Klaus Bauer

Deep structure of the Namibia continental margin as derived from integrated geophysical studies

During the Geophysical Measurements Across the Continental Margin of Namibia (MAMBA) experiments, offshore and onshore refraction and reflection seismic as well as magnetic data were collected. Together with the existing free-air gravity data, these were used to derive two crustal sections across the ocean-continent transition. The results show that the Early Cretaceous continental breakup and the separation of South Africa and South America were accompanied by excessive igneous activity offshore. Off Namibia we found a 150–200 km wide zone of igneous crust up to 25 km thick. The upper part of this zone consists of an extrusive section comprising three units of basaltic composition: two distinct wedges of seaward dipping reflectors (SDRs) separated by flat-lying volcanic flows. The inner wedge of SDRs can be modeled as the source of a long-wavelength magnetic anomaly that borders long parts of both South Atlantic margins (anomaly G). The crust underneath these extrusives is characterized by high-velocity and high-density material (average values 7 km s−1, 3×103 kg m−3). Free-air gravity anomalies along both sides of the high-density crust are interpreted as edge effects resulting from juxtaposition with normal oceanic and continental crust on either side. We define the abrupt landward termination of this zone as the continent-ocean boundary, and consequently, the crust seaward is interpreted as exclusively igneous material and not intruded continental crust. Extrapolation of the interpreted geophysical features along the southwest African margin suggests a fast prograding narrow rift zone and sharp lithospheric rupture leading to the formation of a margin-parallel magmatic belt south of the Walvis Ridge. The influence of the Tristan da Cunha mantle plume may explain the widening of this thick igneous crust near the Walvis Ridge.

Classification of lithology from seismic tomography: A case study from the Messum igneous complex, Namibia

[1]xa0Combined analysis of seismic P and S velocity information provides a reasonable and efficient basis for the petrologic interpretation of seismic cross sections. In this paper, a methodology is presented which allows extraction of prominent features related to well-defined P velocities and Poissons ratios from a tomographic velocity model using a classification approach. We used first-arrival travel time data from a near-vertical seismic experiment and independently determined P and S velocities by forward and inverse modeling. Resolution and uncertainties were estimated from inverting synthetic data. The classification procedure was carried out in two subsequent steps. First, prominent classes were identified in the parameter space spanned by Poissons ratios and P wave velocities. For this purpose, a probability density function was calculated from the tomograms. A function measuring the topography of the probability density was then determined, and a histogram analysis was carried out to detect significant classes. In the second step, the results from principal component analysis for the identified classes were used to map their distribution along the seismic profile. We applied the method to the Messum intrusive complex of Namibia and identified three prominent classes. On the basis of the integration of petrophysical data and comparison with surface geology, we conclude that quartz-syenite composition dominates the upper 800 m of the crust under the complex. Outside of the intrusion the upper crust has properties corresponding to felsic metasediments and granites which are abundant in the local basement. This material shows strong depth-dependent changes of seismic properties which are ascribed to decreasing porosity and fluid saturation with depth.

Crustal structure of northwest Namibia: Evidence for plume-rift-continent interaction

The causes for the formation of large igneous provinces and hotspot trails are still a matter of considerable dispute. Seismic tomography and other studies suggest that hot mantle material rising from the core-mantle boundary (CMB) might play a significant role in the formation of such hotspot trails. An important area to verify this concept is the South Atlantic region, with hotspot trails that spatially coincide with one of the largest low-velocity regions at the CMB, the African large low shear-wave velocity province. The Walvis Ridge started to form during the separation of the South American and African continents at ca. 130 Ma as a consequence of Gondwana breakup. Here, we present the first deep-seismic sounding images of the crustal structure from the landfall area of the Walvis Ridge at the Namibian coast to constrain processes of plume-lithosphere interaction and the formation of continental flood basalts (Parana and Etendeka continental flood basalts) and associated intrusive rocks. Our study identified a narrow region (<100 km) of high-seismic-velocity anomalies in the middle and lower crust, which we interpret as a massive mafic intrusion into the northern Namibian continental crust. Seismic crustal reflection imaging shows a flat Moho as well as reflectors connecting the high-velocity body with shallow crustal structures that we speculate to mark potential feeder channels of the Etendeka continental flood basalt. We suggest that the observed massive but localized mafic intrusion into the lower crust results from similar-sized variations in the lithosphere (i.e., lithosphere thickness or preexisting structures)

