regional analysis of d velocities from the ray parameters of diffracted p profiles in examining the slownesses of linear profiles of diffracted p arrivals we have discerned regional variations in the compressional velocity structure of d the base of the mantle three methods of determining the slownesses were tested concluding that the waveform cross correlation xc technique is more reliable over a wide range of epicentral distances than methods involving a linear regression through either the first motion or first peak arrival times xc slownesses suggest a statistically conservative range in d average velocities of 3 1 and reveal regions of very fast velocities under asia and the eastern pacific basin and of very slow velocities under the new guinea solomon islands region slownesses were found to correlate well with the tomographic model v3 of morelli and dziewonski 1986 derived from a different technique and data set synthetic seismograms were generated using a summation of spheroidal normal modes and at periods greater than 45 s the diffracted waves show much less variation and are very well modeled by prem

interaction of mantle dregs with convection lateral heterogeneity at the core mantle boundary preliminary numerical models indicate that chemically denser material dregs at the base of the mantle would have substantial lateral variations in thickness induced by convection of the overlying mantle and might well form discontinuous aggregations below mantle upwellings a model with a density contrast of about 2 per cent and an initial uniform thickness of the denser layer of 100 km yields a discontinuous distribution with maximum thickness 230 km and bottom topography of several kilometers amplitude in reasonable accord with recent seismological observations of vertical and lateral structure heat flux out of the core is probably strongly modulated laterally by mantle convection while mantle dregs will complicate and possibly amplify this effect such modulation may be relevant to long term 10 7 10 8 year variations in the magnetic field

structural controls on anomalous transport in fractured porous rock anomalous transport is ubiquitous in a wide range of disordered systems notably in fractured porous formations we quantitatively identify the structural controls on anomalous tracer transport in a model of a real fractured geological formation that was mapped in an outcrop the transport determined by a continuum scale mathematical model is characterized by breakthrough curves btcs that document anomalous or non fickian transport which is accounted for by a power law distribution of local transition times t within the framework of a continuous time random walk ctrw we show that the determination of t is related to fractures aligned approximately with the macroscopic direction of flow we establish the dominant role of fracture alignment and assess the statistics of these fractures by determining a concentration visitation weighted residence time histogram we then convert the histogram to a probability density function pdf that coincides with the ctrw t and hence anomalous transport we show that the permeability of the geological formation hosting the fracture network has a limited effect on the anomalous nature of the transport rather it is the fractures transverse to the flow direction that play the major role in forming the long btc tail associated with anomalous transport this is a remarkable result given the complexity of the flow field statistics as captured by concentration transitions this article is protected by copyright all rights reserved

anomalous transport in heterogeneous media demonstrated by streamline based simulation 1 we use fine grid streamline simulation to demonstrate that anomalous non gaussian transport arises from purely advective movement through heterogeneous systems the simulation model represents a channeled sandstone north sea oil field and contains over one million grid blocks we find the average location and spreading of the plume and the breakthrough curves these features are consistent with anomalous transport described by an exponent that characterizes the long time tail of the transit time distribution depends on the degree of heterogeneity and whether the tracer was injected or originally uniformly distributed across the domain incorporating dispersion to account for sub grid block heterogeneity does not affect the results

lateral heterogeneity and azimuthal anisotropy in the north atlantic determined from ss s differential travel times to explore upper mantle heterogeneity beneath the north atlantic we have measured 70 ss s differential travel times using the waveform cross correlation method both the two way preliminary reference earth model prem and j b jeffreys bullen residuals exhibit a maximum variation of 17 s ranging from 10 to 7 and 9 to 8 s respectively the lack of correlation between ss s and s residuals along with the strong correlation between ss s and ss residuals suggests that ss s differential travel times eliminate source and receiver effects and sample the upper mantle anomalies near the ss bounce points without being strongly affected by lower mantle heterogeneities we find evidence for a strong local anomaly in the north atlantic residuals for bounce points immediately to the north of the azores gibraltar plate boundary average about 4 s faster than for the bounce points to the south of the boundary we infer that this velocity contrast exists in the lithosphere and continues below a depth of 200 km a series of linear multiple regression experiments demonstrate that variation in velocity with increasing age of the seafloor and azimuthal anisotropy are both significant contributors to the prem and j b residuals assuming that travel time delays are a linear function of the square root of the seafloor age we find the coefficient for age to be about 1 0 s m y this is smaller than but not significantly different from the corresponding slope for the s delays of intraplate earthquakes in the atlantic reported by j d duschenes and s c solomon 1977 and favors the presence of small amounts of water e g 0 1 in the upper mantle in modeling anisotropy we carry out an experiment using synthetic seismograms to examine the shear wave splitting phenomena in the anisotropic medium and investigate three possible forms for sh wave delay as a function of azimuth sinusoidal variations as a function of 2 4 and a combination of 4 predominant and 2 that mimics the delay expected in olivine crystals when the axis is aligned horizontally our regression models strongly prefer the predominance of a 4 to a 2 variation and the last type of anisotropy explains our data best from the preferred anisotropy model the olivine axis or the upper mantle shearing flow is inferred to be oriented n s the magnitude of the azimuthal variation suggests that anisotropy extends over a depth range of several hundred kilometers in the upper mantle

the changing nature of mantle hotspots through time implications for the chemical evolution of the mantle the melting products of mantle plumes can place constraints on the geochemical structure of the mantle and show how it has varied through time the highest temperature melts komatiites and picrites derived from each plume are assumed to represent the best available sample of the thermal boundary layer from which the plume originates a survey of the geochemistry of picrites and komatiites shows that they underwent a fundamental change at about the archean proterozoic boundary archean komatiites have depleted or neutral geochemistry whereas most protero zoic and younger picrites have originated from enriched oib type mantle although rare depleted plumes persist to the tertiary e g gorgona if plumes originate from a thermal boundary layer near the core mantle interface the composition of this layer must have changed from a mixture of depleted and neutral mantle during the archean to mainly enriched mantle in the proterozoic this change coincides with a drop in the maximum mgo content of the ma

low p wave velocity at the base of the mantle a tool for investigating p wave vp structure at the base of the mantle is presented sks waves at distances around 107 are incident upon the core mantle boundary cmb with a slowness that allows for coupling with diffracted p waves pd at the base of the mantle the p wave diffraction occurs at both the sks entrance and exit locations of the outer core the resulting phase spdks arrives slightly later in time than sks having a wave path through the mantle and core very close to sks the difference time between sks and spdks most strongly depends on vp at the base of the mantle near sks core entrance and exit points digitized long period 5 15s observations from deep focus tonga events recorded by north american wwssn and canadian seismographic network stations and south american events recorded by european and eurasian wwssn stations exhibit anomalously large spdks sks difference times sks and the later arriving spdks are separated by several seconds more than predictions made by the global average prem model models having a pronounced low velocity zone in vp at the base of the mantle best predict the observations these models are perturbations of the prem model whereby the lowermost 50 100 km of the d layer has a negative vp gradient with the mantle side cmb vp reduced from prem by 5 to 13 0 km s at the base of the mantle

aspherical structure of the core mantle boundary from pkp travel times maps of aspherical structure in the vicinity of the core mantle boundary have been constructed from a tomographic analysis of p df and p ab travel times in the distance range 150 180 these maps reveal long wavelength features not previously detected by mantle tomography the peak to peak amplitude of these features is on the order of 1 s in vertical travel time too large to be explained by conventional thermal boundary layer models or dynamically supported topography on the core mantle boundary we hypothesize the existence of one or more chemical boundary layers whose large scale accumulations on the surface of the core may in some ways be analogous to continents at the earth s surface

double beam analysis of anomalies in the core mantle boundary region seismic p waves from nuclear explosions at the kazakhstan test site in eastern kazakhstan kts observed at the yellowknife array yka canada are used to analyze the structure at the base of the mantle under severnaya zemlya arctic sea 18 events at the degelen subsite and 9 events at the shagan river subsite of kts are studied the simple waveforms and well controlled source parameters of nuclear explosions allow the use of the events as source arrays in addition to the usual receiver array configuration a new method double beamforming integrating both concepts is presented this method increases the resolution considerably for the degelen yka path one anomalous arrival 1 9 s before pcp slowness between p and pcp slowness is observed smaller anomalous arrivals are found 6 5 s degelen yka and 8 1 s before pcp shagan river yka these anomalous phases cannot be explained with standard earth models they are produced by reflections from an inhomogeneity in the lowermost mantle at 82 3n 107 5e under the area north of severnaya zemlya the strong variations between the wavefields for the two source receiver combinations indicate a heterogeneous structure in the lower mantle the anomaly found here is located close to an anomaly in the lowermost mantle beneath the nansen basin

