The CQU Regional Seismic Network and applications to underground mining in Central Queensland,Australia

Digital seismograms continuously recorded from 1988 to 1992 by two stations of the RESNOM seismic network in northern Baja California, Mexico, were used to search for probable shear-wave anisotropic characteristics in the region of the Cerro Prieto fault. Shear-wave splitting was identified in many of the three-component records analyzed. We measured the polarization direction of the leadingS wave inside theS-wave window as well as the delay times between fast and slow phases on those records displaying shear-wave splitting. For station CPX, which is nearest the Imperial Valley region to the north, the preferred polarization direction found in this study (azimuth 180°±10°) coincides with the direction of the regional maximum compressive stress determined for the region. This polarization direction can be interpreted in terms of the “Extensive Dilatancy Anisotropy” model as the effect of vertical parallel aligned cracks. The preferred polarization direction measured at LMX, however, gives an azimuth of 45°±5°. Thus, it appears that faults and fractures aligned oblique to the main tectonic trend have a greater influence on the anisotropic characteristics of the crust south of Cerro Prieto volcano than that of the regional stress field. Time delays between slow and fastS waves observed at CPX appear constant from 1988 to 1992 while delays measured at LMX for the same interval indicate a small increase with time which cannot be attributed to azimuthal variations of paths.     

Hydrothermal origin of metals in some East African Rift Lakes

The chemical composition of all Western Rift valley lakes is controlled by emanations of saline and metalliferous hydrothermal solutions. Center of present day activity is Lake Kivu. Sediments of the last 15,000 years carry the distinct chemical fingerprint of hydrothermal springs in the form of stratiform ore deposits. The geological record reveals periods of volcanic and hydrothermal activity and quiescence which coincide with the African climatic sequence of pluvials and interpluvials. It is concluded that the hydrothermal solutions are meteoric waters which on passage through the volcanic wall rock have derived their metal and salt load. Presence of volcanic CO₂ has enforced the leaching process.

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Zusammenfassung Die chemische Zusammensetzung der großen Seen im Bereich des Western Rift ist maßgeblich durch Einschüttungen salziger und metallhaltiger hydrothermaler Lösungen bestimmt. Zentrum heutiger hydrothermaler Tätigkeit ist der Kivusee. Sedimente der letzten 15 000 Jahre weisen einen deutlichen hydrothermalen Fingerabdruck in der Form schichtgebundener Erzlagen auf. Die Sedimentabfolge zeigt einen mehrfachen Wechsel von Perioden vulkanisch-hydrothermaler Tätigkeit und Stille. Diese stimmen zeitlich überein mit Pluvial- bzw. Interpluvialzeiten. Wir vermuten, daß die hydrothermalen Lösungen ehemalige Regenwässer sind, die beim Durchfluß durch das vulkanische Nebengestein Salze und Metalle aufgenommen haben. Die Anwesenheit von vulkanischem CO₂ hat die Auslaugungsprozesse verstärkt.

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Résumé La composition chimique des lacs de la vallée du «Western Rift» est contrôlée par des émanations de solutions hydrothermales salines et métallifères. Le centre de l'activité actuelle est le lac Kivu. Dans les sédiments des derniers 15000 ans ces émanations hydrothermales ont formé des enrichissements minéralisés stratiformes. Les annales géologiques indiquent des périodes d'activité et de quiétude volcaniques et hydrothermales qui coïncident avec la séquence climatique africaine d'époques pluviales et interpluviales. On conclût que les solutions hydrothermales sont des eaux météoriques qui ont dérivé leur charge de métal et de sel en traversant les roches volcaniques encaissantes. La présence du CO₂ volcanique a facilité cet enrichissement.

