late pleistocene and holocene palaeohydrology of lake rukwa tanzania inferred from diatom analysis diatom analysis has been used to provide information on the lake level fluctuations of lake rukwa during the last 21 7 ka diatom inferred chemical variables conductivity ph anion and cation ratios indicate that at the lgm lake level was low probably driven by a low p e ratio but with sufficient water to avoid desiccation following the lgm deep water conditions were obtained through a series of steps until maximum water depth was reached by at least 13 5 ka the high lake level phase lasted through the early holocene until after 6 7 ka a shallow saline lake has occupied the basin from ca 5 5 ka until the present day the pattern of reconstructed lake level fluctuation in lake rukwa is similar to that from lake tanganyika and other lakes from the equatorial and northern tropical zone we can confirm that the n tropical mode of millennial scale lake level change since the lgm extended to at least 8 s

mechanical and petrophysical properties of north sea shales one of the problems when evaluating potential borehole stability problems in the drilling of hydrocarbon exploration and production wells is the lack of relevant data to describe the lithologies this relates in particular to the shales since these constitute most of the overburden and are also the most troublesome this paper presents a set of data from a variety of deeply cored shales from the north sea the shales have been extensively characterised and span a wide range of both petrophysical and mechanical properties depending on for example burial history and diagenesis special characteristics of shales have been tested and described including pore size distribution permeability anisotropy and temperature dependence appropriate procedures for sample handling testing and interpretation have been developed or modified from other related sciences

late quaternary vegetation and fire dynamics on mount kenya abstract pollen and charcoal data generated from a 1469 cm core radiocarbon dated to 26 430 14 c yr bp recovered from rumuiku swamp on the southeast of mount kenya are used to document changes in the distribution and composition of montane vegetation and fire regimes over the late quaternary throughout the transition from the last glacial maximum lgm high resolution sub centennial scale analysis documents a highly dynamic ecosystem and fire regime the pollen record shows that under a cool but rather moist lgm climate ericaceae and stoebe species shifted down slope more than 1000 m relative to the present day rather than simple altitudinal lowering of current vegetation zonation these taxa formed a vegetation assemblage that mixed high altitude components with relatively lowland taxa in particular juniperus that is presently found at altitudes lower than the study site but on the drier side of mount kenya there is noticeable addition and co dominance of hagenia to the ecosystem from 20 500 14 c yr bp until around 14 000 14 c yr bp when a mix of ericaceous belt and upper montane forest taxa such as artemisia polycias schefflera and stoebe dominated the initial development of montane forest reduced levels of hagenia juniperus olea and podocarpus are recorded about the time of the younger dryas with highly variable presence of more mesic taxa such as polyscias and schefflera this development of montane forest over the late pleistocene to holocene transition reflects a significant reorganization of the ecosystem composition that was heavily influenced by a variable fire regime shifts in vegetation composition reflect the onset of a warmer moist climate from the beginning of holocene as mixed montane forest became more established the latter part of the holocene registers human impact and forest clearance with increased anthropogenic impact marked by a transition to open vegetation and increased fire frequency

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

hydrogen index and carbon isotopes of lacustrine organic matter as lake level indicators abstract optical and geochemical techniques were applied to organic matter of late quaternary sediments from lakes victoria and rukwa east africa variations in total organic carbon toc and hydrogen content expressed as hydrogen index hi can be related to known changes in lake level and used to identify periods of very low water or subaerial exposure in basinal sequences toc and hi both decline as an exposure surface is approached mainly due to the selective removal of unstable components by bacterial respiration and inorganic oxidation inert organic material of very low hi is typically concentrated at the overlying transgression surface the carbon isotopic composition of the particulate organic matter tends to be less negative in sediments associated with exposure surfaces although weathering may be partially responsible isotopic variations are mainly due to changes in the relative importance of material from plants using c4 photosynthesis such as grasses and sedges growing around the shore and on the exposed lake muds and of material from planktonic algae and terrestrial c3 plants

intrinsic elasticity of a textured transversely isotropic muscovite aggregate comparisons to the seismic anisotropy of schists and shales 1 the anisotropic properties of phyllosilicate rocks are investigated through the modeling of the pore free intrinsic elasticity of a textured muscovite aggregate the elastic constants were calculated for textures as defined using orientation distribution functions odfs ranging from perfectly aligned to completely random the odfs were described using a gaussian distribution the width of which is controlled by the standard deviation of the orientations of the crystal c axis with respect to the sample foliation plane under the assumption that the sample has hexagonal transversely isotropic symmetry the values were obtained using the widely used voigt reuss hill averaging techniques as well as the geometric mean average gma the gma is more physically meaningful in that it requires that the elastic constants determined in either the stiffness or the compliance domains be invertible comparisons between these different averages indicate that strictly the voigt and reuss values do not necessarily provide the limiting bounds as is expected for the isotropic case despite this some characteristics of the elastic wave p wave anisotropy are only weakly dependent on the averaging procedure employed the calculated elastic constants are converted to the anisotropic and parameters in order to allow for comparison to existing measurements in the literature however these intrinsic results are not intended to provide a measure of the properties of phyllites per se but to provide a reference against which more sophisticated models that incorporate porosity layering and composition may be constructed

crustal stress determination from boreholes and rock cores fundamental principles abstract obtaining quantitative information on the state of stress in the crust can only be accomplished by accessing the zone of interest by drilling the borehole cavity however disrupts the virgin stress state by concentrating stresses in predictable patterns with a number of consequences at sufficiently low stress magnitudes the concentrated stresses amplify the elastic anisotropy azimuthally around the borehole because the elastic properties of most rocks depend nonlinearly on stress at higher levels the stresses damage then fracture the rock near the wellbore wall or within a growing core stub indeed almost all of the borehole techniques indirectly measure stress through these manifestations of the concentrated stresses this contribution reviews at a fundamental level the concentration of stresses by the borehole the effects on the materials in the borehole s vicinity and how these are used to infer stress states stress concentrations applied to rocks which are generally nonlinear elastic materials induce both radial and azimuthal variations in elastic wave speeds near the borehole that can be used to infer stress directions from advanced acoustic logging methods hydraulic fractures initiate propagate and remain propped open by measured pressures that can be interpreted with knowledge of the stress concentrations to obtain quantitative stress magnitudes at higher stress levels the rock fails producing borehole breakouts or drilling induced tensile fractures both of which are indicative of stress directions and can be used to constrain stress magnitudes similarly the various styles of drilling induced core fractures indicate both stress directions and the faulting environment encountered unfortunately no method currently allows for complete determination of the stress tensor using a number of different but complementary techniques is necessary to best constrain in situ stress states

