an algebraic reconstruction technique for weakly anisotropic velocity laboratory measurements and field experiments indicate that many sedimentary rocks display velocity anisotropy robertson and corrigan 1983 white et al 1983 thomsen 1986 winterstein 1986 gaiser et al 1987 accounting for anisotropy is proving important in several areas including the processing and interpretation of shear wave data lynn and thomsen 1986 bush and crampin 1987 and the integration of vertical seismic profile vsp data and surface seismic data chiu and stewart 1987a

anisotropy and transverse isotropy a number of authors in the exploration literature have written about anisotropy but have restricted their discussions to wave propagation through rock having transverse isotropy with a vertical symmetry axis this note shows that there are fundamental differences between transverse isotropy when the symmetry axis is vertical normal to the free surface and more general anisotropy with an azimuthal variation of properties these differences are important now that effective azimuthal shear wave anisotropy resulting from aligned cracks and pores is becoming recognized as a significant property of crustal rocks

paraxial ray tracing in 3d inhomogeneous anisotropic media1 many investigations of the propagation elastic waves within the earth require a technique for producing synthetic seismograms which is capable of modelling 3d propagation effects ray methods are an excellent option for these problems because they can be made fully 3d and allow a relatively quick and flexible computation of synthetic seismograms however the two point problem of finding the ray which connects exactly a specific source and receiver may still be difficult and time consuming therefore application of the paraxial method which allows extrapolation of the information on a given ray to nearby receiver locations is very valuable with this approach great savings in computation time and significant simplification of computer codes are possible we investigate the application of the paraxial ray method to two problems in which the effects of 3d seismic wave propagation are important the first is a model of a reef structure in this case we consider synthetic seismograms for a vsp experimental configuration when the sv source and well are located along the axis of the reef only 2d propagation effects are observed if the source receiver plane is located to the side of the reef however the 3d shape of the reef causes significant amplitudes to be predicted for shear wave arrivals on the transverse component of the synthetic seismograms the second example is a id layered earth model but it includes two layers which are azimuthally anisotropic due to the presence of aligned vertical fractures this anisotropy leads to 3d raypaths synthetic seismographs are presented for a cross hole geometry both for an equivalent isotropic model and for the direction parallel to the fractures and at an angle of 45 to the fractures in the anisotropic case these synthetics show that the differences between the isotropic case and the case for source and receivers aligned with the plane of the fractures are small and subtle on the other hand the predictions for the direction at 45 to the cracks show shear wave splitting and significant transverse component signal these results have important implications for both modelling and for applications such as tomography it is clear that for some of the cases considered a 2d algorithm will lead to errors in interpretation of data in addition ray based tomographic techniques will have great difficulty in obtaining a well defined 2d planar image when the signals are propagating in regions outside the image plane

global correlation of the radiolarian faunal change across the triassic jurassic boundary precise comparison of the change in radiolarian faunas 3 5 m above a u pb zircon dated 199 6 0 3 ma tuff and approximately coincident with a negative 13 c anomaly in the queen charlotte islands b c canada with inuyama japan sequences indicates that major global changes occurred across the triassic jurassic t j boundary nearly 20 genera and over 130 rhaetian species disappeared at the end of the triassic the index genera betraccium and risella disappear and the final appearance of globolaxtorum tozeri livarella valida and pseudohagiastrum giganteum sp nov are also diagnostic for the end of the triassic the low diversity hettangian survival fauna immediately above the boundary is composed mainly of small primitive spumellarians with spongy or irregularly latticed meshwork and rod like spines and new genera charlottea udalia and parahsuum s l first appear in the lowest hettangian in both localities irrespective of different sedimentation rates and sedimentary environments such as shelf to upper slope queen charlotte islands and deep sea below carbonate compensation depth ccd inuyama radiolarians show a similar turnover pattern at the t j boundary

observations of the internal tide in monterey canyon abstract data from two shipboard experiments in 1994 designed to observe the semidiurnal internal tide in monterey canyon reveal semidiurnal currents of about 20 cm s 1 which is an order of magnitude larger than the estimated barotropic tidal currents the kinetic and potential energy evidenced by isopycnal displacements of about 50 m was greatest along paths following the characteristics calculated from linear theory these energy ray paths are oriented nearly parallel to the canyon floor and may originate from large bathymetric features beyond the mouth of monterey bay energy propagated shoreward during the april experiment itex1 whereas a standing wave that is an internal seiche was observed in october itex2 the difference is attributed to changes in stratification between the two experiments higher energy levels were present during itex1 which took place near the spring phase of the fortnightly 14 8 days cycle in sea level while itex2 occurred close to the neap phase further evide