travel times of s and sks implications for three dimensional lower mantle structure beneath the central pacific the travel times of s and sks phases from deep focus fiji tonga events recorded in north america by canadian station network and world wide seismographic station network stations exhibit a strong azimuthal dependence the differential times of s sks exhibit relatively little scatter and are used to investigate three dimensional mantle models for the central pacific region s sks times are corrected for earth s ellipticity and residuals are computed with respect to the iasp91 reference model anomalously large s sks times ranging up to 9 s larger than iasp91 predictions for 100 suggest a slow lower mantle beneath the central pacific these results are compared to the predictions of three tomographic models mdlsh of tanimoto 1990 model sh12 wm13 of su et al 1992 and model sh 10c 17 of masters et al 1992 the three dimensional whole mantle models have been parameterized into 11 spherical shells qualitative agreement between the tomographic model predictions and observations is encouraging varying from fair to good however inconsistencies are present and suggest anomalies in the lower mantle of scale length smaller than the present 2000 km scale resolution of tomographic models laterally varying outer core structure is not necessary to explain the anomalies in this data set although such a scenario cannot be resolved with this data

dynamical effects on the core mantle boundary from depth dependent thermodynamical properties of the lower mantle a common assumption in modelling dynamical processes in the lower mantle is that both the thermal expansivity and thermal conductivity are reasonably constant recent work from seismic equation of state leads to substantially higher values for the thermal conductivity and much lower thermal expansivity values in the deep mantle the dynamical consequences of incorporating depth dependent thermodynamic properties on the thermal mechanical state of the lower mantle are examined with the spherical shell mean field equations we find that the thermal structure of the seismically resolved anomalous zone at the base of the mantle d is strongly influenced by these variable properties and in particular that the convective distortion of the core mantle boundary cmb is reduced with the decreasing thermal expansivity such a reduction of the dynamically induced topography from pure thermal convection would suggest that some other dynamical mechanism must be operating at the cmb

multiple phase analysis of the shear velocity structure in the d region beneath alaska a large data set more than 990 total waveforms comprised of north american station recordings of a variety of long period s phases s scs sdiff ss sscs sks skks from 36 earthquakes in western pacific subduction zones is analyzed to determine characteristics of the shear velocity structure in the d region beneath alaska three hypotheses that have been proposed to explain observed scs precursors and s waveform distortions for this region are appraised with the multiple phase data set and the radial and lateral variations of the shear velocity structure in the 1500 km broad portion of d sampled are also assessed the observed travel time and waveform variations may result from diffraction by near source slab structure scattering by low velocity heterogeneity in the lower mantle or interaction with complex velocity gradients in d associated with a deep mantle velocity discontinuity we utilize waveform modeling and differential time analysis to resolve greater detail of the structure than can presently be obtained by standard travel time tomography techniques and find that the discontinuity model is most consistent with the data subdivision of the data by source region and source to receiver azimuth establishes that the observations exhibit fairly stable behavior which is consistent with extensive local stratification of d the data show sufficient coherence over the 1500 km broad region that we find it useful to present a local average radial model the preferred model sylo has a 2 75 velocity increase 243 km above the core mantle boundary and a negative velocity gradient within the d layer analysis of sks s sks scs and sks skks differential times also indicates that the outermost 100 km of the core below the fast d region underlying alaska must be slower than the prem structure the observations exhibit significant scatter most of which can be attributed to unmodeled heterogeneity in the midmantle and near source regions as well as lateral variations in the d region the complex structure of d that we prefer may be associated with large scale downwelling in the lower mantle but it is not certain whether it is a purely thermal or a combined chemical and thermal effect

streamline based simulation of solute transport we propose streamline based simulation as a possible alternative to particle tracking for modeling solute transport like particle tracking the pressure field is computed on an underlying grid using conventional techniques the flow velocity at cell edges is then computed using darcy s law and this information is used to trace streamlines throughout the domain in particle tracking mass is transported by moving particles along streamlines in the method we describe a one dimensional conservation equation is solved numerically along each streamline if the flow field changes the solute concentration is mapped onto the underlying grid and the streamlines are recomputed the concentration is then mapped back onto the new streamlines and the simulation proceeds as before the method is suited for modeling advectively dominated multispecies transport in heterogeneous aquifers we illustrate the streamline approach with synthetic example problems in fully saturated confined aquifers conservative sorbing and decaying tracer a four component radioactive decay chain and saltwater intrusion where the flow field changes with time where possible we compare the results with analytical solutions and results from particle tracking codes

coupling of the core dynamo and mantle thermal or topographic static features in the earth s magnetic field and fixed sources of secular variation suggest the dynamo is anchored to the lowermost mantle via either thermal or topographic coupling here we show a correspondence with dynamic core topography which is related to mantle temperature and suggest further effects due to topography associated with subducting slabs the two coupling mechanisms may occur on different time scales providing a potential means for discriminating between them

hotspots mantle plumes flood basalts and true polar wander persistent long lived stationary sites of excessive mantle melting are called hotspots hotspots leave volcanic trails on lithospheric plates passing across them the global constellation of fixed hotspots thus forms a convenient frame of reference for plate motions through the orientations and age distributions of volcanic trails left by these melting anomalies hotspots appear to be maintained by whole mantle convection in the form of upward flow through narrow plumes evidence suggests that plumes are deflected little by horizontal flow of the upper mantle mantle plumes are largely thermal features and arise from a thermal boundary layer most likely the mantle layer just above the core mantle boundary experiments and theory show that gravitational instability drives flow beginning with the formation of diapirs such a diapir will grow as it rises fed by flow through the trailing conduit and entrainment of surrounding mantle the structure thus develops a large spherical plume head and a long narrow tail on arrival at the base of the lithosphere the plume head flattens and melts by decompression producing enormous quantities of magma which erupt in a short period these are flood basalt events that have occurred on continents and in ocean basins and that signal the beginning of major hotspot tracks the plume supported hotspot reference frame is fixed in the steady state convective flow of the mantle and is independent of the core generated axial dipole paleomagnetic reference frame comparison of plate motions measured in the two frames reveals small but systematic differences that indicate whole mantle motion relative to the earth s spin axis this is termed true polar wander and has amounted to some 12 since early tertiary time the direction and magnitude of true polar wander have varied sporadically through the mesozoic probably in response to major changes in plate motions particularly subduction zone location that change the planet s moments of inertia

lower mantle structure from scs s differential travel times knowledge of the spectrum of structure in the lower mantle is crucial to our understanding of the dynamical evolution of the earth and in principle can be inferred from the analysis of global seismic data sets the long wavelength compressional velocity structure has usually been constrained through tomographic inver sion of the isc catalogue of p wave travel times1 3 whereas shear velocity has been inferred from waveform modelling of long period shear waves4 6 the models that have been produced are similar in only the largest scale features7 and explain little of the variance of the raw data used in their construction thus there is a legitimate concern that the current generation of global scale models give only a crude approximation to the largest scale structure and that there may be significant aliasing of short wavelength heterogeneity here we use scs s differential travel times to demonstrate that the three dimensional structure of the lower mantle is indeed dominated by continental scale features particularly convincing is the fact that the features are apparent in the raw data and are not the product of a complicated modelling procedure

structure of axisymmetric mantle plumes results of high resolution numerical calculations of axisymmetric thermal plumes in an infinite prandtl number fluid with thermally activated viscosity are used to infer the structure of axisymmetric subsolidus thermal plumes in the earth s lower mantle we calculate the velocity and temperature distribution for axisymmetric convection above a heated disk in an incompressible fluid cylinder 2400 km in height and 1200 km in diameter initially at a uniform temperature t the lower boundary representing the plume source region near the core mantle boundary is stress free and maintained at temperature tc t the upper boundary is permeable allowing the plume to escape the cylinder we determine the sensitivity of plume structure to viscosity variations c ranging from 102 to more than 104 and temperature differences t tc t in the range 400 850 k starting plumes consist of a large leading diapir and a trailing conduit connected to the thermal boundary layer above the heated surface the leading diapir expands during ascent to about 400 km diameter at 2200 km height in steady state the plume consists of a narrow high velocity conduit imbedded within a broader thermal halo the width of both these structures is proportional to height above the heated boundary and inversely proportional to t and the viscosity ratio heat transport in the plume is nearly independent of viscosity variations a simplified boundary layer model yields a heat transfer law nu ra where nu is the plume nusselt number and ra is the rayleigh number based on t and the radius of the heated circular region feeding the plume calculated steady state plume heat transports fit this law to within a few percent we present several calculations of plumes with heat transport in the range 100 400 gw similar to the advective heat transport by the hawaiian hotspot at the top of the lower mantle plumes with large viscosity variations have high velocity conduits 50 60 km in diameter and 125 km diameter thermal halos a centerline temperature anomaly of about 500 k and a centerline velocity of about 75 cm yr 1 plumes with weak viscosity variations are approximately twice as broad and have centerline temperature anomalies less than 160 k at the same height and much smaller velocity large viscosity variations and a minimum heat transport are both

