a continental drift flipbook abstract forty six miniature plate tectonic reconstructions are presented that can be assembled into a flipbook that illustrates the movement of the continents since the late precambrian 750 m yr ago six principal lines of evidence have been used to reconstruct the past positions of the continents 1 linear magnetic anomalies produced by sea floor spreading 2 paleomagnetism 3 hotspot tracks and large igneous provinces 4 the tectonic fabric of the ocean floor mapped by satellite altimetry 5 lithologic indicators of climate e g coals salt deposits tillites and 6 the geologic record of plate tectonic history i discuss the probable uncertainties associated with the plate tectonic reconstructions and give an estimate of the uncertainty in the positions of the continents back through time

constraining seismic rock property logs in organic shale reservoirs abstract one of the major challenges of unconventional shale reservoirs is to understand the effects of organic richness total organic carbon toc mineralogy microcracks pore shape and effective stress on elastic properties the generation of petrophysical parameters such as toc and quantification of total and organic porosities through a physically consistent petrophysical model are described rock matrix density which is a key parameter in determining total porosity is estimated as a function of the amount of toc and its level of maturity then the petrophysical parameters are used as inputs for rock physics modeling to constrain the bedding normal compressional wave velocity as a function of various parameters e g toc porosity mineralogy pore shapes and microcrack density in combination with effective stress modeling results on three shale plays from north america show that compressional wave velocity in these specific formations is controlled mainly by variations in toc mineralogy

new somali basin magnetic anomalies and a plate model for the early indian ocean abstract the oldest portions of the indian ocean formed via the breakup of gondwana and the subsequent fragmentation of east gondwana we present a constrained plate model for this early indian ocean development for the time period from gondwana breakup until the start of the cretaceous normal superchron the motions of the east gondwana terranes are determined using new geophysical observations in the somali basin and existing geophysical interpretations from other coeval indian ocean basins within the somali basin recent satellite gravity data clearly resolve traces of an east west trending extinct spreading ridge and north south oriented fracture zones a thorough compilation of somali basin ship track magnetic data allows us to interpret magnetic anomalies m24bn through m0r about this extinct ridge our magnetic interpretations from the somali basin are similar in age spreading rate and spreading directions to magnetic anomalies previously interpreted in the neighboring mozambique basin and riiser larsen sea the similarity between the two magnetic anomaly datasets allows us to match both basin s older magnetic anomaly picks by defining a pole of rotation for a single and cohesive east gondwana plate however following magnetic anomaly m15n we find it is no longer possible to match magnetic picks from both basins and maintain plausible plate motions in order to match the post m15n geophysical data we are forced to model the motions of madagascar india and east antarctica australia as independent plates the requirement to utilize two independent plates after anomaly m15n provides strong circumstantial evidence that suggests east gondwana breakup began around 135 ma

light lithophile elements in martian basalts evaluating the evidence for magmatic water degassing whether water has played a role in the petrogenesis of martian basalts remains a subject of significant debate estimates of pre eruptive water concentrations in martian magmas are impeded by the effects of degassing and shock however zoning trends of light lithophile elements lle in pyroxene have been interpreted as evidence for the degassing of magmatic water on the basis of the soluble behavior of these elements in hydrothermal fluids we provide ion microprobe analyses of lle in pyroxene in the zagami and shergotty martian basalts complemented by detailed electron microprobe analyses and major element x ray maps to independently verify the zoning of lle and its relationship to texture and major element variations our results corroborate previous results specifically that li concentrations are lower in rims than cores of shergotty and zagami pyroxene in contrast we see no evidence for a core to rim decrease in b in the absence of further data we interpret the decrease in li as reflecting either loss after crystallization of pyroxene cores consistent with magmatic degassing or the diffusive preferential loss of li from pyroxene rims possibly as a result of shock because the partitioning behavior of li between hydrous fluid minerals and melt under relevant conditions of pressure temperature and melt composition is unknown the viability of the water degassing hypothesis depends on experiments establishing the compatibility of li in hydrous fluid associated with martian basaltic melt and the incompatibility of li in pyroxene at elevated pressures

modeling offset dependent reflectivity for time lapse monitoring of water flood production in thin layered reservoirs the objective of this case study is to predict whether 5 years of water flood production from a thinly layered gulf of mexico reservoir will change its seismic amplitude variation with offset avo response in a detectable manner density and velocity profiles were computed from in situ wireline logs for 100 oil gas and brine saturations and for a 5 year prediction that was based on a fluid flow and production simulation analytical avo curves for simple half space models did not match avo curves extracted from synthetic seismograms computed with a full waveform layer stack algorithm several different amplitude corrections were tried to reduce the avo curves from the synthetic data to the analytical ones but ultimately none was deemed satisfactory instead avo change attributes based on relative changes polarity changes or ratios were used attributes based on the change of avo gradient were perceived to be most diagnostic of the water flood but they were also overly sensitive to interference noise and amplitude correction errors for field data from the study area a large decrease in intercept magnitude may be the best indicator of the waterfront

evidence for relative motions between the indian and australian plates during the last 20 m y from plate tectonic reconstructions implications for the deformation of the indo australian plate we use plate tectonic reconstructions to establish whether motions between india and australia occurred since chron 18 43 ma we test the africa antarctica australia india plate circuit closure at chrons 5 10 ma 6 21 ma and 13 36 ma using a compilation of magnetic anomalies and fracture zone traces from the southeast southwest central indian and the carlsberg ridges additional reconstructions at chrons 23 55 ma and 26 61 ma are used to estimate the overall motion between india and australia relative motions between the indian and australian plates are estimated using the plate circuit india africa australia a new statistical approach based on spherical regression analyses is used to assess the uncertainty of the best fitting finite rotations from the uncertainties in the data the uncertainty in a rotation is described by a covariance matrix directly related to the geometry of the reconstructed plate boundary to the distribution and estimated errors of the data points along it our parameterization of the rotations allows for simple combination of the rotation uncertainties along a plate circuit path results for chron 5 are remarkably consistent with present day kinematics in the indian ocean except that the arabian and indian plates are found to be separate plates comparisons of the motions between the indian and african plates across the carlsberg ridge with that between the australian and african plates across the central indian ridge evidence a significant counterclockwise rotation of the australian plate relative to the indian plate about a pole located in the central indian basin the determinations are consistent for chrons 26 13 6 and 5 determination at chron 23 is different but questionable due to the small number of available data we propose two alternative solutions that both predict convergence within the wharton and central indian basins and extension in the vicinity of the chagos laccadive ridge the first solution assumes that all the deformation in the equatorial indian ocean started 7 ma ago as found by ocean drilling program leg 116 hence all the determinations at different times represent the total motion between india and australia the averaged india australia euler vector chrons 5 6 13 and 26 11 1 s 78 0 3 54 lies within the central indian basin and yields a n s contraction of 46 52 km at 85 and 80 63 km at 90 however the difference of the india australia euler vectors at chrons 5 and 6 suggests that the india australia convergence started between 10 and 21 ma following the continent continent collision of india with asia in the early miocene the second averaged solution chrons 6 13 and 26 5 2 s 74 3 e 5 93 predicts a total n s contraction of 123 73 km at 85 e and 178 91 km at 90 e both models are compatible with the deformation pattern observed in the equatorial indian ocean