estimation of elastic stiffness coefficients of an orthorhombic physical model using group velocity analysis on transmission data abstractwe acquired 3c ultrasonic transmission seismograms measured group velocities associated with the three quasi body wave types and determined density normalized stiffness coefficients aij cij over an orthorhombic physical laboratory model the estimation of aij is based on an approximate relationship between the nine orthorhombic aij and group velocities estimation of the anisotropic aij is usually done using phase velocities with well known formulas expressing their theoretical dependence on aij however on time domain seismograms arrivals are observed traveling with group velocities group velocity measurements are found to be straightforward reasonably accurate and independent of the size of the transducers used in contrast the accuracy of phase velocities derived from the p transform analysis was found to be very sensitive to small differences in picked arrival times and to transducer size theoretical phase and group velocities calculated in a forward manner from the aij est

climate change during the last deglaciation in antarctica greenland ice core records provide clear evidence of rapid changes in climate in a variety of climate indicators in this work rapid climate change events in the northern and southern hemispheres are compared on the basis of an examination of changes in atmospheric circulation developed from two ice cores high resolution glaciochemical series covering the period 10 000 to 16 000 years ago from a central greenland ice core and a new site in east antarctica display similar variability these findings suggest that rapid climate change events occur more frequently in antarctica than previously demonstrated

comment on anomalous ocean load tide signal observed in lake level variations in tierra del fuego by a richter et al 1 where z is the deviation of the sea level from the equilibrium tide due to the dynamic forces hendershott 1972 for this small enclosed lake these dynamic forces are very small u2 is the tidal potential of degree two g the gravity value on the earth s surface and a a constant p is the self potential of the tidal water plus the potential field created by the earth s elastic deformation under the weight of the tide this potential at a particular point r can be computed by convolving the tides in the lake and all surrounding ocean tides h with a green s function g

velocity anisotropy of shales and depth estimation in the north sea basin it is known that in the north sea basin the depths to major reflectors as determined from surface seismic data are often larger than the well log depths from a study of data sets which tie 21 wells i found a strong correlation between the occurrence of the depth error and the presence of shales in the subsurface assuming that the error is caused by elliptical velocity anisotropy in shales i measured the anisotropy from a comparison of the well log sonic data and the interval velocity profile obtained from the surface seismic data and also from a comparison of the seismic depth and the well log depth it was found that the two methods of measurements agree with each other and also agree qualitatively with the previous laboratory measurements of anisotropy in shale samples the results strongly suggest that the depth anomaly in the north sea basin is caused by the velocity anisotropy of shales a simple method to correct the seismic depth is given

azimuthal velocity variations caused by borehole stress concentrations in laboratory experiments borehole stress concentrations caused by far field uniaxial stress produce measurable variations in compressional wave velocities near the borehole uniaxial stress was applied perpendicular to the axis of a borehole drilled through a sandstone block inside the fluid filled borehole refracted compressional waves were propagated in the rock parallel to the borehole axis at discrete azimuths producing a scan of velocity versus azimuth as uniaxial stress increased significant variations of compressional wave velocity with azimuth developed of the order of 10 variation the azimuth of velocity minima provides an excellent indicator of the direction of maximum far field uniaxial stress accurate to 5 the detailed variation of velocity with azimuth while approximately sinusoidal has sufficient character to provide new information about the rock one interpretation yields the curvature of the velocity versus stress curve of the rock

seismic anisotropy in sedimentary rocks part 2 laboratory data part one of this paper presents a method for measuring seismic velocities and transverse isotropy in rocks using a single core plug this method saves at least two thirds of the time for preparing core samples and measuring velocities in transversely isotropic ti rocks using this method we have measured velocity and anisotropy of many shale and reservoir rocks from oil and gas fields around the world we present some of the data in this paper which include seismic velocity and anisotropy in 17 brine saturated shale samples 1 gas and brine saturated coal sample 8 brine saturated sands 12 gas saturated sands 32 gas saturated carbonate samples and 25 brine saturated carbonate samples the results show that clays and fine layering in sedimentary rocks are the main causes of seismic anisotropy very little intrinsic anisotropy exists in unfractured reservoir rocks such as sands sandstones and carbonates under reservoir conditions in contrast all shales were found seismically anisotropic anisotr

sediment geochemical records of productivity and oxygen depletion along the margin of western north america during the past 60 000 years teleconnections with greenland ice and the cariaco basin abstract many sediment records from the margins of the californias alta and baja collected in water depths between 60 and 1200 m contain anoxic intervals laminated sediments that can be correlated with interstadial intervals as defined by the oxygen isotope composition of greenland ice dansgaard oeschger d o cycles these intervals include all or parts of oxygen isotope stage 3 ois3 60 24 cal ka the bolling allerod warm interval b a 15 13 cal ka and the holocene this study uses organic carbon corg and trace element proxies for anoxia and productivity namely elevated concentrations and accumulation rates of molybdenum and cadmium in these laminated sediments to suggest that productivity may be more important than ventilation in producing changes in bottom water oxygen bwo conditions on open highly productive continental margins the main conclusion from these proxies is that during the last glacial interval lgi 24 15 cal ka and the younger dryas cold interval yd 13 11 6 cal ka productivity was lower and bwo levels were higher than during ois3 the b a and the holocene on all margins of the californias the corg and trace element profiles in the lgi b a holocene transition in the cariaco basin on the margin of northern venezuela are remarkably similar to those in the transition on the northern california margin correlation between d o cycles in greenland ice with gray scale measurements in varved sediments in the cariaco basin also is well established synchronous climate driven changes as recorded in the sediments on the margins of the californias sediments from the cariaco basin and in the gisp 2 greenland ice core support the hypothesis that changes in atmospheric dynamics played a major role in abrupt climate change during the last 60 ka millennial scale cycles in productivity and oxygen depletion on the margins of the californias demonstrate that the california current system was poised at a threshold whereby perturbations of atmospheric circulation produced rapid changes in circulation in the eastern north pacific ocean it is likely that the pacific and atlantic oceans were linked through the atmosphere warmer air temperatures during interstadials would have strengthened hadley and walker circulations which in turn would have strengthened the subtropical high pressure systems in both the north pacific and the north atlantic producing increased rainfall over the cariaco basin and increased upwelling along the margins of the californias

