Identifying causes of ground-penetrating radar reflections using time-domain reflectometry and sedimentological analyses

Ground-penetrating radar (GPR) is a geophysical technique widely used to study the shallow subsurface and identify various sediment features that reflect electromagnetic waves. However, little is known about the exact cause of GPR reflections because few studies have coupled wave theory to petrophysical data. In this study, a 100- and 200-MHz GPR survey was conducted on aeolian deposits in a quarry. Time-domain reflectometry (TDR) was used to obtain detailed information on the product of relative permittivity (ɛ_r) and relative magnetic permeability (μ_r), which mainly controls the GPR contrast parameter in the subsurface. Combining TDR data and lacquer peels from the quarry wall allowed the identification of various relationships between sediment characteristics and ɛ_rμ_r. Synthetic radar traces, constructed using the TDR logs and sedimentological data from the lacquer peels, were compared with the actual GPR sections. Numerous peaks in ɛ_rμ_r, which are superimposed on a baseline value of 4 for dry sand, are caused by potential GPR reflectors. These increases in ɛ_rμ_r coincide with the presence of either organic material, having a higher water content and relative permittivity than the surrounding sediment, or iron oxide bands, enhancing relative magnetic permeability and causing water to stagnate on top of them. Sedimentary structures, as reflected in textural change, only result in possible GPR reflections when the volumetric water content exceeds 0·055. The synthetic radar traces provide an improved insight into the behaviour of radar waves and show that GPR results may be ambiguous because of multiples and interference.     

Fahien reconsidered: Pleistocene exploitation of wild bananas and Holocene introduction of Musa cultivars to Sri Lanka

A recent publication on the phytolith assemblage at Fahien rockshelter, Sri Lanka (Premathilake and Hunt) is argued to represent: the exploitation of wild Musa acuminata and M. balbisiana during the Late Pleistocene; the introduction of edible diploid cultivars from the Southeast Asia–New Guinea region during the early to mid‐Holocene; the generation and cultivation of triploid banana hybrids on Sri Lanka before 6194–5994 cal bp ; and the subsequent spread of derived triploid cultivars to mainland India and westward to Africa. A careful review of the archaeobotanical research presented by Premathilake and Hunt, in the context of broader multidisciplinary evidence (agronomy, archaeobotany, genetics and linguistics) for the domestication and spread of banana cultivars, indicates that three main aspects of their argument are problematic: the lack of clarity in the characterization of banana domestication in the past; the methods used to discriminate phytoliths into banana taxa; and the promotion of Sri Lanka as a source region rather than a recipient of banana cultivars. Following reconsideration, the Fahien evidence is consistent with previous interpretations for the origins of diploid and significant triploid cultivars outside of Sri Lanka and dispersal to that island. © 2019 John Wiley & Sons, Ltd.     

Solar Image Restoration By Use Of Multi-frame Blind De-convolution With Multiple Objects And Phase Diversity

An extension of Joint Phase Diverse Speckle image restoration is presented. Multiple realizations of multiple objects having known wavefront relations with each other can now be restored jointly. As the alignment of the imaging setup does not change, near-perfect alignment can be achieved between different objects, thus greatly reducing false signals in the determination of derived quantities, such as magnetograms, Dopplergrams, etc. The method was implemented in C++ as an image restoration server, to which worker clients can connect and disconnect randomly, so that a large number of CPUs can be used to speed up the restorations. We present a number of examples of applications of the restoration method to observations obtained with the Swedish 1-m Solar Telescope on La Palma.     

Nitrogen geochemistry as a tracer of fluid flow in a hydrothermal vent complex in the Karoo Basin, South Africa

We have investigated the N geochemistry of minerals and rocks from contact metamorphic aureoles and hydrothermal vent complexes (HVC) in the Karoo Basin in South Africa. The HVC formed during phreatic eruptions associated with rapid devolatilization and pressure build-up in contact aureoles around early Jurassic sill intrusions. By combining outcrop data from a HVC and core data from contact aureoles, we investigate the relationship between light element release during metamorphism and vertical fluid migration. Sandstone and breccia from the HVC contain early-diagenetic ammonium -bearing feldspar (buddingtonite) and illite. Ammonium occupies up to 95% of the A site in feldspar, corresponding to concentrations up to 5.2 wt% N. Bulk-rock N isotope data for rocks from inside and outside the hydrothermal vent complex fall into two distinct groups. Background samples have δ¹⁵N_air between +1.5‰ and +4.9‰, whereas minerals from the vent complex have δ¹⁵N in the range +7.5 to +10.6‰. The N geochemistry of contact metamorphic shale from the lower stratigraphic units of the Karoo Basin shows that the vitrinite reflectance and δ¹⁵N values are positively correlated. Shale with reflectivity values >4%Ro are enriched in ¹⁵N, with δ¹⁵N values between +6‰ and +14‰, implying the release of isotopically light N into metamorphic fluids (probably as N₂). We suggest that the relatively high δ¹⁵N values of the early-diagenetic buddingtonite in the HVC reflect exchange of buddingtonite with N-bearing fluids ascending from greater depth after their release during contact metamorphism and dehydration. We present a qualitative model whereby hydrothermal vent complexes represent fluid flow structures after their formation, focusing N-bearing metamorphic fluids sourced in deeper levels of the basin. The release of organic N from sediments at depth in volcanic basins could play a role in the geochemical cycle of N, becoming particularly important during periods of intense volcanic activity.     

