invariant and changing transitional field configurations in a sequence of geomagnetic reversals in the past few years the increasing number of detailed records of geomagnetic transitions has provided new and important information about the behaviour of the earth s magnetic field during a reversal surprisingly however no attempt has been made to obtain information about the evolution of the transitional field characteristics over a long period of time we report here the results of a palaeomagnetic study of four different geomagnetic reversals three of which are successive recorded at different stratigraphical levels of two upper tortonian marine clay sections in western crete greece this sequence represents 1 5 myr of geomagnetic field history the results obtained suggest that the process of reversal from either starting polarity may remain invariant for a long period of time then change rather abruptly between successive reversals

kinematics of shot geophone migration recent analysis and synthetic examples have shown that many prestack depth migration methods produce nonflat image gathers containing spurious events even when provided with a kinematically correct migration velocity field if this velocityfieldishighlyrefractive thispathologyoccursinall migration methods that produce partial images as independent migrations of data bins shot geophone prestack depth migration is an exception to this pattern each point in the prestack image volume depends explicitly on all traces within the migration aperture using a ray theoretical analysis we have found that shot geophone migration produces focusedsubsurface offsetdomainorflatscattering angledomain image gathers provided there is a curvilinear coordinate system defining pseudodepth with respect to which the rays carrying significant energy do not turn and that the acquisitioncoverageissufficienttodetermineallsuchrays although the analysis is theoretical and idealized a synthetic example suggests that its implications remain valid for practical implementations and that shot geophone prestack depth migration could be a particularly appropriate tool for velocityanalysisinacomplexstructure

non dipole and regional effects on the geomagnetic dipole moment estimation the study of the temporal evolution of the dipole moment variations is a forefront research topic in earth sciences it constrains geodynamo simulations and is used to correct cosmogenic isotope production which is evidence of past solar activity and it is used to study possible correlations between the geomagnetic field and the climate in this work we have analysed the main error sources in the geomagnetic dipole moment computation from palaeomagnetic data the influence of the non dipole terms in the average approach the inhomogeneous distribution of the current palaeomagnetic database and the averaging procedure used to obtain the evolution of the dipole moment to evaluate and quantify these effects we have used synthetic data from a global model based on instrumental and satellite data the international geomagnetic reference field 11th generation results indicate that the non dipole terms contribute on a global scale of 6 in the averaged dipole moment whereas the regional non dipole contribution can show deviations of up to 35 in some regions such as oceania and different temporal trends with respect to the global dipole moment evolution in other ones such as europe and asia a regional weighting scheme seems the best option to mitigate these effects in the dipole moment average approach but when directional and intensity palaeomagnetic information is available on a global scale and in spite of the inhomogeneity of the database global modelling presents more reliable values of the geomagnetic dipole moment

new geomagnetic paleosecular variation results from the old crow basin yukon territory and their use in stratigraphic correlation a total of 626 fully orientated samples were collected from two laterally equivalent vertical sections each approximately 20 m thick in pleistocene sediments exposed in riverbank outcrops on the old crow river yukon territory the paleomagnetic records recovered reveal a sequence of three features correlatable with results obtained from previously sampled sections some 5 km upstream this correlation which is confirmed by magnetic susceptibility measurements allows the stratigraphic position of the old crow tephra to be inferred even though it does not actually occur at this locality it also provides strong support for the reality of these paleosecular variation signals the most prominent of these take the form of repeating linear perturbations which are here interpreted as evidence of stationary sources of reversely directed flux in the outer core such reverse flux patches exist elsewhere in the modern field but a simple magnetostatic model suggests that the paleosources were apparently an ord

complex sedimentation of the holocene mud deposits in a ria type coastal area eastern korea strait abstract the innermost shelf of the eastern korea strait is a ria type coastal sea comprising islands intervening passageways and embayments a detailed analysis of high resolution 1 10 khz subbottom profiles and core sediments from this area reveals complicated depositional and distributional patterns of the holocene mud deposits related to complex topography with varying supply of the adjacent nakdong riverine sediments sediments from the nakdong river were bifurcated around gadeok island forming two proximal systems nakdong and western gadeok systems these proximal systems prograded offshore southward by active sediment supply from the nakdong river in the early stage suspended sediments passing through the nakdong system formed the distal gadeok waterway and eastern geoje systems in the area between the northern geoje and gadeok islands these distal systems show a northwestward onshore prograding tendency to jinhae bay the biggest bay in the vicinity of the nakdong estuary in which the jinhae bay system developed in the late stage a remarkable decrease of sediment supply from the nakdong river has caused retrograding geometry of the two proximal systems however the most distal jinhae bay system has continuously prograded bayward due to the persistent supply of sediments resuspended by strong tidal currents from nearby distal gadeok waterway and eastern geoje systems these complex depositional features indicate that topography has an important influence on depositional developments of the holocene mud deposits by controlling path and intensity of sediment dispersal and resuspension processes

