flow of dense avalanches past obstructions one means of preventing areas from being hit by avalanches is to divert the flow by straight or curved walls or tetrahedral or cylindrical type structures thus there arises the question how a given avalanche flow is changed regarding the diverted flow depth and flow direction in this paper a report is given on laboratory experiments performed for gravity driven dense granular flows down an inclined plane obstructed by plane wall and tetrahedral wedge it was observed that these flows are accompanied by shocks induced by the presence of the obstacles these give rise to a transition from super to subcritical flow of the granular avalanche associated with depth and velocity changes it is demonstrated that with an appropriate shock capturing integration technique for the savage hutter theory the shock formation for a finite mass granular flow sliding from an inclined plane into a horizontal run out zone is well described as is the shock formation of the granular flow on either side of a tetrahedral protection structure

simplified models of spreading flow of dry granular material shallow flow integrated numerical models of landslide and avalanche motion have recently experienced rapid development an important aspect of model development is verification to show that models

acoustic properties of neogene carbonates and siliciclastics from the subsurface of the florida keys implications for seismic reflectivity abstract results of measurements of compressional wave velocity and shear wave velocity have been compared with other rock properties xrd and thin section analyses for over fifty minicores from two cores drilled through sedimentary rocks on stock island and long key florida keys the comparisons indicate that sonic velocity in siliciclastics and carbonates is mainly controlled by three factors 1 porosity and primary pore type 2 quartz content and 3 dolomite content in the case of two pliocene prograding formations the combination of depositional fabric and diagenetic alterations produced patterns and values of almost identical impedance for fine grained carbonates and siliciclastics consequently seismic sections can not be used to distinguish between the prograding quartz sands and the prograding fine grained carbonates compressional and shear wave velocities in the limestones are strongly controlled by total porosity and the primary pore type generally high porosity rocks have lower velocities than low porosity rocks but pore types can significantly influence velocities overall an increasing percentage of dolomite in carbonate rocks results in an increase in velocity some dolomitic fabrics deviate from this trend fabric preserving mimetic dolomites yield the highest velocities whereas samples that are of purely sucrosic dolomites produce lower velocities which however are still higher in average than non or only partly dolomitized samples quartz sandstones and quartz bearing limestones have generally low and uniform velocities the low velocities of the sandstones containing 80 quartz is a result of their primary interparticle porosity due to the low diagenetic potential of quartz grains a high quartz content inhibits major diagenetic alterations which would result in cementation and higher velocities synthetic seismograms and nearby seismic sections reveal two major seismic facies as a result of the observed velocity distribution 1 carbonates of the stock island formation and siliciclastics of the long key formation are both characterized by low seismic reflectivity or even seismic transparency this seismic facies is caused by a very similar uniform vertical distribution of low velocities with almost identical impedance values in the two formations thus the transition or an interfingering between the time equivalent siliciclastic long key formation and the carbonate stock island formation might not be detectable on a seismic section 2 the over and underlying shallow marine carbonates in contrast have higher velocities with a wider range of velocities resulting in a succession of high amplitude reflections

4 d constrained depth conversion for reservoir compaction estimation application to ekofisk field ekofisk field in the norweigian sector of the north sea is a remarkable example of a strongly compacted chalk reservoir since the start of production in 1971 the water bottom has subsided by over 8 m as a result of reservoir compaction hermansen et al the field wide waterflood begun in 1989 was expected to halt pressure depletion and reservoir compaction after a few years of waterflooding ekofisk reservoir pressure began increasing but compaction and subsidence continued laboratory studies on chalk cores indicated that high porosity oil filled chalk compacts strongly when flushed with seawater these studies along with reservoir flow simulation and geomechanic modeling led to a model of a water weakening compaction sylte et al ongoing efforts deployed to monitor the reservoir compaction and seabed subsidence include twice a year compaction logging in a monitoring well daily gps measurements at the platforms and periodic bathymetry surveys nine surveys from 1970 to 1999

gassmann s fluid substitution and shear modulus variability in carbonates at laboratory seismic and ultrasonic frequencies carbonates have become important targets for rock property research in recent years because they represent many of themajoroilandgasreservoirsintheworld someareundergoing enhanced oil recovery most laboratory studies to understand fluid and pressure effects on reservoir rocks have been performed on sandstones but applying relations developed for sandstones to carbonates is problematic at best we measure in the laboratory nine carbonate samples from the same reservoir at seismic 3 3000 hz and ultrasonic 0 8 mhz frequencies samples are measured dry humidified and saturated with liquid butane and brine our carbonate samples showed typical changes in moduli as a function of porosity andfluid saturation however we explore the applicability of gassmann s theory on limestone and dolomite rocks in the context of shear and bulk modulus dispersion andgassmann stheoryassumptions forourcarbonatesetat high differential pressures and seismic frequencies the bulk modulus of rocks with high aspect ratio pores and dolomite mineralogy is predicted by gassmann s relation we also explore in detail some of the assumptions of gassmann s relation especiallyrock framesensitivitytofluidsaturation our carbonate samples show rock shear modulus change from dry to brine saturation conditions and we investigate several rock fluid mechanisms responsible for this change to our knowledge these are the first controlled laboratory experimentsoncarbonatesintheseismicfrequencyrange