amplitudes of diffracted long period p and s waves and the velocities and q structure at the base of the mantle frequency and time domain analysis of amplitudes of long period 20 s p and s waves diffracted around the earth s core and registered at the wwnss and csn stations show that the velocities at the base of the mantle have slightly positive gradients this result substantiates the previous ray parameter study of mula and muller 1980 such velocity gradients at the base of the mantle imply that there is no significant departure from homogeneity and or adiabaticity of the base of the mantle from the rest of the lower mantle as suggested by for example bolt 1972 jones 1977 jeanloz and richter 1979 and doornbos and mondt 1979b in addition to this the q factor for the base of the mantle is found not to be less than 250

lateral variations in d thickness from long period shear wave data we explore global variations in d shear wave structure by examining 12 years of long period global digital seismograph network gdsn data to identify reflected phases from the d layer in the lowermost mantle we restrict our search to epicentral distances between 63 and 74 where a precritical d reflection will lie between the s and scs arrivals with gdsn long period records a d signal at these ranges is generally obscured by the stronger neighboring phases so to isolate the d reflections we develop a technique for stripping away the interfering s and scs waveforms we interpret travel time variations in the observed sds phases in terms of variations in d thickness and generate maps of inferred d thickness for areas of good seismic coverage the regions of observed d reflections are beneath australasia north central asia the arctic alaska and central america we find considerable variation in d structure in these regions a result consistent with previous studies which have found evidence for lateral heterogeneity in d on average the estimated d thickness is 260 km but there is a fairly uniform distribution of thicknesses between 150 km and 350 km a possible correlation is observed between d thickness and regions of predicted strong horizontal mantle flow

lateral variations near the core mantle boundary differential travel times between s and sks ts sks and skks and sks tskks sks along with amplitude behavior have been used to study lateral variations at the core mantle boundary the data were recorded by the world wide seismographic station network wwssn and the canadian seismograph network csn from source regions in south america and the fiji tonga area a core mantle boundary region traversed by sks and s beneath the mid pacific from the fiji tonga area to north american stations shows the most anomalous behavior in ts sks sks and s are anomalously separated by up to 8 seconds in addition to ts sks anomalies sks and skks are anomalously separated by 2 to 3 seconds a model fitting ts sks for this anomalous region requires one or more of the following 1 up to 5 increase in the top 300 km of outer core velocity 2 a large scale high vs region 2 increase in the lower mantle ne of tonga 3 lateral variations in d shear wave velocities of up to 3 however 1 above predicts a decrease in tskks sks contradicting the observations this suggests that 2 and 3 are more likely explanations in modeling this data set

streamline based simulation of carbon dioxide storage in a north sea aquifer 1 we model carbon dioxide co2 storage into a deep north sea aquifer using streamline based simulation we assume incompressible flow of liquid like co2 and aqueous phases we simulate dissolution of co2 and a rate limited precipitation reaction advective transport and reactions are solved along streamlines while dispersion and flow due to gravity segregation of the phases are solved on the underlying grid geological storage is modeled on a one million cell model the distribution of co2 after injection is dominated by advective transport due to multiphase flow and co2 moves preferentially through high permeability channels without reaction the regional groundwater flow causes the co2 to continue to migrate until it reaches residual saturation where it continues slowly to dissolve precipitation leads to a decrease in porosity and permeability while co2 is stored in the solid phase the storage efficiency is low around 2 because of aquifer heterogeneity

dynamical influences from thermal chemical instabilities at the core mantle boundary two dimensional time dependent calculations show that the thermal and flow structures can be influenced by the lower mantle thermal chemical instabilities the style of the core mantle boundary cmb deformation in thermal chemical convection is different from that obtained in thermal convection the amplitude of cmb hills is reduced greatly from the depth dependence of thermal expansivity found in recent high pressure experiments

tomographic inversions for mantle p wave velocity structure based on the minimization of l 2 and l 1 norms of international seismological centre travel time residuals we use international seismological centre isc p arrival data 1964 1987 and iterative algorithms which minimize the l1 and l2 residual norms to solve simultaneously for three dimensional p velocity variations in earth s mantle source mislocations and station corrections we find that the maximum velocity perturbations produced by the l1 minimization approximately 4 in our final model are relatively insensitive to smoothing and damping constraints therefore using an l1 norm criterion allows us to keep the bias introduced to the inversion to a minimum among the well resolved features contained in both the l1 and the l2 velocity models are a fast anomaly in the lower mantle beneath the tonga new hebrides subduction zone to a depth of 1670 km and another fast anomaly beneath the japanese island arc and eastern asia continuity between these anomalies and shallower fast anomalies is not clear a fast anomaly extending from 670 km to 2070 km depth appears beneath eastern north america the caribbean and north central south america a broad fast anomaly appears beneath eastern asia just above the core mantle boundary as well as several slow anomalies under the pacific basin of comparable size both models contain a circum pacific ring of 2 lower velocities in the depth range 0 200 km associated with back arc basins high velocities over 2 associated with the continental shields tend to disappear below 400 km though a significant region of high velocity remains beneath the siberian platform in the 400 to 670 km depth interval

the structure of the core mantle boundary from diffracted waves diffracted p and s waves pd sd traveling around the core mantle boundary cmb of the earth give us information about the velocity structure and therefore the thermochemistry of d the base of the earth s mantle by examining pd and sdarrivals we determined the apparent ray parameter for different regions at the base of the mantle by comparing the data slownesses to those found from reflectivity synthetic seismograms we were able to quantify d average velocities using these averaged velocities with a thermochemical modeling of lower mantle minerals using a birch mumaghan equation of state we have been able to make chemical and physical inferences as to the causes of lateral variations at the cmb examinations found significant lateral heterogeneity at the base of the mantle amounting to 4 for both p and s velocities these velocities did not always vary in parallel and the poisson ratio varied regionally by almost 6 the most unusual region of the cmb found was under indonesia where velocities 3 slower than the preliminary reference earth models were found adjacent to a region of faster than average velocities these regions currently correspond to areas of core up welling and down welling respectively found by voorhies 1986 which if mostly held in place by core mantle coupling might cause a flux of heat and iron into the mantle making the anomaly both thermally and chemically derived at the cmb under the northern pacific rim the fastest shear velocities were found but the same region yielded slower than average p velocities while the presence of fast shear velocities here would support the idea that we are seeing the cold dregs of mantle convection perhaps continuing down from the north pacific subduction zones the presence of slow p velocities suggests additional complications our thermochemical modeling suggests that the d poisson ratio is very sensitive to variations in the silicate oxide ratio and that a decrease in the amount of perovskite relative to magnesiowustite may play an important role in this region

mapping the upper mantle three dimensional modeling of earth structure by inversion of seismic waveforms a method is presented for the inversion of waveform data for the three dimensional distribution of seismic wave velocities the method is applied to data from the global digital networks international deployment of accelerometers global digital seismograph network the selected data set consists of some 2000 seismograms corresponding to 53 events and 870 paths the moment tensors of the earthquakes are determined through an iterative procedure which minimizes the corrupting influence of lateral heterogeneity a global model is constructed for shear wave velocity expanded up to degree and order 8 in spherical harmonics and described by a cubic polynomial in depth for the upper 670 km of the earth s mantle although no a priori information is incorporated the model predictions reproduce much of what is known about the dispersion of mantle waves for example high phase velocities for shields low velocities at ridges and a strong degree 2 pattern for rayleigh waves since the method makes use of complete waveforms overtone data are also included it is shown that the model is reproducible in that substantially the same model can be constructed from each half of the total data set considered independently the model shows that shields and ridges are major features in the depth interval 25 250 km the ridges of the southern pacific and the larger shields persist to 350 km but the southeast indian rise is underlain by a high velocity anomaly at this depth as is much of the mid atlantic ridge at 450 650 km the major features are a broad region of high velocities incorporating south america much of the south atlantic and parts of west africa a broad region of low velocities in the central and eastern pacific high velocities in the western pacific and a low velocity anomaly beneath the red sea and the gulf of aden in the absence of a crustal correction degrees 2 and 3 show a high positive correlation with the geoid paradoxically this is largely destroyed when the distribution in crustal thickness is taken into account spherical harmonic degrees 4 7 show a significant negative correlation

