detection of mantle plumes in the lower mantle by diffraction tomography hawaii abstract a new method is proposed to detect mantle plumes in the lower mantle the method is akin to diffraction tomography and relies on the scattering of long period seismic body waves by nearly vertical heterogeneities the theory is described in a companion paper ying ji h c nataf earth planet sci lett this issue here we apply this method to an actual set of long period digital seismograms recorded on the world wide network of seismic stations between 1980 and 1994 we select seismograms that illuminate the lower mantle in a 20 20 region around hawaii we construct an image of plume like heterogeneities using a cell size of 1 1 by a lsqr inversion of the scattered waves this image shows a strong slow anomaly about 200 km northwest of hawaii although the image is somewhat noisy resolution tests indicate that this feature is fairly robust the amplitude of the anomaly is between 30 and 60 times larger than what we predict from a simple thermal plume model built with a 600 k maximum temperature excess and a gaussian horizontal cross section with an 1 e diameter of 250 km it is the first time that such a method is applied to the mantle while we think that the existence of this anomaly is real we do not know how to explain such a large amplitude if real this observation has a number of important geodynamical consequences it indicates that a mantle plume is indeed responsible for the hawaii hotspot as speculated by morgan w j morgan nature 230 1971 42 43 the plume is nearly vertical it originates from the d region at the base of the lower mantle the amplitude of the anomaly suggests that partial melt or and a chemical anomaly must be present the plume source is to the northwest of its surface expression as predicted by some models of plume advection in the mantle wind

shear coupled pl summary observed teleseismic shear coupled pl waves spl display a variety of waveforms depending on factors such as source depth source type and velocity structure using a wkbj spectral method for sv wave propagation synthetic seismograms of spl are produced to examine the factors important in sv and spl excitation results show that spl is preferentially excited by shallow sources compared to deep sources this is due to large source area reverberations which consequently leak as sv waves into the mantle interaction at the receiver area then sets up the classic prograde elliptical motion by which spl can be identified this is in accordance with the teleseismic observations and indicates that most previous models for the propagation of spl were not appropriate for shallow source since emphasis was placed on wave interactions occurring only near the receiver

what is noise the concept of noise plays a crucial role in the statistical analysis of data as an example of a noisy record consider figure 1 that shows the ground motion of the seismological station ne51 in st petersburg after an earthquake in egypt in earthquake seismology periods may be orders of magnitude larger than in exploration seismology but the principles are the same this time series shows no distinct arrivals or other apparent signatures of an organized nature given the proximity of the recording station to a major population center and to the coast such a noisy record does not seem to be very surprising

upper mantle structure under western europe from fundamental and higher mode surface waves using the nars array summary a generalized inversion of a new data set consisting of surface wave phase velocities and sn traveltimes has resulted in a new shear velocity and density model for the west european platform wepl 2 the surface wave phase velocity data set consists of fundamental and higher mode mode 1 6 measurements from 25 to 70mhz using the network of autonomously recording stations nars array which was specifically designed for higher mode surface wave measurements the sn traveltime data set consists of isc readings for selected european events major features of wepl 2 are a high velocity lid of 80 140 km depth underlain by a pronounced low velocity zone between 160 and 220 km depth the transition zone 400 650 km depth is characterized by high shear velocities with respect to prem the density shows a positive gradient from 60 to 140 km depth followed by an approximately constant value down to 400 km depth a model for the baltic shield scsh 2 is obtained by combination of nolet s 1977 higher mode phase velocity data set for western europe including the baltic shield and the present phase velocities for the platform this shield model shows an absence of the low velocity layer and low shear velocities in the top of the transition layer with respect to wepl 2 density although less well resolved shows lower values for the shield structure between 200 and 350 km depth

the computation of body wave synthetic seismograms in laterally homogeneous media synthetic seismograms computed for realistic horizontally stratified media are now routinely used as an aid to seismic interpretation this paper reviews the theoretical background to these methods and presents comparisons of the two popular algorithms reflectivity and wkbj seismograms for a variety of earth models the transformed wave equations are developed from the equations for a spherical gravitating medium in a symmetric form suitable for body wave calculations four methods of solving these equations in general inhomogeneous layers are described the wkbj and langer asymptotic expansions and the wkbj and langer iterative solutions together with the earth flattening transformation and the ray expansion transformed solutions for body waves can then be obtained for realistic layered media four methods of inverting the frequency and wave number transformations are also described the real and complex spectral and slowness methods although realistic seismic models are normally sufficiently complicated that numerical calculations are essential before proceeding with numerical comparisons we have included a review of the canonical signals included in body wave seismograms these analytic results for direct rays partial and total reflections turning rays on forward and reversed branches head waves interface waves airy caustics and fresnel and interface shadows are useful to anticipate and understand numerical problems and results finally a comparison of green s functions for crustal mantle and whole earth models calculated using the wkbj and reflectivity algorithms is included

the complete synthesis of seismic crustal phases at regional distances using a new method to evaluate green s functions for elastic layered media we compute complete synthetic seismograms at regional distances for an earthquake in the earth crust we choose as source parameters the ones inferred for an earthquake in central france and compare the computed seismograms to the ones recorded at several short period stations of the french seismic network the major features of the records including complexity and duration are well reproduced by the synthetics lg waves which are the prominant phase are shown to be guided waves made up of sh and sv waves incident on the moho at angles of incidence more grazing than the critical angle and multiply reflected within the crust the length of the computed lg wave coda extends between group velocities of 3 5 and 2 8 km s and is in good agreement with the data

synthetic pn and sn phases and the frequency dependence of q of oceanic lithosphere the oceanic lithosphere is an extremely efficient waveguide for high frequency seismic energy in particular the propagation of the regional to teleseismic oceanic pn and sn phases is largely controlled by properties of the oceanic plates the shallow velocity gradient in the sub moho lithosphere results in a nearly linear travel time curve for these oceanic phases and an onset velocity near the material velocity of the uppermost mantle the confinement of pn sn to the lithosphere imposes a constraint on the maximum range that a normally refracted wave can be observed the rapid disappearance of sn and the discontinuous drop in pn sn group velocity beyond a critical distance dependent upon the local thickness of the lithosphere are interpreted as a shadowing effect of the low q asthenosphere wave number integration was used to compute complete synthetic seismograms for a model of oceanic lithosphere the results were compared to data collected during the 1983 ngendei seismic experiment in the southwest pacific the pn sn coda is successfully modeled as a sum of leaky organ pipe modes in the sediment layer and oceanic water column while scattering is present to some degree it is not required to explain the long duration and complicated nature of the pn sn wave trains the presence of extremely high frequencies in pn sn phases and the greater efficiency of sn than pn propagation are interpreted in terms of an absorption band rheology a shorter high frequency relaxation time for p waves than for s waves results in a rheology with the property that q q at low frequency while q q at high frequency consistent with the teleseismic pn sn observations the absorption band model is to viewed as only an approximation to the true frequency dependence of q in the oceanic lithosphere for which analytic expressions for the material dispersion have been developed

