ultrasonic velocity and anisotropy of hydrocarbon source rocks an experimental study of the physical properties of black kerogen rich shales also including maturation analysis scanning electron microscope sem observations and physical modeling revealed fairly peculiar petrophysical parameters specifically these rocks have very low porosity and density but most importantly both p and s ultrasonic velocities normal to bedding are extremely low whereas they are much higher parallel to bedding giving rise to a strong anisotropy even at high confining pressures we found that these parameters primarily reflect kerogen content microstructure and maturation level of these rocks we found also that microcracks inferred from ultrasonic velocity measurements occur only in mature shales these microcracks are parallel to the bedding plane and further enhance strong intrinsic anisotropy notably at low effective pressure our results show that on a small scale kerogen rich shales are transversely isotropic rocks and can be effectively modeled using the thin laye

attenuation anisotropy in shale at elevated confining pressures shale has been estimated to comprise approximately two thirds of all sedimentary rocks 1 nevertheless studies of the physical properties of shale are relatively uncommon compared to other sedimentary lithologies especially rare are studies of shale attenuation anisotropy at elevated confining pressure an understanding of attenuation and attenuation mechanisms is particularly important for seismic exploration vsp and cross well experiments have used attenuation information to help determine the lithology of subsurface formations 2 4 attenuation is also believed to have a significant affect on avo responses 5 and may affect lithological interpretations based on avo analyses measurements of attenuation have been correlated with fluid content e g 6 and porosity e g 7 and may be used to determine in situ hydraulic properties seismic attenuation has also been linked to rock strength 8 9 and may be useful in relation to blasting and the location of landslide material accurate values of attenuation are also necessary to estimate ground motion associated with earthquakes 10 11 shale has been shown to have significant velocity anisotropy e g 12 that persists at elevated confining pressure it is unclear however whether attenuation displays the same anisotropic relationships with pressure previous studies of attenuation in shale have involved either measurements in a single direction measurements at atmospheric pressure or measurements on unsaturated samples only e g 13 15 this study provides the first measurements of shale attenuation in multiple directions at elevated confining pressure under both saturated and unsaturated conditions laboratory measurements have been made on four samples up to 250 mpa velocity and attenuation have been measured parallel and perpendicular to

microcrack induced elastic wave anisotropy of brittle rocks the failure of brittle rocks during compression is preceded by the formation growth and coalescence of microcracks elastic wave velocities are reduced in the presence of open microcracks and fractures and may therefore be used to monitor the progressive damage of the rock in general these microcracks are not randomly oriented and the rock displays an elastic anisotropy the elastic anisotropy due to cracks can be expressed in terms of a second rank and fourth rank crack density tensor for open cracks the contribution of the fourth rank crack density tensor to the elastic wave velocities is small these results are compared with recent measurements of the ultrasonic compressional and shear wave velocities for propagation parallel and perpendicular to an increasing axial stress applied at constant confining stress to berea sandstone inversion of the velocity measurements indicates that the microcracks propagate parallel to the maximum compressive stress in agreement with current rock mechanics theory a reasonable fit to the data is obtained using only the second rank crack density tensor even though at high confining stress the cracks are expected to be in partial contact along their length this is consistent with the model of elastic wave propagation in a medium containing partially contacting fractures published by white however measurements of off axis wave velocities are required to fully quantify the contribution of the fourth rank crack density tensor

geostatistical approaches for incorporating elevation into the spatial interpolation of rainfall this paper presents three multivariate geostatistical algorithms for incorporating a digital elevation model into the spatial prediction of rainfall simple kriging with varying local means kriging with an external drift and colocated cokriging the techniques are illustrated using annual and monthly rainfall observations measured at 36 climatic stations in a 5000 km 2 region of portugal cross validation is used to compare the prediction performances of the three geostatistical interpolation algorithms with the straightforward linear regression of rainfall against elevation and three univariate techniques the thiessen polygon inverse square distance and ordinary kriging larger prediction errors are obtained for the two algorithms inverse square distance thiessen polygon that ignore both the elevation and rainfall records at surrounding stations the three multivariate geostatistical algorithms outperform other interpolators in particular the linear regression which stresses the importance of accounting for spatially dependent rainfall observations in addition to the colocated elevation last ordinary kriging yields more accurate predictions than linear regression when the correlation between rainfall and elevation is moderate less than 0 75 in the case study q 2000 elsevier science b v all rights reserved