sedimentation in lake malawi east africa during the past 10 000 years a continuous paleoclimatic record from the southern tropics a suite of piston cores recovered from lake malawi 9 14 s 34 35 e east africa in 1986 has been analyzed for major and minor elements organic c and n calcium carbonate and diatoms an internally consistent stratigraphy was constructed from calcium carbonate abundance and variations in the two most abundant diatom genera stephanodiscus and melosira with age control obtained primarily from 14c dating of the carbonate differences with time in fe abundance in a transect of cores from different water depths have been interpreted to reflect changes in chemocline depth the depth to the chemocline was on the order of 100 m shallower than present prior to 3500 yr b p indicating less seasonality carbonate production and preservation appears to be related to climatically induced changes in both salinity and chemical distributions in the water column the carbonate which precipitates from surface waters is most abundant during the interval from about 10 000 to 6000 yr b p this micrite most likely represents periods of low lake level when salinity increased and carbonate precipitation was enhanced sedimentary evidence suggests that lake levels were 100 150 m lower than present during this period this record is different from climatic trends in northern intertropical africa but appears to also be related to changes in insolation and monsoon circulation this is the northernmost basin in africa reported to exhibit a southern hemisphere response to the early holocene northern hemisphere summer insolation maximum the climatic hingeline north of lake malawi 9 s implied by our results is significantly south of that indicated by general circulation model simulations however the cores show evidence for periods of abrupt climate change during the interval of generally arid climate

the palaeolimnology of northern lake malawi over the last 25 ka based upon the elemental and stable isotopic composition of sedimentary organic matter abstract sediments accumulating in offshore areas of northern lake malawi typically contain 1 4 organic carbon this organic matter om is of mainly phytoplankton origin with varying mixtures of terrestrial and degraded reworked material high resolution stable carbon and nitrogen isotopic elemental and rock eval pyrolysis analyses of the bulk om contained in three cores have been used to characterise the om and provide detailed insights into the palaeolimnology and palaeoclimatology of northern lake malawi over the last 25 ka preservation of the om as reflected by its rock eval hydrogen index hi is highly variable periods characterised by notably low hi 15 n during the mid and late holocene suggest intervals of stable stratification due to significantly reduced wind driven mixing in the northern part of the lake when n fixing cyanophytes formed a major component of the phytoplankton the geochemical data support recent suggestions of lowstand conditions in lake malawi at the time of the lgm and provide additional evidence of periods of intense but localised aridity in the terminal pleistocene earliest holocene and at the time of the little ice age the change in mixing regime at ca 11 8 ka seems to have been the result of a radical reorganisation of the dominant surface wind flow over south eastern africa the transition was within the resolution of currently available chronologies apparently contemporaneous with marked changes in the circulation patterns of adjacent oceans and the climate of more distant parts of the northern hemisphere

paleo environmental change in amazonian and african rainforest during the lgm abstract the paper provides new and comparative insight into the ecological history of the two largest continental tropical forest areas during the last glacial maximum lgm the tropical forest regions are of particular interest because they present a large source of heat and have been shown to have significant impact on the extra tropical atmospheric circulation they are also the most intense land based convective centers thus especially from the tropics paleoecological information is needed as benchmarks for climate modeling the african data for lgm climates were published earlier including the reconstructed paleoprecipitation patterns deduced from ssts the tropical south american lgm data were interpreted from pollen geochemical and 18o stable oxygen isotope data from brazil and selected surrounding areas the available terrestrial data are consistent with the sst derived precipitation data for the tropical forests in brazil and for africa however the impact of lgm climate extremes was less severe in the amazon than in the congo basin the lgm humid forest area including evergreen and semi deciduous forest types in africa was probably reduced by 84 in contrast the amazon humid forest area probably shrank to 54 of their present day extension still there are different interpretations with respect to the amount of reduction of the amazon forest area during the lgm although direct information about lgm climates in amazonia is still limited the more detailed map obtained in the present work however allows a more reliable characterization of the last glacial tropical environment than previously published for the amazon region

anomalous ocean load tide signal observed in lake level variations in tierra del fuego 1 we demonstrate the application of a 100 km long lake as a sensor for studying the tidal effects on tierra del fuego main island the lake level variations observed in lago fagnano reflect both the direct response to the tidal potential and the indirect effect of the ocean tidal loading modeling both contributions explains the observed tidal signal in the lake to about 70 underestimated model load tide amplitudes are found to be probably responsible for the remaining difference we interpret this discrepancy as a hint for regional elastic lithosphere properties differing substantially from those represented by currently available global models