complete pp waveform modelling for determining crust and upper mantle structure summary we analyse the pp phase at upper mantle distances 25 to 60 to quantify its propagation characteristics and potential for determining crust and upper mantle velocity structure upper mantle distance pp waveforms are very complex involving interference between a variety of arrivals traversing different depth ranges in the earth even for laterally homogeneous structures complete synthetic seismograms such as provided by reflectivity calculations are required to adequately model the full suite of arrivals that contribute to upper mantle pp phases the complexity of pp waveforms can be exploited to extract crust and upper mantle structural information for relatively homogeneous paths while pp p differential traveltimes and pplp amplitude ratios provide constraints on average upper mantle velocities above the transition zone early pp coda comprized of crustal reflections and conversions and precursors to the pp phase comprized of multiple moho underside reflections can provide information on crustal thickness and velocity gradients in the uppermost mantle positive velocity gradients below the crust generate large amplitude moho underside reflections that are particularly visible for short period and broad band seismograms at distances from 25 to 38 we demonstrate the potential use of the pp phase for determining upper mantle structure by modelling long period signals traversing relatively pure paths across north america a continuum of p wave velocity models with varying lid structure is appropriate for the upper mantle beneath north america the highest velocities are found beneath the northern canadian shield with the lid becoming progressively slower beneath the southern shield continental platform and tectonically active regions respectively most of the heterogeneity in p velocity structure is concentrated above a depth of 250 km although some data are consistent with models that are relatively slow at greater depths

evidence for a sharp 670 km discontinuity as inferred from p to s converted waves the fine structure of the upper mantle discontinuities is investigated using observations of converted waves on short period and broadband seismograms using a stacking technique to analyze the p wave coda of teleseismic records evidence is found for coherent near receiver p to s converted phases generated by the 400 and 670 km discontinuity beneath a number of the stations used in this study variations in travel time slowness and amplitude of these phases as observed among the stacks for stations of the regional seismic test network in the united states and of the network of autonomously recording seismographs in western europe are very likely the expression of upper mantle heterogeneity observations of coherent converted phases from the 400 km discontinuity are fewer in number than of phases converted at the 670 km discontinuity suggesting that the latter is more pronounced some of the seismograms especially of station rscp show extremely high amplitude p to s converted phases from the 670 km discontinuity these seismograms allow a detailed waveform comparison of the converted phase with the direct p phase and present evidence for a sharp 670 km discontinuity

s to p scattering at the 650 km discontinuity summary a search of seismograms recorded at the warramunga seismic array wra from events occurring below the izu bonin islands shows an arrival on some but not all of the records with an onset at 25 30 s after p which is not predicted by the standard travel time tables the slowness and azimuth of the phase show that it is generated almost in line with p and the variation of arrival time with the hypocentral depth of the earthquake indicates that its origin lies on the receiver side of the source it appears in fact to be an s to p conversion at a depth of 650 700 km which is seen only when the receiver is close to a node of the p radiation pattern and an antinode for s so that its amplitude compared with that of p is at a maximum finally the duration of the phase indicates that it is not simply a refracted wave but that it has a coda of scattered arrivals from lateral heterogeneity in the neighbourhood of 650 km below the izu bonin islands

are long period body wave coda caused by lateral heterogeneity summary data from two broad band arrays in western europe nars and grf are used to study the character of long period body wave coda the events studied are at epicentral distances of 40 to 60 in the hindu kush region and on the mid atlantic ridge sampling the upper mantle to a depth of about lo00 km the periods studied are 5 50 s the long period coda of p at grf interstation distance about 10 km are strongly coherent whereas the long period coda of pp ppp and s waves are incoherent this indicates that the latter coda consists of scattered waves to investigate the nature of the scattering process the data of grf are analysed for slowness and azimuth variations in the coda intervals a new beamforming algorithm is presented to deal with the low frequences and relatively short time intervals the method is based on backus gilbert inverse theory the results show that the incoherent long period coda intervals almost entirely consist of surface waves these waves are scattered from the preceding body waves some calculations with linearized theory for body wave to rayleigh wave conversion at topography at the surface or at the moho show that realistic scatterers can account for the observed constant coda level the beamforming results show that the phases in the p coda all arrive along the great circle as scattering calculations point out that body wave to body wave scattering is inefficient it is concluded that the long period p coda does not contain a significant amount of scattered energy synthetic seismograms obtained with the reflectivity method show that spherically symmetric upper mantle models can explain these coda waves for events in the hindu kush region an upper mantle with a thick lid overlying a pronounced low velocity zone lvz is necessary to explain the character of the p coda at grf such an upper mantle agrees with previous studies of similar great circle paths the strong coherence of the p coda at grf is lost on the scale of nars station separation about 200 km this suggests lateral variations in the upper mantle at a scale of about 200 km it appears from previous studies of the upper mantle under europe that these variations must be sought in the lvz it is shown that the long period p coda is sensitive to variations in the p velocity structure of the lvz this suggests the p coda i e pdp phases as a tool for monitoring lateral variations in the lvz and possibly to prove the existence or absence of a lvz in the p velocity

effective medium modeling of gas hydrate filled fractures using the sonic log in the krishna godavari basin offshore eastern india 1 indian national gas hydrate program expedition 01 has established that clay rich marine sediment from the krishna godavari kg basin in the eastern indian margin hosts one of the richest gas hydrate deposits in the world resistivity at bit images and pressure cores reveal that the gas hydrate morphology in clay rich sediment varies from complex vein structures grain displacing to invisible pore filling existing rock physics models which relate acoustic data to in situ gas hydrate concentrations generally assume isotropic pore filling gas hydrate which yields misleading concentration estimates for fractured fine grained sediments the anisotropic kg basin sediment presents additional complications here we apply differential effective medium theory to incorporate grain displacing morphologies by which gas hydrate is included as vertical ellipsoids with aspect ratios ranging from those of thin veins up to those of nodules in an elastic anisotropic background we have estimated gas hydrate concentrations from sonic velocities at hole 10d in the kg basin considering three basic gas hydrate morphologies i pore filling ii grain displacing and iii a combination of grain displacing and pore filling average gas hydrate saturations for these three cases are 35 42 27 30 and 33 41 of the total porosity respectively in the depth range 60 140 m below seafloor mbsf saturation is highest at 67 mbsf for any morphology but the values differ between morphologies for the pore filling morphology the maximum gas hydrate saturation of 56 is 18 22 higher than the grain displacing morphology and 2 9 higher than the combined morphology estimates differ by 6 of the sediment volume with rotations of gas hydrate veins from vertical to horizontal

finite difference simulations of seismic scattering implications for the propagation of short period seismic waves in the crust and models of crustal heterogeneity synthetic seismograms produced by the finite difference method are used to study the scattering of elastic and acoustic waves in two dimensional media with random spatial variations in seismic velocity the results of this study provide important insights about the propagation of short period 5 the self similar medium is characterized by a scattering q that is constant with frequency whereas theory predicts that the apparent q in an exponential medium is proportional to frequency these alternative models of crustal heterogeneity can thus be tested by improved measurements of the frequency dependence of crustal q at frequencies greater than about 1 hz assuming that scattering is responsible for most of the attenuation at these frequencies measurements of the time decay of the synthetic coda waves clearly show that the single scattering model of coda decay is not appropriate in the presence of moderate amounts of scattering attenuation scattering q 200 in these cases q values derived from the coda decay rate using the single scattering theory do not correspond to the transmission q of the medium the cross correlation of synthetic waveforms observed for an array of receivers along the free surface is observed to be dependent on the correlation distance of the medium the self similar random medium proposed here for the crust produces waveform variations at high frequencies 15 30 hz similar to those reported for actual small scale seismic arrays with apertures of hundreds of meters