impact of fabric microcracks and stress field on shale anisotropy summary few data are available on shales in terms of seismic to ultrasonic properties and anisotropy although all are important with regards to imaging problems often encountered in such lithologies additionally mechanisms causing changes in these properties are not well documented due to the fine grain size of such materials and time required for testing under controlled pore pressure conditions the results presented here are derived from a set of experiments run on muderong shale with pore pressure control in order to evaluate the effect of stress magnitude and stress anisotropy on ultrasonic response this shale was noted to have a linear velocitymean effective stress response and extremely high anisotropy both likely the result of the presence of fluid filled cracks in a low permeability porous medium changes in velocity and vp vs ratios are dependent on both stress and smectite content s wave velocity is significantly affected by the presence of smectite in this and other shales and at low stress 20 mpa is less sensitive to stress change than p wave velocity vp vs ratios are noted to increase in this shale up to 20 mpa effective stress then decrease slightly due to stress induced loss of interlayer water in smectite intrinsic anisotropy comes from composition a strong compaction fabric and the presence of microfractures changes to ultrasonic anisotropy are the result of the magnitude of the stresses their orientation with respect to the fractures and the degree of stress anisotropy

classification of drainage basins according to their physical characteristics an application for flood frequency analysis in scotland abstract if the standardised floods at all sites within a given region are assumed to be samples from the same distribution all data may be combined to yield an estimate of a regional dimensionless flood frequency relationship the identification of these hydrologically homogeneous regions can be achieved on the basis of the physical characteristics of the basin which control the hydrological processes classification of the basins independent of the discharge data avoids the problems of regionalisation based on the highly variable individual site estimates of flood frequency classification of the basins can be performed using the normix multivariate clustering algorithm the homogeneity of the basins can be tested using a likelihood ratio test of whether standardised data for all basins within a region may be adequately represented by a single regional flood frequency relationship the method was applied to 168 basins in scotland and five regions were identified corresponding to physically reasonable basin types and all but one of which yield homogeneous distributions of flood frequency

a regional monthly precipitation simulation model based on an l moment smoothed statistical regionalization approach summary using rain gauge station records for the statistical characterization and simulation modeling of spatio temporal precipitation field involves many issues and simplistic assumptions one major issue is related to dealing with uncertainty at site sample statistical inference because of the limited length of records regional frequency analysis uses the idea of substituting space for time in order to reduce uncertainty it assumes equal shapes of the precipitation statistical distributions in a region however this assumption limits the area of the analyzed region where this assumption is valid the extension is dependent on terrain complexity this work presents a new approach for the statistical regionalization of a large precipitation field replacing the shape constancy assumption for the hypothesis of smooth spatial variation the approach accounts for every uncertainty on site information using an l moment method for inference analysis additionally the orographic effect is introduced in the regionalization which substantially improves the interpolation performance and estimation of areal precipitation the approach is used for modeling the monthly precipitation field in the jucar river basin authority demarcation spain incorporating its stochastic structure and spatial dependency coming from a geostatistical analysis issues related to the estimation of regional precipitation and mean areal precipitation are discussed in the exposition

