brittle ductile transition shear failure and leakage in shales and mudrocks abstract this paper presents an experimental study on brittle ductile transition shear behaviour the formation of shear fractures in shales and mudrocks and the hydraulic properties of shales and mudrocks with shear induced fractures the experimental study is based on triaxial tests on different mudrocks and shales from north sea reservoirs and adjacent areas sheared at different effective confining stresses it is shown that during burial mudrocks behave as normally consolidated materials and exhibit ductile response to increased load however several processes like uplift chemical diagenesis and overpressure build up turn mudrocks into overconsolidated materials which exhibit brittle behaviour during loading triaxial tests with different loading conditions have been performed to establish the brittle to ductile transition of various mudrocks based on the results of the tests it is shown that the brittle to ductile transition can be related to the overconsolidation ratio a relationship between normalized undrained shear strength and overconsolidation ratio is established in combination with this relationship a correlation between compressional wave velocity and apparent pre consolidation stress which accounts for both mechanical and chemical diagenesis may be used as a tool to evaluate possible leakage of hydrocarbon seals

equatorial vertical drift modulation by the lunar and solar semidiurnal tides during the 2013 sudden stratospheric warming during the 2013 stratospheric sudden warming ssw period the jicamarca unattended long term investigations of the ionosphere and atmosphere julia radar at jicarmarca peru observed low latitude vertical drift modulation with lows of 0 12 m s daytime maximum drifts between 6 13 and 22 25 january and enhanced drifts up to 43 m s between 15 snd 19 january the ncar thermosphere ionosphere mesosphere electrodynamics general circulation model reproduces the prevailing vertical drift feature and is used to examine possible causes the simulations indicate that the modulation of the vertical drift is generated by the beating of the semidiurnal solar sw2 and lunar m2 tides during the ssw period the beating is observable since the magnitudes of lunar and solar semidiurnal tidal amplitudes are comparable the theoretical beating frequency between sw2 and m2 is 1 15 13 day which may be modified due to phase changes this study highlights the importance of the lunar tide during ssw periods and indicates that the equatorial vertical drift modulation should be observable at other longitudes as well

electrical conductivity of the earth s mantle from the first swarm magnetic field measurements we present a 1 d electrical conductivity profile of the earth s mantle down to 2000 km derived from l1b swarm satellite magnetic field measurements from november 2013 to september 2014 we first derive a model for the main magnetic field correct the data for a lithospheric field model and additionally select the data to reduce the contributions of the ionospheric field we then model the primary and induced magnetospheric fields for periods between 2 and 256 days and perform a bayesian inversion to obtain the probability density function for the electrical conductivity as function of depth the conductivity increases by 3 orders of magnitude in the 400 900 km depth range assuming a pyrolitic mantle composition this profile is interpreted in terms of temperature variations leading to a temperature gradient in the lower mantle that is close to adiabatic

can we probe the conductivity of the lithosphere and upper mantle using satellite tidal magnetic signals thesis s m massachusetts institute of technology department of earth atmospheric and planetary sciences 2015

tidal signals in ocean bottom magnetic measurements of the northwestern pacific observation versus prediction summary motional induction in the ocean by tides has long been observed by both land and satellite measurements of magnetic fields while these signals are weak 10 nt when compared to the main magnetic field their persistent nature makes them important for consideration during geomagnetic field modelling previous studies have reported several discrepancies between observations and numerical predictions of the tidal magnetic signals and those studies were inconclusive of the source of the error we address this issue by 1 analysing magnetometer data from ocean bottom stations where the low noise and high signal environment is most suitable for detecting the weak tidal magnetic signals 2 by numerically predicting the magnetic field with a spatial resolution that is 16 times higher than the previous studies and 3 by using four different models of upper mantle conductivity we use vector magnetic data from six ocean bottom electromagnetic obem stations located in the northwestern pacific ocean the obem tidal amplitudes were derived using an iteratively re weighted leastsquares irls method and by limiting the analysis of lunar semidiurnal m2 lunar elliptic semidinurnal n2 anddiurnal o1 tidalmodestothenight time usinga3 delectromagnetic induction solver and the tpx07 2 tidal model we predict the tidal magnetic signal we use earth models with non uniform oceans and four 1 d mantle sections underneath taken from kuvshinov and olsen shimizu et al and baba et al to compare the effect of upper mantle conductivity we find that in general the predictions and observations match within 10 70percent across all the stations for each of the tidal modes the median normalized percent difference npd between observed and predicted amplitudes for the tidal modes m2 n2 and o1were15percent 47percentand98percent respectively forallthestationsandmodels at the majority of stations and for each of the tidal modes the higher resolution 0 25 0 25 modelling gave amplitudes consistently closer to the observations than the lower resolution 1 1 modelling the difference in lithospheric resistance east and west of the izu bonin trench system seems to be affecting the model response and observations in the o1 tidal mode this response is not seen in the m2 and n2 modes thereby indicating that the o1 mode is more sensitive to lithospheric resistance

