Permeability and stress in crystalline rocks

Groundwater from crystalline rocks is a significant resource in many areas of the world. It is also an important medium for contaminant transport from, for example, deep nuclear waste repositories. Stress distributions in fractured rocks are important in controlling groundwater flow in several ways: (i) palaeostress fields are responsible for the evolution of fracture systems which transmit groundwater; (ii) current in situ stress fields will influence the shape and aperture of fractures; (iii) humans can influence the natural stress field in a rock mass to enhance fracture flows. The significance of stresses for groundwater flow can be investigated by field techniques (hydraulic fracturing), laboratory techniques (stress cells) or by numerical modelling.     

The World Wide Web and university education in remote sensing

The World Wide Web (YAW) has a potential for improving university education in remote sensing, as the WWW technology may be applied in a way which reduces the amount of resources which are needed to achieve a certain educational objective. The paper presents a survey of this new technology. The main components of the WWW technology are outlined. Reference is made to information resources which are available through the WWW which relates to remote sensing. The technology can be applied not only by individual staff members, but, also, according to decision at the departmental or faculty level. The use of information technology for education is a research field in its own right. This research field is presented as a basis for suggestion of decision issues and decision criteria regarding the use of the WWW for education in remote sensing. The paper may have an European bias.     

Microzonation of the city of Basel

During the past centuries, the city of Basel has suffered damage caused by earthquakes. One extraordinary event described in historical documents is the strong earthquake which occurred in 1356. The 1356 event, one of the strongest earthquakes in northwest-Europe, was obviously much stronger than the low-magnitude earthquakes observed in the area during this century. Even though the present seismicity in the Basel area is low, strong earthquakes have to be expected due to the city's geographical location close to the northern boundary of the African-European convergence zone, at the southern end of the Rhinegraben. A crucial step towards preparedness for future events and mitigation of earthquake risk involves a microzonation study of the city. The study is carried out in three steps: (1) a detailed mapping of the geology and geotechnical properties of the area, (2) measurement, interpretation and modelling of ambient noise data, and (3) numerical modelling of expected ground motions during earthquakes. A qualitative microzonation of the centre of Basel is presented, and it is discussed by comparing it to the historically reported damage of the 1356 earthquake.     

Dual aspects of mantle convection through time: Changes in global upper mantle temperature, Earth’s rotation, and global plate motion acceleration induced by mantle avalanches

The mean depth of the mantle transition zone depends on the global mantle temperature through the Clapeyron slope. Numerical models of mantle convection with endothermic phase change at 670 km are characterized by quiet periods of partial layering alternating with catastrophic events. During an avalanche, global thermal anomalies, reaching several tens of degrees, and lasting several tens of million years occur that induce a global deepening of the mantle transition zone. The resulting inertia tensor perturbations lead to acceleration of the Earth’s rotation. The bottom heat fluxes remains strong as a consequence of the steep thermal gradient at the CMB level; whereas it decreases to a low level during quiet periods. The upper surface heat flux displays a more complex behavior due to the re-heating of upper mantle by plumes generated at the transition zone and at the CMB. Following these numerical results, two significantly different regimes of mantle convection may have alternated several times during the Earth’s history. The avalanche periods are also associated with a global increase of plate tectonic activity (ridge accretion and continental break-up) due to an increase of the surface velocity during the avalanche process. This work shows the richness of the behavior in surfaces and bottom heat fluxes that arises from the alternation of quiet periods of partial layering with catastrophic avalanche events. Complexities in the real Earth displays are surprisingly simular to those caused by the consequences of mantle avalanches.     

