Inversion Reconstruction of Gravity Potential Based on Gravity Gradients

A new method was developed for the inversion reconstruction of gravity potential. This method offers the possibility to determine the potential function and all of its important derivatives using the common inversion of gravity gradients and the first derivatives of potential. Gravity gradients can be originated from Torsion balance measurements, while the first derivatives of potential can be derived from the deflections of the vertical data. Different fields having great importance can be originated from this reconstructed potential function at any points of the investigated area. The advantage of this method is that the solution can be performed by a significantly over-determined inverse problem. Test computations were performed for the inversion reconstruction of gravity potential. There were 248 torsion balance measurements and 13 points where the deflections of the vertical are known in our test area. This inversion algorithm is rather stable. Gravity potential, the first and the second derivatives of the potential were determined for the test area by this suggested method. This method gives a good possibility for a useful geodetic application; deflections of the vertical based on torsion balance measurements can be determined for the whole area for each torsion balance stations.     

Petrogenesis of Permian alkaline lamprophyres and diabases from the Spanish Central System and their geodynamic context within western Europe

Basic to ultrabasic alkaline lamprophyres and diabases intruded within the Spanish Central System (SCS) during Upper Permian. Their high LREE, LILE and HFSE contents, together with positive Nb–Ta anomalies, link their origin with the infiltration of sublithospheric K-rich fluids. These alkaline dykes may be classified in two distinct groups according to the Sr–Nd isotope ratios: (1) a depleted PREMA-like asthenospheric component, and (2) a BSE-like lithospheric component. A slight enrichment in radiogenic ²⁰⁷Pb and ²⁰⁸Pb allows the contribution of a recycled crustal or lithospheric component in the mantle sources. The intrusion of this alkaline magmatism is likely to have occurred due to adiabatic decompression and mantle upwelling in the context of the widespread rifting developed from Carboniferous to Permian in western Europe. The clear differences in the geochemical affinity of Lower Permian basic magmas from north-western and south-western Europe might be interpreted in terms of a more extensive separation of both regions during that period, until they were assembled during Upper Permian. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Investigation of a shallow alluvial aquifer using geoelectrical methods: a case from Turkey

Near-surface geophysical methods are commonly used to solve a wide class of geological, engineering and environmental problems. In this study, a geoelectrical survey was performed to investigate an alluvial aquifer. The study area is located in the southwest of the Çubukluda? graben, situated in the south of ?zmir, Turkey. The geophysical studies included the electrical resistivity imaging and self-potential (SP) methods. The resistivity data were acquired along eight profiles in the northern part of the study area by a Wenner-Schlumberger electrode configuration and the data processing was achieved by a tomographic inversion technique. The SP data were collected by gradient technique along 16 profiles. Total field values were calculated for each profile by addition of the successive gradient values, then a total field SP map was obtained. The water-saturated zone in the northern part of the study area was clearly revealed by the electrical resistivity imaging and the SP survey yielded useful information on the subsurface fluid movement.     

Palaeohydrogeology: A methodology based on fracture mineral studies

Two sites on the east coast of Sweden (Forsmark and Laxemar/Simpevarp) are currently being investigated as potential geologic hosts for a deep repository isolating high-level nuclear waste. In this paper, a methodology for fracture mineral studies is suggested with focus on the variation in depth of the fresh/saline water interface and location of the redox front in the bedrock. The most commonly precipitated fracture minerals in crystalline rocks are chlorite, calcite, quartz, K-feldspar, Ca–Al-silicates like epidote, prehnite and laumontite, sulphides and Fe-oxides. Of these, calcite is the mineral best suited for palaeohydrological studies since it precipitates during a wide range of conditions including low-temperature conditions during the Pleistocene and Holocene epochs. Sulphides and Fe-oxides/hydroxides provide information on the position of the redox front. In order to carry out palaeohydrological studies, a number of prerequisites are required such as; high quality drill core material, geological knowledge of the sequence of fracture mineralizations; the post-glacial (Holocene) evolution in the area; high quality groundwater chemistry, including stable isotopes; and a conceptual model of the hydrogeochemistry that is to be tested. The choice of methods used here is based on the fact that both the Forsmark and Laxemar/Simpevarp sites are situated in Palaeoproterozoic crystalline rocks with reactivation of fractures over at least 1.5 Ga, and they have been exposed to glaciations/deglaciations and transgressions/regressions of the Baltic Sea during the Quaternary. This has resulted in a palaeohydrology with a range of groundwaters of quite different chemistry and stable isotopic composition. The suggested scheme for solving the variation in depth of the fresh/saline water interface focuses on fracture calcite. It includes a step-by-step procedure with;