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

cross section restoration of salt related deformation best practices and potential pitfalls abstract cross section restoration typically assumes plane strain deformation and area conservation constraints that are usually invalid for salt because of its characteristic three dimensional flow and possible dissolution thus restoration of salt related deformation provides added challenges and uncertainty in this review paper we summarize the historical development of ideas methods and applications of restoration in salt basins while most published restorations do not maintain salt area constraints on its variation range from arbitrary assumptions to quantitatively incorporating isostatic calculations we illustrate several scenarios in which the presence of salt adds ambiguity to restoration primarily because it can hide deformation diapirs can widen during extension and narrow during shortening translating overburden can move into salt and drive allochthonous advance secondary minibasin subsidence can be accommodated at both shallow and deep salt levels and allochthonous salt can record evacuation of deeper salt although we caution against using restoration to test and validate small scale details of interpretations we emphasize that sequential restoration remains an essential tool in structural and basin analyses however because of the uncertainties a regional three dimensional approach and sound geological reasoning are critical for deriving meaningful and useful results from cross section restoration of salt structures

numerical analysis of how sedimentation and redistribution of surficial sediments affects salt diapirism abstract two dimensional finite element models are used to study how sedimentation and redistribution of sediments on the upper surface affects the development of subsurface salt diapirs a rising diapir creates a bulge flanked by topographic lows in a generally accumulating sedimentary pile we find that the rate at which this topography is flattened by erosion and redeposition controls the style of diapirism this is because the redistribution of material from topographic highs to flanking lows is equivalent to changing the effective forces acting on the salt redistributing a potential topography modulates diapiric growth rate the main effects of including surficial sediment redistribution in numerical models of diapirism are 1 1 diapirs grow 10 100 times faster 2 2 diapirs may rise above their level of neutral buoyancy and extrude 3 3 diapirs assume finger or stock like shapes rather than mushroom or balloon on string shapes 4 4 layers in the surrounding sediments remain nearly horizontal and only steepen sharply near the diapir in effect the rate of redistribution of surficial overburden strongly controls the mode of diapirism sediment redistribution referred to as erosion for brevity is modeled using a one dimensional diffusion equation we show the results of two different erosion rates infinitely slow no erosion and extremely fast which redistributes surficial sediments but does not remove them from the system we show that the shapes of model diapirs rising beneath surfaces subjected to rapid erosion simulate salt diapirs in the gulf of mexico columnar diapirs indicate rapid deposition on the shelf and plug like diapirs slow sedimentation on the abyssal plane diapirs rising beneath surfaces with negligible erosion have the mushroom shapes interpreted for salt diapirs in central iran

comprehensive atlas of stress trajectory patterns and stress magnitudes around cylindrical holes in rock bodies for geoscientific and geotechnical applications abstract this study presents a comprehensive atlas comprising maps and graphs which quantify the wide range of possible stress states around circular cylindrical cavities when subjected to the full range of far field stress conditions and internal net pressures a greater understanding of the principal stress trajectories and related stress magnitudes around stressed cylindrical holes in rock formations significantly aids the design of safer and more stable wellbores advances our capacities in hydraulic fracturing and provides additional insights into the nature and stability of volcanic dikes salt domes and caverns used for hydrocarbon and other fluid storage the algorithms used in our study systematically non dimensionalize the analytical kirsch equations using two critical parameters to scale the variable boundary conditions 1 the bi axial stress scalar describes the far field stresses at all depths and for each of the formations penetrated by a cylindrical borehole or natural liquid filled pipe by describing the anisotropy of the two far field principal stresses and 2 the frac number f specifies the net pressure on the cylindrical hole as it relates to the natural background stress in the host rock and the pressure in the penetrated formation at each depth manipulation of these two governing parameters f enables us to view and monitor the stress trajectories in the host rock around a cylindrical hole at any given depth additionally the neutral point locations and their effects on the principal stress orientations near the cylindrical hole have been more thoroughly investigated than in earlier studies the neutral points form the boundaries for the stress cage in overbalanced cylindrical cavities and for the complimentary fracture cage in underbalanced cavities the understanding of stresses induced around cylindrical holes in a rock formation is paramount in geotechnical analyses and stability appraisals of manmade cylindrical holes as well as for the geoscientific interpretation of piercing by cylindrical fluid filled pipes our stress atlas aims to provide fundamental insight and practical support for such analyses

cm5 a pre swarm comprehensive geomagnetic field model derived from over 12 yr of champ orsted sac c and observatory data a comprehensive magnetic field model named cm5 has been derived from champ rsted and sac c satellite and observatory hourly means data from 2000 august to 2013 january using the swarm level 2 comprehensive inversion ci algorithm swarm is a recently launched constellation of three satellites to map the earth s magnetic field the ci technique includes several interesting features such as the bias mitigation scheme known as selective infinite variance weighting sivw a new treatment for attitude error in satellite vector measurements and the inclusion of 3 d conductivity for ionospheric induction sivw has allowed for a much improved lithospheric field recovery over cm4 by exploiting champ along track difference data yielding resolution levels up to spherical harmonic degree 107 and has allowed for the successful extraction of the oceanic m2 tidal magnetic field from quiet nightside data the 3 d induction now captures anomalous solar quiet features in coastal observatory daily records cm5 provides a satisfactory continuous description of the major magnetic fields in the near earth region over this time span and its lithospheric ionospheric and oceanic m2 tidal constituents may be used as validation tools for future swarm level 2 products coming from the ci algorithm and other dedicated product algorithms

the effect of the salt viscosity on future evolution of the gorleben salt diapir germany abstract the gorleben diapir which has been targeted for radioactive waste disposal contains large blocks of anhydrite numerical models that depict the geometrical configuration of the gorleben diapir are used to understand internal structure of diapir caused by movement of the anhydrite blocks for various salt rheologies it is shown that the rheology of the salt plays a significant role in how and at which rate the anhydrite blocks sink within the diapir the mobility of anhydrite blocks depends on the effective viscosity of salt which has to be lower than threshold value of around 1018 1019 pa s decreasing salt viscosity allows the previously stationary anhydrite blocks to sink if the effective viscosity of salt in post depositional stage of the gorleben diapir falls below this threshold value induced internal flow due to the present anhydrite layer might disturb any repository within the diapir