radiation from point sources in general anisotropic media summary a procedure to evaluate the geometrical properties of the slowness surface for arbitrary anisotropic media is presented this allows us to use results for computing the displacement field from point sources in general anisotropic media utilizing the gaussian curvature of the slowness surface of the considered waves since the formulae used are based on high frequency asymptotics the procedure gives reliable results as long as no parabolic points of the slowness surface are encountered radiation patterns of point sources for models representing rocks in the crust and upper mantle are presented and discussed the radiation from point sources in anisotropic models is compared with the radiation from point sources in average isotropic models to emphasize the differences anisotropy is either caused by aligned cracks crustal rocks or by aligned olivine crystals upper mantle rocks considering models of aligned cracks differing only in the physical parameters of the crack material water or gas significantly different radiation patterns are obtained models with wet cracks exhibit focusing of quasi s wave energy for some directions whereas this effect is not observed for the corresponding model with dry cracks the quasi s wave radiation pattern from explosions are multilobed and the wet crack models exhibit more lobes than the dry crack models the radiation of single forces in olivine shows strong energy focusing wave tuning of quasi s waves however for realistic compositions of upper mantle rocks with some 10 per cent of preferentially orientated olivine this wave tuning is less pronounced

offscraping accretion of jurassic chert clastic complexes in the mino tamba belt central japan abstract detailed structural and biostratigraphical analysis of the jurassic inuyama sequence a coherent chert clastic complex in the mino tamba belt central japan clarifies the evolution of accretionary processes at shallow structural levels the inuyama sequence is characterized by a series of stacked thrust sheets each sheet consists of an early triassic to middle jurassic oceanic plate stratigraphy composed of four lithologic units which are in ascending order siliceous claystone ribbon chert siliceous mudstone and clastic rocks the structural features of the inuyama sequence demonstrate a four stage progressive deformation 1 a decollement was initiated within the siliceous mudstone when this sequence was just seaward of the deformation front clastic dikes and sills formed at the horizon just above the decollement at this time 2 the stratigraphic section above the decollement was imbricated by in sequence thrusting in the frontal part of the wedge this initial stage of thrusting and imbrication was followed by 3 the formation of duplex structures with fault related folds within the lower stratigraphic section as the decollement stepped down section to the lowest siliceous claystone interval finally 4 these thrust packages were overprinted by secondary prism thickening in the form of out of sequence thrust faulting

a fast robust method for detecting and characterizing azimuthal anisotropy with marine ps converted waves and its application to the west svalbard continental slope summary a method for using ps converted waves to detect and characterize azimuthal seismic anisotropy is developed and applied to four component ocean bottom seismometer data acquired near the base of the continental slope west of svalbard the method determines the distribution of coherent signal vibrating perpendicular to the sagittal plane the transverse direction and employs an optimized rotation technique to separate the effects of anisotropy from other contributions to the seismic amplitude the magnitude of the ps wave transverse amplitude when normalized by its radial counterpart is shown to correspond to the degree of ellipticity of particle motions and is used to constrain the anisotropic symmetry class in the near surface sediments results indicate the presence of orthorhombic or horizontal transversely isotropic symmetry that is consistent with the presence of near vertical aligned cracks from its orientation the observed anisotropy is associated with 1 gravitational downslope stresses in the sediment sequence which dips by approximately two degrees to the sw and 2 tectonic stresses associated with the segmented spreading centres of the northern knipovich ridge which spread in the nw se direction aligned cracks are likely to influence the local distribution of gas hydrate and free gas in these sediments an investigation of the effect of seismic noise on optimized rotation shows that the vectorial combination of independent geophone records has variable sensitivity to source related noise this sensitivity peaks where the sagittal azimuth bisects the deployment orientation of the geophones on the seabed a method to reduce the sensitivity to directional noise is developed and shown to improve the transverse amplitude plots this approach to noise reduction may be used to improve the quality of seismic images derived from multicomponent data

low latitude and southern hemisphere origin of anisian triassic bedded chert in the inuyama area mino terrane central japan paleomagnetic and paleontologic studies of middle triassic to earliest jurassic radiolarian bedded chert in the inuyama area southern mino terrane of central japan have shown that the chert was deposited in an equatorial region with a paleolatitude between 10 n and 10 s and in particular the lower to middle anisian chert depositional basin was situated in the southern hemisphere the chert has four characteristic components of remanent magnetization the highest blocking temperature component carried by hematite shows positive fold and reversal tests and is regarded as the primary remanent magnetization acquired just after deposition the other three components are interpreted as a present field overprint and two postfolding remagnetizations the absolute tilt corrected inclinations of the primary component are less than 20 for all the samples of middle triassic to earliest jurassic age with the mean inclination of the lower to middle anisian chert being 11 0 4 3 paleomagnetic polarity reversals observed for the lower to middle anisian chert are well correlated with those of the european triassic sections under paleontologic age constraints using radiolarians and conodonts provided that the anisian chert in inuyama was deposited in the southern hemisphere this conclusion illuminates the paleogeographic evolution of the chert bearing accretionary complexes not only in japan but along the eastern margin of asia as well

an investigation into possible facies changes across the triassic jurassic boundary in southern tibet abstract a complete marine succession across the triassic jurassic boundary is reported from southern tibet on the northern flanks of the himalayas both youngest triassic and oldest jurassic ammonites together with palynological data allow a combined facies and gamma ray study indicating no significant change across the system boundary unlike elsewhere in the world