fluid substitution for thinly interbedded sand shale sequences using the mesh method abstractgassmann s fluid substitution model is intended for a monomineralic homogeneous porous rock in which pore pressures induced by applied loads can equilibrate throughout the pore space these assumptions are violated when gassmann s equations are applied to measurements that represent effective medium averages over subresolution layers of alternating sand and shale the conventional procedure for treating this problem has been to first downscale i e estimate the properties of the fine scale sand and shale endmembers from the coarse scale measurements apply gassmann s fluid substitution to the sand only and then backus average back to the original scale this procedure however is very sensitive to errors in estimated sand fraction and shale properties and becomes particularly unstable at small sand fractions a new method for fluid substitution combines rock physics models for dispersed and interbedded sand shale systems which are often approximated with reuss or lower hashin shtrikman inte

motion between the indian antarctic and african plates in the early cenozoic we used a three plate best fit algorithm to calculate four sets of euler rotations for motion between the india capricorn africa somali and antarctic plates for 14 time intervals in the early cenozoic each set of rotations had a different combination of data constraints the first set of rotations used a basic set of magnetic anomaly picks on the central indian ridge cir southeast indian ridge seir and southwest indian ridge swir and fracture zone constraints on the cir and seir but did not incorporate data from the carlsberg ridge and did not use fracture zones on the swir the second set added fracture zone constraints from the region of the bain fracture zone on the swir which were dated with synthetic flowlines based on the first data set the third set of rotations used the basic constraints from the first rotation set and added data from the carlsberg ridge the fourth set of rotations combined both the swir fracture zone constraints and the carlsberg ridge constraints data on the indian plate side of the carlsberg ridge arabian basin were rotated to the capricorn plate before being included in the constraints plate trajectories and spreading rate histories for the cir and swir based on the new rotations document the major early cenozoic changes in plate motion on the cir and seir there was a large but gradual slowdown starting around chron 23o 51 9 ma and continuing until chron 21y 45 3 ma followed 2 or 3 myr later by an abrupt change in spreading azimuth which started around chron 20o 42 8 ma and which was completed by chron 20y 41 5 ma no change in spreading rate accompanied the abrupt change in spreading direction on the swir there was a continuous increase in spreading rates between chrons 23o and 20o and large changes in azimuth around chrons 24 and 23 and again at chron 21 unexpectedly we found that the two sets of rotations constrained by the carlsberg ridge data diverged from the other two sets of rotations prior to anomaly 22o when compared to rotations for the cir that are simultaneously constrained by data from all three branches of the indian ocean triple junction there is a progressively larger separation of anomalies on the carlsberg ridge with a roughly 25 km misfit for anomaly 23o and increasing to over 100 km for anomaly 26y these data require that there was previously unrecognized convergence somewhere in the plate circuit linking the indian capricorn and somali plates prior to chron 22o we quantify this motion by summing our new capricorn somalia rotations with previously published rotations for neogene india capricorn motion and for early cenozoic somali india motion based solely on carlsberg ridge data the most likely possibility is that there was motion within the somalia plate due to a distinct seychelles microplate as young as chron 22o the sense of the misfit on the carlsberg ridge is consistent with roughly 100 150 km of convergence across a boundary passing through the amirante trench and extending north to the carlsberg ridge axis between anomalies 26y and 22o alternatively there may have been convergence within the indian plate either along the western margin of indian or east of the cir in the region of the current capricorn indian diffuse plate boundary our work sharpens the dating of the two major eocene changes in plate motion recognized in the indian ocean

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

a rock physics strategy for quantifying uncertainty in common hydrocarbon indicators we present a strategy for quantifying uncertainties in rock physics interpretations by combining statistical techniques with deterministic rock physics relations derived from the laboratory and theory a simple example combines gassmann s deterministic equation for fluid substitution with statistics inferred from log core and seismic data to detect hydrocarbons from observed seismic velocities the formulation identifies the most likely pore fluid modulus corresponding to each observed seismic attribute and the uncertainty that arises because of natural variability in formation properties in addition to the measurement uncertainties we quantify the measure of information in terms of entropy and show the impact of additional data about s wave velocity on the uncertainty of the hydrocarbon indicator in some cases noisy s data along with noisy p data can convey more information than perfect p data alone while in other cases s data do not reduce the uncertainty we apply the formulation to a well log example for detecting the most likely pore fluid and quantifying the associated uncertainty from observed sonic and density logs the formulation offers a convenient way to implement deterministic fluid substitution equations in the realistic case when natural geologic variations cause the reference porosity and velocity to span a range of values

large scale mantle convection and the history of subduction a large scale pattern of mantle convention is evident in the shape of the earth s gravity field1 lateral variations in seismic velocity2 3 the distribution of hotspots4 and long wavelength bathv metric anomalies5 paradoxically these signals of mantle convection bear little resemblance to present day plate motions1 4 although they seem to be related to past subduction zone configurations6 here we quantify this relationship by correlating a time integrated history of subduction calculated in the hotspot reference frame for the past 180 myr with regions of the deep mantle inferred from other geophysical observations to be colder than average our results support suggestions6 9 that a direct relationship exists between large scale thermal heterogeneity in the lower mantle and subduction during the cenozoic and mesozoic

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

rock properties of smectite and ooze rich claystones abstractparameters measured by well logs define rock properties and seismic reflections at lithology interfaces parameters from standard embedding claystones are normally used when calculating the top sand amplitude variation with offset avo response but this might give erroneous results when the real claystone rock properties deviate from the standard using cuttings and high quality wireline logs from well 34 8 a 33 h above the visund field in the northern north sea we evaluated rock properties of the pleistocene glaciomarine claystones lower miocene and upper oligocene oozy claystones lower oligocene and eocene smectite rich claystones and two interbedded sands glaciomarine claystones fit best with the greenberg castagna equation and have the highest measured velocities even though they are the shallowest buried sediments environmental scanning electron microscope analysis proves the lower miocene and upper oligocene claystones to be oozy the amount of low density oozy material causes signif

