ALBERT

Notizen: The paper discusses the variations of Elevation Correction Factors (E.C.F.) across various stratigraphical and structural occurrences with particular reference to the dipping bed, fault and horst. Graphs are prepared giving E.C.F. variations for various angles of dip and a method is suggested for application in the field. Errors in the linear variation that is commonly used for small angles of dip are discussed.The second part of the paper consists of the application to interpretation of (I) Change of gravity dip and (2) The gravity inversion. It is shown how from a gravity map the dip, position of outcrop and thickness of a dipping bed can be determined. The last part shows how anticlines can give rise to negative anomalies and that in the direction of dip similar structures can at one point have no surface expression and afterwards invert to positive anomalies. The danger of attempting to predict the size of structures in such areas is emphasised.

Notizen: When gravimetric prospecting is concerned with a region featuring broken relief the calculation of topographical corrections becomes vitally important, for these rectify apparent anomalies completely. It is essential that they should be calculated as exactly as possible. By the classic method this takes an extremely long time and may in the long run involve an outlay actually higher than that expended on operations on the terrain.It has thus been felt necessary to seek methods which would speed up calculation, and, bearing that in mind, the use of a polar integrator has been envisaged. With this the contour lines of a map can be followed between two fixed radii, the pole corresponding to the location of the position. Two integrator roulettes will give the values of the first two terms of the expansion in terms of the quotient of the height divided by the distance, the second of the terms resulting from the curvature of the earth. Unless there are escarpments very close to the position, the use of these two terms will be sufficient.Polar integration will deal successively with different contour lines and a subsidiary graph will be plotted, giving the value of each of the integrals in terms of the height. By means of this subsidiary graph the values of the corrections will be obtained with a planimeter and a moment integrator. It is envisaged that the same operation could be repeated in the case of two maps with different scales, for instance, 1–20,000 and 1–200,000 the effect of the areas in the immediate neighbourhood of the position being taken into account by the observer. It is hoped that in this way the calculation of topographical corrections will be made quicker, more exact and less laborious.

Notizen: The problems connected with reflexion surveys in areas of rough topography consist of choosing proper equipment and of making careful near-surface corrections. This paper illustrates some solutions to these problems that have been adopted in a survey which has recently started in a hily coastal area, in Southern Italy.As the area is characterized by a youthful and very actively eroded topography of Post-Miocene clays and litoral clastic sediments, both rotary and percussion drilling were necessary whilst all the seismic equipment had to be light to allow straight lines to be shot as far as possible. Details are given of the truckmounted, light equipment which has been designed for this special purpose.In connection with the youthful topography of the area being surveyed, the criteria for the proper computation of the near-surface corrections is discussed. The great differences in elevation, combined with variations in the surface and near-surface materials, necessitated a careful analysis before deciding the thickness of the correction zone. The results are shown of the preliminary tests carried out in order to determine the average thickness of the low-velocity zone. The solution adopted for drawing the reference surface determining the base of the correction zone for the whole of the area is subseqently explained.Reference datum is drawn so as to follow gently, and with segments of constant slopes, the general features of the rough topographic relief at a depth ranging from 30 to 60 metres from the surface. Criteria for the routine checking of velocity variations in the correction zone are illustrated. Such checks are based on both uphole time analyses and refraction “first breaks” plots, with deeper shot holes at constant intervals.Unsatisfactory results necessitate longer spreads for “low velocity tests” and uphole shooting from deeper holes and the reference datum is deepened for the particular area where this becomes necessary.The final cross-sections are shown and although, as yet, no deep holes, are available to check the seismic data, the general appearance of the cross-sections seems to indicate that subsurface data are free from the influence of topography and of the velocities in the near-surface formations.

Notizen: The paper describes the determination of the chlorine content of deep ground water by means of the direct resistivity method using the Wenner electrode configuration. The purpose was to investigate the distribution of salinity in the brackish waters within a permeable sand of thickness 250 metres and which forms the deeper subsoil of the Ysselmeer areas.Such a knowledge is of great value, firstly because the seepage of salt water into the proposed deep “polders” may reduce the crop yield, and secondly because it allows areas, where potable waters may occur, to be delineated.To test the method, measurements were made near existing boreholes which extended to a depth of 350 metres. From these borings water samples from various depths were collected and it was found that the resistivity pl of the porewater was closely related to their chlorine content. The resistivity p of the saturated sand was determined from the geo-electrical measurements. It was found that the resistivity p was given by p= 4, 5 pl. From this relation, which is independent of the salinity, the chlorine content of the porewater could be estimated. These results are in close agreement with those of other observers. In the area p varied from 1 to 200 ohmmetres corresponding to a chlorine content changing from 12.000 to 15 mg per litre.The method was adapted for working over water. Each of fourteen insulated conductors in a special cable terminated in an electrode, and these electrodes were spaced along the cable so that nine different Wenner spacings could be selected. The electrode separations varied from 44 metres up to 306 metres.

Notizen: The authors develop an optimum filtering theory for seismic reflection recording. A criterion of record improvement is applied to a mathematical model consisting of a set of wavelets superimposed on a random noise backgruond. This leads to the design of electrical filters and geophone and shot-hole patterns with optimum filtering characteristics.

Notizen: Errors in acoustic logs have a significant influence on the assessment of formations, the description of oil deposits and modelling using synthetic seismograms. The elimination of these errors is a problem that many researchers have been trying to solve for many years. We derive theoretical formulae for calculating the radial depth of investigation of acoustic logging, and analyse the relationship between the error in acoustic interval transit time and the altered zone thickness. We explain in detail how mud affects the acoustic interval transit time. We also describe the variation of altered zone thickness with time and a method for calculating the altered zone thickness. Finally, we present a set of new methods that can be directly applied to correct the disturbing influence on acoustic curves. Our results are obtained based on the fact that acoustic velocities in the altered zone show a small lateral variation. Examples are given of analysing the acoustic log data, which are collected from a BS well at three different times. This paper is significant both for selection of the acoustic logging device with proper transmitter-to-receiver distance (spacing) and improvement of the device.

Notizen: We characterize the seismic response of Lake Vostok, an Antarctic subglacial lake located at nearly 4 km depth below the ice sheet. This study is relevant for the determination of the location and morphology of subglacial lakes. The characterization requires the design of a methodology based on rock physics and numerical modelling of wave propagation. The methodology involves rock-physics models of the shallow layer (firn), the ice sheet and the lake sediments, numerical simulation of synthetic seismograms, ray tracing, τ–p transforms, and AVA analysis, based on the theoretical reflection coefficients. The modelled reflection seismograms show a set of straight events (refractions through the firn and top-ice layer) and the two reflection events associated with the top and bottom of the lake. Theoretical AVA analysis of these reflections indicates that, at near offsets, the PP-wave anomaly is negative for the ice/water interface and constant for the water/sediment interface. This behaviour is shown by AVA analysis of the synthetic data set. This study shows that subglacial lakes can be identified by using seismic methods. Moreover, the methodology provides a tool for designing suitable seismic surveys.