the range of effects of azimuthal isotropy and eda anisotropy in sedimentary basins summary combinations of bedding or lithology induced azimuthal isotropy with an axis of symmetry perpendicular to the bedding plane and crack induced extensivedilatancy anisotropy eda with a horizontal axis of symmetry are believed to be common in sedimentary basins and cause the widely observed phenomenon of shear wave splitting combinations of two such transversely isotropic forms of anisotropy with orthogonal axes of cylindrical symmetry lead to orthorhombic symmetry this has two major effects 1 the polarizations of the faster split shear waves may no longer be parallel to the strike of the cracks or fractures even for near vertical propagation and 2 such orthorhombic symmetry systems necessarily have a number of directions called shear wave point singularities where shear waves display disturbed or anomalous behaviour again possibly in near vertical directions unless these effects are correctly identified they could be interpreted mistakenly for the effects of structural irregularities or discontinuities in contrast recognition of the 3 d geometry of this behaviour places comparatively tight constraints on possible combinations of anisotropy in the rockmass in order to give some understanding of the geometry of these phenomena this paper presents 3 d patterns of the behaviour of shear wave splitting that have been computed for a range of combinations of crack and bedding induced anisotropy

magnetization carriers and remagnetization of bedded chert abstract article history received 22 october 2010received in revised form 11 january 2011accepted 25 february 2011editor p demenocalkeywords bedded chertremanencebiogenic magnetitehematite bedded chert is the only sediment type representative of the paleozoic to early mesozoic pelagic marineenvironment becauseoftheirassociationwithophiolitesandislandarcrocks presenceofdatablemicrofossilsandpaleohorizontalreferenceprovidedbywell developedbeddingsurfaces beddedchertsequencesareoftentargeted for paleomagnetic tectonic studies however processes of magnetization acquisition in biosiliceoussediments and consequently the signi cance of their magnetic record are not well understood our rockmagnetic study of a triassic jurassic radiolarian chert sequence the mino terrane central japan shows thatthe ferrimagnetic assemblage of the gray chert units is of detrital origin while the red chert s assemblage isdominated by authigenic phases pigmentary hematite and biogenic magnetite which contribute to thenatural remanent magnetization the presence of magnetofossils places red oxic chert in the category ofprospective environmental archives magnetite producing magnetotactic bacteria were apparently able totolerate elevated concentrations of dissolved silica as well as a steep redox gradient in sedimentary pore waters during the deposition of red chert layers a strong uniaxial anisotropy due to chain alignment of thebiogenic magnetite grains likely contributes to the acquisition of anomalously stable partial thermoviscousmagnetization by chert even at low metamorphic temperatures 2011 elsevier b v all rights reserved

assessing the record and causes of late triassic extinctions accelerated biotic turnover during the late triassic has led to the perception of an end triassic mass extinction event now regarded as one of the big five extinctions close examination of the fossil record reveals that many groups thought to be affected severely by this event such as ammonoids bivalves and conodonts instead were in decline throughout the late triassic and that other groups were relatively unaffected or subject to only regional effects explanations for the biotic turnover have included both gradualistic and catastrophic mechanisms regression during the rhaetian with consequent habitat loss is compatible with the disappearance of some marine faunal groups but may be regional not global in scale and cannot explain apparent synchronous decline in the terrestrial realm gradual widespread aridification of the pangaean supercontinent could explain a decline in terrestrial diversity during the late triassic although evidence for an impact precisely at the boundary is lacking the presence of impact structures with late triassic ages suggests the possibility of bolide impact induced environmental degradation prior to the end triassic widespread eruptions of flood basalts of the central atlantic magmatic province camp were synchronous with or slightly postdate the system boundary emissions of co2 and so2 during these eruptions were substantial but the contradictory evidence for the environmental effects of outgassing of these lavas remains to be resolved a substantial excursion in the marine carbon isotope record of both carbonate and organic matter suggests a significant disturbance of the global carbon cycle at the system boundary release of methane hydrates from seafloor sediments is a possible cause for this isotope excursion although the triggering mechanism and climatic effects of such a release remain uncertain d 2003 elsevier b v all rights reserved

biogeochemical constraints on the triassic jurassic boundary carbon cycle event 1 the end triassic mass extinctions represent one of the five most severe biotic crises in earth history yet remain one of the most enigmatic ongoing debate concerns the environmental effects of the central atlantic magmatic province camp eruptions and their linkage with the mass extinction event across the triassic jurassic boundary there is conflicting paleo evidence for changes in atmospheric pco2 during the extrusion of the camp basalts studies on sediments from european and pacific localities have however identified a substantial negative isotopic anomaly up to 3 5 across the tr j boundary providing an important indicator of changes in the operation of the ancient global carbon cycle we sought to explain the paleo evidence by utilizing a carbon cycle model for the hothouse world of the end triassic that emphasizes the chemical weathering of silicate and carbonate rocks and the ocean carbonate chemistry we find that volcanic co2 outgassing fails to fully account for either a sufficient rise in atmospheric pco2 indicated by the stomata of fossil leaves or the sedimentary isotopic fingerprint instead the scenario that best fits all of the geologic evidence is a positive feedback loop in which warming due to a buildup of volcanically derived co2 triggers destabilization of seafloor methane hydrates and the catastrophic release of ch4 palfy et al 2001 we calculate that this carbon cycle perturbation was huge involving the release of 8000 9000 gt c as co2 during the camp basaltic eruptions and 5000 gt c as ch4 in the model the initial isotopic excursion is assumed to take place over 70 kyr while complete reequilibration of the ocean atmosphere system with respect to co2 is accomplished over 700 1000 kyr our results thus provide a preliminary theoretical explanation for the bioevents estimated pco2 changes and isotopic excursions observed in marine and continental sediments at this time