effect of carbonate pore structure on dynamic shear moduli elastic shear moduli from ultrasonic measurements of carbonate rocks show an increase or decrease when saturated from the dry state with brine the induced changes on the moduli have been attributed to several rock fluid interaction effects including viscous coupling reduction in free surface energy and dispersion caused by local flow with a subsequent effect on acoustic velocities as many studies have recognized that acoustic velocity in carbonate rocks is dependent on pore structure a relationship between the pore structure and observed changes must to a certain extent exist we make use of quantitative digital image analysis parameters such as dominant pore size and perimeter over area to demonstrate how changes in dynamic shear moduli upon saturation relate to the geologic make up of the rock samples with a small dominant pore size and large perimeter over area show abnormal decrease in shear moduli with saturation from the dry state this is attributed to the large surface area available for matrix fluid interaction samples with large dominant pore size and small perimeter over area show an increase in shear moduli with saturation this may be a dispersion effect as high frequency and high permeability may cause the fluid in the saturated samples to move out of phase with the solid during propagation of the acoustic wave and thus cause a stiffening effect

origin of gassmann s equations gassmann s relations are receiving more attention as seismic data are increasingly used for reservoir monitoring correct interpretation of underground fluid migration from seismic data requires a quantitative understanding of the relationships among the velocity data and fluid properties in the form of fluid substitution formulas and these formulas are very commonly based on gassmann s equations nevertheless confusion persists about the basic assumptions and the derivation of gassmann s 1951 well known equation in poroelasticity relating dry or drained bulk elastic constants to those for fluid saturated and undrained conditions it is frequently stated for example but quite incorrect to say that gassmann assumes the shear modulus is constant i e mechanically independent of the presence of the saturating fluid this note clarifies the situation by presenting an unusually brief derivation of gassmann s relations that emphasizes the true origin of the constant shear modulus result while also cla

controls on sonic velocity in carbonates compressional and shear wave velocities v p and v s of 210 minicores of carbonates from different areas and ages were measured under variable confining and pore fluid pressures the lithologies of the samples range from unconsolidated carbonate mud to completely lithified limestones the velocity measurements enable us to relate velocity variations in carbonates to factors such as mineralogy porosity pore types and density and to quantify the velocity effects of compaction and other diagenetic alterations

estimates of velocity dispersion between seismic and ultrasonic frequencies it is generally accepted that acoustic velocities in fluid saturated rocks vary with frequency evidence comes from experimental measurements and from theoretical causality arguments we have developed a simple analysis technique that gives estimates of total velocity dispersion between zero frequency and any measurement frequency the technique requires compressional p and shear s wave velocity measurements on dry and fully saturated rock assuming that the dry velocities are independent of frequency the biot gassmann equations are used to calculate the zero frequency velocities in the fully saturated rock any difference between the measured velocities and the calculated zero frequency velocities is interpreted as evidence of dispersion application of this analysis technique to a variety c ultrasonic data sets gives consistent results in many rocks dispersion between zero frequency and ultrasonic frequencies is on the order of 10 percent at low effective stress and it decreases to only a few pe

elastic piezosensitivity of porous and fractured rocks understanding the stress dependencies of seismic velocities is important for interpreting a variety of seismic data ranging from amplitude versus offset avo and velocity analysis to overpressure prediction and 4d seismic monitoring of reservoirs sometimes rather complex forms of these dependencies based on specific models of porous space geometry are used for example spherical contacts models duffy and mindlin 1957 merkel et al 2001 duffy merkel 2001 and crack contacts models gangi and carlson 1996 carcione and tinivella 2001 have been used in different studies gangi96 however usually the pore pressure velocity dependence along with the velocity dependence on the confining pressure are phenomenologically described by the following simple relationship zimmerman et al 1986 eberhart phillips et al 1989 freund 1992 jones 1992 prasad and manghnani 1997 khaksar et al 1999 carcione and tinivella 2001 kirstetter and macbeth 2001

the origin and earliest state of the earth s hydrosphere the origin and earliest history of the earth s hydrosphere the inventory of excess volatiles defined by rubey in 1951 can be constrained within wide but useful limits by a consideration of empirical and theoretical evidence from astrophysics and geology models for the evolution of the solar system from the protoplanetary nebula and for the growth of the earth to its present dimensions suggest quite strongly that the hydrosphere came into being during accretion its format with h2o mostly in the oceans co2 mostly in sediments and a residual atmosphere dominated by n2 co2 and h2o was established at a very early date and has persisted without large destabilizing climatic excursions until the present day alternative accounts of early history in which the earth either loses a massive primordial atmosphere or acquires its secondary atmosphere by gradual degassing seem improbable on the basis of a series of circumstantial but cumulatively persuasive arguments the difficulty of dissipating a massive atmosphere of solar composition in reasonable times the likelihood that accretion was a highly energetic process and that it triggered early segregation of the core and the tendency of the planet to accumulate volatiles preferentially in the later stages of accretion are examples of arguments favoring an early origin for the hydrosphere several geological isotope systems which can be sampled today require early separation of the atmosphere and probably the hydrosphere as a whole these systems record radiogenic enrichment patterns in the noble gases and stable isotope fractionations which suggest an early origin of the biosphere certain geological indicators of atmospheric composition and the broadly equable character of the rock record are also consistent with a hydrosphere established in the earliest stages of history and having an initial neutral or weakly reduced composition the climatic stability of such a hydrosphere poses crucial questions especially since the early sun was less luminous we find that it is not paradoxical to have an early earth with surface temperature close to that of the present day if the major hydrospheric constituent is liquid water a near global ocean and a cloudy atmosphere provide thermal feedbacks which suffice to maintain climatic stability against the perils of the deep freeze and the runaway greenhouse large quanltities of atmospheric greenhouse gases are not required for climatic stability although co2 may well have been present in amounts larger than today s large expanses of dry land are however difficult to accommodate in the picture which we present but the evidence favors differentiation of the continents later in geological history than the early times which we consider localized glacier ice is permitted and perhaps likely and may provide a milieu favorable for chemical evolution