tracer tests in a fractured dolomite 2 analysis of mass transfer in single well injection withdrawal tests we investigated multiple rate diffusion as a possible explanation for observed behavior in a suite of single well injection withdrawal swiw tests conducted in a fractured dolomite we first investigated the ability of a conventional double porosity model and a multirate diffusion model to explain the data this revealed that the multirate diffusion hypothesis model is consistent with available data and is capable of matching all of the recovery curves second we studied the sensitivity of the swiw recovery curves to the distribution of diffusion rate coefficients and other parameters we concluded that the swiw test is very sensitive to the distribution of rate coefficients but is relatively insensitive to other flow and transport parameters such as advective porosity and dispersivity third we examined the significance of the constant double log late time slopes 2 1 to 2 8 which are present in several data sets the observed late time slopes are significantly different than would be predicted by either conventional double porosity or single porosity models and are believed to be a distinctive feature of multirate diffusion fourth we found that the estimated distributions of diffusion rate coefficients are very broad with the distributions spanning a range of up to 3 6 orders of magnitude fifth when both heterogeneity and solute drift are present late time behavior similar to multirate mass transfer can occur although it is clear that multirate diffusion occurs in the culebra the number of orders of magnitude of variability may be overestimated because of the combined effects of drift and heterogeneity

the steady part of the secular variation of the earth s magnetic field the secular variation of the earth s magnetic field results from the effects of magnetic induction in the fluid outer core and from the effects of magnetic diffusion in the core and the mantle adequate observations to map the magnetic field at the core mantle boundary extend back over three centuries providing a model of the secular variation at the core mantle boundary here we consider how best to analyze this time dependent part of the field we propose that the first step should be to seek the steady core flow that best fits the field maps isolating what we call the steady secular variation the part of the secular variation explained by steady core flow to calculate steady core flow over long time periods we introduce an adaptation of our earlier method of calculating the flow in order to achieve greater numerical stability we perform this procedure for the periods 1840 1990 and 1690 1840 and find that well over 90 of the variance of the time depedent field can be explained by simple steady core flow the core flows obtained for the two intervals are broadly similar to each other and to flows determined over much shorter recent intervals although we can explain a large part of the signal with these flows none of them provides an adequate explanation of the secular variation in particular the fit to the original observations from which the field models were derived is inadequate we examine the residual secular variation the part of the signal that remains we argue that it is dominated by the effects of nonsteady flow and magnetic diffusion we do not find any compelling evidence for torsional oscillations as the dominant ingredient of this nonsteady part of the secular variation

comparison of deterministic with stochastic fracture models in water flooding numerical simulations determination of multiphase flow properties considering the variation of fracture patterns i e number of fracture sets their orientation length distribution spacing and in situ aperture remains a key challenge in reservoirs in reservoir engineering one way is by studying outcrop analogs with comparable petrophysical properties and a similar geological history and incorporating these data into model building discretization and numerical simulation the limitation of directly incorporating attributes measured on outcrops is that this method is error prone because of postburial processes mineralized fracture vein attributes are good candidates to use as analogs for open fractures formed under in situ conditions to establish the relationship between fracture length and aperture and help to reveal the conditions at the time of their formation and to quantify fracture induced porosity in rock masses vein attributes determined from scan lines and window samples were combined to condition the stochastic generation of fractures using the discrete fracture network code fracman comparison of water breakthrough time and oil saturation at breakthrough was then determined by applying a constant pressure gradient for each realization to simulate water flooding numerical simulation using the combined finite element finite volume method the different stochastic realizations were compared with discrete fracture and matrix models and we show how the uncertainty in these fracture attributes affects multiphase flow behavior in naturally fractured rocks uncertainty in quantifying these attributes has a profound impact for predicting the oil recovery and water breakthrough time based on limited information from boreholes

an anomaly in the amplitude ratio of skks sks in the range 100 108 from portable teleseismic data we have examined the amplitude ratio skks sks from two deep focus events recorded by the apt89 portable seismic experiment for one of the events beneath the solomon islands we obtain a profile of 18 stations in the distance range 99 108 we observe large systematic variations in this ratio as a function of distance from 99 to 106 the ratio is fairly constant and in the range 0 1 to 0 5 but it then rapidly increases to the range 0 5 3 5 between 106 and 108 such variations are in fact predicted by standard earth models such as prem although this feature of the wavefield has not previously been observed it is due to the momentary drop in sks amplitude at the ray parameter where the reflected phase scp is critically reflected because ray theory is not valid in this distance range we use full wave theory to model the amplitudes we have been able to match the distance pattern reasonably well if we use a model with the p velocity at the base of the mantle reduced by 1 5 compared to prem although fewer stations are available the second event shows the same basic distance pattern except that the abrupt increase in the ratio occurs at a closer distance this suggests an even lower p velocity for this region about 3 5 slower than prem and is consistent with lateral heterogeneity at the base of the mantle of a few percent

effect of mantle plumes on the growth of d by reaction between the core and mantle the core mantle boundary is a fundamental compositional discontinuity in the earth where molten iron alloy from the core meets solid silicate minerals from the mantle heat flow from the core to the mantle creates a thermal boundary layer at the base of the mantle at the same time chemical reactions may create a layer of different composition and density than the overlying mantle using a finite element model of convection in a spherical shell that includes formation of dense material at the base of the shell we investigate how a layer forms at the base of the mantle development of a layer at the base of the mantle by this method depends both on the composition of the material forming at the core mantle boundary and on the rate at which the material diffuses into the mantle if the material is less than 3 6 denser than the overlying mantle assuming reasonable choices of lower mantle thermodynamic parameters the reactant material will be swept away by upwelling plumes more dense material on the other hand forms an internally convecting layer that entrains material from above the varying thickness and composition of the layer is consistent with geophysical observations of the characteristics of the deep mantle

on convection in the earth s core driven by lateral temperature variations in the lower mantle summary lateral variations in heat flux or temperature in the lowermost mantle associated with mantle convection will drive fluid flow in the liquid core the effect is modelled by a rotating spherical shell with uniform lower boundary temperature and a laterally varying upper surface temperature and the solutions found by numerical calculation some for a density stratified fluid the problem depends upon two dimensionless numbers a horizontal rayleigh number and a stratification parameter the calculations are restricted to surface temperatures which are symmetric about the equator and steady solutions are found induced toroidal flows near the surface are closely linked with the applied temperature profile through the thermal wind equation and coriolis forces make the convection penetrate into the shell stratification acts mainly to suppress radial flow but the surface toroidal flow is relatively unaffected the results illustrate the powerful influence exerted by the surface temperature but are difficult to apply directly to the earth s core because molecular values of the thermal diffusivity entail a very long time scale assuming a turbulent thermal diffusivity equal to the magnetic diffusivity gives a more realistic time scale and it is possible to recover for specific simple imposed temperature profiles flows similar to those found in the earth s core from inversion of geomagnetic secular change

plume formation in the d double prime layer and the roughness of the core mantle boundary numerical simulations of the formation of thermal plumes in the d layer at the base of the earth s mantle show that plumes are initiated by coalescence of small scale convective instabilities within the low viscosity region immediately above the core mantle boundary cmb these instabilities support topographic roughness on the cmb having horizontal scales of 20 50 km and provide a source for scattered p waves seen as precursors to the phases pkikp and pkkp the calculated structure of fully developed plumes emerging from d consists of 5 50 cm yr 1 flow confined to 50 100 km thick vertical conduits with strongly temperature dependent viscosity plumes exhibit time dependent behaviour including upward propagating solitary conduit waves which may contribute to episodicity in hotspot volcanism

observational evidence for diffracted sv in the shadow of the earth s core on the basis of observational evidence it has been assumed for a long time in the seismological literature that the diffracted s wave in the deep shadow of the earth s core is polarized as sh in good agreement with models that do not contain a well pronounced low velocity zone at the base of the mantle while this seems often to be the case broad band horizontal records from the geoscope network show that for some wavepaths the diffracted sv is strong enough to be clearly observed at distances between 107 and 117 moreover in some cases the diffracted sv is shifted with respect to sh by approximately a quarter period such observations should put additional constraints on the structure near the core mantle boundary among other possibilities lateral variations of the gradient at the base of the mantle allowing for regions with negative gradient and possibly azimuthal anisotropy in d should be considered in order to explain these data

