intracratonic conditions. A genetic relationship between them and contemporaneously intruded Karlshamn-group granites in Blekinge, eastern Scania and Småland is supported by intrusion age of 1,460±3 Ma obtained from Pb-Pb isotope ratios measured on single zircons of the G 14 granite.     

Gem corundum deposits of Madagascar: A review

Madagascar is one of the most important gem-producing countries in the world, including ruby and sapphires. Gem corundum deposits formed at different stages in the geological evolution of the island and in contrasting environments. Four main settings are identified: (1) Gem corundum formed in the Precambrian basement within the Neoproterozoic terranes of southern Madagascar, and in the volcano-sedimentary series of Beforona, north of Antananarivo. In the south, high-temperature (700 to 800 °C) and low-pressure (4 to 5 kbar) granulites contain deposits formed during the Pan-African orogenesis between 565 and 490 Ma. They accompany mafic and ultramafic complexes (ruby deposits of the Vohibory group), skarns at the contact between Anosyan granites and the Proterozoic Tranomaro group (sapphire deposits of the Tranomaro–Andranondambo district), and shear-zone corridors cross-cutting feldspathic gneisses, cordieritites and clinopyroxenites in the Tranomaro, Vohimena and Androyan metamorphic series (biotite schist deposits of Sahambano and Zazafotsy, cordieritites of Iankaroka and Ambatomena). The circulation of fluids, especially along discontinuities, allowed in-situ alkaline metasomatism, forming corundum host rocks related to desilicified granites, biotitites, “sakenites” and “corundumites”. (2) Gem corundum also occurs in the Triassic detrital formations of the Isalo group, as giant palaeoplacers in the Ilakaka–Sakaraha area. Here, sapphires and rubies may come from the metamorphic granulitic terranes of southern Madagascar. (3) Gem corundum deposits occur within the Neogene-Quaternary alkali basalts from Ankaratra (Antsirabe–Antanifotsy area) and in the Ambohitra Province (Nosy Be, Ambato and Ambondromifehy districts). Primary deposits are rare, except at Soamiakatra where ruby in gabbroic and clinopyroxenite xenoliths within alkali-basalts probably derive from mantle garnet peridotites. The blue-green-yellow sapphires typical of basaltic fields are always recovered in palaeoplacer (in karst formed upon Jurassic limestones from the Montagne d'Ambre, Antsiranana Province) and alluvial and soil placers (Ankaratra volcanic massif). (4) Deposits occur within Quaternary eluvial, colluvial and alluvial concentrations, such as high-quality rubies from the Andilamena and Vatomandry deposits.     

Inferring protoliths of high-grade metamorphic gneisses of the Erzgebirge using zirconology,geochemistry and comparison with lower-grade rocks from Lusatia (Saxothuringia,Germany)

Protoliths of highly metamorphosed gneisses from the Erzgebirge are deduced from the morphology, age and chemistry of zircons as well as from whole rock geochemistry and are compared with lower-grade rocks of Lusatia. Gneisses with similar structural appearance and/or geochemical pattern may have quite different protoliths. The oldest rocks in the Erzgebirge are paragneisses representing meta-greywackes and meta-conglomerates. The youngest group of zircon of meta-greywackes that did not undergo Pb loss represents the youngest igneous component for source rocks (about 575 Ma). Similar ages and zircon morphology reflect the subordinate formation of new zircon grains or only zircon rims in the augengneiss from Bärenstein and Wolkenstein, which probably represent metamorphic equivalents to Lower Cambrian two-mica granodiorites from Lusatia. Bulk rock chemistry, intense fracturing and high U and Th concentrations of zircons suggest deformation-induced and fluid-enhanced recrystallisation of zircon grains. Temperatures during tectonic overprinting—too low to reset zircon ages—indicate mid- or upper crustal levels for shearing recorded in these augengneisses. Lower Cambrian (~540 Ma) granodiorites are widespread in Lusatia but are exclusively represented by the Freiberg gneiss dome in the Eastern Erzgebirge. Ordovician protolith ages were recorded by zircons from the augengneisses of the Reitzenhain–Catherine dome and the Schwarzenberg dome (Western Erzgebirge) documenting significant regional differences between the eastern and the western Erzgebirge (~540 vs. ~490 Ma). In the Western Erzgebirge, most meta-volcanic rocks (muscovite gneisses) and meta-granites (mainly red augengneisses) yield Ordovician zircon ages, whereas in the Eastern part, similar rocks mainly recorded Lower Cambrian protolith ages. Zircon overprinting was highest within discrete tectonic zones where the combination of fluid infiltration and deformation induced variable degrees of recrystallisation and formation of a new augengneiss structure. Variable degrees of Pb loss caused age shifts that do not correspond to changes in zircon morphology but may be associated with U and Th enrichments. Major changes in bulk rock composition appear to be restricted to discrete zones and to (U)HP nappes, whereas gneisses with a MP–MT metamorphic overprint basically show no geochemical modifications.     

Estimation of industrial and commercial asset values for hazard risk assessment

For risk analyses not only knowledge about the impact of different types of hazards, but also information about the elements and values at risk is necessary. This article introduces a methodology for a countrywide estimation of asset values for commercial and industrial properties using Germany as an example. It consists of a financial appraisal of asset values on the municipal level and a further disaggregation by means of land use data. Novelties are the distinction of 60 economic activities, the consideration of production site sizes and the application of a dasymetric mapping technique for a sector-specific estimation and disaggregation of asset values. A validation with empirical data confirms the feasibility of the calculation. The resulting maps can be used for loss estimations e.g. in the framework of cost–benefit analyses that aim to evaluate hazard mitigation measures or for portfolio analyses by banks and insurance companies. The approach can be used for other countries if the necessary data is available (mainly in industrialized countries). In any case, it reveals the critical points when estimating commercial and industrial asset values.     

Thermal, mechanical and chemical influences on the performance of optical fibres for distributed temperature sensing in a hot geothermal well

Structural well-bore integrity is an important issue for sustainable provision of geothermal energy. Raman scattering based fibre optic distributed temperature sensing (DTS) can help to monitor the status of a well and therefore help to optimize expensive work-over activities. This study reports on the installation of a fibre-optic cable in the cemented annulus behind the anchor casing in the high temperature geothermal well HE-53, Hellisheiði geothermal field, SW Iceland. Although the cable has been damaged during the installation, temperature data could be acquired during the entire length of installation down to 261.3 m. Temperature measurements were performed during the installation in spring 2009, during the onset of a flow test in summer 2009 and after a 8.5 month shut-in period in summer 2010. During the flow test, maximum temperatures of 230 °C were measured after 2 weeks fluid production. Using optical time domain reflectometry (OTDR), attenuation measurements at 850 and 1,300 nm enabled to identify mechanical, thermal, and chemical degradation along the optical fibre. The observed degradation led to erroneous temperature readings and limits, due to the optical budget of the DTS system, the accessible length of the fibre. The characteristics and the influence of the different degradation mechanisms on the accuracy of the DTS measurements are discussed and recommendations for an optimized installation are given.