elastic properties of anisotropic synthetic calcite muscovite aggregates 1 a set of synthetic aggregates containing mixtures of calcite and muscovite were compacted with varying uniaxial loads ranging from 20 mpa up to 400 mpa their elastic properties have been measured using compressional and shear waves in confining hydrostatic pressures up to 475 mpa measured seismic velocities are shown to depend on 1 the ratio of calcite to muscovite 2 the uniaxial load used during sample manufacturing and 3 the porosity amount prior to velocity measurements the matrix framework may also affect the seismic velocities but this effect is not easily quantifiable in general measured seismic velocities decrease with increasing muscovite content and porosity elastic properties have been calculated based on texture measurements which were obtained by neutron diffraction the calculated velocities are based on the calcite and muscovite single crystal elastic tensors their orientation distribution functions and their modal fractions a large discrepancy is apparent between calculated and measured velocities where the former always overestimate the actual velocities shear waves display less of a difference between calculated and measured values than do compressional waves indicating that waves propagating by shearing are less affected by sample porosity and matrix framework a better agreement between measured and calculated seismic velocities arises when the initial porosity is taken into account using a differential effective medium model seismic anisotropy is evident from both measured and calculated velocities and is low for samples containing pure calcite but becomes prominent as the muscovite concentration increases the intensity of anisotropy further depends on the uniaxial load used during sample compaction and the initial porosity

first ever in situ observations of venus polar upper atmosphere density using the tracking data of the venus express atmospheric drag experiment vexade on its highly elliptical 24 h orbit around venus the venus express vex spacecraft briefly reaches a periapsis altitude of nominally 250 km recently however dedicated and intense radio tracking campaigns have taken place in august 2008 october 2009 february and april 2010 for which the periapsis altitude was lowered to the 186 176 km altitude range in order to be able to probe the upper atmosphere of venus above the north pole for the first time ever in situ as the spacecraft experiences atmospheric drag its trajectory is measurably perturbed during the periapsis pass allowing us to infer total atmospheric mass density at the periapsis altitude a precise orbit determination pod of the vex motion is performed through an iterative least squares fitting process to the doppler tracking data acquired by the vex radioscience experiment vera the drag acceleration is modelled using an initial atmospheric density model vts3 model hedin a e niemann h b kasprzak w t seiff a 1983 j geophys res 88 73 83 a scale factor of the drag acceleration is estimated for each periapsis pass which scales hedin s density model in order to best fit the radio tracking data reliable density scale factors have been obtained for 10 passes mainly from the second october 2009 and third april 2010 vexade campaigns which indicate a lower density by a factor of about 1 8 than hedin s model predicts these first ever in situ polar density measurements at solar minimum have allowed us to construct a diffusive equilibrium density model for venus thermosphere constrained in the lower thermosphere primarily by spicav soir measurements and above 175 km by the vexade drag measurements muller wodarg et al in preparation the preliminary results of the vexade campaigns show that it is possible to obtain with the pod technique reliable estimates of venus upper atmosphere densities at an altitude of around 175 km future vexade campaigns will benefit from the planned further lowering of vex pericenter altitude to below 170 km 2011 elsevier inc

ultrasonic velocities in cretaceous shales from the williston basin compressional and shear wave velocities were measured in the laboratory from 1 bar to 4 kbar confining pressure for wet undrained samples of cretaceous shales from depths of 3200 and 5000 ft in the williston basin north dakota these shales behave as transversely isotropic elastic media the plane of circular symmetry coinciding with the bedding plane for compressional waves the velocity is higher for propagation in the bedding plane than at right angles to it and the anisotropy is greater for the 5000 ft shale for shear waves the sh wave perpendicular to bedding and the sv wave parallel to bedding propagate with the same speed which is about 25 percent lower than that for the sh wave parallel to bedding in general compressional and shear velocities are higher for the indurated 5000 ft shale than for the friable 3200 ft shale all velocities increase with in increasing confining pressure to 4 kbar the 3200 ft shale exhibits velocity hysteresis as a function of pressure whereas this effect is a

microcracking based rock classification using ultrasonic and porosity parameters and multivariate analysis methods abstract this work aims at presenting a new methodology based on ndt ultrasonic techniques and water porosity measurements to characterize the microcracking state of rocks and classify them in microcracking based equivalent groups the measurements of ultrasonic pulse velocity the attenuation coefficient and the porosity by water saturation under vacuum conditions make it possible to compare and validate all these techniques as good practices to classify aggregates and ornamental stones with regard to their rock matrix compactness beyond the fact that these developments give new approaches to assess the rock microcracking it was shown that these parameters have a direct relationship indeed the classification methodology was applied to a database containing 56 cores coming from blocks sampled in an aggregate production quarry for these cores ultrasonic parameters wave velocity attenuation and anisotropy coefficients and porosity parameters total water crack and pore porosities were measured two multivariate statistical methods principal component analysis and cluster analysis were applied on this database to assess the relationship between all these parameters and to classify the cores into micro structurally similar groups the application of the setup methodology on the core database allows us to study the main correlations between the measured microcracking rock properties on the other hand it was shown that the method can be used as an effective way to characterize the differences in terms of microstructure between rock samples

