the influence of fluid type on elastic wave velocity and attenuation in volcanic rocks abstract volcano seismic signatures are the convolution of source and path effects the source signature provides important information about the dynamic rock deformation and fluid resonance processes within the volcano but path effects are the result of the volcanic plumbing the edifice physical conditions and the complex geological structure complicating the inversion of seismic data further is the influence of fluid properties here we use ultrasonic experiments to understand the effect of variable fluid viscosity and bulk modulus on p wave attenuation and velocity for volcanic rocks from white island volcano whakaari new zealand our experimental conditions for high porosity ash tuff and low porosity lava resemble shallow and deep volcano facies respectively p wave velocity vp and quality factor qp increase with the viscosity and bulk modulus of the saturating fluid and decrease with porosity and fracture volume tuffs are highly sensitive to saturating fluid with variations in vp of up to 67 and qp of 383 vp and qp in fractured lavas are slightly less sensitive to fluids 24 and 296 respectively but vp can be used to identify lithology for these tuff vs lavas comparison of the experimental data with rock physics models suggests that squirt flow is the dominant attenuation mechanism for viscosities greater than that of water while scattering losses may be important in gas saturated rocks our results have significant implications for the interpretation of seismic tomography studies and for volcano hazard monitoring based on the characteristics of detected seismicity

imaging permeability structure within the highly permeable carbonate earth inverse theory and experiment in most oil reservoir rocks the squirt flow characteristic frequency fsq range of 10 100 khz is much less than the biot characteristic frequency f0 range of 1 mhz 1 ghz in contrast carbonate aquifers have very high permeability 10 3000 d and the two characteristic frequencies are reversed f0 from 10 hz to 10 khz and fsq from 1 mhz to 100 mhz the principal objective of this paper is to develop a pilot inverse method for robust imaging of the high permeability structure within carbonate aquifers an analytical approximate model called the super k model is developed the super k model coincides numerically with the combined biot and squirt flow mechanism model when permeability is higher than 100 md and the frequency is lower than 100 khz the super k regime in the super k regime the pore fluid is always relaxed so that the attenuation due to the biot mechanism is roughly four times larger than that of the biot model also empirical equations are developed that relate the stiffness and rigidit

incorporating full attenuation mechanisms of poroelastic media for realistic subsurface sensing porous materials are ubiquitous in the subsurface formations of the earth where acoustic and seismic waves are used for remote sensing however it is not well understood how the dissipation and the dispersion of poroelastic waves are caused by the viscoelastic and viscous properties of the constituents such as solid grains and pore fluid and by the viscoelastic dissipation of the solid frame as well as the viscodynamic coupling of the pore fluid to the solid frame due to its global and local flows relative to the solid grains such attenuation mechanisms have seldom been incorporated in subsurface sensing simulations although they can be very important to applications in this paper we propose a complete attenuation model including both full stiffness and viscodynamic dissipation for poroelastic media in seismic wave simulations completely based on a generalized zener model the effects associated with physical dissipation and frequency dependent dispersion are accurately simulated by a finite difference time domain algorithm verifications with analytical solutions show the accuracy efficiency and flexibility of our method numerical results demonstrate that the attenuation of biot s model in the sediment of the seafloor has significant effects on acoustic wave scattering from complex geologic structures

four dimensional seismology and true direct detection of hydrocarbons the petrophysical basis figure 1 published by m king hubbert in 1962 and again in 1969 shows the exploitation of fossil fuels in a historical perspective according to this graph about half of the fossil energy on earth is being used up in a period of about 200 years half of which is by now over

a generalized biot gassmann model for the acoustic properties of shaley sandstones1 we obtain the wave velocities of clay bearing sandstones as a function of clay content porosity and frequency unlike previous theories based simply on slowness and or moduli averaging or two phase models we use a biot type three phase theory that considers the existence of two solids sand grains and clay particles and a fluid the theory which is consistent with the critical porosity concept uses three free parameters that determine the dependence of the dry rock moduli of the sand and clay matrices as a function of porosity and clay content testing of the model with laboratory data shows good agreement between predictions and measurements in addition to a rock physics model that can be useful for petrophysical interpretation of wave velocities obtained from well logs and surface seismic data the model provides the differential equation for computing synthetic seismograms in inhomogeneous media from the seismic to the ultrasonic frequency bands

full frequency range transient solution for compressional waves in a fluid saturated viscoacoustic porous medium1 an analytical transient solution is obtained for propagation of compressional waves in a homogeneous porous dissipative medium the solution based on a generalization of biot s poroelastic equations holds for the low and high frequency ranges and includes viscoelastic phenomena of a very general nature besides the biot relaxation mechanism the viscodynamic operator is used to model the dynamic behaviour associated with the relative motion of the fluid in the pores at all frequency ranges viscoelasticity is introduced through the standard linear solid which allows the modelling of a general relaxation spectrum the solution is used to study the influence of the material properties such as bulk moduli porosity viscosity permeability and intrinsic attenuation on the kinematic and dynamic characteristics of the two compressional waves supported by the medium we also obtain snapshots of the static mode arising from the diffusive behaviour of the slow wave at low frequencies lntroduction the acoustics of porous media within the framework of petroleum geophysics have been receiving more attention in recent years regional exploration seismology aimed at the discovery of hydrocarbon reservoirs is based on simplified rheological models and ray representations of the wavefield in the past these approximations were sufficient since the degree of resolution required was not high in the present time when most reservoirs are delimited the need to increase the resolution and use not only the traveltimes but also the information contained in the amplitude and phase of the seismic wavefield has gradually emerged the new exploration scenario is confined to reservoir areas and involves the presence of oil wells and data from seismic logs and well seismics which have enough resolution to see the effects of bulk properties porosity permeability fluid content and fluid solid interaction on the seismic pulse in this case the correct

slow waves in fractures filled with viscous fluid stoneley guided waves in a fluid filled fracture generally have larger amplitudes than other waves and therefore their properties need to be incorporated in more realistic models in this study a fracture is modeled as an infinite layer of viscous fluid bounded by two elastic half spaces with identical parameters for small fracture thickness i obtain a simple dispersion equation for wave propagation velocity this velocity is much smaller than the velocity of a fluid wave in a biot type solution in which fracture walls are assumed to be rigid at seismic prospecting frequencies and realistic fracture thicknesses the stoneley guided wave has wavelengths on the order of several meters and an attenuation q factor exceeding 10 which indicates the possibility of resonance excitation in fluid bearing rocks the velocity and attenuation of stoneley guided waves are distinctly different at low frequencies for water and oil the predominant role of fractures in fluid flow at field scales is supported by permeability data showing an increase of several orders of magnitude when compared to values obtained at laboratory scales these data suggest that stoneley guided waves should be taken into account in theories describing seismic wave propagation in fluid saturated rocks