detection of noninteracting single domain particles using first order reversal curve diagrams we present a highly sensitive and accurate method for quantitative detection and characterization of noninteracting or weakly interacting uniaxial single domain particles unisd in rocks and sediments the method is based on high resolution measurements of first order reversal curves forcs unisd particles have a unique forc signature that can be used to isolate their contribution among other magnetic components this signature has a narrow ridge along the hc axis of the forc diagram called the central ridge which is proportional to the switching field distribution of the particles therefore the central ridge is directly comparable with other magnetic measurements such as remanent magnetization curves with the advantage of being fully selective to sd particles rather than other magnetic components this selectivity is unmatched by other magnetic unmixing methods and offers useful applications ranging from characterization of sd particles for paleointensity studies to detecting magnetofossils and ultrafine authigenically precipitated minerals in sediments

observation and modelling of fault zone fracture seismic anisotropy i p sv and sh travel times summary three component vsp borehole seismograms taken in the vicinity of an active normal fault in california show strong systematic shear wave splitting that increases with proximity to the fault using cerven s method of characteristics for ray tracing in anisotropic heterogeneous media and hudson s formulation of elastic constants for media bearing aligned fractures we have fitted a suite of p sv and sh hanging wall and foot wall travel times with a simple model of aligned fractures flanking the fault zone the dominant fracture set is best modelled as parallel to the fault plane and increasing in density with approach to the fault the increase in fracture density is non uniform power law or gaussian with respect to distance to the fault although the hanging wall and the foot wall rock are petrologically the same unit the fracture halo is more intense and extensive in the hanging wall than in the foot wall upon approach to the fault plane the fracture density or fracture density gradient becomes too great for the seismic response to be computed by hudson cerven procedures the maximum fracture density that can be modelled is about 0 08 within this 25 m fracture domain it appears more useful to model the fault and near field fractures as a low velocity waveguide we observe production of trapped waves within the confines of the intense fracture interval

borehole seismic source radiation pattern in transversely isotropic media we extend previous discussions on crosswell tomography in anisotropic formations by deriving the radiation patterns of three typical downhole seismic sources impulsive air gun or dynamite wall clamped vertical vibrators and cylindrical bender inside a fluid filled borehole embedded in a transversely isotropic ti formation the method of steepest descents in conjuncture with the low frequency and far field assumptions is applied to the exact displacement integrals of these sources to obtain their radiation patterns asymptotically in spite of complications caused by quasi p and quasi sv wave coupling and wavefront triplication in homogeneous ti media the final results can still be expressed in slowness components determined by a ray direction which is desired when source radiation effects are to be accounted for by ray based tomography techniques tests with the radiation patterns show that while the effect of anisotropy on p waves is moderate its effect on the s wave pattern is significant eve

in situ seismic measurements in claystone at tournemire france compressional and shear wave seismic measurements were performed in an old railway tunnel and in galleries excavated in a 250 m thick toarcian claystone formation in the tournemire experimental station france three component 3c geophones and three orthogonal orientations of the vibroseismic force source were used additionally vertical seismic profiling vsp measurements were recorded with a 3c borehole geophone a hydrophone and a microphone in a 159 m deep borehole id180 in the tunnel the seismic data show that toarcian claystone has strong transverse isotropy ti with a vertical symmetry axis the qp sh and qsv wave propagation velocities in horizontal directions the plane of isotropy of the ti medium are measured as 3550 1850 and 1290 m s 1 respectively the zero offset vsp reveals that only one shear wave propagates in the vertical depth direction and the p and s wave velocities are 3100 and 1375 m s 1 respectively four elastic moduli of the ti medium are determined from the seismic velocities and from the bulk density of 2 53 g cm 3 c11 31 9 gpa c33 24 3 gpa c44 4 5 gpa and c66 8 7 gpa a walkaway vsp with the borehole geophone at 50 m depth in borehole id180 and shot points in the galleries leads to oblique seismic ray paths which allow us to determine the fifth elastic modulus of the ti medium to c13 16 gpa the tube wave recorded by a hydrophone in the water filled lower part of the borehole propagates with 1350 m s 1 which confirms the estimate of the elastic constant c66 the analysis of body wave and surface wave data from a seismic experiment in galerie est shows reflections from several fracture zones in the gallery floor the thickness of the excavation damaged zone edz in the floor of galerie est is estimated to 0 7 m the authors 2014 published by oxford university press on behalf of the royal astronomical society

