circulations between mesoscale convective systems along a cold front abstract a frontal squall line that passed over the central united states on 14 15 june 1985 consisted of several primary mesoscale convective systems mcss with a prominent gap several hundred kilometers wide between two of them over central kansas and oklahoma an upper level jet streak crossed the front at right angles with the jet exit region centered in the gap surface upper air radar and satellite data are used to examine conditions contributing to this squall line gap all surface indicators e g a sharp wind shift line and strong low level moisture convergence pointed toward the development of new deep convection in the gap computations of vertical velocity using sounding data showed that the low level vertical motion was indeed upward there however strong subsidence existed aloft the demarcation between subsidence aloft and ascent at low levels was near the 0 c level where there was a concurrent inflow into the mcss a strong inversion existed near this level which effectively ca

a diagenesis model for geomechanical simulations formulation and implications for pore pressure and development of geological structures forward basin modelling is routinely used in many geological applications with the critical limitation that chemical diagenetic reactions are often neglected or poorly represented here a new temperature dependent kinetic diagenesis model is formulated and implemented within a hydromechanical framework the model simulates the macroscopic effects of diagenesis on 1 porosity loss 2 sediment strength 3 sediment stiffness and compressibility 4 change in elastic properties 5 increase in tensile strength due to cementation and 6 overpressure generation a brief overview of the main diagenetic reactions relevant to basin modelling is presented and the model calibration procedure is demonstrated using published data for the kimmeridge clay formation the calibrated model is used to show the implications of diagenesis on prediction of overpressure development and structural deformation the incorporation of diagenesis in a uniaxial burial model results in an increase in overpressure of up to 9 mpa due to both stress independent porosity loss and overpressure generated by disequilibrium compaction caused by a reduction in permeability finally a compressional model is used to show that the incorporation of diagenesis within geomechanical models allows the transition from ductile to brittle behaviour to be captured due to the increase in strength that results in an over consolidated stress state this is illustrated by comparison of the present day structures predicted by a geomechanical only model where a ductile fold forms and a geomechanical model accounting for diagenesis in which a brittle thrust structure is predicted

experimental analysis of the water retention behaviour of shales shales are among the most commonly considered geomaterials in current energy related geomechanical investigations as they are involved in engineering applications such as the extraction of natural gas co2 sequestration and nuclear waste storage a deep understanding of their behaviour with regard to variations in the degree of saturation is of significance for such applications with the aim to establish a sound protocol for shale retention behaviour analysis this paper presents an experimental methodology that takes into account the specificities of shales such as small pore size dimensions and the large variations in water potential to be applied to induce significant changes in the degree of saturation the technique involves the direct control of the shale water content and the subsequent measurement of the suction at equilibrium by a psychrometer a fluid displacement technique with a non polar liquid is also used to assess the volume changes of the shale and to compute the degree of saturation selected test results are presented for various shale samples that have been cored at different depths the results indicate that important features of the retention behaviour of geomaterials are fundamental aspects to be considered when analysing the retention behaviour of shales such as the existence of main wetting and drying paths the hysteresis domain and the dependency of the air entry value on the void ratio 2014 elsevier ltd all rights reserved

secondary initiation of multiple bands of cumulonimbus over southern britain ii dynamics of secondary initiation summary the convective storm initiation project csip pilot field programme took place in the summer of 2004 in southern england during this field campaign a case of a cold pool outflow initiating an arc of convection downwind of the generating storm the primary storm was observed three further arcs were initiated further downwind of this first arc these arcs all later gave significant rainfall over south east england results from the modelling studies described in this paper show that gravity waves generated by the primary storm may have been responsible for initiating the further three arcs of convective showers that were observed the modelled primary storm generates waves with a range of vertical wavelengths these are separated by dispersion with the higher order modes with the largest vertical wavelengths travelling fastest approximately 30 m s 1 the fastest two modes suppressed convection and later modes increased boundary layer depth and so initiated convection the multiple peaks from non hydrostatic effects in the third mode may have been responsible for the two main arcs observed and the fourth mode may have been responsible for the third arc the amplitude of the modelled waves at low levels was much larger downwind of the primary storm than upwind and only initiated convection there which agrees with the observations of the arcs forming downwind of the primary storm this asymmetry appears to be caused by the moving wave source and asymmetric wave trapping finally we discuss the implications for numerical weather prediction nwp reducing the resolution of the model and varying the magnitude of the wind speed used shows that a non hydrostatic high resolution 1k m nwp model should be able to capture such wave processes if the generating storm is forecast or analysed correctly