a comprehensive model of the quiet time near earth magnetic field phase 3 summary the near earth magnetic field is caused by sources in the earth s core ionosphere magnetosphere lithosphere and from coupling currents between the ionosphere and the magnetosphere and between hemispheres traditionally the main field low degree internal field and magnetospheric field have been modelled simultaneously with fields from other sources being modelled separately such a scheme however can introduce spurious features especially when the spatial and temporal scales of the fields overlap a new model designated cm3 comprehensive model phase 3 is the third in a series of efforts to coestimate fields from all of these sources this model has been derived from quiet time magsat and pogo satellite and observatory hourly means measurements for the period 1960 1985 it represents a significant advance in the treatment of the aforementioned field sources over previous attempts and includes an accounting for main field influences on the magnetosphere main field and solar activity influences on the ionosphere seasonal influences on the coupling currents a priori characterization of the influence of the ionosphere and the magnetosphere on earth induced fields and an explicit parametrization and estimation of the lithospheric field the result is a model that describes well the 591 432 data with 16 594 parameters implying a data to parameter ratio of 36 which is larger than several popular field models

wave velocities and attenuation of shaley sandstones as a function of pore pressure and partial saturation we obtain the wave velocities and quality factors of clay bearing sandstones as a function of pore pressure frequency and partial saturation the model is based on a biot type three phase theory that considers the coexistence of two solids sand grains and clay particles and a fluid mixture additional attenuation is described with the constant q model and viscodynamic functions to model the high frequency behaviour we apply a uniform gas fluid mixing law that satisfies the wood and voigt averages at low and high frequencies respectively pressure effects are accounted for by using an effective stress law by fitting a permeability model of the kozeny carman type to core data the model is able to predict wave velocity and attenuation from seismic to ultrasonic frequencies including the effects of partial saturation testing of the model with laboratory data shows good agreement between predictions and measurements

anelasticity and dispersion in dry unconsolidated sands laboratory measurements reveal frame modulus dispersion and creeping behavior in room dry samples of unconsolidated quartzo feldspathic upper miocene turbidites from the wilmington field ca a single standard linear solid model predicts both the dispersion and the creep response similar behavior is observed in ottawa sand samples when mixed with wetted montmorillonite in this case the magnitude of dispersion and creep increase with increasing clay content we suggest that the anelastic behavior of the wilmington sands is due to the presence of micas and a small amount of clay the transition frequency of 2 10 4hz i e between high and low frequency behavior suggests that seismic sonic and ultrasonic measurements of rock moduli are quite similar in the absence of fluids the static frame modulus should be used to model tectonic processes and long term production related changes in oil field environments

analysis of multiscale scattering and poroelastic attenuation in a real sedimentary rock sequence compressional waves in heterogeneous permeable media experience attenuation from both scattering and induced pore scale flow of the viscous saturating fluid for a real finely sampled sedimentary sequence consisting of 255 layers and covering 30 meters of depth elastic and poroelastic computer models are applied to investigate the relative importance of scattering and fluid flow attenuation the computer models incorporate the known porosity permeability and elastic properties of the sand shale sequence in a binary medium plane layered structure the modeled elastic scattering attenuation is well described by stochastic medium theory if two length scale statistics are applied to reflect the relative thickness of the shale layers when compared to the sand layers under the poroelastic biot squirt flow model fluid flow attenuation from the moderate permeability 10 14 m2 sands may be separated in the frequency domain from the attenuation due to the low permeability 5 10 17 m2 shale layers based on

unique problems associated with seismic analysis of partially gas saturated unconsolidated sediments abstract gas hydrate stability conditions restrict the occurrence of gas hydrate to unconsolidated and high water content sediments at shallow depths because of these host sediments properties seismic and well log data acquired for the detection of free gas and associated gas hydrate bearing sediments often require nonconventional analysis for example a conventional method of identifying free gas using the compressional shear wave velocity v p v s ratio at the logging frequency will not work unless the free gas saturations are more than about 40 the p wave velocity dispersion of partially gas saturated sediments causes a problem in interpreting well log velocities and seismic data using the white j e 1975 computed seismic speeds and attenuation in rocks with partial gas saturation geophysics 40 224 232 model for partially gas saturated sediments the difference between well log and seismic velocities can be reconciled the inclusion of p wave velocity dispersion in interpreting well log data is therefore essential to identify free gas and to tie surface seismic data to synthetic seismograms

geochemistry and evolution of the qingyuan archaean granite greenstone terrain ne china the qingyuan archaean granite greenstone terrain is a complex synclinal belt surrounded by granites the first grade structure is an s type regional fold the metavolcano sedimentary series of amphibolite facies is divided into 3 formations from the lower to the upper basic volcanic formation shipenzi fm calc alkaline volcanic formation hongtoushan fm and sedimentary formation nantianmen fm according to the ree distributions the qingyuan basaltic komatiites belong to groups i and ii of jahn et al most tholeiitic rocks belong to the th1 dat type of condie that exhibits lree depletion only a few samples of the hongtoushan fm belong to the th2 eat type the distinguishing feature of the f1 dsv type felsic rocks of the shipenzi fm are their strong depletion in hree but the felsic rocks of the hongtoushan fm belong to the f2 type usv with which the famous hongtoushan massive sulphide copper ore deposit is closely associated the supracrustal rocks overlie a basement of high grade gneissic rocks a lot of tonalitic and monzonitic granites of 2 7 2 9 ga age intruded the greenstone belt the rock association of the qingyuan greenstone succession is similar to the volcanic association in a continental rift setting it is possible that the archaean greenstone belt erupted in a palaeo continental rift which was generated above hot spots or hot lines in the mantle 2 7 2 9 ga ago

the effect of pore pressure depletion and injection cycles on ultrasonic velocity and quality factor in a quartz sandstone passing seismic waves generate transient pore pressure changes that influence the velocity and attenuation characteristics of porous rocks compressional ultrasonic wave velocities vp and quality factors qp in a quartz sandstone were measured under cycled pore pressure and uniaxial strain conditions during a laboratory simulated injection and depletion process the objectives were to study the influence of cyclical loading on the acoustic characteristics of a reservoir sandstone and to evaluate the potential to estimate pore fluid pressure from acoustic measurements the values of vp and qp were confirmed to increase with effective stress increase but it was also observed that vp and qp increased with increasing pore pressure at constant effective stress the effective stress coefficient n was found to be less thanone and dependent on the pore pressure confining stress and load at low pore pressures n approached one and reduced nonlinearly at high pore pressures the change in vp and qp with respect

computation of dynamic seismic responses to viscous fluid of digitized three dimensional berea sandstones with a coupled finite difference method the seismic response of saturated porous rocks is studied numerically using microtomographic images of three dimensional digitized berea sandstones a stress strain calculation is employed to compute the velocities and attenuations of rock samples whose sizes are much smaller than the seismic wavelength of interest to compensate for the contributions of small cracks lost in the imaging process to the total velocity and attenuation a hybrid method is developed to recover the crack distribution in which the differential effective medium theory the kuster toksoz model and a modified squirt flow model are utilized in a two step monte carlo inversion in the inversion the velocities of p and s waves measured for the dry and water saturated cases and the measured attenuation of p waves for different fluids are used by using such a hybrid method both the velocities of saturated porous rocks and the attenuations are predicted accurately when compared to laboratory data the hybrid method is a practical