quantitative measurements of shear wave polarizations at the anza seismic network southern california implications for shear wave splitting and earthquake prediction we analyze shear wave polarizations from local earthquakes recorded by the anza network in southern california using an automated method which provides unbiased and quantitative measurements of the polarization and the duration of linear motion following the shear wave arrival the linearity interval initial shear wave particle motions are strongly aligned at four stations a feature that is not predicted by focal mechanisms the particle motion alignment is most likely caused by shear wave splitting due to anisotropy beneath these stations a result supported by the clear shear wave splitting seen in a borehole recording near one of the anza stations these results are consistent with an earlier analysis of these data by peacock et al 1988 however our analysis does not support claims by crampin et al 1990 that shear wave splitting delay times at station knw exhibit temporal variations which can be correlated with the occurrence of the north palm springs earthquake ml 5 6 of july 8 1986 automatically determined linearity intervals scatter widely from 0 02 to 0 15 s and exhibit no clear temporal trends we find a correlation between earthquake moment and the linearity interval possibly a result of longer effective source time functions for the larger events the inability to identify a distinct slow quasi shear wave pulse for the vast majority of these events indicates that scattering strongly affects the particle motion even in the very early shear wave coda analysis of earthquake clusters with similar waveforms recorded at knw shows that seismic green functions are stable throughout the observational period and that most linearity interval variation is due to source and ray path differences between events if shear wave splitting is causing the observed delay times between horizontal components the waveform stability for events in these clusters restricts any temporal changes in shear wave splitting delay times to less than 5 10

ray series modeling of seismic wave travel times and amplitudes in three dimensional heterogeneous anisotropic crystalline rock borehole vertical seismic profiling seismograms from the mojave desert california a 600 m borehole vertical seismic profiling vsp survey conducted in crystalline rock in the mojave block stress province southern california yielded 100 oriented three component seismograms from a fan shooting geometry with radius of 100 m using both vertical and horizontal vibrators and a vertical impact source the seismograms show up to 12 ms of shear wave splitting and 5 lateral velocity heterogeneity first and second order ray tracing was used to model the travel times and amplitudes of p sv and sh waves in a three dimensional mildly heterogeneous medium with elastic anisotropy induced by aligned fractures a least squares inverse procedure was used to improve model parameters determined by forward modeling the final root mean square travel time residual was 2 ms for purposes of amplitude computation we assume that the wavelets observed in a vsp section derive from a smooth intermediate interface base of weathering layer or sedimentary basement interface that is illuminated by a narrow range of ray angles originating at a point source on the surface source derived amplitude complications are thus minimized our results strongly indicate a population of vertical cracks with an average crack density of 0 035 oriented in the n31 5 w direction such a fracture population is consistent with regional principal strain and stress determinations ray series geometric amplitudes were consistent with the observation that wave motion parallel to the aligned fractures decayed more slowly than wave motion transverse to the aligned fractures we estimate that q 75 100 we were unable to measure a polarization dependent q

a decade of shear wave splitting in the earth s crust what does it mean what use can we make of it and what should we do next it is 10 years since shear wave splitting throught to be diagnostic of some form of seismic anisotropy was first positively identified in the earth s crust from the beginning it was argued that the splitting was probably associated with the presence of stress aligned cracks inclusions in the crust and that this would provide the opportunity for monitoring the in situ geometry of cracks and stress in a variety of different circumstances and in a variety of different applications

a new chronology for the end triassic mass extinction article i nfo the transition from the triassic to jurassic period initiating the age of the dinosaurs approximately 200 ma is marked by a profound mass extinction with more than 50 genus loss in both marine and continental realms this event closely coincides with a period of extensive volcanism in the central atlantic magmatic province camp associated with the initial break up of pangaea but a causal relationship is still debated the triassic jurassic t j boundary is recently proposed in the marine record at the first occurrence datum of jurassic ammonites post dating the extinction interval that concurs with two distinct perturbations in the carbon isotope record the continental record shows a major palynological turnover together with a prominent change in tetrapod taxa but a direct link to the marine events is still equivocal here we develop an accurate chronostratigraphic framework for the t j boundary interval and establish detailed trans atlantic and marine continental correlations by integrating astrochronology paleomagnetism basalt geochemistry and geobiology we show that the oldest camp basalts are diachronous by 20 kyr across the atlantic ocean and that these two volcanic pulses coincide with the end triassic extinction interval in the marine realm our results support the hypotheses of phanerozoic mass extinctions resulting from emplacement of large igneous provinces lips and provide crucial time constraints for numerical modelling of triassic jurassic climate change and global carbon cycle perturbations

anisotropy effects in p wave and sh wave stacking velocities contain information on lithology depths calculated from s wave stacking velocities and event times almost always exceed actual depths sometimes by as much as 25 percent in contrast depths from corresponding p wave information are often within 10 percent of actual depths discrepancies in depths calculated from p and s wave data are attributed to velocity anisotropy a property of sedimentary rocks that noticeably affects s wave moveout curves but leaves the p wave relatively unaffected two careful studies show that discrepancies in depths and hence in constituent layer thicknesses correlate with lithology discrepancies ranged from an average of 13 percent midland basin to greater than 40 percent paloma field in shales but were within expected errors in massive sandstones or carbonates hence anisotropy effects are indicators of lithology analysis of seismic data involved determining interval velocities from stacking velocities calculating layer thicknesses and then comparing layer thicknesses from s wave data with thickne

estimation of near surface elastic parameters using multicomponent seismic data in multicomponent seismic reflection surveys surface and near surface effects can severely deteriorate the quality of reflection data such effects are more pronounced on shear wave than on compressional wave profiles amplitude anomalies statics and strong coherent source generated noise i e surface waves are often associated with inhomogeneous poorly compacted near surface sediments of the weathering layer the magnitude of such effects increases when sources and receivers are deployed at or near the surface in proximity to such inhomogeneities near surface effects can be investigated with respect to their depth of occurrence by burying seismic sources and or receivers at various depths below the inhomogeneous weathering layer in this context an experiment was conducted to collect multicomponent seismic field data on a borehole controlled test site in east texas data were recorded on three component surface and buried receivers using a full vector wavefield surface source although the geolo