Notizen: A walkaround VSP is a novel acquisition geometry comprising multiple sources azimuthally distributed around a fixed multi-component receiver at approximately the same offset. Such a configuration allows the robust measurement of fracture-induced anisotropy by the analysis of certain seismic attributes with respect to azimuth.

Notizen: We investigate the interactions between the elastic parameters, VP, VS and density, estimated by non-linear inversion of AVA data, and the petrophysical parameters, depth (pressure), porosity, clay content and fluid saturation, of an actual gas-bearing reservoir. In particular, we study how the ambiguous solutions derived from the non-uniqueness of the seismic inversion affect the estimates of relevant rock properties. It results that the physically admissible values of the rock properties greatly reduce the range of possible seismic solutions and this range contains the actual values given by the well. By means of a statistical inversion, we analyse how approximate a priori knowledge of the petrophysical properties and of their relationships with the seismic parameters can be of help in reducing the ambiguity of the inversion solutions and eventually in estimating the petrophysical properties of the specific target reservoir. This statistical inversion allows the determination of the most likely values of the sought rock properties along with their uncertainty ranges. The results show that the porosity is the best-resolved rock property, with its most likely value closely approaching the actual value found by the well, even when we insert somewhat erroneous a priori information. The hydrocarbon saturation is the second best-resolved parameter, but its most likely value does not match the well data. The depth of the target interface is the least-resolved parameter and its most likely value is strongly dependent on a priori information. Although no general conclusions can be drawn from the results of this exercise, we envisage that the proposed AVA–petrophysical inversion and its possible extensions may be of use in reservoir characterization.

Notizen: The main objective of the AVO inversion is to obtain posterior distributions for P-wave velocity, S-wave velocity and density from specified prior distributions, seismic data and well-log data. The inversion problem also involves estimation of a seismic wavelet and the seismic-noise level. The noise model is represented by a zero mean Gaussian distribution specified by a covariance matrix. A method for joint AVO inversion, wavelet estimation and estimation of the noise level is developed in a Bayesian framework. The stochastic model includes uncertainty of both the elastic parameters, the wavelet, and the seismic and well-log data. The posterior distribution is explored by Markov-chain Monte-Carlo simulation using the Gibbs' sampler algorithm. The inversion algorithm has been tested on a seismic line from the Heidrun Field with two wells located on the line. The use of a coloured seismic-noise model resulted in about 10% lower uncertainties for the P-wave velocity, S-wave velocity and density compared with a white-noise model. The uncertainty of the estimated wavelet is low. In the Heidrun example, the effect of including uncertainty of the wavelet and the noise level was marginal with respect to the AVO inversion results.

Notizen: We use a poroelastic modelling algorithm to compute numerical experiments on wave propagation in a rock sample with partial saturation using realistic fluid distribution patterns from tomography scans. Frequencies are in the range 10 to 500 kHz. The rock is a homogeneous isotropic sandstone partially filled with gas and water, which are defined by their characteristic values of viscosity, compressibility and density. We assume no mixing and that the two different pore-fills occupy different macroscopic regions. The von Kármán self-similar correlation function is used, employing different fractal parameters to model uniform and patchy fluid distributions, respectively, where effective saturation is varied in steps from full gas to full water saturation.Without resorting to additional matrix–fluid interaction mechanisms, we are able to reproduce the main features of the variation in wave velocity and attenuation with effective saturation and frequency, as those of published laboratory experiments. Furthermore, the behaviour of the attenuation peaks versus water saturation and frequency is similar to that of White's model. The conversion of primary P-wave energy into dissipating slow waves at the heterogeneities is shown to be the main mechanism for attenuating the primary wavefield. Fluid/gas patches are shown to affect attenuation more than equivalent patches in the permeability or solid-grain properties.

Notizen: 3D Kirchhoff migration (KM) smears a trace's time sample along a quasi-ellipsoid in the model space. This is a costly and sometimes noisy process as reflection energy is smeared far away from the actual reflector position, introducing far-field migration artefacts. As a reduced form of 3D KM, 3D wavepath migration (WM) smears a picked reflection arrival to a small Fresnel zone portion centred about the specular reflection point, leading to fewer migration artefacts and reduced computation time. Both the traveltime and the angle of incidence are required by WM for locating the specular reflection point. Our results with 3D prestack synthetic data show that WM generates fewer migration artefacts and can sometimes define complex structure better than KM. Our results with 3D prestack field data show that WM can mostly suppress migration artefacts and can sometimes resolve reflection interfaces better than KM. The CPU comparison shows that, for both the synthetic and field data examples, WM can be more than an order of magnitude faster than KM. The limitation with 3D WM is that the angle of incidence calculation is sensitive to the recording geometry and the signal-to-noise (S/N) ratio, which can lead to blurred images.

Notizen: An accurate and efficient 3D finite-difference (FD) forward algorithm for DC resistivity modelling is developed. In general, the most time-consuming part of FD calculation is to solve large sets of linear equations: Ax=b, where A is a large sparse band symmetric matrix. The direct method using complete Choleski decomposition is quite slow and requires much more computer storage. We have introduced a row-indexed sparse storage mode to store the coefficient matrix A and an incomplete Choleski conjugate-gradient (ICCG) method to solve the large linear systems. By taking advantage of the matrix symmetry and sparsity, the ICCG method converges much more quickly and requires much less computer storage. It takes approximately 15 s on a 533 MHz Pentium computer for a grid with 46 020 nodes, which is approximately 700 times faster than the direct method and 2.5 times faster than the symmetric successive over-relaxation (SSOR) conjugate-gradient method. Compared with 3D finite-element resistivity modelling with the improved ICCG solver, our algorithm is more efficient in terms of number of iterations and computer time. In addition, we solve for the secondary potential in 3D DC resistivity modelling by a simple manipulation of the FD equations. Two numerical examples of a two-layered model and a vertical contact show that the method can achieve much higher accuracy than solving for the total potential directly with the same grid nodes. In addition, a 3D cubic body is simulated, for which the dipole–dipole apparent resistivities agree well with the results obtained with the finite-element and integral-equation methods. In conclusion, the combination of several techniques provides a rapid and accurate 3D FD forward modelling method which is fundamental to 3D resistivity inversion.

Notizen: We present results from the resolution and sensitivity analysis of 1D DC resistivity and IP sounding data using a non-linear inversion. The inversion scheme uses a theoretically correct Metropolis–Gibbs' sampling technique and an approximate method using numerous models sampled by a global optimization algorithm called very fast simulated annealing (VFSA). VFSA has recently been found to be computationally efficient in several geophysical parameter estimation problems. Unlike conventional simulated annealing (SA), in VFSA the perturbations are generated from the model parameters according to a Cauchy-like distribution whose shape changes with each iteration. This results in an algorithm that converges much faster than a standard SA. In the course of finding the optimal solution, VFSA samples several models from the search space. All these models can be used to obtain estimates of uncertainty in the derived solution. This method makes no assumptions about the shape of an a posteriori probability density function in the model space. Here, we carry out a VFSA-based sensitivity analysis with several synthetic and field sounding data sets for resistivity and IP. The resolution capability of the VFSA algorithm as seen from the sensitivity analysis is satisfactory. The interpretation of VES and IP sounding data by VFSA, incorporating resolution, sensitivity and uncertainty of layer parameters, would generally be more useful than the conventional best-fit techniques.