extracting ocean generated tidal magnetic signals from swarm data through satellite gradiometry ocean generated magnetic field models of the principal lunar m2 and the larger lunar elliptic n2 semidiurnal tidal constituents were estimated through a comprehensive inversion of the first 20 5 months of magnetic measurements from european space agency s esa swarm satellite constellation mission while the constellation provides important north south along track gradiometry information it is the unique low spacecraft pair that allows for east west cross track gradiometry this latter type is crucial in delivering an m2 estimate of similar quality with that derived from over 10 years of champ satellite data but over a shorter interval at higher altitude and during more magnetically disturbed conditions recovered n2 contains nonoceanic signal but is highly correlated with theoretical models in regions of maximum oceanic amplitude thus satellite magnetic gradiometry may eventually enable the monitoring of ocean electrodynamic properties at temporal resolutions of 1 to 2 years which may have important implications for the inference of ocean temperature and salinity

detection and assessment of a large and potentially tsunamigenic periglacial landslide in barry arm alaska the retreat of glaciers in response to global warming has the potential to trigger landslides in glaciated regions around the globe landslides that enter fjords or lakes can cause tsunamis which endanger people and infrastructure far from the landslide itself here we document the ongoing movement of an unstable slope total volume of 455 106 m3 in barry arm a fjord in prince william sound alaska the slope moved rapidly between 2010 and 2017 yielding a horizontal displacement of 120 m which is highly correlated with the rapid retreat and thinning of barry glacier should the entire unstable slope collapse at once preliminary tsunami modeling suggests a maximum runup of 300 m near the landslide which may have devastating impacts on local communities our findings highlight the need for interdisciplinary studies of recently deglaciated fjords to refine our understanding of the impact of climate change on landslides and tsunamis

spherical harmonic analysis of geomagnetic tides 1964 1965 the mathematical forms chosen for analyses of s q and l by seventeen different authors are reviewed on the basis of the review and on consideration of the ionospheric dynamo theory and the hough function structure of wind velocities in the upper atmosphere a mathematical model is chosen that includes two more terms than the recent analysis by malin 1973 hourly mean values from 130 magnetic observatories for the i q s ys international quiet solar years 1964 and 1965 were prepared in machine readable form and analysed the solar and lunar transient magnetic variations were evaluated together with the lunar elliptic magnetic tide and the seasonal change at one and two cycles per year for all magnetic tides spherical harmonic analyses were made of the phase law tides and the smaller partial tides altogether 94 different spherical harmonic analyses were completed for a total of ten different magnetic tides two of the ten tides are the solar and lunar magnetic tides usually denoted s and l respectively the results are tabulated in a form that distinguishes between eastward and westward moving terms which is suitable for the evaluation of phase angle differences and amplitude ratios between internal and external parts malin s ocean dynamo calculation has been applied to the lunar and lunar elliptic magnetic tides the spherical harmonic coefficients are compared in some detail with those of malin 1973 for years of high sunspot activity the magnetic tidal potential associated with a given atmospheric tidal mode is derived theoretically and used to obtain an estimate of the hough function components of the solar and lunar semi diurnal atmospheric tides at ionospheric levels from the corresponding magnetic tidal potential

lunar tidal winds between 80 and 110 km from uars hrdi wind measurements 1 recent observations and models show the considerable effects of upward propagating lunar atmospheric tides on many aspects of the ionosphere thermosphere system a key aspect of atmosphere ionosphere coupling is the wind field in the ionospheric e region circa 100 150 km where the dynamo generation of electric fields occurs yet global specifications of lunar tidal winds exist only in theoretical predictions although years of satellite observations on atmospheric winds exist with almost global coverage in the present paper we report on quasi global semidiurnal lunar tidal winds up to 115 km from data gathered by the high resolution doppler imager hrdi on board the upper atmosphere research satellite during the early years from 1991 to 1999 the observed lunar semidiurnal tidal winds are consistent with global scale wave model gswm simulations in both amplitude and phase with maximum amplitudes located between about 40 n 60 n around the solstices the peak amplitudes of the lunar tidal winds lie in the 10 15 ms 1range whereas those simulated by gswm lie between 15 and 25 ms 1 the smaller hrdi amplitudes are likely due in part to amplitude suppression in construction of the climatological mean resulting from year to year phase variability in the lunar tide it is also possible that wave dissipation is underestimated to some degree in the gswm

lunar daily geomagnetic variations in new zealand summary lunar daily geomagnetic variations l exhibit distinctive behaviour at many sites near coasts in this study data from an array of three component magnetometers that operated for a four month summer period at 22 sites throughout new zealand are used to examine l the chapman miller method is used to determine the four main luni solar harmonics of l at each site analysis of 3 5 years of data from the geomagnetic observatory at eyrewell indicates that the array data produce representative values of l separation of the dominant lunar semidiurnal harmonic l2 into parts of oceanic and ionospheric origin indicates near uniform ionospheric contributions to the x and y field components at all sites with peak values occurring within 3 hours of lunar transit the ionospheric contribution to the 2 field component is smaller and suggests secondary electric currents flowing in the deep water to the north east and south west of new zealand the oceanic contribution to l tends not to be significant for x and y its amplitude for 2 is greatest in the eastern north island and in the south western south island and its phase angle for z is locally consistent throughout the north island and in the south western south island the difference between the phase angles that characterize these northern and southern regions is about 180 this pattern suggests near parallel in phase electric currents flowing in the deep water to the north east and south west of new zealand in response to tidal flows this is in agreement with both the general pattern seen in the united kingdom and lunar daily geomagnetic variations observed under the tasman sea and in eastern australia where the magnetic effects of tidal flows in shallow coastal waters are secondary to those from more distant deep waters the oceanic contribution to l2 might be used to estimate the poorly known tidal flows in the deep water off new zealand

