field scale acoustic investigation of a damaged anisotropic shale during a gallery excavation in the opalinus clay formation at the mont terri underground rock laboratory the gallery ga08 was excavated in august 2008 to join the end face of the pre existing gallery ga04 the aim of the present work was to perform in situ acoustic experiments to monitor the evolution of the excavation damaged zone edz induced during the gallery construction the end face of ga04 was instrumented with two arrays of acoustic transducers allowing for the active and passive seismic monitoring i e acoustic survey and micro seismicity from the acoustic survey data which required a high energy acoustic source to emit high frequency signals 21 25 31 and 38 khz the rock mass was observed to be anisotropic and heterogeneous at the scale of the experiment p wave velocities were determined to be in average between 3300 m s along a structural bedding plane and 2700 m s at 70 incidence relative to that assuming a transversely isotropic shale formation the p wave velocity dependence versus was modeled using thomsen s weak transverse isotropy model thomsen 1986 36 thomsen s p wave anisotropy parameter was found to be 0 15 and the fifth thomsen s parameter controlling the deviation of the wave front from an ellipsoidal geometry was found to be 0 16 the s wave velocity was estimated along a single direction of aligned receivers and turned out to be around 1560 m s at 30 we also show that the rock mass acts as a frequency filter for acoustic waves related to the rock mass heterogeneities i e the inter bedding structure which induces wave scattering and refraction from the micro seismicity data we identified a large number of micro seismic events mses detected on the acoustic arrays during and following the excavation most of the mses were induced on the excavated face but we also located some mses inside the rock mass itself we show that these events are located close to a major fault which seems to be reactivated by the excavation process

thickness of sets of cross strata and planar strata as a function of formative bed wave geometry and migration and aggradation rate cross strata and planar strata formed by the migration of ripples and dunes or low relief bed waves on upper stage plane beds respectively are the most common internal structures in fluvial sands and sandstones the thickness of cross sets or planar laminae at a given location is controlled by 1 aggradation rate relative to downstream migration rate of bed waves and 2 the succession of bed waves of varying height passing the location a quantitative understanding of this relationship is now appearing based on new theory and experimental studies with ripples dunes and upper stage plane beds in many cases the effects of control 1 can be ignored in such cases only the largest bed waves in a population form cross sets or planar laminae the ratio of mean cross set thickness mean height of formative bed waves is less than 0 4 and the ratio of mean cross set thickness mean height of all bed waves is less than 0 5 a simple quantitative model a modified form of the paola borgman model is presented for predicting the height distributions of bed waves ripples dunes low relief bed waves from the thickness distributions of cross sets or planar laminae if the aggradation rate is appreciable more of the smaller bed waves in a population can form cross sets or planar laminae and the preservation ratios increase to 1 in the case of supercritical climbing ripple cross strata in this case it is necessary to determine the angle of climb and length of the bed waves in order to use the model to predict bed wave height distributions

structure and stratigraphy of home plate from the spirit mars exploration rover home plate is a layered plateau observed by the mars exploration rover spirit in the columbia hills of gusev crater the structure is roughly 80 m in diameter and the raised margin exposes a stratigraphic section roughly 1 5 m in thickness previous work has proposed a pyroclastic surge possibly followed by aeolian reworking of the ash for the depositional origin for these beds we have performed a quantitative analysis of the structure stratigraphy and sedimentology at this location our results are consistent with an explosive volcaniclastic origin for the layered sediments analysis of bedding orientations over half of the circumference of home plate reveals a radially inward dipping structure consistent with deposition in the volcanic vent or topographic draping of a preexisting depression detailed observations of the sedimentology show that grain sorting varies significantly between outcrops on the east and west sides observations on the western side show a well sorted population of sand sized grains which comprise the bedrock while the eastern margin shows a wider range of grain sizes including some coarse granules these observations are consistent with primary deposition by a pyroclastic surge however aeolian reworking of the upper stratigraphic unit is not ruled out identification of explosive volcanic products on mars may implicate magma interaction with subsurface hydrologic reservoirs in the past

model based amplitude versus offset and azimuth inversion for estimating fracture parameters and fluid content abstractthe normal to shear fracture compliance ratio is commonly used as a fluid indicator in the seismic frequency range the fluid indicator lies between the values for isolated fluid filled fractures and dry fractures and it is not easy to discriminate the fluid content assuming that the fracture surfaces are smooth we use 1 n ndry with n and ndry representing the normal fracture weakness of the saturated and dry rock to indicate fluid types and to define a fluid influencing factor the fluid influencing factor is sensitive to the fluid properties the aspect ratio of the fractures and the frequency conventionally the amplitude versus offset and azimuth avoa inversion is formulated in terms of the contrasts of the fracture weaknesses across the interface assuming that the fractures are vertical with the same symmetry axis we consider fractures with arbitrary azimuths and develop a method to estimate fracture parameters from wide azimuth seismic data the proposed avoa inversion algo

a partial articulated acanthodian from the silurian of new brunswick canada vertebrates preserved in calcareous concretions collected from silurian marginal marine deposits near nerepis southern new brunswick include agnathan thelodonts a heterostracan and possibly an a

how do bedform patterns arise new views on the role of bedform interactions within a set of boundary conditions one explanation for bedform patterns is self organization in which the pattern emerges because of interactions among the bedforms themselves models remote images field studies and lab experiments have identified bedform interactions that involve whole bedforms only bedform defects or that are remote interactions between bedforms it is proposed that bedform interactions form a spectrum from constructive to regenerative in pattern development constructive interactions including merging lateral linking cannibalization and remote transfer of sediment push the system toward fewer larger more widely spaced bedforms regenerative interactions including bedform splitting defect creation and calving push the system back toward a more initial state other interactions including off center collision defect migration and bedform and defect repulsion cause pattern change but may not be strongly constructive or regenerative although bedform interactions are ubiquitous to any field of bedforms their dynamics flow field modification and impact upon measurable pattern parameters are yet poorly understood most bedform interactions span bedform types and fluids supporting the hypothesis that pattern emerges from dynamics at the bedform level in a hierarchy that includes lower levels of bedform flow and grain fluid interactions bedform interactions alone however cannot account for the rich diversity of bedform patterns in nature it is proposed that field diversity arises because of boundary conditions which are the environmental variables within which a field evolves conceptually boundary conditions modify the shape of the attractor toward which a field evolves possibly by altering the type and frequency of bedform interactions boundary conditions are broadly similar within system types but are unique for each bedform field so that no two are ever exactly alike although aeolian and fluvial systems share some types of boundary conditions flow depth is a unique boundary condition in shallow fluvial systems copyright 2009 john wiley sons ltd

ultrasonic moduli for fluid saturated rocks mavko jizba relations rederived and generalized mavko and jizba propose a quantitative model for squirt dispersion of elastic wave velocities between seismic and ultrasonic frequencies in granular rocks their central results are the expressions for the so called unrelaxed frame bulk and shear moduli computed under an assumption that the stiff pores are drained or dry but the soft pores are filled with fluid mavko jizba expressions are limited to liquid saturated rocks but become inaccurate when the fluid bulk modulus is small e g for gas saturated rocks we have derived new expressions for unrelaxed moduli of fluid saturated porous rocks using sayers kachanov discontinuity formalism the derived expressions generalize the established mavko jizba relations to gas saturated rocks reduce to mavko jizba results when the pore fluid is liquid and yield dry moduli when fluid bulk modulus tends to zero we tested this by comparing our model and the model of mavko and jizba against laboratory measurements on a sample of westerly granite

large wind ripples on mars a record of atmospheric evolution wind blowing over sand on earth produces decimeter wavelength ripples and hundred meter to kilometer wavelength dunes bedforms of two distinct size modes observations from the mars science laboratory curiosity rover and the mars reconnaissance orbiter reveal that mars hosts a third stable wind driven bedform with meter scale wavelengths these bedforms are spatially uniform in size and typically have asymmetric profiles with angle of repose lee slopes and sinuous crest lines making them unlike terrestrial wind ripples rather these structures resemble fluid drag ripples which on earth include water worked current ripples but on mars instead form by wind because of the higher kinematic viscosity of the low density atmosphere a reevaluation of the wind deposited strata in the burns formation about 3 7 billion years old or younger identifies potential wind drag ripple stratification formed under a thin atmosphere

path dependence of acoustic velocity and attenuation in experimentally deformed westerly granite brittle failure in rocks is generally preceded by a localization of deformation in the region of the final macrofracture this process is associated with a non uniform distribution of subcritical microcracks which should be reflected in the distribution of acoustic velocities and surface deformation the temporal and spatial development of such deformation zones has been studied by monitoring these quantities during the deformation of westerly granite samples