Notizen: The nth-order moments of the electromagnetic impulse response are useful for interpreting electromagnetic data. We have derived an analytic expression for the half-order moment of a conductive half-space. By inverting this expression, the measured half-order moment can be used to estimate an apparent conductivity of the ground. The first-order moment can also be used to estimate the half-space conductivity. A sensitivity analysis indicates that for an airborne EM configuration, the half-order moment will be most sensitive to material in the top 26–48 m, while the first-order moment will be sensitive to deeper material (down to depths between 66 and 127 m).

Notizen: Spectral factorization is a computational procedure for constructing minimum-phase (stable inverse) filters required for recursive inverse filtering. We present a novel method of spectral factorization. The method iteratively constructs an approximation of the minimum-phase filter with the given autocorrelation by repeated forward and inverse filtering and rearranging of the terms. This procedure is especially efficient in the multidimensional case, where the inverse recursive filtering is enabled by the helix transform.To exemplify a practical application of the proposed method, we consider the problem of smooth two-dimensional data regularization. Splines in tension are smooth interpolation surfaces whose behaviour in unconstrained regions is controlled by the tension parameter. We show that such surfaces can be efficiently constructed with recursive filter preconditioning and we introduce a family of corresponding two-dimensional minimum-phase filters. The filters are created by spectral factorization on a helix.

Notizen: Fractured rock is often modelled under the assumption of perfect fluid pressure equalization between the fractures and equant porosity. This is consistent with laboratory estimates of the characteristic squirt-flow frequency. However, these laboratory measurements are carried out on rock samples which do not contain large fractures. We consider coupled fluid motion on two scales: the grain scale which controls behaviour in laboratory experiments and the fracture scale. Our approach reproduces generally accepted results in the low- and high-frequency limits. Even under the assumption of a high squirt-flow frequency, we find that frequency-dependent anisotropy can occur in the seismic frequency band when larger fractures are present. Shear-wave splitting becomes dependent on frequency, with the size of the fractures playing a controlling role in the relationship. Strong anisotropic attenuation can occur in the seismic frequency band. The magnitude of the frequency dependence is influenced strongly by the extent of equant porosity. With these results, it becomes possible in principle to distinguish between fracture- and microcrack-induced anisotropy, or more ambitiously to measure a characteristic fracture length from seismic data.

Notizen: We consider a layered heterogeneous viscoelastic transversely isotropic medium with a vertical symmetry axis (a viscoelastic TIV medium) and parameters that depend on depth only. This takes into account intrinsic attenuation, anisotropy and thin layering. The seismic wavefield is decomposed into up- and downgoing waves scaled by the vertical energy flux. This scaling gives important symmetry relationships for both reflection and transmission (R/T) responses. For a stack of homogeneous layers, the exact reflection response can be computed in a numerically stable way by a simple layer-recursive algorithm. We derive exact plane-wave R/T coefficients and several linear and quadratic approximations between two viscoelastic TIV media, as functions of the real-valued horizontal slowness. The approximations are valid for pre- and post-critical values of horizontal slowness provided that the proper complex square roots are used when computing the vertical slowness. Numerical examples demonstrate that the quadratic approximations can be used for large differences in medium parameters, while the linear approximations can be used for small differences. For weak anisotropy it is sufficient to use an isotropic background medium, while for strong anisotropy it is necessary to use a weak TIV or TIV background medium. We also extend the O'Doherty–Anstey formula to the P- and SV-wave transmission responses of a stack of viscoelastic TIV layers, taking into account intrinsic attenuation, anisotropy and thin layering.

Notizen: The first-order perturbation theory is used for fast 3D computation of quasi-compressional (qP)-wave traveltimes in arbitrarily anisotropic media. For efficiency we implement the perturbation approach using a finite-difference (FD) eikonal solver. Traveltimes in the unperturbed reference medium are computed with an FD eikonal solver, while perturbed traveltimes are obtained by adding a traveltime correction to the traveltimes of the reference medium. The traveltime correction must be computed along the raypath in the reference medium. Since the raypath is not determined in FD eikonal solvers, we approximate rays by linear segments corresponding to the direction of the phase normal of plane wavefronts in each cell. An isotropic medium as a reference medium works well for weak anisotropy. Using a medium with ellipsoidal anisotropy as a background medium in the perturbation approach allows us to consider stronger anisotropy without losing computational speed. The traveltime computation in media with ellipsoidal anisotropy using an FD eikonal solver is fast and accurate. The relative error is below 0.5% for the models investigated in this study. Numerical examples show that the reference model with ellipsoidal anisotropy allows us to compute the traveltime for models with strong anisotropy with an improved accuracy compared with the isotropic reference medium.

Notizen: Measurements of seismic anisotropy in fractured rock are used at present to deduce information about the fracture orientation and the spatial distribution of fracture intensity. Analysis of the data is based upon equivalent-medium theories that describe the elastic response of a rock containing cracks or fractures in the long-wavelength limit. Conventional models assume frequency independence and cannot distinguish between microcracks and macrofractures. The latter, however, control the fluid flow in many subsurface reservoirs. Therefore, the fracture size is essential information for reservoir engineers. In this study we apply a new equivalent-medium theory that models frequency-dependent anisotropy and is sensitive to the length scale of fractures. The model considers velocity dispersion and attenuation due to a squirt-flow mechanism at two different scales: the grain scale (microcracks and equant matrix porosity) and formation-scale fractures. The theory is first tested and calibrated against published laboratory data. Then we present the analysis and modelling of frequency-dependent shear-wave splitting in multicomponent VSP data from a tight gas reservoir. We invert for fracture density and fracture size from the frequency dependence of the time delay between split shear waves. The derived fracture length matches independent observations from borehole data.

Notizen: The paper describes and discusses the results of an experimental gravity survey which was carried out underground on different levels of a mine, in the mine shafts, and on the surface above the mine workings.The paper is composed of three complementing sections. The part dealing with gravity measurements in the shafts gives also attention to the particular problem of the terrain corrections underground, due to the surface topography. The interval densities from gravity measurements in the shafts are computed and adjusted in accordance with known geology and compared with the stratigraphical columns of the shafts. The effect of the ore body on the stations in the shaft is derived theoretically and compared with the observed one.The gravity contours are constructed on different levels in the mine workings and discussed in relation to the known extent of the ore body. The gravity profile across a fault underground is presented and discussed. Another gravity profile was run underground in the same plan position as a surface traverse 1000 feet above it. The line of boreholes along this traverse gives good account of geology which includes step-faulting. This known geology is compared with the deductions based on the gravity results. This is also done in the case of another gravity profile run over a known geological section. A number of gravity measurements were also taken in the same plan position, separated by the vertical distance of 800–1,100 feet. These points were placed by the boreholes previously drilled in the area. Attempt in correlation of these and gravity results is made.The densities computed from the gravity measurements are compared with the laboratory determinations of the densities, carried out on samples from different parts of the mine.The contours on the top of the base formation are constructed from the information obtained from the boreholes, and are compared with the gravity contours on the surface above.A simple method of computation of the effects of slabs and blocks is presented as applied to the calculation of the corrections for underground drifts and faults. A table is appended for use with this method.