is earth s magnetic field reversing abstract earth s dipole field has been diminishing in strength since the first systematic observations of field intensity were made in the mid nineteenth century this has led to speculation that the geomagnetic field might now be in the early stages of a reversal in the longer term context of paleomagnetic observations it is found that for the current reversal rate and expected statistical variability in polarity interval length an interval as long as the ongoing 0 78 myr brunhes polarity interval is to be expected with a probability of less than 0 15 and the preferred probability estimates range from 0 06 to 0 08 these rather low odds might be used to infer that the next reversal is overdue but the assessment is limited by the statistical treatment of reversals as point processes recent paleofield observations combined with insights derived from field modeling and numerical geodynamo simulations suggest that a reversal is not imminent the current value of the dipole moment remains high compared with the average throughout the ongoing 0 78 myr brunhes polarity interval the present rate of change in earth s dipole strength is not anomalous compared with rates of change for the past 7 kyr furthermore there is evidence that the field has been stronger on average during the brunhes than for the past 160 ma and that high average field values are associated with longer polarity chrons there is no evidence from recent millennial scale time varying paleofield models to indicate that the field is entering a polarity transition nevertheless it remains a reasonable supposition that the magnetic field will eventually reverse even though the time scale is unpredictable a more immediate concern is that ongoing secular variation in the magnetic field may be expected to moderate the current high dipole strength on centennial to millennial time scales it would not be surprising if it dropped substantially returning closer to the average without necessarily reversing this could have important consequences for space weather and also highlights the need for improved understanding of the impact of geomagnetic field strength on the production rates of cosmogenic isotopes that are used to estimate past solar variability

geographical distribution of transitional vgps evidence for non zonal equatorial symmetry during the matuyama brunhes geomagnetic reversal abstract paleomagnetic records of the matuyama brunhes polarity reversal obtained recently from the northern equatorial and southern latitudes of the atlantic sector display a distinct symmetry with respect to the equator the virtual geomagnetic pole vgp paths from the mid latitude sites in the northern and southern hemispheres dsdp site 609 and core v16 58 are nearly coincident tracking northward through the americas the vgp path from the equatorial site odp site 664 tracks northward through eastern asia nearly antipodal to the other two paths modeling results indicate that superimposing an h 1 3 geometry onto a reversing axial dipole explains the sense of equatorial symmetry exhibited by these three records these results are consistent with the bulk of the data available from other records of this polarity transition suggesting that the transitional vgps during this reversal tend to fall along two distinct longitudinal bands the grouping of transitional vgps suggests that there is a strong geographical control over the reversal process an apparent correlation between the longitudinal bands of transitional vgps and the location of the major flux concentrations in the radial component of the historical geomagnetic field at the core mantle boundary 1 suggests that the processes which cause these flux concentrations may have influenced the geometry of the matuyama brunhes transitional field

longitudinally confined geomagnetic reversal paths from non dipolar transition fields it has long been thought that conditions at the boundary between the core and mantle influence the earth s magnetic field but the supporting evidence is rather indirect1 3 recent palaeomagnetic results suggesting that there are persistent preferred longitudinal paths for the virtual geomagnetic pole vgp during reversals4 would provide the first direct evidence of the solid mantle s influence on the core although their statistical significance has been disputed5 6 the results are potentially exciting because the preferred paths lie close to the pacific rim where the present geomagnetic secular variation changes character2 7 here we present a simple model based on an extension of a previous theory8 that produces reversals with vgp paths confined within relatively narrow longitude bands despite the transition field having a substantially non dipolar structure thus although longitude bias of the vgp paths is definitive evidence for core mantle interaction simple vgp paths are not evidence of near dipolar transition fields

evidence for a 20 tilting of the earth s rotation axis 110 million years ago true polar wander tpw the shift of the earth s rotation axis with respect to the entire globe is most probably due to mass redistribution in the earth s mantle as a result of convection using a new rigorously selected palaeomagnetic database gathering only directions obtained from magmatic rocks we find that tpw has been clearly intermittent over the last 200 ma with two long periods of strict standstill from the present to 80 ma and from approximately 150 to 200 ma a single period of shifting is observed between 80 and about 150 ma ago this period culminates around 110 ma ago in an 20 abrupt tilting during which an angular speed exceeding 5 ma 0 5m yr may have been reached assuming that the time averaged geomagnetic field is axial our results indicate that the changes in the position of the rotation axis and therefore in the inertia tensor of the earth are intermittent we suggest that a major reorganization of the mass distribution in the earth s mantle occurred in the lower cretaceous this event concomitant with plume hyperactivity at the earth s surface and probable drastic changes at the core mantle boundary attested by the inhibition of geomagnetic reversals suggests unmixing of upper and lower mantle by avalanching of upper mantle material down to the core mantle boundary the astonishingly strict stability of the time averaged position of the rotation axis before and after this episode of shifting implies the existence of some steady convection which does not modify the large scale distribution of mass within the mantle given the intermittence of mantle avalanching we suggest that these long periods of stability correspond to the temporary reestablishment of a basically two layered convection system within the mantle