evidence for a shear velocity discontinuity in the lower mantle beneath india and the indian ocean abstract sh and ssh seismograms are modeled to determine the shear velocity structure in the d region beneath india and the indian ocean the signals show waveform complexities similar to those observed in data sampling the d region beneath alaska the caribbean and eurasia lay and helmberger which have been attributed to a 2 7 shear velocity discontinuity 280 km above the core mantle boundary the new data set consists of long period tangential component recordings at wwssn stations in africa the middle east and europe for 11 intermediate and deep focus indonesian earthquakes in the distance range 70 82 the waveforms show an arrival between sh and scsh with systematic moveout from 89 to 94 there is a strong distortion of the sh waveforms indicating the arrival of several phases closely spaced in time the relative time shifts of similar complexity in the corresponding ssh phases requires a deep mantle origin the depth dependence and moveout of the interference effects are well predicted for both sh and ssh phases by a model with a lower mantle discontinuity alternative explanations of the interference as resulting from receiver reverberations sks contamination multiple source complexity or near source multipathing are ruled out by systematic tests while it is apparent that lateral variations in the lower mantle velocity structure prevent any single model from fitting all of the data synthetic waveform modeling using generalized ray theory and reflectivity shows that the data can be well fit by a model with a discontinuity similar in size and depth to that proposed for the previously investigated regions lay and helmberger but with a negative velocity gradient within the d layer

tracer transport in fractured crystalline rock evidence of nondiffusive breakthrough tailing extended tailing of tracer breakthrough is often observed in pulse injection tracer tests conducted in fractured geologic media this behavior has been attributed to diffusive exchange of tracer between mobile fluids traveling through channels in fractures and relatively stagnant fluid between fluid channels along fracture walls or within the bulk matrix we present a field example where tracer breakthrough tailing apparently results from nondiffusive transport tracer tests were conducted in a fractured crystalline rock using both a convergent and weak dipole injection and pumping scheme deuterated water bromide and pentafluorobenzoic acid were selected as tracers for their wide range in molecular diffusivity the late time behavior of the normalized breakthrough curves were consistent for all tracers even when the pumping rate was changed the lack of separation between tracers of varying diffusivity indicates that strong breakthrough tailing in fractured geologic media may be caused by advective transport processes this finding has implications for the interpretation of tracer tests designed to measure matrix diffusion in situ and the prediction of contaminant transport in fractured rock

streamline based dual porosity simulation of reactive transport and flow in fractured reservoirs 1 we present a streamline based formulation to model two processes transport of a tracer undergoing rate limited sorption and two phase water oil or air water transport in fractured systems using a dual porosity approach we show that these two processes can be simulated using mathematically equivalent formulations in both cases the system conceptually has two components a flowing fraction connected to stagnant regions with transfer between the two domains streamlines capture movement through the flowing fraction fluid transfer between flowing and stagnant regions enters as a source sink term in the one dimensional transport equations along a streamline to model flow and transport in fractured systems we develop a new formulation for the transfer function that matches experimental imbibition data then we illustrate the streamline approach with synthetic reservoir problems we use a finely gridded over one million grid blocks three dimensional domain with a highly heterogeneous permeability field to study both fracture flow and tracer transport we find breakthrough curves that are consistent with anomalous transport described by an exponent that characterizes the longtime tail of the transit time distribution for fracture flow we demonstrate that the speed of fluid advance in the fractures is controlled by the imbibition rate the run times for the simulations scale approximately linearly with system size making the method appropriate for the simulation of large numerical models

slab diffraction of s waves seismograms are synthesized by superposition of gaussian beams for teleseismic s waves radiated by deep focus earthquakes located within the zone of high seismic velocities that define a descending lithospheric slab the s wave synthetics demonstrate a phenomenon of slab diffraction that acts to lengthen the apparent pulse width of the s wave low frequency energy leaks out of the slab and travels to the receivers along paths in the lower velocity region surrounding the slab seismograms synthesized in fully three dimensional models of slab structure show that slab diffraction strongly depends on the azimuth of the receiver with respect to strike of the slab slab diffraction lengthens the pulse width of s waves in the entire azimuthal range on the dipping side of the slab as the azimuth approaches the strike of the slab the diffracted phase may evolve into a higher frequency impulsive phase which arrives after the minimum time arrival some slab models predict the slab phase to become a superposition of multipathed ray theoretical arrivals as the azimuth approaches the strike of the slab as the azimuth varies through the strike of the slab toward the side opposite the direction of dip the slab phase is rapidly extinguished rapid variations in the complexity of s waveforms recorded along azimuths parallel to the strike of the slab are consistent with the predictions of synthetic seismograms evidence of slab diffraction in the s waveforms of the deepest focus earthquakes supports the hypothesis of slab penetration beyond 650 km depth broadband s waveforms from deep focus earthquakes in the kuril kamchatka slab suggest that this slab penetrates below 650 km and that it may suffer some distortion in shape and advective thickening below this depth

lateral variations in p velocity and attenuation in the d layer from diffracted p waves abstract diffracted p waves recorded at two stations along the same great circle are analysed to specify the lateral variations in velocity and velocity gradient in the d layer at the base of the mantle data come from long period and broadband geoscope and gdsn stations the variations in both apparent ray parameter and amplitude decay are studied as a function of the dominant period of the p wave regional variations in the ray parameter indicate a 2 5 variation for the p velocity in d with high velocities beneath the north pacific central america and northern asia and low velocities beneath the south pacific south atlantic and northern indonesia the absence of correlation between these velocity variations and the lateral variations in p attenuation favour a chemical origin rather than a thermal origin for these heterogeneities on the other hand lateral variations in the velocity gradient are suggested from the frequency dependence of both ray parameter and p amplitude decay

the p wave velocity structure near the base of the mantle travel times from 16 events recorded at two temporary networks of short period seismometers deployed across australia have been used to study the p wave velocity structure of the mantle below depths of 1700 km the slowness distance curve derived from these times shows small deviations from the slownesses predicted from the preliminary reference earth model of dziewonski and anderson 1981 that may be significant the most important feature is a rapid decrease of more than 0 5 s degree between distances of 85 and 88 velocity filtering of seismograms from 5 nuclear explosions in kazakh recorded at these australian networks indicates the presence of two arrival branches at distances between 83 and 91 whose slownesses differ by 0 40 to 0 55 s degree and which intersect at a distance of 87 processed p wave seismograms from the yellowknife array in canada also indicate the presence of a similar phenomenon in the same distance range in two separate regions the results can be explained by a widespread rapid or sharp increase in velocity of 2 5 to 3 0 about 200 km above the core mantle boundary

the dynamical and thermal structure of deep mantle plumes abstract if the interpretation of the d layer at the base of the mantle as a thermal boundary layer with a temperature increment in the order of 800 k is correct then the formation of deep mantle plumes to vent material from it appears inevitable we demonstrate quantitatively that the strong temperature dependence of viscosity guides the upward flow into long lived chimneys that are 20 km in diameter near the base of the mantle and decrease in width with progressive upward softening and partial melting of plume material the speed of flow up the axis of the plume is correspondingly fast 1 6 m y 1 at the base and 4 8 m y 1 at 670 km depth thermal diffusive spreading of a heated plume is compensated by a slow horizontal convergence of mantle material toward the chimney in response to the lower pressure there this convergence which contributes only a small increment to the flux of material up the plume also serves to throttle the flow in the chimney the global plume mass flux necessary to transport 1 6 10 12 w of core heat upward through the mantle is 1 8 10 6 kg s 1 at its base plume material is probably still significantly below its solidus or eutectic temperature but substantial partial melting is very likely as it rises we speculate that a small fraction of this fluid component eventually emerges at the surface in hot spots with the fate of the remainder being unknown the behaviour and properties of d and of plumes are closely coupled not only are plumes a necessary consequence of a thermal boundary layer but their existence is impossible without that layer

