Brandes C.,Leibniz University of Hanover |
Tanner D.C.,Leibniz Institute for Applied Geophysics
Earth-Science Reviews | Year: 2014
Folding that is directly related to fault activity is an important deformation feature that occurs all over the world in mountain belts, accretionary wedges, fold-and-thrust belts, and intra-plate settings in either strike-slip, compressional, or extensional regimes. Due to their widespread occurrence, knowledge about the development of these structures is important to a broad spectrum of geoscience sub-disciplines, such as structural geology, seismology, geomorphology, petroleum geology, and Quaternary geology. Fault-related folding has been analyzed intensively over the last 140. years. For the sake of this review, we divide the folds according to the way the faults and the folds form; that is into detachment, fault-bend, and fault-propagation folds.All fault-related folds are caused by changes in fault parameters. The simplest method to produce folds is to transport material along faults that have stepped, flat-ramp-flat geometries (fault-bend fold). Alternatively the slip can decrease along the length of the fault, and depending on whether the fault remains within a detachment layer or steps up through mechanical stratigraphy, either a detachment fold or a fault-propagation fold is formed, respectively.Detachment folding was first investigated in the early 20th Century, whereas the full significance of fault-propagation folds was recognized quite late in the 1980s. Seminal work on fault-related folding was carried out in the 1930s, but quantitative kinematic models have only been available in the last 30. years. These models are extremely valuable, because they allow a comprehensive understanding of the evolution of fault-related folds and lead to more accurate predictions of the sub-surface structure. From the mid-1990s onwards, numerical simulations have been used to identify how fault parameters (such as dip and fault-bend angle, propagation-to-slip ratio, and shape of the trishear zone) influence the geometry of the related folding. This is directly applicable to the analysis of the shape of anticlines produced. However, this does not mean that fold geometry is uniquely related to fault geometry; on the contrary, different kinematic approaches can lead to a similar fold shape. © 2014 Elsevier B.V.
Rucker C.,TU Berlin |
Gunther T.,Leibniz Institute for Applied Geophysics
Geophysics | Year: 2011
Direct-current resistivity surveys usually are performed using steel rods of finite extent and grounding resistance. However, in modeling, electrodes are commonly treated as ideal point sources. We present an approach for numerical computation applying the complete electrode model (CEM), which is known from medical imaging. The electrode surface was discretized, and the partial-differential equations were extended by additional relations incorporating a contact impedance and a condition for the current flow through the electrode surface. We verified the modeling of the electrical potential using an analytical solution for a perfectly coupled half-ellipsoid current source. To quantify the influence of a finite electrode, we computed the electrode effect as the ratio between CEM and point-source solution and investigated its dependence on geometry and contact impedance. Surface measurements using rods of typical spatial extent showed electrode effects on the order of the measuring accuracy for an electrode length/spacing ratio lower than 0.2. However, the effects are more significant for closed geometries such as experimental tanks. A comparison with a point approximation for finite electrodes using point-source locations along the electrode axis showed the best agreement, with points at about 60% of the electrode extension. The contact impedance played a minor role for four-point measurements, contributing only a few percent to the electrode effect. In addition to penetrating electrodes, we investigated surface electrodes with galvanic or capacitive coupling, showing electrode effects on the same order as for penetrating electrodes. An inhomogeneous resistivity distribution clearly increased the size of the effects. We also investigate the use of CEM to simulate current injected through steel-cased boreholes. Finally, we applied the approach with buried ring electrodes to calculate effects caused mainly by geometric disturbances from the borehole. © 2011 Society of Exploration Geophysicists.