wave equation migration velocity analysis i theory we present a migration velocity analysis mva method based on wavefield extrapolation similarly to conventional mva our method aims at iteratively improving the quality of the migrated image as measured by the flatness of angle domain common image gathers adcigs over the aperture angle axis however instead of inverting the depth errors measured in adcigs using ray based tomography we invert image perturbations using a linearized wave equation operator this operator relates perturbations of the migrated image to perturbations of the migration velocity we use prestack stolt residual migration to define the image perturbations that maximize the focusing and flatness of adcigs our linearized operator relates slowness perturbations to image perturbations based on a truncation of the born scattering series to the first order term to avoid divergence of the inversion procedure when the velocity perturbations are too large for born linearization of the wave equation we do not invert directly the image perturbations obtained by residual migration but a linearized version of the image perturbations the linearized image perturbations are computed by a linearized prestack residual migration operator applied to the background image we use numerical examples to illustrate how the backprojection of the linearized image perturbations i e the gradient of our objective function is well behaved even in cases when backprojection of the original image perturbations would mislead the inversion and take it in the wrong direction we demonstrate with simple synthetic examples that our method converges even when the initial velocity model is far from correct in a companion paper we illustrate the full potential of our method for estimating velocity anomalies under complex salt bodies

earth s magnetic field is probably not reversing the geomagnetic field has been decaying at a rate of 5 per century from at least 1840 with indirect observations suggesting a decay since 1600 or even earlier this has led to the assertion that the geomagnetic field may be undergoing a reversal or an excursion we have derived a model of the geomagnetic field spanning 30 50 ka constructed to study the behavior of the two most recent excursions the laschamp and mono lake centered at 41 and 34 ka respectively here we show that neither excursion demonstrates field evolution similar to current changes in the geomagnetic field at earlier times centered at 49 and 46 ka the field is comparable to today s field with an intensity structure similar to today s south atlantic anomaly saa however neither of these saa like fields develop into an excursion or reversal this suggests that the current weakened field will also recover without an extreme event such as an excursion or reversal the saa like field structure at 46 ka appears to be coeval with published increases in geomagnetically modulated beryllium and chlorine nuclide production despite the global dipole field not weakening significantly in our model during this time this agreement suggests a greater complexity in the relationship between cosmogenic nuclide production and the geomagnetic field than is commonly assumed

lower mantle convection and geomagnetism the statistical properties of recent marine magnetic anomaly time scales covering the period from 165 ma to the present are analyzed via the methods presented in the companion paper mcfadden this issue the data no longer support the hypothesis that during the last 86 ma there were two periods separated by a discontinuity at about 45 ma when the properties were stationary instead there is strong evidence that the rate of reversals decreased in an approximately linear manner with time from 165 ma until it reached zero resulting in the cretaceous normal polarity interval at about 86 ma ago the process of reversals began again and the rate of reversals increased linearly with time there is a slight indication that the rate of reversals reached a maximum about 10 ma ago and is again decreasing analysis of the reversal sequences shows there is no good evidence for the concept of differences in the relative stabilities of the two polarity states nor need such a concept be invoked to explain long intervals without reversals such as the cretaceous normal polarity interval quite clearly there has been some form of thermal perturbation at the base of the mantle with a characteristic time of the order of 108 years our interpretations require convection deep within the mantle and suggest that the temperature at the base of the mantle attains a maximum during long magnetic quiet intervals

the implications of non suppressed geomagnetic secular variation during the permo carboniferous reversed superchron abstract we have investigated variations in the natural remanent magnetisation nrm in red beds which were deposited during the permo carboniferous reversed superchron pcrs near the paleo equator these red beds are reliable recorders of the geomagnetic field because the nrm is carried by detrital hematite we analysed the angular standard deviation asd and spectral content of the directional records to quantify the amount of paleosecular variation psv during the time of deposition the asd of a high resolution directional record of the dome de barrot which spans approximately 27 kyr is 10 this is comparable to recent 0 5 ma low latitude lavas from the psvrl database mcelhinny and mcfadden 1997 and to recent lake sediments from the secvr database mcelhinny and lock 1996 the frequency spectra of declination and inclination are also similar to those from the recent lake sediments spectral power significant at the 95 level is concentrated in periods of 1 5 kyr higher frequencies are not significant from the asd and spectral analyses it appears that the amount of psv during at least part of the pcrs was comparable to that of recent times several heuristic and numerical geodynamo models are discussed in the context of superchrons and psv there is a discrepancy between the amount of sv shown by our data and that suggested by numerical models which use lateral heat flux variations at the core mantle boundary cmb however computational limitations still force some model parameters to have values that are unrealistic for the earth for the phenomenological geodynamos low energy state models which suggest that psv is suppressed during a superchron are not supported by the dome de barrot data rather a high energy state of the geodynamo in which outer core convection is vigorous is favoured as a prerequisite for a superchron to occur

migration velocity analysis from locally coherent events in 2 d laterally heterogeneous media part i theoretical aspects we present a new method based on migration velocity analysis mva to estimate 2 d velocity models from seismic reflection data with no assumption on reflector geometry or the background velocity field classical approaches using picking on common image gathers cigs must consider continuous events over the whole panel this interpretive step may be difficult particularly for applications on real data sets we propose to overcome the limiting factor by considering locally coherent events a locally coherent event can be defined whenever the imaged reflectivity locally shows lateral coherency at some location in the image cube in the prestack depth migrated volume obtained for an a priori velocity model locally coherent events are picked automatically without interpretation and are characterized by their positions and slopes tangent to the event even a single locally coherent event has information on the unknown velocity model carried by the value of the slope measured in the cig the velocity is e