Self-diffusion of water and its dependence on temperature and ionic strength in highly compacted montmorillonite, illite and kaolinite

The effect of temperature and ionic strength on the diffusion of HTO parallel to the direction of compaction through 5 highly compacted clay minerals (bulk dry density, ρ_b,d = 1.90 ± 0.05 Mg/m³), namely montmorillonite (Na- and Ca-form), illite (Na- and Ca-form), and kaolinite, was studied. The diffusion experiments were carried out at temperatures between 0 °C and 60 °C and at ionic strengths of 0.01 M and 1 M NaCl for the Na-form clays and kaolinite, and of 0.005 M and 0.5 M CaCl₂ for the Ca-form. The ionic strength had an insignificant influence on the values of the effective diffusion coefficient (variation by less than 10%) for the clays under study at this degree of compaction. The effective diffusion coefficients followed the order Na-montmorillonite < Ca-montmorillonite < Ca-illite < Na-illite  kaolinite. It is thought that the differences between Na- and Ca-montmorillonite originate from the larger size particles, and thus the lower tortuosity of the latter; whereas the differences between Na- and Ca-illite are related to the different degree of solvation of the Na and Ca cations. The activation energies were successfully calculated using the Arrhenius law. Swelling clays (Na- and Ca-montmorillonite) had slightly larger activation energy values (20 kJ/mol) compared to bulk water (17 kJ/mol); Ca-illite (16 kJ/mol), Na-illite (13 kJ/mol) and kaolinite (14.4 kJ/mol) lower values than that of bulk water. The low activation energies of the last three clays may be related to weaker H-bonds between water and the clay surfaces compared to those in bulk water.     

A static drilling riser model using free boundary conditions

A static three-dimensional analytical method for drilling risers experiencing large displacements and slip at the top joint is presented. The riser is described in cylindrical coordinates as a three-dimensional tensioned string, without bending or torsional stiffness. The vertical vessel displacement is not taken into account. The equilibrium configuration is obtained from the stationary condition of the total potential energy functional. Kinematic boundary conditions are introduced for the radial displacement only. The riser orientation at the top directly follows from the natural boundary conditions based on the variational treatment of the energy functional.     

Sedimentary record of redox-sensitive elements (U, Mn, Mo) in a transitory anoxic basin (the Thau lagoon, France)

In order to further document the relation between redox conditions and the sedimentary record of Mn, U and Mo in a transitory anoxic water basin, their distribution has been studied along two profiles in the Thau lagoon (France). Sediments and pore-water have been sampled at two contrasting sites located, respectively, in the shellfish-farming area and in the centre of the lagoon. In the shellfish-farming area, the particulate organic carbon (POC) data indicate a more rapid organic matter mineralisation compared to the centre of the lagoon. This results in a sharper redox gradient characterized by the appearance of H₂S in pore-water a few millimetres below the sediment–water interface. In the centre of the lagoon, H₂S appears at a depth of 35 cm.In both cores, sedimentary Mn is relatively depleted through out the whole sedimentary column and varies with the proportion of clay minerals. After an initial release into solution at the sediment–water interface in relation to Mn-oxide reductive dissolution, authigenic U is immobilized when sulphides appear. Despite the occurrence of anoxic conditions at the sediment–water interface at the site influenced by shellfish farming, the burial of U is reduced by bioturbation, which raises reducing sediments to the surface. In the centre of the lagoon, Mo profiles reflect continuous diffusion into pore water and immobilization at 15 cm probably in anoxic microenvironments. At shellfish farms, dissolved Mo undergoes removal with sulphides but contrary to U, sedimentary Mo does not appear to be strongly affected by bioturbation. The profile indicates an increase in the frequency of anoxia crises during the second half of the 20th century.     

A gamma‐ray spectroscopy survey of Omani meteorites

The gamma‐ray activities of 33 meteorite samples (30 ordinary chondrites, 1 Mars meteorite, 1 iron, 1 howardite) collected during Omani‐Swiss meteorite search campaigns 2001–2008 were nondestructively measured using an ultralow background gamma‐ray detector. The results provide several types of information: Potassium and thorium concentrations were found to range within typical values for the meteorite types. Similar mean ²⁶Al activities in groups of ordinary chondrites with (1) weathering degrees W0‐1 and low ¹⁴C terrestrial age and (2) weathering degree W3‐4 and high ¹⁴C terrestrial age are mostly consistent with activities observed in recent falls. The older group shows no significant depletion in ²⁶Al. Among the least weathered samples, one meteorite (SaU 424) was found to contain detectable ²²Na identifying it as a recent fall close to the year 2000. Based on an estimate of the surface area searched, the corresponding fall rate is ~120 events/10⁶ km²*a, consistent with other estimations. Twelve samples from the large JaH 091 strewn field (total mass ~4.5 t) show significant variations of ²⁶Al activities, including the highest values measured, consistent with a meteoroid radius of ~115 cm. Activities of ²³⁸U daughter elements demonstrate terrestrial contamination with ²²⁶Ra and possible loss of ²²²Rn. Recent contamination with small amounts of ¹³⁷Cs is ubiquitous. We conclude that gamma‐ray spectroscopy of a selection of meteorites with low degrees of weathering is particularly useful to detect recent falls among meteorites collected in hot deserts.