plate kinematics of the south atlantic chron c34 to present a high resolution seafloor spreading history of the south atlantic since chron c34 is constrained by a combination of seasat altimeter data and underway marine geophysical data a set of 45 finite rotation poles defines the relative position of africa and south america at roughly 2 m y intervals a set of 12 stage poles constrain the relative motion of these two plates at 5 to 10 m y intervals the position of the stage poles continuously migrates reflecting the continuously changing azimuths of the fracture zones major changes in spreading direction are observed in the late cretaceous and early cenozoic as the fracture zones sweep out broad s shaped curves similar to the pattern seen on the kane fracture zone in the central atlantic small offset fracture zones were found to be the most accurate recorders of changes in plate motion large offset fracture zones such as the agulhas falkland fracture zone were the least reliable recorders at 30 s spreading rates decrease throughout the late cretaceous from a high of 75 mm yr at the end of chron c34 to a low of 30 mm yr around chron c27 a period of slow spreading between chron c30 and chron c20 corresponds to a zone of fracture zone proliferation an increase in the amplitude of geoid anomalies over fracture zones greater relief on topographic profiles and locally evidence of intraplate crustal deformation spreading rates increase at chron c20 to about 50 mm yr and then gradually decrease during the late paleogene and neogene a comparison of synthetic fracture zones based on the south atlantic stage poles to the observed trends of fracture zones in the equatorial atlantic indicates that the vema and marathon fracture zones were generated by south atlantic spreading as opposed to central atlantic spreading at least as far back as 35 m y ago and possibly 50 m y ago

quantifying subresolution saturation scales from time lapse seismic data a reservoir monitoring case study the goal of this paper is to interpret and analyze time lapse seismic data quantitatively to better understand subsurface fluid saturations and saturation scales we present a case study of a time lapse seismic survey water and gas were injected into an oil producing reservoir and repeat seismic surveys were collected to monitor the subsurface fluids over a period of 14 years in this study we show that the subresolution spatial distribution of fluids not captured by traditional flow simulators can impact the seismic response although there is a good qualitative match between the fluid changes predicted by the flow simulator and the fluid changes interpreted from the seismic data the simulator predicts smooth saturation profiles that do not quantitatively match the time lapse seismic changes we find that downscaling smooth saturation outputs from the flow simulator to a more realistic patchy distribution is required to provide a good quantitative match with the near and far offset time lapse data

ocean basin evolution and global scale plate reorganization events since pangea breakup we present a revised global plate motion model with continuously closing plate boundaries ranging from the triassic at 230 ma to the present day assess differences among alternative absolute plate motion models and review global tectonic events relatively high mean absolute plate motion rates of approximately 9 10 cm yr 1 between 140 and 120 ma may be related to transient plate motion accelerations driven by the successive emplacement of a sequence of large igneous provinces during that time an event at 100 ma is most clearly expressed in the indian ocean and may reflect the initiation of andean style subduction along southern continental eurasia whereas an acceleration at 80 ma of mean rates from 6 to 8 cm yr 1 reflects the initial northward acceleration of india and simultaneous speedups of plates in the pacific an event at 50 ma expressed in relative and some absolute plate motion changes around the globe and in a reduction of global mean plate speeds from about 6 to 4 5 cm yr 1 indicates that an increase in collisional forces such as the india eurasia collision and ridge subduction events in the pacific such as the izanagi pacific ridge play a significant role in modulating plate velocities

dynamic topography of passive continental margins and their hinterlands since the cretaceous abstract even though it is well accepted that the earth s surface topography has been affected by mantle convection induced dynamic topography its magnitude and time dependence remain controversial the dynamic influence to topographic change along continental margins is particularly difficult to unravel because their stratigraphic record is dominated by tectonic subsidence caused by rifting we follow a three fold approach to estimate dynamic topographic change along passive margins based on a set of seven global mantle convection models we first demonstrate that a geodynamic forward model that includes adiabatic and viscous heating in addition to internal heating from radiogenic sources and a mantle viscosity profile with a gradual increase in viscosity below the mantle transition zone provides a greatly improved match to the spectral range of residual topography end members as compared with previous models at very long wavelengths spherical degrees 2 3 we then combine global sea level estimates with predicted surface dynamic topography to evaluate the match between predicted continental flooding patterns and published paleo coastlines by comparing predicted versus geologically reconstructed land fractions and spatial overlaps of flooded regions for individual continents since 140 ma modelled versus geologically reconstructed land fractions match within 10 for most models and the spatial overlaps of inundated regions are mostly between 85 and 100 for the cenozoic dropping to about 75 100 in the cretaceous regions that have been strongly affected by mantle plumes are generally not captured well in our models as plumes are suppressed in most of them and our models with dynamically evolving plumes do not replicate the location and timing of observed plume products we categorise the evolution of modelled dynamic topography in both continental interiors and along passive margins using cluster analysis to investigate how clusters of similar dynamic topography time series are distributed spatially a subdivision of four clusters is found to best reveal end members of dynamic topography evolution along passive margins and their hinterlands differentiating topographic stability long term pronounced subsidence initial stability over a dynamic high followed by moderate subsidence and regions that are relatively proximal to subduction zones with varied dynamic topography histories along passive continental margins the most commonly observed process is a gradual motion from dynamic highs towards lows during the fragmentation of pangea reflecting the location of many passive margins now over slabs sinking in the lower mantle our best fit model results in up to 500 150 m of total dynamic subsidence of continental interiors while along passive margins the maximum predicted dynamic topographic change over 140 million years is about 350 150 m of subsidence models with plumes exhibit clusters of transient passive margin uplift of about 200 200 m but are mainly characterised by long term subsidence of up to 400 m the good overall match between predicted dynamic topography to geologically mapped paleo coastlines makes a convincing case that mantle driven topographic change is a critical component of relative sea level change and indeed the main driving force for generating the observed geometries and timings of large scale continental inundation through time

mapping lithofacies and pore fluid probabilities in a north sea reservoir seismic inversions and statistical rock physics reliably predicting lithologic and saturation heterogeneities is one of the key problems in reservoir characterization in this study we show how statistical rock physics techniques combined with seismic information can be used to classify reservoir lithologies and pore fluids one of the innovations was to use a seismic impedance attribute related to the vp vs ratio that incorporates far offset data but at the same time can be practically obtained using normal incidence inversion algorithms the methods were applied to a north sea turbidite system we incorporated well log measurements with calibration from core data to estimate the near offset and far offset reflectivity and impedance attributes multivariate probability distributions were estimated from the data to identify the attribute clusters and their separability for different facies and fluid saturations a training data was set up using monte carlo simulations based on the well log derived probability distributions fluid substitution by ga

rock physics based relations for density and s velocity versus p velocity in deepwater subsalt gulf of mexico shales which is commonly used to estimate s wave velocity s from p wave velocity p where the units of velocity are km s and the empirical relations for estimating the shear wave velocity from the compressional wave velocity given by greenberg and castagna 1992 figure 1 compares the relationship between density and sonic velocity for two deepwater subsalt wells in the green canyon area of the gulf of mexico to gardner s predic tion assuming values for parameters a and b given by castagna et al 1993 for sands and shales while the gardner estimate is reasonable for the sands the shales at any given density indicate a significantly lower velocity than that predicted by gardner s relation similarly figure 2 compares p wave versus s wave ve locity data for the same two wells to the prediction of equation 2 and of the greenberg castagna relation for shales it is seen that these relations tend to overestimate s wave velocity for the shales at a given p wave velocity in this paper these discrepancies are investigated using a rock physics model this model not only gives a good descrip tion of the data but also allows the effective pore aspect ratio to be estimated rock physics model to model the elastic moduli of shales the shales were considered as a mixture of clay and quartz with properties taken as those given by carmichael 1989 for quartz and gulf clays han et al 1986 the properties used are listed in table 1 pore spaces were assumed filled by brine having a bulk mod ulus of 3 4 gpa appropriate for the pressure and temperature in the depth range of interest since quartz grains in shales are embedded within a softer continuous clay matrix the elastic moduli of the solid phase were estimated using the hashin sh trikman lower bound hashin and shtrikman 1963 the up per and lower hashin shtrikman bounds for the elastic moduli in table 1 are plotted in figure 3