the application of synthetic seismograms to the interpretation of the upper mantle p wave velocity structure in northern australia abstract during the period 1974 to 1977 a long range seismic refraction project was conducted in central australia along a profile extending south from darwin earthquakes from the banda sea region were used as seismic sources for this experiment an analysis by hales and co workers of the resulting data based on travel times and using geometric raytracing techniques has resulted in the construction of an upper mantle velocity model using synthetic seismograms to model amplitudes it is shown that additional constraints can be placed on the derived velocity profile the low velocity zone beneath the 200 km discontinuity is found to have a more abrupt onset than was previously suggested a smaller discontinuity at 325 km depth is now implied the analysis suggests that the 400 km discontinuity is a first order velocity increase whereas all other observed upper mantle discontinuities are more satisfactorily modelled as second order type structures

comparative analysis of teleseismic p coda and lg waves from underground nuclear explosions in eurasia this study has investigated the characteristics of p coda and lg waves recorded at the norsar and graefenberg arrays from presumed nuclear explosions in western russia lg waves were found to be more energetic relative to the preceding coda in the mid band frequency range 0 2 to 1 0 hz recorded on the broadband graefenberg instruments than in the high frequency band 0 6 to 3 0 hz at either array comparison of the log rms coda envelopes at the two arrays revealed that the early high frequency p coda at norsar is stronger relative to lg than at graefenberg also the amplitudes of p waves and early p coda waves within 55 sec after p recorded at the centers of the norsar subarrays are positively correlated indicating the effect of focusing defocusing on the p and coda waves however for subarrays which cluster spatially there is an inverse correlation which seems to be due to scattering attenuation suggesting that much of the early p coda is generated by local p wave scattering beneath the norsar array also the norsar coda envelopes flatten between about 219 and 352 sec after p whereas the graefenberg coda decays monitonically from the p wave out to the sn and lg arrival times the standard deviation across the norsar array of the log rms amplitudes of lg and s coda phases are higher by about 0 05 to 0 06 magnitude units than those of p coda phases these differences are probably related to differential site and propagation path effects on p s and lg waves in the p coda the flattening of the norsar coda envelopes appears to be a burst of energy not associated with any known regional phases the burst may be caused by forward scattering of lg to p waves along the path between the source and norsar based on a regional travel time curve for western russia the start and end time of the coda flattening is consistent with lg to p scattering in the distance range of 23 to 29 from norsar which brackets the ural mountains the lg propagation path from the source region to graefenberg does not appear to traverse the ural mountains which may explain why the coda flattening is not observed at graefenberg

limitations of waveform modelling of long period seismograms summary in recent years the use of synthetic seismograms calculated for radially stratified models has gained increasing popularity as a means of placing further constraints on the velocity structure of the earth s mantle such synthetics do however have a number of limitations at short periods 1 s the amplitudes as well as the wave shapes of travel time branches are affected by seismograph siting the structure immediately beneath the seismograph and any laterally heterogeneous structure in the mantle later arrivals can also be masked by signal generated noise and by extended source times at longer periods 20 s the larger wavelengths reduce the sensitivity of amplitudes and waveforms to contarninating effects as a result the use of long period synthetics can only lead to the resolution of the gross features of the earth s interior

frequency dependence of q in the mantle underlying the shield areas of eurasia part iii the q model summary a large set of teleseismic body wave data covering a broad range in frequency was analysed to determine the frequency arid depth dependence of q for p and s waves under the northern shield areas of eurasia based on numerous f estimates for p and s waves covering the seismic band between 0 02 and 8 hz a q model of the eurasian shield was constructed the data require a model in which q increases with frequency and which is characterized by q values in the upper mantle that are generally higher than those of global average models the model with the best fit includes a minimum in q between about 100 and 200 km depth and high q values on the order of thousands throughout most of the deeper mantle these results are generally consistent with proposed models of attenuation as a thermally activated process which is influenced by the temperature and pressure in the earth preliminary results suggest that t under tectonic regions is higher than t under shield regions for all frequencies over the 0 02 to 8 hz frequency range suggesting that q varies regionally as well as with frequency and depth

the elastic anisotrophy of shales shales constitute about 75 of the clastic fill sedimentary basins and have a decisive effect on fluid flow and seismic wave propagation because of their low permeability and anisotropic microstructure the elastic stiffnesses of a shale with partially oriented clay particles is expressed in terms of the coefficients w sub lmn in an expansion of the clay particle orientation distribution function in generalized legendre functions application is made to the determination of the anellipticity of shales for transverse isotrophy the anellipticity quantifies the deviation of the p wave slowness curve from an ellipse and is shown to depend on a single coefficient w sub 400 in the expansion of the clay particle orientation distribution function if w sub 400 is small the anellipticity may be neglected as is apparently the case for a near surface late tertiary shale studied by winterstein and paulson strongly aligned clay particles result in a positive value of w sub 400 and a positive anellipticity in agreement with the majority of the field measurements however less well ordered shales could have a significantly positive second moment w sub 200 but only a small positive or even negative value of w sub 400 for such shales the anellipticity would be small or negative despite amore preferred alignment of clay particles in the bedding plane numerical examples of clay particle orientation distribution functions leading to zero or negative anellipticity are given less

effect of grain scale alignment on seismic anisotropy and reflectivity of shales the elastic properties and anisotropy of shales are strongly influenced by the degree of alignment of the grain scale texture in general an orientation distribution function odf can be used to describe this alignment which in practice can be characterized by two legendre coefficients we discuss various statistical odfs that define the alignment by spreading from a mean value in particular the gaussian fisher and bingham distributions we compare the statistical models with an odf resulting from pure vertical compaction no shear strain of a sediment the compaction odf may be used to estimate how the elastic properties and anisotropy evolve due to burial of clayey sediments our study shows that the three statistical odfs produce almost identical correspondence between the two legendre coefficients as a function of the spreading parameter so that the spreading parameter of one odf can be converted to the spreading parameter of another odf in most cases it is then sufficient to apply the spreading parameter for the odf instead of the two legendre coefficients the effect of compaction on the odf gives a slightly different correspondence between the two legendre coefficients from that for the other models in principle this opens up the possibility of distinguishing anisotropy effects due to compaction from those due to other processes we also study reflection amplitudes versus angle of incidence ava for all wave modes where shales having various odfs overlie an isotropic medium the ava responses are modelled using both exact and approximation formulae and their intercepts and gradients are compared the modelling shows that the s wave velocity is sensitive to any perturbation in the spreading parameter while the p wave velocity becomes increasingly sensitive to a perturbation of a less ordered system similar observations are found for the ava of the p p and p sv waves modelling indicates that a combined use of the amplitude versus offset of p p and p sv reflected waves may reveal certain grain scale alignment properties of shale like rocks

