a newly discovered skull of the temnospondyl amphibian dendrerpeton acadianum owen a virtually complete three dimensional skull of a subadult of the temnospondyl amphibian dendrerpeton acadianum provides new information on the structure of the palate and lower jaw the left stapes appears to lie in a natural position on the quadrate ramus of the pterygoid the proportionately large otic ossicle probably precluded it from having acted as a transmitter of airborne vibrations from a tympanum to the inner ear the use of the term otic notch should be restricted to amphibians or reptiles in which the embayment of the posterior margin of the cheek is accompanied by the presence of a slender rod like stapes that could have functioned as part of an impedance matching system in those species in which a notch is present but the stapes is a massive supporting element the term squamosal embayment should be used rather than otic notch the squamosal embayment in early tetrapods may have been inherited directly from their fish ancestors and had no functional relationship to hearing

empirical relationships among seismic velocity effective pressure porosity and clay content in sandstone we use a multivariate analysis to investigate the influence of effective pressure pe porosity and clay content c on the compressional velocity vp and shear velocity vs of sandstones laboratory measurements on water saturated samples of 64 different sandstones provide a large data set that was analyzed statistically for each sample relationships between effective pressure and vp and vs have been determined all samples were well fit by relationships that have an exponential increase in velocity at low pe tapering to a linear increase with pe for pe greater than 0 2 kbar there are differences in the pressure dependences of velocity for different rocks particularly at very low pressures however the differences cannot be attributed to or c for the combined set of measurements from all samples the best fitting formulations are vp 5 77 6 94 1 73c 0 446 pe e 16 7pe and vs 3 70 4 94 1 57c 0 361 pe e 16 7pe while this is admittedly a very simplified parameterization it is remarkable how well

effective pressure or what is the effect of pressure using time lapse seismics as a reservoir monitoring tool geophysics can help distinguish different reservoir production scenarios for example eiken et al 2000 successfully detected fluid saturation changes after co2 injection using time lapse seismics at sleipner field over the cycle of a reservoir life oil saturation usually decreases reservoir pressure declines and gas breakout may occur these changes cause rock property changes that are detectible in time lapse seismics therefore it is important to understand the effects of pressure and saturation changes on rock properties while the effects of saturation changes are often well described by gassmann 1951 brown and korringa 1975 and mavko 1975 the effects of pressure changes are less understood here we focus on understanding the effects of fluid pressure on velocities

caco3 precipitation kinetics in waters from the great bahama bank implications for the relationship between bank hydrochemistry and whitings the source of whitings on the great bahama bank and their relationship to major changes in the chemistry of bank waters have been among the longest and most hotly debated topics in carbonate geochemistry in this paper we demonstrate that the reaction kinetics of calcite with bank waters for a given saturation state are similar to but somewhat slower 2 to 3 times than with gulf stream water the interpretation of the reaction kinetics of suspended bank sediment with bank water requires that the precipitating phase be about twice as soluble as aragonite good agreement at equivalent saturation states was found between experimental precipitation rates and those calculated for the rate of change of bank water chemistry in the region of whitings these results indicate that the dominant mode of carbonate removal is via precipitation on resuspended sediments rather than the rapid pseudo homogeneous precipitation of calcium carbonate in the water column resulting in the formation of a whiting estimates indicate that single aragonite needles may be resuspended many times over a period of decades during which they experience repeated overgrowth a major portion 98 of suspended calcium carbonate is outside the visually dramatic whitings thus as visually spectacular as they are whitings do not represent a short term locally massive precipitation of carbonate on the great bahama bank nor are they even likely to be the dominant sites of carbonate removal in this region although future refinements are needed that include seafloor processes we have at this point arrived at a mechanistic kinetic model that provides a reasonably quantitative explanation for the hydrochemistry of the carbonate system on the northern great bahama bank

