Use of terrestrial photosieving and airborne topographic LiDAR to assess bed grain size in large rivers: a study on the Rhine River

Most grain size monitoring is still being conducted by manual sampling in the field, which is time consuming and has low spatial representation. Due to new remote sensing methods, some limitations have been partly overcome, but methodological progress is still needed for large rivers as well as in underwater conditions. In this article, we tested the reliability of two methods along the Old Rhine River (France/Germany) to estimate the grain size distribution (GSD) in above-water conditions: (i) a low-cost terrestrial photosieving method based on an automatic procedure using Digital Grain Size (DGS) software and (ii) an airborne LiDAR topo-bathymetric survey. We also tested the ability of terrestrial photosieving to estimate the GSD in underwater conditions. Field pebble counts were performed to compare and calibrate both methods. The results showed that the automatic procedure of terrestrial photosieving is a reliable method to estimate the GSD of sediment patches in both above-water and underwater conditions with clean substrates. Sensitivity analyses showed that environmental conditions, including solar lighting conditions and petrographic variability, significantly influence the GSD from the automatic procedure in above-water conditions. The presence of biofilm in underwater conditions significantly altered the GSD estimation using the automatic procedure, but the proposed manual procedure overcame this problem. The airborne LiDAR topographic survey is an accurate method to estimate the GSD of above-water bedforms and is able to generate grain size maps. The combination of terrestrial photosieving and airborne topographic LiDAR methods is adapted to assess the GSD over several kilometers long reaches of large rivers. © 2020 John Wiley & Sons, Ltd.     

Observations on New Approaches for the Determination of Platinum-Group Elements, Gold and Silver in Different Geochemical Samples from Siberia and the Far East

Some recent experiments on the determination of Au and the platinum-group elements (PGE) in geochemical samples are reviewed. Emphasis is given to the determination of ultra-low levels of PGE concentrations in resistant matrices, including chromites, molybdenites and ultrabasic ores. The problems and features of PGE determination in samples of various chemical composition are considered. For each sample type studied, a series of sample preparation techniques are proposed. These techniques included acid digestion, fusion with sodium peroxide, cold sintering with an oxidizing mixture of Na₂O₂+ Na₂CO₃ and also oxidizing fluorination with bromine trifluoride. A new approach for preparing geochemical material prior to digestion, based on mechano-chemical activation with simultaneous hyperfine grinding, is proposed and studied. The instrumental determination of PGE contents was carried out directly by AAS from extracted organic phases. It was found that a combination of digestion processes was required to achieve geochemical background levels of Au and PGE concentrations with the following detection limits: Pd, Rh - 1 ng g⁻¹, Pt, Ru - 10 ng g⁻¹, Au - 0.2 ng g⁻¹, Ag - 0.1 ng g^−1. The uncertainty in PGE and Au determination in geochemical samples is dependent on metal concentration, and also on their distribution in samples. The total analytical uncertainty of the proposed method is between 15-30%.     

Magnetotelluric survey in the bradyseismic area of Phlegraean Fields

A magnetotelluric survey was carried out in order to contribute to the definition of the deep structure of the Phlegraean Fields area affected by the bradyseismic crisis begun in the summer 1982. In a preliminary campaign carried out in autumn 1983, the MT measurements were made in eight sites, located between 0.3 and 5 km from the Harbour of Pozzuoli and affected by different degrees of uplift.In a second campaign (autumn, 1984) seven of the first eight MT soundings were repeated and four new sites were investigated, one of which located in a stable zone 10 km away from Pozzuoli.MT measurements show the presence of a thick conductive layer (<1 m) thinned underneath the town of Pozzuoli, where its top reaches the minimum depth (1.2–2 km). The upper horizon of this layer has a greater concavity than that produced by surface deformation. Below the conductive layer, the substratum became progressively more resistive with depth. This resistive basement is well defined in the soundings located far away from Pozzuoli. Here, on the contrary, a zone or body much more conductive than its surroundings is detected. This conductive body could correspond to a high temperature magmatic intrusion, probably already solidified, with a rough cylindrical shape with a base diameter of 1.5±0.5 km.Introducing into the obtained model the depth of the conductive horizon desumed from the deep geothermal wells, the top of the intrusive body would lye at 3–4 km depth.A small uplift of this body related to tectonic readjustments eventually induced by strong earthquakes recently occurred in near zones could explain the surface deformation observed during the present bradyseismic crisis.     

Use of geoprocessing in the study of land degradation in urban environments: the case of the city of São Carlos,state of São Paulo,Brazil

This paper presents a methodology for the geological engineering survey of land degradation in urban environments using both remote sensing and geoprocessing tools. The area under study was the city of São Carlos, state of São Paulo, Brazil (urban and expansion area). The data presented here were obtained from earlier studies, photointerpretation and geological engineering mapping. The Envi 4.1 software package was used to prepare the digital orthophotos that served as a reference base for the information. Orthorectification of the Ikonos image (PSM, 1 m) was done and compared with other orthophotos from studies of environmental degradation in urban areas. The evolution of land degradation processes was analyzed based on the photointerpretation of aerial photographs taken on different dates and using Ikonos image. This study allowed to conclude that most of the degradation occurring in the city has been caused by unplanned land occupation, in disregard of environmental conditions, resulting in environmental degradation and thus impacting the quality of life of the urban population.     