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

borehole stability analysis accounting for anisotropies in drilling to weak bedding planes abstract borehole instabilities pose significant challenges to drilling and completion operations particularly in regions with weak bedding planes and pre existing fractures where formations have strong anisotropies the bedding planes rock anisotropy and their impacts on horizontal stresses are considered in the proposed model to improve borehole stability modeling this improved model enables to calculate borehole failures and minimum mud weight along borehole trajectories with various drilling orientations versus bedding directions laboratory test data of rock compressive strengths are analyzed and a new correlation is developed to allow for predicting uniaxial compressive strengths in weak rocks from sonic velocities time dependent rock compressive strength is also examined to analyze the wellbore failure evolution with time the slip failure gradient in the weak planes is derived which can be used to model wellbore sliding shear failure in the planes of weakness the mud weight applied to prevent borehole shear failures in both intact rocks and ones with weak bedding planes can be obtained from the proposed model

the effect of anisotropy on the young s moduli and poisson s ratios of shales young s modulus and poisson s ratio are required for geomechanics applications such as hydraulic fracture design analysis of wellbore stability and rock failure determination of in situ stress and assessment of the response of reservoirs and surrounding rocks to changes in pore pressure and stress shales are usually anisotropic and models that neglect shale anisotropy may fail to describe geomechanical behaviour correctly anisotropy in shales results from a partial alignment of anisotropic clay particles kerogen inclusions microcracks low aspect ratio pores and layering for shales the young s modulus measured parallel to bedding e1 is usually greater than the young s modulus measured perpendicular to bedding e3 however the poisson s ratio 31 corresponding to stress applied perpendicular to bedding and strain measured parallel to bedding can be greater than equal to or less than the poisson s ratio 12 for stress applied parallel to bedding and strain measured parallel to bedding for transverse isotropy the elastic anisotropy resulting from a partial alignment of clay particles can be written in terms of the coefficients w200 and w400 which describe the impact of clay particle orientation on the anisotropy of shales disorder in the orientation of clay particles acts to reduce w400 faster than w200 since w400 is a higher order moment of the clay particle orientation distribution function than w200 this is confirmed by analysis of measured anisotropy parameters for shales a partial alignment of clay particles is consistent with the measured young s moduli for shales and with values of poisson s ratio 31 12 but not with values 31 12 these values can be explained if there exist kerogen inclusions microcracks or low aspect ratio pores aligned parallel to the bedding plane

lunar tides in loch ness scotland 1 measurements have been made of the astronomical tide in loch ness scotland which is not directly connected to marine tides our measurements of the loch tide are so far as we know the first in a european lake where the tide originates primarily from ocean tide loading loch ness is a readily accessible lake and is in a region for which the neighboring ocean tides are large and described well by modern global ocean tide models the principal tidal constituent m2 was observed to have an amplitude of approximately 1 5 mm and to be in antiphase at each end of the loch these values are in close agreement with the theoretical combined effects of the direct gravitational tide body tide and the tilt effects due to ocean tide loading computed using green s functions based on conventional elastic earth models by analyzing over long periods for coherent tidal signals we are able to significantly improve the signal to noise ratio in the tilt values compared with values obtained by direct level differencing our tilt accuracy of better than 10 8 measured over 35 km demonstrates loch ness as one the world s longest and most accurate tiltmeters despite this unprecedented accuracy earth tidal models are still at least as accurate as our ability to measure them

geomechanical and ultrasonic characterization of a norwegian sea shale anisotropy of velocity in shaly overburden is known to cause significant problems for geophysical interpretation including depth conversion and fluid identification in addition mechanical and dynamic elastic shale behavior is not well understood because few tests have been performed on well preserved samples multiple stage triaxial tests were performed upon horizontal core plugs of a shale from the norwegian sea with a view to evaluating rock strength and the evolution of ultrasonic response during rock deformation in addition standard rock physical properties were characterized as well as composition the shale microfabric is seen to be strongly laminated with alternating thick clay rich laminae and thin silt rich laminae occasional microfractures are also noted parallel to these laminations the shale has low friction coefficient and cohesive strength and shows anisotropy of these parameters when the maximum principal stress is oriented parallel to and at 45 to the microfabric the orientation of the maximum principal stress parallel to the intrinsic fabric and microcracks was seen to significantly impact on velocity normal to the fabric as stress parallel to the fabric increased s wave anisotropy was significantly affected by the increasing stress anisotropy stress orientation with respect to fabric orientation was therefore found to be an important control on the degree of anisotropy of dynamic elastic properties in this shale

the shear properties of oil shales a vast unexploited source of hydrocarbons is oil shales i e shales rich in kerogen the schlumberger oilfield glossary defines kerogen as the naturally occurring solid insoluble organic matter that occurs in source rocks and can yield oil upon heating the original organic constituents of kerogen are algae and woody plant material kerogens have a high molecular weight relative to bitumen or soluble organic matter bitumen forms from kerogen during petroleum generation estimates vary as to how much oil is contained in oil shale reserves the united states office of naval petroleum and oil shale reserves estimates some 1 6 trillion barrels of oil are contained in oil shales around the world with 60 70 of reserves 1 0 1 2 trillion barrels in the united states most u s oil shale is concentrated in the green river formation in wyoming utah and colorado covering 16 000 square miles

environmental changes at lake cheshi zambia since 40 000 years b p abstract sediment and microfossil analyses of a 7 5 m core from lake cheshi suggest that south central africa experienced late quaternary climate changes similar to those in east africa the lake formed around 34 000 yr b p after a prelacustrine phase of at least 6000 yr from climatic or tectonic causes ratios of precipitation to evaporation were probably similar to those of today until a decline about 15 000 13 000 yr b p when the lake shrank and became chemically concentrated maximal lake levels occurred between 8000 and 4000 yr b p and were followed by a low stand under presumably arid conditions about 3500 yr b p encroachment of sudd vegetation contributed to shallowing during the last 3000 yr a phase of microfossil dilution may reflect human activity in the basin or climatic or hydrological changes melosira valve morphology seems to reflect mixing regimes sponge and testate amoeba remains were most numerous relative to diatoms during low water phases