hydrocarbon generation induced microcracking of source rocks laboratory measurements of ultrasonic velocity and anisotropy in kerogen rich black shales of varying maturity suggest that extensive bedding parallel microcracks exist in situ in most mature source rocks undergoing the major stage of hydrocarbon generation and migration given the normal faulting regime with the vertical stress being the maximum principal stress typical of most sedimentary basins this microcrack alignment cannot be accounted for using simplified fracture mechanics concepts this subhorizontal microcrack alignment is consistent with 1 a model of local principal stress rotation and deviatoric stress reduction within an overpressured formation undergoing hydrocarbon generation and with 2 a strong mechanical strength anisotropy of kerogen rich shales caused by bedding parallel alignment of kerogen microlayers microcracks originate within kerogen or at kerogen illite interfaces when pore pressure exceeds the bedding normal total stress by only a few mpa due to the extremely low fractu

microcrack induced versus intrinsic elastic anisotropy in mature hc source shales recent experimental studies of ultrasonic velocity anisotropy in kerogen rich shales indicate that these rocks are characterized by a very strong anisotropic response related to a very fine bedding parallel lamination of organic matter and preferred orientation of clay particles in the rock matrix vernik and nur 1992 this intrinsic anisotropy is further enhanced in thermally mature shales by bedding parallel microcracks caused by the processes of hydrocarbon generation vernik paper in preparation however the potential of recognizing mature source rocks in situ using downhole sonic crosshole seismic or vsp data clearly depends on our ability to discriminate between these two major causes of elastic anisotropy remove the effect of the intrinsic anisotropy and estimate crack density and crack porosity from velocity measurements

the thermal structure of the venus atmosphere intercomparison of venus express and ground based observations of vertical temperature and density profiles the venus international reference atmosphere vira model contains tabulated values of temperature and number densities obtained by the experiments on the venera entry probes pioneer venus orbiter and multi probe missions in the 1980s the instruments on the recent venus express orbiter mission generated a significant amount of new observational data on the vertical and horizontal structure of the venus atmosphere from 40 km to about 180 km altitude from april 2006 to november 2014 many ground based experiments have provided data on the upper atmosphere 90 130 km temperature structure since the publication of vira in 1985 the thermal structure of the venus atmosphere team was supported by the international space studies institute issi bern switzerland from 2013 to 2015 in order to combine and compare the ground based observations and the vex observations of the thermal structure as a first step towards generating an updated vira model results of this comparison are presented in five latitude bins and three local time bins by assuming hemispheric symmetry the intercomparison of the ground based and vex results provides for the first time a consistent picture of the temperature and density structure in the 40 km 180 km altitude range the venus express observations have considerably increased our knowledge of the venus atmospheric thermal structure above 40 km and provided new information above 100 km there are however still observational gaps in latitude and local time above certain regions considerable variability in the temperatures and densities is seen above 100 km but certain features appear to be systematically present such as a succession of warm and cool layers preliminary modeling studies support the existence of such layers in agreement with a global scale circulation the intercomparison focuses on average profiles but some vex experiments provide sufficient global coverage to identify solar thermal tidal components the differences between the vex temperature profiles and the vira below 0 1 mbar 95 km are small there is however a clear discrepancy at high latitudes in the 10 30 mbar 70 80 km range the vex observations will also allow the improvement of the empirical models vts3 by hedin et al 1983 and vira by keating et al 1985 above 0 03 mbar 100 km in particular the 100 150 km region where a sufficient observational coverage was previously missing the next steps in order to define the updated vira temperature structure up to 150 km altitude are 1 define the grid on which this database may be provided 2 fill what is possible with the results of the data intercomparison and 3 fill the observational gaps an interpolation between the datasets may be performed by using available general circulation models as guidelines an improved spatial coverage of observations is still necessary at all altitudes in latitude longitude and at all local solar times for a complete description of the atmospheric thermal structure in particular on the dayside above 100 km new in situ observations in the atmosphere below 40 km are missing an altitude region that cannot be accessed by occultation experiments all these questions need to be addressed by future missions

