Line-broadening effects in the powder infrared spectrum of apatite

The crystallinity of natural and synthetic apatite samples is often determined from the broadening of ν ₄ PO₄ infrared absorption bands. However, various physical mechanisms contribute to the observed linewidth. In the present study, the factors determining the linewidth in the powder spectrum of synthetic fluorapatite and hydroxyapatite samples are investigated. The temperature dependence of the infrared spectrum (10–270 K) is used to assess the respective contributions of homogeneous broadening, related to the decay of phonons through anharmonic coupling, and heterogeneous broadening related to elastic strain and macroscopic electrostatic effects. This latter contribution is dominant in the investigated samples and depends on the shape of powder particles. It is discussed under the light of the theoretical modeling of the low-frequency dielectric properties of apatite based on first-principles density functional theory calculations. The linewidth of the weak ν ₁ PO₄ absorption band provides a reliable information on microscopic sources of broadening, i.e., apatite crystallinity. In comparison, the other more intense PO₄ bands are more sensitive to long-range electrostatic effects.     

Lithospheric tectonic structures developed under high-grade metamorphism in the Southern part of Madagascar

Abstract

This paper describes the tectono-metamorphic evolution of a segment of the Precambrian deep crust, in the southern Madagascar island. This crust corresponds to an Archaean basement reworked by a widespread, late panAfrican event (550–580 Ma) during the formation of the Mozambican belt. The finite geometry and associated metamorphism are depicted by satellite imaging, field mapping and P-T estimations using both conventional thermobarometric methods and TWEEQ software program with internally consistent thermodynamic data and uniform set of solution models. The structural pattern developed during high-grade metamorphism shows the juxtaposition of domains with complex fold geometries separated by a 15 km wide ductile shear zone. Within the folded domains, kilometre scale interference patterns associated with strongly dipping metamorphic stretching lineations can be described as superposed folding (F1 and F2 folds). The tight and upright F2 folds result from East-West horizontal shortening. The shear zone is defined by homogeneous orientations of steep foliations, sub-horizontal stretching lineations, and kilometre scale strain gradient. Within the shear zone, we observe dominant non-coaxial criteria at various scales that are consistent with a sinistral strike-slip system during D2 deformation stage. Nevertheless, we have also found in the shear zone, geometries typical of a horizontal shortening. Such a strain pattern is characteristic of transpression tectonics.

The synkinematic metamorphic conditions are estimated on mafic garnetiferous metabasites. Results show that regional transpression tectonics has developed under very high and constant thermal regime (about 800°C). A pressure gap, of about 3 kbar between the domains separated by the shear zone is identified. This implies tectonic coupling of two different structural levels during tranpressive tectonic.     

Comprehension and hydrogeological conceptualization of aquifer in arid and semi-arid regions using integrated hydrogeological information system: case of the deep aquifer of Zéramdine-Béni Hassen (east-central Tunisia)

In arid and semi-arid regions, the groundwater overexploitation caused drawdown in piezometric levels and a degradation of chemical water quality. That is why the groundwater monitoring needs a good comprehension of the hydrogeological aquifer properties. This is specially the case of Zéramdine–Béni Hassen deep aquifer (east-central Tunisia). Seismic profiles interpretation highlights the existence of the Zéramdine fault corridor, the Boumerdès anticline, the Moknine and Mahdia grabens that represent lateral boundaries for the study aquifer. The outcrop of the aquifer is located in the Zéramdine, Béni Hassen and Ain Ben Jannet regions, where two lithostratigraphic sections were realized. The piezometric study shows that the principal groundwater flow is from west to east. A secondary flow is from NW to SE. The hydrochemical study of 22 sample shows that the aquifer is characterized by freshwater, dominated by Na–Ca–Cl–SO4 facies. The salinity increase is from the west to the east, which coincides with the principal water flow direction. The integration of all results deduced from the hydrogeophysic, hydrodynamic and hydrochemical studies is developed to investigate hydrological processes of Zéramdine–Béni Hassen aquifer and consequently to propose a conceptual model, which will help to propose a rescue plan for the studied aquifer and to implement a spatial hydrogeological database using the global information system and then to characterize the complex aquifer system.     

Rock magnetic study of basic intrusions and massive sulphides in the Hercynian central Jebilets Massif (Occidental-Meseta), Morocco

This paper describes a multidisciplinary study approach (petrography and rock magnetism) conducted on samples collected from the study area to characterise the magnetic mineralogy and to determine if the magnetisation of both lithologies were induced or retain a remnant component. Petrophysical, mineralogical and geochemical analyses confirm bimodal aspects, particularly in basic rocks; the two magnetic modes depend essentially on the mineralogical and geochemical characteristics of the samples. The ultramafic rocks comprise a highly altered primary mineralogy with chromite and magnetite as magnetic phases. The second type is of mafic composition with a less altered primary mineralogy and essentially magnetite and/or (hemo-ilmenite) as a carrier of magnetic mineralisation. Sulphides are characterised by high concentrations of Cu, Zn and Pb. The mineralogy is composed mainly of pyrrhotite (85% to 90%), sphalerite, galena, chalcopyrite, arsenopyrite and, occasionally, stannite. Monocline pyrrhotite seems to be the magnetic carrier of magnetisation in both Draa Sfar and Koudiat Aïcha. However, we suspect a different amount of hexagonal pyrrhotite as the cause of different magnetic behaviour. Paleomagnetic and thermomagnetic analyses reveal different. The calculated characteristic direction of natural remnant magnetisation for sulphides was used to model the magnetic anomaly of Draa Sfar. The proposed model match the geological features concluded from geological mapping and boreholes. Results from this work can be very useful for any modelling processes of magnetic anomalies suspected due to a sulphide mineralisation in an area with poor outcrops and no presence of boreholes information or of any geological or geochemical data.     