compaction behavior of argillaceous sediments as function of diagenesis the paper presents the results of laboratory compaction tests on samples of kimmeridge clay taken from two different locations in the uk the main objective of the laboratory tests is to determine how diagenesis affects the hydro mechanical properties and compaction behavior of argillaceous sediments the two kimmeridge clays used in the tests are essentially from the same parent material but have undergone different degrees of mechanical and chemical diagenesis the laboratory tests are designed to determine the effects of mechanical loading and chemical processes on changes in porosity permeability compressibility strength and effective horizontal stress in argillaceous materials another objective of the study is to investigate the adequacy of soil mechanical constitutive models in predicting the response of argillaceous materials to conditions corresponding to burial depths in sedimentary basins the tests on kimmeridge clay are carried out to as high as 150 mpa effective vertical stress corresponding to about 9 km in burial the results indicate that mechanical compaction and burial depth cannot explain the differences in the hydro mechanical behavior of the tested materials the differences in hydro mechanical response are attributed to chemical diagenesis by precipitation of minerals in the pores of the sediment other observed effects of chemical diagenesis on hydro mechanical behavior of lithified sediments are 1 increase in apparent pre consolidation stress 2 decrease in compressibility 3 decrease in permeability and 4 decrease in effective horizontal stress implications of the results to fluid flow and overpressure development in sedimentary basins are discussed

improvements to seismic monitoring of the european arctic using three component array processing at spits abstract the detectability of low magnitude seismic events in the european arctic is determined primarily by the small aperture international monitoring system arrays arces and spits in august 2004 the spits array was upgraded to a broadband array with an increase in the sampling rate from 40 to 80 hz most important however for the detection and location of small magnitude seismic events was the deployment of three component instruments at six of the nine sites detection and correct classification of secondary phases are of paramount importance for events observed by only a small number of stations at regional distances and in the absence of the strong lg phases typically observed for continental propagation paths multiple three component stations were deemed necessary to exploit the higher s phase amplitudes anticipated on the horizontal sensors we demonstrate improved signal to noise ratios snrs for s phases on horizontal beams for several events close to novaya zemlya horizontal component f k analysis improves direction estimates and phase classification for low snr signals we demonstrate secondary phases that are misidentified by vertical only f k analysis but which are correctly classified by three component array processing a significant problem with array processing at spits is the overlap in slowness space of regional p and s phases phase identification is improved greatly by comparing the coherence between vertical traces with the coherence between horizontal traces considerations in the routine array processing of spits data are reviewed including the need for elevation corrections in slowness estimation and the need to take into account azimuth dependent variation of apparent velocity estimates for regional phases

compaction driven evolution of porosity and permeability in natural mudstones an experimental study this paper describes a series of experiments designed to investigate the influence of lithology on the compactional loss of porosity and permeability in mudstones two intact samples of london clay with clay fractions of 40 and 67 were compacted to 33 mpa effective stress clay fraction permeability porosity pore size distribution and specific surface area were measured and their evolution was monitored throughout the compaction process electron microscopy was combined with mercury porosimetry to trace the collapse of the pore structure with increasing effective stress in both cases porosity loss occurred primarily by the collapse of large pores this process is more obvious in the coarser grained sample because throughout the compaction process it has a much broader range of pore radii and a much greater mean pore radius consistent with the pore size distributions the permeability of the coarser sample ranges from 10 10 m s 1 to 10 12 m s 1 while that of the finer grained sample ranges from 4 10 12 m s 1 to 5 10 14 m s 1 during progressive compaction from 2 to 33 mpa the compressibility of the finer grained sample is greater than that of the coarser grained sample 0 15 as opposed to 0 07 however in both cases the compressibility is much lower than that inferred for lithologically similar samples compacted over geological timescales the demonstration that both porosity and lithology clay fraction influence the permeability of mudstones should allow the development of more realistic porosity permeability relationships which take into account lithological variations exhibited by mudstones