laboratory micro seismic signature of shear faulting and fault slip in shale this article reports the results of a triaxial deformation experiment conducted on a transversely isotropic shale specimen this specimen was instrumented with ultrasonic transducers to monitor the evolution of the micro seismic activity induced by shear faulting triaxial failure and subsequent fault slip at two different rates the strain data demonstrate the anisotropy of the mechanical quasi static compliance of the shale the p wave velocity data demonstrate the anisotropy of the elastic dynamic compliance of the shale the spatio temporal evolution of the micro seismic activity suggests the development of two distinct but overlapping shear faults a feature similar to relay ramps observed in large scale structural geology the shear faulting of the shale specimen appears quasi aseismic at least in the 0 5 mhz range of sensitivity of the ultrasonic transducers used in the experiment concomitantly the rate of micro seismic activity is strongly correlated with the imposed slip rate and the evolution of the axial stress the moment tensor inversion of the focal mechanism of the high quality micro seismic events recorded suggests a transition from a non shear dominated to a shear dominated micro seismic activity when the rock evolves from initial failure to larger and faster slip along the fault the frictional behaviour of the shear faults highlights the possible interactions between small asperities and slow slip of a velocity strengthening fault which could be considered as a realistic experimental analogue of natural observations of non volcanic tremors and very low frequency earthquakes triggered by slow slip events

acoustic emission and velocities associated with the formation of compaction bands in sandstone received 31 may 2005 revised 2 may 2006 accepted 22 june 2006 published 7 october 2006 1 a series of laboratory experiments has been conducted in which three dimensional 3 d locations of acoustic emissions ae were recorded and used to analyze the development of compaction bands in bleurswiller sandstone which has a porosity of 25 results were obtained for saturated samples deformed under triaxial compression at three different confining pressures 60 80 and 100 mpa a pore pressure of 10 mpa and room temperature we recorded acoustic emissions compressional and shear wave velocities and porosity reduction under hydrostatic condition and under triaxial loading conditions at a constant axial strain rate our results show that seismic velocities and their amplitude increased during hydrostatic pressure build up and during initial axial loading during shear enhanced compaction axial and radial velocities decreased progressively indicating an increase of stress induced damage in the rock in experiments performed at confining pressures of 80 and 100 mpa during triaxial loading acoustic emissions were localized in clusters during progressive loading ae clusters grow horizontally perpendicular to the maximum principal stress direction indicating formation of compaction bands throughout the specimens microstructural analysis of deformed specimens confirmed a spatial correspondence of ae clusters and compaction bands for the experiment performed at a confining pressure of 60 mpa ae locations and microstructural observations show symmetric compaction bands inclined to the cylinder axis of the specimen in agreement with predictions from recent theoretical models

observation of atmospheric tides in the martian exosphere using mars reconnaissance orbiter radio tracking data 1 based on the perturbations of the trajectory of the mars reconnaissance orbiter due to atmospheric drag we estimate using precision orbit determination the density of the martian exosphere near 250 km altitude with much better temporal and therefore spatial resolution than previously possible with this method small longitudinal density variations are observed to be consistent over long time periods although they appear fixed with respect to the planet we interpret them as non migrating tides because of the sun synchronous spacecraft orbit similar tides were observed previously by mgs at much lower altitude 125 km and our results agree to first order with the mgs findings citation mazarico e m t zuber f g lemoine and d e smith 2008 observation of atmospheric tides in the martian exosphere using mars reconnaissance orbiter radio tracking data geophys res lett 35 l09202 doi 10 1029 2008gl033388

