Geophysical surveys

This paper includes a discussion of the validity of theobjections which are usuallyraised against the physical plausibility of the correlationsbetween extraterrestrialphenomena and changes in circulation in the lower atmosphere.The behaviour of the winterlower troposphere in the Northern Hemisphere was analysed bothon a month-to-month timescale and on a day-to-day time scale. The mechanisms which cansatisfactorily explainthe behaviour of the winter lower troposphere on these timescales have been described.The basic features of the results presented were explained bymeans of a mechanism basedon the propagation of planetary waves. Attention was focussedon studying the connectionsamong the changes in the pressure and temperature fields, thechanges in solar/geomagnetic activity and QBO phases by the method of composites. It hasbeen found that the compositeswithout considering QBO phases are similar to the QBO-eastpatterns under the highestactivities. Pressure deviations during QBO-west high solaractivity or low geomagneticactivity and temperature deviations during QBO-east/west lowgeomagnetic activity provedto be of negligible statistical significance.

The discovery of an excess of Ir and other noble metals in approximately chondritic proportions at numerous sites, synchronous with the Cretaceous/Tertiary (K/T) boundary, provided hard evidence for earlier suggestion that extraterrestrial envents may have coincided with many global geologic boundaries. Further support for the impact hypothesis arose from the identification of abundant microspherules and shocked quartz in K/T boundary marine and nonmarine deposits, respectively, and the discovery of an apparent periodicity in the megaextinction event record. However continuing doubts about the impact origin of the K/T spherical forms and associated geochemical anomalies, uncertainty about the range and extent of the shocked quartz distribution, and the lack of a large, suitably-aged crater, has led to an alternative hypothesis calling upon purely endogenic processes to account for the boundary layer phenomena. Advocates of this opposing theory generally call upon large scale, possibly periodic injection of mantle material directly into the lithosphere. While a number of arguments can be made from the the existing data base for and against both competing theories, other evidence suggests that additional phenomena, including sudden changes in the physical and chemical conditions of the oceans, natural combustion on a global scale, and/or widespread sediment-contaminated volcanism, may have contributed to the dramatic transition recorded in the K/T boundary rock record.

Unlike current manned systems, NASA's next generation SLR2000 Satellite Laser Ranging (SLR) station is fully autonomous, eye-safe, relatively compact and inexpensive, and, during daytime tracking, operates at signal-to-noise ratios several orders of magnitude below unity. Tiny, passivelyQ-switched microlasers generate ultra-short pulses with output energies on the order of 100 μJ at few kHz rates to achieve mm-levelranging precision to satellite altitudesof 20,000 km. Special ranging receivers, combined with Poisson statistical analysis of the received photon distribution, enable the system to rapidly and reliably identify and extract the single photon laser echoes from the solar background. The enhanced rate of return, combined with a uniform signal strength, can actually drive down both systematic and random range errors. The new SLR2000 technology has already spawned exciting new applications. Compact microlaser altimeters, capable of mapping the surface of a planet or other celestial body at multikilohertz rates, is one such application, and a high altitude, airborne version is currently being developed under NASA's Instrument Incubator Program. Interplanetary microlaser transponders would be capable of performing decimeter ranging or subnanosecond time transfer to spacecraft throughout the inner Solar System, resulting in improved knowledge of planetary motions and librations and enhanced General Relativity experiments.

Two types of systems of units have been formed. The commercial is a system of weights and measures, such as the metric, defined by civil law. The scientific is a system of physical units generated through use of scientific laws which link corresponding quantities. The International System of units, denoted SI, was established in 1960 by the General Conference of Weights and Measures. The set of SI units included therein is the only comprehensive, coherent system of physical units. There is only one SI unit, whose value is unambiguous, for each physical quantity. Included in the SI, but not in any CGS system, are the volt, ampere, ohm, and watt, which are the only units used for measurement of the corresponding electrical quantities. Also included in the SI are prefixes used to form non-coherent, decimal multiples and sub-multiples of SI units. A few other units, such as the day, degree of arc, and degree Celsius may be used with SI units. Precepts for use of the SI, conversion tables, and geophysical constants expressed in SI units are given.

A two-dimensional thermo-mechanicalplane-strain finite element model forsnow is presented. Snow is modeled asa two component porous medium consisting ofa solid ice matrix and interstitial pore air.The ice and air phases are not always in thermalequilibrium. Therefore, heat transport is governedby two non-stationary energy conservation equationswhich are coupled by free convection heat exchangesat the interfacial ice-air boundary. The icematrix deforms viscoelastically according to anexperimentally-based temperature dependent constitutivelaw. Creep deformation rates are governed by a powerlaw with a density dependent exponent n.The highly nonlinear character of the mechanical modelis illustrated by simulating snowcovers with layersof variable height and density. Weak layerinterfaces – believed to be the location of initiationof snow slab fracture – are modeled using specialfinite elements which transfer normal stresses buthave little or no shear resistance. Stress andstrain-rate concentrations at the boundaries ofweak zones are calculated and compared withbrittle fracture strain-rates.

The two last decades of research in paleoceanography are characterized by extensive data collection and by the development of numerous hypotheses based on these data. Both the nature of the thermohaline circulation and the nature of the surface circulation during past times are currently being debated. Physical models of the ocean circulation are beginning to play an important role in these debates. The models range from highly simplified mathematical representations intended to illustrate physical principles to three-dimensional time-dependent global models intended to simulate temperature, salinity and flow fields. At the very least such models can be applied to explore the limitations or appropriateness of various assumptions in paleoceanography. At the very best, paleoceanographic data may be used as a verification of simulations of past oceans.

As part of the European project CADZIE, the interactionbetween powder avalanches and dams has been studied. First, we simulated a powder avalancheusing a heavy salt solution in a water tank. These experiments showed that (1) the ratio betweenthe maximum velocity parallel to the slope and the front velocity is greater than 1 (up to 1.5),and that (2) the velocity normal to the slope can rise to 74% of the front velocity. The presence ofa dam leads to a reduction of front velocity. The ratio between the maximum horizontal velocityand the front velocity increases with the dam height and reaches a maximum value of 1.9. Aswe did not have access to the density distribution in our experimental set-up, we calibrateda numerical model thanks to the experiments and we deduced the dynamic pressure. Our resultsshowed that, for an avalanche without obstacle, AVAER (Rapin, 1995) and Beghin and Closet'shypothesis (1990) lead to an overestimation of the dynamic pressure in the lower partof the flow as well as in the upper part, and that the use of front velocity for estimating the lossof kinetic energy leads to an overestimation of the dam's effectiveness.