high resolution crosswell imaging of a west texas carbonate reservoir part 5 core analysis we conducted a core analysis program to provide supporting data to a series of crosswell field experiments being carried out in mcelroy field by stanford university s seismic tomography project the objective of these experiments is to demonstrate the use of crosswell seismic profiling for reservoir characterization and for monitoring co2 flooding for these west texas carbonates we estimate that co2 saturation causes p wave velocity to change by 1 9 pooled average range 6 3 to 0 1 s wave velocity by 0 6 range 0 to 2 7 and the p to s velocity ratio by 2 4 range 6 4 to 0 3 when we compare these results to the precisions we can expect from traveltime tomography about 1 for p and s wave velocity and about 2 for the p to s velocity ratio we conclude that time lapse traveltime tomography is sensitive enough to resolve changes in the p wave velocity s wave velocity and p to s velocity ratio that result from co2 saturation we concentrated here on the potential for co2 sat

effective stress coefficient for seismic velocities in carbonate rocks effects of pore characteristics and fluid types the concept of effective stress is key for understanding the dependence of rock elastic and compaction behaviors on stress and pore fluid pressure previous studies on the concept largely used data acquired on siliciclastic rocks carbonate rocks however display elastic and compaction behaviors that can be very different than those of siliciclastic rocks for example applying most velocity to pore pressure transforms in the context of carbonate reservoirs can be quite challenging our study used an experimental approach a disequilibrium compaction scenario to assess effective stress coefficient n for velocities in three carbonate samples displaying comparable porosities but different dominant pore types in terms of shape and compliance different saturating fluids nitrogen and distilled water were used one at a time which allowed us to compare both pore and fluid type effects on the coefficient n between these rocks we found that n is generally bounded by unity the exception is with nvs n derived from shear wave velocities obtained under nitrogen saturated conditions nvs is higher than 1 on the three studied samples under nitrogen saturated conditions the less compliant the main pore types in a given rock are the higher the value of nvs is higher than unity values of nvs indicate a deviation from the behavior predicted by existing theories this could stem from i the fact that theoretical analyses assume a pore fluid whose properties are not comparable to those of nitrogen and or ii the way the bulk volumetric strain a main factor in elastic wave propagation is incorporated into those theories

pressure and temperature dependence of acoustic wave speeds in bitumen saturated carbonates implications for seismic monitoring of the grosmont formation abstractrecent time lapse seismic observations in carbonate reservoirs subject to steam assisted enhanced oil recovery display substantial changes in seismic reflectivity due to the combined effects of saturation pressure and temperature understanding these field seismic observations requires knowledge of the effects on the seismic wave speeds in bitumen saturated carbonates we have conducted ultrasonic measurements of p and s wave velocities in bitumen saturated dolomite taken from the grosmont formation in northern alberta wave speeds are measured under a variety of conditions of constant pore pressure constant effective pressure and with varying temperature to map the various controlling factors the temperature dependent declines of 12 and 9 for the p and s wave speeds respectively with temperatures from 10 c to 102 c are most notable unlike oil sands at times the dolomite retains its structure upon removal of the bitumen allowing for measurement of the dry and water saturated frame pr

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

the savage hutter avalanche model how far can it be pushed the savage hutter sh avalanche model is a depth averaged dynamical model of a fluid like continuum implementing the following simplifying assumptions i density preserving ii shallowness of the avalanche piles and small topographic curvatures iii coulomb type sliding with bed friction angle delta and iv mohr coulomb behaviour in the interior with internal angle of friction phi or delta and an ad hoc assumption reducing the number of mohr s circles in three dimensional stress states to one we scrutinize the available literature on information regarding these assumptions and thus delineate the ranges of validity of the proposed model equations the discussion is limited to relatively large snow avalanches with negligible powder snow component and laboratory sand avalanches starting on steep slopes the conclusion of the analysis is that the sh model is a valid model for sand avalanches but its mohr coulomb sliding law may have to be complemented for snow avalanches by a second velocity dependent contribution for very small snow avalanches and for laboratory avalanches starting on moderately steep and bumpy slopes it may not be adequate