Effective velocities in fractured media: a numerical study using the rotated staggered finite‐difference grid

The modelling of elastic waves in fractured media with an explicit finite‐difference scheme causes instability problems on a staggered grid when the medium possesses high‐contrast discontinuities (strong heterogeneities). For the present study we apply the rotated staggered grid. Using this modified grid it is possible to simulate the propagation of elastic waves in a 2D or 3D medium containing cracks, pores or free surfaces without hard‐coded boundary conditions. Therefore it allows an efficient and precise numerical study of effective velocities in fractured structures. We model the propagation of plane waves through a set of different, randomly cracked media. In these numerical experiments we vary the wavelength of the plane waves, the crack porosity and the crack density. The synthetic results are compared with several static theories that predict the effective P‐ and S‐wave velocities in fractured materials in the long wavelength limit. For randomly distributed and randomly orientated, rectilinear, non‐intersecting, thin, dry cracks, the numerical simulations of velocities of P‐, SV‐ and SH‐waves are in excellent agreement with the results of the modified (or differential) self‐consistent theory. On the other hand for intersecting cracks, the critical crack‐density (porosity) concept must be taken into account. To describe the wave velocities in media with intersecting cracks, we propose introducing the critical crack‐density concept into the modified self‐consistent theory. Numerical simulations show that this new formulation predicts effective elastic properties accurately for such a case.     

Numerical simulation of flow around a circular cylinder close to a flat seabed at high Reynolds numbers using a k–ε model

High Reynolds number flows around a circular cylinder close to a flat seabed have been computed using a two-dimensional standard high Reynolds number k–ε turbulence model. The effects of gap to diameter ratio, Reynolds number and flat seabed roughness for a given boundary layer thickness of the inlet flow upstream of the cylinder have been investigated. Hydrodynamic quantities and the resulting bedload transport have been predicted, and the vortex shedding mechanisms have been investigated. Predictions of hydrodynamic quantities around a cylinder located far away from the bed (so that the effect of the bed is negligible) are in satisfactory agreement with published experimental data and numerical results obtained for the flow around an isolated cylinder. Results for lower Reynolds number flows have also been computed for comparison with the high Reynolds number flow results. Overall it appears that the present approach is suitable for design purposes at high Reynolds numbers which are present near the seabed in the real ocean.     

Wave run-up on slender circular cylindrical foundations for offshore wind turbines in nonlinear random waves

A practical method for estimating the wave run-up height on a slender circular cylindrical foundation for wind turbines in nonlinear random waves is provided. The approach is based on the velocity stagnation head theory and Stokes second order wave theory by assuming the basic harmonic wave motion to be a stationary Gaussian narrow-band random process. Comparisons are made with measurements by De Vos et al. (2007), and some of the highest wave run-up events that were predicted agree with those measured.     

Surface deformation analysis of dense GPS networks based on intrinsic geometry: deterministic and stochastic aspects

In this contribution, the regularized Earth’s surface is considered as a graded 2D surface, namely a curved surface, embedded in a Euclidean space . Thus, the deformation of the surface could be completely specified by the change of the metric and curvature tensors, namely strain tensor and tensor of change of curvature (TCC). The curvature tensor, however, is responsible for the detection of vertical displacements on the surface. Dealing with eigenspace components, e.g., principal components and principal directions of 2D symmetric random tensors of second order is of central importance in this study. Namely, we introduce an eigenspace analysis or a principal component analysis of strain tensor and TCC. However, due to the intricate relations between elements of tensors on one side and eigenspace components on other side, we will convert these relations to simple equations, by simultaneous diagonalization. This will provide simple synthesis equations of eigenspace components (e.g., applicable in stochastic aspects). The last part of this research is devoted to stochastic aspects of deformation analysis. In the presence of errors in measuring a random displacement field (under the normal distribution assumption of displacement field), the stochastic behaviors of eigenspace components of strain tensor and TCC are discussed. It is applied by a numerical example with the crustal deformation field, through the Pacific Northwest Geodetic Array permanent solutions in period January 1999 to January 2004, in Cascadia Subduction Zone. Due to the earthquake which occurred on 28 February 2001 in Puget Sound (M _w > 6.8), we performed computations in two steps: the coseismic effect and the postseismic effect of this event. A comparison of patterns of eigenspace components of deformation tensors (corresponding the seismic events) reflects that: among the estimated eigenspace components, near the earthquake region, the eigenvalues have significant variations, but eigendirections have insignificant variations.