Engineering geological mapping developed in the Fortaleza Metropolitan Region, State of Ceara, Brazil

Engineering geological mapping was carried out, at a scale of 1:100,000, in the Fortaleza Metropolitan Region, located in northeastern Brazil, to provide engineering geological information as an aid in planning land use and occupation. In this study, eight basic maps were produced drawing: urban areas, lithology, unconsolidated materials in addition to geomorphological map showing slopes, water resources, engineering geological zoning and distribution of geological–geotechnical environmental problems based on information collected through field and laboratory investigations. The region was divided into nine engineering geological zones analyzed in terms of geohazard susceptibility and foundation, excavation and waste disposal conditions.     

Numerical study of tides in Ontario Lacus,a hydrocarbon lake on the surface of the Saturnian moon Titan

In the context of the emergence of extra-terrestrial oceanography, we adapted an existing oceanographic model, SLIM (www.climate.be/slim), to the conditions of Titan, a moon of Saturn. The tidal response of the largest southern lake at Titan’s surface, namely Ontario Lacus, is simulated. SLIM solves the 2D, depth-averaged shallow water equations on an unstructured mesh using the discontinuous Galerkin finite element method, which allows for high spatial resolution wherever needed. The impact of the wind forcing, the bathymetry, and the bottom friction is also discussed. The predicted maximum tidal range is about 0.56 m in the southern part of the lake, which is more than twice as large as the previous estimates (see Tokano, Ocean Dyn 60:(4) 803–817 10.1007/s10236-010-0285-3 (Tokano 2010)). The patterns and magnitude of the current are also markedly different from those of previous studies: the tidal motion is not aligned with the major axis of the lake and the speed is larger nearshore. Indeed, the main tidal component rotates clockwise in an exact period of one Titan day and the tidal currents can reach 0.046 ms ^?1 close to the shores depending on the geometry and the bathymetry. Except for these specific nearshore regions, the current speed is less than 0.02 ms ^?1. Circular patterns can be observed offshore, their rotational direction and size varying along the day.     

Analogue models of caldera collapse in strike-slip tectonic regimes

Regional-scale faulting, particularly in strike-slip tectonic regimes, is a relatively poorly constrained factor in the formation of caldera volcanoes. To examine interactions between structures associated with regional-tectonic strike-slip deformation and volcano-tectonic caldera subsidence, we made scaled analogue models. Tabular (sill-like) inclusions of creamed honey in a sand/gypsum mix replicated shallow-level granitic magma chambers in the brittle upper crust. Lateral motion of a base plate sited below half the sand/gypsum pack allowed simulation of regional strike-slip deformation. Our experiments modelled: (1) strike-slip deformation of a homogeneous brittle medium; (2) strike-slip deformation of a brittle medium containing a passive magma reservoir; (3) caldera collapse into sill-like magma reservoirs without regional strike-slip deformation; and (4) caldera collapse into sill-like magma reservoirs after regional strike-slip deformation. Our results show that whilst the magma chamber shape principally influences the development and geometry of volcano-tectonic collapse structures, regional-tectonic strike-slip faults (Riedel shears and Y-shears) may affect a caldera’s structural evolution in two main ways. Firstly, regional strike-slip faults above the magma chamber may form a pre-collapse structural grain that is exploited and reactivated during subsidence. Our experiments show that such faults may preferentially reactivate where tangential to the collapse area and coincident with the chamber margins. In this case, volcano-tectonic extension in the caldera periphery tends to localise on regional-tectonic faults that lie just outside the chamber margins. In addition, volcano-tectonic reverse faults may link with and reactivate pre-collapse regional-tectonic faults that lie just inside the chamber margins. Secondly, where regional-tectonic strike-slip faults define corners in the magma chamber margin, they may halt the propagation of volcano-tectonic reverse faults. The experiments also highlight the potential difficulties in assessing the relative contributions of volcano-tectonic and regional-tectonic subsidence processes to the final caldera structure seen in the field. Disruption of the pre-collapse surface by regional-tectonic faulting was preserved during coherent volcano-tectonic subsidence to produce a caldera floor of differentially-subsided fault blocks. Without definitive evidence for syn-eruptive growth faulting, thickness changes in caldera fill across such regional-tectonic fault blocks in nature could be mistaken as evidence for piecemeal volcano-tectonic collapse.     