constraints on lower mantle composition and temperature from density and bulk sound velocity profiles recent studies comparing density from the seismological model prem to that predicted for various perovskite and magnesiowustite phase assemblages under lower mantle conditions suggest that the lower mantle is a few percent denser than reasonable candidates for upper mantle composition such as pyrolite this has been interpreted as evidence for an iron enriched lower mantle in addition to density bulk sound velocity provides an important constraint upon proposed lower mantle compositions being independent of the shear modulus has several desirable characteristics experimentally it can be determined in the laboratory by density measurements under static compression seismologically it is not affected by attenuative dispersion and is expected to be much less laterally heterogeneous than p or s wave velocities we have calculated and along lower mantle adiabats as functions of iron magnesium content xmg and silica content xpv and compared them to values obtained from prem for a reasonable temperature estimate at the top of the lower mantle tlm of 2000 k we find that the data are most compatible with a lower mantle enriched in fe in agreement with previous studies in addition the data require that silica enrichment accompany this fe enrichment furthermore increasing tlm increases both the required fe and si content with a high assumed tlm 2700 k the data can be satisfied by low xmg values but even a pure perovskite lower mantle is too slow compared to thus assuming no free sio2 in the lower mantle this provides an upper bound on tlm of approximately 2700 k finally conclusions regarding the composition and temperature of the lower mantle are strongly dependent upon uncertainties in estimated thermoelastic parameters and upon choice of a seismological model most importantly neither fe nor si enrichment is required if the zero pressure volume coefficient of thermal expansion of perovskite is about 2 5 10 5k 1 rather than 4 0 10 5k 1 at high temperatures alternatively but less probably fe and si enrichment are not required if the average of the lower mantle is 1 0 less than given by prem the si enrichment is not required if the average of the lower mantle is 0 5 slower than pkem closer to the value given by the body wave model derived from the jeffreys bullen tables or if the lower mantle is anomalously cold with tlm 1700 k

models of mantle convection one or several layers numerical model calculations are used to determine if convection in the earth s mantle could be organized in two or more layers with only limited mass exchange in between the seismic discontinuity at 670 km depth and the top of the d layer at the bottom of the mantle are considered as candidates for internal boundaries if the 670 km discontinuity is caused by an isochemical phase transition it has to have a clapeyron slope of dp dt 6 mpa k 1 to prevent convection currents from crossing this value is improbably low if the discontinuity represents a chemical boundary the intrinsic density difference has to exceed 3 to prevent subducted lithospheric slabs from penetrating deeply into the lower mantle also the condition is possibly hard to meet the least improbable mechanism for a mid mantle barrier for convection currents would be a combination of endothermic phase transition and chemical change the boundary between upper and lower mantle would show considerable topography and a limited material exchange is to be expected at any rate the possibility of a downward segregation of former oceanic crust transformed to dense eclogite is studied in a further model series it requires a region of low viscosity as the delayer probably is and is faciliated by the decrease of the thermal expansion coefficient with pressure about 20 of subducted oceanic crust could accumulate at the core mantle boundary the dense material would concentrate underneath rising therm al plumes and some of it is entrained into the plumes possibly affecting their geochemical signature

rate limited mass transfer or macrodispersion which dominates plume evolution at the macrodispersion experiment made site we present a model of solute transport that explains the large scale behavior of the solute tracer test plumes at the macrodispersion experiment made site as the result of advection and rate limited mass transfer between mobile and small scale immobile domains this model does not consider the process of dispersion and yet provides an alternative explanation of the evolution of the observed concentration profiles compared to the macrodispersion model the mass transfer model better represents the change in mobile dissolved mass with time the peak of the concentration profile and the profile asymmetry specifically unlike the macrodispersion model the mass transfer model explains the facts that the observed mass of the plume was greater than the injected mass in early snap shots of the plume and less than the injected mass at late times we suggest that the injected mass advects through the mobile domain and diffuses into and out of the immobile domain the immobile domain consists of a combination of low permeability zones on the scale of centimeters to decimeters the darcy scale immobile domain and intragranular porosity dead end pores and surface sorption the pore scale immobile domain we suggest that the mobile domain was sampled preferentially when water was extracted therefore at early times relatively clean water in the immobile domain was not sampled and incorrectly assumed to contain high solute concentrations similarly the mass at late times was underestimated because solute trapped in the pore scale immobile domain was not extracted during sampling and therefore ignored the combination of advection and slow mass transfer is consistent with the fact that the peak of the plume migrated only 5 m by the termination of the experiment as well as the different behavior of bromide and tritium tracers

seismological mapping of fine structure near the base of the earth s mantle the earth s core mantle boundary cmb juxtaposes liquid iron and crystalline silicates and is a region of large vertical thermal gradients the d region which extends up to 200 300 km above the cmb often has elevated shear wave velocity and suggestions of lateral variations in structure1 recent improvements in our ability to assemble and analyse records from regional seismic networks have allowed us to examine long profiles of travel times amplitudes and waveforms from more than a thousand short period seismometers2 we observe across canada and the united states p waves that have grazed the cmb from the powerful nuclear test in lop nor china on 21 may 1992 first arrival travel times and large secondary arrivals are consistent with a 1 5 compressional velocity increase with depth 130 km above the cmb about half the thickness of d in this locality3 our observations together with evidence for the absence of such a thin fast layer in neighbouring regions suggest the presence of lateral heterogeneity in composition or phase at the base of the mantle

catastrophic overturn of the earth s mantle driven by multiple phase changes and internal heat generation the effects of phase changes and strong internal heat generation may combine to bring about brief but extremely intense episodes of rapid thermal convection in the earth s mantle numerical calculations using realistic thermodynamic properties for the exothermic olivine spinel and endothermic spinel perovskite magnesiowustite phase transitions suggest the transition region of the earth s mantle may act as a capacitor for subducting slabs slab material accumulates in the transition region until a threshold level of thermal buoyancy is reached and is then rapidly discharged into the lower mantle in my calculations this occurs as a catastrophic burst of convection lasting 10 myr with elevated heat transfer rates lasting 100 myr such episodes may be analogs to superplume activity which has been hypothesized to give rise to an intense episode of intra plate volcanism and stabilize the geodynamo against reversals the topography on the two phase change boundaries is found to be negatively correlated with the correlation becoming more negative during periods of rapid convection furthermore the results of this study suggest the transition region of the earth s mantle could be 250 k cooler on average and consequently more viscous than the surrounding mantle

lateral temperature variations at the core mantle boundary deduced from the magnetic field recent studies of the secular variation of the earth s magnetic field over periods of a few centuries have suggested that the pattern of fluid motion near the surface of earth s outer core may be strongly influenced by lateral temperature variations in the lowermost mantle in this paper we introduce a self consistent method for finding the temperature variations near the core surface by assuming that the dynamical balance there is geostrophic and that lateral density variations there are thermal in origin the calculation is accomplished by using the radial and horizontal components of the magnetic field at the core mantle boundary to calculate the fluid flow and its radial derivative near the core surface which are related to the pattern of lateral temperature variations by the thermal wind equation as expected these lateral temperature variations are very small we find some agreement between this pattern and the pattern of topography of the core mantle boundary but are unable to answer conclusively to what extent core surface motions are controlled by the mantle rather than being determined by processes in the core

the secular variation of earth s magnetic field models of the magnetic field at the core mantle boundary at selected epochs from 1715 0 to 1980 0 reveal novel features in the field at the core these suggest that the flow of core fluid is coupled to the mantle and that magnetic diffusion is significant

mapping the lowermost mantle using core reflected shear waves a map of laterally varying d velocities is obtained for the region from 50 s to 50 n in latitude and 70 e to 190 e in longitude velocities are found using an analysis of the differential travel time residuals from 481 scs s and 266 sscs ss phase pairs the long period data are taken from the global digital seismograph network digital waveform catalog for the time period of january 1980 to march 1987 each differential travel time is found by a cross correlation of the s phase ground displacement corrected to simulate differential attenuation with all following phases travel times are corrected for ellipticity and mantle heterogeneity outside of their d paths and the remaining residuals are interpreted as the result of d heterogeneity ray tracing tests are made to check the validity of converting travel time residuals into velocity path anomalies the resulting map reveals significant long wavelength d structure including a 3 low velocity region beneath northeastern indonesia surrounded by three identified high velocity zones beneath northwestern pacifica 4 southeast asia 3 and australia 3 5 this structure is of continent ocean spatial scales and is most likely created by dynamic processes dominant in the lower mantle the low velocity region may have both chemical and thermal origins and is very possibly the site of an incipient lower mantle plume where mature d rock which has been heated by the core has become gravitationally unstable and begun to rise a chemical component possibly exists as a chemical boundary layer is dragged laterally toward the plume site much the way continents are dragged toward subduction zones the high velocity zones possibly result from the downward convection of cold lower mantle plumes which pond at the core mantle boundary these seismic anomalies may also contain a chemical signature from faster iron poor materials brought down through the lower mantle or the additional presence of sio2 stishovite perhaps in its higher pressure polymorph