multi spatial analysis of aeolian dune field patterns abstract aeolian dune fields are composed of different spatial scales of bedform patterns that respond to changes in environmental boundary conditions over a wide range of time scales this study examines how variations in spatial scales of dune and ripple patterns found within dune fields are used in environmental reconstructions on earth mars and titan within a single bedform type different spatial scales of bedforms emerge as a pattern evolves from an initial state into a well organized pattern such as with the transition from protodunes to dunes additionally different types of bedforms such as ripples coarse grained ripples and dunes coexist at different spatial scales within a dune field analysis of dune field patterns at the intersection of different scales and types of bedforms at different stages of development provides a more comprehensive record of sediment supply and wind regime than analysis of a single scale and type of bedform interpretations of environmental conditions from any scale of bedform however are limited to environmental signals associated with the response time of that bedform large scale dune field patterns integrate signals over long term climate cycles and reveal little about short term variations in wind or sediment supply wind ripples respond instantly to changing conditions but reveal little about longer term variations in wind or sediment supply recognizing the response time scales across different spatial scales of bedforms maximizes environmental interpretations from dune field patterns

misalignment of the orientation of fractures and the principal axes for p and s waves in rocks containing multiple non orthogonal fracture sets summary azimuthal anisotropy in rocks can result from the presence of one or more sets of partially aligned fractures with orientations determined by the stress history of the rock a shear wave propagating in an azimuthally anisotropic medium splits into two components with diverent polarizations if the source polarization is not aligned with the principal axes of the medium in the presence of two or more non orthogonal sets of vertical fractures the symmetry of the medium may be approximated as monoclinic with a horizontal plane of mirror symmetry if in the absence of fractures the rock is transversely isotropic ti with the symmetry axis perpendicular to the bedding plane for such a medium the fast and slow polarization directions for vertically propagating shear waves are not parallel or perpendicular to any of the fracture planes but lie in directions given by the principal axes of a second rank fracture compliance tensor this tensor is independent of the normal compliance of the fractures it follows that for vertical propagation of shear waves in a vertically fractured ti medium any number of arbitrarily oriented vertical fracture sets is equivalent to two mutually perpendicular fracture sets provided that the seismic wavelength is large compared to the size of the fractures for ovsets typical of surface seismic acquisition the p wave velocity at fixed ovset varies with azimuth as cos 2 w w 0 where w is the azimuth measured with respect to the fast polarization direction for a vertically polarized shear wave w 0 depends on both the normal and the shear compliance of the fractures and may diver from zero if the ratio of the normal to shear compliance of the fractures varies significantly between fracture sets if this ratio is similar for all fractures w 0 0 and the principal axes of the variation in p wave velocity with azimuth for fixed ovset are determined by the principal axes of the second rank fracture compliance tensor

bed thickness distributions on mars an orbital perspective studies on earth show that sedimentary bed thickness and bed thickness distributions record information about the processes controlling sediment deposition high resolution digital terrain models dtms such as those derived from the high resolution imaging science experiment hirise now provide the opportunity to quantify bed thickness properties on mars over several orders of magnitude down to the submeter scale this study uses hirise dtms and visible images to measure bed thickness distributions at 10 deposits on mars with the aim of determining whether statistical techniques can provide useful criteria for distinguishing sedimentary depositional processes basic statistics including mean thickness and range are examined as are histograms cumulative frequency plots and log log plots statistical tests interrogate these deposits for thinning or thickening upward trends and the presence of normal lognormal and exponential distributions although there are challenges associated with these methods the statistical analysis of bed thickness coupled with morphological and mineralogical interpretations has the potential to be a powerful tool for characterizing and classifying sedimentary rocks on mars in particular bed thickness statistics are particularly well suited for examining changes in sediment supply and accommodation within martian sedimentary sequences

observations of an aeolian landscape from surface to orbit in gale crater abstract landscapes derived solely from aeolian processes are rare on earth because of the dominance of subaqueous processes in contrast aeolian derived landscapes should typify mars because of the absence of liquid water the long exposure times of surfaces and the presence of wind as the default geomorphic agent using the full range of available orbital and mars science laboratory rover curiosity images wind formed features in gale crater were cataloged and analyzed in order to characterize the aeolian landscape and to derive the evolution of the crater wind regime over time inferred wind directions show a dominance of regional northerly winds over geologic time scales but a dominance of topography driven katabatic winds in modern times landscapes in gale crater show a preponderance of aeolian features at all spatial scales interpreted processes forming these features include first cycle aeolian abrasion of bedrock pervasive deflation organization of available sand into bedforms abundant cratering and gravity driven wasting all of which occur over a background of slow physical weathering the observed landscapes are proposed to represent a spectrum of progressive surface denudation from fractured bedrock to retreating bedrock capped mesas to remnant hills capped by bedrock rubble to desert pavement plains this model of landscape evolution provides the mechanism by which northerly winds acting over 3 ga excavated tens of thousands of cubic kilometers of material from the once sediment filled crater thus carving the intra crater moat and exhuming mount sharp aeolis mons the current crater surface is relatively sand starved indicating that potential sediment deflation from the crater is greater than sediment production and that most exhumation of mount sharp occurred in the ancient geologic past

incremental linear elastic response of rocks containing multiple rough fractures similarities and differences with traction free cracks fractures in the subsurface serve as conduits for fluids and gas connecting remote hydrocarbon reservoir sections to production wells seismic and sonic data are popular sources for information on fracture properties the most commonly used model to extract fracture information from such data is based on the paradigm of the displacement discontinuity interface without a direct link to relevant characteristics such as the surface roughness properties of a fracture indeed fractures can be modeled as displacement discontinuity surfaces and in this sense they resemble traction free cracks in literature cracks and fractures are not always properly distinguished perhaps because the terms are often perceived as synonyms however microstructural parameters that control magnitudes of the discontinuities and thus the effective stiffnesses are entirely different statistics of contacts for fractures versus crack density for traction free cracks we explore the effective elasticity of rocks containing multiple fractures using a model of a fracture as two rough surfaces with isolated contacts this is done in the context of the incremental linear elastic response to small stress changes typical in wave propagation problems fractures are dry or may have diverse orientations and contacts may or may not be hertzian a link exists between contact characteristics and effective stiffness of single and multiple fractures our work examines and accounts for the strong effect of interactions between individual contacts by means of a double sum over mutual positions as well as outlines the differences and similarities between theories for cracks and fractures

an analytic model for the stress induced anisotropy of dry rocks one of the main causes of azimuthal anisotropy in sedimentary rocks is anisotropy of tectonic stresses in the earth s crust we have developed an analytic model for seismic anisotropy caused by the application of a small anisotropic stress we first considered an isotropic linearly elastic medium porous or nonporous permeated by a distribution of discontinuities with random isotropic orientation such as randomly oriented compliant grain contacts or cracks the geometry of individual discontinuities is not specified instead their behavior is defined by a ratio b of the normal to tangential excess compliances when this isotropic rock is subjected to a small compressive stress isotropic or anisotropic the number of cracks along a particular plane is reduced in proportion to the normal stress traction acting on that plane this effect is modeled using the sayers kachanov noninteractive approximation the model predicts that such anisotropic crack closure yields elliptical anisotropy regardless of

sedimentological evidence for a deltaic origin of the western fan deposit in jezero crater mars and implications for future exploration abstract we examine the stratigraphic architecture and mineralogy of the western fan deposit in the jezero crater paleolake on mars to reassess whether this fan formed as a delta in a standing body of water as opposed to by alluvial or debris flow processes analysis of topography and images reveals that the stratigraphically lowest layers within the fan have shallow dips

characterizing the effect of elastic interactions on the effective elastic properties of porous cracked rocks elastic interactions between pores and cracks reflect how they are organized or spatially distributed in porous rocks the principle goal of this paper is to understand and characterize the effect of elastic interactions on the effective elastic properties we perform finite element modelling to quantitatively study how the spatial arrangement of inclusions affects stress distribution and the resulting overall elasticity it is found that the stress field can be significantly altered by elastic interactions compared with a non interacting situation stress shielding considerably stiffens the effective media while stress amplification appreciably reduces the effective elasticity we also demonstrate that the t matrix approach which takes into account the ellipsoid distribution of pores or cracks can successfully characterize the competing effects between stress shielding and stress amplification numerical results suggest that when the concentrations of cracks increase beyond the dilute limit the single parameter crack density is not sufficient to characterize the contribution of the cracks to the effective elasticity in order to obtain more reliable and accurate predictions for the effective elastic responses and seismic anisotropies the spatial distribution of pores and cracks should be included additionally such elastic interaction effects are also dependent on both the pore shapes and the fluid infill

anisotropic azimuthal surface wave inversion for a vertically fractured and transverse isotropy medium theory and examples from a controlled field experiment abstract fractures in elastic media add compliance to a rock in the direction normal to the fracture strike therefore elastic wave velocities in a fractured rock will vary as a function of the energy propagation direction relative to the orientation of the aligned fracture set anisotropic thomson haskell matrix rayleigh wave equations for a vertically transverse isotropic media can be used to model surface wave dispersion along the principal axes of a vertically fractured and transversely isotropic medium furthermore a workflow combining first break analysis and azimuthal anisotropic rayleigh wave inversion can be used to estimate p wave and s wave velocities thomsen s e and thomsen s along the principal axes of the orthorhombic symmetry in this work linear slip theory is used to map our inversion results to the equivalent vertically fractured and transversely isotropic medium coefficients we carried out this inversion on a synthetic example and a field example the synthetic data example results show that joint estimation of s wave velocities with thomsen s parameters e and along normal and parallel to the vertical fracture set is reliable and when mapped to the corresponding vertically fractured and transversely isotropic medium provides insight into the fracture compliances when the inversion was carried out on the field data results indicated that the fractured rock is more compliant in the azimuth normal to the visible fracture set orientation and that the in situ normal fracture compliance to tangential fracture compliance ratio is less than half which implies some cementation may have occurred along the fractures such an observation has significant implications when modelling the transport properties of the rock and its strength both synthetic and field examples show the potential of azimuthal anisotropic rayleigh wave inversion as the method can be further expanded to a more general case where the vertical fracture set orientation is not known a priori