angle dependent reflectivity by means of prestack migration most present day seismic migration schemes determine only the zero offset reflection coefficient for each grid point depth point in the subsurface in matrix notation the zero offset reflection coefficient is found on the diagonal of a reflectivity matrix operator that transforms the illuminating source wave field into a reflected wave field however angle dependent reflectivity information is contained in the full reflectivity matrix our objective is to obtain angle dependent reflection coefficients from seismic data by means of prestack migration multisource multioffset after downward extrapolation of source and reflected wave fields to one depth level the rows of the reflectivity matrix representing angle dependent reflectivity information for each grid point at that depth level are recovered by deconvolving the reflected wave fields with the related source wave fields this process is carried out in the space frequency domain in order to preserve the angle dependent reflectivity in the im

high resolution record of the upper olduvai transition from po valley italy sediments support for dipolar transition geometry abstract a detailed record of the upper olduvai polarity transition composed of 100 transitional directions has been obtained from the crostolo section in northern italy a careful examination of the rock magnetic properties of the sediments using standard paleomagnetic techniques thermomagnetic scanning electron microscopy microprobe x ray diffraction and mossbauer analysis shows that greigite is the main magnetic carrier of the remanence the correlation of a shift in the magnetic record to a small scale sedimentary feature observed in the section indicates that the magnetization was acquired at deposition or very shortly after the virtual geomagnetic pole vgp path is largely confined along a great circle over north and south america 90 west of the site and consists of three stages first the vgp moves to southern mid latitudes then after a period of standstill it comes back to almost true north and finally the south pole is reached in a third step although different from a record from the southern indian ocean this path is virtually identical to those obtained for the same transition from north pacific deep sea cores and partly coincides with a north atlantic record which suggests that a dipolar component may be present in the transitional field during the upper olduvai reversal a review of the recently obtained records of various transitions shows that in more than two thirds of the cases the vgp paths are similarly confined along a meridian over the americas or antipodal to them irrespective of the sampling site and of the sense of the transition although not deterministic this tendency suggests that a similar dipolar component might be present in the transitional fields of other reversals of different ages

toroidal flow in the outer core and the thermal regime below the core mantle boundary numerical calculations of fluid dynamos are used to constrain the probable origin of toroidal flows in the outer core azimuthal drift of the core fluid relative to the mantle is the sum of three components 1 uniform differential rotation of the core relative to the mantle produced by electromagnetic torques on the mantle 2 cylindrical flow driven by electromagnetic torques in the core and 3 buoyantly driven thermal wind flow produced by lateral variations of temperature and composition dynamo calculations indicate that electromagnetic torques on the mantle contribute to long term drift but are not likely to be its primary cause zonal flow driven by electromagnetic torques in the core and thermal wind flows driven by zonal temperature gradients are both sufficiently energetic to account for azimuthal flows with angular velocities of 0 1 0 5 yr 1 as inferred from frozen flux analyses of secular variation

geomagnetic reversal frequency since the late cretaceous models of geomagnetic reversals as a stochastic or gamma renewal process have generally been tested for the heirtzler et al 1 magnetic polarity time scale which has subsequently been superseded examination of newer time scales shows that the mean reversal frequency is dominated in the cenozoic and late cretaceous by a linearly increasing trend on which a rhythmic fluctuation is superposed subdivision into two periods of stationary behavior is no longer warranted the distribution of polarity intervals is visibly not poissonian but lacks short intervals the labrecque et al 2 polarity time scale shows the positions of 57 small wavelength marine magnetic anomalies which may represent short polarity chrons after adding these short events the distribution of all polarity intervals in the age range 0 40 myr is stationary and does not differ significantly from a poisson distribution a strong asymmetry develops in which normal polarity chrons are poisson distributed but reversed polarity chrons are gamma distributed with indexk 2 this asymmetry is of opposite sense to previous suggestions and results from the unequal distribution of the short polarity chrons which are predominantly of positive polarity and concentrated in the late cenozoic if short wavelength anomalies arise from polarity chrons the geomagnetic field may be more stable in one polarity than the other alternative explanations of the origin of short wavelength marine magnetic anomalies cast doubt on the inclusion of them as polarity chrons however the observed behavior of reversal frequency suggests that core processes governing geomagnetic reversals possess a long term memory

enabling affordable omnidirectional subsurface extended image volumes via probing image gathers as a function of subsurface offset are an important tool for the inference of rock properties and velocity analysis in areas of complex geology traditionally these gathers are thought of as multidimensional correlations of the source and receiver wavefields the bottleneck in computing these gathers lies in the fact that one needs to store compute and correlate these wavefields for all shots in order to obtain the desired image gathers therefore the image gathers are typically only computed for a limited number of subsurface points and for a limited range of subsurface offsets which may cause problems in complex geological areas with large geologic dips we overcome increasing computational and storage costs of extended image volumes by introducing a formulation that avoids explicit storage and removes the customary and expensive loop over shots found in conventional extended imaging as a result we end up with a matrix vector formulation from which different image gathers can be formed and with which amplitude versus angle and wave equation migration velocity analysis can be performed without requiring prior information on the geologic dips aside from demonstrating the formation of two way extended image gathers for different purposes and at greatly reduced costs we also present a new approach to conduct automatic wave equation based migration velocity analysis instead of focusing in particular offset directions and preselected subsets of subsurface points our method focuses every subsurface point for all subsurface offset directions using a randomized probing technique as a consequence we obtain good velocity models at low cost for complex models without the need to provide information on the geologic dips