waveform inversion for three dimensional density and s wave structure long period seismic data longer than about 100 s are sensitive to density as well as p wave and s wave structure in the earth seismograms containing overtone and fundamental mode signals are inverted directly for density and s wave velocity structure a simple relation between p wave and s wave velocity perturbations is assumed in order to reduce the number of unknowns a model is parameterized by spherical harmonics up to degree and order 8 and by five knots in depth at 30 220 450 670 and 882 km with linear interpolation between adjacent knots density resolution is good only at shallow depths in fact only the average in the upper 200 km is marginally resolved s wave resolution is generally good throughout the depth range s wave results show good correlation to surface geological features and are similar to previous studies density results show quite different patterns from s wave maps high density regions appear along the subduction zones surrounding the pacific ocean and low density regions appear in most major continents most continents have low density and high s wave velocity which suggests that continental mantle consists of compositionally different materials since thermal effects can never produce such perturbations high s wave velocity also suggests a relatively cold geotherm for continents and thus continents should consist of significantly low density material such as depleted mantle rocks as proposed by many researchers before the density map also correlates well with a geoid map for spherical harmonic components l 4 6 but not for l 2 3 it suggests that sources for these geoid undulations l 4 6 are in the upper mantle probably in the upper 300 400 km but those for l 2 3 are in deeper regions

a search for anomalies in the lowermost mantle using seismic bulletins abstract for deep events and epicentral distances around 75 the bulletins of the international seismological centre bisc contain many secondary arrivals between p and pcp the reflection from the core mantle boundary cmb which cannot be explained by current spherical symmetric earth models assuming that these arrivals in the p coda are reflections from p velocity anomalies in the lower mantle we calculate the associated reflection points geographical coordinates and distance from the cmb and map both strength relative density of reflection points in bins in the lower mantle and distance of the reflectors from the cmb on a global scale the regions with these anomalous bins cover 3 of the 50 of the lowermost mantle that can be analysed with this data set the most significant regions with anomalous bins values in parentheses give a preliminary estimate of the distance of the reflector from the cmb are under the north atlantic ridge 250 km northern central siberia 230 km japan 180 km the central mid atlantic ridge 310 and 260 km and off the coast of ecuador 340 km the presence of reflectors in one of these regions has already been confirmed by detailed seismic studies using waveform data from arrays and networks a slight 2 3 p velocity contrast is sufficient to produce measurable reflection amplitudes in the distance range of 70 80 ten percent of the lower mantle sampled by this study is covered by bins with no or very few secondary arrivals in the p coda the most significant regions with such a normal lower mantle are under central america the caroline and marshall islands in the eastern pacific the area of hawaii and the easter islands and the northern atlantic identification of the regions with anomalous bins serves to locate the areas of the lowermost mantle which should be analysed further by using the full information content of seismograms the study of the travel time the amplitude and the form of the wavefield recorded preferably at seismic arrays dense networks will allow the determination of the anomalous velocity structure of the core mantle transition zone in more detail the most promising areas for such a study are given here but further anomalies are likely to exist

interaction of plumes with a compositional boundary at 670 km the seismic discontinuity observed at a depth of 670 km may result from a phase change a compositional boundary or both likewise it may mark a change in density viscosity or both between the upper and lower mantle this paper addresses whether upwelling plumes in the lower mantle penetrate into the upper mantle and what effect lower mantle plumes have on convection in the upper mantle if there is a density jump at 670 km due to a change in composition with depth a finite element model of double diffusive convection is used to model flow in a spherical axisymmetric shell heated from below with a rayleigh number of 106 the ability of plumes to rise into the upper mantle is governed by the buoyancy number b c t which is the ratio of the compositional to thermal buoyancy when b 1 flow in the upper layer is shear coupled to flow in the lower layer that is a plume impinging on the compositional boundary will induce a down welling in the overlying layer when b 1 the coupling is more complex strong plumes in the lower layer are shear coupled to flow in the overlying layer but as plumes evolve and weaken the flow in the upper layer switches to thermal coupling with entrainment of lower mantle material

geophysical inferences of thermal chemical structures in the lower mantle lateral variations of the temperature field in the lower mantle have been reconstructed using new results in mineral physics and seismic tomographic data we show that with the application of high pressure experimental values of thermal expansivity and of sound velocities the slow seismic anomalies in the lower mantle under the pacific and africa can be converted into realistically looking plume structures with large dimensions of 0 10 km the outer fringes of the plumes have an excess temperature of around 400 k in the core of the plumes are found tongue like structures with extremely high thermal anomalies these values can exceed 1200 k and are too high to be explained on the basis of thermal anomalies alone we suggest that these major plumes in the deep mantle may be driven by both thermal and chemical buoyancies or that enhanced conductive heat transfer may be important there

time dependent mapping of the magnetic field at the core mantle boundary we consider the problem of constructing a time dependent map of the magnetic field at the core mantle boundary we use almost all the available data from the last 300 years to produce two maps one for the period 1690 1840 and the other for 1840 1990 we represent the spatial dependency of the field using spherical harmonics the time dependency using a cubic b spline basis and seek the smoothest solutions compatible with the observations particular attention must be paid to the effects of the crustal field in the data we argue that for observations from permanent magnetic observatories the most efficient strategy is to use first differences of annual means for satellite data the most efficient strategy is simply to limit the number of data used so as to minimize any tendency to map the crustal field into the core field the resulting model fits the observatory data better than any previous model yet has less power in the secular variation than previous models demonstrating that very simple models fit the data the resulting time dependent field map exhibits much of the same structure in the field and its secular variation identified in earlier studies

models of the core mantle boundary and the travel times of internally reflected core phases topographic maps of the core mantle boundary cmb obtained by inverting different subsets of international seismological centre arrival time data involving either waves refracted through the boundary pkp or waves reflected from the topside pcp do not agree we report on the implications for models of cmb structure when data from bottomside reflections notably pkkp are included in the inversion travel time residuals with respect to a reference earth model prem of all core phases are positively biased but the mean residual is different for different phases and must be accounted for prior to inversion the new data illustrate that models of cmb topography can explain only a small fraction of the data variance the new cmb model involves both topography and a laterally varying boundary layer the cmb itself is smooth compared to previous results

further evidence for the lay discontinuity beneath northern siberia and the north atlantic from short period p waves recorded in france abstract we analyze data from the french laboratoire de detection geophysique ldg network of digital short period seismic stations the p waves from several earthquakes in the kuril okhotsk region display clear secondary arrivals in the 75 85 range which are not explained by smooth lower mantle models the move out of the arrivals across the network indicates a deep mantle origin we interpret them as waves reflected off or transmitted at the lay discontinuity about 300 km above the core mantle boundary the bounce points at this discontinuity are located beneath northern siberia slightly to the northwest of the region for which a similar structure has been reported we propose tools for investigating the lateral variations of the d structure on the scale 100 1000 km the method we propose is based on the deconvolution of the data or convolution of synthetics by a source time function derived from the data we show that the method helps identification of the waves that interact with the lay discontinuity and enables a more objective mapping of its lateral variations evidence for bumps a few kilometers high on the lateral scale of a few tens of kilometers is proposed we also probe a spot of the lower mantle beneath the north atlantic a detailed analysis of the waveforms from a chiapas earthquake suggests that the lay discontinuity is also present in that part of the world

spatially varying hydraulic and solute transport characteristics of a fractured till determined by field tracer tests funen denmark a natural gradient tracer test that simulated one dimensional flow was conducted in a large 4 m 4 8 m surface area isolated block of fractured till in funen denmark to determine both bulk and spatially distributed hydraulic properties hydraulic conductivity based on free flow tests was higher in the upper 2 5 m 5 81 10 5 m s 1 than in the upper 4 m 8 00 10 6 m s 1 of till indicating the more highly fractured and bioturbated nature of the upper part of the till chloride was introduced at the surface of the block as nacl solution 490 mg l 1 cl and breakthrough of cl was monitored in 13 horizontal screens at depths of 2 5 and 4 m arrival times of half of the input cl concentrations c c0 0 5 ranged about sevenfold among various screens at each sampling depth 5 35 hours at 2 5 m and 11 68 hours at 4 m chloride breakthrough data for the two depths were fitted to both an equivalent porous media epm model based on the convective dispersive equation and a discrete fracture model these results indicate differences in transport characteristics of the till with depth at the 2 5 m depth the epm model fit the initial part of the breakthrough curves quite well for most samplers but did not predict the long tails adequately the converse was generally true for the discrete fracture model thus it appears that transport in the upper 2 5 m of till is controlled by a complex interconnected system of fractures and macropores field measurements of fractures and macropores at this site substantiate these results at the 4 m depth the discrete fracture model described solute transport in all samplers better than the epm model indicating that vertical fracture flow dominates in the lower portion of the block fracture apertures and spacings were estimated based on the cubic law and fitting the breakthrough data to the discrete fracture model fitted fracture spacings at the 4 m depth 0 025 0 08 m were slightly smaller than the minimum spacing 0 09 m mapped in the field calculated apertures at the 4 m depth were large compared with other studies and ranged from 65 to 127 m average value of 94 m