anisotropic effective medium modeling of the elastic properties of shales shales are complex porous materials normally consisting of percolating and interpenetrating fluid and solid phases the solid phase is generally comprised of several mineral components and forms an intricate and anisotropic microstructure the shape orientation and connection of the two phases control the anisotropic elastic properties of the composite solid we develop a theoretical framework that allows us to predict the effective elastic properties of shales its usefulness is demonstrated with numerical modeling and by comparison with established ultrasonic laboratory experiments the theory is based on a combination of anisotropic formulations of the self consistent sca and differential effective medium dem approximations this combination guarantees that both the fluid and solid phases percolate at all porosities our modeling of the elastic properties of shales proceeds in four steps first we consider the case of an aligned biconnected clay fluid composite composed of ellipsoidal inclusion

a note on lateral variation in upper mantle shear wave velocity across the alpine front summary we have detected what appear to be strong lateral variations in upper mantle shear wave velocities under europe and western eurasia the evidence suggests that the alpine belt of southern europe is underlain by a tectonic upper mantle with a thin lid and thick low velocity layer whereas the eastern europe western russia precambrian platform is underlain by shield like upper mantle the transition zone between the two appears to be sharply defined and coincident with the northern boundary of the alpine belt the techniques used in the analysis include the construction of synthetic seismograms for long period s and ss waves in the distance range 30 60 where differences in upper mantle shear wave velocity most strongly affect the differential times ss s and the ratio of ss to s amplitudes as a function of epicentral distance a comparison with upper mantle shear wave structure in western north america as reported by grand helmberger indicated that under the alpine belt the upper mantle is considerably faster than under western north america a situation that may be interpreted as due to the much larger and widespread thermal anomalies under western north america as a consequence of the existence ofa subducted ridge

partitioned waveform inversion and two dimensional structure under the network of autonomously recording seismographs nonlinear optimization methods are very effective in finding models that predict optimal waveform fits to observed seismograms provided the number of model parameters is small to reduce the size of the large scale two or three dimensional seismic modelling problem path integrals over seismic velocities may be defined as model parameters instead of the seismic velocities themselves this partitions the large scale optimization problem into a number of independent small scale problems and reduces the computational effort by several orders of magnitude the path integral parameters are transformed such that their correlation matrix becomes diagonal a linear inversion performs the mapping from the space of path integral parameters into the model space of seismic velocities and makes backus gilbert resolution analysis possible the method has been applied to body wave and surface wave data from the broadband portable network of autonomously recording seismographs nars in western europe using a surface wave ray approximation to generate synthetics by mode summation waveforms were fitted for frequencies as high as 50 mhz variations in s velocity as large as 6 1 have been established with a vertical resolution of 100 150 km and a horizontal resolution as small as 400 km in parts of the model although a low velocity layer is present under the southern part of the nars array such layer is very weak or absent north of the paris basin horizontal velocity gradients are well resolved down to 400 km depth

regional velocity structure in northern california from inversion of scattered seismic surface waves seismic surface waves recorded by the berkeley digital seismic network have been analyzed in order to constrain three dimensional lateral heterogeneity of the upper mantle under northern california a total of 2164 seismograms from 173 teleseismic events were windowed for the fundamental mode rayleigh wave followed by estimation of complex amplitude spectra over the period range 16 to 100 s using a multiple taper method since rayleigh waves at shorter periods particularly below 35 s suffer from serious multipathing or non plane wave arrivals these amplitude spectra have been interpreted as the product of wavefront distortion along the teleseismic propagation path and seismic structure beneath the network the amplitude spectra are first modeled in terms of non plane incoming wavefields and structural phase velocity perturbations period by period after corrections for moho and surface topography the phase velocity maps are inverted for three dimensional shear velocity perturbations s down to a depth of 200 km the s maps are in good agreement with the results of body studies over a broad spatial scale the dominant signals are associated with the thermal effects of the active gorda and fossil farallon subducted slab stretching from mount shasta through the western sierran foothills to the southern great valley and asthenospheric upwelling beneath the northern coast ranges the southern sierra nevada range is characterized by fast s down to 50 km and slow velocities between 60 and 120 km depth in agreement with independent inferences of a cold crust and warm upper mantle which may provide the buoyancy forces necessary to support the elevation of the range

seismic imaging of upper mantle structure with new evidence for a 520 km discontinuity stacked images of long period seismic data reveal many phases some not previously observed which are caused by reflections and conversions at discontinuities in the earth s upper mantle these images support the existence of discontinuities at 410 520 and 660km depth but no evidence is found for a significant interface at 220 km

generation of the teleseismic p wave coda from aleutian earthquakes summary we investigate large amplitude phases arriving in the p wave coda of broad band seismograms from teleseisms recorded by the grafenberg array the german regional seismic network and the global seismic network the data set consists of all events mb 5 6 from the aleutian arc between 1977 and 1992 earthquakes with large amplitude coda waves correlate with the presence of oceanic crust in the source region the amplitudes sometimes approach those of the p wave much larger than predicted by theory modelling indicates that phases in the p wave coda cannot be p wave multiples beneath the source and receiver or underside reflections which precede pp from upper mantle discontinuities among the events seismograms are very similar where the arrival times of the unusual phases agree approximately with the predicted times of s to p conversions from the upper mantle discontinuities under the source because the large amplitude phases in the p wave coda have little if any dependence on event depth and have predominantly an sv wave radiation pattern towards the receiver we suggest that they originate as sv and or rayleigh waves and are enhanced by lateral heterogeneity and multipathing from the subducting aleutian slab

velocity shift in heterogeneous media with anisotropic spatial correlation summary converted waves analysis is used to map the anisotropy as a function of depth in the upper mantle stacking of sv a ndt component records of several seismic events well distributed in azimuth helps to detect these converted waves weighting coe cients are used to extract the anisotropic signal in the records synthetic examples show that a harmonic analysis helps to separate azimuthal anisotropy eiects from the eiects of lateral heterogeneity such as those resultingfrom dipping structures a good azimuthal coverage of the data set is important in order to separate the diierent eiects in the stack sections obtained for diierent azimuthal orders the procedure is applied to data observed at geoscope station can and gives a model with two anisotropic layers in the upper mantle the upper layer located between the crust and a depth of140 km has its fast axis in an east west direction the lower layer 40 km thick has its fast axis in a north south direction the fast axis directions are in agreement with results obtained from surface wave analysis and tectonic information

observational and theoretical constraints on crustal and upper mantle heterogeneity abstract the effects of crustal and mantle heterogeneity are reflected in the relative complexity of short period seismograms at regional to teleseismic distances observations of events at regional ranges from indonesia and new guinea at the warramunga array in northern australia allow a separation of the propagation through the mantle by concentrating on the coherent signal across the medium aperture array most of the incoherent features on the seismograms arise from signal generated noise in the general vicinity of the array with scale lengths of the order of 1 2 km or smaller partially coherent phases with relatively rapid changes in waveform across the array are to be associated with organised heterogeneity this is most likely to arise from features in the crust or at the crust mantle boundary near the receiver but could in some cases be due to structural effects near the source the coherent signals show considerable complexity for phases travelling large portions of their path above 80 km as the depth of sources increases this coherent portion of the seismic field tends to become simpler which suggests that the scale length of heterogeneity is larger at depth the use of horizontally stratified models has provided a convenient framework for the understanding of many seismic wave phenomena via the calculation of synthetic seismograms however further development is needed for calculating the response of the laterally heterogeneous earth the convenience of physical interpretation provided by working with the reflection and transmission properties of the medium can be retained if the structure can be split into a stratified reference model with superimposed heterogeneity with a plane wave decomposition of the wavefield the effect of lateral variations can be introduced by forcing mixing of wavenumber components a wide variety of phenomena can be simulated by varying the statistical description of the heterogeneity field in the wavenumber domain this varies the cross coupling between wavenumber components and so changes the nature of the resulting theoretical seismograms such an approach allows a quantitative study of the degradation of the major seismic phases compared to the predictions for stratified models and also the characterisation of the codas of these phases