velocity anisotropy in shale determined from crosshole seismic and vertical seismic profile data crosshole and vertical seismic profile vst data made possible accurate characterization of the elastic properties including noticeable velocity anisotropy of a near surface late tertiary shale formation shear wave splitting was obvious in both crosshole and vsp data in crosshole data two orthologonally polarrized shear s waves arrived 19 ms in the uppermost 246 ft 75 m vertically traveling s waves of the vsp separated about 10 ms in the uppermost 300 ft 90 m but remained at nearly constant separation below that level a transversely isotropic model which incorporates a rapid increase in s wave velocities with depth but slow increase in p wave velocities closely fits the data over most of the measured interval elastic constants of the transvesely isotropic model show spherical p and s2 wave velocity surfaces but an ellipsoidal s1 wave surface with a ratio of major to minor axes of 1 15 the magnitude of this s wave anisotropy is consistent with and lends credence to s wave anisotropy magn

migration of noisy crustal seismic data a slowness weighted diffraction stack algorithm is used to migrate crustal seismic reflection data slowness information is extracted from the data using a slant stack beam forming process the main purpose of beam forming is to improve the signal to noise ratio over a given range of slowness values localized slant stacks are used to design a coherency weighted slowness filter in the time distance domain through migration the subsurface structure and the observed seismic data are tied via slownessselective diffraction curves a combination of nonlinear slowness filtering with migration gives excellent noise rejection and leads to an improved image in the depth domain the method which is particularly effective for migrating moderate to steep dipping events with low signal to noise ratio is applied to various poststack and prestack synthetic data examples and to a deep crustal data set collected on vancouver island as part of the canadian lithoprobe project

shear wave anisotropy in the parkfield varian well vsp abstract a vertical seismic profile vsp survey was run to 1334 m depth in the instrumented varian well 1 4 km from the san andreas fault trace at parkfield california to test the sensor string shortly after its permanent installation the cable subsequently failed near the 1000 m level so the test survey represents the deepest data acquired in the study a shear wave vibrator source was used at three ofsets and two orthogonal orientations and the data have been processed for p and s wave velocities and for s wave velocity anisotropy velocities are well determined 3 3 and 1 9 km sec respectively at the deeper levels and the s waves are seen clearly to be split by anisotropy below about 400 m some 8 per cent velocity difference is apparent between polarizations parallel to and perpendicular to the san andreas fault faster and slower respectively and the difference seems to decrease with distance from the fault suggesting that the cause may be the fabric of the fault zone repeated surveys at the 1000 m depth are being conducted as part of the parkfield monitoring program

a multi proxy study of argentina loess marine oxygen isotope stage 4 and 5 environmental record from pedogenic hematite abstract a 12 m loess paleosol section la angostura from tafi del valle tucuman argentina was studied using rock magnetic measurements geochemistry mossbauer effect particle size and nd isotopes in order to elucidate changes in paleoenvironment at the site combining the results of these experiments allowed us to separate the effects of the two major controls parent material and pedogenesis on the measured parameters two behavior regimes were identified in the sediment 1 in the lower half of the section pedogenic processes though weak dominate the variation of the sediment s physical properties and 2 for the upper half of the section subtle changes in parent material control the majority of the measured parameters even those typically controlled by pedogenesis e g clay fraction applying rock magnetic mossbauer and bulk geochemistry measurements in concert we calculated an iron budget for the loess 80 of the iron in the sediment is in paramagnetic minerals 20 of the iron is in hematite 1 6 of the sediment s mass and 1 of the total iron is present as magnetite maghemite 0 1 of the sediment s total mass neither the pedogenic formation nor the depletion of ferrimagnetic material with increasing pedogenesis was consistently observed in this sediment throughout the entire section the concentration of hematite is argued to be sensitive to in situ alteration and it is further surmised to be sensitive to the available moisture at the site finally we present a curiosity within the error of our chronology the concentration of hematite seems to correlate with the coeval climate record from antarctica vostok it is possible that the sedimentary magnetic record from la angostura is a proxy for the strength of the south american summer monsoon

cavitation induced seismic events caused by small explosions in boreholes an electrolytically recharged sleeve exploder was fired at depths between 15 2 m and 51 8 m in a 200 mm diameter steel cased water filled shallow borehole in hopkins county texas oxy hydrogen explosive gas charges were varied between 30 and 170 kj seismic traces revealed three distinct delayed anomalous arrivals two of these moved out nonlinearly with increasing charge the fact that the anomalies were not peculiar to gaseous sleeve explosions was confirmed by repeating part of the experiment using small high explosive charges the anomalies appear to stem from the collapse of cavities that are created by shock waves a model is advanced in which up and downgoing shock fronts sweep away from the explosion at the tube wave velocity they reflect from the free and plugged ends of the borehole reflection at the free end transforms an upgoing overpressure into a downgoing rarefaction momentum transfer surges the fluid surges accumulate two rarefaction shocks eventually collide at the shot point