effects of strongly temperature dependent viscosity on time dependent three dimensional models of mantle convection numerical simulations of thermal convection in a wide 8 8 1 cartesian box heated from below with temperature dependent viscosity contrasts of 1000 and rayleigh number 105 show that boundary conditions and aspect ratio have an enormous effect on the preferred flow pattern with rigid upper and lower boundaries spoke pattern flow with small diameter 1 5 cells is obtained consistent with laboratory experiments and previous numerical results however with the arguably more realistic stress free boundaries the flow chooses the largest possible wavelength forming a single square cell of aspect ratio 8 with one huge cylindrical downwelling surrounded by upwelling sheets the addition of stress dependence to the rheology weakens the stiff upper boundary layer resulting in smaller cells though still with upwelling sheets and downwelling plumes

development of diapiric structures in the upper mantle due to phase transitions solid state phase transitions in time dependent mantle convection can induce diapiric flows in the upper mantle when a deep mantle plume rises toward phase boundaries in the upper mantle the changes in the local thermal buoyancy local heat capacity and latent heat associated with the phase change at a depth of 670 kilometers tend to pinch off the plume head from the feeding stem and form a diapir this mechanism may explain episodic hot spot volcanism the nature of the multiple phase boundaries at the boundary between the upper and lower mantle may control the fate of deep mantle plumes allowing hot plumes to go through and retarding the tepid ones

evidence from earthquakes for bookshelf faulting at large non transform ridge offsets migrating non transform offsets which occur along mid ocean ridges when a propagating segment gradually elongates and takes over spreading from a neighbouring doomed rift segment1 3 represent a significant modification of the plate tectonic paradigm4 the migrating offset zone between the two spreading segments is clearly a locus of significant deformation but the mechanism of this deformation has been controversial here we use the source parameters and locations of earthquakes at such offsets to discriminate between previously proposed models2 4 7 earthquakes at large non transform offsets at medium and fast spreading rates show strike slip mechanisms with nodal planes rotated relative to the expected transform fault orientation one set of nodal planes presumably corresponding to the fault planes is parallel to curved sea floor lineaments which show increased rotation as a function of position in the deforming zone a bookshelf faulting model in which initially ridge parallel faults are rotated by simple shear is consistent with the observed lineament orientations focal mechanisms and earthquake distributions

traveltime based descriptions of transport and mixing in heterogeneous domains 1 modeling mixing controlled reactive transport using traditional spatial discretization of the domain requires identifying the spatial distributions of hydraulic and reactive parameters including mixing related quantities such as dispersivities and kinetic mass transfer coefficients in most applications breakthrough curves btcs of conservative and reactive compounds are measured at only a few locations and spatially explicit models are calibrated by matching these btcs a common difficulty in such applications is that the individual btcs differ too strongly to justify the assumption of spatial homogeneity whereas the number of observation points is too small to identify the spatial distribution of the decisive parameters the key objective of the current study is to characterize physical transport by the analysis of conservative tracer btcs and predict the macroscopic btcs of compounds that react upon mixing from the interpretation of conservative tracer btcs and reactive parameters determined in the laboratory we do this in the framework of traveltime based transport models which do not require spatially explicit costly aquifer characterization by considering btcs of a conservative tracer measured on different scales one can distinguish between mixing which is a prerequisite for reactions and spreading which per se does not foster reactions in the traveltime based framework the btc of a solute crossing an observation plane or ending in a well is interpreted as the weighted average of concentrations in an ensemble of non interacting streamtubes each of which is characterized by a distinct traveltime value mixing is described by longitudinal dispersion and or kinetic mass transfer along individual streamtubes whereas spreading is characterized by the distribution of traveltimes which also determines the weights associated with each stream tube key issues in using the traveltime based framework include the description of mixing mechanisms and the estimation of the traveltime distribution in this work we account for both apparent longitudinal dispersion and kinetic mass transfer as mixing mechanisms thus generalizing the stochastic convective model with or without inter phase mass transfer and the advective dispersive streamtube model we present a nonparametric approach of determining the traveltime distribution given a btc integrated over an observation plane and estimated mixing parameters the latter approach is superior to fitting parametric models in cases wherein the true traveltime distribution exhibits multiple peaks or long tails it is demonstrated that there is freedom for the combinations of mixing parameters and traveltime distributions to fit conservative btcs and describe the tailing a reactive transport case of a dual michaelis menten problem demonstrates that the reactive mixing introduced by local dispersion and mass transfer may be described by apparent mean mass transfer with coefficients evaluated by local btcs

inhomogeneities near the core mantle boundary evidenced from scattered waves a review statistical properties of small scale inhomogeneities wavelengths between 20 and 70 km near the core mantle boundary are inferred from scattered core waves observations of scattered core waves at large seismic arrays and worldwide networks indicate that the inhomogeneities have a global nature with similar characteristics however there may exist a few regions having markedly stronger or weaker strengths scattering by volumetric inhomogeneities of about 1 inp wave velocity in the lower mantle or by about 300 m of topographic relief of the core mantle boundary can explain the observations at present it is not possible to rule out either of these two alternatives or a combination of both

the nature of the earth s core the properties of the earth s core are overviewed with emphasis on seismologically determined regions and pressures and seismologically measured density elastic wave velocities and gravitational acceleration attention is given to solid state convection of the inner core and it is noted that though seismological results do not conclusively prove that the inner core is convective the occurrence and magnitude of seismic anisotropy are explained by the effects of solid state convection igneous petrology and geochemistry of the inner core a layer at the base of the mantle and contact metasomatism at the core mantle boundary and evolution of the core mantle system are discussed it is pointed out that high pressure melting experiments indicate that the temperature of the core is ranging from 4500 to 6500 k and a major implication of such high temperature is that the tectonics and convection of the mantle as well as the resulting geological processes observed at the surface are powered by heat from the core as a result of the high temperatures along with the compositional contrast between silicates and iron alloy the core mantle boundary is considered to be most chemically active region of the earth

mantle layering from scs reverberations 4 the lower mantle and core mantle boundary this is the final installment in a four part sequence that examines the nature of mantle layering required by the multiple scs phases and internal reflections observed within the reverberative interval of sh polarized seismograms in this paper long period sh polarized multiple scs phases reflected once from a mantle discontinuity first order reverberations are used to search for abrupt shear wave impedance contrasts in the lower mantle beneath the geographic regions sampled the depth interval of 1000 2600 km bullen s region d appears free of any distinct radial layering in agreement with the majority of recent seismic models and the notion of near adiabatic compression of a compositionally homogeneous lower mantle to pass undetected discontinuities in d must be either small sh polarized reflection coefficient r0 1 0 highly intermittent or subject to lateral depth variations well in excess of 50 km at greater depths corresponding to the d region of the lowermost mantle we find evidence for a reflector of long period seismic energy situated an average of 325 km above the core mantle boundary cmb similar to the discontinuity proposed by t lay and coworkers our results indicate a 4 4 increase in shear wave impedance and a 1 7 increase in density if the 2 75 increase in shear velocity proposed by lay and helmberger 1983 holds true for our study area the discontinuity is observed beneath less than one third of the 18 seismic corridors examined and does not appear to be a ubiquitous feature of the lower mantle this plus an observed correlation of discontinuity depth and lower mantle velocity heterogeneity and the lack of experimental evidence for deep mantle phase transformations strongly favor a compositional origin mantle side layering nearer the cmb perhaps associated with a thin chemical boundary layer would mimic the effects of the surface crust producing a complex wave train of first and higher order reverberations superposed upon all other reverberative interval arrivals if present and unaccounted for cmb structure would drive estimates of crustal structure and qscs obtained by multiple scs waveform inversion away from their true values compared to independent estimates our results admit little bias monte carlo tests are used to assess the sensitivity of this technique to cmb layering which decreases with decreasing boundary layer thickness nonetheless the results limit cmb transition zones in excess of 20 km to impedance contrasts less than 12 8 for a 40 km transition beneath much of the western pacific australia melanesia and indonesia