Hydrochemical characteristics of groundwater and surface water for domestic and irrigation purposes in Vea catchment,Northern Ghana

The Vea catchment, mainly underlain by crystalline basement rocks, is located in Northern Ghana. Hydrogeochemical studies were carried out in this area with the objective of identifying the geochemical processes influencing water quality and suitability of surface and groundwater for agricultural and domestic uses. Sixty-one groundwater and four surface water samples were collected from boreholes, dams and rivers and analysed for Ca²⁺, Mg²⁺, Na⁺, K⁺, HCO₃ ^?, Cl^?, and SO₄ ^2?, Fe_tot, PO₄ ^3?, Mn_tot, NH₄ ⁺, NO₃ ^?, NO₂ ^?. In addition, pH, total dissolved solids, electrical conductivity, total hardness, turbidity, colour, salinity and dissolved oxygen were analysed. Chloro-alkaline indices 1 and 2, and characterization of weathering processes suggest that the chemistry of groundwater is dominated by the interaction between water and rocks. Cation exchange and silicate weathering are the dominant processes controlling the chemical composition of the groundwater in the area studied. Mineral saturation indices indicate the presence of at least three groups of groundwater in the Vea catchment with respect to residence time. The meteoric genesis index suggests that 86% of the water samples belong to the shallow meteoric water percolation type. The findings further suggest that the groundwater and surface water in the basin studied are mainly Ca–Mg–HCO₃ water type, regardless of the geology. Compared to the water quality guidelines of WHO, the study results on sodium absorption ratio, sodium percentage, magnesium hazard, permeability index and residual sodium carbonate indicate that groundwater and surface water in the Vea catchment are generally suitable for drinking and irrigation purposes.     

An Origin for Harrisitic and Granular Olivine in the Rum Layered Suite, NW Scotland: a Crystal Size Distribution Study

Dendritic crystal morphologies occur in a number of igneousrocks and are thought to originate from the rapid growth ofcrystals, yet many examples of dendritic morphologies are foundin plutonic igneous rocks where cooling rates should be low.Results from crystal size distribution (CSD) measurements onharrisitic olivines from Rum, Scotland, combined with estimatedolivine growth rates, suggest that the characteristic skeletalhopper and branching olivines of harrisitic cumulates that areup to centimetres long, may have exceptionally short crystalgrowth times (several hours to several hundreds of days). This,together with very low calculated nucleation densities for harrisiticolivine, supports the interpretation of harrisite being a disequilibriumtexture, developed in response to supersaturation of the magmain olivine. We propose that this supersaturation arose throughundercooling of thin picrite sheets emplaced along the Rum magmachamber floor, beneath cooler resident magma. It is envisagedthat the picrite sheets were largely free of suspended olivinecrystals. Coupled with the olivine-enriched composition of themelt and the increasing cooling rate, this allowed homogeneousnucleation of olivine to set in at deeper undercooling and greaterolivine supersaturation than if there had been plentiful suspendedolivines to act as heterogeneous nuclei. The enhanced supersaturationcaused rapid growth of olivine once nucleation began, with skeletaland dendritic shapes. It is suggested that the observed, interlayeredsequences of harrisite and cumulus peridotite found throughoutthe Rum Layered Suite are a result of multiple episodes of harrisitecrystallization resulting from picrite emplacement that alternatedwith periods of crystal growth and accumulation in the mainbody of magma at lesser degrees of undercooling. KEY WORDS: crystal size distribution; harrisite; crystal growth rates; Rum Layered Suite     

Carbonate Assimilation at Merapi Volcano, Java, Indonesia: Insights from Crystal Isotope Stratigraphy

Recent basaltic andesite lavas from Merapi volcano contain abundant,complexly zoned, plagioclase phenocrysts, analysed here fortheir petrographic textures, major element composition and Srisotope composition. Anorthite (An) content in individual crystalscan vary by as much as 55 mol% (An_40–95) across internalresorption surfaces with a negative correlation between highAn mol% (>70), MgO wt% and FeO wt%. In situ Sr isotope analysesof zoned plagioclase phenocrysts show that the ⁸⁷Sr/⁸⁶Sr ratiosof individual zones range from 0·70568 to 0·70627.The upper end of this range is notably more radiogenic thanthe host basaltic andesite whole-rocks (< 0·70574).Crystal zones with the highest An content have the highest ⁸⁷Sr/⁸⁶Srvalues, requiring a source or melt with elevated radiogenicSr, rich in Ca and with lower Mg and Fe. Recent Merapi eruptiverocks contain abundant xenoliths, including metamorphosed volcanoclasticsediment and carbonate country rock (calc-silicate skarns) analysedhere for petrographic textures, mineralogy, major element compositionand Sr isotope composition. The xenoliths contain extremelycalcic plagioclase (up to An₁₀₀) and have whole-rock ⁸⁷Sr/⁸⁶Srratios of 0·70584 to 0·70786. The presence ofthese xenoliths and their mineralogy and geochemistry, coupledwith the ⁸⁷Sr/⁸⁶Sr ratios observed in different zones of individualphenocrysts, indicate that magma–crust interaction atMerapi is potentially more significant than previously thought,as numerous crystal cores in the phenocrysts appear to be inheritedfrom a metamorphosed sedimentary crustal source. This has potentiallysignificant consequences for geochemical mass-balance calculations,volatile saturation and flux and eruptive behaviour at Merapiand similar island arc volcanic systems elsewhere. KEY WORDS: assimilation; isotopes; Merapi; xenolith; calc-silicate