Modeling long-period noise in kinematic GPS applications

We develop and test an algorithm for modeling and removing elevation error in kinematic GPS trajectories in the context of a kinematic GPS survey of the salar de Uyuni, Bolivia. Noise in the kinematic trajectory ranges over 15 cm and is highly autocorrelated, resulting in significant contamination of the topographic signal. We solve for a noise model using crossover differences at trajectory intersections as constraints in a least-squares inversion. Validation of the model using multiple realizations of synthetic/simulated noise shows an average decrease in root-mean-square-error (RMSE) by a factor of four. Applying the model to data from the salar de Uyuni survey, we find that crossover differences drop by a factor of eight (from an RMSE of 5.6 to 0.7 cm), and previously obscured topographic features are revealed in a plan view of the corrected trajectory. We believe that this algorithm can be successfully adapted to other survey methods that employ kinematic GPS for positioning.     

Spatial Periodic Boundary Condition for MODFLOW

Small‐scale hyporheic zone (HZ) models often use a spatial periodic boundary (SPB) pair to simulate an infinite repetition of bedforms. SPB's are common features of commercially available multiphysics modeling packages. MODFLOW's lack of this boundary type has precluded it from being effectively utilized in this area of HZ research. We present a method to implement the SPB in MODFLOW by development of the appropriate block‐centered finite‐difference expressions. The implementation is analogous to MODFLOW's general head boundary package. The difference is that the terms on the right hand side of the solution equations must be updated with each iteration. Consequently, models that implement the SPB converge best with solvers that perform both inner and outer iterations. The correct functioning of the SPB condition in MODFLOW is verified by two examples. This boundary condition allows users to build HZ‐bedform models in MODFLOW, facilitating further research using related codes such as MT3DMS and PHT3D.     

Sea‐level rise impact on fresh groundwater lenses in two‐layer small islands

The fresh groundwater lenses (FGLs) of small islands can be highly vulnerable to climate change impacts, including sea‐level rise (SLR). Many real cases of atoll or sandy islands involve two‐layer hydrogeological conceptualizations. In this paper, the influential factors that affect FGLs in two‐layer small islands subject to SLR are investigated. An analytical solution describing FGLs in circular islands, composed of two geological layers, is developed for the simplified case of steady‐state and sharp‐interface conditions. An application of the developed model is demonstrated to estimate the FGL thickness of some real‐world islands by comparison with existing FGL thickness data. Furthermore, numerical modelling is applied to extend the analysis to consider dispersion effects and to confirm comparable results for both cases. Sensitivity analyses are used to assess the importance of land‐surface inundation caused by SLR, relative to other parameters (i.e. thickness of aquifer layers, hydraulic conductivity, recharge rate and land‐surface slope) that influence the FGL. Dimensionless parameters are used to generalize the findings. The results demonstrate that land‐surface inundation has a considerable impact on a FGL influenced by SLR, as expected, although the FGL volume is more sensitive to recharge, aquifer thickness and hydraulic conductivity than SLR impacts, considering typical parameter ranges. The methodology presented in this study provides water resource managers with a rapid‐assessment tool for evaluating the likely impacts of SLR and accompanying LSI on FGLs.     

Getting ‘holistic’ about land rehabilitation in hot arid regions

The paper describes Western Australian examples and causes of land degradation. It outlines shortcomings in the methodologies used to rehabilitate these areas. From this a protocol is suggested for an ‘holistic’ approach to land rehabilitation.     