predictive modelling of structure evolution in sandbox experiments in this paper a computational approach is presented that is able to forward model complex structural evolution with multiple intersecting faults that exhibit large relative movement the approach adopts the lagrangian method complemented by robust and efficient automated adaptive meshing techniques a constitutive model based on critical state concepts and global energy dissipation regularized by inclusion of fracture energy in the equations governing state variable evolution the efficacy of the approach is benchmarked by forward simulation of two extensional analogue experiments that exhibit the development of a roll over anticline with a series of superimposed crestal collapse graben systems these sandbox experiments are excellent benchmarks for computational models as both intersecting localisations with different rates of slip and large relative movement on localisations must be represented while other complex phenomena associated with structural evolution over geological timeframes are not present

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

joint inversion of satellite detected tidal and magnetospheric signals constrains electrical conductivity and water content of the upper mantle and transition zone we present a new global electrical conductivity model of earth s mantle the model was derived by using a novel methodology which is based on inverting satellite magnetic field measurements from different sources simultaneously specifically we estimated responses of magnetospheric origin and ocean tidal magnetic signals from the most recent swarm and champ data the challenging task of properly accounting for the ocean effect in the data was addressed through full three dimensional solution of maxwell s equations we show that simultaneous inversion of magnetospheric and tidal magnetic signals results in a model with much improved resolution comparison with laboratory based conductivity profiles shows that obtained models are compatible with a pyrolytic composition and a water content of 0 01 wt and 0 1 wt in the upper mantle and transition zone respectively

factors controlling the evolution of the perdido fold belt northwestern gulf of mexico determined from numerical models 1 the perdido fold belt pfb is a prominent saltcored deep water structure in the northwestern gulf of mexico it is characterized by symmetric kink banded folds of a 4 5 km thick prekinematic layer and its vicinity to the extensive sigsbee salt canopy we use 2 d finite element numerical models to study the evolution of the pfb as a gravity driven fold belt both in a local context and in the context of the larger scale passive margin influenced by adjacent allochthonous salt structures we show that parameters such as overburden strength salt geometry or salt viscosity determine timing extent and location of the modeled fold belt simplified models of the gulf of mexico show that toe of slope folding is a viable mechanism to develop diapirs in the deep salt basin and to delay folding of the distal overburden in this scenario the pfb likely represents the terminal folding of a much larger diachronously formed fold belt system citation gradmann s c beaumont and m albertz 2009 factors controlling the evolution of the perdido fold belt northwestern gulf of mexico determined from numerical models tectonics 28 tc2002 doi 10 1029 2008tc002326

initiation of salt diapirs with frictional overburdens numerical experiments we have studied the initiation of salt diapirs with frictional overburdens under lateral compression and extension the overburden is assumed to be elasto plastic with mohr coulomb yield criterion and non associated flow rule plastic flow localizes into narrow shear bands which may represent faults the geometry and the evolution of these shear zones are simulated for various rheological parameters the dilational and frictional properties of frictional overburdens are found to be important controls on the types of shear zones formed in lateral compression and extension the friction and dilation angles influence the angle of shear and the width dip spacing and development history of conjugate zones of shear plus dilation fault zones that define wedges and grabens

mechanics of active salt diapirism abstract an active diapir forcefully intrudes its overburden driven by diapir pressure that overcomes the resistance of the overburden strength possible causes for the driving pressure are differential loading of the source layer and a density contrast with the overburden possible resisting forces derive from the mass of the roof block and resistance to the faulting and folding that accommodate the intrusion for a typical density contrast between salt and a sedimentary overburden a simple force balance demonstrates that the diapir height must be more than two thirds to three quarters the thickness of the surrounding overburden to initiate substantial active diapirism narrow diapirs must be even taller physical and numerical modeling show that typical structures of active diapirism are a central crestal graben flanked by relatively unstrained flaps that rotate upward and outward curved reverse faults can separate the flaps from the regional overburden normal faults in the crestal graben propagate downward with new faults created farther outward as the roof arches over the rising diapir thin wide roofs develop multiple grabens separating a relatively flat roof from the rotating flaps in cases leading to emergence the piercing diapir evolves from a pointed crest to a progressively rounded and widening crest as the flaps rotate outward the numerical models show that overburden flexure contributes to early formation of active diapir structures but piercement does not continue to emergence unless the diapir is very wide or tall relative to the overburden thickness active diapirism becomes progressively more difficult for diapir crests ranging from a rectangle a round cornered rectangle a semicircle to a triangle with a pointed crest