discrimination between pressure and fluid saturation changes from time lapse seismic data explicit expressions for computing saturation and pressure related changes from time lapse seismic data have been derived and tested on a real time lapse seismic data set necessary input is near and far offset stacks for the baseline seismic survey and the repeat survey the method has been tested successfully in a segment where pressure measurements in two wells verify a pore pressure increase of 5 to 6 mpa between the baseline survey and the monitor survey estimated pressure changes using the proposed relationships fit very well with observations between the baseline and monitor seismic surveys 27 of the estimated recoverable hydrocarbon reserves were produced from this segment the estimated saturation changes also agree well with observed changes apart from some areas in the water zone that are mapped as being exposed to saturation changes which is unlikely saturation changes in other segments close to the original oil water contact and the top reservoir interface are also estimated and conf

microbes and ooids abstract the bahamian archipelago is one of the few locations in the world where ooid formation is actively occurring ooid cortices from six locations in the region were incrementally dissolved and analyzed for 14c 18o 13c mg ca and sr ca ratios ooids were examined under sem after each step in the incremental analyses to characterize the nature of dissolution radiocarbon dating indicates that ooids presently at the sediment water interface began forming across the archipelago between 1000 and 2800 yr bp and continue to form today the ooids have the same pattern of microboring alteration across the region the surface and outer cortex of the ooids are punctuated with unfilled microborings whereas the inner cortex contains two morphologies of aragonite cement filling the microborings the two morphologies of cement form in association with two different species of cyanobacteria one is solentia sp the other is interpreted to be hyella sp the chemistry of ooids from across the region is remarkably similar 18o and 13c values for all samples vary directly having a slope of approximately 1 the outer cortex has low 18o and 13c values of 3 4 and 0 2 respectively whereas the 18o and 13c values of the inner cortex are high with values of 1 9 and 6 8 respectively the presence of aragonite cement in microborings in the inner cortex increases the overall isotopic composition of both oxygen and carbon in the ooid causing it to appear close to equilibrium with seawater the isotopic variation in 18o and 13c within the cortex can be characterized as a mixing line between the low values in the unaltered ooid laminae and the aragonite cement in the microborings the most exterior portion of the ooid has very high mg ca values and is interpreted as an amorphous calcium carbonate acc coating there are two other phases in the cortex both being aragonite the outer cortex has a higher mg ca ratio and lower sr ca ratio than the inner cortex this difference in chemistry is a result of the presence of aragonite cement in the inner cortex stable isotopic and trace element results coupled with sem investigations indicate that microbes do not play a role in ooid formation but instead alter the texture and chemistry of ooids after they have formed a new model of ooid formation is proposed whereby a veneer of acc precipitates on an ooid while it is at the sediment water interface the active phase this veneer of acc later recrystallizes to aragonite needles and a new cortex layer is formed the stationary phase observations from this study lead to a deeper understanding of the chemical processes involved in ooid genesis which allow a better understanding of paleoenvironments hosting ooid formation

stress induced seismic azimuthal anisotropy in the upper crust across the north west shelf australia seismic azimuthal anisotropy saa is observed in many areas of the earth and fast polarized directions have been mapped using earthquake surface waves and teleseismic s wave splitting over large regional and tectonic scales higher resolution petroleum exploration data including 3 d seismic surveys and dipole shear logs often exhibit azimuthal anisotropy however we are unaware of any published studies mapping saa from exploration scale data to study large scale regional and tectonic saa trends the physical mechanisms causing saa in earthquake data are a subject of great interest comparing regional trends to the higher resolution exploration trends may help understand these mechanisms we present a saa analysis using seismic exploration data across the north west shelf nws of australia we map the fast polarization directions from 34 dipole shear logs and two 3 d seismic surveys and compare them to in situ maximum horizontal stress orientations h our results show that the and h trends correlate across a geographical area spanning almost 2000 km and are similar to published results in the region from earthquake seismology these results suggest that differential horizontal stress is the likely mechanism causing saa at a wide range of spatial scales on the nws of australia we also show that saa is not observed at exploration scales in some areas of the nws and propose a lithology dependent stress sensitivity model in which saa is strongest in clean unconsolidated quartz dominated sandstones and weaker or nonexistent in well consolidated cemented rocks and shale dominated sediments

