Archaeological and archaeomagnetic dating at a site from the ager Tarraconensis (Tarragona,Spain): El Vila-sec Roman pottery

A very accurate archaeological dating of a Roman site in NE Spain (El Vila-sec) was made based on the typology of pottery artifacts. Three different phases were identified with activity ranging from the mid-1st century BC to the early-3rd century AD. Analyses of bricks from kilns at El Vila-sec produced data on their stored archaeomagnetic vector. These data were compared with the secular variation curve for the Iberian Peninsula and the SCHA.DIF.3K regional archaeomagnetic model. Both, the reference curve and the model, produced probability distributions for the final period of use for two kilns from the second archaeological phase that were not used during the third phase. At a 95% confidence level, both time distributions cover a wide chronological range including the presumed archaeological age. Both the Iberian secular variation curve and the SCHA.DIF.3K regional model proved to be suitable models for dating the site, although on their own they do not produce a single unambiguous solution. This archaeomagnetic approach could also be applied to neighbouring archaeological sites that have an imprecise archaeological age.     

Multi-temporal archaeological analyses of alluvial landscapes using the photogrammetric restitution of historical flights: a case study of Medellin (Badajoz,Spain)

One of the main challenges affecting the archaeological study of alluvial landscapes is the intensity of change these environments experience over time. Quick and dramatic alterations in geomorphological dynamics and land exploitation determine the visibility and conservation of the archaeological record. This study proposes an approach to the problem of studying these evolving environments based on the analysis and treatment of a series aerial photographs taken between the 1950s and the present day. This paper is particularly interested in looking at the process of photogrammetric restitution and in validating and comparing the digital terrain models and orthoimages produced. The quantitative analysis and visual interpretation of these results can provide valuable information about the transformation of landscapes and factors affecting surface evidence. The intended final result is to develop the ability to map the most problematic or best preserved areas. Nevertheless, it is considered in terms of a relative measure of change magnitude, rather than trying to provide absolute figures.     

Modelling the Neolithic transition in a heterogeneous environment

The Neolithic transition, involving the change of subsistence from foraging to agriculture, can fruitfully be modelled mathematically, as, e.g., in the three-population model of Aoki et al. (1996). Here that model is modified to include some features of population dynamics in a realistic, two-dimensional environment, and including population pressure, competition for resources between farmers and foragers, and the dependence of the population carrying capacities and diffusivities on the environmental conditions. This modified model allows for the survival of foragers in regions where environmental conditions do not favour farming. The model is tentatively applied to the Indian subcontinent, which is a complicated example of this transition involving multiple domestication centres. The results are briefly compared with published archaeological data.     

3D modelling of magnetic data from an archaeological site in north-western Tlaxcala state,Mexico

In Archaeology, geophysical methods had been applied usually in a qualitative form, limited only to the use of filters that enhance the data display. The main objective in this work is the implementation of a modelling technique that allows us to reconstruct the geometry of buried bodies and the determination of their depths. This is done by means of the estimation of the magnetic moments of archaeological objects using a three-dimensional mesh of individual magnetic dipoles using the least squares method and the singular value decomposition of a weighted matrix to solve the linear problem. The distribution and shape of the underlying archaeological remains can be inferred. This methodology was applied to an archaeological site called Los Teteles de Ocotitla, in the state of Tlaxcala, Mexico. A high-resolution magnetic prospection was carried out in three selected areas (terraces). The most important total field anomalies found on each area were inverted, obtaining results that were corroborated by archaeological excavations. This investigation demonstrates the potential of quantitative geophysical methods for the characterization of archaeological structures, in extension and in depth.     

Impact of transition zones, variable fluid viscosity and anthropogenic activities on coupled fluid-transport processes in a shallow salt-dome environment