volume change during sediment diagenesis and the development of growth faults abstract the stress system induced the weight of a sediment and the pore fluid it contains termed the passive stress system is capable of deforming the sediment this volumetric deformation is examined with respect to the evolution of porosity and permeability during burial the effect of this self deformation on the shape of growth faults and normal faults in extensional terrains is considered

compression behaviour of reconstituted soils at high initial water contents oedometer tests starting from a very small effective vertical stress of 0 5 kpa were performed on three reconstituted clays with different liquid limits the soils were prepared at various initial water contents ranging from 0 7 to 2 0 times their corresponding liquid limits it is observed that the e log v compression curves show an inverse s shape due to suction pressure resisting deformation similar to that of soft natural clays caused by consolidation yield stress the suction pressure s of the reconstituted clays can be correlated with the ratio of initial void ratio to void ratio at liquid limit e0 el the suction pressure curve defined by a unique relationship between suction pressure s and the normalised void ratio at suction pressure es el is also proposed to distinguish between the pre suction and the post suction states in addition burland s concept of the intrinsic compression line is adopted for correlating the compression curves of various reconstituted clays at high initia

on changes in porosity and volume during burial of argillaceous sediments abstract the porosity and hence volume of argillaceous sediments is determined by 1 the magnitude of the effective stress acting within the sediment 2 the previous stress history of the sediment and 3 at shallow depths of burial by features such as the mineralogy and the nature of the depositional environment stress paths and the critical state diagrams for a number of clays are used to investigate the range of porosities possible in argillaceous sediments as the effective stresses increase it is found that all porosity effective stress curves converge at large stresses the change in porosity is strongly dependent on the mean effective stress but largely independent of the deviatoric stress and thus is largely independent of the nature of the stress field acting on the basin compressional extensional etc because of the dependence of porosity on the mean effective stress no simple relationship exists between porosity and depth of burial but in the absence of overpressured pore fluids and assuming the sediment is not overconsolidated it is possible to contour the porosity effective stress diagram in terms of burial depths these data should assist in recalculating stratigraphic thicknesses for basin reconstruction and stratigraphic correlation studies

influence of early diagenesis on geotechnical properties of clay sediments romania black sea abstract the geotechnical properties of clay sediments were investigated using laboratory and in situ measurements as part of the geohazard assessment in the romanian sector of the black sea affected by landslides and seafloor deformation features the sediments were characterized as predominantly high plastic silty clay of high compressibility low undrained shear strength low cohesion and moderate sensitivity a shallow increase in shear strength exceeding the general trend could have been reconciled with evidence for the precipitation of iron sulfides and calcium carbonates related to early diagenetic reactions of sulfate reduction e g cryptic sulfur cycle and anaerobic oxidation of methane comparison of the compression behaviour of natural and reconstituted samples indicated that precipitation of these materials at particle contacts produced cementation the analysis of deeper sediments sharing similar geochemical properties illustrated that the early precipitation of even low quantity of sulfur and carbonate cements allowed clays to develop sufficient strength to preserve an abnormal high water content even under tens of meters burial depth because this cementation process was observed to be associated with an enhanced sensitivity and a strain softening response to undrained shearing it was concluded that such early diagenesis should be considered as a key environmental factor leading to weak layer formation that may influence the susceptibility of seafloor to failure