morphometric analysis of slipface processes of an aeolian dune implications for grain flow dynamics grain flows are an integral part of sand dune migration they are a direct response to the local windregime and reflect complex interactions between localized over steepening of a dune slipface andcomplex turbulent airflow on the lee slope grain flows are primarily responsible for delivering sediment to the base of a dune thus driving slipface advancement yet there are few constraints on their morphological and spatial characteristics or the amount of sediment that is redistributed bythese flows using a combination of high resolution terrestrial laser scanning and video recordings four distinct grain flow types are identified based on morphology and area on a dune slipface grainflow morphologies range from small superficial flows to larger flows that affect greater portions of the slipface moving significant amounts of sediment detailed field observations are presented of the dynamics of lee slopes including measurements of the initiation location thickness magnitude and frequency statistics of grain flows as well as volume estimates of redistributed sediment for each grain flow observed high resolution laser scans enable accurate quantification of bulk sediment transfer from individual grain flows and can be used to study grain flows in a variety of environments a categorization of grain flow morphologies is presented that links styles of flows with wind strength and direction turbulent airflow sediment deposition and environment

preservation of planar laminae due to migration of low relief bed waves over aggrading upper stage plane beds comparison of experimental data with theory experimental studies of the formation of planar laminae by migration of low relief bed waves over aggrading upper stage plane beds show that the average thickness of laminae at a point increases with both aggradation rate and the variance of the heights of bed waves passing that point in general the preserved laminae represent only a small proportion generally less than 50 of the height of the largest bed waves in the population the theory developed by paola borgman 1991 relating the probability density function of stratal thickness to that of bed wave height for the case of no net aggradation was adapted for aggrading conditions and shown to agree well with data these results suggest that the theory can be used to estimate from the distribution of thickness of planar laminae either the distribution of bed wave heights mean aggradation rate mean bed wave length or mean bed wave celerity provided the other parameters can be estimated

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

microfracturing and microporosity in shales shales are ubiquitous rocks in sedimentary basins where their low permeability makes them efficient seals for conventional oil and gas reservoirs and underground waste storage repositories waste waters co2 nuclear fuels moreover when they contain organic matter they form source rocks for hydrocarbons that may escape towards a more porous reservoir during burial a process referred to as primary migration and when the hydrocarbons cannot escape these rocks can be exploited as oil or shale gas reservoirs while the presence of fractures at the outcrop scale has been described the existence of fractures at smaller scales their link with microporosity the mechanisms that created them their persistence over geological times and their effect on the petrophysical properties of shales represent scientific challenges for which drillings in various sedimentary basins over the past decades may hold timely key data here we review and synthetize the current knowledge on how microfractures and micropores in shales can be imaged and characterized and how they control their anisotropic mechanical properties and permeability one question is whether such microfractures when observed in outcrops or in drilled core samples extracted from boreholes are related to decompaction and do not exist at depth another question is whether veins observed in shales represent microfractures that were open long enough to have acted as flow paths across the formation the mechanisms of microfracture development are described some have an internal origin fracturing by maturation of organic matter dehydration of clays while others are caused by external factors tectonic loading importantly the amount of microfracturing in shales is shown to depend strongly on the content in 1 organic matter and 2 strong minerals the nucleation of microfractures depends on the existence of mechanical heterogeneities down to the nanometer scale their propagation and linkage to create a percolating network will depend on the presence of heterogeneities at the meso to macro scales such percolating microfracture networks could control both the long term sealing capabilities of cap rocks and the further propagation of hydraulic fracturing cracks finally possible areas of research for describing the mechanism of microfracture formation in greater detail and how this impacts the transport and mechanical properties of shales are also discussed

anisotropy pore fluid pressure and low angle normal faults low angle normal faults lanfs are commonly associated with high angle normal faults high pore fluid pressures and foliated fault cores explanations for lanf slip based on high pore fluid pressure critically depend on stress rotations in the fault core zone which have been based on an assumption of isotropy anisotropy fundamentally changes the coupling between pore fluid pressure and effective stress the response of saturated fault core rocks in lanfs can be explored by combining the effective medium theory with anisotropic poroelasticity the nucleation and evolution of lanfs are investigated in a framework of local changes in rock properties and the interaction of a pressurised pore fluid with intrinsic lithological and extrinsic damage anisotropies this quantitative approach enables comparison with detailed field and laboratory measurements of fault rock properties

effects of fluid shear resistance and squirt flow on velocity dispersion in rocks abstractlaboratory measurements of rocks saturated with high viscosity fluids such as heavy oil bitumen magma kerogen etc often exhibit considerable seismic velocity dispersion which is usually underestimated by the biot theory over the years grain scale dispersion mechanisms such as squirt local flow and shear relaxation nonzero shear stress in the pore fluid have been more successful in explaining the measured dispersion we developed a new method to quantify the combined high frequency effects of squirt and shear dispersion on the effective moduli of rocks saturated with viscous fluids viscous fluid at high frequencies was idealized as an elastic solid of finite shear modulus hydraulically locked in stiff and soft pores this method entailed performing solid substitution in stiff pores of a dry rock frame which itself was unrelaxed due to solid filled soft pores the unrelaxed frame stiffness solutions required information on the pressure dependency of the rock stiffness and porosity

quantification of variability in bedform geometry we analyze the variability in bedform geometry in laboratory and field studies even under controlled steady flow conditions in laboratory flumes bedforms are irregular in size shape and spacing also in case of well sorted sediment our purpose is to quantify the variability in bedform geometry we use a bedform tracking tool to determine the geometric variables of the bedforms from measured bed elevation profiles for each flume and field data set we analyze variability in 1 bedform height 2 bedform length 3 crest elevation 4 trough elevation and 5 slope of the bedform lee face each of these stochastic variables is best described by a positively skewed probability density function such as the weibull distribution we find that except for the lee face slope the standard deviation of the geometric variable scales with its mean value as long as the ratio of width to hydraulic radius is sufficiently large if the ratio of width to hydraulic radius is smaller than about ten variability in bedform geometry is reduced an exponential function is then proposed for the coefficients of variation of the five variables to get an estimate of variability in bedform geometry we show that mean lee face slopes in flumes are significantly steeper than those in the field the 95 and 98 values of the geometric variables appear to scale with their standard deviation the above described simple relationships enable us to integrate variability in bedform geometry into engineering studies and models in a convenient way

new species of silurian acanthodians from the mackenzie mountains canada two new acanthodian taxa are described the ischnacanthid xylacanthus kenstewarti is based on large dentigerous jaws and granulacanthus joenelsoni is based on isolated spines the isolated remain

reconstructing random topography from preserved stratification one of the fundamental problems of interpreting the sedimentary record is reconstructing the original heights of palaeotopographical features such as bedforms or river channels this requires an understanding of the relationship between topography and set thickness but at present an exact theory exists only for periodic topography of uniform height the applicability of this simple theory is severely limited by the random variability characteristic of many sedimentary systems in this paper we develop an exact theory for the probability density function pdf of sets generated from topography of random height we focus on the limiting case of zero net deposition in order to provide a lower bound for the set thickness and derive an analytical set thickness pdf that is determined by one parameter of the pdf for topographical height this parameter measures the breadth of the tail of the topographical pdf the mean set thickness is 0 8225 for bedforms and 1 645 for river channels if the topographical height is gamma distributed the preservation ratio defined as mean thickness of preserved sets mean topographical height is 0 8225 r2 for bedforms and 1 645 r2 for river channels where r is the coefficient of variation standard deviation mean of the generating topography in a comparison with data from laboratory current ripples our analytical predictions compare well with observations of both mean set thickness and thickness distribution the preservation ratio for the ripples is about 0 5 whilst measured dune heights give a preservation ratio of about 0 12 depth data from two modern braided streams yield preservation ratios ranging from 0 4 to 0 75 as more data on the distributions of topographical height in modern environments become available calculations such as these should help provide reliable error bounds for quantitative topographical reconstruction