numerical modelling of salt diapirism influence of the tectonic regime abstract in the context of sedimentary basin mechanics salt diapirism has usually been described as a phenomenon initiated and driven by gravity instabilities associated with buoyant forces or differential loading this result holds essentially for an approximation of linear rheologies of both salt and overburden this has been contested by recent analog models and analytical studies buoyancy alone cannot drive salt diapirism through a brittle overburden regional extension or compression could be a necessary mechanism likely to promote salt diapirism numerical models are presented here to describe the relative effects of gravity and lateral forces on the dynamics of salt diapirism the numerical scheme is based on a viscoplastic approximation of deformations and an updated lagrangian description of kinematics which allows a study of large non linear deformations of salt bodies and the evolution of material interfaces this scheme takes into account brittle and ductile deformations of sediments moreover it describes the initiation and evolution of shear bands which play a crucial role in basin mechanics and especially in the dynamics of salt diapirism we show why extension is the most propitious driving mechanism of diapirism necking and faulting induced by extension promote and drive salt diapirism in the case of a brittle overburden this result holds with or without buoyancy forces this demonstrates that buoyancy which alone is not sufficient to break a brittle overburden is not a necessary mechanism either buoyancy however enhances diapirism induced by lateral forces in contrast with extension salt diapirism during compression is strongly constrained by the rheology of the overburden a brittle mode of deformation favours faulting rather than buckling induced thrusting and thickening of the overburden inhibit massive diapirism and favour the confinement of small salt plugs finally the post extension stability of a mature diapir produced by extension is presented

a numerical approach for investigation of stress states induced by salt structures abstract the main oil and gas reserves around the world are found in regions containing evaporitic rocks owing to their low porosity and low permeability these rocks provide favorable conditions for hydrocarbon trapping increasing the probability of success in oil and gas exploration salt rocks exhibit time dependent strain behavior when submitted to deviatoric stresses this behavior is described by creep constitutive laws based on deformation mechanics one of the major challenges in modeling stress states in sedimentary basins in the presence of salt structures is to predict the magnitude of stress perturbations around salt bodies this work presents a geo mechanical approach to take into account the specific weight variation throughout the lithology and the effects of buoyancy in the resulting forces on the interface salt body formation modeling is driven by the fact that salt bodies cannot sustain deviatoric stresses numerical analyses using the finite element method under plane strain conditions provide the results for the comparison of the methodologies found in the literature with the one proposed by the authors two synthetic models with salt dome structures are investigated the proposed approach provided more realistic stress fields in the vicinity of salt structures

a decision analysis framework for the assessment of likely post failure velocity of translational and compound natural rock slope landslides an integral component of the assessment of hazard and risk for landslides from large natural rock slopes is the examination of the likely consequences associated with failure this in turn is inherently related to the post failure velocity of the slide mass this paper presents a decision analysis framework for assessment of the post failure velocity of such slopes the paper includes discussion of characteristics that influence the post failure velocity and presents decision trees and supporting matrices to allow assessment of the likely post failure velocity of translational and internally sheared compound landslides these represent the most common classes of large rock landslides the framework is based on data gathered from a large number of landslides from natural rock slopes and incorporates information from a study of excavated rock slopes these landslides have been studied to determine the factors and characteristics of rock slope failures that influence the post failure velocity the framework

pore pressure and stress regime in a thick extensional basin with active shale diapirism western mediterranean the alboran sea in the mediterranean is a back arc basin developed during the miocene by extensional collapse within an arc shaped orogen a major depocenter 10 km 6 2 mi is located to the west of the basin west alboran basin wab and contains a diapiric province with overpressured shales and mud volcanoes seismic and well data are used to analyze the evolution of the shale structures in the northern margin of the wab and to estimate the in situ stress tensor geomechanical modeling suggests a present day normal faulting stress regime along the northern wab where the maximum horizontal stress is parallel to the coastline pore pressure shows a hydrostatic gradient down to 2000 m 6561 ft where the top of the regional pore pressure ramp is located undercompaction is the dominant mechanism generating overpressures in sediments shallower than 5000 m 16 406 ft at greater depths thermal mechanisms impose an excess of pore pressure on the sediments feeding the diapirs this framework is used to discuss the contribution of thermally generated pressures to the triggering of shale diapirism increasing thermal pressures in the deepest confined units cause tensile failure of the overburden and subsequently promote mud withdrawal and injection in the overburden the magnitude of the overpressure conditions the vertical ascent of shale the more mature structures reuse preexisting normal faults in their ascent toward shallower basin levels results provide insights into the current discussion about the triggering factors behind shale diapirism they also help to explain the differences between shale structures and those shaped by salt tectonics

lunar tide in the equatorial electrojet in relation to stratospheric warmings 1 the relationship between sudden stratospheric warmings ssws and large amplitude lunar tides in the equatorial electrojet eej is studied analysis of ground magnetometer data shows that the lunar tide in the eej is maximum during the northern winter season except in the pacific ocean region since ssws are also a northern winter phenomenon it is suggested that the relation between the large lunar tide in the eej and the ssw may possibly be coincidental the lunar tide in the geomagnetic variations at huancayo is anomalously large compared with other eej stations an examination of geomagnetic variations at eej stations during ssw events shows that afternoon counter electrojets are frequently present at new moon and full moon though the relationship is sometimes broken the observation of large lunar eejs when no ssw is present and of various different delay times suggests that other atmospheric processes are likely to be in play