In the Schleswig–Holstein region (S–H) of Germany, most observed near-surface saline ground waters originate from dissolution of shallow salt domes. Previous numerical simulations of thermohaline flow clarified the major mechanisms controlling large-scale density-driven flow. It has been found that, in addition to topographically driven flow, gravitational and thermohaline convection are the primary mechanisms for extensive solute exchange between shallow and deep aquifers. Geological features such as glacial channels control recharge/discharge processes at the surface. Here we address several previously unresolved issues: (i) the impact of a permeable unit (transition zone) between the salt and adjacent units; (ii) the role of variable brine viscosity in affecting regional- (i.e. km-) scale heat and mass patterns; and (iii) the influence of anthropogenic activities such as pumping stations on density-driven flow. We found that geophysical factors play a major role in determining the dynamics of fluid processes. The transition zone significantly influences the flow field and the distribution of heat, slowing the formation of highly concentrated salty plumes. The impact of variable fluid viscosity on the coupled heat and brine flow is twofold. In a colder and highly concentrated environment, such as a shallow salt-dome crest, it retards brine flow. In a less saline environment, variable fluid viscosity enhances thermally induced upward fluid flow. Groundwater extraction from production wells only affects brine and heat flow locally within the upper aquifers.     

Hydrogeochemical modelling of CO2 equilibria and mass transfer induced by organic–inorganic interactions in siliciclastic petroleum reservoirs

Different feldspar types control complex hydrogeochemical processes in hydrocarbon‐bearing siliciclastic reservoirs, which have undergone different degrees of degradation. To test such processes generically, carbon dioxide equilibria and mass transfers induced by organic–inorganic interactions have been modelled for different hydrogeochemical scenarios. The approach is based on and compared with data from the Norwegian continental shelf ( Smith & Ehrenberg 1989 ) and assumes local thermodynamic equilibrium among solids and fluids. Equilibrating mineral assemblages (different feldspar types, quartz, kaolinite, calcite) are based on the primary reservoir composition. Equilibration and coupled mass transfer were triggered by the addition and reaction of different amounts of CO₂, CH₄ and H₂ (plus acetic acid) at temperatures between 50 and 95°C (323 and 368 K). These components occur in oil fields as products of anaerobic bacterial degradation, hydrolytic disproportionation of hydrocarbons and/or thermal maturation of kerogen. We apply two different computer codes and two different thermodynamic data bases to calculate the results. Reaction of 0.32–0.6 mol CO₂, 0.16–0.3 mol CH₄ and 0.8–1.5 mol H₂ with K‐feldspar, quartz, kaolinite and calcite in 1 l of pore water results in modelled values of 0.3–2.3 mol% CO₂ in a multicomponent gas phase that resembles measured data (0.2–1.5 mol%). Similar CO₂ contents result from acetic acid addition (CO₂, CH₄, H₂ + 0.016 mol CH₃COOH). Equilibration with albite or anorthite reduces the release of CO₂ into the multicomponent gas phase dramatically, by 1 or 4 orders of magnitude compared with the equilibration with K‐feldspar. Minor differences in the modelled CO₂ content (0.1–0.2 mol%) result from calculations with different computer codes if the same thermodynamic data base is applied. Relevant differences (up to 1.9 mol% CO₂) result from calculations using different thermodynamic data bases.     

Mechanisms of strain-rate- and reaction-dependent permeability in the mid-crust of the Southern Alps: insight from three-dimensional mechanical models

We used three-dimensional mechanical modelling to explore the interplay of rheology, modes of permeability creation and fluid flow in the mid-crust of an oblique orogen. We used the central Southern Alps and the Otago Schist of New Zealand to constrain our models. We also compared our models with the magnetotelluric survey along the Rangitata–Whataroa rivers that imaged a U-shaped zone of high conductivity, interpreted as interconnected fluids, beneath the central Southern Alps. Modelling was carried out using the numerical code FLAC^3D. We used a number of simple assumptions: an initially homogeneous starting material, deformation boundary conditions based on the tectonics of the South Island, the capability of the modelled material to develop an anisotropic permeability structure, strain rate and reaction-induced permeability increases, initial saturation and lithostatic pore pressures as a basis for our models. The initial isotropic permeability was 10⁻¹⁸ m². We modelled two possible mechanisms of permeability increase: (i) strain-rate dependent and (ii) reaction dependent. For a strong mid-crust, the models showed enhanced permeability and hence fluid interconnectivity in a symmetric region beneath the model main divide, both ends were turned up towards the brittle–ductile transition and fluid flow was the greatest in the across strike direction. For a weak mid-crust, the region of enhanced permeability was asymmetric and turned up towards the brittle–ductile regime close to the Alpine Fault. The strong mid-crust model reproduces the features that are common to all interpretations of the MT soundings and is our preferred model for the central Southern Alps.     