frequency dependence of anisotropy in fluid saturated rocks part ii stress induced anisotropy case a major cause of attenuation in fluid saturated media is the local fluid flow or squirt flow induced by a passing wave between pores of different shapes and sizes several squirt flow models have been derived for isotropic media for anisotropic media however most of the existing squirt flow models only provide the low and high frequency limits of the saturated elastic properties we develop a new squirt flow model to account for the frequency dependence of elastic properties and thus gain some insight into velocity dispersion and attenuation in anisotropic media in a companion paper we focused on media containing aligned compliant pores embedded in an isotropic background matrix in this paper we investigate the case for which anisotropy results from the presence of cracks with an ellipsoidal distribution of orientations due to the application of anisotropic stress the low and high frequency limits of the predicted fluid saturated elastic properties are respectively consistent with gassmann theory and mukerji mavko squirt flow model in the most important case of liquid saturation analytical expressions are derived for elastic properties and thomsen anisotropy parameters the main observations drawn from this model are as follows crack closure perpendicular to the applied stress leads to an increase in seismic velocities as a function of stress in the direction of applied stress and a decrease in squirt flow induced dispersion and attenuation in this direction the anisotropy of squirt flow dispersion engenders a decrease in the degree of anisotropy with frequency the stress induced anisotropy remains elliptical even in saturated media for all frequency ranges

stratigraphic analysis of eolian interactions with marine and fluvial deposits middle jurassic page sandstone and carmel formation colorado plateau u s a abstract the eolian page sandstone middle jurassic in south central utah and adjacent arizona consists of multiple mostly eolian sequences and sequence bounding unconformities super surfaces the super surfaces are a powerful correlation tool that provide the basis for a detailed regional stratigraphic analysis of the page sandstone and coeval parts of the marine and coastal plain carmel formation some page sandstone super surfaces correlate with sharp lithologic breaks in the carmel formation that are interpreted as marine flooding surfaces others correlate with fluvial surfaces in the carmel formation the lower part of the page carmel interval encompasses seven onlapping accumulation sequences bounded by marine flooding surfaces the uppermost two represent the middle jurassic marine high water mark these seven units comprise marine transgressive and marine highstand system tracts each interval is composed of progradational eolian deposits over marine deposits major tectonic and sedimentologic changes affected accumulation sequences 8 and 9 the two intervals are continental in origin fluvial to the west and eolian to the east a major slightly angular regional unconformity marks the top of accumulation sequence 9 the unconformity reflects regional tilting up to the northeast and ensuing erosion of older units in the page sandstone accumulation sequence 10 the top of the s udy interval displays complexly juxtaposed continental and marine facies that buried the unconformity as regional subsidence continued the broad patterns described above can be correlated with thick marine carbonate and minor siliciclastic deposits of the twin creek limestone 400 km north of the study area the resulting depositional patterns define three broad depositional events the oldest of these sequences defines a regional marine transgression and highstand the second sequence reflects strong siliciclastic influx and slight regional tilting that resulted in widespread continental deposition regional tilting and erosion closed out this event the youngest sequence indicates a partial return to marine transgressive conditions but was complicated by regional uplift and tilting related to are tectonism this study demonstrates that a sequence stratigraphic framework can be applied to sparsely fossiliferous eolian sabkha and restricted marine deposits in a marine coastal setting using detailed sedimentologic and stratigraphic studies tectonic eustatic and possibly climatic signals can be differentiated within the complex cyclic facies patterns displayed in the page sandstone and carmel formation

archean to recent aeolian sand systems and their sedimentary record current understanding and future prospects the sedimentary record of aeolian sand systems extends from the archean to the quaternary yet current understanding of aeolian sedimentary processes and product remains limited most preserved aeolian successions represent inland sand sea or dunefield erg deposits whereas coastal systems are primarily known from the cenozoic the complexity of aeolian sedimentary processes and facies variability are under represented and excessively simplified in current facies models which are not sufficiently refined to reliably account for the complexity inherent in bedform morphology and migratory behaviour and therefore cannot be used to consistently account for and predict the nature of the preserved sedimentary record in terms of formative processes archean and neoproterozoic aeolian successions remain poorly constrained palaeozoic ergs developed and accumulated in relation to the palaeogeographical location of land masses and desert belts during the triassic widespread desert conditions prevailed across much of europe during the jurassic extensive ergs developed in north america and gave rise to anomalously thick aeolian successions cretaceous aeolian successions are widespread in south america africa asia and locally in europe spain and the usa several eocene to pliocene successions represent the direct precursors to the present day systems quaternary systems include major sand seas ergs in low lattitude and mid latitude arid regions pleistocene carbonate and holocene modern siliciclastic coastal systems the sedimentary record of most modern aeolian systems remains largely unknown the majority of palaeoenvironmental reconstructions of aeolian systems envisage transverse dunes whereas successions representing linear and star dunes remain under recognized research questions that remain to be answered include i what factors control the preservation potential of different types of aeolian bedforms and what are the characteristics of the deposits of different bedform types that can be used for effective reconstruction of original bedform morphology ii what specific set of controlling conditions allow for sustained bedform climb versus episodic sequence accumulation and preservation iii can sophisticated four dimensional models be developed for complex patterns of spatial and temporal transition between different mechanisms of accumulation and preservation and iv is it reasonable to assume that the deposits of preserved aeolian successions necessarily represent an unbiased record of the conditions that prevailed during episodes of earth history when large scale aeolian systems were active or has the evidence to support the existence of other major desert basins been lost for many periods throughout earth history

exploring trends in microcrack properties of sedimentary rocks an audit of dry core velocity stress measurements rock physics models are used increasingly to link fluid and mechanical deformation parameters for dynamic elastic modeling we explore the input parameters of an analytical stress dependent rock physics model to do this we invert for the stressdependent microcrack parameters of more than 150 sedimentary rock velocity stress core measurements taken from a literature survey theinversionschemeisbasedonamicrostructuraleffective medium formulation defined by a second rank crack density tensor scalar crack model or by a second and fourth rank crack density tensor joint inversion model then the inversion results are used to explore and predict the stress dependent elastic behavior of various sedimentary rock lithologies using an analytical microstructural rock physics model via the initial model inputparameters initialcrackaspectratioandinitialcrackdensity estimates of initial crack aspect ratio are consistent among most lithologies with a mean of 0 0004 but for shales they differ uptoseveraltimesinmagnitudewithameanof0 001 estimates of initial aspect ratio are relatively insensitive to the inversion method although the scalar crack inversion becomes less reliableatlowvaluesofnormal to tangentialcrackcomplianceratio bn bt initialcrackdensityissensitivetothedegreeofdamage as well as the inversion procedure an important implication is thatthefourth rankcrack densitytermisnotnecessarilynegligible for most sedimentary rocks and evaluation of this term or bn bt is necessary for accurate prediction of initial crack density this is especially important because recent studies suggest thatbn btcanindicatefluidcontentincracks

ancient martian aeolian processes and palaeomorphology reconstructed from the stimson formation on the lower slope of aeolis mons gale crater mars reconstruction of the palaeoenvironmental context of martian sedimentary rocks is central to studies of ancient martian habitability and regional palaeoclimate history this paper reports the analysis of a distinct aeolian deposit preserved in gale crater mars and evaluates its palaeomorphology the processes responsible for its deposition and its implications for gale crater geological history and regional palaeoclimate whilst exploring the sedimentary succession cropping out on the northern flank of aeolis mons gale crater the mars science laboratory rover curiosity encountered a decametre thick sandstone succession named the stimson formation unconformably overlying lacustrine deposits of the murray formation the sandstone contains sand grains characterized by high roundness and sphericity and cross bedding on the order of 1 m in thickness separated by sub horizontal bounding surfaces traceable for tens of metres across outcrops the cross beds are composed of uniform thickness cross laminations interpreted as wind ripple strata cross sets are separated by sub horizontal bounding surfaces traceable for tens of metres across outcrops that are interpreted as dune migration surfaces grain characteristics and presence of wind ripple strata indicate deposition of the stimson formation by aeolian processes the absence of features characteristic of damp or wet aeolian sediment accumulation indicate deposition in a dry aeolian system reconstruction of the palaeogeomorphology suggests that the stimson dune field was composed largely of simple sinuous crescentic dunes with a height of ca 10 m and wavelengths of ca 150 m with local development of complex dunes analysis of cross strata dip azimuths indicates that the general dune migration direction and hence net sediment transport was towards the north east the juxtaposition of a dry aeolian system unconformably above the lacustrine murray formation represents starkly contrasting palaeoenvironmental and palaeoclimatic conditions stratigraphic relationships indicate that this transition records a significant break in time with the stimson formation being deposited after the murray formation and stratigraphically higher mount sharp group rocks had been buried lithified and subsequently eroded

asymmetry in the time lapse seismic response to injection and depletion large changes in seismic reflection amplitude have been observed around injectors and result from the decrease in elastic wave velocity due to the increase in pore pressure in the reservoir in contrast the velocity change resulting from the decrease in pore pressure in depleting reservoirs is observed to be smaller in magnitude elastic wave velocities in sandstones vary with stress due to the presence of stress sensitive grain boundaries within the rock grain boundary stiffness increases non linearly with increasing compressive stress due to increased contact between opposing faces of the boundary this results in a change in velocity due to a decrease in pore pressure that is smaller than the change in velocity caused by an increase in pore pressure in agreement with time lapse seismic observations the decrease in porosity resulting from depletion is not fully recovered upon re pressurization and this leads to an additional steepening of the velocity vs effective stress curve for injection relative to depletion this difference is enhanced by any breakage of cement or weakening of grain contacts that may occur during depletion and by the reopening or formation of fractures or joints and dilation of grain boundaries that may occur during injection