monte carlo simulation of stress associated scattering attenuation from laboratory ultrasonic measurements abstract seismic coda waves scattered by small scale heterogeneities contain information on stress changes of the medium because of changes in the physical state of materials based on the ultrasonic measurements under different stresses for a cylindrical sandstone sample we investigate the influence of stress changes on ultrasonic s coda attenuation and aim to characterize its stress frequency dependent pattern considering the complexity of ultrasonic coda waveforms measured from finite size rock samples in laboratory experiments the monte carlo simulation is employed to synthesize ultrasonic envelopes which act by incorporating the effect of multiple scatterings and boundary reflections on coda waves the optimal simulation parameters estimated by minimizing the residual between the observed and synthesized envelopes indicate that the rock sample under study presents moderate heterogeneities the relationship between attenuation and stress is similar for direct and coda s waves and remains stable in the range of high effective stresses around 30 60 mpa with less stress sensitivity enhanced attenuation for both types of waves occurs at lower effective stresses but with coda attenuation much faster and stronger presenting a quite different nonlinear behavior with respect to stress coda attenuation increases drastically at extremely low effective stresses below 15 mpa because of the increase in rock compliance showing much greater sensitivity to high pore pressure than intrinsic attenuation this study improves our understanding of the mechanism of ultrasonic coda attenuation and its scaling dependence on stress and frequency

seismic wave properties in time dependent porosity homogeneous media summary it is quantified the properties of seismic waves in fully saturated homogeneous porous media within the framework of sahay s modified and reformulated poroelastic theory the computational results comprise amplitude attenuation velocity dispersion and seismic waveforms they show that the behaviour of all four waves modelled as a function of offset frequency porosity fluid viscosity and source bandwidth depicts realistic dissipation within the sonic ultrasonic band therefore it appears that there is no need to include material heterogeneity to model attenuation by inference it is concluded that the fluid viscosity effects may be enhanced by dynamic porosity

acoustic measures of partial gas saturation in tight sandstones the acoustic properties of tight sandstones are very sensitive to partial gas water saturation measurements at 5 khz show that compressional velocity drops sharply with undersaturation extensional and shear losses at 5 khz are large in both fully and partially saturated sands the shear loss qs 1 declines linearly with increasing gas content the extensional loss qe 1 peaks at 0 80 water saturation q 1 is frequency dependent the attenuation is well explained by hydrodynamic relaxation in the grain contacts the measurements indicate velocity dispersions consistent with this mechanism ultrasonic velocities at 500 khz are roughly 10 25 higher than acoustic velocities at 5 khz the ultrasonic velocities exhibit a weak dependence on saturation frame moduli are unrelaxed at these frequencies

seismic evidence for ultralow velocity zones beneath africa and eastern atlantic sks waveforms recorded at distances of about 110 are extremely useful to constrain seismic velocity structure at the base of the mantle sks waves near this distance develop a complicated interference pattern with the phases sp dks and skp ds we report anomalous behavior of this interference in a number of recordings of deep earthquakes beneath south america from stations in europe and africa we model these data with two dimensional dome like structures at the base of the mantle which extend laterally by a few hundred kilometers and in which the shear velocity is up to 30 lower than in the preliminary reference earth model prem the spatial extent of these structures their position with respect to the sks core exit points and their seismic characteristics can not be uniquely determined however the presence of a dipping or a concaved upper interface is a key attribute of successful models models that invoke flat layers are insufficiently complex to explain the most erratic waveform behavior the most anomalous data correspond to sampling regions at the base of the mantle beneath the east african rift and beneath the iceland where possibly whole mantle upwellings form

acoustic and elastic characterization of marine sediments by analysis modeling and inversion of ultrasonic p wave transmission seismograms ultrasonic p wave transmission seismograms recorded on sediment cores have been analyzed to study the acoustic and estimate the elastic properties of marine sediments from different provinces dominated by terrigenous calcareous and diatomaceous sedimentation instantaneous frequencies computed from the transmission seismograms are displayed as gray shaded images to give an acoustic overview of the lithology of each core centimeter scale variations in the ultrasonic waveforms associated with lithological changes are illustrated by wiggle traces in detail cross correlation multiple filter and spectral ratio techniques are applied to derive p wave velocities and attenuation coefficients s wave velocities and attenuation coefficients elastic moduli and permeabilities are calculated by an inversion scheme based on the biot stoll viscoelastic model together with porosity measurements p and s wave scatter diagrams are constructed to characterize different sediment types by their velocity and attenuation porosity relationships they demonstrate that terrigenous calcareous and diatomaceous sediments cover different velocity and attenuation porosity ranges in terrigenous sediments p wave velocities and attenuation coefficients decrease rapidly with increasing porosity whereas s wave velocities and shear moduli are very low calcareous sediments behave similarly at relatively higher porosities foraminifera skeletons in compositions of terrigenous mud and calcareous ooze cause a stiffening of the frame accompanied by higher shear moduli p wave velocities and attenuation coefficients in diatomaceous ooze the contribution of the shear modulus becomes increasingly important and is controlled by the opal content whereas attenuation is very low this leads to the opportunity to predict the opal content from nondestructive p wave velocity measurements at centimeter scale resolution

compressional wave dispersion due to rock matrix stiffening by clay squirt flow the standard biot gassmann theory of poroelasticity fails to explain strong compressional wave velocity dispersion experimentally observed in 12 tight siltstone with clay filled pores in order to analyze and understand the results we developed a new double porosity model of clay squirt flow where wave induced local fluid flow occurs between the micropores in clay aggregates and intergranular macropores the model is validated based on the combined study of ultrasonic experiments on specimens at different saturation conditions and theoretical predictions the presence of a sub pore scale structure of clay micropores contained in intergranular macropores where the fluid does not have enough time to achieve mechanical equilibrium at ultrasonic frequencies and thus stiffens the rock matrix provides a suitable explanation of the experimental data moreover the model provides a new bound for estimating the compressional wave velocity of tight rocks saturated with two immiscible liquids the theoretical predictions indicate that the velocity variation between gas and liquid saturated specimens is predominantly induced by the clay squirt stiffening effect on the rock matrix and not by fluid substitution the effect contributes more than 90 to the variation in the porosity range of 0 5 thus clay squirt flow dominates the relationships between compressional wave velocity and pore fluid in tight rocks

archean sedimentation on dharwar craton india and its implications abstract the dharwar craton is the largest archean cratonic nuclei of india and is made up of western and eastern blocks separated by the 2 5 ga closepet granite batholith the western dharwar craton consists mostly of 3 3 3 0 ga ttg the peninsular gneissic complex older mesoarchean greenstone sargur type and younger neorchean greenstone the dharwar supergroup comprising of lower bababudan and upper chitradurga groups in contrast the eastern dharwar craton is mostly made of granitoid gneisses and felsic volcanics with some neorchean supracrustal enclaves sandur ramgiri kolar kadri schist belts recently it has been suggested that the dharwar cratonic block is formed by assembly of western central and eastern micro blocks separated by shear zones jayananda et al 2018 the greenstone successions are present in all three dharwar micro blocks however the proportion of greenstones is much higher and the quality of preservation is much better in the western micro block the mesoarchean sargur group hosts barite deposit and consists of fuchsite quartzites carbonates metapelites banded iron formations and metamorphosed basalts and komatiites researchers have postulated a platformal orthoquartzite carbonate pelite association for the sargur group some researchers examined the barite deposits of the sargur group and inferred a subaerial volcanism during the sargur sedimentation the neoarchean bababudan group comprises largely terrestrial to shallow marine sedimentary successions glacial deposits bif volcanic and volcaniclastics rocks in contrast the overlying chitradurga group comprises conglomerates greywackes phyllites carbonates bifs cherts shales associated with a variety of volcanic rocks ranging from pillowed basalts to rhyolites and indicates relatively deep water sedimentation in a subsiding basin in an active continental margin setting stromatolites are abundant in the chitradurga group as also in other neoarchean supracrustal successions of the dharwar craton in this review we have critically synthesized published sedimentological data from both older as well as younger greenstone successions of the dharwar craton and have presented the archean sedimentation history of the dharwar craton we have highlighted the research gaps that researchers may find helpful