the role of texture cracks and fractures in highly anisotropic shales organic shales generally have low permeability unless fractures are present however how gas oil and water flows into these fractures remains enigmatic the alignment of clay minerals and the alignment of fractures and cracks are effective means to produce seismic anisotropy thus the detection and characterization of this anisotropy can be used to infer details about lithology rock fabric and fracture and crack properties within the subsurface we present a study characterizing anisotropy using s wave splitting from microseismic sources in a highly anisotropic shale we observe very strong anisotropy up to 30 with predominantly vti vertical transverse isotropy symmetry but with evidence of an hti horizontal transverse isotropy overprint due to a ne striking vertical fracture set parallel to the maximum horizontal compressive stress we observe clear evidence of a shear wave triplication due to anisotropy which to our knowledge is one of only a very few observations of such triplications in field scale data we use modal proportions of minerals derived from x ray fluorescence data combined with realistic textures to estimate the contribution of intrinsic anisotropy as well as possible contributions of horizontally aligned cracks we find that aligned clays can explain much of the observed anisotropy and that any cracks contributing to the vertical transverse isotropy vti must have a low ratio of normal to tangential compliance zn zt typical of isolated cracks with low hydraulic connectivity subhorizontal cracks have also been observed in the reservoir and we propose that their reactivation during hydraulic fracturing may be an important mechanism to facilitate gas flow

topography of the 400 km discontinuity from observations of long period p400p phases summary new evidence is presented for lateral variation of the depth of upper mantle discontinuities this evidence is derived from an analysis of amplitude of p400p phases underside reflections from the 400 km discontinuity for great circle paths with bounce points in the upper mantle near the hawaii emperor hotspot chain the data are long period from wwssn sro and rstn stations the epicentral distance ranges from 95 to 115 which ensures that no other major phases arrive in the time window of interest observed p400p amplitudes are compared with synthetic amplitudes for both a laterally homogeneous model and models including 3 d topography on the discontinuities the observations permit models of topography that can be characterized by undulations of the 400 km discontinuity with wavelengths in the range 600 2000 km and peak to peak amplitude of the order of 30 40 km such long wavelength structure causes quite strong de focusing of pdp phases synthetic pdp waveforms reveal that topography not only affects its amplitude but also smears out the pdp wave causing overlap between different pdp phases this mechanism is proposed as the reason for the lack of well developed pdp phases reported in the literature the fact that the waveforms can be seriously distorted prohibits the use of pdp phases in a waveform inversion the curvature of the discontinuity can be explained as the signature of convection currents crossing the discontinuity although the data cover the hawaii hotspot no signature of a plume structure can be detected in the data probably because the resolving power of the data is too low

crust and upper mantle structure along the dss baltic profile in se finland summary the 430 km long dss baltic profile in se finland runs through the rapakivi intrusion the svecokarelian geosynclinal complex including the ladoga bothnian bay zone and the archean basement complex a brief description of field operations in 1982 is presented record sections of p and s waves were used for the interpretations a starting model was constructed by inverse methods and improved by dynamic ray tracing modelling three blocks are indentified on the cross section from sw to ne these are the southern associated with the rapakivi massif depth to moho m about 40 km the central ladoga bothnian bay zone with m of approximately 55 60 km and the northern archean crystalline basement with m about 40 45 km the average crustal velocity is 6 6 6 7 kms the crustal structure along the baltic profile is compared with the surrounding dss profiles finnish sveka and soviet kern tulos and ladoga profiles the eastern border of the ladoga bothnian bay zone was located and specific features of the crust and uppermost mantle along the profile area are discussed

scattering of teleseismic waves in the lower crust observations in the massif central france abstract high frequency coda signals consistently recorded by a temporary seismic network of 29 short period stations during a 6 month survey in the french massif central are concordant with independent evidence for a heterogeneous lower crust obtained from wide angle and near vertical reflection experiments in the same region the teleseismic recordings of 22 events have been analysed in record sections rather than as single station seismograms following the low frequency lf 0 5 1 5 hz first p arrival a high frequency hf coda 2 4 hz dominant frequency extends over several seconds duration the hf signals become clearly visible after bandpass filtering but can also be identified in the original seismograms in event sections the hf coda forms a consistent pattern of reverberations which is characterized by the following properties 1 the duration is typically longer on the radial 4 15 s than on the vertical 3 11 s component 2 the beginning of the hf coda referred to the picked lf first p arrivals varies between 0 2 5 s with a dominance around 1 5 2 s 3 the amplitude of the hf coda is about 3 10 of the primary lf p phase amplitude 4 the signals are incoherent between neighbouring stations and over the network 5 the hf coda arrives dominantly around the plane of incidence of the teleseismic wave in the first 2 s to maximum 4 s afterwards a widening of the particle motion to an elliptical shape is observed indicating arrivals off the sagittal plane the data parameters 1 to 5 point to the conclusion that the hf signals are generated by a scattering process wide angle and cdp reflection experiments in the same region measured a similar reverberating coda pattern and located the origin also in the lower crust these observations and the data parameters of the hf coda suggest that the hf teleseismic waves are scattered at heterogeneities in the lower crust in the massif central in this contribution we mainly concentrate on the presentation of the teleseismic recordings and conclude with a preliminary structural model which contains randomly distributed scatterers in the lower crust

texture and anisotropy analysis of qusaiba shales scanning and transmission electron microscopy synchrotron x ray diffraction microtomography and ultrasonic velocity measurements were used to characterize microstructures and anisotropy of three deeply buried qusaiba shales from the rub alkhali basin saudi arabia kaolinite illite smectite illite mica and chlorite show strong preferred orientation with 001 pole figure maxima perpendicular to the bedding plane ranging from 2 4 6 8 multiples of a random distribution m r d quartz feldspars and pyrite crystals have a random orientation distribution elastic properties of the polyphase aggregate are calculated by averaging the single crystal elastic properties over the orientation distribution assuming a nonporous material the average calculated bulk p wave velocities are 6 2 km s maximum and 5 5 km s minimum resulting in a p wave anisotropy of 12 the calculated velocities are compared with those determined from ultrasonic velocity measurements on a similar sample in the ultrasonic experiment which measures the effects of the shale matrix as well as the effects of porosity velocities are smaller p wave maximum 5 3 km s and minimum 4 1 km s the difference between calculated and measured velocities is attributed to the effects of anisotropic pore structure and to microfractures present in the sample which have not been taken into account in the matrix averaging

