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Sudha S.,University of Cologne | Tezkan B.,University of Cologne | Siemon B.,BGR Hanover
Near Surface Geoscience 2012 | Year: 2012

The use of ground-based electromagnetic methods (transient electromagnetics (TEM) and radiomagnetotellurics (RMT)) is increased in last decades for hydrogeological purposes. However, airborne electromagnetic methods were also proposed as a possible tool for near surface investigations due to the capability to cover large areas in short time. By jointly inverting different kinds of geophysical measurements at a site, the ambiguity inherent in different geophysical methods can be avoided. In order to couple spatial data from helicopter-borne electromagnetics (HEM) with ground-based electromagnetics, a common 1D joint inversion algorithm for HEM, TEM and RMT data is developed. The depth of investigation of HEM data is rather limited compared to time-domain EM sounding methods. In order to improve the accuracy of model parameters of shallow as well as of the deeper subsurface, the HEM, TEM, and RMT measurements are combined using joint inversion methodology. The 1D joint inversion algorithm is verified for the synthetic HEM, TEM and RMT data. The proposed concept of joint inversion takes advantage of each single method, which provides the capability to resolve near surface (RMT and HEM) and deeper electrical structures (TEM). Furthermore, the joint inversion is realized for the field data (HEM and TEM) from Cuxhaven area, Germany. Source


Volker D.,Leibniz Institute of Marine Science | Geersen J.,Leibniz Institute of Marine Science | Contreras-Reyes E.,University of Chile | Sellanes J.,Catolica del Norte University | And 6 more authors.
International Journal of Earth Sciences | Year: 2014

The continental shelf and slope of southern Central Chile have been subject to a number of international as well as Chilean research campaigns over the last 30 years. This work summarizes the geologic setting of the southern Central Chilean Continental shelf (33°S–43°S) using recently published geophysical, seismological, sedimentological and bio-geochemical data. Additionally, unpublished data such as reflection seismic profiles, swath bathymetry and observations on biota that allow further insights into the evolution of this continental platform are integrated. The outcome is an overview of the current knowledge about the geology of the southern Central Chilean shelf and upper slope. We observe both patches of reduced as well as high recent sedimentation on the shelf and upper slope, due to local redistribution of fluvial input, mainly governed by bottom currents and submarine canyons and highly productive upwelling zones. Shelf basins show highly variable thickness of Oligocene-Quaternary sedimentary units that are dissected by the marine continuations of upper plate faults known from land. Seismic velocity studies indicate that a paleo-accretionary complex that is sandwiched between the present, relatively small active accretionary prism and the continental crust forms the bulk of the continental margin of southern Central Chile. © 2012, Springer-Verlag. Source


Graupner T.,BGR Hanover | Muhlbach C.,Sallstr. 79 | Schwarz-Schampera U.,BGR Hanover | Henjes-Kunst F.,BGR Hanover | And 3 more authors.
Ore Geology Reviews | Year: 2015