rock magnetic record of the triassic jurassic transition in pelagic bedded chert of the inuyama section japan the end triassic mass extinction event is regarded as one of the fi ve largest extinction events of the phanerozoic the emerging consensus points to volcanic activity at the central atlantic magmatic province camp as the ultimate cause of the extinction yet the underlying mecha nisms and the nature of global environmental changes that accompanied the biotic turnover remain elusive we present a rock magnetic study of the extinction interval found within a continuous chert sequence that provides an uninterrupted record of pelagic sedimentation in the panthalassa ocean the variations in the relative abundances and characteristics of authi genic magnetic phases indicate that the triassic jurassic transition progressed in two stages the initial stage characterized by a disappearance of the previously ubiquitous magnetofos sils started a few tens of thousands of years to 100 k y prior to the formal triassic jurassic boundary as identifi ed by the diagnostic radiolarian species the second stage defi ned by signifi cant changes in optical and magnetic properties of hematite pigment lasted a few tens of thousands of years the stepwise change in magnetic properties is suggestive of the protracted environmental deterioration likely prompted by the early episodes of the camp volcanism which was followed by a sudden ocean acidifi cation event perhaps triggered by a catastrophic release of gas hydrates

multiple phases of carbon cycle disturbance from large igneous province formation at the triassic jurassic transition the end triassic mass extinction ca 201 4 ma coincided with a major carbon cycle perturbation based on an 5 6 negative excursion in 13 c toc total organic carbon records both events coincided directly with the onset of massive flood basalt volcanism in the central atlantic magmatic province camp organic carbon isotope data from the western tethys ocean austria and the germanic basin uk and germany however demonstrate earlier disruption of the global carbon cycle preceding camp eruptions a 2 3 late rhaetian precursor negative excursion in marine and continental 13 c toc records is matched by a negative perturbation in 13 c leaf data suggesting multiple events of rhaetian atmospheric 13 c depletion intruding dike and sill systems preceding camp eruptive volcanic activity may have released 3000 7000 gt of isotopically light carbon as thermogenic methane from subsurface organic rich strata this possibly caused an end triassic atmospheric p co 2 increase and reduced ecosystem stability before the actual onset of eruptive volcanic activity in the camp region we present a model that identifies three phases of disturbances in global biogeochemical cycles related to the formation of this large igneous province

radiation patterns of a shear wave vibrator in near surface shale two shallow surface to borehole field tests were conducted to investigate the radiation patterns of a shear wave vibrator in near surface shale in the in line direction for which baseplate motion is transverse to the source geophone raypath the vibrator produces a broadband high amplitude signal of the sh type and virtually no signal of the sv or p types the amplitude of the sh radiation is approximately constant as a function of polar angle but it falls off as the cosine of the azimuthal angle measured with respect to the in line direction in the broadside direction the vibrator produces a strong uncontaminated signal of the sv type the radiation pattern of the sv signature is lobate in the polar plane and peaks at an incident angle of 30 to 35 degrees in the horizontal plane the sv radiation varies as the sine of the azimuthal angle measured with respect to the in line direction the field tests were conducted on a permian shale in oklahoma and an eocene shale in east texas comparison between sh and sv wave surfaces indicates that both shales behave approximately like transversely isotropic solids with a vertical axis of symmetry theoretical wave surfaces are fit to the observed data by using elastic moduli determined from the field measurements and by making the assumption that the near surface shale at each field site is a single homogeneous inherently transversely isotropic material the analyses indicate that horizontal sh group velocity is about 30 percent greater than horizontal sv group velocity in the permian shale and about 25 percent greater in the eocene shale

petrophysics studies of sedimentary rocks from a cross hole seismic test site in order to provide a site for studying in detail seismic wave propagation over a large range of frequencies through layered sedimentary rocks four vertical boreholes of approximately 260 m depth and maximum distance apart of 95 m have been drilled through five cyclical sequences of shallow dipping sandstone mudrock and limestone formations in the carboniferous limestone series in northern england two of the boreholes were fully cored and a number of core samples from these were carefully preserved at 1 m intervals in the first and at selected intervals in the second bulk density porosity and p and s wave velocities and attenuation in the vertical and horizontal directions have been made on a total of 180 specimens from the preserved core samples permeability and electrical properties have been measured for the sandstone samples multiple linear regression equations have been derived relating the p and s wave velocities to porosity and clay content these equations indicate the relative importance of increases in porosity and in clay content on decreases in all velocities and the relative insensitivity to changes in clay content of ph and sh wave velocities for the samples tested the average anisotropy observed under in situ stress conditions is approximately 0 5 for limestones porosities to 12 and 5 for sandstones porosities in the range 2 to 18 for mudrocks porosities in the range 2 to 15 the average anisotropy is 21 for p waves and 27 for s waves the results have been used to confirm the magnitudes of anisotropy observed in cross hole seismic tomography at the site in modelling the seismic response of sedimentary rocks with an appreciable clay content and in modelling velocity dispersion and attenuation for rocks at the site over a wide range of frequencies