the relationship between acoustic properties and the petrographic character of carbonate rocks laboratory studies of the detailed relationships between acoustic properties and the petrographic character of brine and air saturated carbonate rocks with a wide range of facies porosities lithologies and rock fabrics indicate that porosity is the major factor influencing both p and s wave impedance and velocity primary lithology and secondary mineralogy have only a small influence on impedance and velocity combined use of p and s wave velocity data discriminates porosity changes from lithologic changes all other variables including pore fluid type and petrographic fabric have no significant influence on velocities laboratory measurements of p wave velocity under simulated in situ conditions reproduce well log velocity values reliably laboratory porosity velocity trends agree with the time average equation when the correct matrix velocities are used rock property results were used to interpret porosity lithology variations for an inverted seismic section from the williston basin where well

limiting stress states in granular avalanches the savag e hutter theory for granular avalanches assumes that the granular material is in either of two limiting stress states depending on whether the motion is convergent or divergent at transitions between convergent and divergent regions ajump in stress occurs which necessarily implies that there is ajump in the ava lanche velocity and or its thickness in this paper a regularization scheme is used which smoothly switches from one stress state to the other and avoids the generation of such singular surfaces the resulting algorithm is more stable than previous numerical meth ods but shocks can still occur during rapid convergence in the run out zone results are presented from two di mensional calculations on complex geometry which illustrate that some necking features observed in laboratory experiments can be explained by the regu larized savage rutter model

oxygen and ozone in the early earth s atmosphere abstract the precise amount of o2 and o3 in the earth s prebiological paleoatmosphere has been a topic of considerable discussion in the past since the photolysis of h2o and co2 the prebiological mechanisms to produce o2 depends on the ultraviolet uv flux from the sun a reliable quantification of the problem requires detailed knowledge of such flux using the most recent astronomical observation of young stars from the international ultraviolet explorer as well as a detailed photochemical model of the paleoatmosphere we find that the amount of o2 in the prebiological paleoatmosphere may have been as much as 106 times greater than previously estimated some of the implications of this new value are discussed

a laboratory study of the retarding effects of braking mounds on snow avalanches a series of laboratory experiments in a 6 m long chute using glass particles of mean diameter 100 m were performed to investigate the interaction of a supercritical granular flow with obstacles it was found that the collision of the flow with a row of mounds led to the formation of a jet whereby a large fraction of the flow was launched from the top of the mounds and subsequently landed back on the chute the retarding effect of the mounds was investigated quantitatively by direct measurements of the velocity of the flow its runout length and the geometry of the jet the effects of several aspects of the layout of the mounds on their retarding effects were examined it was observed that a row of steep mounds with an elongated shape in the transverse direction to the flow and with a height several times the flow depth led to dissipation of a large proportion of the kinetic energy of the flow

high resolution crosswell imaging of a west texas carbonate reservoir using crosswell data collected at a depth of about 3000 ft 900 m in west texas carbonates one of the first well to well reflection images of an oil reservoir was produced the p and s brute stack reflection images created after wavefield separation tied the sonic logs and exhibited a vertical resolution that was comparable to well log resolution both brute stacks demonstrated continuity of several reflectors known to be continuous from log control and also imaged an angular unconformity that was not detected in log correlations or in surface seismic profiling the brute stacks particularly the s wave reflection image also exhibited imaging artifacts we found that multichannel wavefield separation filters that attenuated interfering wavemodes were a critical component in producing high resolution reflection images in this study the most important elements for an effective wavefield separation were the time alignment of seismic arrivals prior to filter application and the implementation of wavefield separation filters in multiple domains particularly in common offset domain the effectiveness of the multichannel filtering was enhanced through the use of extremely fine wellbore sampling intervals in this study 2 5 ft 0 76 m vertical sampling intervals for both source and receiver were used whereas most previous crosswell data sets were collected with much coarser sampling intervals resulting in spatial aliasing and limiting the utility of the data for reflection processing the wavefield separation techniques employed in this study used data volumes and associated filtering operations that were several orders of magnitude larger than those encountered in conventional vsp data analysis

evidence for the transition to an oxygen rich atmosphere during the evolution of red beds in the lower proterozoic sequences of southern africa abstract strata 2 5 to 1 8 ga in southern africa appear to span the transition from a non oxidizing atmosphere to one characterized by free oxygen red beds in 2 3 to 2 1 ga unconformity bounded sequences have matrices of stained clay minerals and are spatially associated with volcanic rocks ironstones or banded iron formations which evidently were sources of iron for the matrices however red beds in 1 9 ga unconformity bounded sequences not only matrix staining but also hematitic coatings on grains these beds also have far fewer interstratified ferruginous lithologies because red beds with stained clay matrices banded iron formations pyritic witwatersrand type conglomerates and iron depleted paleosols all occur in the 2 2 ga pretoria group formation of these lithologies may not have been time dependent until atmospheric oxygen approached 10 2 atm by 1 9 ga atmospheric oxygen was 10 2 atm causing lateritic weathering and red beds with hematitic coatings on grains