preservation of autogenic processes and allogenic forcings in set scale aeolian architecture i numerical experiments a reduced complexity aeolian dune stratification model is developed and applied to explore the role of dune morphodynamics in the creation of synthetic sections of aeolian stratigraphy originating from three sets of environmental forcing 1 steady wind transport capacity 2 steady bed aggradation and variable wind transport capacity and 3 steady wind transport capacity and bed aggradation in each scenario the forward motion of initial highly disorganized dunes generates a significant record exclusively containing autogenic signals that arise from early dune growth deformation and merger however continued dune growth scours deeply and shreds all records of early dunes afterward dunes self organize into quasi stable groups forward motion of dune groups create truncate and amalgamate sets and co sets of cross strata quickly forming a second significantly more robust stratigraphic record which preserves a comingling of signals sourced from ongoing autogenic processes and each scenario s specific set of environmental forcings the allogenic boundary conditions of the sand sea although the importance of self organization on modeled aeolian stratification is clear in the few presented scenarios self organization may be throttled via variability within environmental forcings as thoroughly documented within a companion paper cardenas et al this issue therefore additional work is warranted as this numerical experiment only begins to sample possible sets of environmental forcing boundary conditions and initial conditions geomorphic responses and consequential preservation possible within the presented surface stratigraphic dune modeling framework

reassessment of ischnacanthus scheii spjeldnaes acanthodii ischnacanthiformes from the latest silurian or earliest devonian of ellesmere island arctic canada articulated specimens of jawed fishes and assemblages of disarticulated elements that can be assigned to a single biological species are extremely rare from pre devonian deposits the acanthodian species ischnacanthus scheii spjeldnaes is based on a monospecific assemblage comprising fin spines dentigerous jaw bone fragments and scales from the siluro devonian boundary beds of the devon island formation in central west ellesmere island canadian arctic archipelago nunavut a new examination of the type material in particular by scanning electron microscopy and thin sectioning of scales shows that the species is a porosiform poracanthodid that is now assigned to radioporacanthodes scheii comb nov scales of the same species are also recognized from the upper pridoli of cornwallis island and the pridoli or lochkovian of north greenland

seismic anisotropy and its impact on imaging the shallow alpine fault an experimental and modeling perspective the transpressional alpine fault in new zealand has created a thick shear zone with associated highly anisotropic rocks low seismic velocity zones lvz and high seismic reflectivity are recorded in the alpine fault zone but no study has explored the underlying physical rock parameters of the shallow crust that control these observations protomylonites are the volumetrically dominant lithology of the fault zone here we combine experimental measurements of p wave speeds with numerical models of elastic wave anisotropy of protomylonite samples to explore how the fault zone can be seismically imaged numerical models that account for the porosity free real samples fabric elastic tensors from ebsd are calculated by mtex and a finite element model fem while microfractures are modeled with differential effective media dem theory at effective pressures representative of the alpine fault brittle zone experimental wave speeds are lower than those predicted by mtex fem a possible dem model suggests that a combination of random and aligned microfractures with aspect ratios increasing with pressure can explain the experimental wave speeds for pressures 70 mpa such micro porosity in the form of foliation and mica basal plane parallel microfractures and grain boundaries is validated with synchrotron x ray microtomography and tem images finally by modeling anisotropy of seismic reflection coefficients with angle of incidence we demonstrate that the high reflectivity and lvz observed at the alpine fault can only be explained if this microporosity is accounted for throughout the brittle fault zone even at depths of 7 10 km

fluid dependent shear wave splitting in a poroelastic medium with conjugate fracture sets the dependence of shear wave splitting in fractured reservoirs on the properties of the filling fluid may provide a useful attribute for identifying reservoir fluids if the direction of wave propagation is not perpendicular or parallel to the plane of fracturing the wave polarized in the plane perpendicular to the fractures is a quasi shear mode and therefore the shear wave splitting will be sensitive to the fluid bulk modulus the magnitude of this sensitivity depends upon the extent to which fluid pressure can equilibrate between pores and fractures during the period of the deformation in this paper we use the anisotropic gassmann equations and existing formulations for the excess compliance due to fracturing to estimate the splitting of vertically propagating shear waves as a function of the fluid modulus for a porous medium with a single set of dipping fractures and with two conjugate fracture sets dipping with opposite dips to the vertical this is achieved using two alternative approaches in the first approach it is assumed that the deformation taking place is quasi static that is the frequency of the elastic disturbance is low enough to allow enough time for fluid to flow between both the fractures and the pore space throughout the medium in the second approach we assume that the frequency is low enough to allow fluid flow between a fracture set and the surrounding pore space but high enough so that there is not enough time during the period of the elastic disturbance for fluid flow between different fracture sets to occur it is found that the second approach yields a much stronger dependence of shear wave splitting on the fluid modulus than the first approach this is a consequence of the fact that at higher wave frequencies there is not enough time for fluid pressure to equilibrate and therefore the elastic properties of the fluid have a greater effect on the magnitude of the shear wave splitting

active aeolian processes on mars a regional study in arabia and meridiani terrae 1 we present evidence of widespread aeolian activity in the arabia terra meridiani region mars where different kinds of aeolian modifications have been detected and classified passing from the regional to the local scale we describe one particular dune field in meridiani planum where two ripple populations are distinguished by means of different migration rates moreover a consistent change in the ripple pattern is accompanied by significant dune advancement between 0 4 1 meter in one martian year that is locally triggered by large avalanche features this suggests that dune advancement may be common throughout the martian tropics

deltaic deposits indicative of a paleo coastline at aeolis dorsa mars abstract recent work at aeolis dorsa mars has identified exposure of fluvial sedimentary outcrop deposited early in martian history likely during the late hesperian or earlier here we examine a 1200 km2 exposure of sedimentary outcrop in southeast aeolis dorsa total thickness of the stratigraphic section exceeds 100 m we identify eight discrete complexes each of which is primarily composed of branching channel filling fluvial deposits associated with a primary feeder or trunk channel deposit relative ages for the branching complexes are derived from observed stacking relationships and there is a pattern of eastward younging using digital elevation models derived from context camera ctx stereo pair images we measure the strike and dip of stratal surfaces mapped throughout the study area there is a trend of increasing dip magnitude from north to south the branching complexes are interpreted as exhumed remnants of the lobes of an aggrading river delta that shifted its depo center from west to east over time the increasing dip magnitude of stratal surfaces towards the south records topset foreset transitions an architectural feature associated with river deltas and indicates paleo transport was locally southward the interpreted deltaic lobes indicate that aeolis dorsa was a coastal region not presently bound by a modern crater basin and the studied outcrop is part of a larger set of coastal sedimentary deposits identified in the region

measurement of the normal tangential fracture compliance ratio zn zt during hydraulic fracture stimulation using s wave splitting data we develop a method to invert s wave splitting sws observations measured on microseismic event data for the ratio of normal to tangential compliance zn zt of sets of aligned fractures we demonstrate this method by inverting for zn zt using sws measurements made during hydraulic fracture stimulation of the cotton valley tight gas reservoir texas when the full sws data set is inverted we find that zn zt 0 74 0 04 windowing the data by time we were able to observe variations in zn zt as the fracture stimulation progresses most notably we observe an increase in zn zt contemporaneous with proppant injection rock physics models and laboratory observations have shown that zn zt can be sensitive to 1 the stiffness of the fluid filling the fracture 2 the extent to which this fluid can flow in and out of the fracture during the passage of a seismic wave and 3 the internal architecture of the fracture including the roughness of the fracture surfaces the number and size of any asperities and the presence of material filling the fracture these factors have direct implications for modelling the fluid flow properties of fractures consequently the ability to image zn zt using sws will provide useful information about fractured rocks and allow additional constraints to be placed on reservoir behaviour

dune field pattern formation and recent transporting winds in the olympia undae dune field north polar region of mars high resolution imaging science experiment hirise imagery of the central olympia undae dune field in the north polar region of mars shows a reticulate dune pattern consisting of two sets of nearly orthogonal dune crestlines with apparent slipfaces on the primary crests ubiquitous wind ripples areas of coarse grained wind ripples and deflated interdune areas geomorphic evidence and dune field pattern analysis of dune crest length spacing defect density and orientation indicates that the pattern is complex representing two constructional generations of dunes the oldest and best organized generation forms the primary crestlines and is transverse to circumpolar easterly winds gross bed form normal analysis of the younger pattern of crestlines indicates that it emerged with both circumpolar easterly winds and ne winds and is reworking the older pattern mapping of secondary flow fields over the dunes indicates that the most recent transporting winds were from the ne the younger pattern appears to represent an influx of sediment to the dune field associated with the development of the olympia cavi reentrant with ne katabatic winds channeling through the reentrant a model of the pattern reformation based upon the reconstructed primary winds and resulting secondary flow fields shows that the development of the secondary pattern is controlled by the boundary condition of the older dune topography