the p wave upper mantle structure beneath an active spreading centre the gulf of california summary detailed analysis of short period travel time dt d and wave form data reveals the upper mantle structure beneath an oceanic ridge to depths of 900 km more than 1400 digital seismograms from earthquakes in mexico and central america recorded at scarlet yield 1753 travel times and 58 direct measurements of dt d as well as high quality stable waveforms the 29 events combine to form a continuous record section from 9 to 40 with an average station spacing of less than 5 km first the travel times are inverted using the tau method of bessonova et al the resultant model is adjusted to agree with the experimental p values further constraints arise from the observed relative amplitudes of mantle phases which are modelled by trial and error using wkbj synthetic seismograms chapman wiggins model gca which is appropriate for western mexico north of 20 latitude is similar to existing upper mantle models for shield tectonic continental and arc trench regimes below 400 km but differs significantly in the upper 350 km gca velocities are very low in this depth range the model catches up with the others with a very large velocity gradient from 225 to 390 km this well resolved feature is consistent with the shear wave model tna for western north america of grand helmberger the abundant data from 20 to 30 control the detailed shape of the 660 km discontinuity very large velocity gradients lie both above 620 660 km and below 661 680km a 2 8 per cent velocity change

elastic properties of hydrate bearing sediments using effective medium theory accurate and detailed models of the seismic velocity structure of gas hydrate bearing sediments may be determined by careful analysis of controlled source seismic data however interpretation of these velocities in terms of hydrate saturation of the pore space has hitherto relied on semiempirical formulas and or simple effective medium theory we develop a rigorous theoretical scheme to relate the seismic properties of a clay rich hydrate bearing sediment to its porosity mineralogy microstructural features and hydrate saturation we consider separately the two possible end members for the distribution of hydrate in the pore space 1 hydrates are unconnected and located in the pore voids without appreciable grain contact and 2 connected hydrates are forming cement binding around the grains the scheme is transversely isotropic to allow for anisotropy due to alignment of clay platelets and is based on a combination of a self consistent approximation a differential effective medium theory and a method of smoothing for crystalline aggregates we have applied the scheme to lithological and seismic velocity data from ocean drilling program site 995 on the blake ridge southeastern u s continental margin to make estimates of the hydrate saturation it was found that the hydrates are probably unconnected and their volume concentration varies between approx 0 at 100 m below the seabed and approx 9 at 400 m depth just above the bottom simulating reflector if the clay platelet orientation distribution resembles the function we have used

large scale waveform inversions of surface waves for lateral heterogeneity 2 application to surface waves in europe and the mediterranean linear surface wave scattering theory is used to reconstruct the lateral heterogeneity under europe and the mediterranean using surface wave data recorded with the network of autonomously recording seismographs nars the waveform inversion of the phase and the amplitude of the direct surface wave leads to a variance reduction of approximately 40 and results in phase velocity maps in the period ranges 30 40 s 40 60 s and 60 100 s a resolution analysis is performed in order to establish the lateral resolution of these inversions using the phase velocity perturbations of the three period bands a two layer model for the s velocity under europe and the mediterranean is constructed the s velocity perturbations in the deepest layer 100 200 km are much more pronounced than in the top layer 0 100 km which confirms that the low velocity zone exhibits pronounced lateral variations in both layers the s velocity is low under the western mediterranean while the s velocity is high under the scandinavian shield in the deepest layer a high s velocity region extends from greece under the adriatic to northern italy several interesting smaller features such as the massif central are reconstructed one of the spectacular features of the reconstructed models is a sharp transition in the layer between 100 and 200 km near the tornquist tesseyre zone this would indicate that there is a sharp transition at depth between central europe and the east european platform the waveform inversion of the surface wave coda leads to good waveform fits but the reconstructed models are chaotic this is due both to a lack of sufficient data for a good imaging of the surface wave energy on the heterogeneities and to an appreciable noise component in the surface wave coda

lateral heterogeneity of europe s upper mantle as inferred from modelling of broad band body waves summary the upper mantle velocity structure beneath europe is investigated by body wave modelling of broad band seismograms from earthquakes in the eastern mediterranean recorded by stations in north west europe the gross features of the p and s wave velocity distribution as inferred from the data show a high velocity lid a low velocity zone and two discontinuities at depths of approximately 420 and 650 km the seismograms also present evidence for lateral heterogeneity of at least a few per cent 2 5 per cent for p 4 per cent for s to a depth of approximately 400 km many of the inferred velocity variations can be related to the tectonic setting of the region between source and receiver broad band seismograms are essential for such detailed study of upper mantle structure using body wave modelling

assessment of a rapid method for quantitative reach scale estimates of deposited fine sediment in rivers abstract despite increasing concerns about the negative effects that increased loads of fine grained sediment are having on freshwaters the need is clear for a rapid and cost effective methodology that gives precise estimates of deposited sediment across all river types and that are relevant to morphological and ecological impact to date few attempts have been made to assess the precision of techniques used to assemble data on fine sediment storage in river channels accordingly we present an investigation into the sources of uncertainty associated with estimates of deposited fine grained sediment in rivers using a sediment resuspension technique an approach that provides an instantaneous measure of deposited fine sediment surface and subsurface in terms of quantity and quality we investigated how variation associated with river type spatial patchiness within rivers sampling and individual operators influenced estimates of deposited fine sediment using this approach and compared the precision with that of visual estimates of river bed composition a commonly applied technique in rapid river surveys we have used this information to develop an effective methodology for producing reach scale estimates with known confidence intervals by using a spatially focussed sampling strategy that captured areas of visually high and low deposition of fine grained sediment the dominant aspects of small scale spatial variability were controlled and a more precise instantaneous estimate of deposited fine sediment derived the majority of the remaining within site variance was attributable to spatial and sampling variability at the smallest patch scale the method performed as well as visual estimates of percentage of the river bed comprising fines in its ability to discriminate between rivers but unlike visual estimates was not affected by operator bias confidence intervals for reach scale measures of deposited fine grained sediment were derived for the technique and these can be applied elsewhere

reflections of p p from upper mantle discontinuities beneath the mid atlantic ridge summary precursors to p p observed in japan from a large deep earthquake mb 6 4 in the fiji islands are analysed to investigate reflection properties of upper mantle discontinuities beneath an area close to the mid atlantic ridge mar in the equatorial atlantic ocean traveltime analysis indicates that the precursors are interpreted as underside reflections of p p from discontinuities at depths of 655 and 415 km in the mantle the latter reflection is much weaker and more intermittent than the former slant stacking and beam forming are attempted to measure the slowness and incidence azimuth of p p and its precursors the obtained slownesses are 3 4 and 2 8 s deg 1 for p p and p p respectively little deviations of the ray incidence off the diametral azimuth are found for both phases amplitude ratio p dp p p here d 655 or 415 is studied as a function of the wave period and the distance of reflection point of p p to the axis of mar the overall trend of the spectral amplitude ratio for p 655p is consistent with the theoretical value for the 670 km discontinuity of the prem model dziewonski anderson 1981 p 415p signals were very weak and it was difficult to obtain an unbiased estimate of p 415p p p the amplitude ratios p 655p p p of filtered seismograms with a passband of 0 5 4 0 s have a mean value of 0 13 and a standard deviation of 0 03 this amplitude ratio is in good agreement with those of previous studies on the reflections from the 650 km discontinuity beneath antarctica and surrounding seas when the reflection points of p p are calculated under the assumption of a spherically symmetric earth the observed p 655p p p seems to decrease as the points become close to the axis of mar thus it could be inferred that some changes in the reflectivity of the discontinuity might occur at or near the spreading center another interpretation may also be possible the low velocity beneath the mar region would cause focussing of p p rays