petrophysical implications of source rock microfracturing abstract hydrocarbon generation induced microfracturing in source rocks is important affecting both primary migration of hydrocarbons and unconventional shale plays in this study petrographic observations rock eval and total organic carbon content toc data and log derived physical properties were analyzed in order to evaluate eventual petrophysical implications of microfracturing induced by hydrocarbon generation petrographically evidences of microfracturing both normal and parallel to bedding have been found horizontal microfractures typically cross cut each other at an angle between 30 and 90 horizontal microfractures are readily imaged compared to vertical microfractures vertical microfractures were clearly observed only in rare cases in samples with a high degree of microfracturing in the case of moderate microfracturing the vertical component of microfractures may be too small to be detectable using conventional microscopy techniques longer vertical components of the microfractures may be facilitated by higher toc and particularly dense laminated organic matter fabric as well as higher thermal maturity levels clay minerals particularly smectites are also playing a role in reducing permeability and thereby increasing the local pressure generated by hydrocarbon generation eventually resulting in microfracturing log derived petrophysical properties particularly p wave velocity vp and formation resistivity can give information on microfracturing potential even though the techniques can t resolve the microfractures directly vp has been postulated to be sensitive to organic matter quantity and fabric whereas formation resistivity is largely governed by fluid content and clay minerals a reverse trend of increasing resistivity with decreasing vp was found and is linked to higher hydrocarbon saturation and toc and or dense laminated fabric of organic matter favoring microfractures the implications of the log derived petrophysical results are in agreement with the direct petrographic observations formation resistivities in the source rocks however are lower than those observed in non clay dominated hydrocarbon saturated intervals this indicate that low permeable organic rich source rocks with a high clay content particularly smectites may facilitate formation of the hydrocarbon filled vertical microfractures even at relatively low thermal maturities

climate change at the end of the old kingdom in egypt around 4200 bp new geoarchaeological evidence the paper presents compilation of geological and geoarchaeological data based on excavations at the saqqara necropolis to denote climate variability in egypt during the late old kingdom around 2200 bc a change in climate in that time was expressed firstly by aridification and low floods of the nile but also by occasional heavy rainfalls in northern egypt low nile floods were probably a consequence of decreased summer precipitation in the ethiopian highlands that resulted in catastrophically low discharges into the blue nile drainage basin these weaker summer monsoons in ethiopia and gradual aridification in egypt that started about 5000 cal bp were coincident with a southward progressing shifting of the summer intertropical convergence zone in africa simultaneous intensive rainfalls resulted in wide spread sheet flood accumulations attested by archaeological evidence in northern egypt these rainfalls could be triggered by the north atlantic oscillation both these reasons caused a rapid collapse of the old kingdom at about 4200 cal bp

pollen based vegetation changes in southern tanzania during the last 4200 years climate change and or human impact the age constrained pollen data of a sedimentary sequence from the crater lake masoko southern tanzania 9 20ps 33 45pe 770 m display a continuous record of vegetation for the past 4200 years this record provides evidence that wetter zambezian woodlands always occupied this area during the late holocene reaching a maximum extent between 2800 and 1650 cal yr bp related to increase in summer monsoon intensity however three main episodes of decline have been detected between 3450 and 2800 cal yr bp between 1650 and 1450 cal yr bp and from 1200 to 500 cal yr bp for which a climatic interpretation decrease in the summer monsoon strength was preferentially advanced the first is synchronous with lowstand of many tropical african lakes and so mainly induced by increased aridity in contrast the abrupt change in the pollen record at 1650 1550 cal yr bp is marked by a large extension of grasslands at the expense of arboreal cover further by an increase in ricinus communis and an intensification of burning it could thus indicate local clearance of vegetation by man however at the same time the decline of montane forest suggests the impact of a more regional change during the last episode between 1200 and 500 cal yr bp dry climatic conditions are inferred from a combination of pollen diatom and magnetic proxies although the occurrence of late iron age settlements in the region means that local human interference cannot be excluded this study illustrates the difficulties in deciphering ecological and anthropological changes from pollen data in african tropical regions a 2003 elsevier b v all rights reserved

noncontacting benchtop measurements of the elastic properties of shales we evaluated a laser based noncontacting method to measure the elastic anisotropy of horizontal shale cores whereas conventional transducer data contained an ambiguity between phase and group velocity measurements small laser source and receiver footprints on typical core samples ensured group velocity information in our laboratory measurements with a single dense acquisition of group velocity versus group angle on a horizontal core we estimated the elastic constants c11 c33 and c55 directly from ultrasonic waveforms and c13 from a least squares fit of modeled to measured group velocities the observed significant p wave velocity and attenuation anisotropy in these dry shales were almost surely exaggerated by delamination of clay platelets and microfracturing but provided an illustration of the new laboratory measurement technique although challenges lay ahead to measure preserved shales at in situ conditions in the lab we evaluated the fundamental advantages of the proposed method over conventional transducer measurements

sediments soils and the expansion of farmers into a forager s world a geoarchaeological study of the mid to late holocene in hwange national park zimbabwe abstract the transition from foraging to food production in interior southern africa is still not well understood despite being central to hypothesized migration and diffusion routes connecting the earliest agropastoralist sites in central africa with the southern african subcontinent hwange national park 14 650 sq km zimbabwe is located along these proposed routes and has both late stone age and early iron age sites that can address this transition to better understand this change in lifeways and any environmental and or topographic factors that influenced it geoarchaeological investigations characterized the soils sediments and landforms in six different areas of the park forty eight stratigraphic sections varying from two to four m thick were described six of which are detailed here these stratigraphic sections include dated material that allows correlation between basins of different substrates and comparisons of carbon isotopic signatures to reveal whether conditions were locally favorable to grasses hot dry or woodlands cool wet through time examined river cuts and augered samples show that some river basins have sediment textures soil types and topographic relief that should have been attractive to farmers however radiocarbon ages reveal variable potential for finding remains of any early farmers from basin to basin with some bedrock constrained valleys having no preserved deposits of appropriate age the repeated scouring of these river basins testifies to sometimes volatile conditions that would have challenged sustained food production in fact in the study area landscape instability and variable environmental conditions were commonplace during the past three millennia drought between ca 2 8 and 2 6 ka may have lessened the risk of disease vectors for the earliest agropastoralists that arrived during more mesic conditions between 1 9 and 1 65 ka at this time basins began to accumulate fill and soil development suggests landscape stability even in headland basins changes in soil carbon isotope values imply relatively hot dry conditions prevailed around 0 73 ka and 0 41 ka and cool wet conditions around 0 53 ka but much information has been lost due to the numerous erosional events that wiped away swaths of sediments during intervening droughts the landscape in hwange national park continues to change rapidly as reflected by several meters of river downcutting in response to recent man made disturbance and or drought this downcutting both reveals and endangers the few remaining archaeological sites that are critical to understanding the first food producers in the region