seismic anisotropy of shales the seismic properties and preferred clay mineral orientation of a suite of shales are investigated using laboratory velocity measurements as a function of confining pressure x ray diffraction techniques and electron microprobe backscatter bse imaging the velocity measurements indicate that these shales are transversely isotropic with the main symmetry axis perpendicular to bedding anisotropy at elevated pressures caused mainly by preferred orientation of clays illite parallel to bedding ranges from 20 vp and 19 vs for a sample of new albany shale to 30 vp and 35 vs for a sample of chattanooga shale the degree of clay mineral alignment in the shales is constrained by orientation indices produced using simple x ray diffraction techniques a strong positive correlation is found between the degree of preferred orientation as expressed in the orientation indices and seismic anisotropy bse images of the shale fabrics confirm in a qualitative manner the results of the x ray study to investigate wave propagation in the shales elastic constants of each sample are calculated and used to produce phase and group wave velocity surfaces which describe variation in velocity as a function of angle to the bedding normal the calculated velocity surfaces constrained by independent velocity measurements display a lack of shear wave splitting at near normal incidence in even the most anisotropic shales for the highly anisotropic chattanooga shales group velocity surfaces differ significantly from corresponding phase velocity surfaces

physical properties of shale at temperature and pressure deformational properties p wave and s wave velocities and electrical resistivity were measured for three north sea malm shales in the laboratory under pressures to 800 bars and temperatures to 100 c these data were used to evaluate how factors such as mineralogy microstructure compaction and pore fluid conductivity affect a shale s seismic and electrical responses deformation in the shales is dominated by inelastic processes which cause time dependent changes in velocity resistivity and pore pressure overall shales are less sensitive to pressure changes as compared to sandstones of similar porosity however changes in temperature result in large changes in physical properties as compared to sandstones or shaly sands p wave and s wave velocities may decrease by as much as 10 percent over the temperature range studied and calculated activation energies for surface conduction are nearly twice those observed in shaly sands these comparisons emphasize fundamental differences in fabric among the

anisotropy correction for deviated well sonic logs application to seismic well tie borehole sonic logs acquired in deviated wells penetrating the hrz and colville shales in the niakuk field in alaska s north slope are seen to be significantly faster than vertical well logs these differences are attributed to shale anisotropy an iterative inversion scheme was created to invert for shale anisotropy parameters using multiple wells penetrating shale sections at different angles the inversion involves fitting the sonic log data at a range of borehole angles to the compressional wave group velocity surface the result is an estimate of the anisotropy parameters e and and the vertical p wave velocity the results show that the shales are strongly anisotropic with compressional wave anisotropy thomsen s parameter e on the order of 40 and the anisotropy parameter relates vertical velocity to short offset nmo velocity around 10 this large anisotropy can affect seismic imaging avo and time depth calculations a procedure was created to estimate the anisotropy corrected vertical s

stress dependent seismic anisotropy of shales a simple theory for the stress dependent seismic anisotropy of shales can be obtained in terms of a second rank tensor and a fourth rank tensor that depend on the orientation distribution of contacts between clay platelets the theory allows the normal and shear stiffness of the contact regions between clay platelets to be obtained as a function of stress from measurements of seismic p and s wave velocities for shales the ratio of the normal to shear compliance bn bt of the contact regions between clay particles is found to be sensitive to the saturation state of the shale inversion of velocity measurements for fully saturated shales shows a low value of bn bt when compared with measurements on air dry shales consistent with the expected reduction in normal compliance in a fluid saturated low permeability rock for all shales considered bn bt is found to be less than unity the contacts between clay particles are therefore more compliant in shear than in compression