model based shear wave velocity estimation versus empirical regressions modelling of avo signatures for reservoir characterization requires v s estimation from other available logs when shear wave data are not available we tested various models for predicting v s from p wave velocity porosity and shale volume measured in well logs effective medium models which characterize the pore space in terms of ellipsoidal inclusions were compared with statistical v p v s regressions the inclusion models were calibrated by non linear minimization of the difference between model predicted velocities and actual measured velocities the quality of the v s prediction was quantified in terms of the rms error by comparison with shear wave data in wells where both v p and v s were measured the linear regressions were found to be more robust and the rms error in the prediction was comparable to effective medium model based predictions

compressional and shear wave velocities from gas hydrate bearing sediments examples from the india and cascadia margins as well as arctic permafrost regions abstract shear wave velocity data have been acquired at several marine gas hydrate drilling expeditions including the india national gas hydrate program expedition 1 nghp 01 the ocean drilling program odp leg 204 and integrated ocean drilling program iodp expedition 311 x311 in this study we use data from these marine drilling expeditions to develop an understanding of general grain size control on the p and s wave properties of sediments a clear difference in the downhole trends of p wave vp and s wave vs velocity and the vp vs ratio from all three marine regions was observed the northern cascadia margin iodp x311 shows the highest p wave and s wave velocity values overall and those from the india margin expedition nghp 01 are the lowest the southern cascadia margin odp leg 204 appears to have similar low p wave and s wave velocity values as seen off india s wave velocity values increase relative to the sites off india but they are not as high as those seen on the northern cascadia margin such regional differences can be explained by the amount of silt sand or lack thereof occurring at these sites with northern cascadia being the region of the highest silt sand occurrences this grain size control on p wave and s wave velocity and associated mineral composition differences is amplified when compared to the arctic permafrost environments where gas hydrate predominantly occurs in sand and silt dominated formations using a cross plot of gamma ray values versus the vp vs ratio we compare the marine gas hydrate occurrences in these regions offshore eastern india margin offshore cascadia margin the ignik sikumi site in alaska and the mallik 5l 38 site in the mackenzie delta the log data from the arctic permafrost regions show a strongly linear vp vs relationship similar to the previously defined empirical relationships by greenberg and castagna 1992 p and s wave velocity data from the india margin and odp leg 204 deviate strongly from these linear trends whereas data from iodp x311 plot closer to the trend of the arctic data sets and previously published relationships three new linear relationships for different grain size marine sediment hosts are suggested a mud dominated mahanadi basin odp leg 204 nghp 01 17 vs 1 5854 vp 2 1649 b silty mud kg basin vs 0 8105 vp 1 0223 c silty sand iodp x311 vs 0 5316 vp 0 4916 we investigate the relationship of gas hydrate saturation determined from electrical resistivity on the vp vs ratio and found that the sand dominated arctic hosts show a clearly decreasing trend of vp vs ratio with gas hydrate saturation though limited due to lower overall gh saturations a similar trend is seen for sites from iodp x311 and at the ash dominated nghp 01 17 sediment in the andaman sea gas hydrate that occurs predominantly in fractured clay hosts show a different trend where the vp vs ratio is much higher than at sand dominated sites and remains constant or increases slightly with increasing gas hydrate saturation this trend may be the result of anisotropy in fracture dominated systems where p and s wave velocities appear higher and archie based saturations of gas hydrate are overestimated gas hydrate concentrations were also estimated in these three marine settings and at arctic sites using an effective medium model combining p and s wave velocities as equally weighted constraints on the calculation the effective medium approach generally overestimates s wave velocity in high porosity clay dominated sediments but can be accurately used in sand rich formations

predicting trench and plate motion from the dynamics of a strong slab the motion of oceanic plates is commonly related to the subduction of cold and dense oceanic material into the mantle these models predict plate velocities from subduction velocities but the trench motion is not directly included in the computation here using a recent compilation of a global data set we found that the motion of trenches either advancing or retreating with respect to upper plates scales with their corresponding subducting plates motion based on simple experimental tests we found that subduction of strong slabs inside the upper mantle correctly predicts these kinematic relationships we deduce that the motion of the trenches represent the surface manifestation of the resistance encountered by the subducting lithosphere to bend and penetrate within the upper mantle

constraining south atlantic growth with seafloor spreading data recent models of south atlantic opening history focus on early plate divergence by incorporating intracontinental deformation which is poorly constrained aiming to avoid the uncertainties in this approach we model the entire divergence history with a joint inversion for seafloor spreading data for this history the pre campanian motion parameters are the first to feature formal uncertainty estimates we date the onset of spreading at 138 ma with movement along intracontinental accommodation zones leading to the assembly of south america by 123 ma and africa by 106 ma part of the ridge in the agulhas basin jumped westward soon afterward toward the bouvet plume initiating the motion of a short lived malvinas plate the ne georgia and maud rises and agulhas plateau formed as a large igneous province over the plume farther north part of the ridge jumped eastward toward the tristan plume around 94 93 ma but seems not to have resulted in independent plate motion our results show that the south atlantic grew by diachronous breakup of continents on just two plates cretaceous intracontinental deformation in south america and africa can be interpreted in terms of the accommodation of stress associated with northward propagation of this process the pattern of accommodation is usually envisaged as focusing all of the strain in narrow belts with our rotations a commonly used set of such belts accounts instead for just 42 67 of the implied total strain we suggest that the remainder was accommodated at all scales within the continental interiors and the extended continental margins

comparison of avo indicators a modeling study a worldwide collection of 25 sets of velocity and density measurements from adjacent shales brine sands and gas sands was acquired with full waveform sonic dipole sonic and conventional well logging devices and or in the laboratory these data provide theoretical shale over brine sand and shale over gas sand p wave and s wave normal incidence reflection coefficients r p and r s avo intercepts a avo gradients b the avo indicators r p r s reflection coefficient difference and a b avo product the reflection coefficient difference is found to be a more universal indicator than the avo product in clastic stratigraphic intervals for shale over brine sand re flections the average r p r s tends to be near zero and relatively invariant with depth irrespective of gas sand impedance r p r s is always negative for shale over reservoir quality gas sand reflections and more negative than for the corresponding brine sand reflections in comparison the avo product may be positive near zero or negative for gas sands depend ing on the impedance contrast with the overlying shale these measurements also verify that r p r s is well approximated by a simple linear combination of a and b