anisotropic elastic moduli of carbonates and evaporites from the weyburn midale reservoir and seal rocks assuming vertical transverse isotropy vti symmetry the elastic anisotropy as a function of confining pressure of four carbonates and one evaporite from the williston sedimentary basin in saskatchewan canada is investigated using the ultrasonic pulse transmission method ultrasonic p and s wave velocities are obtained from cylindrical plugs cut from a main sample along horizontal vertical and 45 orientations with respect to the sample s presumed vertical axis of symmetry the elastic constants were then calculated from the measured velocities and densities anisotropy was quantified by estimating thomsen parameters thomsen 1986 from elastic constants the results show that the samples are at the best weakly anisotropic the presence of microcracks and pores as well as the heterogeneity of the samples play an important role in defining the p and s wave velocities the weak anisotropy found in these samples suggests that intrinsic properties of these rocks negligibly contribute to the anisotropy observed at the seismic scale

climatic and volcanic forcing revealed in a 50 000 year diatom record from lake massoko tanzania the interactions between climatic and volcanic forcing on diatom communities contained in a 50 000 year sedimentary sequence from lake massoko tanzania were examined at the century scale 19 discrete tephra inputs to the lake isolated the sedimentary nutrient supply and shifted the diatom communities to those tolerant of low phosphorus levels whereas at the millennial scale diatom inferred shifts in precipitation evaporation based on conductivity optima and diatom life form ratios were broadly similar to lake level reconstructions from lake rukwa lake malawi and others in the region some fluctuations of lake massoko are consistent with the precession driven changes in insolation but the major climate shifts do not relate directly to orbital forcing of summer insolation south of the equator and show more consistency with records from the equatorial and northern tropics that receive rainfall from the passing of the intertropical convergence zone sea surface temperatures are strongly correlated to multimillennial scale climate patterns over this region of africa

palynological evidence of climate change and land degradation in the lake baringo area kenya east africa since ad 1650 abstract paleoenvironmental records derived from pollen fungal spores and microscopic charcoal from lake baringo kenya reveal a largely dry environment in the east african region since ad 1650 the dry environment is punctuated by a succession of centennial to decadal scale wet and dry episodes disjointed by sharp transitions including two intense dry episodes that led to drying of the lake at ca ad 1650 and ad 1720 which coincide with the little ice age lia period in europe the baringo record shows that land degradation in the area began prior to the colonial period in east africa and has persisted to the present land degradation and increased soil erosion in the lake baringo drainage basin was severe enough to significantly age the lake sediments due to influx of old carbon resulting in the dating inversion

the southwest indian monsoon over the last 18 000 years previously published results suggest that the strength of the sw indian monsoon can vary significantly on century to millenium time scales an observation that has important implications for assessments of future climate and hydrologic change over densely populated portions of asia we present new well dated multi proxy records of past monsoon variation from three separate arabian sea sediment cores that span the last glacial maximum to late holocene to a large extent these records confirm earlier published suggestions that the monsoon strengthened in a series of abrupt events over the last deglaciation however our data provide a somewhat refined picture of when these events took place and suggest the primacy of two abrupt increases in monsoon intensity one between 13 and 12 5 ka and the other between 10 and 9 5 ka this conclusion is supported by the comparisons between our new marine data and published paleoclimatic records throughout the african asian monsoon region the comparison of data sets further supports the assertion that maximum monsoon intensity lagged peak insolation forcing by about 3000 years and extended from about 9 5 to 5 5 ka the episodes of rapid monsoon intensification coincided with major shifts in north atlantic european surface temperatures and ice sheet extent this coincidence coupled with new climate model experiments suggests that the large land sea thermal gradient needed to drive strong monsoons developed only after glacial conditions upstream of and on the tibetan plateau receded cold north atlantic sea surface temperatures european ice sheets and extensive asian snow cover it is likely that abrupt changes in seasonal soil hydrology were as important to past monsoon forcing as were abrupt snow related changes in regional albedo our analysis suggests that the monsoon responded more linearly to insolation forcing after the disappearance of glacial boundary conditions decreasing gradually after about 6 ka our data also support the possibility that significant century scale decreases in monsoon intensity took place during the early to mid holocene period of enhanced monsoon strength further highlighting the need to understand paleomonsoon dynamics before accurate assessments of future monsoon strength can be made

model proxy and isotopic perspectives on the east african humid period abstract both north and east africa experienced more humid conditions during the early and mid holocene epoch 11 000 5000 yr bp 11 5 ka relative to today the north african humid period has been a major focus of paleoclimatic study and represents a response of the hydrological cycle to the increase in boreal summer insolation and associated ocean atmosphere and land surface feedbacks meanwhile the mechanisms that caused the coeval east african humid period are poorly understood here we use results from isotope enabled coupled climate modeling experiments to investigate the cause of the east african humid period the modeling results are interpreted alongside proxy records of both water balance and the isotopic composition of rainfall our simulations show that the orbitally induced increase in dry season precipitation and the subsequent reduction in precipitation seasonality can explain the east african humid period and this scenario agrees well with regional lake level and pollen paleoclimate data changes in zonal moisture flux from both the atlantic and indian ocean account for the simulated increase in precipitation from june through november isotopic paleoclimate data and simulated changes in moisture source demonstrate that the western east african rift valley in particular experienced more humid conditions due to the influx of atlantic moisture and enhanced convergence along the congo air boundary our study demonstrates that zonal changes in moisture advection are an important determinant of climate variability in the east african region

