Georg Kaufmann

Reservoir‐induced deformation and continental rheology in vicinity of Lake Mead, Nevada

Lake Mead is a large reservoir in Nevada, formed by the construction of the 221-m-high Hoover Dam in the Black Canyon of the Colorado River. The lake encompasses an area of 635 km2, and the total volume of the reservoir is 35.5 km3. Filling started in February 1935. On the basis of a first-order leveling in 1935, several levelings were carried out to measure the deformation induced by the load of the reservoir. Subsidence in the central parts of the lake relative to the first leveling was around 120 mm (1941), 218 mm (1950), and 200 mm (1963). The subsidence pattern clearly shows relaxation of the underlying basement due to the water load of the lake, which ceased after 1950. Modeling of the relaxation process by means of layered, viscoelastic, compressible flat Earth models with a detailed representation of the spatial and temporal distribution of the water load shows that the thickness of the elastic crust underneath Lake Mead is 30±3 km. The data are also consistent with a 10-km-thick elastic upper crust and a 20-km-thick viscoelastic lower crust, with 1020 Pa s as a lower bound for its viscosity. The subcrust has an average viscosity of 1018±0.2 Pa s, a surprisingly low value. The leveling data constrain the viscosity profile down to ∼200 km depth.

Karst aquifer evolution in fractured rocks

We study the large-scale evolution and flow in a fractured karst aquifer by means of a newly developed numerical method. A karst aquifer is discretized into a set of irregularly spaced nodal points, which are connected to their set of natural neighbors to simulate a network of interconnected conduits in two dimensions. The conduits are allowed to enlarge by solutional widening. The geometric flexibility of this method, along with a simplified model for the dissolution kinetics within the system water-carbon dioxide-calcite, enables us to study both laminar and turbulent flow in a karst aquifer during its early phase of evolution. A sensitivity analysis is conducted for parameters such as conduit diameter, hydraulic pressure differences, and recharge conditions along the surface of the aquifer and shows that passage evolution depends strongly on the recharge condition and the amount of water available. Under fixed hydraulic head boundary conditions an early single-passage system develops under laminar conditions and is transformed into a maze-like passage system after the onset of turbulence. Fixed recharge boundary conditions are more likely to result in a branchwork-like passage system, although the addition of distributed recharge may lead to a maze-like system of secondary passages.

Stalagmite growth and palaeo-climate: the numerical perspective

Abstract The growth of stalagmites can be approximated by a simple mathematical model, which depends on growth rate and equilibrium radius. These two parameters are controlled by the climate. Temperature variations derived from ice and deep-sea core data, together with models for changes in precipitation and soil cover, are used to derive stalagmite stratigraphies, which reflect the palaeo-climate variations imposed. In general, stalagmite growth is strongly correlated to temperature and the amount of carbon dioxide available in the soil. Furthermore, precipitation is correlated to the stalagmite diameter. However, several assumptions need to be made: (i) A functional relationship between temperature on the one hand and precipitation and soil cover on the other needs to be established. (ii) The kinetics of calcite dissolution and precipitation needs to be assigned, either under soil and epikarst conditions open to the atmosphere or under fractured rock conditions closed from the atmosphere. These assumptions are difficult to access from field data, and therefore a stalagmite stratigraphy can be ambiguous and not easily converted back into an unknown palaeo-climate signal.

Glacial isostatic adjustment in Fennoscandia with a three-dimensional viscosity structure as an inverse problem

Abstract Glacial isostatic adjustment data are commonly used to invert for the radial viscosity structure of the mantle. However, the effects of lateral variations in mantle viscosity in such inversions are not yet accounted for. Here we analysed synthetic sea-level data for the Fennoscandian region, which are derived from a three-dimensional (3D) earth model with realistic lateral and vertical viscosity variations deduced from seismological and geological information. The inversion of the 3D synthetic data for a best-fitting 1D viscosity profile reveals that (i) lateral lithospheric thickness variations can be detected with 1D model predictions, if the data are grouped into regional subsets, (ii) combined lateral variations in lithospheric thickness and asthenospheric viscosity are not properly resolved with 1D model predictions, and (iii) the spatial and temporal distribution of observational data strongly affects the resulting 1D viscosity profile.

Lateral asthenospheric viscosity variations and postglacial rebound: A case study for the Barents Sea

The effect of lateral asthenospheric viscosity variations on observable signatures related to postglacial rebound in the Barents Sea is studied. Using a finite-element approach to discretize the problem, a comparison between a laterally homogeneous reference earth model and a laterally heterogeneous earth model is performed. The results indicate that a change in asthenospheric viscosity of about three orders of magnitude influences predictions of land uplift up to 10–20 m, present-day velocities up to 0.5–1.5 mm/a, and present-day gravity anomalies up to 0.4–0.8 mGal in the northwestern part of the Barents Sea region.