gravity waves compensating subsidence and detrainment around cumulus clouds abstract gravity waves play an important role in the redistribution of heat and moisture in a deep convecting cloud field we explore this role in a two dimensional numerical experiment on a simple moist convecting system consisting of an isolated long lasting nonprecipitating cloud in a calm atmosphere with no surface forcing the cloud develops a horizontally averaged density variation with height which is neutrally buoyant with respect to a moist adiabatic the buoyancy difference between the cloud and the undisturbed sounding produces circulations that can be understood as spreading gravity waves which adjust the environmental buoyancy to be equal to the cloud buoyancy by compensating subsidence unlike the circulations inside clouds this adjustment takes place without turbulent mixing hence the buoyancy adjustment time t1 during which the environment comes into rough buoyant equilibrium with the clouds is much shorter than the mixing time t2 which it takes a tracer initially concentrated at s

the mechanical behaviour of weak mudstone opalinus clay at low stresses abstract over the last 10 years argillaceous rocks mudrocks such as the jurassic claystone opalinus clay have gained acceptance as a potential host formation for geological disposal of nuclear waste observations from tunnels in opalinus clay at the mont terri rock laboratory switzerland suggest that the excavation induced response is not linear elastic the micro structure of the opalinus clay has been identified as a source for this response it has been demonstrated that diagenetic processes over the last 180 ma have been responsible for the formation of strong diagenetic bonds locking latent strain energy into the micro structure of opalinus clay a conceptual model of the micro structure including the effects of diagenetic processes has been developed as a framework to interpret the mechanical behaviour of the opalinus clay samples recovered at mont terri rock laboratory have been subjected to processes that have resulted in bond breakage and the release of locked in latent strain energy this tends to reduce the strength and deformation properties of samples and induces low stiffness non linearity of the stress strain curve at low stress levels numerical modelling of the sampling stress path at mont terri rock laboratory shows that unloading was the dominant factor resulting in this response as a result samples can shed light on the unloading response of the rockmass to excavation this non linear behaviour has been quantified by means of routine laboratory compressive tests

evolution of microstructures and of macroscopic properties of some clays during experimental compaction abstract in order to understand the evolution of clay sediments during their burial to three or four kilometers depth remoulded and non remoulded clay samples were oedometrically consolidated under loads ranging from 0 1 50 mpa and their microstructural mechanical and transport properties were studied for this study four clays were selected on mineralogical grounds a kaolinite st austell clay two illitic clays la bouzule clay and salins 14 clay and an illite smectite mixed clay marais poitevin clay among those materials two are almost pure clays st austell clay and salins 14 clay and were compacted in a remoulded state the other two have a clay fraction about 60 and were studied in non remoulded state for marais poitevin clay and both remoulded and non remoulded for la bouzule clay the oedometric tests have been performed in classical oedometers and in a new oedometric cell specially designed to measure lateral stresses and to perform hydraulic conductivity measurements under compaction load the two pure clays show an evolution of their void ratio differing from what is expected considering their liquid limits the behaviour seems to be governed by the size of the clay particles around 1 m for st austell clay and around 0 1 m for salins 14 clay marais poitevin clay very slightly compacted in its natural state and remoulded la bouzule clay have consolidation coefficients cc and void ratios consistent with their liquid limits a relationship reported in soil mechanics literature skempton 1944 burland 1990 nagaraj et al 1994 the natural la bouzule clay shows very low void ratios and cc due to overconsolidation the microstructure observed by tem showed that clay particles are platy and made of aggregates of individual crystals except for salins 14 clay where individual crystals do not aggregate the evolution of the microstructure with applied stress shows no change in the particles structure but a progressive reorientation of the particles perpendicular to the load axis these microstructural observations explain the transport properties measured and their evolution the thermal conductivity of kaolinite measured in the axial and radial direction reflects the evolution of anisotropy with compaction observed on particles orientation the different hydraulic conductivities observed on the various samples are compared to those obtained with the kozeny carman formula using two types of specific surface determination this comparison emphasizes the importance of inter and intraparticule pore size and of their intimate organisation