reverse time migration migration of stacked or zero offset sections is based on deriving the wave amplitude in space from wave field observations at the surface conventionally this calculation has been carried out through a depth extrapolation we examine the alternative of carrying out the migration through a reverse time extrapolation this approach may offer improvements over existing migration methods especially in cases of steeply dipping structures with strong velocity contrasts this migration method is tested using appropriate synthetic data sets

image domain wavefield tomography with extended common image point gathers waveform inversion is a velocity model building technique based on full waveforms as the input and seismic wavefields as the information carrier conventional waveform inversion is implemented in the data domain however similar techniques referred to as image domain wavefield tomography can be formulated in the image domain and use a seismic image as the input and seismic wavefields as the information carrier the objective function for the image domain approach is designed to optimize the coherency of reflections in extended common image gathers the function applies a penalty operator to the gathers thus highlighting image inaccuracies arising from the velocity model error minimizing the objective function optimizes the model and improves the image quality the gradient of the objective function is computed using the adjoint state method in a way similar to that in the analogous data domain implementation we propose an image domain velocity model building method using extended common image point space and time lag gathers constructed sparsely at reflections in the image the gathers are effective in reconstructing the velocity model in complex geologic environments and can be used as an economical replacement for conventional common image gathers in wave equation tomography a test on the marmousi model illustrates successful updating of the velocity model using commonimage point gathers and resulting improved image quality

mechanism for geomagnetic polarity reversals recent studies of old magnetic observations1 and the seismic velocity of the lower mantle2 suggest that at least part of the geomagnetic secular variation is driven by temperature anomalies in the solid mantle3 in particular a patch of flux of opposite sign to that expected for a dipole field occurs beneath southern africa it has intensified throughout the twentieth century by a process that is believed to be flux expulsion associated with a particularly hot part of the lowermost mantle and is drifting westwards a similar patch lies beneath south america here i show that the present fall in the dipole moment is directly related to the intensification and southward movement of these patches and suggest the fall occasionally leads to polarity reversal the almost linear increase in the frequency of polarity reversals that has occurred since the cretaceous quiet interval is attributed to the growth of the hot patch of mantle presently under southern africa

geomagnetic polarity reversals a connection with secular variation and core mantle interaction geomagnetic polarity reversal remains one of nature s most enigmatic phenomena dynamo theory admits solutions in pairs with reversed magnetic fields b and b but detailed calculations are required to understand how the field can change sign theory also admits separate solutions with different symmetry across the equatorial plane the symmetric es quadrupole and the antisymmetric ea dipole solutions which may be important in the reversal process and which offer a simple framework for interpreting small paleomagnetic data sets ordinary secular variation leads to very large changes in the magnetic field over several centuries and could easily develop into full reversal the theory of secular variation which is relatively well developed may therefore help in understanding reversals other clues to geomagnetic reversals come from the sun whose magnetic field reverses every 11 years paleomagnetic data show the earth s magnetic field reverses every million years or so with each transition taking about a thousand years during which the intensity may fall by as much as 1 order of magnitude reversal frequency undergoes a modulation on the long timescale 107 years of mantle convection and there have been two long intervals in the past with no reversals such behavior is typical of a highly nonlinear dynamical system but the very long timescale of changes in reversal frequency and its close proximity to the overturn time of mantle convection suggests some control of the dynamo by the mantle short term phenomena such as change in the length of day and secular variation have been studied extensively for evidence of core mantle interactions and we may draw on this body of evidence in order to understand long term effects three physical mechanisms have been proposed topographic electromagnetic and thermal with the last two being most significant for long term effects symmetries allow the dynamo to generate an ea field with the major component a dipole but lateral variations on the core mantle boundary may lead to magnetic fields with no symmetry reflecting the structure of the boundary anomalies changes in reversal frequency on the mantle convection timescale could arise either from changes in total heat flux from the core to the mantle or from instabilities associated with lateral variations at the core mantle boundary neither mechanism is well understood but the former involves significant changes to the earth s overall heat budget whereas the latter must always arise as a natural consequence of deep mantle convection recent measurements of transition fields show pole paths that lie close to two preferred longitudes near 90 w and 90 e if substantiated the result would provide the first definitive evidence of long term mantle control of the geomagnetic field further evidence suggests that the geomagnetic pole during stable polarity also lies along these two longitudes and that magnetic flux at the core surface tends to concentrate along the same longitudes as does the present field a simple theory is proposed relating changes in the core field to apparent transition paths measured at the earth s surface the model shows that longitude confinement of the transition paths can occur for quite complicated core fields and that surface intensities can drop by 1 order of magnitude on average simply because of the reduction in length scale of the transitional field simple transition paths may be an indication of some organization of flux at the core surface but not of large scale or small amplitude core fields