southern high latitude dune fields on mars morphology aeolian inactivity and climate change in a study area spanning the martian surface poleward of 50 s 1190 dune fields have been identified mapped and categorized based on dune field morphology dune fields in the study area span 116 400 km2 leading to a global dune field coverage estimate of 904 000 km2 far less than that found on earth based on distinct morphological features the dune fields were grouped into six different classes that vary in interpreted aeolian activity level from potentially active to relatively inactive and eroding the six dune field classes occur in specific latitude zones with a sequence of reduced activity and degradation progressing poleward in particular the first signs of stabilization appear at 60 s which broadly corresponds to the edge of high concentrations of water equivalent hydrogen content observed by the neutron spectrometer that have been interpreted as ground ice this near surface ground ice likely acts to reduce sand availability in the present climate state on mars stabilizing high latitude dunes and allowing erosional processes to change their morphology as a result climatic changes in the content of near surface ground ice are likely to influence the level of dune activity spatial variation of dune field classes with longitude is significant suggesting that local conditions play a major role in determining dune field activity level dune fields on the south polar layered terrain for example appear either potentially active or inactive indicating that at least two generations of dune building have occurred on this surface many dune fields show signs of degradation mixed with crisp brinked dunes also suggesting that more than one generation of dune building has occurred since they originally formed dune fields superposed on early and late amazonian surfaces provide potential upper age limits of 100 my on the south polar layered deposits and 3 ga elsewhere at high latitudes no craters are present on any identifiable dune fields which can provide a lower age limit through crater counting assuming all relatively stabilized dune fields represent a single noncontiguous surface of uniform age their estimated crater retention age is 10 000 years an average sized uncratered dune field 94 km2 has a crater retention age 8 my this apparent youth suggests that present day climate conditions are responsible for the observed degradation and reduced level of aeolian activity a lack of observed transport pathways and the absence of large dune fields in the largest basins hellas and argyre planitiae are consistent with the previously proposed idea that dune sands are not typically transported far from their source regions on mars

boundary condition controls on the high sand flux regions of mars wind has been an enduring geologic agent throughout the history of mars but it is often unclear where and why sediment is mobile in the current epoch we investigated whether eolian bed form dune and ripple transport rates are depressed or enhanced in some areas by local or regional boundary conditions e g topography sand supply availability bedform heights migration rates and sand fluxes all span two to three orders of magnitude across mars but we found that areas with the highest sand fluxes are concentrated in three regions syrtis major hellespontus montes and the north polar erg all regions are located near prominent transition zones of topography e g basins polar caps and thermophysical properties e g albedo variations these are not known to be critical terrestrial boundary conditions the two regions adjacent to major impact basins hellas and isidis planitia showed radially outward upslope winds driving sand movement although seasonally reversing wind regimes were also observed the northern polar dunes yielded the highest known fluxes on the planet driven by summer katabatic winds modulated by the seasonal co2 cap retreat processes not known to affect terrestrial dunes in contrast southern dune fields 45 s were less mobile likely as a result of seasonal frost and ground ice suppressing sand availability results suggest that unlike on earth large scale topographic and thermophysical variabilities play a leading role in driving sand fluxes on mars

how does wave impact generate large boulders modelling hydraulic fracture of cliffs and shore platforms abstract boulder quarrying by large waves along steep high energy coastlines contributes to erosion both by causing inland migration of cliff faces and by vertical lowering of coastal platform surfaces it also leads to the formation of coastal boulder deposits cbd above and inland of the high water mark we describe mechanisms by which hydraulic fracture creates boulders from cliffs and bedrock platforms intense fluid pressures are induced by wave impact on the base of overhanging ledges and also along interior surfaces as cracks fill with water during wave runup and overtopping these processes impose large loads on the overlying rock and bending stresses thus induced can create or propagate microcracks in the rock repeated loading leads to complete fracture and detachment of bedrock slabs which are liberated as boulders we consider these processes using the aran islands ireland as a type locality both because cbd there are well documented and because flat lying strata and orthogonal joint patterns yield tabular bedrock geometry which lends itself to modelling the loaded rock is modelled as a beam with a microcrack that is fractured by a bending stress from an applied hydrodynamic load this simple approach allows us to examine general scalings and dependencies we determine a condition for fracture relating the fluid pressure loaded area fracture stress microcrack position and geometry in some situations the fluid pressure need only be a fraction of the fracture stress fracture is most likely to occur during extreme wave conditions and when the exposed rock has a large eroded area for loading the balance of forces suggests that larger beams break more easily facilitating the creation of large boulders

analysis of dynamic fracture compliance based on poroelastic theory part ii results of numerical and experimental tests in part i a dynamic fracture compliance model dfcm was derived based on the poroelastic theory the normal compliance of fractures is frequency dependent and closely associated with the connectivity of porous media in this paper we first compare the dfcm with previous fractured media theories in the literature in a full frequency range furthermore experimental tests are performed on synthetic rock specimens and the dfcm is compared with the experimental data in the ultrasonic frequency band synthetic rock specimens saturated with water have more realistic mineral compositions and pore structures relative to previous works in comparison with natural reservoir rocks the fracture pore geometrical and physical parameters can be controlled to replicate approximately those of natural rocks p and s wave anisotropy characteristics with different fracture and pore properties are calculated and numerical results are compared with experimental data although the measurement frequency is relatively high the results of dfcm are appropriate for explaining the experimental data the characteristic frequency of fluid pressure equilibration calculated based on the specimen parameters is not substantially less than the measurement frequency in the dynamic fracture model the wave induced fluid flow behavior is an important factor for the fracture wave interaction process which differs from the models at the high frequency limits for instance hudson s un relaxed model

complex bedding geometry in the upper portion of aeolis mons gale crater mars abstract the upper formation of aeolis mons in gale crater exhibits curvilinear bedding patterns on the surfaces of several erosional benches that have been interpreted as cross bedding we use high resolution imaging science experiment hirise stereo topography to test this hypothesis by measuring the bedding geometry within these benches the bedding geometry is consistent with aeolian cross beds measured dips rarely exceed the angle of repose and the distribution of dip azimuths is non random allowing dune morphology and paleo transport directions to be inferred using computer models of bedforms the inferred dune type and transport direction vary between the benches of the upper formation indicating that the benches are separated by sufficient time for the wind regime to change the paleo wind directions derived from bedding geometry measurements differ from modern wind modeling results suggesting that the conditions during deposition of the upper formation were unlike modern conditions the concentric bedding patterns in some locations indicate that the rate of deposition approached the rate of bedform migration the evidence for lithified hundred meter scale dunes in the upper formation of aeolis mons indicates that the area was a sediment sink at the time of formation and any hypothesis for the formation of aeolis mons must be compatible with these results we present one possible sequence of events for the formation of aeolis mons

the effect of remote sensing resolution limits on aeolian sandstone measurements and the reconstruction of ancient dune fields on mars numerical experiment using the page sandstone earth the distribution of cross set thicknesses is important field collected data for reconstructing ancient aeolian dune fields from the strata they accumulated but most aeolian strata on mars must be observed with remote sensing we hypothesize that remote sensing resolution limits will affect cross set thickness measurements and the dune field reconstructions that follow here we test this hypothesis using a numerical experiment mimicking the effects of remote sensing resolution limits performed on a distribution of aeolian cross set thicknesses measured in the field from the page sandstone arizona usa page set thicknesses are exponentially distributed representing the accumulations of dry no water table dune fields in a state of net sediment bypass set thickness measurements are progressively blended into adjacent sets based on the map view distance between their upper and lower bounding surfaces on exposures with different dips this is termed the exposure distance of a cross set and in this experiment is a function of 1 set thickness 2 the dip of the outcrop surface and 3 assumptions of the number of remote sensing image pixels required to detect a set detection limit using outcrop dips from 1 to 60 and detection limits from 3 to 10 hirise pixels gently sloping surfaces 13 allow most of the page sandstone sets to be measured conserving the bypass interpretation made from the true set thicknesses at all detection limits although these results are specific to the page they can be used as a rule of thumb for future mars work

modeling of elasticity effects of sandstone compaction using coated inclusions compaction of siliciclastic sediments leads to an increase in their stiffness parameters and seismic velocities although mechanical compaction implies a reduction of porosity and closing of compliant pores chemical compaction may alter the mineral properties the cementing of grain contacts and the pore volume the ability of rock physics models to quantify such effects on seismic observables will aid hydrocarbon exploration a framework was designed for modeling compaction effects by use of a so called coated inclusion model that eliminates the need of using a hybrid approach through combining different theories a basic feature of the model is that the inclusion is defined by a kernel representing the pore which is surrounded by shells that may individually have different elastic properties from those of the pore filling material and the background matrix the modeling can be designed to explore seismic effects of various texture perturbations including contact cementing and pore filling processes the numerical modelings seem to be consistent with the results obtained from other rock physics models the model allows for the possibility of including small scale heterogeneities within the rock texture and estimating frequency dispersion together with attenuation due to pore fluid flow a basic weakness of the method is the relatively large number of parameters needed to describe a porous rock which will always limit its practical usage however its basic physical foundation may provide a reference for understanding the qualitative and quantitative effects of various cementation scenarios on seismic parameters