Directional permeability of porous media to homogeneous fluids

Summary Investigations have shown that in groundwater- and oil-bearing strata there are preferential directions of flow that are often maintained over wide areas.Johnson & Hughes (1948, see Ref.) analysed a series of oil well cores by cutting them into small horizontal plugs and they obtained directional permeabilities which they plotted in the form of polar graphs. They were not able to give a physical explanation of this phenomenon. On the other hand, there exists a theory of permeability in which the latter is represented as a symmetric tensor. This theory has been developed byFerrandon (1948, see Ref.), but no experimental substantiation of it seems ever to have been attempted.In the present paper, the author undertakes to compare the two sets of findings. FromFerrandon's theory, the directional permeabilities (denoted byk) corresponding to the experiments ofJohnson & Hughes are calculated and it is shown thatk ^–1/2 if plotted as polar graph, should form an ellipse. The data ofJohnson & Hughes are then are drawn. In this manner, a substantiation of the tensor theory ofFerrandon is obtained.

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Zusammenfassung Untersuchungen der Permeabilität von Grundwasser und Erdöl führenden Gesteinsschichten haben gezeigt, dass dieselbe in vielen Fällen richtungsabhängig ist. Hierbei bleibt die Richtung extremaler Permeabilität oft über weite Gebiete konstant.Johnson & Hughes untersuchten eine Reihe von Bohrkernen von Oelquellen auf Richtungsabhängigkeit der Permeabilität. Hierzu schnitten sie aus denselben kleine, waagrechte Stücke, bestimmten deren Permeabilität und stellten das Ergebnis ihrer Messungen in der Form von Permeabilitätspolardiagrammen dar. Sie waren nicht im Stande, eine theoretische Erklärung der erhaltenen Kurven zu geben. Auf der anderen Seite existiert eine Theorie der Permeabilität, wobei die letztere als symmetrischer Tensor behandelt wird. Diese Theorie wurde vonFerrandon vorgeschlagen; es scheint aber, dass keine experimentelle Bestätigung davon je versucht worden ist.In der vorliegenden Arbeit vergleicht der Verfasser die zwei Typen von Untersuchungen. Nach derFerrandon'schen Theorie wird die «gerichtete» Permeabilität (mitk bezeichnet), die den Experimenten vonJohnson & Hughes entspricht, berechnet. Es wird gezeigt, dassk ^–1/2, als Polardiagramm dargestellt, die Gestalt einer Ellipse haben sollte. Die Resultate vonJohnson & Hughes werden dann in die Form vonk ^–1/2 umgerechnet und als entsprechende Polardiagramme dargestellt. In dieser Weise wird eine experimentelle Bestätigung der Tensortheorie vonFerrandon erhalten.

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Published by Permission of Imperial Oil Limited, Calgary, Alberta (Canada).     

A comment on the continuity equation model of slope profile development and its boundary conditions

The constant base boundary condition used by Kirkby to derive characteristic form solutions to the continuity equation are unlikely in practice. Constant form solutions relying on constant basal erosion are however more probable.     

Rock Magnetic Investigation of Historical Lavas Used in Palaeointensity Studies

Five samples from historical lava flows on Mt. Etna, which had previously been used in a palaeointensity study, were examined using a combination of rock magnetic and microscopic techniques to elucidate the causes of failure of palaeointensity determination. The samples were characterised using a combination of low-temperature susceptibility (LT-), Bitter pattern imaging and scanning electron microscope (SEM). High-temperature susceptibility curves and hysteresis loops had been previously measured by Calvo et al. (2001). Of the five samples only one gave an accurate palaeointensity. This sample was deuterically oxidised and consisted of large exsolved ore grains. It was only possible to distinguish this sample either optically or magnetically from a similar sample by the LT- warming curves; the unsuccessful sample displayed alteration in the LT- warming curves, which was not readily observed in the high temperature susceptibility curves. It is proposed that the measurement of LT- curves before and after heating could be a more sensitive method of determining suitability for palaeointensity determination than previous rock magnetic pre-selection techniques.     

The significance of surface joints

This brief review demonstrates the significance, on a global scale, of joints as shearing surfaces in the neotectonic stress field. Thus, the principal directions of the neotectonic stress field can be determined by a statistical treatment of joint orientation data. These principal stress directions agree with those postulated from plate tectonic theory in every case, on five continents.