     

Isotopic evolution of common lead

The isotopic composition of lead from galenas and feldspars of granitic rocks covering a time span of 3600 m.y. is reviewed. Hew data are combined with data from the literature, all ratios being normalized to a fixed value for the isotopic composition of lead in an inter-laboratory reference sample. Comparison of feldspar and galena results show that the isotopic composition of initial lead in granitic rocks may be identical to, or more radiogenic than, that in coeval stratiform (or ‘single stage’) ore deposits. The isotopic evolution of terrestrial lead over the past 3600 m.y. deduced from the stratiform ore data does not fit the closed system source. The data are best fitted to evolution models in which the value of μ (U²³⁸/Pb²⁰⁴ normalized to the present day) in the source has increased either over approximately the past 3.6 billion years or since the time of formation of the Earth. A limited number of high precision data for strontium likewise indicate open system isotopic evolution. In this system the Rb/Sr ratio appears to have decreased as a function of time. Physical models that explain the data are presented. Either mantle differentiation or crustal contamination processes could account for the change in μ in the source for the lead. The strontium data and some detailed studies of the isotopic composition of lead in oceanic volcanic rocks support mantle differentiation rather than crustal contamination. The exact nature of the mantle differentiation process cannot be specified, although empirical data favoring the existence of such a process are given. The best estimate of the age of the Earth, calculated with the data from 2750 and 3290 m.y.-old lead ores, is 4660 m.y.     

Meteorological fields from remote infrared sensing

Modern numerical weather prediction techniques require global observations of the atmospheric state and structure parameters. The current meteorological observing system, which is based on radiosonde balloon observations, has extensive gaps. Remote sensing of the Earth atmosphere emission spectrum from satellites can fill these gaps. The physical basis for extracting information on meteorological fields from such remote observations is explained. The problem reduces to that of solving a linear Fredholm equation of the first kind in the presence of noisy data. There is no unique solution to such a problem. The mathematical techniques-inversion techniques-that are currently used to solve the problem are reviewed. Examples are given of meteorological fields obtained from remote infrared sensing from satellites. Results indicate that meteorological parameters such as temperature and geopotential height of constant pressure surfaces can be measured-in conditions of clear skies-to accuracies approaching that of the radiosonde system. Other meterological variables, e.g., water vapor and ozone, can be determined to a lesser degree of accuracy. Applications of the remotely sensed fields are described. Problem areas and suggested solutions are discussed.     

Synoptic and mesoscale weather conditions during air pollution episodes in Athens,Greece

Based on regular climatological and air quality data from the Greater Athens Area (GAA), the air pollution episodes observed in Athens during the period 1983–1990 were analysed and classified. The main characteristics of atmospheric conditions during days with high air pollution concentrations are summarized too. Model simulations show that the worst air pollution episodes in Athens occur during days with a critical balance between synoptic and mesoscale circulations and/or during days with warm advection in the lower troposphere.     

Geomorphology and hydrology of upper sinking cove,cumberland plateau,Tennessee

Upper Sinking Cove, dissecting the eastern escarpment of the Cumberland Plateau, is characterized by a multiple aquifer, predominantly vadose hydrologic system with minor surface components. There is a central trunk channel along the axis of the cove and a network of independent tributaries. Aquitards within the limestones, particularly Hartselle Formation shales, have influenced both cave and surface landform development by perching ground waters and slowing the vertical growth of closed depressions. Long-term solutional denudation in the portion of the cove underlain by limestones (40 per cent) is an estimated 56 mm per 1000 years, suggesting that karst development began 15–16 million years ago. Despite lower soil CO₂ and spring water hardness, 61 per cent of annual denudation occurs in the six winter months when 76 per cent of yearly runoff occurs. Landform development in Upper Sinking Cove appears to have begun as stream erosion carved a valley first in the sandstone caprock of the escarpment and later in the underlying Pennington Formation limestones containing numerous shale layers which promoted surface stream flow. Eventually stream erosion exposed the massive Bangor limestones which allowed deep ground water flow. Surface streams were pirated underground with the eventual formation of the chain of three closed depressions which constitute Upper Sinking Cove.     

Earth orientation determinations by short duration VLBI observations

Summary In May 1989 and April 1990 the radio telescopes of the Wettzell Geodetic Fundamental Station in Germany and of the Shanghai Observatory near Seshan in China observed two series of daily VLBI experiments of short duration for precise determination of UT1. In 1990 a few experiments were complemented by the Hartebeesthoek Radio Astronomy Observatory in South Africa and the Medicina telescope of the Bologna Istituto di Radioastronomia in Italy. Employing the South African station together with the east-west baseline formed by the observatories of Seshan and Medicina permitted simultaneous determinations of UT1 and polar motion. Here we report on the results of these observations. Comparing the UT1 results with those of the IRIS Intensive series gives a clear indication of the absolute accuracy of such short duration VLBI measurements which is estimated to be of the order of ±60µs.