ionosphere variability during the 2009 ssw influence of the lunar semidiurnal tide and mechanisms producing electron density variability to investigate ionosphere variability during the 2009 sudden stratosphere warming ssw we present simulation results that combine the whole atmosphere community climate model extended version and the thermosphere ionosphere mesosphere electrodynamics general circulation model time gcm the simulations reveal notable enhancements in both the migrating semidiurnal solar sw2 and lunar m2 tides during the ssw the sw2 and m2 amplitudes reach 50 m s 1 and 40 m s 1 respectively in zonal wind at e region altitudes the dramatic increase in the m2 at these altitudes influences the dynamo generation of electric fields and the importance of the m2 on the ionosphere variability during the 2009 ssw is demonstrated by comparing simulations with and without the m2 time gcm simulations that incorporate the m2 are found to be in good agreement with jicamarca incoherent scatter radar vertical plasma drifts and constellation observing system for meteorology ionosphere and climate cosmic observations of the maximum f region electron density the agreement with observations is worse if the m2 is not included in the simulation demonstrating that the lunar tide is an important contributor to the ionosphere variability during the 2009 ssw we additionally investigate sources of the f region electron density variability during the ssw the primary driver of the electron density variability is changes in electric fields changes in meridional neutral winds and thermosphere composition are found to also contribute to the electron density variability during the 2009 ssw the electron density variability for the 2009 ssw is therefore not solely due to variability in electric fields as previously thought

3 d modelling the electric field due to ocean tidal flow and comparison with observations 1 the tidal motion of the ocean water through the ambient magnetic field generates secondary electric field this motionally induced electric field can be detected in the sea or inland and has a potential for electrical soundings of the earth a first goal of the paper is to gain an understanding of the global distribution of the electric signal due to tidal ocean flow we simulate the electric signals for two tidal constituents lunar semidiurnal m2 and diurnal o1 tides we assume a realistic earth s conductivity model with a surface thin shell and 1 d mantle underneath simulations demonstrate that in some coastal regions the amplitudes of the electric field can reach 100 mv km and 10 mv km for m2 and o1 tides respectively the changes of lithosphere resistance produce detectable changes in the tidal electric signals we show that our predictions are in a good agreement with observations

impact of rock salt creep law choice on subsidence calculations for hydrocarbon reservoirs overlain by evaporite caprocks accurate forward modeling of surface subsidence above producing hydrocarbons reservoirs requires an understanding of the mechanisms determining how ground deformation and subsidence evolve here we focus entirely on rock salt which overlies a large number of reservoirs worldwide and specifically on the role of creep of rock salt caprocks in response to production induced differential stresses we start by discussing available rock salt creep flow laws we then present the subsidence evolution above an axisymmetric finite element representation of a generic reservoir that extends over a few kilometers and explore the effects of rock salt flow law choice on the subsidence response we find that if rock salt creep is linear as appropriate for steady state flow by pressure solution the subsidence response to any pressure reduction history contains two distinct components one that leads to the subsidence bowl becoming narrower and deeper and one that leads to subsidence rebound and becomes dominant at later stages this subsidence rebound becomes inhibited if rock salt deforms purely through steady state power law creep at low stresses we also show that an approximate representation of transient creep leads to relatively small differences in subsidence predictions most importantly the results confirm that rock salt flow must be modeled accurately if good subsidence predictions are required however in practice large uncertainties exist in the creep behavior of rock salt especially at low stresses these are a consequence of the spatial variability of rock salt physical properties which is practically impossible to constrain a conclusion therefore is that modelers can only resort to calculating bounds for the subsidence evolution above producing rock salt capped reservoirs

comparison of evolutionary and static modeling of stresses around a salt diapir abstract we compare an evolutionary with a static approach for modeling stress and deformation around a salt diapir we show that the two approaches predict different stress histories and very different strains within adjacent wall rocks near the base of a rising salt diapir significantly higher shear stresses develop when the evolutionary analysis is used in addition the static approach is not able to capture the decrease in the hoop stress caused by the circumferential diapir expansion nor the increase in the horizontal stress caused by the rise of the diapir hence only the evolutionary approach is able to predict a sudden decrease in the fracture gradient and identify areas of borehole instability near salt furthermore the evolutionary model predicts strains an order of magnitude higher than the strains within the static model more importantly the evolutionary model shows significant shearing in the horizontal plane as a result of radial shortening accompanied by an almost equivalent hoop extension the evolutionary analysis is performed with elfen and the static analysis with abaqus we model the sediments using a poro elastoplastic model overall our results highlight the ability of forward evolutionary modeling to capture the stress history of mudrocks close to salt diapirs which is essential for estimating the present strength and anisotropic characteristics of these sediments

upper mesospheric lunar tides over middle and high latitudes during sudden stratospheric warming events in recent years there have been a series of reported ground and satellite based observations of lunar tide signatures in the equatorial and low latitude ionosphere thermosphere around sudden stratospheric warming ssw events this lower atmosphere ionosphere coupling has been suggested to be via the e region dynamo in this work we present the results of analyzing 6 years of hourly upper mesospheric winds from specular meteor radars over a midlatitude 54 n station and a high latitude 69 n station instead of correlating our results with typical definitions of ssws we use the definition of polar vortex weaking pvw used by zhang and forbes 2014 this definition provides a better representation of the strength in middle atmospheric dynamics that should be responsible for the waves propagating to the e region we have performed a wave decomposition on hourly wind data in 21 day segments shifted by 1 day in addition to the radar wind data the analysis has been applied to simulations from whole atmosphere community climate model extended version and the thermosphere ionosphere mesosphere electrodynamics general circulation model our results indicate that the semidiurnal lunar tide m2 enhances in northern hemispheric winter months over both middle and high latitudes the time and magnitude of m2 are highly correlated with the time and associated zonal wind of pvw at middle high latitudes m2 in the upper mesosphere occurs after before the pvw at both latitudes the maximum amplitude of m2 is directly proportional to the strength of pvw westward wind we have found that m2 amplitudes could be comparable to semidiurnal solar tide amplitudes particularly around pvw and equinoxes besides these general results we have also found peculiarities in some events particularly at high latitudes these peculiarities point to the need of considering the longitudinal features of the polar stratosphere and the upper mesosphere and lower thermosphere regions for example during ssw 2009 we found that m2 enhances many days before pvw which is not in agreement with most of our results