Notizen: The gravity difference between two stations, one at the surface and the other underground vertically below the former and at a given distance from it, depends on the mean density of the earth, σm as well as on the density of the layer of rock contained between the two stations. When one of these densities is known, the other can be computed from this gravity difference. The reliability of this determination depends on the relative accuracies with which σm and σ can be obtained.These accuracies are discussed in the paper. The mean density of. the earth has been determined with an accuracy of approximately 0.01 gr/cm3. The determination of the density of a layer of rock depends on density determinations of rock samples which are not representative of the layer as a whole. Thus the accuracy of the value of σ based on a number of sample determinations will depend on many factors, including the method of averaging the density values obtained from the samples and the degree of uniformity in the geology.To investigate the problem discussed above, three sets of gravity measurements were made under differing conditions. In each instance a number of pairs of stations vertically above each other were occupied on the surface and underground. The results computed from the data on each pair of stations in a set of measurements were considered as repeated measurements of the same quantity, and the most probable value was calculated.The results demonstrated that the accuracy varied with the conditions prevailing in the area where the observations were made. In Godstone Quarries the dip of the strata was negligible, the rocks fairly uniform and structural conditions undisturbed. Consequently, although the rock layer between the surface and underground stations was only of the order of a hundred feet, the mean density of the earth computed from the average density of the rock samples, was very close to the accepted standard value of 5.52 gr./cm3. This agreement, however, was easily upset when only one random sample density was assumed as representative of a given formation.In a different locality in Cumberland the observations were made in a mine and on the surface. The rock layer between the surface and the underground stations was approximately a thousand feet thick. One set of measurements followed a line parallel to a fault, the other a line crossing this fault. The results differed appreciably from the standard value of σm, particularly in the latter case.It is concluded that the gravity difference between a surface and an underground station can be used satisfactorily to determine the average density of a rock layer in situ and en bloc, using the standard value for the mean Earth density.

Notizen: In this paper we describe a protractor which provides a very simple method of constructing refracted rays in a vertical plane. Another advantage of this apparatus is that it can be made locally in any computing office.

Notizen: A method is described for computing master resistivity curves. It is a modification of that devised by Ehrenburg and Watson, but is superior as a shorter time is required for the computations. A collection of 31 master curves for the three layer case is presented.

Notizen: On the basis of results of extensive geoelectrical investigations in East Frisia as well as of theoretically calculated data, it is shown to what extent geoelectrics can furnish a contribution to success during the preliminary work for the establishment of water works, or for the enlargement of existing water works, in coastal areas. One of the most important tasks in this sphere is the search for permeable beds suitable for intake, the delimitation of their thickness and extent (catchment areas), and then the location of favourable places for wells. This task is rather difficult because of thickness variations in and change of facies of the old-pleistocene basin clay (Lauenburg clay) and the presence of Tertiary clay and fine clayey sands which, acting as impervious layers, often divide a permeable bed into several storage horizons. Further difficulties arise from the occurrence of brine bearing deposits near the coast, and water from the depths with a high salt content as a result of leaching processes around salt domes.The problems arising in this connection are illustrated with the aid of examples from the Rheiderland. The following questions seem to be the most important:a) the physical interpretation of the measurements with special regard to the principle of equivalence;b) the correlation of the physical data with the subsurface geology.

Notizen: This paper describes a highly portable geophone, designed for large-scale refraction surveying, which uses a novel suspension system to obtain a natural frequency of about 1.85 c/s. The nominal output is 0.7 volts/cm/second into a 500 ohm load, the damping then being 0.7 of critical. Some typical recorded arrivals obtained with this geophone are illustrated.

Notizen: To improve the ratio of signal to noise resulting from the shot, seismologists have increased the number of geophones in each recording channel and have also increased the length of each spread. As a rule, however, the effectiveness of this multiple recording is based on theoretical considerations, and it appeared desirable to carry out an experimental investigation to obtain a direct measurement of the reduction in noise as a function of the geophone pattern etc. The work was done in the Landes where experience had shown that recording was difficult.The experimental arrangement, which allowed the accurate measurement of the noise without interference from reflected energy, is described.Shots, repeated under apparently identical conditions, revealed the random nature of the noise and pointed to the necessity of a large number of experiments to yield an accurate result. Two geophone patterns were employed, one of 12 geophones in a straight line and the other of two parallel lines of geophones, the directions of the geophone lines being either along the shot line, perpendicular to the shot line or at some intermediate angle.The results obtained, in two widely spaced locations were in good agreement and allowed definite conclusions to be drawn. When the 12 geophone line is long enough the noise is reduced by a factor of 0.36 and with two lines, each 55 metres long and containing 12 geophones, the reduction was 0.29, if the lines were sufficiently widely separated. The results agree with the factor K N-½ for the reduction in noise where N is the number of geophones per trace and K is a factor close to unity. The results also show the influence of the length of the geophone line its orientation and its mean distance from the shot.

Notizen: For the first time a method of observing the effects of hammering and of small blasts has been applied in a series of experiments on various soils, using a three component set of small mechanical leaf spring seismographs. The seismograms have been subjected to a thorough harmonic analysis. By means of these experiments, the theoretical concept of Rayleigh waves generated in a layer overlying a semi infinite medium has been confirmed. The dispersion curves resulting from different experiments correspond well with theoretical dispersion curves.It may be. concluded that every seismic pulse generates a specific Rayleigh wave mechanism in loose layers. The depth of penetration of the Rayleigh wave mechanism and its frequency spectrum depend upon the delivery of energy and upon the consistence of the particular soil.Most of the energy delivered is transformed into surface-waves, and it is obvious that the direct reading of pulses in seismograms effects mainly group velocities of the Rayleigh wave mechanism. Thus, all endeavours to procure information about near surface layers of loose soils by refraction seismic work, as the present investigations show, meet with far more difficulties than has been assumed in the past.With regard to the determination of the critical frequencies of the generated surface waves, the method of harmonic analysis of seismograms is superior to the usual dynamic method of soil mechanics, since rotating weights do not give the characteristic phase velocities and transversal wave velocities necessary for judging the consistency of loose soils.Finally, a method of determining the absorption of surface waves is discussed.