stiff sedimentary clays geological origins and engineering properties this keynote paper summarises and extends earlier work on the characterisation of clays with particular reference to stiff sedimentary clays examples of clay characterisation are given that include the various main geological events from sedimentation to erosion and weathering and other phenomena such as diagenesis and brecciation the examples demonstrate the value of plotting basic data such as the current stress state and undrained strength in terms of the void index these three parameters permit an immediate simple categorisation in terms of geological history of sedimentary clays further examples illustrate the engineering consequences of weathering of stiff clays and the associated phenomenon of brecciation produced by the processes of cambering and valley bulging it is emphasised that at all stages in a clay s geological history it is the bond strength between the soil particles not the geological preconsolidation stress that controls the mechanical behaviour of sedimentary clays

volume changes during diagenesis abstract the literature pertaining to volume change during diagenesis of clastic sediments is reviewed with respect to the problem of calculating pre and syn compaction thicknesses of sediments for basin reconstruction and stragraphic correlation studies four major mechanisms for volume change are identified mechanical compaction mechanical dissolution chemical dissolution and phase change the first two of these are found to be strongly dependent on the effective stress whilst the latter two show at least a pressure dependence quantification of the relationships between porosity and depth of burial of a sediment seems to be possible only for specific examples of the first of these processes at present this quantification is dealt with in the accompanying related publication

the general and congruent effects of structure in natural soils and weak rocks the engineering properties of naturally occurring sedimentary and residual deposits which are usually treated in geotechnical engineering as soils are reviewed and it is shown that usually they have characteristics due to bonded structure which are similar to those of porous weak rock while this structure can arise from many causes its effects follow a simple general pattern that involves stiff behaviour followed by yield this yield can be described in a similar way to that occurring due to overconsolidation although it is a separate phenomenon the effects of structure are as important in determining engineering behaviour as are the effects of initial porosity and stress history which are the basic concepts of soil mechanics as it can be described in a general way it is concluded that structure and its effects should be treated as a further basic concept of equal importance l article passe en revue les proprietes des depots sedimentaires et residuels naturels qui sont nor malement traites comm

measurement of in situ stress in weak rocks at mont terri rock laboratory switzerland in situ stress measurements in weak rocks such as clay shales that respond strongly to environmental changes are particularly difficult an extensive in situ stress measurement program has been conducted at the mont terri rock laboratory in northwestern switzerland hydraulic fracturing undercoring around a 600 mm diameter borehole using csiro triaxial strain cells and the borehole slotter were used to establish the state of stress for the opalinus clay at this location conflicting orientations and magnitudes resulted from the measurement programs three dimensional elastic modelling was used in conjunction with tunnel and borehole observations to establish the most likely stress tensor a stress measurement program using a borehole deformation gauge is currently underway to check the stress tensor resulting from the observational modelling

multi scale modeling of shale laminas and fracture networks in the yanchang formation southern ordos basin china abstract outcrops and core observations show that sandy and tuffaceous laminas and natural fractures are well developed in the continental shale gas reservoir this study reports the distribution of laminas and natural fracture networks at different scales in cases of outcrops wells and cores from yanchang formation southern ordos basin firstly based on two dimensional fracture field investigations a 3 d joint network model of the study area was built using the monte carlo simulation method then the laminations were identified from macroscopic scales to microscopic scales with multiple probe techniques statistical analysis of multi scale layer thickness suggested that the average thickness of layers at multiple scales showed a fractal feature and that the layer thickness at each scale followed with an exponential probability distribution whose exponential coefficient was 3 38 1 91 1 65 1 21 corresponding to meter scale decimeter scale centimeter scale and millimeter scale respectively the fitting curves indicated that the majority of laminas at each scale were relatively thin and the thicker the layer the less the number of layers contained in shale finally the lamination models were constructed and superposed over the joint network model to generate 3 d geological structure models at various scales the models were validated by reproducing fracture and lamina parameters fairly close to those obtained from realistic geological bodies in the study area the findings of this work could provide a more reliable geological model for the numerical simulation and physical model test of hydraulic fracturing in shale gas reservoir