hydraulic stimulation of natural fractures as revealed by induced microearthquakes carthage cotton valley gas field east texas we produced a high resolution microseismic image of a hydraulic fracture stimulation in the carthage cotton valley gas field of east texas we improved the precision of microseismic event locations four fold over initial locations by manually repicking the traveltimes in a spatial sequence allowing us to visually correlate waveforms of adjacent sources the new locations show vertical containment within individual targeted sands suggesting little or no hydraulic communication between the discrete perforation intervals simultaneously treated within an 80 m section treatment i e fracture zone lengths inferred from event locations are about 200 m greater at the shallow perforation intervals than at the deeper intervals the highest quality locations indicate fracture zone widths as narrow as 6 m similarity of adjacent source waveforms along with systematic changes of phase amplitude ratios and polarities indicate fairly uniform source mechanisms fracture plane orientation and sense of slip over

phased array compaction cell for measurement of the transversely isotropic elastic properties of compacting sediments sediments undergoing compaction typically exhibit transversely isotropic ti elastic properties we present a new experimental apparatus the phased array compaction cell for measuring the ti elastic properties of clay rich sediments during compaction this apparatus uses matched sets of p and s wave ultrasonic transducers located along the sides of the sample and an ultrasonic p wave phased array source together with a miniature p wave receiver on the top and bottom ends of the sample the phased array measurements are used to form plane p waves that provide estimates of the phase velocities over a range of angles from these measurements the five ti elastic constants can be recovered as the sediment is compacted without the need for sample unloading recoring or reorienting this paper provides descriptions of the apparatus the data processing and an application demonstrating recovery of the evolving ti properties of a compacting marine sediment sample

altimetry of the venus cloud tops from the venus express observations 1 simultaneous observations of venus by visible and infrared thermal imaging spectrometer and venus monitoring camera onboard the venus express spacecraft are used to map the cloud top altitude and to relate it to the ultraviolet uv markings the cloud top altitude is retrieved from the depth of co2 absorption band at 1 6 m in low and middle latitudes the cloud top is located at 74 1 km it decreases poleward of 50 and reaches 63 69 km in the polar regions this depression coincides with the eye of the planetary vortex at the same latitude and hour angle cloud top can experience fast variations of about 1 km in tens of hours while larger long term variations of several kilometers have been observed only at high latitudes uv markings correlate with the cloud altimetry however the difference between adjacent uv dark and bright regions does not exceed several hundred meters surprisingly co2 absorption bands are often weaker in the dark uv features indicating that these clouds may be a few hundred meters higher or have a larger scale height than neighboring clouds ultraviolet dark spiral arms which are often seen at about 70 correspond to higher altitudes or to the regions with strong latitudinal gradient of the cloud top altitude cloud altimetry in the polar region reveals the structure that correlates with the thermal emission maps but is invisible in uv images this implies that the uv optically thick polar hood is transparent in the near ir

morphology and dynamics of the upper cloud layer of venus esa s venus express probe has been in orbit since april 2006 eight research papers in this issue present new results from the mission covering the atmosphere polar features interactions with the solar wind and the controversial matter of venusian lightning hakan svedham et al open the section with a review of the similarities and mostly differences between venus and its twin the earth andrew ingersoll considers the latest results and also how the project teams plan to make the most of the probe s remaining six years of life venus is completely covered by a thick cloud layer with the cloud tops in fast retrograde rotation global and small scale properties of these clouds and their temporal and latitudinal variations are investigated and the wind velocities are derived the southern polar region is highly variable and can change dramatically on time scales as short as one day perhaps arising from the injection of so2 into the mesosphere venus is completely covered by a thick cloud layer of which the upper part is composed of sulphuric acid and some unknown aerosols1 the cloud tops are in fast retrograde rotation super rotation but the factors responsible for this super rotation are unknown2 here we report observations of venus with the venus monitoring camera3 on board the venus express spacecraft we investigate both global and small scale properties of the clouds their temporal and latitudinal variations and derive wind velocities the southern polar region is highly variable and can change dramatically on timescales as short as one day perhaps arising from the injection of so2 into the mesosphere the convective cells in the vicinity of the subsolar point are much smaller than previously inferred4 5 6 which we interpret as indicating that they are confined to the upper cloud layer contrary to previous conclusions7 8 but consistent with more recent study9