microfracturing damage and failure of brittle granites 1 the evolution of stress induced damage and the eventual brittle failure are experimentally analyzed for mount scott granite of oklahoma we quantify the damage intensity in two methods and directly compare model predictions and actual damage the 14 samples of the medium grain size granite were loaded triaxially at dry conditions room temperature and under 41 mpa confining pressure microfractures were mapped in five samples and the majority of them 80 belong to two groups tensile microfractures trending subparallel to the loading axis and shear microfractures trending 11 40 off the loading axis the tensile microfractures dominate the low stress stage and they remain intragranular with a stress increase the relative density of shear microfractures increases with increasing stresses and they formed elongated intergranular zones of coalescing microfractures we compared two independent values of damage intensity 1 the macroscopic experimentally measured reduction of the deformation modulus and 2 the expected reduction of this modulus calculated with several damage models for the density of the mapped microfractures our fracture density data best fit the model of noninteracting cracks of kachanov 1992

effective elasticity of fractured rocks a snapshot of the work in progress exploration and development of naturally fractured reservoirs rely on understanding and interpretation of certain signatures associated with seismic waves propagating through cracked rocks this understanding comes primarily from the effective media theories that predict an overall elastic behavior of a solid containing many inhomogeneities cracks in particular whose sizes are too small to be seen individually by the waves to model seismic responses of fractured formations a geophysicist typically has a choice between the effective media schemes of hudson and schoenberg while the two predictions usually deviate slightly for liquid filled cracks the differences are significant when the fractures are dry explaining the origin of these differences and selecting a more accurate scheme is the first goal of this tutorial our second more challenging task is to prove that simply adding the compliance contributions of cracks as if they were isolated and noninteracting remains sufficiently accurate even for fractures that grossly violate the basic theoretical assumption of penny shaped cracks real fractures have notoriously irregular shapes might be partially closed and often form interconnected networks yet these details of fracture microgeometry turn out to be unimportant for the effective elasticity given a typical noise level in seismic data no closed form theory exists for irregular fracture shapes however take into account finite element simulations on so called digital rocks demonstrate which features of the crack geometry have to be taken into account because they influence propagation of long compared to the size of fractures seismic waves and which features can be ignored

brittle deformation and damage induced seismic wave anisotropy in rocks summary we study the relations between rock fracturing non linear deformation and damage and stress induced anisotropy of seismic waves by comparing theoretical predictions of a damage rheology model to results of laboratory experiments with granite samples the employed damage model provides a generalization of hookean elasticity to a non linear continuum mechanics framework of cracked media incorporating degradation and recovery of the effective elastic properties along with gradual accumulation of irreversible deformation beyond the elastic regime the model assumes isotropic distribution of local microcracks expressed in terms of a scalar damage variable but the non linear elastic response caused by the opening closure and evolution of the internal cracks is predicted to lead to seismic wave anisotropy we develop relations between the seismic wave anisotropy internal rock damage and stress field and test the viscoelastic damage rheology against sets of laboratory experiments with cylindrical granite samples the observed data include measurements of stress and strain in three loading cycles culminating in a final macroscopic failure together with measured wave velocities along and perpendicular to the axis of the cylinder using a single set of parameters the model fits well the overall evolution of the axial and transversal stress strain relations as well as the anisotropic elastic wave velocities during all cycles from the onset of fracturing in the first cycle until the macroscopic failure in the final cycle

preservation of autogenic processes and allogenic forcings in set scale aeolian architecture ii the scour and fill dominated jurassic page sandstone arizona u s a the stratigraphic architecture of aeolian sandstones is thought to record signals originating from both autogenic dune behavior and allogenic environmental boundary conditions within which the dune field evolves mapping of outcrop scale surfaces and sets of cross strata between these surfaces for the jurassic page sandstone near page arizona usa demonstrates that the stratigraphic signature of autogenic behavior is captured by variable scour depths and subsequent fillings whereas the dominant signatures of allogenic boundary conditions are associated with antecedent surface topography and variable water table elevations at the study area the page sandstone is 60 m thick and is separated from the underlying navajo sandstone by the j 2 regional unconformity with meters of relief thin climbing sets of cross strata of the basal page representing early dune field accumulations fill j 2 depressions in contrast the overlying lower and middle page consist of cross strata that are one to a few meters thick and packaged between outcrop scale bounding surfaces these bounding surfaces have been previously correlated to high stand deposits of the adjacent carmel sea and at this site possess meters of erosional relief produced by dune scour notably absent within packages of cross strata bounded by these outcrop scale surfaces are strata of early dune field accumulations any interdune deposits and climbing dune strata instead these packages preserve a scour and fill architecture created by large migrating dunes migrating within a dry mature dune field undergoing negligible bed aggradation any record of early phases of dune field construction for the lower and middle page are interpreted to have been cannibalized by the deepest scours of later large dunes interpretations are independently supported by the relatively large coefficients of variation in lower and middle page set thicknesses which are consistent with set production by successive deepest trough scours and the relatively low coefficient of variation for the depression filling basal page sets consistent with a significant component of bed aggradation numerical modeling presented here and more completely in the companion paper demonstrates how this cannibalization of early phase stratigraphy is an expected outcome of autogenic dune growth processes and that early phase strata can be preserved within antecedent depressions relative rise of the inland water table from basin subsidence and carmel sea level forced preservation of multiple stacked packages composed of scour and fill architecture without these allogenic forcings the page would be little more than an erosional surface

ultrahigh resolution topographic mapping of mars with mro hirise stereo images meter scale slopes of candidate phoenix landing sites 1 the objectives of this paper are twofold first to report our estimates of the meter to decameter scale topography and slopes of candidate landing sites for the phoenix mission based on analysis of mars global surveyor mgs mars orbiter camera moc images with a typical pixel scale of 3 m and mars reconnaissance orbiter mro high resolution imaging science experiment hirise images at 0 3 m pixel 1 and second to document in detail the geometric calibration software and procedures on which the photogrammetric analysis of hirise data is based a combination of optical design modeling laboratory observations star images and mars images form the basis for software in the u s geological survey integrated software for imagers and spectrometers isis 3 system that corrects the images for a variety of distortions with single pixel or subpixel accuracy corrected images are analyzed in the commercial photogrammetric software socet set bae systems yielding digital topographic models dtms with a grid spacing of 1 m 3 4 pixels that require minimal interactive editing photoclinometry yields dtms with single pixel grid spacing slopes from moc and hirise are comparable throughout the latitude zone of interest and compare favorably with those where past missions have landed successfully only the mars exploration rover mer b site in meridiani planum is smoother moc results at multiple locations have root mean square rms bidirectional slopes of 0 8 4 5 at baselines of 3 10 m hirise stereopairs one per final candidate site and one in the former site yield 1 8 2 8 slopes at 1 m baseline slopes at 1 m from photoclinometry are also in the range 2 3 after correction for image blur slopes exceeding the 16 phoenix safety limit are extremely rare

aeolian slipface dynamics and grainflow morphologies on earth and mars abstract in 2015 an active dune field on mars was visited up close by the curiosity rover in gale crater providing the first high resolution ground images of fine scale windblown features not previously resolved from orbital based imagery for the first time these images allow for direct comparison with terrestrial aeolian dynamics and provide critical ground truth data to bridge the gap between model predictions and satellite observations the image data from the slipface on the namib dune within the bagnold dune field shows grainflow morphologies that are similar to dunes on earth quantitative estimates of flow thickness based on shadow length are presented for the grainflows on the namib dune slipface and compared to grainflow characteristics measured by terrestrial laser scans from the maspalomas dune field located in gran canaria spain using observations from maspalomas to support interpretations of martian slipface dynamics we discuss implications for the local wind regime style of grainflow seasonal activity and dune migration the presence of multiple large magnitude grainflows on the namib slipface suggests an active aeolian environment capable of delivering enough sediment to the slipface to initiate these flows and transport sediment to the bottom of the lee slope however the thinness of grainflows on the namib dune the formation of smaller grainflows directly below the dune brink and limited grainfall suggest a lower wind energy environment at least for the most recent slipface activity large actively migrating stoss ripples obliquely oriented to the dune crest regularly deposit sediment on to the upper slipface and may be a mechanism in which larger grainflow occur during seemingly low energy wind events this mechanism of sediment delivery may also explain the existence of a variety of slipface morphologies both young and old which are otherwise quickly erased on earth due to sediment redistribution and grainfall

wind blown sandstones cemented by sulfate and clay minerals in gale crater mars gale crater contains mount sharp a 5km thick stratigraphic record of mars early environmental history the strata comprising mount sharp are believed to be sedimentary in origin but the specific depositional environments recorded by the rocks remain speculative we present orbital evidence for the occurrence of eolian sandstones within gale crater and the lower reaches of mount sharp including preservation of wind blown sand dune topography in sedimentary strata a phenomenon that is rare on earth and typically associated with stabilization rapid sedimentation transgression and submergence of the land surface the preserved bedforms in gale are associated with clay minerals and elsewhere accompanied by typical dune cross stratification marked by bounding surfaces whose lateral equivalents contain sulfate salts these observations extend the range of possible habitable environments that may be recorded within gale crater and provide hypotheses that can be tested in situ by the curiosity rover payload