laboratory measurements of the effect of fluid saturation on elastic properties of carbonates at seismic frequencies abstract a significant portion of the world s hydrocarbon reserves are found in carbonate reservoirs yet analysis of the petrophysical properties of these reservoirs is associated with a number of challenges some of these challenges stem from physical and chemical interactions between the carbonate rock matrix and pore fluids which can affect elastic properties of the rock hence the study of the pore fluid effects on the elastic properties of carbonates is important for understanding a change of the field performance properties of carbonate reservoir caused by fluid movements during hydrocarbon extraction in producing fields in this laboratory study we investigate the applicability of gassmann s model for predictions of the elastic moduli of water and hydrocarbon saturated savonnieres limestone and the influence of partial water saturation on elastic and anelastic properties of the rock we present the results of two sets of laboratory experiments on the savonnieres oolitic limestone where we i evaluate the effect of full water and n decane saturation on elastic moduli and attenuation at seismic 0 1 hz 120 hz and ultrasonic 0 5 mhz frequencies and ii quantify the dependence of elastic moduli and extensional attenuation on water saturation at two seismic frequencies of 1 hz and 10 hz we demonstrate that the change in the bulk modulus of limestone fully saturated either with n decane or water is in agreement with gassmann s fluid substitution theory whereas the shear modulus is noticeably reduced the measurements with partial saturation show that the bulk modulus decreases with increasing water saturation to a lesser extent than the young s and shear moduli our results show that extensional attenuation in the samples with closed boundaries is insignificant under dry and fully saturated conditions but is influenced greatly by the liquid content when saturation is between 0 and 20 or 95 and 100

evidence for the transition to an oxygen rich atmosphere in the rooiberg group south africa a note abstract the rooiberg felsite group is a rare example of a conformable succession that apparently spans the period of evolution of an oxygen rich atmosphere this conclusion is reached indirectly from a review of the proterozoic stratigraphy of south africa and directly from a consideration of the rooiberg itself the atmospheric transition coincides with the stratigraphic interval between the dark silicic lavas and associated grey volcaniclastics of the damwal formation and the overlying red rhyodacites and intercalated red bed sedimentary rocks of the selonsrivier formation radiometric age measurements indicate that this transition occurred after 2224 21 ma but before 2090 40 ma

effect of pore geometry on gassmann fluid substitution although there is no assumption of pore geometry in derivation of gassmann s equation the pore geometry is in close relation with hygroscopic water content and pore fluid communication between the micropores and the macropores the hygroscopic water content in common reservoir rocks is small and its effect on elastic properties is ignored in the gassmann theory however the volume of hygroscopic water can be significant in shaly rocks or rocks made of fine particles therefore its effect on the elastic properties may be important if the pore fluids in microspores cannot reach pressure equilibrium with the macropore system assumption of the gassmann theory is violated therefore due to pore structure complexity there may be a significant part of the pore fluids that do not satisfy the assumption of the gassmann theory we recommend that this part of pore fluids be accounted for within the solid rock frame and effective porosity be used in gassmann s equation for fluid substitution integrated study of ultrasonic laboratory measurement data petrographic data mercury injection capillary pressure data and nuclear magnetic resonance t2 data confirms rationality of using effective porosity for gassmann fluid substitution the effective porosity for gassmann s equation should be frequency dependent knowing the pore geometry if an empirical correlation between frequency and the threshold pore throat radius or nuclear magnetic resonance t2 could be set up gassmann s equation can be applicable to data measured at different frequencies without information of the pore geometry the irreducible water saturation can be used to estimate the effective porosity

gravity driven granular free surface flow around a circular cylinder snow avalanches and other hazardous geophysical granular flows such as debris flows lahars and pyroclastic flows often impact on obstacles as they flow down a slope generating rapid changes in the flow height and velocity in their vicinity it is important to understand how a granular material flows around such obstacles to improve the design of deflecting and catching dams and to correctly interpret field observations in this paper small scale experiments and numerical simulations are used to investigate the supercritical gravity driven free surface flow of a granular avalanche around a circular cylinder our experiments show that a very sharp bow shock wave and a stagnation point are generated in front of the cylinder the shock standoff distance is accurately reproduced by shock capturing numerical simulations and is approximately equal to the reciprocal of the froude number consistent with previous approximate results for shallow water flows as the grains move around the cylinder the flow expands and the pressure gradients rapidly accelerate the particles up to supercritical speeds again the internal pressure is not strong enough to immediately push the grains into the space behind the cylinder and instead a grain free region or granular vacuum forms on the lee side for moderate upstream froude numbers and slope inclinations the granular vacuum closes up rapidly to form a triangular region but on steeper slopes both experiments and numerical simulations show that the pinch off distance moves far downstream

a simple analytical model for pressure on obstacles induced by snow avalanches the forces snow avalanches are able to exert on protection dams or buildings are of crucial interest in order to improve avalanche mitigation measures and to quantify the mechanical vulnerability of structures likely to be damaged by snow avalanches this paper presents an analytical model that is able to calculate these forces taking into account dead zone mechanisms first we present a 2 d analytical hydrodynamic model describing the forces on a wall overflown by gravity driven flows down an inclined plane second the 2 d model is successfully validated on discrete simulations of granular flows third we provide ingredients to extend the 2 d model to flows of dry and cold snow fourth we propose a simplified 3 d analytical model taking into account lateral fluxes finally the predictions from the simplified 3 d analytical model are successfully compared to recent measurements on two full scale snow avalanches released at the lautaret site in france