velocity anisotropy in shales a petrophysical study using ultrasonic velocity and anisotropy measurements on a variety of shales with different clay and kerogen content clay mineralogy and porosity at a wide range of effective pressure we find that elastic anisotropy of shales increases substantially with compaction the effect is attributed to both porosity reduction and smectite to illite transformation with diagenesis a means of kerogen content mapping using velocity versus porosity crossplot for shales is shown matrix anisotropy of shales dramatically increases with kerogen reaching the maximum values of about 0 4 at total organic carbon toc 15 20 a strong chemical softening effect was found in shales containing even minor amounts of swelling smectite clay when saturated with aqueous solution this effect results in a significant p wave anisotropy reduction as compared to dry and oil saturated shales since mature black shales are normally oil wet this effect can only have a local significance restricted to the wellbore wall accurate measurements of phase velocities including velocities at a 45 direction to the bedding plane allow us to immediately calculate elastic stiffnesses and anisotropic parameters intrinsic high pressure properties of shales display an e 0 relation introduction of the bedding parallel microcracks in overpressured shales results in a decrease when fully fluid saturated and a increase when partially gas saturated with a characteristic effect on the shape of the p wave velocity surface at small angles of incidence filtering the contribution of the intrinsic anisotropy of shales it is possible to estimate the pore fluid phase microcrack density and aspect ratio parameters using seismic anisotropy measurements

anisotropy of elastic wave velocities in deformed shales part 1 experimental results elastic wave velocity measurements in the laboratory are used to assess the evolution of the microstructure of shales under triaxial stresses which are representative of in situ conditions microstructural parameters such as crack aperture are of primary importance when permeability is a concern the purpose of these experiments is to understand the micromechanical behavior of the callovo oxfordian shale in response to external perturbations the available experimental setup allows for the continuous simultaneous measurement of five independent elastic wave velocities and two directions of strain axial and circumferential performed on the same cylindrical rock sample during deformation in an axisymmetric triaxial cell the main results are 1 identification of the complete tensor of elastic moduli of the transversely isotropic shales using elastic wave velocity measurements 2 assessment of the evolution of these moduli under triaxial loading and 3 assessment of the evolution of the elastic ani

anisotropic approximations for mudrocks a seismic laboratory study an experimental technique for determining the in situ elastic properties of mudrocks with horizontal alignments in the microstructure is used to study the accuracy of a set of three nested scalar anisotropic approximations for transversely isotropic ti media each subsequent approximation adds one more velocity parameter and includes the previous as a special case these approximations are convenient and robust because of their close relationship to standard geophysical measurements there exists no good theory to predict the effects of an imposed stress on the elasticity of mudrocks in this study the tensor of elastic moduli of a single test specimen of mudrock subjected to an anisotropic stress field is determined from ultrasonic group velocity measurements involving pointlike transducers the mechanical characterization performed at constant pore pressure is accompanied by detailed microscopic observations and analysis the method was used to obtain accurate elastic constants for five well defi

experimental laboratory determination of the dynamic elastic properties of wet drained shales the ultrasonic anisotropic elastic properties of drained saturated shales were measured as a function of confining pressure two shales were characterized in this study a jurassic outcrop shale retrieved under the sea in a saturated state and a kimmeridge clay shale cut from a north sea borehole both shales were highly anisotropic both texturally as revealed by scanning electron microscopy analysis and elastically as measured ultrasonically in the laboratory the strongest anisotropy was seen in the kimmeridge clay shale where up to 38 compressional wave anisotropy thomsen s parameter e and up to 58 shear wave anisotropy thomsen s parameter were observed in addition for both shales e and were found to decrease as a function of increasing confining pressure with the pore pressure drained to atmosphere while the anellipticity deviation of the slowness surfaces from ellipses was found to be positive and decreased as a function of increasing confining pressure accompanying these changes in elastic properties was a decrease in porosity with increasing confining pressure from 10 5 to 8 5 for the jurassic shale the decrease in overall anisotropy of the shales with increasing confining pressure was found to be consistent with theoretical modeling of shale properties where the shale anisotropy and anellipticity were predicted to decrease as a function of decreasing fluid filled porosity