offset and angle domain common image point gathers for shot profile migration prestack depth migration of shot profiles by downward continuation is a practical imaging algorithm that is especially cost effective for sparse shot wide azimuth geometries the interpretation of offset as the displacement between the downward propagating shot wavefield and upward propagating receiver wavefield enables us to extract offset domain common image point cip gathers during shot profile migration the offset domain gathers can then be transformed to the angle domain with a radial trace mapping originally introduced for shot geophone migration the computational implications of this procedure include both the additional cost of multioffset imaging and an implicit transformation from shot geophone to midpoint offset coordinates although this algorithm provides a mechanism for imaging angle dependent reflectivity via shot profile migration for sparse shot geometries the fundamental problem of shot aliasing may severely impact the quality of cip gathers

on the analysis of paleomagnetic secular variation modern methods of geomagnetic field analysis can be applied to paleomagnetic data provided account is taken of the relatively sparse and inaccurate nature of paleomagnetic results paleomagnetism tends to produce data sets dominated by inclination measurements which do not provide a unique field model as an example two magnetic fields are found with closely similar inclinations but completely different declinations and strengths directional measurements are difficult to deal with because they are nonlinearly related to the field model the equations are simplified by linearizing the field about an axial dipole the approximation is adequate for paleomagnetic data and can be used to show that the vgp scatter function is not an appropriate estimate of the non axial dipole field even when averaged over longitude four assertions made in the paleomagnetic literature are shown to be correct within this linearization but are not true in general in particular equatorial symmetry of the vgp scatter functions for the separate symmetric quadrupole field holds within the approximation the virtual geomagnetic pole vgp scatter functions for the modern geomagnetic field has strong north south asymmetry but historical field models show it to be a relatively recent feature the vgp scatter function for the symmetric part of the field sq is dominated by the equatorial dipole and is approximately equal to the angular distance between the geomagnetic and geographic poles that for the antisymmetric part of the field sd is dominated by geomagnetic coefficients of degree 2 and order 1 surprisingly the axisymmetric quadrupole and octupole terms contribute very little highlighting the dangers of modeling the paleomagnetic field without regard to departures from axial symmetry

angle domain common image gathers for migration velocity analysis by wavefield continuation imaging we analyze the kinematic properties of offset domain common image gathers cigs and angle domain cigs adcigs computed by wavefield continuation migration our results are valid regardless of whether the cigs were obtained by using the correct migration velocity they thus can be used as a theoretical basis for developing migration velocity analysis mva methods that exploit the velocity information contained in adcigs we demonstrate that in an adcig cube the image point lies on the normal to the apparent reflector dip that passes through the point where the source ray intersects the receiver ray the image point position on the normal depends on the velocity error when the velocity is correct the image point coincides with the point where the source ray intersects the receiver ray starting from this geometric result we derive an analytical expression for the expected movements of the image points in adcigs as functions of the traveltime perturbation caused by velocity errors by applying this ana

full azimuth subsurface angle domain wavefield decomposition and imaging part i directional and reflection image gathers we present a new subsurface angle domain seismic imaging systemforgeneratingandextractinghigh resolutioninformation about subsurface angle dependent reflectivity the system enables geophysicists to use all recorded seismic data in a continuousfashiondirectlyinthesubsurfacelocalangledomainlad resulting in two complementary full azimuth common imageangle gather systems directional and reflection the complete setofinformationfrombothtypesofanglegathersleadstoaccurate high resolution reliable velocity model determination and reservoir characterization the directional angle decomposition enables the implementation of specular and diffraction imaging in real 3d isotropic anisotropic geological models leading to simultaneous emphasis on continuous structural surfaces and discontinuous objects such as faults and small scale fractures structural attributes at each subsurface point e g dip azimuth andcontinuity canbederiveddirectlyfromthedirectionalangle gathers the reflection angle gathers display reflectivity as a function of the opening angle and opening azimuth these gathers are most meaningful in the vicinity of actual local reflecting surfaces wherethereflectionanglesaremeasuredwithrespectto the derived background specular direction the reflection angle gathers are used for automatic picking of full azimuth angle domainresidualmoveoutsrmowhich togetherwiththederived background orientations of the subsurface reflection horizons provide a complete set of input data to isotropic anisotropic tomography the full azimuth angle dependent amplitude variations are used for reliable and accurate amplitude versus angle andazimuthavazanalysisandreservoircharacterization the proposed system is most effective for imaging and analysis below complex structures such as subsalt and subbasalt high velocity carbonate rocks shallow low velocity gas pockets and others in addition it enables accurate azimuthal anisotropic imaging and analysis providing optimal solutions for fracture detectionandreservoircharacterization

complex behavior of the geomagnetic field during three successive polarity reversals 11 12 m y b p detailed paleomagnetic records of both the directional and relative field intensity changes occurring during three sequential polarity reversals have been obtained from marine clays of serravallian tortonian boundary age in the island of zackinthos greece the three reversals are characterized by a complicated geometrical and temporal structure periods of rapid changes of the geomagnetic field vector alternate with periods of near stationarity different episodes of collapse and recovery of the field intensity and the presence of rebounds have been documented overall the records obtained here are reminiscent of volcanic records and appear much more complex and quite different from those previously obtained from upper tortonian marine clays in crete in the same geographical area the bearing of these results on some of the published reversal models is discussed and it is also suggested that the greater complexity of the zackinthos records might be related to the fact that they are exactly coeval with the last reversal spurt documented by reversal frequency analyses