on the propagation of a disturbance in a heterogeneous deformable porous medium saturated with two fluid phases the coupled modeling of the flow of two immiscible fluid phases in a heterogeneous elastic porous material is formulated in a manner analogous to that for a single fluid phase an asymptotic technique valid when the heterogeneity is smoothly varying is used to derive equations for the phase velocities of the various modes of propagation a cubic equation is associated with the phase velocities of the longitudinal modes the coefficients of the cubic equation are expressed in terms of sums of the determinants of 3 3 matrices whose elements are the parameters found in the governing equations in addition to the three longitudinal modes there is a transverse mode of propagation a generalization of the elastic shear wave estimates of the phase velocities for a homogeneous medium based upon the formulas in this paper agree with previous studies furthermore predictions of longitudinal and transverse phase velocities made for the massilon sandstone containing varying amounts of air and water are compatible with laboratory observations

numerical models of the geodynamo and observational constraints 1 the past few years have seen the emergence of a large number of numerical simulations of the geodynamo in parallel both new and old geomagnetic archeomagnetic and paleomagnetic observations have been interpreted as actual geomagnetic features and used as constraints for dynamo models naturally model predictions should be tested against actual characteristics of the geomagnetic field despite huge differences sometimes in excess of a billion between the values of parameters used in the simulations and those estimated for the earth it is intriguing that many available simulations succeed in producing largely axial dipolar magnetic fields with weaker nondipolar structures in agreement with the first order characteristics of the geomagnetic field yet when considering finer characteristics there are significant differences and failures to actually produce a number of fundamental characteristic features in this presentation we first review numerical results obtained to date then we attempt to summarize which field characteristics derived from observational data sets can be considered robust on the basis of simple criteria used to evaluate the degree of confidence that can be placed in each datum we sort presumably characteristic geomagnetic features into three categories robust controversial and unlikely we conclude that numerical models should be illustrated with a number of key predictions averaged over at least 10 dipole diffusion times these predictions should be tested against the subset of robust observations only controversial observations should await additional confirmation

rock anelasticity due to patchy saturation and fabric heterogeneity a double double porosity model of wave propagation heterogeneity of rock s fabric can induce heterogeneous distribution of immiscible fluids in natural reservoirs since the lithological variations mainly permeability may affect fluid migration in geological time scales resulting in patchy saturation of fluids therefore fabric and saturation inhomogeneities both affect wave propagation to model the wave effects attenuation and velocity dispersion we introduce a double double porosity model where pores saturated with two different fluids overlap with pores having dissimilar compressibilities the governing equations are derived by using hamilton s principle based on the potential energy kinetic energy and dissipation functions and the stiffness coefficients are determined by gedanken experiments yielding one fast p wave and four slow biot waves three examples are given namely muddy siltstones clean dolomites and tight sandstones where fabric heterogeneities at three different spatial scales are analyzed in comparison with experimental data in muddy siltstones where intrapore clay and intergranular pores constitute a submicroscopic double porosity structure wave anelasticity mainly occurs in the frequency range 104 107 hz while in pure dolomites with microscopic heterogeneity of grain contacts and tight sandstones with mesoscopic heterogeneity of less consolidated sands it occurs at 103 107 hz and 101 103 hz seismic band respectively the predicted maximum quality factor of the fast compressional wave for the sandstone is the lowest approximately 8 and that of the dolomite is the highest the results of the diffusive slow waves are affected by the strong friction effects between solids and fluids the model describes wave propagation in patchy saturated rocks with fabric heterogeneity at different scales and the relevant theoretical predictions agree well with the experimental data in fully and partially saturated rocks

local and global fluid effects on sonic wave modes abstractmost subsurface formations of value to exploration contain a heterogeneous fluid filled pore space where local fluid pressure effects can significantly change the velocities of passing seismic waves to better understand the effect of these local pressure gradients on borehole wave propagation we combined chapman s squirt flow model with biot s poroelastic theory we applied the unified theory to a slow and fast formation with permeable borehole walls containing different quantities of compliant pores these results are compared with those for a formation with no soft pores the discrete wavenumber summation method with a monopole point source generates the wavefields consisting of the p s leaky p stoneley and pseudo rayleigh waves the resulting synthetic wave modes are processed using a weighted spectral semblance wss algorithm we found that the resulting wss dispersion curves closely matched the analytical expressions for the formation compressional velocity and solutions to the peri

the squirt flow mechanism macroscopic description we introduce a poroelasticity model that incorporates the two most important mechanisms of solid fluid interaction in rocks the biot mechanism and the squirt flow mechanism this combined biot squirt bisq model relates compressional velocity and attenuation to the elastic constants of the drained skeleton and of the solid phase porosity permeability saturation fluid viscosity and compressibility and the characteristic squirt flow length squirt flow length is a fundamental rock property that does not depend on frequency fluid viscosity or compressibility and is determined experimentally we find that the viscoelastic response of many sandstones is dominated by the squirt flow component of the bisq mechanism and that the viscoelastic properties of these rocks can be expressed through a single dimensionless parameter omega r 2 kappa where omega is angular frequency r is the characteristic squirt flow length and kappa is hydraulic diffusivity the biot mechanism alone does not give an adequate explanation of the observed velocity dispersion and attenuation and the viscoelastic behavior of many sandstones

nonlinear acoustic scattering from a gassy poroelastic seabed a model for difference frequency backscatter from trapped bubbles in sandy sediments was developed a nonlinear volume scattering coefficient was computed via a technique similar to that of ostrovsky and sutin nonlinear sound scattering from subsurface bubble layers in natural physical sources of underwater sound edited by b r kerman kluwer dordrecht 1993 pp 363 373 which treats the case of bubbles surrounded by water biot s poroelastic theory is incorporated to model the acoustics of the sediment biot fast and slow waves are included by modeling the pore fluid as a superposition of two acoustic fluids with effective densities that differ from the pore fluid s actual density and account for its confinement within sediment pores the principle of acoustic reciprocity is employed to develop an expression for the backscattering strength model behavior is consistent with expectations based on the known behavior of bubbles in simpler fluid media

relating p wave attenuation to permeability to relate p wave attenuation to permeability we examine a three dimensional 3 d theoretical model of a cylindrical pore filled with viscous fluid and embedded in an infinite isotropic elastic medium we calculate both attenuation and permeability as functions of the direction of wave propagation attenuation estimates are based on the squirt flow mechanism permeability is calculated using the kozeny carman relation we find that in the case when a plane p wave propagates perpendicular to the pore orientation q 90 1 attenuation is always higher than when a wave propagates parallel to this orientation q 0 1 the ratio of these two attenuation values q 90 1 q 0 1 increases with an increasing pore radius and decreasing frequency and saturation by changing permeability varying the radius of the pore we find that the permeability attenuation relation is characterized by a peak that shifts toward lower permeabilities as frequency decreases therefore the attenuation of a low frequency wave d

