Crystal chemistry of perovskite-type compounds in the tausonite-loparite series, (Sr1−2 x Na x La x )TiO3

Perovskite-type compounds in the series tausonite-loparite, (Sr_{1?2 x} Na _x La _x )TiO₃, were synthesized by solid-state reaction (final heating at 1200–1300?°C), and studied using “conventional” and synchrotron X-ray powder diffractometry. The structures of intermediate compositions were determined using the Rietveld profile refinement method. In the compositional range 0?≤x?≤ 0.1, the series comprises perovskites characterized by an undistorted cubic structure (space group Pm3¯m, a?≈ 3.905–3.902?Å, Z?=?1). Intermediate compounds in the range 0.15?≤?x?≤?0.35 crystallize with tetragonal symmetry (I4/mcm, a?≈? , c?≈? , Z?=?4) derived from the cubic aristotype by antiphase rotation of the TiO₆ octahedra about a fourfold axis. The angle of rotation estimated from the positional parameters of oxygen atoms ranges from 2.5(7)° to 5.5(4)°. The cubic-to-tetragonal transition arises from substitution of Sr²⁺ by the comparatively smaller Na¹⁺ and La³⁺ cations. A further transition from the tetragonal to rhombohedral symmetry (R3¯c, a?≈? , c?≈?2 , Z?=?6) occurs between x?=?0.35 and 0.40, and apparently does not involve formation of perovskite with an intermediate two-tilt structure (Imma). The rhombohedral structure is characterized by a multicomponent octahedral tilt about a threefold axis ranging in magnitude from 6.5(2)° to 7.7(2)°. In the series (Sr_{1?2 x} Na _x La _x )TiO₃, the unit-cell dimensions decrease, and the degree of structural distortion increases with x.     

Estimating backscattering strength for a correlation log

A correlation log is a type of sonar system used for measuring the velocity of a surface vessel or of a submersible vehicle. The operation is similar to that of a conventional echo sounder, and the direction of acoustic propagation is vertically downwards. Since a correlation log can be used for either bottom tracking or water tracking, it is of interest to relate the transmitted power level to the echo levels from both bottom and volume scatterers, which is a means of predicting the log's performance. This is done by modeling the transmission channel in terms of the sonar equations     

Alluvial sediment sources in a glaciated catchment: The voidomatis basin,Northwest Greece

X-ray diffraction (XRD) of the fine matrix component of four alluvial units and modern channel sediments in the Voidomatis River Basin of northwest Greece shows that fine sediment sources have changed considerably during the late Quaternary. The matrix fraction of the modern channel sediments is derived predominantly from erosion of local flysch rocks and soils. During the last glaciation, however, the fine sediment load of the Voidomatis River was dominated by glacially-ground, finely comminuted limestone materials. Limestone-derived fine sediment is not produced in significant amounts under modern climatic conditions. By combining this XRD work with a detailed programme of clast lithologic 1 analysis we have reconstructed former bedload and fine sediment load composition. The lithological properties of both the coarse (8-256 mm) and fine (< 63 μm) elements of the sediment load have varied markedly during the late Quaternary. A simple, semiquantitative assessment of fine sediment mineralogy, using diffractogram peak-height data, has provided a valuable complement to the information gathered from more traditional clast lithological techniques. Together, in favourable geological settings, fine fraction mineralogy and clast lithological analysis can provide a valuable tool for the reconstruction of late Quaternary alluvial environments.     

The geology of dam spillways

Over the past 25 years, embankment dam design has shown trends towards increased use of poorer quality rockfill and omission of concrete lining and energy dissipation structures in spillways. The most economic design may be an unlined rockcut spillway, supplying the total rockfill requirement for the embankment construction. However, such a design does impose significant restrictions upon construction program flexibility, and the apparent cost advantage can quickly disappear if geological problems are encountered during construction. Geological criteria for the acceptability of rockfill and for the omission of concrete lining in spillways have, therefore, become increasingly important in recent years. The decision to omit concrete lining and energy dissipation structures from the spillway should only be taken after detailed consideration of the site geology, the probable erosion mechanism and the operational characteristics of the spillway. It should be recognised that frequently operating spillways with severe discharge conditions require special consideration and a certain degree of conservatism over and above that required for spillways where discharge conditions are only moderate.     

Contributions to a shallow aquifer study by reprocessed seismic sections from petroleum exploration surveys, eastern Abu Dhabi, United Arab Emirates

The US Geological Survey, in cooperation with the National Drilling Company of Abu Dhabi, is conducting a 4-year study of the fresh and slightly saline groundwater resources of the eastern Abu Dhabi Emirate. Most of this water occurs in a shallow aquifer, generally less than 150 m deep, in the Al Ain area. A critical part of the Al Ain area coincides with a former petroleum concession area where about 2780 km of vibroseis data were collected along 94 seismic lines during 1981–1983. Field methods, acquistion parameters, and section processing were originally designed to enhance reflections expected at depths ranging from 5000 to 6000 m, and subsurface features directly associated with the shallow aquifer system were deleted from the original seismic sections. The original field tapes from the vibroseis survey were reprocessed in an attempt to extract shallow subsurface information (depths less than 550 m) for investigating the shallow aquifer.A unique sequence of reproccessing parameters was established after reviewing the results from many experimental tests. Many enhancements to the resolution of shallow seismic reflections resulted from: (1) application of a 20-Hz, low-cut filter; (2) recomputation of static corrections to a datum nearer the land surface; (3) intensive velocity analyses; and (4) near-trace muting analyses. The number, resolution, and lateral continuity of shallow reflections were greatly enhanced on the reprocessed sections, as was the delineation of shallow, major faults. Reflections on a synthetic seismogram, created from a borehole drilled to a depth of 786 m on seismic line IQS-11, matcheddprecisely with shallow reflections on the reprocessed section. The 33 reprocessed sections were instrumental in preparing a map showing the major structural features that affect the shallow aquifer system. Analysis of the map provides a better understanding of the effect of these shallow features on the regional occurrence, movement, and quality of groundwater in the concession area. Results from this study demonstrate that original seismic field tapes collected for deep petroleum exploration can be reprocessed to explore for groundwater.     

Vertical cross-spectra of horizontal velocity components at the Boulder observatory

Cross-spectra between horizontal wind components at different levels of the Boulder Atmospheric Observatory (BAO) tower lead to the following conclusions:

1. Davenport's hypothesis is satisfied that coherence decays exponentially with the ratio of vertical separation to horizontal wave length, at least to very small values of coherence.
2. The decay coefficients increase with z/L for z/L < 0.5. For larger stabilities, irregular fluctuations with periods of order 10–20 min have considerable vertical coherence. Results at BAO are quite consistent with those elsewhere.
3. Eddy slopes in vertical planes increase with wind shear up to a point where the slope (horizontal delay over vertical separation) is just above 2. Beyond that point, the systematic increase of slopes with shear ceases. Since wind shear decreases upward, slopes tend to decrease upward. Slopes for lateral components are significantly larger than those for u-components.

     

Stability of Slopes with Berms on Rigid Foundations

The finite element method was used to assess the influence of berms on the stability of slopes resting on rigid foundations for three different slope angles. Both purely cohesive soils and soils exhibiting cohesive and frictional strength properties were considered. The results of the analyses are presented in non-dimensional graphical and tabular form for design purposes and show that, for each case considered, there is a unique (berm width) : (slope height) ratio after which further increases in the berm width produce no further increases in stability. Example design calculations are presented and validated using Bishop's simplified method.