rifted margins ductile deformation boudinage continentward dipping normal faults and the role of the weak lower crust the stunningly increased resolution of the deep crustal levels in recent industrial seismic profiles acquired along most of the world s rifted margins leads to the unraveling of an unexpected variety of structures it provides unprecedented access to the processes occurring in the middle and lower continental crust we present a series of so far unreleased profiles that allows the identification of various rift related geological processes such as crustal boudinage ductile shear and low angle detachment faulting and a rifting history that differs from the classical models of oceanward dipping normal faults the lower crust in rifted margins appears much more intensely deformed than usually represented at the foot of both magma rich and magma poor margins we observe clear indications of ductile deformation of the deep continental crust along large scale shallow dipping shear zones these shear zones generally show a top to the continent sense of shear consistent with the activity of continentward dipping normal faults cdnf observed in the upper crust this pattern is responsible for a migration of the deformation and associated sedimentation and or volcanic activity toward the ocean we discuss the origin of these cdnf and investigate their implications and the effect of sediment thermal blanketing on crustal rheology in some cases low angle shear zones define an anastomosed pattern that delineates boudin like structures the maximum deformation is localized in the inter boudin areas the upper crust is intensely boudinaged and the highly deformed lower crust fills the inter boudins underneath the boudinage pattern controls the position and dip of upper crustal normal faults we present some of the most striking examples from the margins of uruguay west africa south china sea and barents sea and discuss their implications for the time temperature history of the margins

plate kinematics of the rocas verdes basin and patagonian orocline the processes of orocline formation are a topic of debate in geosciences the patagonian orocline has been a case in point for over a century large anomalous paleomagnetic pole rotations show that the orocline started to form at the same time as mid cretaceous closure of the rocas verdes basin today known from ophiolitic and basin fill remnants in the patagonian and fuegian andes some studies therefore present bending of the andes and closure of the basin as shared consequences of rotation of a small plate that was driven by subduction related forces at the pacific margin of gondwana an alternative view of the orocline is as a product of cretaceous to paleogene aged sinistral oblique convergence at the plate boundary scale geological data from tierra del fuego have been interpreted in support of both views here i test these suggestions by comparing the rocas verdes basin s tectonostratigraphy to predictions of a plate kinematic model for fragmentation of the western interior of gondwana the model is sufficient to explain the known history of basin opening to a width of 100 300 km during the period 152 141 ma and later closure in oblique plate convergence as this convergence occurred by motion around a distant euler pole it could not have produced the patagonian orocline by rotation of a lithospheric plate on its pacific flank the large anomalous paleomagnetic rotations of tierra del fuego instead are likely to have occurred within the crust by rotation and deformation of regional strike slip faults and the intervening rocks to accommodate oblique convergence of the south american and antarctic plates between albian and paleocene times

seismic imaging of complex geometry forward modeling of sandstone intrusions abstract sandstone intrusions form large bedding discordant sandstones that intruded into finer grained less permeable host strata they form naturally sand propped hydraulic fractures that constitute a connected network of permeable conduits through which fluids escape to the earth s surface saucer shaped sandstone intrusions are among the largest volume intrusions and are commonly resolved on seismic data outcrop analogues of seismically resolved saucers shaped intrusions reveal that many attendant intrusions in particular dikes are undetected in seismic data seismic forward modeling of a detailed outcrop description of a saucer shaped intrusion demonstrates that intrusions steeper than 45 are undetected and that up to 40 of the entire volume of sandstone intrusions is not seismically imaged wedge geometry associated with discordant contacts between different lithologies causes constructive and destructive amplitude interference creating imaging artefacts of sandstone thickness and geometry comparison of the outcrop seismic models with 3d seismic data from volund oilfield demonstrate both the similarity of the saucer shaped intrusions and the distribution and quantity of dikes that may be undetected ca 78 using subsurface data lack of detection of dikes has direct implications on the valuation of upward migration of fluids and an overestimation of seal capacity this therefore has major implications when using seismic data to evaluate waste sequestration or to execute hydrocarbon or groundwater exploration and production

yet another vs equation the classical raymer hunt gardner functional form vp 1 2 vps vpf where vps and vpf denote the p wave velocity in the solid and pore fluid phases respectively and is the total porosity can also be used to relate the s wave velocity in dry rock to porosity and mineralogy as vsdry 1 2 vss where vss is the s wave velocity in the solid phase assuming that the shear modulus of rock does not depend on the pore fluid vs in wet rock is vswet vsdry bdry bwet where bdry and bwet denote the bulk density of the dry and wet rock respectively this new functional form for vs prediction reiterates nur s critical porosity concept the vp vs ratio in dry rock equals that in the solid phase it accurately predicts vs in consolidated clastic and carbonate rock two motivations for using it are 1 it is simple and 2 it predicts vs not from vp but directly from and mineralogy

vp vs relationship and amplitude variation with offset modelling of glauconitic greensand the relationship between vp and vs may be used to predict vs where only vp is known vp vs is also used to identify pore fluids from seismic data and amplitude variation with offset analysis theoretical physical as well as statistical empirical vp vs relationships have been proposed for reservoir characterization when shear wave data are not available in published work the focus is primarily on the vp vs relationship of quartzitic sandstone in order to broaden the picture we present vp vs relationships of greensand composed of quartz and glauconite by using data from the paleocene greensand nini oil field in the north sea a vp vs relationship derived from modelling is compared with empirical vp vs regressions from laboratory data as well as from log data the accuracy of vs prediction is quantified in terms of root mean square error we find that the vp vs relationship derived from modelling works well for greensand shear wave velocity prediction we model the seismic response of glauconitic greensand by using laboratory data from the nini field our studies here reveal that brine saturated glauconitic greensand can have a similar seismic response to that from oil saturated quartzitic sandstone and that oil saturated strongly cemented greensand can have a similar amplitude variation with offset response to that from brine saturated weakly cemented greensand

long wavelength convection poiseuille couette flow in the low viscosity asthenosphere and the strength of plate margins summary mixed heated 3 d mantle convection simulations with a low viscosity asthenosphere reveal relatively short and long wavelength regimes with different scalings in terms of surface velocity and surface heat flux and show that mantle flow in the lithosphere asthenosphere region is a poiseuille couette flow the poiseuille couette velocity magnitude ratio d u allows us to characterize solid state flow in the asthenosphere and to predict the regime transition the transition from dominantly pressure driven poiseuille flow at shorter wavelengths to dominantly shear driven couette flow at long wavelengths depends on the relative strength of lithosphere and asthenosphere and is associated with a switch in the dominant resistance to convective motion in the poiseuille regime significant resistance is provided by plate bending whereas in the couette regime most resistance is due to vertical shear in the bulk mantle the couette case corresponds to classical scaling ideas for mantle convection whereas the poiseuille case with asthenospheric velocities exceeding surface velocities is an example of a sluggish lid mode of mantle convection that has more recently been invoked for thermal history models of the earth our simulations show that both modes can exist for the same level of convective vigour i e rayleigh number but at different convective wavelengths additional simulations with temperature and yield stress dependent viscosity show consistent behaviour and suggest an association of the regime crossover with the relative strength of plate margins our simulations establish a connection between the strength of plate margins solid state flow in the asthenosphere and the wavelength of mantle convection this connection suggests that plate tectonics in the sluggish lid mode is wavelength dependent and potentially more robust than previously envisioned