small scale structure of the geodynamo inferred from oersted and magsat satellite data the geodynamo in the earth s liquid outer core produces a magnetic field that dominates the large and medium length scales of the magnetic field observed at the earth s surface1 2 here we use data from the currently operating danish oersted3 satellite and from the us magsat2 satellite that operated in 1979 80 to identify and interpret variations in the magnetic field over the past 20 years down to length scales previously inaccessible projected down to the surface of the earth s core we found these variations to be small below the pacific ocean and large at polar latitudes and in a region centred below southern africa the flow pattern at the surface of the core that we calculate to account for these changes is characterized by a westward flow concentrated in retrograde polar vortices and an asymmetric ring where prograde vortices are correlated with highs and retrograde vortices with lows in the historical 400 year average magnetic field4 5 this pattern is analogous to those seen in a large class of numerical dynamo simulations6 except for its longitudinal asymmetry if this asymmetric state was reached often in the past it might account for several persistent patterns observed in the palaeomagnetic field7 8 9 10 we postulate that it might also be a state in which the geodynamo operates before reversing

p wave seismic attenuation by slow wave diffusion effects of inhomogeneous rock properties recent research has established that the dominant p wave attenuation mechanism in reservoir rocks at seismic frequencies is because of wave induced fluid flow mesoscopic loss the p wave induces a fluid pressure difference at mesoscopic scale inhomogeneities larger than the pore size but smaller than the wavelength typically tens of centimeters and generates fluid flow and slow diffusion biot waves continuity of pore pressure is achieved by energy conversion to slow p waves which diffuse away from the interfaces in this context we consider a periodically stratified medium and investigate the amount of attenuation and velocity dispersion caused by different types of heterogeneities in the rock properties namely porosity grain and frame moduli permeability and fluid properties the most effective loss mechanisms result from porosity variations and partial saturation where one of the fluids is very stiff and the other is very compliant such as a highly permeable sandstone at shallow depths saturated with small amounts of gas around 10 saturation and water grain and frame moduli variations are the next cause of attenuation the relaxation peak moves towards low frequencies as the background permeability decreases and the viscosity and thickness of the layers increase the analysis indicates in which cases the seismic band is in the relaxed regime and therefore when the gassmann equation can yield a good approximation to the wave velocity

seismic detection of transient changes beneath black rapids glacier alaska u s a ii basal morphology and processes using changes observed in daily seismic reflections we have investigated the basal morphology of black rapids glacier alaska u s a the englacial drainage of ice marginal lakes caused significant changes in the daily reflections as well as dramatic increases in basal motion changes in reflection arrival times and amplitudes indicate that there is a basal till layer at least 5 m thick at some locations beneath this surge type glacier rapid changes in the observed reflection coefficients during the drainage events indicate that changes in till properties must occur throughout the entire 5 m thick layer they must last for several days following the lake drainages and they must be completely reversible over as little as 36 min our seismic analysis shows that changes in effective pressure of the till are unlikely to cause the required changes in the reflection coefficients but that a decrease in till saturation is likely we therefore interpret the cause ofthe seismic anomalies as being a temporary decrease in saturation as water is input to the subglacial hydraulic system and propose that such a change may occur quickly and reversibly by a redistribution ofoverburden pressure higher water pressures within the hydraulic system cause that region to support more of the glacier s weight leaving the remaining areas to support less any till within these areas of decreased normal stress would experience a consequent decrease in pore water pressure causing gas to exolve thus decreasing saturation this decrease in saturation would cause a change in the strength of the basal layer and may affect basal dynamics

velocity anisotropy and attenuation of shale in under and overpressured conditions the seismic velocity and attenuation of fully saturated shales were measured for the first time under overpressured conditions using the ultrasonic reflection technique shale cores from naturally overpressured horizons in the north sea were tested in the laboratory at confining and pore pressures relevant to in situ conditions a single frequency tone burst pulse wave was used to determine the seismic wave velocities and quality factors of the shale samples with errors less than 0 3 and 0 1 db cm respectively at a frequency of 0 75 mhz sample length changes with varying confining and pore pressure were measured and the pore pressure equilibration time was monitored for each sample the anisotropy of the seismic attributes vp vs qp and qs was determined over a range of differential pressures from 5 to 60 mpa with respect to the predominant foliation the ultrasonic velocity data followed a transversely isotropic pattern depending on the direction of wave propagation with respect to the laminations the poisson s ratio was found to rise by 5 as the shale material progressed from a normally pressured to an overpressured state the quality factor q characteristics were interpreted in terms of pore geometry and connectivity as well as the directional permeability of the transversely isotropic shale material the results were converted to bulk and shear loss modulus defects and a positive bulk loss was observed for waves propagating perpendicular to the lamination plane even above differential pressures of 20 mpa this indicates different levels of biot flow and squirt flow attenuation mechanisms acting within the shale structure depending on the wave propagation and vibration directions

experimental observation of water saturation effects on shear wave splitting in synthetic rock with fractures aligned at oblique angles fractured rocks are known to exhibit seismic anisotropy and shear wave splitting sws sws is commonly used for fractured rock characterization and has been shown to be sensitive to fluid type the presence of partial liquid gas saturation is also known to affect the elastic properties of rocks the combined effect of both fractures and partial liquid gas saturation is still unknown using synthetic silica cemented sandstones with aligned penny shaped voids we conducted laboratory ultrasonic experiments to investigate the effect fractures aligned at an oblique angle to wave propagation would have on sws under partial liquid gas saturation conditions the result for the fractured rock shows a saturation dependence which can be explained by combining a fractured rock model and a partial saturation model at high to full water saturation values sws decreases as a result of the fluid bulk modulus effect on the quasi shear wave this bulk modulus effect is frequency dependent as a result of wave induced fluid flow mechanisms which would in turn lead to frequency dependent sws this result suggests the possible use of sws for discriminating between full liquid saturation and partial liquid gas saturation

acoustic signatures of partial saturation the relationship between elastic wave velocities and water saturation in a water gas reservoir depends strongly on whether saturation is heterogeneous patchy or homogeneous heterogeneity in saturation may result from lithologic heterogeneity because under conditions of capillary equilibrium different lithologies within a reservoir can have different saturations depending on their porosities and permeabilities we investigate this phenomenon by generating models of a reservoir in which we control the distribution of lithologic units and theoretically determine the corresponding velocity saturation relationship we assume a state of capillary equilibrium in the reservoir and determine the saturation level of each region within the reservoir from the corresponding capillary pressure curve for the lithologic unit at that location the velocities we calculate for these models show that saturation heterogeneity caused by lithologic variation can lead to a distinct dependence of velocity on saturation in a water gas saturated reservoir a patchy distribution of the different lithologic units is found to cause p wave velocity to exhibit a noticeable and almost continuous velocity variation across the entire saturation range this is in distinct contrast to the response of a homogeneous reservoir where there is only a large change in velocity at water saturations close to 100