The Vergenoeg fluorite deposit in the Bushveld Complex in South Africa is hosted by a volcanic pipe-like body. The distribution characteristics, composition and formation conditions of high-field-strength element (HFSE)-rich minerals in different lithological units of the deposit were investigated by optical and cathodoluminescence microscopy, scanning electron microscopy, X-ray fluorescence, inductively-coupled plasma mass-spectrometry and electron-probe microanalysis. The Vergenoeg host rocks comprise a diverse silica-undersaturated assemblage of fayalite-magnetite-fluorite with variably subordinate apatite and mineral phases enriched in rareearth elements (REEs). The Sm-Nd isotope systematics of the fluorite from the various lithological units of the pipe support the model that the HFSE budget of the Vergenoeg pipe was likely derived from a Lebowa-type granitic magma. Isotopically, there is no evidence for other REE sources. Formation of the pipe, including development of the fluorite mineralization, occurred within the same time frame as the emplacement of other magmatic rock units of the Bushveld Complex (Sm-Nd isochron age for fluorite separates: 2040 ± 46 Ma). Hydrothermal alteration is manifested in strongly disturbed Rb-Sr isotope systematics of the Vergenoeg deposit, but did not affect its HFSE and REE budget. Whole-rock chondrite-normalized REE + Y distribution patterns of two types were observed: (i) flat patterns characteristic of magnetite-fluorite unit, gossan, metallurgical-grade fluorite ("metspar") plugs and siderite lenses, and (ii) U-shaped patterns showing enrichment towards the heaviest REE (Tm-Lu) observed in the fayalite-rich units. Common HFSE minerals are complex Nb-rich oxides (samarskite, fergusonite), REE phosphates and fluorocarbonates. Additionally, fluocerite and REE silicates, whose identification requires further work, were found. Most of the HFSE-rich minerals are spatially associated with Fe-rich phases (e.g., pyrite, magnetite, greenalite and hematite). To a smaller extent, they are found finely disseminated or healingmicro-fractures in fluorite. The whole-rock REE+Y distribution patterns of the individual lithological units aremainly controlled by the distribution of Yb-rich and Y-rich xenotime in these rocks. The common occurrence of bastnäsite-(Ce) in the gossan, "metspar" plugs and especially in the rhyolitic carapace at the pipe-wall-rock contact, controls the REE + Y distribution patterns of these rocks. HFSE minerals in the Vergenoeg pipe rocks have formed in several stages. Samarskite and coarse fluorapatite belong to the primary mineral assemblage. Fergusonite and Yb-rich xenotime formed during high- to moderate-temperature hydrothermal activity. Significant remobilization of the HFSE from the early-crystallized minerals (breakdown of fluorapatite and possibly allanite with release of REE + Y) and subsequent partial redistribution of these elements into near surface rocks are inferred. The late-stage assemblages are characterized by the presence of fine-grained REE fluorocarbonates, monazite-(Ce), monazite-(La) and xenotime-(Y). © 2014 Elsevier B.V. Source


Neumaier M.,Schlumberger | Littke R.,RWTH Aachen | Back S.,RWTH Aachen | Kukla P.,RWTH Aachen | And 3 more authors.
1st EAGE Basin and Petroleum Systems Modeling Workshop: Advances in Basin and Petroleum Systems Modeling in Risk and Resource Assessment | Year: 2014

The Atlantic margin offshore Morocco can be characterized as a frontier area in which hydrocarbon exploration is very immature. While the onshore Essaouira Basin hosts some small oil and gas fields, offshore exploration has so far resulted in sub-commercial discoveries only. Recent deepwater wells failed to find viable reservoirs, however, numerous hydrocarbon shows have been encountered, a variety of different potential traps are related to salt tectonics, and the offshore domain recently gained the interest of international oil companies once again. In addition, the relative success of the conjugate Atlantic margin of Nova Scotia is encouraging. However, even though the Triassic to Jurassic rifting to break-up history is similar, the Moroccan margin has experienced a significantly different geodynamic post-rift evolution. This includes Late Cretaceous uplift, the Cenozoic Atlas orogeny, and the Canary Island hotspot. This study investigates the geodynamic evolution of the Atlantic margin offshore Essaouira (Morocco) and its implication on hydrocarbon exploration. The methods applied are structural interpretation and analysis of recently acquired 2D seismic reflection data of the 2011 MIRROR experiment, crustal-scale gravity modeling integrated in basin modeling, structural restoration, petroleum systems modeling, and probabilistic analysis. Source


Schwalenberg K.,BGR Hanover | Engels M.,BGR Hanover | Rippe D.,BGR Hanover | Scholl C.,CGG Electro Magnetics
76th EAGE Conference and Exhibition 2014, Workshops | Year: 2014

In the past years BGR-the German Federal Institute for Geosciences and Natural Resources has been developing unique marine CSEM systems to explore the electrical attributes of the shallow seafloor. CSEM data are sensitive to the presence of resistive gas and gas hydrate in the sediment, and provide complementary volume information which, if used in connection with seismic and other exploration methods, e.g. drilling, allow for a better evaluation of the gas or gas hydrate resource potential. The gas hydrate setting differs from the exploration of conventional offshore oil and gas reservoirs as typical gas hydrate deposits are smaller in scale und at shallower depths below the seafloor. Therefore instrumentation and survey configurations need to be adapted. HYDRA, the seafloor-towed, multi-receiver system has been recently refined with a new signal generator and receiver units which both allow for online communication and data transfer. ID and 2D inversions of CSEM data collected offshore New Zealand result in highly anomalous resistivities over several methane seep sites within the gas hydrate stability field which are believed to be caused by concentrated gas hydrates below the seeps. Source

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