stress induced seismic azimuthal anisotropy sand shale content and depth trends offshore north west australia abstractseismic azimuthal anisotropy is apparent when p wave velocities vary with source receiver azimuth and downward propagating s waves split into two quasi s waves polarized in orthogonal directions not accounting for these effects can degrade seismic image quality and result in erroneous amplitude analysis and geologic interpretations there are currently no physical models available to describe how azimuthal anisotropy induced by differential horizontal stress varies with sand shale lithology and depth we develop a model that does so in unconsolidated sand shale sequences offshore north west australia our method naturally introduces two new concepts critical anisotropy and anisotropic depth limit critical anisotropy is the maximum amount of azimuthal anisotropy expected to be observed at the shallowest sediment burial depth where the confining pressure and sediment compaction are minimal the anisotropic depth limit is the maximum depth where the stress induced azimuthal anisotropy is ex

acoustic measurements in unconsolidated sands at low effective pressure and overpressure detection shallow water flows and over pressured zones are a major hazard in deepwater drilling projects their detection prior to drilling would save millions of dollars in lost drilling costs i have investigated the sensitivity of seismic methods for this purpose using p wave information alone can be ambiguous because a drop in p wave velocity vp can be caused both by overpressure and by presence of gas the ratio of p wave velocity to s wave velocity vp vs which increases with overpressure and decreases with gas saturation can help differentiate between the two cases since p wave velocity in a suspension is slightly below that of the suspending fluid and vs 0 vp vs and poisson s ratio must increase exponentially as a load bearing sediment approaches a state of suspension on the other hand presence of gas will also decrease vp but vs will remain unaffected and vp vs will decrease analyses of ultrasonic p and s wave velocities in sands show that the vp vs ratio especially at low effective pressures

velocity determination for pore pressure prediction overpressured formations exhibit several of the following properties when compared with a normally pressured section at the same depth dutta 2002 1 higher porosities 2 lower bulk densities 3 lower effective stresses 4 higher temperatures 5 lower interval velocities and 6 higher poisson s ratios

an articulated embolomere skeleton amphibia anthracosauria from the lower pennsylvanian bashkirian of nova scotia recently discovered skeletal material greatly expands our knowledge of the embolomeres from the early pennsylvanian locality of joggins nova scotia these specimens are probably attributable to calligenethlon watsoni steen that has been described previously from this locality but despite their articulated condition and relative completeness this assignment is equivocal inclusion of the most complete specimen nsm 994 gf 1 1 in a previous phylogenetic analysis of the embolomeri results in a sistergroup relationship between nsm 994 gf 1 1 and anthracosaurus however none of the 16 synapomorphies supporting this relationship are unique to these two taxa but rather all occur in one or more of the other taxa in the analysis this indicates extensive homoplasy within embolomeres and emphasizes the difficulties inherent in resolving embolomere interrelationships the dirth of unequivocal synapomorphies is probably also reflects the conservative nature and basal status of embolomeres within the initial pal

improved granular medium model for unconsolidated sands using coordination number porosity and pressure relations we have developed a recipe for using closed form expressions of effective medium models to predict velocities in unconsolidated sandstones the commonly used hertz mindlin effective medium model for granular media often predicts elastic wave velocities that are higher and vp vs ratios that are lower than those observed in laboratory and well log measurements in unconsolidated sediments we use the extended walton model which introduces a parameter to represent the fraction of grain contacts that are perfectly adhered using the extended walton model with ranging from 0 3 to 1 we obtain new empirical relations between the coordination number c porosity and pressure for p and s wave velocities by inverting dynamic measurements on dry unconsolidated sands we propose using the extended walton model 0 6 along with these new c porosity and c pressure relations to study the mechanical compaction of unconsolidated sandstones the model has been tested on two experimental data sets it provide