velocities of compressional and shear waves in limestones carbonate rocks are important hydrocarbon reservoir rocks with complex textures and petrophysical properties porosity and permeability mainly resulting from various diagenetic processes compaction dissolution precipitation cementation etc these complexities make prediction of reservoir characteristics e g porosity and permeability from their seismic properties very difficult to explore the relationship between the seismic petrophysical and geological properties ultrasonic compressional and shear wave velocity measurements were made under a simulated in situ condition of pressure 50 mpa hydrostatic effective pressure at frequencies of approximately 0 85 mhz and 0 7 mhz respectively using a pulse echo method the measurements were made both in vacuum dry and fully saturated conditions in oolitic limestones of the great oolite formation of southern england some of the rocks were fully saturated with oil the acoustic measurements were supplemented by porosity and permeability measurements petrological and pore geometry studies of resin impregnated polished thin sections x ray diffraction analyses and scanning electron microscope studies to investigate submicroscopic textures and micropores it is shown that the compressional and shear wave velocities vp and vs respectively decrease with increasing porosity and that vp decreases approximately twice as fast as vs the systematic differences in pore structures e g the aspect ratio of the limestones produce large residuals in the velocity versus porosity relationship it is demonstrated that the velocity versus porosity relationship can be improved by removing the pore structure dependent variations from the residuals the introduction of water into the pore space decreases the shear moduli of the rocks by about 2 gpa suggesting that there exists a fluid matrix interaction at grain contacts which reduces the rigidity the predicted biot gassmann velocity values are greater than the measured velocity values due to the rock fluid interaction this is not accounted for in the biot gassmann velocity models and velocity dispersion due to a local flow mechanism the velocities predicted by the raymer and time average relationships overestimated the measured velocities even more than the biot model

modeling of two dimensional overland flow a two dimensional hydrodynamic and numerical model for overland flow is developed in this paper the model allows spatial variations in hillslope physical characteristics including surface roughness infiltration and microtopography the accuracy of the model is tested by comparison with characteristic based solutions and experimental data the results of these tests indicate that the model is accurate and has good stability and convergence properties the application of the model to two dimensional overland flow on hillslopes is demonstrated by applying it to surfaces with spatially variable roughness infiltration and microtopography these simulations show that microtopography is the dominant factor causing spatial variation in overland flow depth velocity and direction

variation in dynamic elastic shear modulus of sandstone upon fluid saturation and substitution experimental acoustic measurements on sandstone rocks at both sonic and ultrasonic frequencies show that fluid saturation can cause a noticeable change in both the dynamic bulk and shear elastic moduli of sandstones we observed that the change in dynamic shear modulus upon fluid saturation is highly dependent on the type of saturant its viscosity rock microstructure and applied pressures frequency dispersion has some influence on dynamic elastic moduli too but its effect is limited to the ultrasonic frequency ranges and above we propose that viscous coupling reduction in free surface energy and to a limited extent frequency dispersion due to both local and global flow are the main mechanisms responsible for the change in dynamic shear elastic modulus upon fluid saturation and substitution and we quantify influences

the time bound character of uranium mineralising processes with special reference to the proterozoic of gondwana abstract it is more useful to consider the time bound character of uranium mineralising processes than the time bound character of uranium deposits this is because present uranium deposits are the result of a series of mineralising steps which occurred over hundreds of millions of years we have identified five periods which are characterised by moderately distinct mineralising processes 1 1 before 2800 ma uranium was fractionated into the upper crust during late archaean early proterozoic cratonisation by magmatic and metamorphic processes the granitic terranes were an important source of uranium for subsequent mineralising events 2 2 2800 2200 ma although the atmosphere was probably not reducing at this time the level of oxygen was sufficiently low for uraninite to be transported and concentrated in sedimentary basins cratonisation was a prerequisite for the formation and preservation of long lived basins rapid burial and reducing conditions during diagenesis were important for preserving the uranitite the uranium was mobilised during diagenesis and metamorphism 3 3 2200 1200 ma after 2000 ma when the oxygen level in the atmosphere increased markedly the movement of uranium in solution as uranyl ions became more important the first marine shelf sequences containing chemical and organic sediments to be deposited on the cratons formed excellent sites for the syn sedimentary diagenetic or metamorphic concentration of uranium in some cases this mineralisation was economic in other cases it may have been just a step in a succession of mineralising events continental clastic sediments which overlie these shelf sediments were excellent hosts for large hydrothermal circulation systems which could remobilise the uranium from either granites or sediments the lack of organic and argillaceous material in these continental sequences resulted in the precipitation of uranium near the basal unconformity where there were reducing sediments 4 4 1200 400 ma with the start of modern plate tectonics there was greater potential for the thermally activated remobilisation of uranium the period 1200 900 ma marked the final mineralisation in some important deposits the pan african and brazilian events 700 400 ma mobilised uranium of uncertain origin and concentrated it in previously deposited marginal marine sediments 5 5 400 0 ma the incorporation of organic material into continental clastic sediments allowed uranyl ions to be precipitated before they reached the sea to form the important sandstone type of deposits any time bound mineralising process may be either the final or an intermediate step in forming an economic uranium deposit which of these it is depends on the subsequent history of the area on whether the uranium is preserved dispersed or reconcentrated that many major uranium deposits occur on late archaean early proterozoic cratons emphasises the importance of the long term preservation of sedimentary sequences even though the uranium may be remobilised several times within the sediments