Fluid injection and surface deformation at the KTB location: modelling of expected tilt effects

This investigation is indented to explore the relationship between changes in pore fluid pressure and deformation of the land surface induced by a large‐scale injection experiment at the KTB site. Deformation will be monitored by ASKANIA borehole tiltmeters at five locations. During the year 2003, a network of borehole tiltmeters was installed, data transmission links established and tested, and recording of tilt data started. Our first main interest was to receive data sets of all stations well before the injection experiment to start in May 2004, to be able to evaluate local site effects. Thus, the separation of injection‐induced effects will be more reliable. Principal 3D numerical modelling (poro‐elastic modelling and investigations, using the finite element method, FEM) of poro‐elastic behaviour showed that significant tilt amplitudes can be expected during controlled fluid injection. Observed deformation will be investigated within the framework of the fluid flow behaviour and resulting deformation. Two models have been used: a coupled hydro geomechanical finite element model (abaqus ) and, as a first step, also a multi‐layered poro‐elastic crust (poel ). With the numerical model two effects can be quantified: (i) the deformation of the upper crust (tilt measurements) and (ii) the spatial distribution and the changes of material properties in the KTB area. The main aim of the project is to improve the knowledge of coupled geomechanic–hydraulic processes and to quantify important parameters. Thus, the understanding of fracture‐dominated changes of the hydrogeological parameters will be enhanced, geomechanical parameter changes and the heterogeneity of the parameter field quantified. In addition, the induced stress field variation can be explained, which is believed to be mainly responsible for the increase of local seismic activity. Here, we introduce the tiltmeter array at the KTB site, the modelling for a poro‐elastic crust and the preliminary FEM modelling.     

Computing geochemical mass transfer and water/rock ratios in submarine hydrothermal systems: implications for estimating the vigour of convection

A method of calculating chemical water/rock ratios is presented that enables the estimation of fluid velocities in open, flow‐through hydrologic systems. The approach is based on relating the gain/loss of a chemical species per kilogram of solid phase to the loss/gain of that species in the fluid phase, integrated across a specified length of the flowpath. After examining the underlying approximations of the approach using a one‐dimensional model of seawater moving through a basalt under nonisothermal conditions, the method is applied to representative zones within a two‐dimensional hydrothermal convective system. The method requires that regions within the flow system can be identified in which the direction of flow is steady for an extended period of time. Estimates of fluid velocity are spatial and temporal averages for the length of the flowpath used in the calculation. The location within the flow system and the nature of the alteration reactions determine which species can provide reliable values of the chemical water/rock ratio and useful estimates of fluid velocities. Over the length of the flowpath considered, the calculation of water/rock ratios works best when a species is controlled by a single reaction. Accurate estimates are favoured if the concentration profile of a species along the flowpath increases or decreases monotonically. If the length of the flowpath extends over more than one reaction zone, then erroneous estimates of the water/rock ratio and fluid velocity are more likely. Model calculations suggest that the quartz/silica system should provide reliable estimates for fluid velocity under a wide range of temperature and flow conditions, in particular in those regions of a system at or near quartz equilibrium, so that the aqueous silica concentration is buffered by quartz and correlated with the temperature distribution.     

Spatial Variability of Controls on Downstream Patterns of Sediment Storage: a Case Study in the Lane Cove Catchment,New South Wales,Australia

This study uses GIS techniques to examine the spatial distribution of stream power along major streamlines in the Lane Cove catchment in northern Sydney, Australia. Channel gradient estimates derived from a 5 m resolution digital elevation model (DEM) are combined with streamflow data to estimate stream power along river courses. Stream power and its constituent components are then related to a detailed field‐based assessment of sediment storage along the trunk stream and primary tributaries. At the catchment scale, sediment storage per unit length decreases as channel gradient and gross stream power increase. However, local controls such as variability in valley width and occurrence of confluence zones exert a greater influence upon sediment storage, disrupting systematic catchment‐wide relationships. The total volume of storage along each streamline has a strong linear relationship to the area of the subcatchment, but the distribution of sediment along streamlines varies between subcatchments. The GIS framework employed in this project allows generation of continuous, empirical data, thereby providing catchment‐specific predictive capacity that can accompany theoretical approaches to stream power modelling.