the two dimensional response of a tropical jet to propagating lines of convection this paper uses simple one and two dimensional models to investigate the influence of a propagating line of convective forcing on a tropical jet representative of the african easterly jet the results are used to infer changes in the environment of the forcing region which would in reality tend to influence the evolution of the storm through convective mechanisms which are not resolved here from linear analytical solutions with a rigid lid it is found that the influence of the propagation of the forcing region is to intensify the response on the upstream side of the forcing this result is confirmed in two dimensional nonlinear simulations when linear wave modes are computed in a basic state that includes the jet structure small sensitivities to the basic state jet are found the two dimensional nonlinear model has been used further to compute the change in the environmental structure as a result of the forcing principally it is found that the modes of response to the forcing may be associated with characteristic changes in the basic state shear convective available potential energy cape and convective inhibition cin which would be expected to have significant influence on the convective system itself

on the hydro mechanical behaviour of remoulded and natural opalinus clay shale the geo energy sector makes use of advanced technologies such as shale gas extraction co2 sequestration and nuclear waste geological disposal that rely on the exploitation of shale formations due to the great depths involved in these applications and the difficulties in retrieving intact samples remoulded shale specimens are often adopted for hydro mechanical testing remoulded and intact shales may substantially differ in their hydro mechanical behaviour due to the particular structure of the natural material which is the result of diagenesis and burial history this paper presents an experimental campaign aimed at i characterizing the role of diagenesis and depth for opalinus clay shale and their impact on the hydro mechanical behaviour of the material and ii understanding how representative the behaviour of the remoulded material is with respect to that of the natural shale in this paper the hydro mechanical behaviour of remoulded and intact specimens from the opalinus clay shale formation is investigated by means of a comprehensive oedometric testing campaign the effects of depth and diagenesis on the material behaviour are analysed by testing intact specimens that come from two different sites and depths of the considered shale formation the results highlight a considerably greater stiffness for the natural opalinus clay with respect to the remoulded specimens when the same vertical effective stress is considered together with a significantly lower porosity for the former which is shown to be the result of diagenetic processes rather than solely mechanical compaction a lower swelling sensitivity is also manifested by the natural material with respect to the remoulded specimens as a result of the formation of bonding on the other hand the diagenetic processes do not seem to have a strong impact on the permeability of the material finally creep effects are reduced when diagenesis occurs while they become more relevant when the material is disturbed or remoulded the obtained results provide significant insight into the roles of depth diagenesis and structural disturbance on the hydro mechanical behaviour of the opalinus clay shale

westward generation of eastward moving tropical convective bands in toga coare mechanisms responsible for westward generation of eastward moving tropical convective bands in the tropical ocean and global atmosphere coupled ocean atmosphere response experiment toga coare are investigated using a two dimensional numerical cloud model sequential generation of new convective bands to the west of an old eastward moving convective band is successfully simulated in an environment of a convectively active day during toga coare characterized by west winds at low levels and strong easterlies aloft it is concluded that the westward generation of new convective bands is explained by a gravity wave mechanism two westward propagating modes excited below the convective cells moving westward relative to the convective bands appear to play an important role a slow propagating mode 15 ms 21 excited by a shallow convective band is ducted in the troposphere under an unstable layer of small richardson number containing its critical level a fast propagating mode 25 m s21 excited by a deep convective band is ducted in the troposphere under the remaining region of the convective cell containing its critical level these two modes propagate horizontally to the west and promote the growth of shallow convection into long lived convective bands a dry model with thermal forcing representing the convective cell showed that preferential excitation of westward propagating waves below the convective cell is due to westward motion and ascension of the convective cell a comparative simulation without the critical level confirms the proposed gravity wave mechanism