Application of nondestructive testing methods to study the damage zone underneath impact craters of MEMIN laboratory experiments

Abstract– Within the framework of the Multidisciplinary Experimental and Modeling Impact Research Network (MEMIN) research group, the damage zones underneath two experimentally produced impact craters in sandstone targets were investigated using several nondestructive testing (NDT) methods. The 20 × 20 × 20 cm sandstones were impacted by steel projectiles with a radius of 1.25 mm at approximately 5 km s^?1, resulting in craters with approximately 6 cm diameter and approximately 1 cm depth. Ultrasound (US) tomography and vibrational analysis were applied before and after the impact experiments to characterize the damage zone, and micro‐computer tomography (μ‐CT) measurements were performed to visualize subsurface fractures. The newly obtained experimental data can help to quantify the extent of the damage zone, which extends to about 8 cm depth in the target. The impacted sandstone shows a local p‐wave reduction of 18% below the crater floor, and a general reduction in elastic moduli by between approximately 9 and approximately 18%, depending on the type of elastic modulus. The results contribute to a better empirical and theoretical understanding of hypervelocity events and simulations of cratering processes.     

Global height system unification with GOCE: a simulation study on the indirect bias term in the GBVP approach

One of the main objectives of ESA’s Gravity Field and Steady-State Ocean Circulation mission GOCE (Gravity field and steady-state ocean circulation mission, 1999) is to allow global unification of height systems by directly providing potential differences between benchmarks in different height datum zones. In other words, GOCE provides a globally consistent and unbiased geoid. If this information is combined with ellipsoidal (derived from geodetic space techniques) and physical heights (derived from leveling/gravimetry) at the same benchmarks, datum offsets between the datum zones can be determined and all zones unified. The expected accuracy of GOCE is around 2–3 cm up to spherical harmonic degree n _max ≈ 200. The omission error above this degree amounts to about 30 cm which cannot be neglected. Therefore, terrestrial residual gravity anomalies are necessary to evaluate the medium and short wavelengths of the geoid, i.e. one has to solve the Geodetic Boundary Value Problem (GBVP). The theory of height unification by the GBVP approach is well developed, see e.g. Colombo (A World Vertical Network. Report 296, Department of Geodetic Science and Surveying, 1980) or Rummel and Teunissen (Bull Geod 62:477–498, 1988). Thereby, it must be considered that terrestrial gravity anomalies referring to different datum zones are biased due to the respective datum offsets. Consequently, the height reference surface of a specific datum zone deviates from the unbiased geoid not only due to its own datum offset (direct bias term) but is also indirectly affected by the integration of biased gravity anomalies. The latter effect is called the indirect bias term and it considerably complicates the adjustment model for global height unification. If no satellite based gravity model is employed, this error amounts to about the same size as the datum offsets, i.e. 1–2 m globally. We show that this value decreases if a satellite-only gravity model is used. Specifically for GOCE with n _max ≈ 200, the error can be expected not to exceed the level of 1 cm, allowing the effect to be neglected in practical height unification. The results are supported by recent findings by Gatti et al. (J Geod, 2012).     

Positional accuracy of the Google Earth terrain model derived from stratigraphic unconformities in the Big Bend region,Texas, USA

The Google Earth terrain model could prove beneficial for extraction of positional data in the future. At present, only an aging independent benchmark study (Potere, D., 2008. Horizontal position accuracy of Google Earth's high-resolution imagery archive. Sensors, 8, 7973–7981) provides constraints on positional accuracy for Google Earth imagery. In this investigation, we compared virtually traced positions against high-precision (<1 m) field measurements along three stratigraphic unconformity sub-sections in the Big Bend region to determine current positional accuracy for the Google Earth terrain model. A horizontal position accuracy of 2.64 m RMSE_r was determined for the Google Earth terrain model with mean offset distance being 6.95 m. A vertical position accuracy of 1.63 m RMSE_z with mean offset distance of 2.66 m was also calculated for the terrain model. Results suggest data extracted from the Google Earth terrain model could plausibly be used in future studies. However, we urge caution in using Google Earth data due to limited information disclosures by developers.     

Development of an instantaneous GNSS/MEMS attitude determination system

Global navigation satellite systems (GNSS) are well suited for attitude determination. The key to high-precision GNSS-attitude determination is the ambiguity resolution. In case of kinematic applications, the rapidity of this process is of particular importance. We present a new instantaneous attitude determination system for GNSS-challenged environments. The single-epoch ambiguity resolution is performed by the ambiguity function method aided by a micro-electro-mechanical system (MEMS), leading to success rates above 99 %. The GNSS/MEMS fusion is realized by the use of an extended Kalman filter. When the system is stationary, a state vector augmentation with a shaping filter reduces systematic effects in the GNSS-attitudes. By means of two field experiments, the system was tested successfully. Despite poor GNSS measurement conditions, it provided reliable and accurate results, with empirical standard deviations in the range of 0.03–0.1 deg for the yaw angle.