dynamic and static moduli static and dynamic bulk moduli ks and kd are measured as continuous functions of pressure from zero to 2 3 kilobars for two sandstones a tuff limestone granite and oil shale results for the sandstones and granite are in good agreement with previously reported data with ks kd varying from about 0 5 at atmospheric pressure to close to unity at pressures 2 kilobars and above for rocks behaving elastically under static loading the ks kd ratio is inversely related to the microcrack density for the limestone time dependent deformation associated with pore collapse results in ks kd ratios approaching 0 1 at high pressure upon unloading while initially high 1 0 at high pressures ks kd becomes lower than values obtained during loading at low pressures 1 kilobar due to opening of microcracks generated during pore collapse for the oil shale in which few microcracks exist ks kd remains relatively constant with pressure at a value of about 0 7

organic matter geochemistry and petrography of late cretaceous cenomanian turonian organic rich shales from the belle fourche and second white specks formations west central alberta canada abstract organic rich mudstones with up to 10 wt toc from the upper portion of the belle fourche formation and the lower part of the second white specks formation in the western canada sedimentary basin were evaluated as source rocks both geochemistry and organic petrography indicate an open marine paleoenvironment with deposition of type ii kerogen based on the predominance of marine alginite and amorphous organic matter om limited amounts of terrigenous vitrinite and inertinite macerals the presence of marine fossils and the low ratio of toc to total sulfur 1 26 the prevalence of short chain n alkanes n c13 to n c19 a predominance of c28 h 20s steranes and small concentrations of oleanane confirm the dominantly algal and planktonic origin of om alternating oxic to anoxic paleoenvironmental sedimentary conditions are proposed based on common bioturbation abundant inoceramid prisms and good organic richness biomarker distributions are consistent with intermittent anoxia without unequivocal evidence for water column stratification or hypersalinity the thermal maturity measured in seven sediment cores by different methods consistently indicates a westward increase in maturity according to vitrinite reflectance tmax and hopane and sterane biomarkers two cores are thermally immature 0 42 ro one is early mature 0 65 ro and four cores are within the oil window 0 78 to 0 89 ro all thermally mature cores retain good to very good hydrocarbon potential 248 mg hc g rock and are dominantly oil prone and minor gas prone based on their maceral compositions the upper belle fourche and lower second white specks formations represent potential targets for unconventional light shale oil production

a late holocene paleoclimatic history of lake tanganyika east africa abstract a nearshore core lt03 05 from the north basin of lake tanganyika provides diatom pollen and sedimentary time series covering the last ca 3800 yr at 15 36 yr resolution a chronology supported by 21 ams dates on terrestrial and lacustrine materials allows us to account for ancient carbon effects on 14c ages and to propose refinements of the region s climatic history conditions drier than those of today were followed after ca 3 30 ka by an overall wetting trend several century scale climate variations were superimposed upon that trend with exceptionally rainy conditions occurring 1 70 1 40 ka 1 15 0 90 ka 0 70 0 55 ka and 0 35 0 20 ka around 0 55 0 35 ka during the sporer sunspot minimum drier conditions developed in the northern tanganyika basin while more humid conditions were registered at lakes victoria and naivasha this indicates significant variability in the nature and distribution of near equatorial rainfall anomalies during much of the little ice age

interhemispheric dynamics of the african rainbelt during the late quaternary abstract the spatial pattern of precipitation variability in tropical and subtropical africa over the late quaternary has long been debated prevailing hypotheses variously infer 1 insolation controlled asymmetry of wet phases between hemispheres 2 symmetric contraction and expansion of the tropical rainbelt and 3 independent control on moisture available in southern africa via sea surface temperatures in the indian ocean in this study we use climate model simulations covering the last glacial cycle 120 kyr with hadcm3 and the multi model ensembles from pmip3 the palaeoclimate model intercomparison project to investigate the long term behaviour of the african rainbelt and test these simulations against existing empirical palaeohydrological records through regional model data comparisons we find evidence for the validity of several hypotheses with various proposed processes occurring concurrently but with different regional emphasis e g asymmetric shifts at the seasonal extremes and symmetric expansions contractions towards west equatorial regions crucially variations in rainfall are associated with multiple forcing mechanisms that vary in their dominance both spatially and temporally over the glacial cycle an important consideration when interpreting and extrapolating from often relatively short palaeoenvironmental records

sea surface temperatures during the sw and ne monsoon seasons in the western arabian sea over the past 20 000 years abstract the western arabian sea is influenced by the seasonal southwest and northeast monsoon wind systems in the modern day sw monsoon induced upwelling of cold water leads to lowest ssts in summer while in glacial periods weakened upwelling and increased cooling by ne monsoon winds may have resulted in lowest sst in winter in order to reconstruct the western arabian sea seasonal sea surface temperature sst evolution during the monsoons over the past 20 000 years a novel proxy is used that captures the contrast in seasonal ssts the presented proxy concerns the difference in 18o between two shallow dwelling species of planktonic foraminifera globigerinoides ruber which calcifies throughout the year and globigerina bulloides which mainly calcifies during sw monsoon driven upwelling we convert this 18o difference to the difference in calcification temperature tcrub bul and use it in combination with mg ca derived temperatures to reconstruct seasonal ssts we present the monsoon evolution record of core niop929 located nw of the island of socotra based on our results we distinguish three distinct modes i a glacial monsoon mode 20 13 ka bp characterised by weakly negative tcrub bul values and sw monsoon ssts tsw that are 2 c higher than ne monsoon ssts tne this pattern is indicative for a weak sw monsoon and stronger cooling by glacial ne monsoon winds compared to the modern situation ii a transitional mode 13 8 ka bp characterised by tcrub bul values around 0 and thus a reduced difference between tsw and tne iii the modern monsoon mode which started at 8 ka bp with the main shift toward low summer ssts and high winter ssts the modern mode is characterised by tcrub bul values around 4 c indicative of a strong sw monsoon associated with strong upwelling and weak influence of the ne monsoon on sst we propose that measuring mg ca on both species will improve the accuracy of the sst estimates