dynamo waves and palaeomagnetic secular variation summary a dynamic model is proposed for large amplitude geomagnetic secular variation with periodicity in the range of 1000 3000yr thermal convection in the lower mantle imposes a spatially variable heat flux at the core mantle boundary if the total heat loss from the core exceeds approximately 3 x 10 2w then both i a convective layer and ii large scale quasigeostrophic shear flows are likely to exist beneath the core mantle boundary the combined effects of convection and shear at the top of the outer core can generate three dimensionally propagating magnetic field disturbances called cuw dynamo waves an analysis is made of the surface magnetic field disturbances due to dynamo wave propagation in the outer core in the presence of turbulent convection and an east west shear flow nonaxisymmetric disturbances exhibit the following characteristics i dynamo waves propagate in each hemisphere with a poleward or equatorward component depending on the sign of the a effect ii both eastward and westward propagation is allowed althcugh westward propagation is preferred the apparent velocity at the earth s surface is the sum of propagation and drift velocities in the core iii dynamo wave disturbances are transient with exponential growth and decay phases iv traces of inclination versus declination of individual fourier components obey runcorn s circularity rule a comparison is made with lake sediment secular variation records from central north america measured by lund banerjee the band passed time series and the major inclination declination loops can be modelled from 2600 to 10 000 yr bp by a single dynamo wave periodic in 2400 yr propagating under the site along an azimuth of 345 between 0 and 2600 yr bp the record can be modelled by addition of a wave with 1200yr period propagating along an azimuth of 30 this indicates that the palaeomagnetic secular variation record in north america to 10 000 yr may be dominated by poleward propagating wave like disturbances

automatic velocity analysis via shot profile migration we described an implementation of differential semblance velocity analysis based on shot profile migration and illustrated its ability to estimate complex strongly refracting velocity fields the differential semblance approach to velocity analysis uses waveform data directly it does not require any sort of traveltime picking the objective function minimized by the differential semblance algorithm can measure either focusing of the image in offset or flatness of the image in scattering angle we showed that the offset variant of differential semblance yields somewhat more reliable migration velocity estimates than does the scattering angle variant and we explain why this is so we observed that inconsistency with the underlying model born scattering about a transparent background can lead to degraded velocity estimates from differential semblance and we showed how to augment the objective function with stack power to enhance ultimate accuracy a 2d marine survey over a target obscured by the lensing effects of a gas chimney provides an opportunity for direct comparison of differential semblance with reflection tomography the differential semblance estimate yields a more data consistent model flatter angle gathers than does reflection tomography in this application resulting in a more interpretable image below the gas cloud

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

core mantle interactions abstract recent studies of the magnetic field at the core mantle boundary have revealed fixed sites of either static magnetic features or persistent secular variation this suggests part of the magnetic field behaviour is controlled by the mantle the most plausible mechanism for core mantle interaction is thermal coupling although topography may also be significant the magnetic sites coincide with anomalies in lower mantle seismic velocity as determined from tomography and density as determined by flow models of mantle convection constrained by tomography and the geoid some magnetic features coincide with subduction zones particularly those near indonesia they may be caused by bumps on the core mantle boundary beneath trenches paleomagnetic pole positions suggest the magnetic behaviour has persisted for at least 5 myr as would be expected if it were controlled from the mantle

mapping the lower mantle determination of lateral heterogeneity in p velocity up to degree and order 6 the data from the international seismological centre bulletins for the years 1964 1979 are used to derive a three dimensional model of lateral variations of the p velocity in the lower mantle unlike previous studies in which such perturbations were represented by a blocklike parameterization we seek the solution in the form r k 0k l 0l m 0l k r kalm cos m kblm sin m plm cos some 500 000 travel time residuals for teleseismic distances from 5000 earthquakes are used in an iterative procedure to derive the coefficients a and b the maximum k and l used are 4 and 6 respectively in each iteration in addition to solving for the three dimensional structure we also relocate all the earthquakes perturb the average travel time curve and determine station corrections for over 1000 stations particular attention is given to the problem of weighting the individual observations in order to avoid as much as possible the bias due to the uneven distribution of sources and receivers the resulting model shows a high level of perturbations just below the 670 km discontinuity and just above the core mantle boundary where the maximum perturbations reach 1 1 5 of the average velocity even for this highly smoothed model at a depth of 2000 km the rms perturbations are some 3 to 4 times lower than at either of the two boundaries the model predicts well the large scale pattern of observed travel time residuals for various source regions except for the distinct effects of the subduction zones the most striking large scale three dimensional feature of the model is a ring of high velocities circumscribing the pacific basin within a depth range from 1000 km to the core mantle boundary if the proportionality between perturbations in the velocity and temperature is assumed the derived pattern of anomalies could be consistent with a two boundary layer model of convection in the lower mantle some caution however should be exercised in interpreting the results near the 670 km discontinuity where the resolving power of our data set is limited by the lack of adequate control of the upper mantle structure at the same time a significant global increase in the level of perturbations in the lowermost mantle is established beyond question the results of dziewonski et al 1977 on the correlation between the equivalent geoid and the observed geoid for the angular order numbers 2 and 3 is confirmed but there is no significant correlation for degrees from 4 to 6 these calculations are made for the rigid earth richards and hager this issue show that introduction of finite viscosity can change the sign of a geoid anomaly in addition recent studies of lateral variations in the shear velocity reveal the presence of large scale anomalies in the upper mantle thus while the inferences made here with respect to the origin of the gravest orders of the geopotential field are not conclusive they point the way in which results from seismology can be used to address some of the basic questions of geodynamics