effects of porosity and clay content on wave velocities in sandstones the ultrasonic compressional vp and shear vs velocities and first arrival peak amplitude ap were measured as functions of differential pressure to 50 mpa and to a state of saturation on 75 different sandstone samples with porosities ranging from 2 to 30 percent and volume clay content c ranging from 0 to 50 percent respectively both vp and vs were found to correlate linearly with porosity and clay content in shaly sandstones at confining pressure of 40 mpa and pore pressure of 1 0 mpa the best least squares fits to the velocity data are vp km s 5 59 6 93 2 18c and vs km s 3 52 4 91 1 89c deviations from these equations are less than 3 percent and 5 percent for vp and vs respectively the velocities of clean sandstones are significantly higher than those predicted by the above linear fits about 7 percent for vp and 11 percent for vs which indicates that a very small amount of clay 1 or a few percent of volume fraction significantly reduces the elastic moduli of sandstones for shaly

shear wave velocity estimation in porous rocks theoretical formulation preliminary verification and applications shear wave velocity logs are useful for various seismic interpretation applications including bright spot analyses amplitude versus offset analyses and multicomponent seismic interpretations measured shear wave velocity logs are however often unavailable we developed a general method to predict shear wave velocity in porous rocks if reliable compressional wave velocity lithology porosity and water saturation data are available the precision and accuracy of shear wave velocity prediction are 9 and 3 respectively the success of our method depends on 1 robust relationships between compressional and shear wave velocities for water saturated pure porous lithologies 2 nearly linear mixing laws for solid rock constituents 3 first order applicability of the biot gassmann theory to real rocks we verified these concepts with laboratory measurements and full waveform sonic logs shear wave velocities estimated by our method can improve formation evaluation our method has been successfully tested with data from several locations

estimation of pressure saturation changes for unconsolidated reservoir rocks with high vp vs ratio abstractwe find that conventional time lapse seismic pressure saturation discrimination methods become unstable for high vp vs ratios using first order approximations in the amplitude variation with offset avo gradient and intercept changes and in the rock physics models increase the inaccuracy in pressure saturation changes estimates we propose a new method based on a stepwise linear approximation to the intercept and gradient reflectivity changes to estimate pressure and saturation changes the applicability of the new method is tested on synthetic data over a range of 0 50 gas saturation and 0 3 5 mpa pore pressure changes the new method is more consistent and provides better estimates compared with conventional methods in the presence of random noise up to 15 the estimates are reasonably better for the new method we use this new method to investigate the feasibility of pressure saturation discrimination for a shallow unconsolidated sand reservoir in which an underground blowout occurred

an integrated target oriented prestack elastic waveform inversion sensitivity calibration and application an integrated target oriented prestack elastic wave form inversion procedure is implemented to estimate lithology fluid type and pay thickness of a layered target zone from surface seismic pp reflection data the integrated inversion procedure consists of three parts data processing and preparation iterative seis mic inversion and rock property integration in the data processing and preparation the seismic field gathers are processed and transformed into plane wave seismograms and the well logs are prepared for establishing a well tie the well tie is established using synthetics from a layered model obtained from block ing sonic shear sonic and density logs the layered model is later used as the a priori starting model for the iterative seismic inversion based on the a priori starting model and using the processed plane wave seismograms as input p wave velocity vp s wave velocity vs density p and thickness h of each layer in the layered target zone are estimated at each cdp location in the iterative seismic inversion

rapid south atlantic spreading changes and coeval vertical motion in surrounding continents evidence for temporal changes of pressure driven upper mantle flow the south atlantic region displays 1 a topographic gradient across the basin with africa elevated relative to south america 2 a bimodal spreading history with fast spreading rates in late cretaceous and eo oligocene and 3 episodic regional uplift events in the adjacent continents concentrated in late cretaceous and oligocene here we show that these observations can be linked by dynamic processes within earth s mantle through temporal changes in asthenosphere flow beneath the region the topographic gradient implies westward pressure driven mantle flow beneath the basin while the rapid spreading rate changes on order 10 million years require significant decoupling of regional plate motion from the large scale mantle buoyancy distribution through a mechanically weak asthenosphere andean topographic growth in late miocene can explain the most recent south atlantic spreading velocity reduction arising from increased plate boundary forcing associated with the newly elevated topography but this mechanism is unlikely to explain the late cretaceous tertiary spreading variations as changes in andean paleoelevation at the time are small we propose an unsteady pressure driven flow component in the asthenosphere beneath the south atlantic region to explain the late cretaceous tertiary spreading rate variations temporal changes in mantle flow due to temporal changes in regional mantle pressure gradients imply a correlation of horizontal and vertical motions we find that this prediction from our models agrees with geologic and geophysical observations of the south atlantic region including episodes of passive margin uplift regional basin reactivation and magmatic activity

cretaceous to present kinematics of the indian african and seychelles plates an iterative inverse model of seafloor spreading data from the mascarene and madagascar basins and the flanks of the carlsberg ridge describes a continuous history of indian african plate divergence since 84 ma visual fit modelling of conjugate magnetic anomaly data from near the seychelles platform and laxmi ridge documents rapid rotation of a seychelles plate about a nearby euler pole in palaeocene times as the euler pole migrated during this rotation the amirante trench on the western side of the plate accommodated first convergence and later divergence with the african plate the unusual present day morphology of the amirante trench and neighbouring amirante banks can be related to crustal thickening by thrusting and folding during the convergent phase and the subsequent development of a spreading centre with a median valley during the divergent phase the model fits fracture zone trends in the north arabian and east somali basins suggesting that they formed in india africa plate divergence seafloor fabric in and between the basins shows that they initially hosted a segmented spreading ridge that accommodated slow plate divergence until 71 69 ma and that upon arrival of the deccan re union plume and an increase to faster plate divergence rates in the period 69 65 ma segments of the ridge lengthened and propagated ridge propagation into the indian continental margin led first to the formation of the laxmi basin which accompanied extensive volcanism onshore at the deccan traps and offshore at the saurashtra high and somnath ridge a second propagation episode initiated the ancestral carlsberg ridge at which seychelles india and india africa plate motions were accommodated with the completion of this propagation the plate boundaries in the mascarene basin were abandoned seafloor spreading between this time and the present has been accommodated solely at the carlsberg ridge