tracking geomagnetic impulses at the core mantle boundary abstract the main magnetic field of the earth is generated and maintained by convective motions in the fluid outer core temporal field variations occur on an impressive variety of scales ranging from months to millions of years among these the most intriguing features are geomagnetic impulses or jerks v courtillot j ducruix and j l le mouel sur une acceleration recente de la variation seculaire du champ magnetique terrestre acad sci paris c r d287 1978 1095 1098 j l le mouel j ducruix and c ha duyen geophys res lett 5 1983 369 1 2 here we investigate these very rapid events at the surface of the earth s core using a global model designed for use at this depth we identify two regions of very active variations where wave like structures propagate the geometrical characteristics of these wave like motions provide new constraints on the intricate dynamics of the core

numerical simulation of the biot slow wave in water saturated nivelsteiner sandstone the development of rock acoustic models is important for interpreting seismic data these models should relate seismic properties such as wave velocity and attenuation to the production and lithological properties of reservoir rocks such as porosity permeability fluid type and fluid saturation

on the recent variations of the apparent westward drift rate quantitative study of the westward drift w d was initiated by bullard et al 1950 and then followed by many others using more or less similar methods e g yukutake 1962 harwood and malin 1976 more recently magsat results have enhanced the interest in the subject see for example fig 1 of peddie 1982 in the present paper we compute the w d rate in the 1950 1980 time interval using different methods and we establish that the 1969 1970 secular acceleration impulse the so called jerk clearly shows up in the results

experimental detection of a slow acoustic wave in sediment at shallow grazing angles following recent experimental results at sea n p chotiros proc oceans 89 that suggest the existence of a previously undetected type of acoustic wave in sandy sediments an experiment was designed to detect and measure the speed of acoustic waves in an isolated environment the experiment was conducted in a laboratory tank containing 1 m of unwashed river sediment under a 3 m water column observations were made of the travel time and attenuation of a pulse from an acoustic source located above the water sediment interface to a set of probes below the interface it was observed that at normal incidence the pulse traveled at about 1750 m s while at shallow grazing angles the pulse traveled through the sediment at close to 1200 m s an interesting possible explanation exists in the biot model m a biot j acoust soc am 28 168 191 1956 which predicts a slow acoustic wave in porous materials work funded by onr under noarl management

application of the bisq model to clay squirt flow in reservoir sandstones 1 an understanding of the variations of compressional wave velocity and attenuation in sandstones as a function of pore fluid properties is important for the identification and monitoring of hydrocarbon reservoirs in order for seismic inversion to be well constrained a rock physics model that describes accurately the observed behavior is required a number of such models have been proposed but have not been extensively tested against laboratory data the unified biot and squirt flow model was applied to data obtained from ultrasonic experiments conducted on a wide range of sandstone types when saturated with different pore fluids with viscosities ranging from 0 3 to 1000 cp this model was seen to match well all the observed velocity and attenuation values as opposed to the biot model which was only successful for clean sandstones saturated with low viscosity fluids this has led to the conclusion that a squirt flow mechanism is important at the grain scale in addition an increase in the dynamic uniaxial strain modulus of the dry frame with increasing pore fluid viscosity was observed in the absence of free gas or grain contact microcracks the laboratory data were collected at 60 mpa effective pressure this can be linked to decreasing local flow and decreasing relaxation in the soft pore space of the sandstone the extent of which is in turn connected to the clay content and pore structure of the sandstone these results imply that at ultrasonic frequencies the squirt flow is due to the presence of clay

modelling orthorhombic anisotropic effects for reservoir fracture characterization of a naturally fractured tight carbonate reservoir onshore texas usa in this study we present a step by step theoretical modelling approach using established seismic wave propagation theories in anisotropic media to generate unique anisotropic reflection patterns observed from three dimensional pure mode pressure 3d pp full azimuth and full offset seismic reflection data acquired over a naturally fractured tight carbonate field onshore texas usa our aim is to gain an insight into the internal structures of the carbonate reservoir responsible for the observed anisotropic reflection patterns from the generated model we were able to establish that the observed field seismic reflection patterns indicate azimuthal anisotropy in the form of crack induced shear wave splitting and variation in p wave velocity with offset and azimuth amplitude variation with azimuth avaz analysis also confirmed multi crack sets induced anisotropy which is characteristic of orthorhombic symmetry evident as multiple bright and dim amplitude azimuth directions as well as complete reversal of bright amplitude to dim amplitude azimuth direction as the angle of incidence increases from near 15 to mid 30 offsets finally we fitted the generated p wave velocity into an ellipse to determine the intensity and orientation n26e of the open crack set as well as the direction of the minimum in situ stress axis n116e within the reservoir the derived information served as an aid for the design of horizontal well paths that would intercept open fractures and ensure production optimization of the carbonate reservoir which was on production decline despite reservoir studies that indicate un depleted reserves

assessment of the structural representativeness of sample data sets for the mechanical characterization of deep formations accurate characterization of the mechanical behavior of geomaterials at depth is a fundamental need for geologic and engineering purposes laboratory tests on samples from well cores provide the material characterization in terms of mechanical response and other relevant properties representativeness of a sample data set with respect to the in situ conditions at depth is a key issue which needs to be addressed to extrapolate the laboratory response to the whole rock mass we have developed a procedure aimed at quantitatively evaluating the representativeness of laboratory samples the methodology is based on joint processing of laboratory ultrasonic tests and wellbore sonic logs a structural index is used to quantify the difference between the average structure of the laboratory sample and the structure of the formation at the wellbore scale this index could be used to identify different causes of discrepancies between the behavior of the cored samples and the behavior of the rock formation as documented by well logs then it could also be used to integrate laboratory data for the construction of a reliable geomechanical model with reference to the real in situ state the methodology was applied to three different experimental data sets showing the effectiveness of the method

a cracked porous medium elastic wave theory and its application to interpreting acoustic data from tight formations rocks in the earth s crust usually contain pores and cracks typical examples include tight sandstone and shale rocks that have low porosity but contain abundant microcracks by extending the classic biot s poroelastic wave theory to include the effects of cracks we obtain an elastic wave theory for porous rocks containing cracks adding crack density and aspect ratio as two important parameters to the original theory because the flat or narrow shaped cracks can easily deform under acoustic wave excitation the acoustic property of a cracked porous rock is quite different for different saturation conditions the predicted fluid sensitivity is used to interpret acoustic velocity log data from tight sand and shale gas formations in both scenarios the new theory correctly predicts the trend of velocity variation with gas saturation the results confirm that the presence of cracks in tight rocks can give rise to significant hydrocarbon signature in the acoustic measurement data allowing for identifying hydrocarbons from the data