geomechanical modeling of stresses adjacent to salt bodies part 1 uncoupled models we compare four approaches to geomechanical modeling of stresses adjacent to salt bodies these approaches are distinguished by their use of elastic or elastoplastic constitutive laws for sediments surrounding the salt as well as their treatment of fluid pressures in modeling we simulate total stress in an elastic medium and then subtract an assumed pore pressure after calculations are complete simulate effective stress in an elastic medium and use assumed pore pressure during calculations simulate total stress in an elastoplastic medium either ignoring pore pressure or approximating its effects by decreasing the internal friction angle and simulate effective stress in an elastoplastic medium and use assumed pore pressure during calculations to evaluate these approaches we compare stresses generated by viscoelastic stress relaxation of a salt sphere in all cases relaxation causes the salt sphere to shorten vertically and expand laterally producing extensional strains above and below the sphere and shortening against the sphere flanks deviatoric stresses are highest when sediments are assumed to be elastic whereas plastic yielding in elastoplastic models places an upper limit on deviatoric stresses that the rocks can support so stress perturbations are smaller these comparisons provide insights into stresses around salt bodies and give geoscientists a basis for evaluating and comparing stress predictions

the episodic rise net growing rate and kinematics of radial faults of the salinas de oro diapir using paleoseismological techniques ne spain salt upwelling versus karstic subsidence abstract this paper explores the use of trenching and paleoseismological techniques to determine salt flow rates into a salt diapir for the first time in the literature the salinas de oro diapir located in the northern spanish pyrenees is an oval shaped triassic salt stock that extends vertically for 7 km down to the paleozoic basement salt dissolution subsidence and diapir growth are coeval active processes karstification is responsible for the development of large sinkholes a thick caprock and the monocline folding and ring faulting of the cretaceous and early tertiary limestone rim the evaporite karstic aquifer discharges high concentration water of up to 137 g l of total dissolved solids and a conductivity over 200 ms cm into the salado creek which drains the diapir top the salinity monitoring of this drainage provides a maximum karstic surface lowering rate of around 2 8 mm yr salt upwelling has caused the 150 m uplift of the annular limestone escarpment and the development of 3000 m long radial grabens with up to 90 m of vertical offset that disrupt drainages displace quaternary deposits and overprint concentric faults the 260 cm drag folding of lacustrine facies exposed in a 42 m long and 6 5 m deep trench due to the creep motion of the western radial fault of azanza graben yielded a minimum short term uplift slip rate of 1 75 mm yr for the last 1485 years and provides a minimum salt supply rate of over 5 mm yr considering karstification lowering this value that is several hundred times higher than average evidence the discontinuous growth of salinas de oro diapir

implications of passive salt diapir kinematics for reservoir segmentation by radial and concentric faults abstract reservoirs associated with salt diapirs that are circular or elliptical in plan view can be segmented by radial faults concentric faults both or neither explanations for this variation are offered based on integration of fault kinematics published studies and seismic examples strata around diapirs can be divided into three structural domains with characteristic fault styles 1 roof zone between the present day depositional surface and crest of a buried inactive diapir this zone is characterised by doming radial faults and occasionally conic graben doming is usually caused by differential compaction and increases with depth 2 a steeply dipping sheath of strata surrounds the diapir out to a distance of less than one diapir radius recording near surface diapir growth in poorly lithified sediments these contain early gravity slumps and late concentric extensional faults regional layer bound polygonal fault systems form radial faults within approximately one diapir radius the only significant radial faulting mechanism below the roof zone 3 diapir root zone of strata whose structural architecture records the diapirs initiating mechanism typically rim synclines or graben that detach on the salt source layer concentric faults at radial distances of a kilometre or more have been found only in root zone rim synclines and sheared sheath strata these models and observations can assist seismic interpretation and structural model building in areas of poor geophysical imaging for instance below overhanging salt

geotechnical characteristics of large slow very slow and extremely slow landslides based on a study of 45 large slow moving landslides it is apparent that for a landslide to travel slowly after failure the sliding is most likely to be active or reactivated on a basal rupture surface at or close to residual strength the likelihood of slow movement after failure is also increased when the inclination of the basal rupture surface is less than the residual friction angle the slow moving landslides are all of low rock mass strength with varying degrees of disaggregation or they possess soil strength the influence of lateral margins on landslide restraint is generally small with landslide movement typically controlled by fluctuations in piezometric pressure the most commonly observed slow large landslides are mudslides and translational debris rock slides followed by particular forms of translational rock slides and internally sheared compound slides some mudslides display evidence of short periods of up to moderate velocities

modeling stress evolution around a rising salt diapir abstract we model the evolution of a salt diapir during sedimentation and study how deposition and salt movement affect stresses close to the diapir we model the salt as a solid visco plastic material and the sediments as a poro elastoplastic material using a generalized modified cam clay model the salt flows because ongoing sedimentation increases the average density within the overburden sediments pressurizing the salt stresses rotate near a salt diapir such that the maximum principal stress is perpendicular to the contact with the salt the minimum principal stress is in the circumferential direction and drops near the salt the mean stress increases near the upper parts of the diapir leading to a porosity that is lower than predicted for uniaxial burial at the same depth we built this axisymmetric model within the large strain finite element program elfen our results highlight the fact that forward modeling can provide a detailed understanding of the stress history of mudrocks close to salt diapirs such an understanding is critical for predicting stress porosity and pore pressure in salt systems