shock waves dead zones and particle free regions in rapid granular free surface flows shock waves dead zones and particle free regions form when a thin surface avalanche of granular material flows around an obstacle or over a change in the bed topography understanding and modelling these flows is of considerable practical interest for industrial processes as well as for the design of defences to protect buildings structures and people from snow avalanches debris flows and rockfalls these flow phenomena also yield useful constitutive information that can be used to improve existing avalanche models in this paper a simple hydraulic theory first suggested in the russian literature is generalized to model quasi two dimensional flows around obstacles exact and numerical solutions are then compared with laboratory experiments these indicate that the theory is adequate to quantitatively describe the formation of normal shocks oblique shocks dead zones and granular vacua such features are generated by the flow around a pyramidal obstacle which is typical of some of the defensive structures in use today

changes in dynamic shear moduli of carbonate rocks with fluid substitution to assess saturation effects on acoustic properties in carbonates we measure ultrasonic velocity on 38 limestone samples whose porosity ranges from 5 to 30 under dry and water saturated conditions complete saturation of the pore space with water causes an increase and decrease in compressional and shear wave velocity as well as significant changes in the shear moduli compressional velocities of most water saturated samples are up to 500 m s higher than the velocities of the dry samples some show no change and a few even show a decrease in velocity shear wave velocity vs generally decreases but nine samples show an increase of up to 230 m s water saturation decreases the shear modulus by up to 2 gpa in some samples and increases it by up to 3 gpa in others the average increase in the shear modulus with water saturation is 1 23 gpa the average decrease is 0 75gpa the vp vs ratio shows an overall increase with water saturation in particular rocks displaying shear weakening have disti

a model for the runout analysis of rapid flow slides debris flows and avalanches runout analyses are used for risk assessment and design of remedial measures against rapid landslides such as debris flows debris avalanches rockslide avalanches large scale liquefaction failure

factors controlling elastic properties in carbonate sediments and rocks carbonate sediments are prone to rapid and pervasive diagenetic alterations that change the mineralogy and pore structure within carbonate rocks in particular cementation and dissolution processes continuously modify the pore structure to create or destroy porosity in extreme cases these modifications can completely change the mineralogy from aragonite calcite to dolomite or reverse the pore distribution whereby original grains are dissolved to produce pores as the original pore space is filled with cement to form the rock figure 1 all these modifications alter the elastic properties of the rock and therefore the sonic velocity the result is a dynamic relationship among diagenesis porosity pore type and sonic velocity the result is a wide range of sonic velocity in carbonates in which compressional wave velocity vp ranges from 1700 to 6600 m s and shear wave velocity vs from 600 to 3500 m s

mechanisms affecting ultrasonic wave propagation in fluid containing sandstones under high hydrostatic pressure the elastic and anelastic parameters of sandstones at ultrasonic frequencies under low uniaxial pressure are strongly influenced by the fluid content the two main mechanisms controlling the rock behavior are based on 1 local fluid flow and 2 a grain surface effect which results in a modulus reduction both mechanisms depend fundamentally on the microstructure the critical factor controlling seismic wave velocity and attenuation in rocks under low pressure ultrasonic measurements have been performed on a fluid containing sandstone under hydrostatic pressure up to 200 mpa to investigate the mechanisms affecting rock behavior under in situ pressure conditions at high hydrostatic pressure the measured p wave velocities indicate a change in the rock microstructure and in the influence of the fluid by means of two models the effects of the mechanisms of local fluid flow murphy et al 1986 and global fluid flow biot 1956a b were calculated and the loss mechanisms were compared in their capability of explaining the experimental data the experimental and theoretical results show that for the investigated fluid containing sandstone both the local fluid flow mechanism and in the case of polar fluids the modulus reduction are still effective at high external pressure

simulation of three dimensional rapid free surface granular flow past different types of obstructions using the sph method in nature when hazardous geophysical granular flows e g a snow avalanche impact on an obstacle as they stream down a slope rapid changes in flow depth direction and velocity will occur it is important to understand how granular material flows around such obstacles in order to enhance the design of defense structures in this study a three dimensional 3 d smoothed particle hydrodynamics sph model is developed to simulate granular flow past different types of obstacles the elastic perfectly plastic model with implementation of the mohr coulomb failure criterion is applied to simulate the material behavior which describes the stress states of soil in the plastic flow regime the model was validated by simulating the collapse of a 3 d column of sand with two different aspect ratios the results showed that the sph method is capable of simulating granular flow the model is then applied to simulate the gravity driven granular flow down an inclined surface obstructed by a group of columns with different spacing a circular cylinder and a tetrahedral wedge the numerical results are then compared with experimental results and two different numerical solutions the good agreements obtained from these comparisons demonstrate that the sph method may be a powerful method for simulating granular flow and can be extended to design protective structures