Notizen: Book review in this articleA. S. Eve and D. A. Keys. – Applied geophysics in the search for minerals. Cambridge University Press. Fourth edition 1954.

Notizen: Book review in this articleL. W. Sorokin. – Lehrbuch der Geophysikalischen Methoden zur Erkundung von Erdölvorkommen.M. R. J. Wyllie. – The fundamentals of electric log interpretation. Academic Press Inc. – New York, Sept. 1954. Price $ 3.60.

Notizen: The problem is considered of how to determine, from dip shooting data, the position in space of a reflecting horizon located underneath a refracting interface between two media of different velocities.The author proposes a combination between a geometric solution and an analytic one, to obtain the quickest result.

Notizen: Calculations are presented of reflection coefficients of plane longitudinal waves incident at oblique angles on boundary planes between elastic media. It is shown that the manner in which these reflection coefficients vary with the angle of incidence is strongly affected by the values of the Poisson's ratios of the two media. It appears that, contrary to a conclusion arrived at by Muskat and Meres, the reflection coefficient may vary appreciably with the angle of incidence in the range between O° and 30°. Possibilities of practical application of this phenomenon are discussed.

Notizen: Abstract Optical single transporter recording (OSTR) is an emerging technique for the fluorescence microscopic measurement of transport kinetics in membrane patches. Membranes are attached to transparent microarrays of cylindrical test compartments (TCs) ~0.1-100 mum in diameter and ~10-100 mum in depth. Transport across membrane patches that may contain single transporters or transporter populations is recorded by confocal microscopy. By these means transport of proteins through single nuclear pore complexes has been recorded at rates of 〈1 translocation/s. In addition to the high sensitivity in terms of measurable transport rates OSTR features unprecedented spatial selectivity and parallel processing. This article reviews the conceptual basis of OSTR and its realization. Applications to nuclear transport are summarized. The further development of OSTR is discussed and its extension to a diversity of transporters, including translocases and ATP-binding cassette (ABC) pumps, projected.

Notizen: Abstract Since mid-1990, with cloning and identification of several families of natural killer (NK) receptors, research on NK cells began to receive appreciable attention. Determination of structures of NK cell surface receptors and their ligand complexes led to a fast growth in our understanding of the activation and ligand recognition by these receptors as well as their function in innate immunity. Functionally, NK cell surface receptors are divided into two groups, the inhibitory and the activating receptors. Structurally, they belong to either the immunoglobulin (Ig)-like receptor superfamily or the C-type lectin-like receptor (CTLR) superfamily. Their ligands are either members of class I major histocompatibility complexes (MHC) or homologs of class I MHC molecules. The inhibitory form of NK receptors provides the protective immunity through recognizing class I MHC molecules with self-peptides on healthy host cells. The activating, or the noninhibitory, NK receptors mediate the killing of tumor or virally infected cells through their specific ligand recognition. The structures of activating and inhibitory NK cell surface receptors and their complexes with the ligands determined to date, including killer immunoglobulin-like receptors (KIRs) and their complexes with HLA molecules, CD94, Ly49A, and its complex with H-2Dd, and NKG2D receptors and their complexes with class I MHC homologs, are reviewed here.

Notizen: Abstract Recent years have witnessed a renaissance of fluorescence microscopy techniques and applications, from live-animal multiphoton confocal microscopy to single-molecule fluorescence spectroscopy and imaging in living cells. These achievements have been made possible not so much because of improvements in microscope design, but rather because of development of new detectors, accessible continuous wave and pulsed laser sources, sophisticated multiparameter analysis on one hand, and the development of new probes and labeling chemistries on the other. This review tracks the lineage of ideas and the evolution of thinking that have led to the actual developments, and presents a comprehensive overview of the field, with emphasis put on our laboratory's interest in single-molecule microscopy and spectroscopy.

Notizen: Abstract The lamba integrase, or tyrosine-based family of site-specific recombinases, plays an important role in a variety of biological processes by inserting, excising, and inverting DNA segments. Flp, encoded by the yeast 2-mum plasmid, is the best-characterized eukaryotic member of this family and is responsible for maintaining the copy number of this plasmid. Over the past several years, structural and biochemical studies have shed light on the details of a common catalytic scheme utilized by these enzymes with interesting variations under different biological contexts. The emergence of new Flp structures and solution data provides insights not only into its unique mechanism of active site assembly and activity regulation but also into the specific contributions of certain protein residues to catalysis.

Notizen: Abstract The past decade has witnessed increasingly detailed insights into the structural mechanism of the bacteriorhodopsin photocycle. Concurrently, there has been much progress within our knowledge pertaining to the lipids of the purple membrane, including the discovery of new lipids and the overall effort to localize and identify each lipid within the purple membrane. Therefore, there is a need to classify this information to generalize the findings. We discuss the properties and roles of haloarchaeal lipids and present the structural data as individual case studies. Lipid-protein interactions are discussed in the context of structure-function relationships. A brief discussion of the possibility that bacteriorhodopsin functions as a light-driven inward hydroxide pump rather than an outward proton pump is also presented.

Notizen: Abstract G protein-coupled receptors (GPCRs) are integral membrane proteins that respond to environmental signals and initiate signal transduction pathways activating cellular processes. Rhodopsin is a GPCR found in rod cells in retina where it functions as a photopigment. Its molecular structure is known from cryo-electron microscopic and X-ray crystallographic studies, and this has reshaped many structure/function questions important in vision science. In addition, this first GPCR structure has provided a structural template for studies of other GPCRs, including many known drug targets. After presenting an overview of the major structural elements of rhodopsin, recent literature covering the use of the rhodopsin structure in analyzing other GPCRs will be summarized. Use of the rhodopsin structural model to understand the structure and function of other GPCRs provides strong evidence validating the structural model.

Notizen: Abstract The coupling of high-performance mass spectrometry instrumentation with highly efficient chromatographic and electrophoretic separations has enabled rapid qualitative and quantitative analysis of thousands of proteins from minute samples of biological materials. Here, we review recent progress in the development and application of mass spectrometry-based techniques for the qualitative and quantitative analysis of global proteome samples derived from whole cells, tissues, or organisms. Techniques such as multidimensional peptide and protein separations coupled with mass spectrometry, accurate mass measurement of peptides from global proteome digests, and mass spectrometric characterization of intact proteins hold great promise for characterization of highly complex protein mixtures. Advances in chemical tagging and isotope labeling techniques have enabled quantitative analysis of proteomes, and highly specific isolation strategies have been developed aimed at selective analysis of posttranslationally modified proteins.

Notizen: Abstract Drag reduction in wall-bounded flows can be achieved by transverse motions imposed by passive means, e.g., riblets, or by external forcing, such as wall oscillation or transverse traveling-wave excitation. In this article, we review possible physical mechanisms responsible for turbulent drag reduction and corresponding near-wall flow modification.