A model comparison of karst aquifer evolution for different matrix-flow formulations

The evolution of permeability and flow in a karst aquifer is studied by numerical simulations. The aquifer considered consists of a large central fracture, a network of finer fissures, and a porous rock matrix. Enlargement of both the central fracture and the fissures by chemical dissolution is possible, hence the conductivities in the fracture and the fissure system can increase with time. No dissolution is allowed in the porous rock matrix, which has a constant conductivity. Flow is driven by a simple fixed head boundary condition representative for the initial phase of karstification. A systematic parameter study is carried out by varying the initial width of the fissure network and the conductivity of the rock matrix, while keeping the initial width of the central fracture fixed. Key parameters such as flowrates, breakthrough times, and conductivities for the different models are compared. If either the conductivity of the rock matrix is high enough or the initial width of the fissures is large enough to carry flow, breakthrough times of the aquifer are significantly reduced, when compared to a model with low matrix conductivity and small fissures. However, due to the dissolutional widening of fissures the evolution of the aquifer is distinctively different for models with rock matrix simulated by a porous medium or a fissure network.

Is the long‐wavelength geoid sensitive to the presence of postperovskite above the core‐mantle boundary?

[1] The analysis of seismic data represents today the primary tool in the search for the presence of postperovskite in the lowermost mantle (D 00 ). This work aims at testing whether the inversion of gravitational data can also contribute to the detection of postperovskite in D 00 .W e assume that the transition from perovskite to postperovskite is accompanied by a reduction in viscosity and test the effects of such viscosity change on the prediction of the dynamic geoid with a numerical model of subducted lithosphere. Our results show that the long-wavelength component of the geoid is very sensitive to the presence of postperovskite areas in D 00 , especially if their viscosity is significantly lower than the viscosity of the surrounding perovskite and if these areas are located close to density � � � � � � � � � � � � � � � � � � � � � ��

Cave detection using multiple geophysical methods: Unicorn cave, Harz Mountains, Germany

Unicorn cave in the southern Harz Mountains of Germany is a show cave in dolomitic rocks of the Zechstein Formation. The cave’s trunk passage is interrupted by larger rooms. The overburden is only around 15 m. The passages are filled with sediments that can be up to 50 m thick. We used gravimetry and electrical resistivity imaging over the cave area to identify the subsurface voids and the extent of the sediment infill of the cave passages. Our choice of methods was based on several conditions unique to the cave: (1) well surveyed, (2) shallow overburden, (3) large air-filled passages, and (4) thick sediment cover, concealing true passage size. Using the cave survey as an initial model for the subsurface structure, we successfully identified the air-filled cave with both methods. We then inferred the thickness of the sediment infill by forward modeling and identified a possible southward continuation beyond the currently explored passages.

Modelling unsaturated flow in an evolving karst aquifer

A two-dimensional cross-section of a karst aquifer, in which chemical dissolution enlarges fractures with time, is studied. The karst aquifer is recharged by precipitation and drains towards a resurgence. The initial aquifer has low conductivities both in the rock matrix and the fracture network, and the initial water table is high. As the enlargement of fractures by dissolution increases the fracture conductivity, the water table drops, until it reaches a steady-state along the level of the resurgence. Several parameterisations are discussed for flow in the unsaturated zone above the water table. It is shown that different approaches result in similar cave passage patterns, with a large water-table cave draining the recharge towards the resurgence. However, flow patterns in the unsaturated zone can be very different for the different parameterisations.

Deep conduit flow in karst aquifers revisited

Caves formed in soluble rocks such as limestone, anhydrite, or gypsum are efficient drainage paths for water moving through the aquifer from the surface of the host rock toward a resurgence. The formation of caves is controlled by the physical solution through dissociation of the host rock by water or by the chemical solution through reactions of the host rock with water enriched with carbon dioxide. Caves as large underground voids are simply the end-member of secondary porosity and conductivity characterizing the aquifer. Caves and their relation to a present or past base level are found both close to a past or present water table (water table caves) and extending far below a past or present water table (bathy-phreatic caves). One explanation for this different speleogenetic evolution is the structural control: fractures and bedding partings are preferentially enlarged around more prominent faults, thus the fracture density in the host rock controls the speleogenetic evolution. This widely accepted explanation can be extended by adding other controls, e.g., a hydraulic control: as temperature generally increases with depth, density and viscosity of water change, and particularly the reduction of viscosity due to the increase in temperature enhances flow. This hypothesis was proposed by Worthington (2001, 2004) as a major controlling factor for the evolution of deep bathy-phreatic caves. We compare the efficiency of structural and hydraulic control on the evolution of a cave passage by numerical means, adding a third control, the chemical control to address the change in solubility of the circulating water with depth. Our results show that the increase in flow through deep bathy-phreatic passages due to the decrease in viscosity is by far outweighted by effects such as the decrease in fracture width with depth due to lithostatic stress and the decrease in solubility with depth. Hence, the existence of deep bathy-phreatic cave passages is more likely to be controlled by the structural effect of prominent faults.