the combined effects of pressure and cementation on 4d seismic data abstracttime lapse seismology has proven to be a useful method for monitoring reservoir fluid flow identifying unproduced hydrocarbons and injected fluids and improving overall reservoir management decisions the large magnitudes of observed time lapse seismic anomalies associated with strong pore pressure increases are sometimes not explainable by velocity pressure relationships determined by fitting elastic theory to core data this can lead to difficulties in interpreting time lapse seismic data in terms of physically realizable changes in reservoir properties during injection it is commonly assumed that certain geologic properties remain constant during fluid production injection including rock porosity and grain cementation we have developed a new nonelastic method based on rock physics diagnostics to describe the pressure sensitivity of rock properties that includes changes in the grain contact cement and we applied the method to a 4d seismic data example from offshore australia we found that

rock physics diagnostic of north sea sands link between microstructure and seismic properties velocity in high porosity sands strongly depends on the position and amount of intergranular material velocity is high if the original grains are cemented at their contacts it is low if the pore filling material is placed away from the contacts the amount and type of intergranular material can be theoretically determined from velocity and porosity data by adjusting an effective medium theoretical model curve to a trend in the data and then assuming that the microstructure of the sediment is such as used in the model we apply this diagnostic method to clean sands in the heimdal formation the north sea the results show that the reservoir zone sand in one well under examination has slight contact cementation while it is completely uncemented and friable in another well this conclusion is supported by microscopic grain images the rock physics diagnostic is important for establishing a velocity porosity trend consistent with local geology such trends can improve the accuracy of porosity prediction from seismic data

rock physics modeling of unconsolidated sands accounting for nonuniform contacts and heterogeneous stress fields in the effective media approximation with applications to hydrocarbon exploration by treating contact stiffness as a variable one can extend the effective medium approximation used to obtain elastic stiffness of a random pack of spherical grains more specifically we suggest calibrating effective media approximation based on contact mechanics by incorporating nonuniform contact models the simple extension of the theory provides a better fit for many laboratory and field experiments and can provide insight into the micromechanical bonds associated with unconsolidated sediments this approach is motivated by repeated observations of shear wave measurements in unconsolidated sands where observed shear wave velocities are lower than predicted by the hertz mindlin contact theory we present the calibration process for well log data from a north sea well penetrating a shallow gas discovery and a deepwater well in the gulf of mexico finally we demonstrate the benefit of using this model for amplitude variation with angle ava analysis of shallow sand targets in exploration and reservoir studies

time lapse seismic reservoir monitoring time lapse seismic reservoir monitoring has advanced rapidly over the past decade there are currently about 75 active projects worldwide and more than 100 cumulative projects in the past decade or so the present total annual expenditures on 4 d seismic projects are on the order of 50 100 million us this currently represents a much smaller market than 3 d seismic but the use of 4 d seismic has grown exponentially over the past decade and is expected to continue to do so

overpressure detection from compressional and shear wave data by analyzing experimental data we show that in many room dry rocks the poisson s ratio pr decreases with decreasing differential pressure confining minus pore pressure this means that in gas saturated rocks pr decreases with increasing pore pressure we confirm the generality of the observed effect by theoretically reproducing it via effective medium modeling this effect can be used as a new tool for overpressure detection from surface seismic cross well sonic logs and measurements ahead of the drill bit

oxygen isotope fractionation during synthesis of camg carbonate and implications for sedimentary dolomite formation hydrous camg carbonate was synthesized at temperatures of 40 degrees 60 degrees and 80 degrees c in the laboratory this material has very similar mineralogical characteristics to natural disordered dolomite from the coorong region in south australia besides the dolomite variable amounts of amorphous carbonate are present in all samples the oxygen isotope compositions of synthesized bulk carbonate samples e g amorphous carbonate plus dolomite plot significantly lower than the northrop and clayton 1966 dolomite water equilibrium fractionated degassing of the samples however revealed relatively low oxygen isotope values for fast reacting using 100 h3po4 amorphous carbonate in contrast slow reacting dolomite has more positive oxygen isotope values and calculated carbonate water oxygen isotope fractionation values are close to strongest known dolomite water oxygen isotope fractionation published earlier on variations of reaction stabilization temperatures during synthesis gave evidence for dolomite formation from hypersaline solutions by a dissolution reprecipitation process it is likely that amorphous carbonate has been a problem in defining the dolomite water fractionation in the past moreover dolomite associated amorphous carbonate contents probably led to incorrect speculations about lower oxygen isotope fractionation in a so called protodolomite water system copyright c 2005 elsevier ltd