Notizen: Abstract In this review we describe the aerodynamic problems that must be addressed in order to design a successful small aerial vehicle. The effects of Reynolds number and aspect ratio (AR) on the design and performance of fixed-wing vehicles are described. The boundary-layer behavior on airfoils is especially important in the design of vehicles in this flight regime. The results of a number of experimental boundary-layer studies, including the influence of laminar separation bubbles, are discussed. Several examples of small unmanned aerial vehicles (UAVs) in this regime are described. Also, a brief survey of analytical models for oscillating and flapping-wing propulsion is presented. These range from the earliest examples where quasi-steady, attached flow is assumed, to those that account for the unsteady shed vortex wake as well as flow separation and aeroelastic behavior of a flapping wing. Experiments that complemented the analysis and led to the design of a successful ornithopter are also described.

Notizen: Abstract The issue of the physical mechanism(s) that control the efficiency with which the density field in stably stratified fluid is mixed by turbulent processes has remained enigmatic. Similarly enigmatic has been an explanation of the numerical value of ~0.2, which is observed to characterize this efficiency experimentally. We review recent work on the turbulence transition in stratified parallel flows that demonstrates that this value is not only numerically predictable but also that it is expected to be a nonmonotonic function of the Richardson number that characterizes preturbulent stratification strength. This value of the mixing efficiency appears to be characteristic of the late-time behavior of the turbulent flow that develops after an initially laminar shear flow has undergone the transition to turbulence through an intermediate instability of Kelvin-Helmholtz type.

Notizen: Abstract Recent small-scale turbulence observations allow the mixing regimes in lakes, reservoirs, and other enclosed basins to be categorized into the turbulent surface and bottom boundary layers as well as the comparably quiet interior. The surface layer consists of an energetic wave-affected thin zone at the very top and a law-of-the-wall layer right below, where the classical logarithmic-layer characteristic applies on average. Short-term current and dissipation profiles, however, deviate strongly from any steady state. In contrast, the quasi-steady bottom boundary layer behaves almost perfectly as a logarithmic layer, although periodic seiching modifies the structure in the details. The interior stratified turbulence is extremely weak, even though much of the mechanical energy is contained in baroclinic basin-scale seiching and Kelvin waves or inertial currents (large lakes). The transformation of large-scale motions to turbulence occurs mainly in the bottom boundary and not in the interior, where the local shear remains weak and the Richardson numbers are generally large.

Notizen: Abstract Increasing urbanization and concern about sustainability and quality of life issues have produced considerable interest in flow and dispersion in urban areas. We address this subject at four scales: regional, city, neighborhood, and street. The flow is one over and through a complex array of structures. Most of the local fluid mechanical processes are understood; how these combine and what is the most appropriate framework to study and quantify the result is less clear. Extensive and structured experimental databases have been compiled recently in several laboratories. A number of major field experiments in urban areas have been completed very recently and more are planned. These have aided understanding as well as model development and evaluation.

Notizen: Abstract It is classically assumed that the far field of a round turbulent jet discharging into quiescent fluid has a unique behavior characterized only by its momentum flux. However, there is now considerable evidence that different discharge conditions at the jet nozzle exit can give rise to very different far-field flows. Perhaps the most striking examples of these are the bifurcating and blooming jets produced by appropriate combinations of controlled axial and circumferential excitations at the nozzle exit. With the right excitations, a jet can be made to divide into two separate jets (bifurcating jet), each of which carries half the axial momentum and spreads in a manner similar to a single jet. Trifurcating jets can also be produced. Other excitations can produce blooming jets, in which the jet explodes into a shower of vortex rings, producing a far-field flow that is quite unlike a normal unexcited jet. Bifurcating and blooming jets exhibit much greater mixing than normal jets, suggesting possible applications in flow control. This article summarizes our work on bifurcating and blooming jets, which began with our discovery of them in the early 1980s and continued through the mid- 1990s. One of us (D.E.P.) continued exploration of flow control using excited jets, first at the McDonnell Douglas Corporation, and more recently at the Georgia Institute of Technology. The key to flow control is the manipulation of the large vortical structures in the near field of the jet. Ultimately this work, and that of others, led to full-scale testing of jet engine exhaust mixing control. There it was shown that the jet temperature downstream of the engine can be very significantly reduced by application of well-designed and easily implemented excitation at the engine discharge, thereby solving problems encountered during ground operations. Related jet control work by other investigators is included in this review.

Notizen: Abstract The recent progress in three-dimensional boundary-layer stability and transition is reviewed. The material focuses on the crossflow instability that leads to transition on swept wings and rotating disks. Following a brief overview of instability mechanisms and the crossflow problem, a summary of the important findings of the 1990s is given.

Notizen: Abstract Cyclooxygenases-1 and -2 (COX-1 and COX-2, also known as prostaglandin H2 synthases-1 and -2) catalyze the committed step in prostaglandin synthesis. COX-1 and -2 are of particular interest because they are the major targets of nonsteroidal antiinflammatory drugs (NSAIDs) including aspirin, ibuprofen, and the new COX-2-selective inhibitors. Inhibition of the COXs with NSAIDs acutely reduces inflammation, pain, and fever, and long-term use of these drugs reduces the incidence of fatal thrombotic events, as well as the development of colon cancer and Alzheimer's disease. In this review, we examine how the structures of COXs relate mechanistically to cyclooxygenase and peroxidase catalysis and how alternative fatty acid substrates bind within the COX active site. We further examine how NSAIDs interact with COXs and how differences in the structure of COX-2 result in enhanced selectivity toward COX-2 inhibitors.

Notizen: Abstract Understanding the action of enzymes on an atomistic level is one of the important aims of modern biophysics. This review describes the state of the art in addressing this challenge by simulating enzymatic reactions. It considers different modeling methods including the empirical valence bond (EVB) and more standard molecular orbital quantum mechanics/molecular mechanics (QM/MM) methods. The importance of proper configurational averaging of QM/MM energies is emphasized, pointing out that at present such averages are performed most effectively by the EVB method. It is clarified that all properly conducted simulation studies have identified electrostatic preorganization effects as the source of enzyme catalysis. It is argued that the ability to simulate enzymatic reactions also provides the chance to examine the importance of nonelectrostatic contributions and the validity of the corresponding proposals. In fact, simulation studies have indicated that prominent proposals such as desolvation, steric strain, near attack conformation, entropy traps, and coherent dynamics do not account for a major part of the catalytic power of enzymes. Finally, it is pointed out that although some of the issues are likely to remain controversial for some time, computer modeling approaches can provide a powerful tool for understanding enzyme catalysis.

Notizen: The galaxies of the Local Group serve as important laboratories for understanding the physics of massive stars. Here I discuss what is involved in identifying various kinds of massive stars in nearby galaxies: the hydrogen-burning O-type stars and their evolved He-burning evolutionary descendants, the luminous blue variables, red supergiants, and Wolf-Rayet stars. Primarily I review what our knowledge of the massive star population in nearby galaxies has taught us about stellar evolution and star formation. I show that the current generation of stellar evolutionary models do well at matching some of the observed features and provide a look at the sort of new observational data that will provide a benchmark against which new models can be evaluated.