daily geomagnetic variations on the iberian peninsula summary mean hourly values of the horizontal and vertical components of the geomagnetic field at six observatories on the iberian peninsula were analysed to study seasonal and spatial behaviour of the solar and lunar ionospheric and oceanic daily geomagnetic variations the results are tabulated and their characteristics are discussed the influence of local oceanic effects on the lunar oceanic contribution at the peninsular observatories is noted in particular the spatial behaviour of the lunar oceanic contribution is considered whereas the mean value for the peninsula is consistent with that expected according to an existing model for electric currents in the eastern atlantic ocean individual values are not the influence of tide in the mediterranean sea of water currents in the strait of gibraltar and of local effects in the atlantic ocean is proposed as the reason for this behaviour

satellite observations of magnetic fields due to ocean tidal flow the ocean is an electrically conducting fluid that generates secondary magnetic fields as it flows through earth s main magnetic field extracting ocean flow signals from remote observations has become possible with the current generation of satellites measuring earth s magnetic field here we consider the magnetic fields generated by the ocean lunar semidiurnal m2 tide and demonstrate that magnetic fields of oceanic origin can be clearly identified in satellite observations

hydromechanical modeling of stress pore pressure and porosity evolution in fold and thrust belt systems we present coupled critical state geomechanical fluid flow simulations of the evolution of a fold and thrust belt in nw borneo our modelling is the first to include the effects of both syntectonic sedimentation and transient pore pressure on the development of a fold and thrust belt the present day structure predicted by the model contains the key first order structural features observed in the field in terms of thrust fault and anticline architectures stress predictions in the sediments show two compressive zones aligned with the shortening direction located at the thrust front and back limb between the compressive zones near to the axial plane of the anticline the modelled stress field represents an extensional regime the predicted overpressure distribution is strongly influenced by tectonic compaction with the maximum values located in the two laterally compressive regions we compared the results at three notional well locations with their corresponding uniaxial strain models the 2d thrust model predicted porosities which are as much as 7 5 lower at 2 5 km depth and overpressures which are up to 6 4 mpa higher at 3 km depth these results show that one dimensional methods are not sufficient to model the evolution of pore pressure and porosity in contractional settings finally we performed a drained simulation during which pore pressures were kept hydrostatic the predicted geological structures differ substantially from those of the coupled simulation with no thrust faulting these results demonstrate that pore pressure is a key control on structural development

ocean tidal signals in observatory and satellite magnetic measurements 1 ocean flow moves sea water through the earth s magnetic field inducing electric fields currents and secondary magnetic fields these motionally induced magnetic fields have a potential for the remote sensing of ocean flow variability a first goal must be to gain a better understanding of magnetic field generation by tidal ocean flow we predict the motionally induced magnetic fields for the six major tidal constituents and compare their amplitudes with the spectra of night time observatory and satellite magnetic measurements for the indian ocean the magnetic variations at the solar s2 kl and p1 periods turn out to be dominated by unrelated external fields in contrast observed lunar m2 and n2 tidal signals are in fair agreement with predictions from motional induction the lunar diurnal o1 signal visible at some observatories could be caused by ocean flow but disagrees in amplitude with our predictions

geomechanical analysis of a welding salt layer and its effects on adjacent sediments abstract we simulate welding of the source layer of a salt diapir with a forward finite element model and study stresses and deformation in the salt layer and the diapir as well as in their adjacent sediments welded salt layers are abundant in mature salt basins where most or all of the salt has withdrawn into diapirs however there is little understanding of the stress field in these layers and their adjacent sediments we show that salt flow along the source layer leads to significant stress anomalies inside the layer and in adjacent sediments in the source layer salt pressure becomes higher than overburden stress in nearly welded areas and becomes lower than overburden stress in adjacent thicker areas when the source layer welds stresses increase significantly in sediments near the weld tip which helps compaction of these sediments and possibly their fracturing and faulting our model illustrates that all sediments overlying the weld experience this stress increase and the associated material changes as the weld tip propagates along the weld we present natural examples fitting our predictions and discuss the importance of our results for the exploration characterization and production of reservoirs near welded salt layers

an arbitrary lagrangian eulerian formulation for creeping flows and its application in tectonic models summary this paper presents and analyses numerical techniques developed to investigate viscoplastic stokes flows within a model of lithospheric deformation in particular the techniques are related to a subduction model of compressional orogens the driving mechanism in the model corresponds to the near rigid convergence and subduction of one mantle lithosphere beneath another in plane strain and this boundary condition forces flow in an overlying viscoplastic model crust the numerical techniques use the arbitrary eulerian lagrangian formulation in which flows with free surfaces and large deformation are computed on an evolving eulerian finite element grid that conforms to the material domain a regridding algorithm allows the associated lagrangian motion and fields to be followed and in addition coupled back to the eulerian calculation of the flow mass flux boundary conditions are used so that the effects of erosion and deposition by surface processes and mass loss by subduction can be included in the model calculation the evolving model crustal layer is flexurally compensated using a general elastic beam formulation the applicability of the numerical techniques to problems ranging from accretionary wedges to crustal and lithospheric scale deformation is discussed simple flows a linear viscous subduction model a whirl flow and a quasi convection model are used to show that the mass conservation regridding and surface tracking errors are small the broader applicability of the modelling techniques is reviewed