ultrasonic properties of sedimentary rocks effect of pressure saturation frequency and microcracks seismic velocities and attenuations are influenced by lithology porosity and permeability as well as the kind and quantity of the pore fluids the microstructure of rocks is another important factor influencing seismic properties this influence can be used to investigate the pressure dependent closure of microcracks for this purpose velocities and attenuations of p and s waves have been determined at ultrasonic frequencies in three different rocks vacuum dry and partially saturated with water as a function of hydrostatic pressure up to 200 mpa a new combined model has been developed with this model an explicit description of the microstructure local fluid flow lf and macrostructure gassmann effect global fluid flow is possible assuming a patchy saturation i e inhomogeneous distribution of the fluid in the pores and modulus reduction both the saturation dependent measurements and the pressure dependent data can be explained the modelling and the phenomenological interpretation of the data gained under increasing hydrostatic pressure yields consistent results concerning the amount of crack closure as well as the range in which crack closure appears in high porosity sandstones bentheimer and obernkirchner sandstone a strong closure of cracks already with a small increase of pressure must be concluded in a low porosity harzer greywacke a significantly lower closure of cracks can be concluded within the pressure range of 0 100 mpa it is shown however that there must be open cracks in all rocks even under elevated hydrostatic pressure resulting in a measurable effect due to lf

a new velocity pressure compaction model for uncemented sediments summary knowledge of the pressure dependence of rock properties is useful for a wide range of earth science problems especially related to pore pressure changes caused by fluid injection or withdrawal as often occurs in groundwater hydrocarbon and co2 sequestration reservoirs a long standing problem is that theoretical models of velocity pressure response often do not match laboratory measurements and alternately empirical regressions fit to such data do not extrapolate accurately to wider pressure ranges since they have little or no physical basis accurate determination of the dry rock frame properties at low effective pressure is a key aspect of the problem particularly when ultrasonic laboratory measurements are not available in this pressure range we present a new model to describe the pressure sensitivity of the bulk and shear moduli for uncemented sedimentary rocks our model incorporates effects of sedimentary compaction and critical porosity including a relationship to account for porosityanddensitychangewithpressure themodelistestedonlaboratorymeasurementsfor various rock samples and fits well over a wide range of pressures the new velocity pressure model should be useful for improved prediction and interpretation of pressure dependent rock properties and seismic data

prediction of seismic wave velocities in rock at various confining pressures based on unconfined data abstractlaboratory measurement of p and s wave velocities vp and vs respectively under confining pressure indicates that with an increase in confining pressure vp and vs will increase the trend is exponential at low pressures transitioning to linear above a critical pressure however the trend of the velocity pressure curve for each rock specimen may be determined knowing the coefficients of this curve we first studied how the coefficients of the velocity pressure curve were expected to be functions of elastic moduli then four empirical equations were used to estimate four coefficients of the velocity pressure curve using the rock density and vp and vs at atmospheric pressure unconfined conditions this analysis was carried out based on laboratory experiments on 285 rock specimens of different lithology from around the world namely the united states china germany iran and deep sea drilling projects for each rock specimen vp and vs were measured at different confining stress levels re

new embolomerous tetrapod material and a faunal overview of the mississippian aged point edward locality nova scotia canada embolomerous tetrapods moderately sized to large aquatic predators form a major faunal constituent of permo carboniferous tetrapod communities embolomeres are recognized by their distinct circul