the cretaceous opening of the south atlantic ocean abstract the separation of south america from africa during the cretaceous is poorly understood due to the long period of stable polarity of the geomagnetic field the cretaceous normal superchron cns lasted between 121 and 83 6 myr ago we present a new identification of magnetic anomalies located within the southern south atlantic magnetic quiet zones that have arisen due to past variations in the strength of the dipolar geomagnetic field using these anomalies together with fracture zone locations we calculate the first set of magnetic anomalies based finite rotation parameters for south america and africa during that period the kinematic solutions are generally consistent with fracture zone traces and magnetic anomalies outside the area used to construct them the rotations indicate that seafloor spreading rates increased steadily throughout most of the cretaceous and decreased sharply at around 80 myr ago a change in plate motion took place in the middle of the superchron roughly 100 myr ago around the time of the final breakup i e separation of continental oceanic boundary in the equatorial atlantic prominent misfit between the calculated synthetic flowlines older than anomaly q1 and the fracture zones straddling the african plate in the central south atlantic could only be explained by a combination of seafloor asymmetry and internal dextral motion

compaction rock physics and rock properties of sandstones of the sto formation case study of five wells from the south western barents sea norway abstract five wells containing lower middle jurassic sandstones of sto formation from the hammerfest basin 7120 9 1 7121 7 1 the ringvassoy loppa fault complex 7119 12 1 7119 12 4 and the troms finnmark fault complex 7019 1 1 in the barents sea area are considered in this study the sto formation sandstones contain dominantly very fine to medium grained quartz arenites with occasional coarse grained sandstone layers feldspathic and quartz wackes are also present the effect of compaction and exhumation on reservoir properties porosity and permeability and seismic property p wave velocity of these sandstones have been investigated source of quartz cement has also been investigated forty polished thin sections embedded in blue epoxy were studied using optical microscopy scanning electron microscopy and cathodoluminiscence bulk mineralogy was also analysed using x ray diffraction the studied sandstones have experienced cenozoic exhumation ranging between 820 and 1050 m p wave velocity is higher porosities and permeabilities are lower in the western wells 7019 1 1 7119 12 1 and 7119 12 4 compared to the eastern wells 7120 9 1 and 7121 7 1 rock physics models and diagnostics show that the western wells are diagenetically more mature stiffer more compacted and more cemented than the eastern wells these trends are attributed largely to difference in burial history from the east to the west and less to textural variations quartz cement is the most important authigenic mineral in these sandstones quartz cement in the western well 7119 12 1 is predominantly derived from clay induced dissolution at macrostylolites whereas the eastern wells 7120 9 1 and 7121 7 1 are mostly sourced from clay induced dissolution at grain contacts or microstylolites while cementational porosity loss dominates in the western wells compactional porosity loss dominates in the eastern wells compaction can reduce porosities down to 26 and this might be the reason for better porosity preservation and reservoir quality in the eastern wells than in the western wells

reconciling strong slab pull and weak plate bending the plate motion constraint on the strength of mantle slabs although subducting slabs undergo a bending deformation that resists tectonic plate motions the magnitude of this resistance is not known because of poor constraints on slab strength however because slab bending slows the relatively rapid motions of oceanic plates observed plate motions constrain the importance of bending we estimated the slab pull force and the bending resistance globally for 207 subduction zone transects using new measurements of the bending curvature determined from slab seismicity predicting plate motions using a global mantle flow model we constrain the viscosity of the bending slab to be at most similar to 300 times more viscous than the upper mantle stronger slabs are intolerably slowed by the bending deformation weaker slabs however cannot transmit a pull force sufficient to explain rapid trenchward plate motions unless slabs stretch faster than seismically observed rates of similar to 10 15 s 1 the constrained bending viscosity similar to 2 x 10 23 pa s is larger than previous estimates that yielded similar or larger bending resistance here similar to 25 of forces this apparent discrepancy occurs because slabs bend more gently than previously thought with an average radius of curvature of 390 km that permits subduction of strong slabs this gentle bending may ultimately permit plate tectonics on earth c 2008 elsevier b v all rights reserved

calibration of prestack simultaneous impedance inversion using rock physics this paper represents the third installment in a sequence of reservoir characterization articles that follow a workflow whose ultimate goal is to understand and predict the rock properties of a reservoir away from well control the first article singleton 2008 described a method to detect anomalous seismic attenuation this was important because seismic data at the well location have a steep falloff in amplitude caused by high attenuation in and above the reservoir interval the second article singleton 2009 dealt with the seismic gathers that were input into the inversion process it is of the utmost importance that the quality of data input into a simultaneous prestack inversion be as high as possible errors in seismic gathers low snr resolution loss from excessive nmo stretch nonflat reflectors offset amplitude problems multiples etc are broadcast straight into the inversion impedance volumes so this is a critical step in geophysical reservoir characterization

uncertainty in the breakup spreading history and velocity variations of gondwana abstract there are three major uncertainties in the derivation of plate velocities in general firstly the errors associated with the identification of isochrons and fitting conjugate isochrons secondly various geological choices made in the model that represent the model creator s view of when break up occurred and how tight the fit was and whether certain isochrons should be part of the reconstruction or not this i term model uncertainty finally the choice of timescale while much of the geoscientific community have adopted recent timescale standards such as gts16 the smaller plate kinematic community continue to use legacy timescales typically a composite of kg85 and cent94 in this paper i examine these second and third source of uncertainty and show that they can be significant sources of noise in the signal of plate velocities frequently over 20 of the signal and often as high as 60 for those seeking to connect plate kinematics vertical motions and the mantle dynamics knowing the uncertainties in the kinematic model used is essential

laboratory measurements of porosity permeability resistivity and velocity on fontainebleau sandstones the relations among the resistivity elastic wave velocity porosity and permeability in fontainebleau sandstone samples from the ile de france region around paris france were experimentally revisited these samples followed a permeability porosity relation given by kozeny carman s equation for the resistivity measurements the samples were partially saturated with brine archie s equation was used to estimate resistivity at 100 water saturation assuming a saturation exponent n 2 using self consistent sc approximations modeling with grain aspect ratio 1 and pore aspect ratio between 0 02 and 0 10 the experimental data fall into this theoretical range the sc curve with the pore aspect ratio 0 05 appears to be close to the values measured in the entire porosity range the elastic wave velocity was measured on these dry samples for confining pressure between 0 and 40 mpa aloading and unloading cycle was used and did not produce any significant hysteresis in the velocity pressure behavior for the velocity data using the sc model with a grain aspect ratio 1 and pore aspect ratios 0 2 0 1 and 0 05 fit the data at 40 mpa pore aspect ratios ranging between 0 1 0 05 and 0 02 were a better fit for the data at 0 mpa both velocity and resistivity in clean sandstones can be modeled using the sc approximation in addition a linear fit was found between the p wave velocity and the decimal logarithm of the normalized resistivity with deviations that correlate with differences in permeability combining the stiff sand model and archie for cementation exponents between 1 6 and 2 1 resistivity was modeled as a function of p wave velocity for these clean sandstones

practical seismic petrophysics the effective use of log data for seismic analysis a quick scan of the seg web site shows that the phrase seismic petrophysics has been used explicitly in the title of a paper or abstract six times the earliest of which was by williams et al in 1996 the hugoton cross well survey a direct look at stratigraphy seismic petrophysics and shale anisotropy however the first attempt at a definition and an expanded description of seismic petrophysics was published by wayne pennington in the leading edge in 1997 seismic petrophysics an applied science for reservoir geophysics he defines seismic petrophysics as follows the purposeful application of rock physics theory as calibrated by laboratory and well measurements to the interpretation of seismic data