modeling of elasticity effects of sandstone compaction using coated inclusions compaction of siliciclastic sediments leads to an increase in their stiffness parameters and seismic velocities although mechanical compaction implies a reduction of porosity and closing of compliant pores chemical compaction may alter the mineral properties the cementing of grain contacts and the pore volume the ability of rock physics models to quantify such effects on seismic observables will aid hydrocarbon exploration a framework was designed for modeling compaction effects by use of a so called coated inclusion model that eliminates the need of using a hybrid approach through combining different theories a basic feature of the model is that the inclusion is defined by a kernel representing the pore which is surrounded by shells that may individually have different elastic properties from those of the pore filling material and the background matrix the modeling can be designed to explore seismic effects of various texture perturbations including contact cementing and pore filling processes the numerical modelings seem to be consistent with the results obtained from other rock physics models the model allows for the possibility of including small scale heterogeneities within the rock texture and estimating frequency dispersion together with attenuation due to pore fluid flow a basic weakness of the method is the relatively large number of parameters needed to describe a porous rock which will always limit its practical usage however its basic physical foundation may provide a reference for understanding the qualitative and quantitative effects of various cementation scenarios on seismic parameters

energy flux model of seismic coda separation of scattering and intrinsic attenuation a new model of seismic coda is presented based on the balance between the energy scattered from the direct wave and the energy in the seismic coda this energy flux model results in a simple formula for the amplitude and time decay of the seismic coda that explicitly differentiates between the scattering and intrinsic anelastic attenuation of the medium this formula is valid for both weak and strong scattering and implicitly includes multiple scattering the model is tested using synthetic seismograms produced in finite difference simulations of wave propagation through media with random spatial variations in seismic velocity some of the simulations also included intrinsic dissipation the energy flux model explains the coda decay and amplitude observed in the synthetics for random media with a wide range of scattering q in contrast the single scattering model commonly used in the analysis of microearthquake coda does not account for the gradual coda decay observed in the simulations for media with moderate or strong scattering attenuation scattering q less than or equal to 150 the simulations demonstrate that large differences in scattering attenuation cause only small changes in the coda decay rate as predicted by the energy flux model the coda decay rate is sensitive however more to the intrinsic q of the medium the ratio of the coda amplitude to the energy in the direct arrival is a measure of the scattering attenuation thus analysis of the decay rate and amplitude of the coda can in principle produce separate estimates for the scattering and intrinsic q values of the crust we analyze the coda from two earthquakes near anza california intrinsic q values determined from these seismograms using the energy flux model are comparable to coda q values found from the single scattering theory the results demonstrate that coda q values are at best measures of the intrinsic attenuation of the lithosphere and are unrelated to the scatteing q less

broadband f k analysis of array data to identify sources of local scattering f k analyses of short period recordings of both u s and soviet underground nuclear explosions at the eskdalemuir eka scotland array indicate arrivals from a local source about 15 km due northwest of the array use of residual seismograms derived by subtracting the beamed record from each array channel confirmed the presence of the same scatterer the secondary seismic source appears to be short period surface waves due to the scattering of incident p waves at a nearby deep valley known as moffat water in agreement with key s 1967 results similar analyses of the noress norway array data from u s and soviet nuclear shots also indicate secondary arrivals suggesting a local near surface scatterer about 25 30 km southwest of the array in the region of lake mjosa thus short aperture array data can be useful in identifying and locating sources of near receiver scattering

upper mantle converted waves beneath the nars array several registrations of teleseismic events on the nars array show a phase which can be identified as a p to s conversion at the 670 km discontinuity beneath western europe variations in the amplitude and arrival time of the so called ps phase are observed for different azimuths a single station multiple event stacking technique was applied to the data to determine the consistency of the phase the results are indicative of amplitude arrival time and slowness variations of the converted phase on a local scale these observations are most logically explained by a model with depth variations of the 670 km discontinuity of the order of 20 km over a distance of 200 km

blind deconvolution of multichannel recordings by linearized inversion in the spectral domain abstractin seismology blind deconvolution aims to recover the source wavelet and the green s function or parts of it e g reflectivity series from a recorded seismic trace a multitude of algorithms exist that tackle this ill posed problem by different approaches making assumptions on the phase spectra of the source wavelet and or the statistical distribution of the reflectivity series is useful for single trace the nature of closely spaced multichannel recordings enables a better estimation of a common source wavelet and thus increases the confidence of the results this approach has been exploited in the past although different types of assumptions are used for a variety of algorithms we introduced a new method for simultaneous reconstruction of arbitrary source wavelets and local vertical reflectivity series from teleseismic earthquakes closely spaced receivers record vertically incident earthquake body waves and their surface related multiples which comprise the unknown reflectivity series

the effect of 3 d structure on lg propagation patterns the method of ray diagrams based on the constructive interference of multiple s reflections gives a visual measure of the interaction of guided wavetrains with variations in crustal structure the technique is extended to 3 d heterogeneity and by representing the s ray systems in plan view through their basement reflection points a semi quantitative measure of the disruption of lg at different phase velocities can be made in addition the tilt of the ray systems can be used as an indication of the degree of interconversion between rayleigh and love wave energy two simple crustal models are examined to investigate the effects of oblique incidence and transverse topographic gradients on multiple s reflections using ray diagrams the method is then applied to several real earth models of the crust in central asia using topographic data and assuming isostasy interpretations of the ray diagrams for paths across the major tectonic features compare well with many of the observations from previous studies ruzaikan et al 1977 ni barazangi 1983 and suggest that the gross characteristics of lg propagation can be attributed to the shape of the crustal waveguide

precursors to p p re examined using broad band data summary observations of underside p wave reflections from discontinuities deep within the upper mantle which precede the p p phase have provided important constraints on the existence and physical properties of these dynamically critical features in this study we extend earlier work by examining for the first time broad band recordings of those earthquakes for which one would expect to observe p p and its associated precursors p dp an exhaustive search of 20 yr of carnegie data and 11 yr of grafenberg data uncovered a number of fine p p observations but in only one or perhaps two instances are there evidence of a precursor from near 670 km depth these null results are consistent with what has been reported in earlier studies and provide us with an opportunity to estimate the variability of p velocity near 670 km or alternatively the scale length of discontinuity topography which acts to defocus the precursor this has been done by using the reflectivity and gaussian beam methods to compute synthetic seismograms for a suite of 1 d and 2 d earth models which vary slightly from prem we find that the null data may be satisfied either by i reducing the prem impedance jump at 670 km from 8 5 to 4 5 per cent ii smoothing this 8 5 per cent jump linearly over a zone 15 30km thick or iii imposing deformations in the 670 km discontinuity as small as 10 km in amplitude and 300 km in wavelength for the one case of an observed precursor the relative size of the underside reflection and main p p phase suggests that in this instance the precursor s amplitude is correctly predicted by prem