gas generation and overpressure effects on seismic attributes drilling of deep gas resources is hampered by high risk associated with unexpected overpressure zones knowledge of pore pressure using seismic data as for instance from seismic while drilling techniques will help producers plan the drilling process in real time to control potentially dangerous abnormal pressures we assume a simple basin evolution model with a constant sedimentation rate and a constant geothermal gradient oil gas conversion starts at a given depth in a reservoir volume sealed with faults whose permeability is sufficiently low so that the increase in pressure caused by gas generation greatly exceeds the dissipation of pressure by flow assuming a first order kinetic reaction with a reaction rate satisfying the arrhenius equation the oil gas conversion fraction is calculated balancing mass and volume fractions in the pore space yields the excess pore pressure and the fluid saturations this excess pore pressure determines the effective pressure which in turn determines the skeleton bulk moduli if the generated gas goes into solution in the oil this effect does not greatly change the depth and oil gas conversion fraction for which the hydrostatic pressure approaches the lithostatic pressure the seismic velocities versus pore pressure and differential pressure are computed by using a model for wave propagation in a porous medium saturated with oil and gas moreover the velocities and attenuation factors versus frequency are obtained by including rock frame fluid viscoelastic effects to match ultrasonic experimental velocities for the basin evolution model used here pore pressure is seismically visible when the effective pressure is less than about 15 mpa and the oil gas conversion is about 2 5 percent

joint elastic electrical properties of reservoir sandstones and their relationships with petrophysical parameters we measured in the laboratory ultrasonic compressional and shear wave velocity and attenuation 0 7 1 0 mhz and low frequency 2 hz electrical resistivity on 63 sandstone samples with a wide range of petrophysical properties to study the influence of reservoir porosity permeability and clay content on the joint elastic electrical properties of reservoir sandstones p and s wave velocities were found to be linearly correlated with apparent electrical formation factor on a semi logarithmic scale for both clean and clay rich sandstones p and s wave attenuations showed a bell shaped correlation partial for s waves with apparent electrical formation factor the joint elastic electrical properties provide a way to discriminate between sandstones with similar porosities but with different clay contents the laboratory results can be used to estimate sandstone reservoir permeability from seismic velocity and apparent formation factor obtained from co located seismic and controlled source electromagnetic surveys

finite difference modeling of wave propagation and diffusion in poroelastic media numerical modeling of seismic waves in heterogeneous porous reservoir rocks is an important tool for interpreting seismic surveys in reservoir engineering various theoretical studies derive effective elastic moduli and seismic attributes from complex rock properties involving patchy saturation and fractured media to confirm and further develop rock physics theories for reservoir rocks accurate numerical modeling tools are required our 2d velocity stress finite difference scheme simulates waves within poroelastic media as described by biot s theory the scheme is second order in time contains high order spatial derivative operators and is parallelized using the domain decomposition technique a series of numerical experiments that are compared to exact analytical solutions allow us to assess the stability conditions and dispersion relations of the explicit poroelastic finite differ ence method the focus of the experiments is to model wave induced flow accurately in the vicinity of mesoscopic hete

chemical remnants of the archaean protocrust in the sargur schist belt of karnataka craton india abstract the high grade sargur schist belt in the southern part of karnataka craton is characterised by two contrasting magmatic suites an older suite of metavolcanics and basic granulites of tholeiitic chemistry and a differentiated metabasic serpentinised ultramafic sequence of calc alkaline chemistry the metavolcanics have a distinctive chemistry of their own and are not comparable to modern mid oceanic ridge basalts and island arc tholeiites the chemistry of the metasediments which mostly comprise pelites semipelites minor ca rich marbles and quartzites indicates a mixed source consisting mostly of basic rocks and minor granitic material

full waveform ultrasonic transmission seismograms a fast new method for the determination of physical and sedimentological parameters of marine sediment cores detailed information about the sediment properties and microstructure can be provided through the analysis of digital ultrasonic p wave seismograms recorded automatically during full waveform core logging the physical parameter which predominantly affects the elastic wave propagation in water saturated sediments is the p wave attenuation coefficient the related sedimentological parameter is the grain size distribution a set of high resolution ultrasonic transmission seismograms 50 500 khz which indicate downcore variations in the grain size by their signal shape and frequency content are presented layers of coarse grained foraminiferal ooze can be identified by highly attenuated p waves whereas almost unattenuated waves are recorded in fine grained areas of nannofossil ooze color encoded pixel graphics of the seismograms and instantaneous frequencies present full waveform images of the lithology and attenuation a modified spectral difference method is introduced to determine the attenuation coefficient and its power law a kf applied to synthetic seismograms derived using a constant q model even low attenuation coefficients can be quantified a downcore analysis gives an attenuation log which ranges from 700 db m at 400 khz and a power of n 1 2 in coarse grained sands to few decibels per meter and n s 0 5 in fine grained clays a least squares fit of a second degree polynomial describes the mutual relationship between the mean grain size and the attenuation coefficient when it is used to predict the mean grain size an almost perfect coincidence with the values derived from sedimentological measurements is achieved

an equivalent medium model for wave simulation in fractured porous rocks seismic wave propagation in reservoir rocks is often strongly affected by fractures and micropores elastic properties of fractured reservoirs are studied using a fractured porous rock model in which fractures are considered to be embedded in a homogeneous porous background the paper presents an equivalent media model for fractured porous rocks fractures are described in a stress strain relationship in terms of fracture induced anisotropy the equations of poroelasticity are used to describe the background porous matrix and the contents of the fractures are inserted into a matrix based on the fractured equivalent medium theory and biot s equations of poroelasticity two sets of porosity are considered in a constitutive equation the porous matrix permeability and fracture permeability are analysed by using the continuum media seepage theory in equations of motion we then design a fractured porous equivalent medium and derive the modified effective constants for low frequency elastic constants due to the presence of fractures the expressions of elastic constants are concise and are directly related to the properties of the main porous matrix the inserted fractures and the pore fluid the phase velocity and attenuation of the fractured porous equivalent media are investigated based on this model numerical simulations are performed we show that the fractures and pores strongly influence wave propagation induce anisotropy and cause poroelastic behaviour in the wavefields we observe that the presence of fractures gives rise to changes in phase velocity and attenuation especially for the slow p wave in the direction parallel to the fracture plane

geomagnetic jerks and rapid hydromagnetic waves focusing at earth s core surface geomagnetic jerks are abrupt changes in the second time derivative the secular acceleration of earth s magnetic field that punctuate ground observatory records as their dynamical origin has not yet been established they represent a major obstacle to the prediction of geomagnetic field behaviour for years to decades ahead recent jerks have been linked to short lived temporally alternating and equatorially localized pulses of secular acceleration observed in satellite data associated with rapidly alternating flows at earth s core surface here we show that these signatures can be reproduced in numerical simulations of the geodynamo that realistically account for the interaction between slow core convection and rapid hydromagnetic waves in these simulations jerks are caused by the arrival of localized alfven wave packets radiated from sudden buoyancy releases inside the core as they reach the core surface the waves focus their energy towards the equatorial plane and along lines of strong magnetic flux creating sharp interannual changes in core flow and producing geomagnetic jerks through the induced variations in magnetic field acceleration the ability to numerically reproduce jerks offers a new way to probe the physical properties of earth s deep interior geomagnetic jerks in the earth s magnetic field are caused by the arrival of hydromagnetic waves and could be generated by sudden releases of buoyancy in the earth s core suggest geodynamic numerical model simulations