calculating dense snow avalanche runout using a voellmy fluid model with active passive longitudinal straining a quasi one dimensional dense snow avalanche model has been developed to predict avalanche runout and flow velocity in a general two dimensional terrain the model contains three different dense snow avalanche flow laws these are 1 a voellmy fluid flow law with longitudinal active passive straining 2 a voellmy fluid flow law advanced by russian researchers in which the coulomb like dry friction is limited by a yield stress and 3 a modified criminale ericksen filby fluid model proposed by norwegian researchers the application of the voellmy fluid law with active passive straining to solve practical avalanche dynamics problems is evaluated by applying the model to simulate laboratory experiments and field case studies the model is additionally evaluated by comparing simulation results using the russian and norwegian models in a final analysis the influence of the initial conditions on avalanche runout is investigated we conclude that the model resolves many of the shortcomings of the voellmy salm model which is traditionally used in switzerland to predict avalanche runout furthermore since the model contains the three well calibrated parameters of the swiss guidelines on avalanche calculation it can be readily applied in practice we discuss why we believe the russian and norwegian models are not ready for practical application finally we show that many problems remain such as the specification of the initial release conditions we conclude that numerical models require a more detailed description of initial fracture conditions

channelized free surface flow of cohesionless granular avalanches in a chute with shallow lateral curvature received 29 november 1997 and in revised form 15 february 1999 a series of laboratory experiments and numerical simulations have been performed to investigate the rapid fluid like flow of a nite mass of granular material down a chute with partial lateral connement the chute consists of a section inclined at 40 to the horizontal which is connected to a plane run out zone by a smooth transition the flow is conned on the inclined section by a shallow parabolic cross slope prole photogrammetric techniques have been used to determine the position of the evolving boundary during the flow and the free surface height of the stationary granular deposit in the run out zone the results of three experiments with dierent granular materials are presented and shown to be in very good agreement with numerical simulations based on the savage hutter theory for granular avalanches the basal topography over which the avalanche flows has a strong channelizing eect on the inclined section of the chute as the avalanche reaches the run out zone where the lateral connement ceases the head spreads out to give the avalanche a characteristic tadpole shape sharp gradients in the avalanche thickness and velocity began to develop at the interface between the nose and tail of the avalanche as it came to rest indicating that a shock wave develops close to the end of the experiments

a new reflective optical extensometer rox system for geomechanical deformation measurements we have developed an extensometer for measurement of distance and displacements in geotechnical and civil engineering applications this is an optical extensometer that measures distance using a modulated laser beam in this design reflecting targets are placed at desired measurement locations and distances between each target and an optical head are measure repeatedly using the modulated laser beam moreover all electronic and moving parts are located outside of the hostile or difficult environment as the optical head is connected to the laser switching and signal analysis hardware using optical fibers a reference beam is utilized to provide direct correction of system behavior the system also utilizes movable reflective anchors that can be repositioned if desired we have installed the system in a field experiment currently being conducted for the yucca mountain site characterization project and report preliminary results

fundamentals of seismic rock physics during the past 50 years or so tremendous progress has been made in studying physical properties of rocks and minerals in relation to seismic exploration and earthquake seismology during this period many theories have been developed and many experiments have been carried out some of these theories and experimental results have played important roles in advancing earth sciences and exploration technologies this tutorial paper attempts to summarize some of these results

pressure coefficient in dam break flows of dry granular matter abstractthe propagation of dry granular flows such as rock and snow avalanches can be described by depth averaged models different from classical shallow water equations these models take into account the anisotropy of normal stresses inside the flowing pile through using an earth pressure coefficient in the pressure term a new regularization function for calculating the pressure coefficient in the savage hutter type models at the early stages of dam break flows and collapses is proposed in such circumstances the flow lines are significantly curved with respect to the basal surface and a special treatment of the earth pressure coefficient is required for obtaining a satisfactory agreement with experimental data the comparison between numerical simulations and laboratory experimental data shows an apparent improvement in describing the early stages of dam break waves over rough beds the comparison with experiments over smooth bed surface exhibits minor evidence of improvement nonetheless in this

fluid mobility and frequency dependent seismic velocity direct measurements the influence of fluid mobility on seismic velocity dispersion is directly observed in laboratory measurements from seismic to ultrasonic frequencies a forceddeformation system is used in conjunction with pulse transmission to obtain elastic properties at seismic strain amplitude 10 7 from 5 hz to 800 khz varying fluid types and saturations document the influence of pore fluids the ratio of rock permeability to fluid viscosity defines mobility which largely controls pore fluid motion and pore pressure in a porous medium high fluid mobility permits pore pressure equilibrium either between pores or between heterogeneous regions resulting in a low frequency domain where gassmann s equations are valid in contrast low fluid mobility can produce strong dispersion even within the seismic band here the low frequency assumption fails since most rocks in the general sedimentary section have very low permeability and fluid mobility shales siltstones tight limestones etc most rocks are not in the lowfrequency domain even at seismic frequencies only those rocks with high permeability porous sands and carbonates will remain in the low frequency domain in the seismic or sonic band