Notizen: The first law of theoretical physics, the Newtonian law of gravitation, relies on the concept of action at a distance. The success of this law led to the concept being applied to electricity and magnetism, which were next to be explored in depth. Here the action at a distance had a limited success and ultimately had to be abandoned in favor of the increasingly more popular field theory. Nevertheless, in the 1940s, an attempt was made to revive the concept of action at a distance in a relativistically invariant way by Wheeler & Feynman (1945, 1949). It inspired a series of investigations in both electrodynamics and gravity in which the field concept was not used but the interaction was described as taking place directly between particles. As it impinged very intimately on cosmology, Hoyle was keenly interested in it. This review discusses the work by Hoyle, the author, and others on the development of electrodynamics and gravitation as direct particle theories. In this review, the author discusses how the work was started and went through stages of increasing sophistication, e.g., extending the Wheeler-Feynman electrodynamics to curved spacetime, its consequences in different cosmologies, and the issues arising from its quantization. The resolution of ultraviolet divergences in quantum electrodynamics is also briefly discussed. The parallel development of a Machian theory of gravitation followed the lead from electrodynamics. In both theories one sees a strong link between the large-scale structure of the universe and local physics, as might be expected from an action-at-a-distance framework.

Notizen: Targeted laboratory astrophysics measurements are being conducted to address the needs of X-ray astronomy. The measurements are producing large sets of reliable atomic data, which include ionization and recombination cross sections for charge balance calculations, as well as line lists, excitation cross sections, and dielectronic recombination rates for interpreting X-ray line formation. Additional experiments focus on resolving specific puzzles posed by astrophysical observations, as well as on calibrating existing and developing new X-ray line diagnostics. We discuss the types of data produced and illustrate how the laboratory measurements support such missions as ASCA, EUVE, Chandra, XMM, and ASTRO-E2.

Notizen: Photoionized clouds are ubiquitous. They define the endpoints of stellar evolution (H II regions and planetary nebulae), constitute the interstellar and intergalactic media, and are found in high redshift quasars and star-forming galaxies. The spectra of these objects are dominated by emission lines that are sensitive to details of the emitting gas. These details include the microscopic atomic processes that cause the gas to glow; the density, composition, and temperature of the gas; and the radiation field of the central continuum source. Large-scale numerical codes that incorporate all the needed physics and predict the observed spectrum have become essential tools in understanding these objects. This article reviews the current status of the numerical simulations of emitting gas, with particular emphasis on photoionized clouds and the underlying simplicity that governs these nebulae; the types of questions that can be addressed by today's codes; and the big questions that remain unanswered.

Notizen: Abstract The modern study of a crowd as a flowing continuum is a recent development. Distinct from a classical fluid because of the property that a crowd has the capacity to think, interesting new physical ideas are involved in its study. An appealing property of a crowd in motion is that the nonlinear, time-dependent, simultaneous equations representing a crowd are conformably mappable. This property makes many interesting applications analytically tractable. In this review examples are given in which the theory has been used to provide possible assistance in the annual Muslim Hajj, to understand the Battle of Agincourt, and, surprisingly, to locate barriers that actually increase the flow of pedestrians above that when there are no barriers present. Modern developments may help prevent some of the approximately two thousand deaths that annually occur in accidents owing to crowding.The field of crowd motion, that is, the field of "thinking fluids," is an intriguing area of research with great promise.

Notizen: Abstract The recent avalanche of research activity in the field of granular matter has yielded much progress. The use of state-of-the-art (and other) computational and experimental methods has led to the discovery of new states and patterns and enabled detailed tests of theories and models. The application of statistical mechanical methods and phenomenology has contributed to the understanding of the microscopic a nd macroscopic properties of granular systems. Some previously open problems seem to be solved. Fluidized granular systems (rapid granular flows), recently referred to as granular gases, are often modeled by hydrodynamic equations of motion, some of which are based on systematic expansions applied to the pertinent Boltzmann equation. The undeniable success of granular hydrodynamics is somewhat surprising in view of the lack of scale separation in these systems and the neglect of certain correlations in most derivations of the hydrodynamic equations. Microstructures have been recognized as key features of granular gases; explanations for their existence have been proposed, and many of their properties elucidated. Granular-gas multistability can often be traced back to microstructure dynamics. In spite of these and other impressive advances, this field still poses serious challenges.

Notizen: Abstract Recent advances in achieving textbook multigrid efficiency for fluid simulations are presented. Textbook multigrid efficiency is defined as attaining the solution to the governing system of equations in a computational work that is a small multiple of the operation counts associated with discretizing the system. Strategies are reviewed to attain this efficiency by exploiting the factorizability properties inherent to a range of fluid simulations, including the compressible Navier-Stokes equations. Factorizability is used to separate the elliptic and hyperbolic factors contributing to the target system; each of the factors can then be treated individually and optimally. Boundary regions and discontinuities are addressed with separate (local) treatments. New formulations and recent calculations demonstrating the attainment of textbook efficiency for aerodynamic simulations are shown.

Notizen: Abstract Mass spectrometry has provided a powerful method for monitoring hydrogen exchange of protein backbone amides with deuterium from solvent. In comparison to popular NMR approaches, mass spectrometry has the advantages of higher sensitivity, wider coverage of sequence, and the ability to analyze larger proteins. Proteolytic fragmentation of proteins following the exchange reaction provides moderate structural resolution, in some cases enabling measurements from single amides. The technique has provided new insight into protein-protein and protein-ligand interfaces, as well as conformational changes during protein folding or denaturation. In addition, recent studies illustrate the utility of hydrogen exchange mass spectrometry toward detecting protein motions relevant to allostery, covalent modifications, and enzyme function.

Notizen: Abstract Cations are bound to nucleic acids in a solvated state. High-resolution X-ray diffraction studies of oligonucleotides provide a detailed view of Mg2+, and occasionally other ions bound to DNA. In a survey of several such structures, certain general observations emerge. First, cations bind preferentially to the guanine base in the major groove or to phosphate group oxygen atoms. Second, cations interact with DNA most frequently via water molecules in their primary solvation shell, direct ion-DNA contacts being only rarely observed. Thus, the solvated ions should be viewed as hydrogen bond donors in addition to point charges. Finally, ion interaction sites are readily exchangeable: The same site may be occupied by any ion, including spermine, as well as by a water molecule.

Notizen: Abstract Although protein function is thought to depend on flexibility, precisely how the dynamics of the molecule and its environment contribute to catalytic mechanisms is unclear. We review experimental and computational work relating to enzyme dynamics and function, including the role of solvent. The evidence suggests that fast motions on the 100 ps timescale, and any motions coupled to these, are not required for enzyme function. Proteins where the function is electron transfer, proton tunneling, or ligand binding may have different dynamical dependencies from those for enzymes, and enzymes with large turnover numbers may have different dynamical dependencies from those that turn over more slowly. The timescale differences between the fastest anharmonic fluctuations and the barrier-crossing rate point to the need to develop methods to resolve the range of motions present in enzymes on different time- and lengthscales.