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Initial δ¹⁸O and δ¹³C, analyses and complementary petrographic studies of thin sections and crystal morphology followed by     

Flood risk perceptions and spatial multi-criteria analysis: an exploratory research for hazard mitigation

The conventional method of risk analysis (with risk as a product of probability and consequences) does not allow for a pluralistic approach that includes the various risk perceptions of stakeholders or lay people within a given social system. This article introduces a methodology that combines the virtues of three different methods: the quantifiable conventional approach to risk; the taxonomic analysis of perceived risk; and the analytical framework of a spatial multi-criteria analysis. This combination of methods is applied to the case study ‘Ebro Delta’ in Spain as part of the European sixth framework project ‘Floodsite’. First, a typology for flood hazards is developed based on individual and/or stakeholders’ judgements. Awareness, worry and preparedness are the three characteristics that typify a community to reflect various levels of ignorance, perceived security, perceived control or desired risk reduction. Applying ‘worry’ as the central characteristic, a trade-off is hypothesized between Worry and the benefits groups in society receive from a risky situation. Second, this trade-off is applied in Spatial Multi-Criteria Analysis (SMCA). MCA is the vehicle that often accompanies participatory processes, where governmental bodies have to decide on issues in which local stakeholders have a say. By using risk perception-scores as weights in a standard MCA procedure a new decision framework for risk assessment is developed. Finally, the case of sea-level rise in the Ebro Delta in Spain serves as an illustration of the applied methodology. Risk perception information has been collected with help of an on-site survey. Risk perception enters the multi-criteria analysis as complementary weights for the criteria risk and benefit. The results of the survey are applied to a set of scenarios representing both sea-level rise and land subsidence for a time span of 50 years. Land use alternatives have been presented to stakeholders in order to provide the regional decision maker with societal preferences for handling risk. Even with limited resources a characteristic ‘risk profile’ could be drawn that enables the decision maker to develop a suitable land use policy.     

Simulation of a reactive tracer experiment using stochastic hydraulic conductivity fields

Reactive tracer tests are performed to derive flow, transport and in situ biodegradation parameters. This paper describes the 3D simulation of a reactive tracer test using the transition probability geostatistical approach. Fifty different equally probable aquifer realizations were generated based on the geological information of 107 boreholes in an area of 62,500 m². One realization was chosen for the reactive transport simulation based on the results of groundwater flow modeling and on particle tracking calculations for the site. Field velocities at the site vary between 0.4 and 3 m/d. The transport of the reactive tracers deuterium ring labeled toluene-d₅ and fully deuterated toluene-d₈ was simulated and first-order biodegradation rates of 0.017 d⁻¹ for toluene-d₅ and 0.012 d⁻¹ for toluene-d₈ were determined.     

A priori fully populated covariance matrices in least-squares adjustment—case study: GPS relative positioning

In this contribution, using the example of the Mátern covariance matrices, we study systematically the effect of apriori fully populated variance covariance matrices (VCM) in the Gauss–Markov model, by varying both the smoothness and the correlation length of the covariance function. Based on simulations where we consider a GPS relative positioning scenario with double differences, the true VCM is exactly known. Thus, an accurate study of parameters deviations with respect to the correlation structure is possible. By means of the mean-square error difference of the estimates obtained with the correct and the assumed VCM, the loss of efficiency when the correlation structure is missspecified is considered. The bias of the variance of unit weight is moreover analysed. By acting independently on the correlation length, the smoothness, the batch length, the noise level, or the design matrix, simulations allow to draw conclusions on the influence of these different factors on the least-squares results. Thanks to an adapted version of the Kermarrec–Schön model, fully populated VCM for GPS phase observations are computed where different correlation factors are resumed in a global covariance model with an elevation dependent weighting. Based on the data of the EPN network, two studies for different baseline lengths validate the conclusions of the simulations on the influence of the Mátern covariance parameters. A precise insight into the impact of apriori correlation structures when the VCM is entirely unknown highlights that both the correlation length and the smoothness defined in the Mátern model are important to get a lower loss of efficiency as well as a better estimation of the variance of unit weight. Consecutively, correlations, if present, should not be neglected for accurate test statistics. Therefore, a proposal is made to determine a mean value of the correlation structure based on a rough estimation of the Mátern parameters via maximum likelihood estimation for some chosen time series of observations. Variations around these mean values show to have little impact on the least-squares results. At the estimates level, the effect of varying the parameters of the fully populated VCM around these approximated values was confirmed to be nearly negligible (i.e. a mm level for strong correlations and a submm level otherwise).