late pleistocene paleohydrography and diatom paleoecology of the central basin of lake malawi africa analysis of sedimentary diatom assemblages 10 to 144 ka form the basis for a detailed reconstruction of the paleohydrography and diatom paleoecology of lake malawi lake level fluctuations on the order of hundreds of meters were in ferred from dramatic changes in the fossil and sedimentary archives many of the fossil diatom assemblages we observed have no analog in modern lake malawi cyclotelloid diatom species are a major component of fossil assemblages prior to 35 ka but are not found in significant abundances in the modern diatom communities in lake malawi salinity and alkalin ity tolerant plankton has not been reported in the modern lake system but frequently dominant fossil diatom assemblages prior to 85 ka large stephanodiscoid species that often dominate the plankton today are rarely present in the fossil record prior to 31 ka similarly prior to 31 ka common central basin aulacoseiroid species are replaced by species found in the shallow well mixed southern basin surprisingly tychoplankton and periphyton were not common throughout prolonged lowstands but tended to increase in relative abundance during periods of inferred deeper lake environments a high resolution lake level reconstruction was generated by a principle component analysis of fossil diatom and wetsieved fossil and mineralogical residue records prior to 70 ka fossil assemblages suggest that the central basin was periodically a much shallower more saline and or alkaline well mixed environment the most significant reconstructed lowstands are 600 m below the modern lake level and span thousands of years these conditions contrast starkly with the deep dilute dysaerobic environments of the modern central basin after 70 ka our reconstruction indicates sustained deeper water environments were common marked by a few brief but significant lowstands high amplitude lake level fluctuations appear related to changes in insolation seismic reflection data and additional sediment cores recovered from the northern basin of lake malawi provide evidence that supports our reconstruction

understanding the impacts of late quaternary climate change in tropical and sub tropical regions abstract the understanding of climate impacts on tropical rivers and catchments has developed in part from models developed for temperate landscapes and a common rhythm is often apparent when millennial scale studies are compared at this level issues arising from the complex response of rivers to internal and external factors appear less important than the climate driven signal understanding the fluvial record however depends on the availability and interpretation of other sources of proxy data such as pollen spectra lake levels and ocean cores in many rainforest areas few pollen records exist and many of these reveal a hiatus at the time of the last glacial maximum examples of fluvial archives from africa indonesia and tropical queensland australia are discussed in relation to independent records of vegetation change and inferences are made concerning critical episodes in fluvial history focusing on the transitions from oxygen isotope stage ois 3 towards glacial conditions in ois2 and the pleistocene holocene transition to modern climates links between climate vegetation change and fluvial activity are presented as descriptive models

new evidence for a reduced water balance in east africa during the last glacial maximum implication for model data comparison some syntheses of lake level data for the last glacial maximum lgm in east africa 10 n and 30 s east of 25 e show apparently wetter conditions than present for some basins whereas palaeovegetation reconstruction indicates a generally dry climate pmip gcm simulations for the lgm support both scenarios for this region when run under different boundary conditions here we compare three new records from lakes in the data poor southern part of east africa lake malawi lake massoko and lake rukwa we also re assess previously published lake level data and apply a salinity transfer function to the diatom record from lake manyara our results show that in contrast to previous interpretations these lakes were at least as low as today at the lgm and are thus in agreement with the palaeovegetation data relative drought across east africa is best simulated by gcms that use computed ssts rather than the higher climap values lower ssts and the presence of the northern hemisphere ice sheets must have been dominant over any monsoon precipitation rise caused by astronomically induced summer insolation enhancement in the southern african tropics

late quaternary stream sedimentation in the humid tropics a review with new data from ne queensland australia there is now a wide agreement that temperature depression in the humid tropics during the last glacial maximum lgm was at least 5 c most estimates of precipitation reduction at the lgm range from 25 30 to 50 65 based on proxy data but the recent ccm1 model envisages only around 12 dates obtained from river sediments indicate major changes to fluvial activity in the late quaternary isotope zone 3 sediments 58 28 ka bp are widespread and possibly indicate cooler conditions post 28 ka bp and certainly post 21 ka bp river regimes altered radically towards fan building braiding or major reduction in all activity this paper reports on fan formation in ne queensland between 26 and 14 ka bp and reviews evidence for comparable changes in humid tropical areas of s america w africa and se asia including records of holocene sedimentation within a global rhythm of major changes to river regimes in the humid tropics during the late quaternary it is now possible to detect regional variations in stream response to climatic change at the lgm reductions in stream power may have led to fan formation in ne queensland while vegetation changes may have contributed to increased sediment loads and braiding in some forest marginal areas but in w africa greater aridity may have been responsible for enfeebled streams leaving few records channel cutting then deposition of coarse sediment in braided rivers marked the transition to the early holocene in w africa and fans became entrenched in ne queensland this regime persisted until forest recovery was complete by 9 5 8 5 ka bp when widespread overbank deposition occurred and a change towards meandering channels took place widely across the humid tropical zone followed by several cut and fill episodes in the middle and late holocene

a comparative study of the anisotropic dynamic and static elastic moduli of unconventional reservoir shales implication for geomechanical investigations abstractwe obtained the complete set of dynamic elastic stiffnesses for a suite of shales representative of unconventional reservoirs from simultaneously measured p and s wave speeds on single prisms specially machined from cores static linear compressibilities were concurrently obtained using strain gauges attached to the prism regardless of being from static or dynamic measurements the pressure sensitivity varies strongly with the direction of measurement furthermore the static and dynamic linear compressibilities measured parallel to the bedding are nearly the same whereas those perpendicular to the bedding can differ by as much as 100 compliant cracklike porosity seen in scanning electron microscope images controls the elastic properties measured perpendicular to the rock s bedding plane and results in highly nonlinear pressure sensitivity in contrast those properties measured parallel to the bedding are nearly insensitive to stress this anisotropy to the pressure dependency of the stra

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

geodetic measurement of the local elastic response to the changing mass of water in lago laja chile abstract a geodetic station antc built in bedrock in southern chile is undergoing non steady vertical motion within a range of nearly 50 mm these fluctuations are dominated by the earth s local elastic response to the changing weight of water in a reservoir located about 20 km away there is also an annual periodic component of motion that is attributed to global and regional patterns of loading as well as a steady tectonic signal the local loading response constrains the average elastic structure of the area and implies a stiffness that falls near the lower end of the range observed in rock samples