aeolian dune interactions preserved in the ancient rock record abstract interactions between individual dunes drive the evolution of a dune field pattern ubiquitous in modern systems dune interactions have increasingly been the focus in understanding dune field dynamics through surface models and experiments in contrast how these dynamics are incorporated into the stratigraphic record has remained largely unaddressed current models for the interpretation of the architecture of sets of aeolian cross strata and bounding surfaces do not explicitly incorporate dune interactions recent documentation of a common type of dune interaction and the resulting stratigraphic architecture at the white sands dune field new mexico has established a set of diagnostic criteria by which this architecture could be identified in this study five examples of interpreted interaction architecture were identified in well known jurassic aeolian units in the western united states these results confirm the expected presence of dune interaction architecture in the rock record and highlight the need for a new generation of models that couple complex dune surface dynamics with their stratigraphic expression

effects of borehole stress concentration on elastic wave velocities in sandstones the stress redistribution that occurs in the vicinity of a borehole may lead to damage or failure of the rock if the vertical stress is a principal stress the principal stresses at the wall of a vertical well are the hoop radial and vertical stresses the hoop stress varies with position around the wellbore and takes its maximum compressive value at the azimuth of the far field minimum horizontal stress this may lead to compressive failure at this azimuth if the compressive strength of the rock is exceeded recent developments in sonic logging allow the variation in elastic wave velocities with radius and azimuth around a borehole to be characterized since elastic wave velocities in sandstones are sensitive to changes in stress owing to the presence of stress sensitive grain boundaries within the rock this allows the changes in stress to be monitored an expansion of the elastic compliance of the grain boundaries to first order in stress shows that a radially polarized vertically propagating shear wave is more sensitive to radial stress than to hoop stress close to the wellbore however large changes in stress occur and any nonlinearity in the variation in compliance of the grain boundaries as a function of stress becomes important this nonlinearity increases the sensitivity to the wellbore pressure and can cause significant reductions in the velocity of compressional and shear waves in the vicinity of the wellbore

frozen dynamics of migrating bedforms here we show how a continuum granular flow model can replicate realistic self organizing subaqueous bedforms and we explore how a record of this dynamic topography is transferred into the substrate where it can be fossilized modeled bedform behavior is quantitatively compared to laboratory and field data sediment scouring and deposition by migrating bedforms produce sets of partially preserved bedforms recording nonlinear behavior at the sediment fluid interface model results show the importance of spatial and temporal variations in bedform size and migration rate to the generation of sets addition of net bed aggradation to modeled bedforms allows us to explore the preservation of topography under a wide range of depositional conditions we present new relationships among aggradation rate bedform migration rate and set thickness variability of set thickness is shown to be a product of the competition between bed aggradation and bedform migration rates these results show how using the entire distribution of set thickness for ancient strata can refine estimates of the formative paleoenvironmental conditions

persistent aeolian activity at endeavour crater meridiani planum mars new observations from orbit and the surface abstract aeolian driven bedform activity is now known to occur in many regions of mars based on surface and orbital observation of contemporary martian ripple and dune mobility events many of these sites have only been monitored with sufficient resolution data for the last few mars years when the high resolution imaging science experiment hirise began acquiring images of mars one exception is the well monitored endeavour crater in meridiani planum which was one of the first known sites of unambiguous dune activity migration and deflation however those early detections used lower resolution images over longer temporal baselines versus the hirise data now available leaving some measurements poorly constrained new orbital and surface observations of endeavour show multiple spatial cm m km and temporal seasons mars year scales of aeolian driven surface change which confirms earlier reports dome dunes in the eastern portion of the crater persistently deflate disseminating dark sand across lighter toned regolith and or eroded bright dust and likely contribute to the crater interior s episodic decreases in orbital albedo measurements other dome dunes are detected with the highest migration rates 4 12 m per mars year and volumetric sand fluxes reported yet for mars estimated dune construction times or turnover times here and elsewhere on mars are significantly shorter than martian obliquity cycles implying that it is not necessary to invoke paleoclimate wind regimes to explain current dune morphologies located on the crater rim the opportunity rover detected evidence for near and far field aeolian driven activity with observations of spherules sand movement in the rover workspace bedform albedo alteration and dust lifting events observations of intracrater dunes show periodic shifting dark streaks that significantly constrain local wind regimes directionality and seasonality constraints on wind directions from surface and orbital images show that aeolian bedforms can be extremely active in bi and possibly tri modal wind regimes and during periods without major dust storms

pervasive aeolian activity along rover curiosity s traverse in gale crater mars the nasa mars science laboratory msl rover curiosity has safely landed near a 35 km long dark dune field in gale crater on mars this dune field crosses the landing site from the northeast to the southwest and lies along curiosity s traverse to aeolis mons here we present the first evidence of recent aeolian activity in the form of ripple and dune migration and further estimate wind directions within the dune field through analysis of ripple and dune morphologies and the mars regional atmospheric modeling system mrams we measured a minimum ripple migration rate of 0 66 m per earth year and dune migration rate of 0 4 m per earth year in the southwest portion of the field a strongly bidirectional ripple crestline orientation nearly orthogonal dune slipfaces and linear seif or oblique dunes indicate a bidirectional wind regime with winds mainly coming from the ene and from the northwest however mrams results indicate primary winds from the ene our constraints on the wind regime provide the unique opportunity to use ground measurements from msl to test the accuracy of winds predicted from orbital data

microcracks in rocks a review abstract in the past decade the number of studies about microcracks in rocks has rapidly increased this review of recent work concentrates on microcracks in rock as separate entities emphasizing microcrack morphogenesis kinematics dynamics population statistics and observational techniques cracks are produced when the local stress exceeds the local strength the local stress may be augmented by twin lamellae interactions kink bands and deformation lamellae stress concentrations at grain boundary contacts and around intracrystalline cavities local strength may be reduced along cleavage planes along grain boundaries and along any internal surface as a result of corrosion by chemically active fluids dislocations appear not to be a significant factor for crack nucleation below about 500 c in silicates spatial and temporal changes in temperature can also induce microcracking as a result of differential thermal expansion between grains with different thermoelastic moduli and thermal conductivities the amount of quartz in the rock has a significant effect on thermally induced microcracks because of its large and variable thermal expansivity the application of hydrostatic pressure between 100 and 200 mpa effectively closes most cracks but the closure may not be uniform if crack wall asperities exist hydrostatic pressure appears to stabilize cracks and make crack growth more difficult the number and average size of mechanically induced microcracks is greater in rock deformed at higher pressures the application of a deviatoric stress field on the boundaries of a rock mass results on a microscopic scale in a very complex stress system which greatly affects nucleation and propagation paths the relative amount of intragranular and intergranular cracking appears to depend upon mineralogy rock type and stress state the vast majority of stress induced microcracks in rocks appear to be extensional statistically they are predominantly oriented within 30 of the macroscopic maximum stress direction crack densities increase as macroscopic deviatoric stress increases above a threshold level crack size distributions may be either lognormal or exponential fracture in rock under compressive boundary loads is a result of the coalescence of many microcracks not the growth of a single crack some crack configurations are more favorable for coalescence than others as deviatoric stress increases and rock failure is approached the microcrack population changes spatially from random to locally intense zones of cracking away from the fault the crack density dies off rapidly to the background level a few grains away under lesser deviatoric stresses slow subcritical microcrack growth can occur as a result of stress aided corrosion at the crack tip the rate governing mechanism may be either the chemical reaction rate or the rate at which water can get to the crack tip important details still remain to be worked out

aeolian dune field pattern boundary conditions aeolian dune field patterns reflect the complex external environment within which the pattern evolves these external environmental controls are the boundary conditions on aeolian dune field pattern formation the influences of boundary conditions such as wind regime and sediment supply are well known however boundary conditions represent a relatively unexplored area of dune field pattern formation source area geometry and areal limits are two newly recognized boundary conditions measurements of crest spacing and crest length from satellite images of dune field patterns with point and line source area geometries show an increase in spacing and crest length over distance whereas spacing and crest length in plane sourced patterns emerge equally across the dune field the impact of the size and shape of a dune field on crest spacing is tested using a previously established analytical model model results indicate that the area of a dune field limits the maximum spacing that can occur within a given area and that dune fields that are five times longer in the direction of dune migration than in the crest parallel direction can achieve the greatest spacing within a given area empirical measurements of spacing defect density and dune field area from ten different dune fields ranging over four orders of magnitude in size show that spacing increases and defect density decreases as dune field area increases because the formation of all dune fields involves a wind regime and sediment supply there must be interplay of boundary conditions within the same dune field at different stages of pattern development one boundary condition may dominate over others and with changing climatic eustatic or tectonic parameters boundary conditions may change to modify an existing pattern recognizing boundary condition controls on aeolian dune field pattern formation provides a framework for recognizing the signature of the external environment in which a pattern developed and can be used for reconstructing past dune constructional events and climatic change