Barta G.,ELTE Institute of Geography and Earth science |
Barta G.,Leibniz Institute for Applied Geophysics
Quaternary International | Year: 2014
Micro- and macroscale secondary carbonates were investigated from the loess-paleosol sequence of Sütto, Hungary. As secondary carbonates are formed in pedosedimentary environments, they designate how the microecosystem adapts to dust accretion. Besides pedogenic aspects, the hints of diagenesis can be tracked, especially concerning leaching effects. The main goal of this study was to make an attempt whether the distribution-related signals of secondary carbonates combined with their genetic properties can be used for paleoenvironmental reconstruction. The elaborated method is wet sieving of bulk loess/paleosol samples in order to: 1) separate secondary carbonates to describe morphological properties, partly with the help of scanning electron microscopy and 2) provide a depth-related semiquantitative distribution. The semiquantitative distribution is ordered to the marine isotope stage units of the Sütto sequence (MIS 6 to 2). The MIS 6 section is characterized by presumably slower dust accumulation and raises the possibility of a multiphase leaching history throughout the profile. Lower dust accumulation rates with increasing aridity are assumed upwards in MIS 5. MIS 4 is typified by arid conditions and the alternation of lower and higher dust accumulations. Certain properties of paleosol development and frost deformation related to secondary carbonates can be distinguished in MIS 3. Dust accumulation rates are presumed to be consistent upwards in this section. During MIS 2, different sedimentation stages are distinguished, but generally they show a decreasing trend upwards to the modern soil. This work serves as a complementary method which may help to refine the paleoenviromental signals of the Sütto loess-paleosol sequence to determine relative sedimentary phases. The limitations of the method have to be taken into consideration, as the results are semiquantitative. © 2013 Elsevier Ltd and INQUA.
Costabel S.,Federal Institute for Geosciences and Natural Resources |
Muller-Petke M.,Leibniz Institute for Applied Geophysics
Near Surface Geophysics | Year: 2014
In this paper three different despiking methods for surface-NMR data are investigated and compared. Two of these are applied in the time domain: a threshold is determined that identifies and marks a spiky event. Afterward, the marked time sequence is substituted with zeros or with the mean value of the signal amplitude of the measurement repetitions for the same passage on the time axis. The third despiking approach takes advantage of the wavelet-like nature of spiky events. It isolates and eliminates spiky signals in the wavelet domain, i.e., after transforming a single record with the help of the discrete wavelet transform. The latter is able to reconstruct the original signal content in the (spike-caused) distorted time sequence to some extent. If the spiky noise in surface-NMR measurements consists mainly of single spiky events, the three despiking methods show very similar results and are able to remove spiky noise from data very effectively, as we can show with two real data examples. However, a synthetic study shows that, if a series of spikes within a relatively short period of time occurs, the wavelet-based despiking approach shows significant shortcomings. Because the NMR signal content cannot be restored completely in a single record, the fitting of the signal after stacking leads to underestimation of the initial amplitude up to approximately 10%. Nevertheless, we can show that, in principle, the processing of surface-NMR data in the wavelet domain works and can lead to the same results as straight-forward applications. Moreover, waveletbased strategies have some interesting properties and thus have some potential for further development regarding surface-NMR processing, which is discussed in detail. © 2014 European Association of Geoscientists & Engineers.
Hubner W.,Leibniz Institute for Applied Geophysics
Journal of Applied Geophysics | Year: 2014
Evaluating the formation quality by deriving porosity, pore size, and permeability from cuttings instead of drill cores is a promising and challenging field of research established in the past decade. Challenges with cuttings are their small and irregular size rendering them unsuitable for e.g. standard permeability measurements. Permeability can be estimated from nuclear magnetic resonance (NMR) measurements. NMR measurements on cuttings are especially challenging 1) because the total NMR signal is very low due to small sample sizes and 2) because the high ratio of outer surface to volume leads to a significant contribution of interstitial water to the NMR signal, which thus distorts the informative NMR signal from within the pore space. The aim of the study is to evaluate the use of NMR in combination with micro-computed tomography (μCT) as a method to determine the pore space characteristics of small drill cuttings from the Bückeberg Formation (German Wealden). After accurate removal of interstitial water and a CT based sorting, it was possible to measure NMR signals representative for the individual pore sizes. The representiveness of the measured values was verified by simulations of the NMR signals in pore spaces determined via μCT. Porosity, relaxation time distributions, and permeability were calculated for cuttings assemblages with large, medium, small, and very small pores. © 2014 Elsevier B.V.