morin transition in hematite size dependence and thermal hysteresis 1 hematite is a frequently used mineral in paleomagnetic and environmental magnetic studies just below room temperature it undergoes a magnetic phase transition the morin transition whose nature is an important part of our basic understanding of hematite s magnetism and magnetic memory we have determined the temperature tm of the morin transition from saturation remanence warming curves to be 250 261 k for 0 5 6 mm hematite natural single crystals 257 260 k for 45 600 m sieved crystal fractions and 241 256 k for submicron synthetic hematites with grain sizes between 120 and 520 nm the variation must be due to differences in crystal morphology lattice strain and crystal defects common in both synthetic and natural crystals our results are compatible with published data for 100 nm to 10 mm hematites and show that tm is nearly size independent decreasing very gradually as particle size decreases over this broad range which includes both multidomain md and single domain sd structures however tm decreases sharply between 90 and 30 nm below 20 nm the transition disappears entirely as near surface spins deviate strongly from the antiferromagnetic easy axis our sd and md hematites exhibit a thermal hysteresis in the morin transition the values of tm in cooling and in heating are different for the same cooling warming rate tm is much greater for submicron hematites than for larger crystals we attribute the lag in the transition in both cooling and heating to crystal imperfections and resulting internal stresses and speculate that defects may serve to pin and stabilize surface spins preventing spin rotation in a region large enough to trigger the phase transition would inhibit destabilization of the weakly ferromagnetic phase in cooling and the antiferromagnetic phase in heating the wide distribution of particle sizes in our submicron samples may also play a role

seismic source theory in stratified anisotropic media we extend seismic source theory to general anisotropic media for the numerical evaluation of spectral amplitudes of point sources in multilayered anisotropic crustal models first we obtain an explicit representation of the spectral elastodynamic green s tensor in general homogeneous anisotropic media as a sum of three integrals over the corresponding three slowness surfaces the multidimensional stationary phase principle is then applied to derive an asymptotic approximation at the far field in the case of azimuthally isotropic media we offer an alternative representation of the green s tensor and its ensuing displacements fields in the form of an exact hankel transform over the horizontal wave number variable the total field is specified here in terms of two potentials an sh potential and a mixed quasi transverse quasi longitudinal potential both of which assume the role of two scalar green s functions the availability of the green s tensor in analytical form enables one to obtain readily numerical solutions for a wide selection of media and sources it is shown that the radiation field of an explosion has the following new features 1 quasi transverse waves are created with four and eight lobe patterns 2 quasi longitudinal waves are generated for the collatitudinal displacement with four lobe patterns 3 the energy ratio sv p may reach the value of 20 for more than 50 of the azimuths in crustal structures such as tuff and shales and 4 radiation patterns for vertical shear waves are created which are indistinguishable from corresponding waves produced by earthquake faults our formalism allows us to establish a haskell type matrix algorithm for a multilayered azimuthally isotropic half space which enables us to calculate body waves and surface waves in real earth crustal models

3d modeling of fracture induced shear wave splitting in the southern california basin splitting of shear waves generated by local earthquakes in the southern california basin and their polarization changes during propagation through layers with different orientations of anisotropy are investigated from multicomponent seismograms and their hodograms synthetic data for earthquakes in the southern california basin illustrate the effects on polarization caused by dry vertical microcracks with a crack density of 0 04 at the source and receiver locations hodograms for observed and synthetic data show that the polarization at the receiver depends only on the orientation of the anisotropy within two wavelengths of the receiver which qualitatively explains the frequency dependence of anisotropy in previous studies seismograms are simulated for a model with a range of station azimuths and distances if the anisotropy producing crack orientations do not change between the source and a receiver and the source motion is parallel to the cracks there is no shear wave splitting splitting occurs when the anisotropy orientation at the receiver or between the source and receiver is different from the source orientation

magnetotactic bacterial abundance in pelagic marine environments is limited by organic carbon flux and availability of dissolved iron article i nfo article history magnetotactic bacteria intracellularly biomineralize magnetite of an ideal grain size for recording palaeomagnetic signals however bacterial magnetite has only been reported in a few pre quaternary records because progressive burial into anoxic diagenetic environments causes its dissolution deep sea carbonate sequences provide optimal environments for preserving bacterial magnetite due to low rates of organic carbon burial and expanded pore water redox zonations such sequences often do not become anoxic for tens to hundreds of metres below the seafloor nevertheless the biogeochemical factors that control magnetotactic bacterial populations in such settings are not well known we document the preservation of bacterial magnetite which dominates the palaeomagnetic signal throughout eocene pelagic carbonates from the southern kerguelen plateau southern ocean we provide evidence that iron fertilization associated with increased aeolian dust flux resulted in surface water eutrophication in the late eocene that controlled bacterial magnetite abundance via export of organic carbon to the seafloor increased flux of aeolian iron bearing phases also delivered iron to the seafloor some of which became bioavailable through iron reduction our results suggest that magnetotactic bacterial populations in pelagic settings depend crucially on particulate iron and organic carbon delivery to the seafloor

synthetic seismogram modeling of shear wave splitting in vsp data from the geysers california a vertical seismic profile data set showing shear wave splitting from the geysers california can be approximately fit with synthetic seismograms calculated with geometrical ray theory for anisotropic models of aligned cracks in the uppermost crust limited agreement between data and synthetics can be obtained if the cracks are constrained to be vertical but a much better fit is possible if the cracks dip at an angle of 55 degrees