relationships between compressional wave and shear wave velocities in clastic silicate rocks new velocity data in addition to literature data derived from sonic log seismic and laboratory measurements are analyzed for clastic silicate rocks these data demonstrate simple systematic relationships between compressional and shear wave velocities for water saturated clastic silicate rocks shear wave veloci ty is approximately linearly related to compressional wave velocity and the compressional to shear velocity ratio decreases with increasing compressional velocity laboratory data for dry sandstones indicate a nearly constant compressional to shear velocity ratio with rigidity approximately equal to bulk modulus ideal models for regular packings of spheres and cracked solids exhibit behavior similar to the observed water saturated and dry trends for dry rigidity equal to dry bulk modulus gassmann s equations predict velocities in close agreement with data from the water saturated

south atlantic opening a plume induced breakup upwelling hot mantle plumes are thought to disintegrate continental lithosphere and are considered to be drivers of active continental breakup the formation of the walvis ridge during the opening of the south atlantic is related to a putative plume induced breakup we investigated the crustal structure of the walvis ridge southeast atlantic ocean at its intersection with the continental margin and searched for anomalies related to the possible plume head the overall structure we identify suggests that no broad plume head existed during opening of the south atlantic and anomalous mantle melting occurred only locally we therefore question the importance of a plume head as a driver of continental breakup and further speculate that the hotspot was present before the rifting leaving a track of kimberlites in the african craton

feedback between mountain belt growth and plate convergence while it is generally assumed that global plate motions are driven by the pattern of convection in the earth s mantle the details of that link remain obscure bouyancy forces associated with subduction of cool dense lithosphere at zones of plate convergence are thought to provide significant driving force but the relative magnitudes of other driving and resisting forces are less clear as are the main factors controlling long term changes in plate motion the ability to consider past as well as present plate motions provides significant additional constraints because changes in plate motion are necessarily driven by changes in one or more driving or resisting forces which may be inferred from independent data here we present for the first time a model that explicitly links global mantle convection and lithosphere models to infer plate motion changes as far back as miocene time by accurately predicting observed convergence rates over the past 10 m y we demonstrate that surface topography generated at convergent margins is a key factor controlling the long term evolution of plate motion specifically the topographic load of large mountain belts and plateaus consumes a significant amount of the driving force available for plate tectonics by increasing frictional forces between downgoing and overriding plates

the strength of large scale plate boundaries constraints from the dynamics of the philippine sea plate since 5ma abstract friction controls most of the strength of tectonic plate boundaries and thus the force mutually exchanged between plates estimates of the plate boundary friction coefficient are therefore of paramount importance to our understanding of the lithosphere torque balance however several lines of evidence indicate that the friction coefficient of plate margins is significantly lower than is measured in laboratory experiments on fractured rocks this poses a significant limit to extrapolation from the laboratory scale to the large scale of earth s lithosphere the record of past and present day plate motions represents an efficient probe into the torque balance of plate tectonics because plate motions readjust virtually instantaneously to temporal changes in driving and resisting forces here i derive inferences on the strength of large scale plate boundaries by focusing on the convergence of the fast moving philippine sea plate towards eurasia since subduction initiation 5 myr ago because at the present day the philippine slab reaches depths shallower than the 410 km transition zone in the upper mantle its weight is unlikely to provide sufficient driving force to shear the trailing plate over the viscous mantle at the observed rates using global models of the coupled mantle lithosphere system i argue that frictional coupling with the pacific and eurasia plates is the main driver for the dynamics of the philippine sea plate this allows me to infer that the friction coefficient of large scale plate boundaries is in range 0 01 0 07 and that the amount of sediments entering plate margins efficiently modulates the friction coefficient contributing significantly to its lateral variations importantly i corroborate my conclusions by looking at the present day vertical deformation of the philippine sea eurasia margin evident from observed trench parallel gravity anomalies

explicit use of the biot coefficient in predicting shear wave velocity of water saturated sediments predicting the shear wave s wave velocity is important in seismic modelling amplitude analysis with offset and other exploration and engineering applications under the low frequency approximation the classical biot gassmann theory relates the biot coefficient to the bulk modulus of water saturated sediments if the biot coefficient under in situ conditions can be estimated the shear modulus or the s wave velocity can be calculated the biot coefficient derived from the compressional wave p wave velocity of water saturated sediments often differs from and is less than that estimated from the s wave velocity owing to the interactions between the pore fluid and the grain contacts by correcting the biot coefficients derived from p wave velocities of water saturated sediments measured at various differential pressures an accurate method of predicting s wave velocities is proposed numerical results indicate that the predicted s wave velocities for consolidated and unconsolidated sediments agree well with measured velocities

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

controls on the thermomechanical evolution of hyperextended lithosphere at magma poor rifted margins the example of espirito santo and the kwanza basins high quality long offset seismic data from many distal rifted margins show evidence for hyper extended 10 km thick crust direct observation of such domains is challenging as they lie at great water depth buried beneath thick sedimentary sequences and formed by rock assemblages that are hydrated and geophysically indistinguishable only a few drill holes have penetrated basement at ultradistal rifted margins these observations together with outcrops of preserved analogs exposed in collisional orogens suggest that the complex interaction of detachment faults rooted in a subhorizontal shear zone in the hyperextended crust or in the serpentinized mantle controls the formation of the ocean continent transition while depth dependent thinning controls the early phases of rifting conforming to classical rift models we still have a superficial understanding of how normal faults and subhorizontal shear zones form and evolve during rifting and lithospheric breakup here we develop a rheological parameterization to simulate the formation of and slip on large offset normal faults rooted in growing brittle to ductile shear zones the evolution of these structures leads to the creation of a hyperextended crust and eventually exhumed serpentinized mantle we also propose a simplified formulation to simulate magmatic underplating and seafloor spreading the resulting numerical models provide a self consistent picture for the evolution of magma poor rifted margins from initiation of rifting to seafloor spreading the model results are compared with first order observations of the kwanza and espirito santo conjugate margins in the south atlantic as well as of magma poor margins globally

a comparison between methods that discriminate fluid content in unconsolidated sandstone reservoirs three seismic attributes commonly used to predict pore fluid and lithology are the fluid factor f poisson impedance pi and lambda rho we evaluated the pore fluid sensitivity of these attributes with both well log and seismic data in tertiary unconsolidated sediments from the gulf of mexico where sand and shale are the only expected lithologies while the sensitivity of one attribute versus another to discriminate pore fluid is often debated in the literature the sensitivities of the three attributes are not independent but can be traced back to the fluid factor which is a function of the p and s wave normal incident reflection coefficients interestingly the fluid factor which is a reflectivity attribute at the top of a hydrocarbon saturated reservoir is basically independent of the shale properties above the reservoir it is a function of the brine and hydrocarbon impedances of the reservoir the next attribute poisson impedance is thenequal to the fluid factor times the sum o