relationships among compressional wave attenuation porosity clay content and permeability in sandstones anelastic attenuation is the process by which rocks convert compressional waves into heat and thereby modify the amplitude and phase of the waves understanding the causes of compressional wave attenuation is important in the acquisition processing and interpretation of high resolution seismic data and in deducing the physical properties of rocks from seismic data we have measured the attenuation coefficients of compressional waves in 42 sandstones at a confining pressure of 40 mpa equivalent to a depth of burial of about 1 5 km in a frequency range from 0 5 to 1 5 mhz the compressional wave measurements were made using a pulse echo method in which the sample 5 cm diameter 1 8 cm to 3 5 cm long was sandwiched between perspex lucite buffer rods inside the high pressure rig the attenuation of the sample was estimated from the logarithmic spectral ratio of the signals corrected for beam spreading reflected from the top and base of the sample the results show that for these samples compressio

equatorially dominated magnetic field change at the surface of earth s core slow temporal variations in earth s magnetic field originate in the liquid outer core we analyzed the evolution of nonaxisymmetric magnetic flux at the core surface over the past 400 years we found that the most robust feature is westward motion at 17 kilometers per year in a belt concentrated around the equator beneath the atlantic hemisphere surprisingly this motion is dominated by a single wavenumber and persists throughout the observation period this phenomenon could be produced by an equatorial jet of core fluid by hydromagnetic wave propagation or by a combination of both discrimination between these mechanisms would provide useful constraints on the dynamics of earth s core

dynamic poroelasticity a unified model with the squirt and the biot mechanisms the velocities and attenuation of seismic and acoustic waves in rocks with fluids are affected by the two most important modes of fluid solid interaction 1 the biot mechanism where the fluid is forced to participate in the solid s motion by viscous friction and inertial coupling and 2 the squirt flow mechanism where the fluid is squeezed out of thin pores deformed by a passing wave traditionally both modes have been modeled separately with the biot mechanism treated in a macroscopic framework and the squirt flow examined at the individual pore level we offer a model which treats both mechanisms as coupled processes and relates p velocity and attenuation to macroscopic parameters the biot poroelastic constants porosity permeability fluid com pressibility and viscosity and a newly introduced microscale parameter a characteristic squirt flow length the latter is referred to as a fundamental rock property that can be determined experimentally we show that the squirt flow mechanism dominates the biot mechanism and is responsible for measured large velocity dispersion and attenuation values the model directly relates p velocity and attenuation to measurable rock and fluid properties therefore it allows one to realistically interpret velocity dispersion and or attenuation in terms of fluid properties changes e g viscosity during thermal enhanced oil recovery eor or to link seismic measurements to reservoir properties as an example of the latter transformation we relate permeability to attenuation and achieve good qualitative correlation with experimental data

a new approach to directly determine the secular variation from magnetic satellite observations 1 observatory monthly means provide an excellent opportunity to study the temporal changes of the geomagnetic field at a given location unfortunately determination of the global pattern of changes using observatory data is hampered by their uneven distribution satellite data provide excellent global coverage but the spacecraft movement makes direct comparisons of satellite and observatory data difficult to investigate short period secular variation in observatory and satellite data we developed an approach to extract satellite monthly means for virtual observatories at 400 km altitude using champ magnetic measurements comparison of these virtual observatory monthly means with the corresponding ground values shows a remarkably well correlated signal at time scales of months to years which is beyond the temporal resolution limit of recent global models here we describe this newly developed approach its validation and discuss how it can be used to understand short period changes of the recent geomagnetic field

observation of the biot slow wave in water saturated nivelsteiner sandstone acoustic signals are used extensively in the oil industry to determine the physical properties of reservoir rock in the interpretation of these signals empirical laws play a major role to obtain a more fundamental interpretation of the recorded wavetrains the need for a comprehensive theory for acoustic wave propagation and damping in rocks is obvious in this respect biot s 1956a b theory is a straightforward and effective two phase theory in contrast to biot who derived the macroscopic equations for wave propagation in saturated poro elastic material by postulating definite positive energy density functions burridge and keller 1981 whitaker 1986a b c and pride et al 1992 applied rigorous averaging techniques to derive the poro elastic equations from a microscale de vries 1989 and geerits 1996 used the averaging techniques to derive macroscopic poro elastic equations for the nonviscous case the fundamental feature of all these theoretical descriptions is the existence of both a fa

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

sensitivity of 3d 3c synthetic seismograms to anisotropic attenuation and velocity in reservoir models abstractanisotropic attenuation in fluid saturated reservoirs with high fracture density may be diagnostic for reservoir characterization wave induced mesoscale fluid flow is considered to be the major cause of intrinsic attenuation at exploration seismic frequencies we perform tests of the sensitivity of anisotropic attenuation and velocity to reservoir properties in fractured hti media based on the mesoscale fluid flow attenuation mechanism the viscoelastic t matrix a unified effective medium theory of global and local fluid flow mechanisms is used to compute frequency dependent anisotropic attenuation and velocity for ranges of reservoir properties including fracture density orientation fracture aspect ratio fluid type and permeability the 3d 3c staggered grid finite difference anisotropic viscoelastic modeling with a crank nicolson scheme is used to generate seismograms using the frequency dependent velocity and attenuation computed by the viscoelastic t matrix a standard linear solid mo

a geostatistical framework for incorporating seismic tomography auxiliary data into hydraulic conductivity estimation abstract a geostatistical approach is presented for the inclusion of seismic tomography data and sonic log data into the estimation of hydraulic conductivity the procedure accounts for the errors in seismic tomography inversion and for the correlation of such errors the proposed methodology consists of two steps 1 the cross variogram inference is carried out using only the data at the wellbore both hydraulic and seismic 2 a co kriging procedure takes the cross well data into account to interpolate between boreholes no postulated a priori relationship is needed in order to illustrate the methodology a synthetic data set is generated on the basis of evidence from published case studies and simplified physical considerations the numerical experiments show that the choice of the excitation frequency is critical a trade off exists between the need for a high resolution survey asking for higher frequencies and the need for a good correlation between hydraulic conductivity and seismic properties asking for frequencies below the squirt frequency of the medium in the simulation using seismic data with the best excitation frequency 1 khz in this case the mean squared error of the hydraulic conductivity estimate is two thirds lower than using hydraulic data alone it is important to note also that only a part of the interwell region is adequately sampled by the tomographic experiment such a region can be readily identified by calculating the energy of the quasi null space through singular value decomposition of the tomographic matrix in planning this type of experiments it is necessary to carefully verify case by case whether the adopted range of high frequencies does not prevent the seismic energy from propagating effectively from sources to receivers

on the propagation of a disturbance in a smoothly varying heterogeneous porous medium saturated with three fluid phases abstractfrom the equations governing the deformation of a porous medium containing three fluid phases i derive expressions for the phase velocity of the various modes of displacement these expressions are valid for a medium with smoothly varying heterogeneity there is a single mode of transverse displacement similar in nature to an elastic shear wave the four phase velocities of the longitudinal modes of displacement are derived from the solutions of a quartic equation the coefficients of the polynomial equation are written as linear sums of the determinants of 4 4 matrices the matrices contain various combinations of the parameters from the governing equations the three phase expressions are compared to two phase estimates for the case in which one of the fluid saturations vanishes also in a numerical illustration velocity variations of around 10 are associated with the cyclic injection of carbon dioxide and water into an oil saturated reservoir