saturation pore pressure and effective stress from sandstone acoustic properties 1 acoustic velocity and vp vs ratio show different responses in saturated and dry rocks under increasing differential pressures velocity is also shown to be sensitive to pore pressure not just differential pressure under identical low differential pressure conditions velocity is higher in sandstones when pore pressure is high during simulation of inflationary overpressure conditions vp vs ratios of sandstones with high fluid pressures are shown to lie between dry conditions and saturated conditions with constant and low pore pressure thus the velocity differential pressure relationship is not unique however the use of the measured biot effective stress coefficient results in a unique relationship it is postulated that higher velocity and different vp vs ratios are caused by loss of compliance in microfractures due to stiffening by increasing fluid pressure

bayesian inversion of time lapse seismic data for the estimation of static reservoir properties and dynamic property changes seismic conditioning of static reservoir model properties such as porosity and lithology has traditionally been faced as a solution of an inverse problem dynamic reservoir model properties have been constrained by time lapse seismic data here we propose a methodology to jointly estimate rock properties such as porosity and dynamic property changes such as pressure and saturation changes from time lapse seismic data the methodology is based on a full bayesian approach to seismic inversion and can be divided into two steps first we estimate the conditional probability of elastic properties and their relative changes then we estimate the posterior probability of rock properties and dynamic property changes we apply the proposed methodology to a synthetic reservoir study where we have created a synthetic seismic survey for a real dynamic reservoir model including pre production and production scenarios the final result is a set of point wise probability distributions that allow us to predict the most probable reservoir models at each time step and to evaluate the associated uncertainty finally we also show an application to real field data from the norwegian sea where we estimate changes in gas saturation and pressure from time lapse seismic amplitude differences the inverted results show the hydrocarbon displacement at the times of two repeated seismic surveys

compaction trends for shale and clean sandstone in shallow sediments gulf of mexico compaction depth trends are important in drilling basin modeling and seismic exploration for several purposes 1 to de tect overpressure and hydrocarbon zones and distinguish them from seismic velocity anomalies 2 to calculate interval velocities and depth conversion involving seismic data and earth models 3 to predict seismic signatures of sand shale interfaces as a function of depth and 4 to recognize over compacted zones due to uplift several authors have studied the effects of compaction on the porosity of sands and shales e g magara 1980 ramm and bjorlykke 1994 the effects of compaction on velocity depth trends have been provided by different authors e g al chalabi 1997 faust 1951 japsen 2000 however porosity and velocity depth trends in the shallow section are not well established the main challenge in computing such trends is the paucity of well log data in the shallow subsurface figure 1 a typical well log from the gulf of mexico lacks measurements in the shall

tracking changes in the specific storage of overburden rock during longwall coal mining the hydraulic properties of geologic formations such as specific storage and hydraulic conductivity are known to vary spatially due to formation heterogeneity however they may also change temporally if the formation undergoes mechanical disturbance such as occurs during mining characterising changes in hydraulic properties is important for groundwater systems that are disturbed by mining conventional hydrogeological investigations rely on literature derived values or limited programs of field testing e g pumping test to define specific storage but these approaches do not consider the change in compressibility that may occur due to mechanical disturbance this paper presents for the first time direct in situ measurements of the changes in compressibility and consequently specific storage as a result of mechanical disturbance within geologic formations overlying underground mining operations the in situ measurements of compressibility are derived in this study from the pore pressure response to barometric loading or strains induced by earth tides even prior to disturbance the compressibility obtained from in situ measurements was found to be an order of magnitude lower than that measured on core samples by unconfined strength test in the laboratory the differences in compressibility for the intact formation are likely due to sample disturbance and the differences in strain level imposed by the field and laboratory methods the increase in compressibility of the overburden rock varied with the location of the monitoring site relative to longwall extraction and the level of strain the formation sustained the study found that quantifying the changes in compressibility and consequently specific storage as a result of mine induced disturbance is critical to our understanding of subsidence the extent of depressurization and the impact that mining may have on regional groundwater flow systems

the pennsylvanian tropical biome reconstructed from the joggins formation of nova scotia canada the pennsylvanian langsettian joggins formation contains a diverse fossil assemblage first made famous by lyell and dawson in the mid 19th century collector curves based on c 150 years of observation suggest that the joggins fossil record is relatively complete a key feature of the site is that fossils occur in par autochthonous assemblages within a narrow time interval