Tomographic P wave velocity and vertical velocity gradient structure across the geothermal site Groß Schönebeck (NE German Basin): Relationship to lithology, salt tectonics, and thermal regime

[1]xa0Seismic wide-angle data were collected along a 40-km-long profile centered at the geothermal research well GrSk 3/90 in the Northeast German Basin. Tomographic inversion of travel time data provided a velocity and a vertical velocity gradient model, indicative of Cenozoic to Pre-Permian sediments. Wide-angle reflections are modeled and interpreted as top Zechstein and top Pre-Permian. Changes in velocity gradients are interpreted as the transition from mechanical to chemical compaction at 2–3 km depth, and localized salt structures are imaged, suggesting a previously unknown salt pillow in the southern part of the seismic profile. The Zechstein salt shows decreased velocities in the adjacent salt pillows compared to the salt lows, which is confirmed by sonic log data. This decrease in velocity could be explained by the mobilization of less dense salt, which moved and formed the salt pillows, whereas the denser salt remained in place at the salt lows. We interpret a narrow subvertical low-velocity zone under the salt pillow at GrSk 3/90 as a fault in the deep Permian to Pre-Permian. This WNW-ESE trending fault influenced the location of the salt tectonics and led to the formation of a fault-bounded graben in the Rotliegend sandstones with optimal mechanical conditions for geothermal production. Thermal modeling showed that salt pillows are related to chimney effects, a decrease in temperature, and increasing velocity. The assumed variations in salt lithology, density, and strain must thus be even higher to compensate for the temperature effect.

Lithological control on gas hydrate saturation as revealed by signal classification of NMR logging data

In this paper, nuclear magnetic resonance (NMR) downhole logging data are analyzed with a new strategy to study gas hydrate-bearing sediments in the Mackenzie Delta (NW Canada). In NMR logging, transverse relaxation time (T2) distribution curves are usually used to determine single-valued parameters such as apparent total porosity or hydrocarbon saturation. Our approach analyzes the entire T2 distribution curves as quasi-continuous signals to characterize the rock formation. We apply self-organizing maps, a neural network clustering technique, to subdivide the data set of NMR curves into classes with a similar and distinctive signal shape. The method includes (1) preparation of data vectors, (2) unsupervised learning, (3) cluster definition, and (4) classification and depth mapping of all NMR signals. Each signal class thus represents a specific pore size distribution which can be interpreted in terms of distinct lithologies and reservoir types. A key step in the interpretation strategy is to reconcile the NMR classes with other log data not considered in the clustering analysis, such as gamma ray, hydrate saturation, and other logs. Our results defined six main lithologies within the target zone. Gas hydrate layers were recognized by their low signal amplitudes for all relaxation times. Most importantly, two subtypes of hydrate-bearing shaly sands were identified. They show distinct NMR signals and differ in hydrate saturation and gamma ray values. An inverse linear relationship between hydrate saturation and clay content was concluded. Finally, we infer that the gas hydrate is not grain coating, but rather, pore filling with matrix support is the preferred growth habit model for the studied formation.

Joint Seismic and Magnetotelluric Exploration around the Geothermal Research Well Gross Schönebeck (NE German Basin)

Geophysical exploration for geothermal resources is often challenging because information on the parameters of interest such as porosity, permeability, fluid content, etc., cannot be observed directly. Conventional (seismic) structure images are usually not sufficient to locate potential geothermal targets. Magnetotelluric (MT) and seismic methods provide information about the resistivity and velocity distributions of the subsurface in similar scales and resolution. The lack of a fundamental law linking the two parameters, however, limits a joint interpretation to a more qualitative analysis. Using a statistical approach in which resisitivity and velocity models are investigated in the joint parameter space, we can identify regions of high correlation between the two model parameters. Back-mapping of these regions onto the spatial domain allows us to identify common classes which can then be compared with lithological information. Application of this technique to a seismic - MT profile in the area of the Gros Schonebeck geothermal site, allows us to identify a number of classes in accordance with local geology. In particular, a high velocity - low resistivity class is interpreted as related to salt lows, where highly fractured anhydrite might produce enhanced permeability.