compaction fabrics of pelites experimental consolidation of kaolinite and implications for analysis of strain in slate abstract compaction of clay and shale results in large reductions in volume as pore water is expelled preferred orientation of the platy minerals increases with compaction strain and loss of porosity according to the march owens model this relationship has been studied quantitatively by experimentally consolidating kaolinite clay from slurries and analyzing the resulting fabrics with the x ray pole figure goniometer and scanning electron microscope sem initial porosity corresponds to the onset of the strain recorded by the preferred orientation and the values of 0 78 for dispersed slurries and 0 76 for flocculated slurries reflect the electrostatic forces between the clay platelets initial porosities of recently deposited fine silt and clay are in the range of 0 60 0 90 and are a function of grain size and mineralogy loss of this initial porosity has a large effect on the subsequent development of slaty cleavage matrix methods were used to model deformation paths for slates in the welsh slate belt preferred orientation of mica and ellipsoidal shapes of reduction spots were simulated for one locality by loss of a 0 60 initial porosity a 6 tilt of the beds and horizontal shortening involving plane strain strain determinations for shales and slates should include the large reduction in volume

phased array compaction cell for measurement of the transversely isotropic elastic properties of compacting sediments sediments undergoing compaction typically exhibit transversely isotropic ti elastic properties we present a new experimental apparatus the phased array compaction cell for measuring the ti elastic properties of clay rich sediments during compaction this apparatus uses matched sets of p and s wave ultrasonic transducers located along the sides of the sample and an ultrasonic p wave phased array source together with a miniature p wave receiver on the top and bottom ends of the sample the phased array measurements are used to form plane p waves that provide estimates of the phase velocities over a range of angles from these measurements the five ti elastic constants can be recovered as the sediment is compacted without the need for sample unloading recoring or reorienting this paper provides descriptions of the apparatus the data processing and an application demonstrating recovery of the evolving ti properties of a compacting marine sediment sample

numerical analytical interfacing in two dimensions with applications to modeling nts seismograms a new method for interfacing numerical and integral techniques allows greater flexibility in seismic modeling specifically numerical calculations in laterally varying structure are interfaced with analytic methods that enable propagation to great distances such modeling is important for studying situations containing localized complex regions not easily handled by analytic means the calculations involved are entirely two dimensional but the use of an appropriate source in combination with a filter applied to the resulting seismograms produces synthetic seismograms which are point source responses in three dimensions the integral technique is called two dimensional kirchhoff because its form is similar to the classical three dimensional kirchhoff data from yucca flat at the nevada test site are modeled as a demonstration of the usefulness of the new method in this application both local and teleseismic records are modeled simultaneously from the same model with the same finite difference run this application indicates the importance of locally scattered rayleigh waves in the production of teleseismic body wave complexity and coda

the velocity structure of the crust and upper mantle under china from broad band p and pp waveform analysis summary we model 45 p and pp waveforms at epicentral distances of 14 to 40 primarily sampling western and central china to determine regionalized p wave velocity variations for the crust and upper mantle in this region observed p and pp waveforms are matched by 1 d forward modelling using the reflectivity technique our approach is to first find a homogeneous average velocity model for china which can match the observed broad band waveforms filtered with a long period instrument response this model is then used as a starting model for the matching of the broad band waveforms to investigate the lateral velocity variations our preferred average model wch has a 50 km thick crust with a velocity of 6 4 km s a p velocity of 8 15 km s and a low velocity zone between 100 km and 175 km depth it has intermediate characteristics between an active tectonic region and a stable shield compatible with previous s wave modelling results modelling of the broad band waveforms indicates significant lateral deviations from model wch for the tibetan plateau we observe larger crustal thickness 55 70 km and p velocity 8 15 8 25 km sc in combination with a low velocity zone that varies in magnitude throughout the tibetan region lower velocities than our average model may be present below 200 km depth our modelling indicates a thin lid of about 30 45 km under tibet we infer from our results and previous work that crustal shortening and thickening is the most likely process responsible for the central and northern tibetan plateau s thick crust and high average elevation and that the indian plate has probably only underplated the southernmost part of the plateau mantle convection associated with the crustal shortening process has led to strong lateral heterogeneity in the upper mantle under tibet higher p velocities in western tibet might indicate that this region is presently undergoing crustal shortening paths traversing eastern china require a constant velocity lid of about 100 km thickness a p velocity of 8 12 8 14kms and a crustal thickness between 27 and 35 km a preliminary model for northern china suggests a crustal thickness between 43 and 50km a p velocity of about 8 0kms and a 100km thick lid with a positive velocity gradient for this region

detection of underside p reflections at mantle discontinuities by stacking broadband data underside p reflections at mantle discontinuities are difficult to observe directly on seismograms mainly because of their low amplitude it is shown that stacking seismograms obtained at several epicentral distances but having their pp bounce points in the same region enables the identification of such phases this method is applied to data from japanese and taiwan events recorded by the nars array and other broadband european stations discontinuities at about 670 and 300 km are revealed beneath northern eurasia

anisotropic velocity analysis1 results from walkaway vsp and shale laboratory experiments show that shale anisotropy can be significantly anelliptic heterogeneity and anellipticity both lead to non hyperbolic moveout curves and the resulting ambiguity in velocity analysis is investigated for the case of a factorizable anisotropic medium with a linear dependence of velocity on depth more information can be obtained if there are several reflectors the method of dellinger et al for anisotropic velocity analysis in layered transversely isotropic media is examined and is shown to be restricted to media having relatively small anellipticity a new scheme based on an expansion of the inverse squared group velocity in spherical harmonics is presented this scheme can be used for larger anellipticity and is applicable for horizontal layers having monoclinic symmetry with the symmetry plane parallel to the layers the method is applied to invert the results of anisotropic ray tracing on a model sand shale sequence for transversely isotropic media with small anisotropy the scheme reduces to the method of byun et al and byun and corrigan the expansion in spherical harmonics allows the p phase slowness surface of each layer to be determined in analytic form from the layer parameters obtained by inversion without the need to assume that the anisotropy is weak

experimental laboratory determination of the dynamic elastic properties of wet drained shales the ultrasonic anisotropic elastic properties of drained saturated shales were measured as a function of confining pressure two shales were characterized in this study a jurassic outcrop shale retrieved under the sea in a saturated state and a kimmeridge clay shale cut from a north sea borehole both shales were highly anisotropic both texturally as revealed by scanning electron microscopy analysis and elastically as measured ultrasonically in the laboratory the strongest anisotropy was seen in the kimmeridge clay shale where up to 38 compressional wave anisotropy thomsen s parameter e and up to 58 shear wave anisotropy thomsen s parameter were observed in addition for both shales e and were found to decrease as a function of increasing confining pressure with the pore pressure drained to atmosphere while the anellipticity deviation of the slowness surfaces from ellipses was found to be positive and decreased as a function of increasing confining pressure accompanying these changes in elastic properties was a decrease in porosity with increasing confining pressure from 10 5 to 8 5 for the jurassic shale the decrease in overall anisotropy of the shales with increasing confining pressure was found to be consistent with theoretical modeling of shale properties where the shale anisotropy and anellipticity were predicted to decrease as a function of decreasing fluid filled porosity