Notizen: Abstract The OB-fold domain is a compact structural motif frequently used for nucleic acid recognition. Structural comparison of all OB-fold/nucleic acid complexes solved to date confirms the low degree of sequence similarity among members of this family while highlighting several structural sequence determinants common to most of these OB-folds. Loops connecting the secondary structural elements in the OB-fold core are extremely variable in length and in functional detail. However, certain features of ligand binding are conserved among OB-fold complexes, including the location of the binding surface, the polarity of the nucleic acid with respect to the OB-fold, and particular nucleic acid-protein interactions commonly used for recognition of single-stranded and unusually structured nucleic acids. Intriguingly, the observation of shared nucleic acid polarity may shed light on the longstanding question concerning OB-fold origins, indicating that it is unlikely that members of this family arose via convergent evolution.

Notizen: Abstract The observation of liquid-liquid immiscibility in cholesterol-phospholipid mixtures in monolayers and bilayers has opened a broad field of research into their physical chemistry. Some mixtures exhibit multiple immiscibilities. This unusual property has led to a thermodynamic model of "condensed complexes." These complexes are the consequence of an exothermic, reversible reaction between cholesterol and phospholipids. In this quantitative model the complexes are sometimes concentrated in a separate liquid phase. The phase separation into a complex-rich phase depends on membrane composition and intensive variables such as temperature. The properties of defined cholesterol-phospholipid mixtures provide a conceptual foundation for the exploration of a number of aspects of the biophysics and biochemistry of animal cell membranes.

Notizen: Astrophysics has been an important part of my personal and scientific life three times. The first was in 1938 when I did work on stellar energy production. The second was a joyful period nearly 30 years later when that work was rewarded with the Nobel Prize in physics. And the third has lasted over the time since my retirement in 1975 during which Gerry Brown and I have had a very satisfactory collaboration exploring various aspects of supernovae and, more recently, binary pairs.

Notizen: Blueshifted absorption lines in the UV and X-ray spectra of active galaxies reveal the presence of massive outflows of ionized gas from their nuclei. The "intrinsic" UV and X-ray absorbers show large global covering factors of the central continuum source, and the inferred mass loss rates are comparable to the mass accretion rates. Many absorbers show variable ionic column densities, which are attributed to a combination of variable ionizing flux and motion of gas into and out of the line of sight. Detailed studies of the intrinsic absorbers, with the assistance of monitoring observations and photoionization models, provide constraints on their kinematics, physical conditions, and locations relative to the central continuum source, which range from the inner nucleus (~0.01 pc) to the galactic disk or halo (~10 kpc). Dynamical models that make use of thermal winds, radiation pressure, and/or hydromagnetic flows have reached a level of sophistication that permits comparisons with the observational constraints.

Notizen: Old, cool white dwarfs convey valuable information about the early history of our Galaxy. They have been used to determine the age of the galactic disk, several open clusters, and a globular cluster. We review the current understanding of the physics of cool white dwarfs, including their mass distribution, chemical evolution, magnetism, and cooling. We also examine the role of white dwarfs as tracers of various stellar populations, both in terms of observational searches and theoretical models.

Notizen: Giant planets have now been discovered around other stars, and it is only a matter of time until Earth-sized planets are detected. Whether any of these planets are suitable for life depends on their volatile abundances, especially water, and on their climates. Only planets within the liquid-water habitable zone (HZ) can support life on their surfaces and, thus, can be analyzed remotely to determine whether they are inhabited. Fortunately, current models predict that HZs are relatively wide around main-sequence stars not too different from our sun. This conclusion is based on studies of how our own planet has evolved over time. Earth's climate has remained conducive to life for the past 3.5 billion years or more, despite a large increase in solar luminosity, probably because of previous higher concentrations of CO2 and/or CH4. Both these gases are involved in negative feedback loops that help to stabilize the climate. In addition to these topics, we also briefly discuss the rise of atmospheric O2 and O3, along with their possible significance as indicators of life on other planets.

Notizen: Stellar clusters are born embedded within giant molecular clouds (GMCs) and during their formation and early evolution are often only visible at infrared wavelengths, being heavily obscured by dust. Over the past 15 years advances in infrared detection capabilities have enabled the first systematic studies of embedded clusters in galactic molecular clouds. In this article we review the current state of empirical knowledge concerning these extremely young protocluster systems. From a survey of the literature we compile the first extensive catalog of galactic embedded clusters. We use the catalog to construct the mass function and estimate the birthrate for embedded clusters within ~2 kpc of the sun. We find that the embedded cluster birthrate exceeds that of visible open clusters by an order of magnitude or more indicating a high infant mortality rate for protocluster systems. Less than 4-7% of embedded clusters survive emergence from molecular clouds to become bound clusters of Pleiades age. The vast majority (90%) of stars that form in embedded clusters form in rich clusters of 100 or more members with masses in excess of 50 Mo. Moreover, observations of nearby cloud complexes indicate that embedded clusters account for a significant (70-90%) fraction of all stars formed in GMCs. We review the role of embedded clusters in investigating the nature of the initial mass function (IMF) that, in one nearby example, has been measured over the entire range of stellar and substellar mass, from OB stars to substellar objects near the deuterium burning limit. We also review the role embedded clusters play in the investigation of circumstellar disk evolution and the important constraints they provide for understanding the origin of planetary systems. Finally, we discuss current ideas concerning the origin and dynamical evolution of embedded clusters and the implications for the formation of bound open clusters.

Notizen: In this contribution, a review is presented on the ample data obtained on post-AGB stars, both on the central stars and their circumstellar material. The fast evolutionary phase is characterized by a rapid change in the properties of the objects, but the variety is so large that there is yet no clear consensus on how the detailed studies of individual objects are linked together by evolutionary channels. The absence of strong molecular veiling in the photospheres of the central stars, together with a spread in intrinsic metallicity make post-AGB stars very useful in constraining AGB chemical evolutionary models. We discuss the surprisingly wide variety of chemical signatures observed. The onset in the creation process of the panoply of structures and shapes observed in planetary nebulae occurs during the short post-AGB evolution, but the physical nature of the processes involved is still badly understood. In the rapidly growing field of circumstellar mineralogy, post-AGB stars have their story to tell and also the molecular envelope changes significantly due to dilution and hardening of the stellar radiation. The real-time evolution of some objects suffering a late thermal flash is reviewed and their possible link to other hydrogen-deficient objects is discussed. Any review on stellar evolution has a section on binaries and this contribution is no exception because binaries make up a significant fraction of the post-AGB stars known to date.

Notizen: Abstract Full-scale icing experiments and, therefore, certification time and cost can be significantly reduced by developing calculation methods to evaluate the aircraft and system performance for a wide range of icing conditions. This article summarizes calculation methods for icing that include ice accretion, ice system performance, and icing effects on aircraft.