persistent high paleosecular variation activity in southern hemisphere for at least 10 000 years abstract direct observations of the geomagnetic field show that secular variation is strong in the atlantic hemisphere and comparatively reduced in the pacific region the dipole has been decaying since at least 1840 ad driven by growth and migration of reverse flux patches in the southern hemisphere we investigate whether anything like this modern pattern of geomagnetic secular variation persists and can be detected in global paleomagnetic field models synthesis of results from two new time varying spherical harmonic models shows that geographically distinct geomagnetic secular variation extends to at least 10 000 bp the models use the same database but differ in methodology leading to some regional differences in results consistent large scale surface features include strong average fields in the northern hemisphere and weaker fields with greater overall variability in the south longitudinal structure is present with weaker average fields in the western pacific than in the east and prominent negative inclination anomalies extending beneath indonesia across africa and to brazil but weaker anomalies in the central pacific marginally positive inclination anomalies occur west of the americas paleosecular variation activity peaks at high southern latitudes and there is a pattern of reduced activity at equatorial and mid latitudes beneath the pacific although the dipole has exhibited both growth and decay over the interval 0 10 000 bp our results show that geomagnetic paleosecular variation is preferentially focused in similar geographic regions to secular variation seen in the modern field

magnetic boundary layers in numerical dynamos with heterogeneous outer boundary heat flux abstract it has been proposed that magnetic flux expulsion due to outer core fluid upwellings may affect the geomagnetic secular variation on the core mantle boundary bloxham 1986 in this process intense horizontal field lines are concentrated below the outer boundary introducing small radial length scales and consequently strong radial diffusion here we explore such magnetic boundary layers in numerical dynamo simulations with heterogeneous outer boundary heat flux inferred from a tomographic model of lower mantle seismic shear waves velocity anomalies our scheme associates magnetic boundary layers to peak horizontal magnetic fields at the top of the shell in our models mean magnetic boundary layer thickness ranges 200 400 km and decreases with increasing magnetic reynolds number extrapolation or interpolation to earth s core conditions based on total core flow amplitude or its poloidal part gives mean magnetic boundary layer thickness of 220 and 260 330 km respectively we find magnetic boundary layers associated with the azimuthal field at the equatorial region whereas magnetic boundary layers associated with the meridional field are found at mid latitudes negative outer boundary heat flux anomalies yield preferred locations of expulsion of azimuthal field below africa and the pacific while positive outer boundary heat flux anomalies yield preferred locations of expulsion of meridional field below the americas and east asia furthermore we find a tendency of the azimuthal field to low latitudes of the northern hemisphere our results suggest that the local diffusion time is on the order of several kyr and the local magnetic reynolds number is on the order of 10 both much smaller than classical estimates

geomagnetic palaeointensities during the cretaceous normal superchron measured using submarine basaltic glass high quality palaeointensity data have been obtained from thellier thellier experiments on recent and cretaceous submarine basaltic glasses whereas the recent samples faithfully yield today s geomagnetic intensity at the site the palaeointensities for the beginning and end of the cretaceous normal superchron are only 45 and 25 respectively of today s value the data thus extend the mesozoic dipole low into the cretaceous superchron and confirm that submarine basaltic glass is an excellent material for palaeointensity studies

velocity inversion by differential semblance optimization differential semblance optimization dso is an approach to inversion of reflection seismograms which avoids the severe convergence difficulties associated with nonlinear least squares inversion the method exploits both moveout and amplitude characteristics of reflections we have implemented a version appropriate to plane wave p tau seismograms and layered constant density acoustic earth models theoretical and numerical analyses of this version of dso indicate that stable and reasonably accurate estimates of both velocity trend and reflectivity can be derived to test dso further we applied it to a marine data set from the gulf of mexico where the method produced results which compare favorably to well log information the method can be extended to incorporate laterally heterogeneous velocity models

thermal core mantle interactions maps of the magnetic field at the core mantle boundary for 1715 1980 reveal static features in the field and an absence of westward drift from much of the core mantle boundary the static features suggest that flow in the core is coupled to the mantle we examine different coupling mechanisms and predict lateral inhomogeneity at the core mantle boundary comparison with models of the lower mantle and core mantle boundary derived seismically and from the geoid is encouraging though their resolution is currently inadequate for a full comparison to be made with the much better resolved magnetic field

how persistent is the present trend of the geomagnetic field to decay and possibly to reverse abstract recent studies provide evidence for a possible imminent change of polarity or an excursion of the geomagnetic field in this paper we explore the possibility that the present trends are persistent looking at the behaviour of some physical quantities of the recent geomagnetic field with particular attention to the last century analysis of the mean square value of the field over the last 400 years shows a linear decay that if extrapolated will be zero in around 1000 years while if we extrapolate the field over antarctica it will go to zero in around 300 years the information content of the geomagnetic field has been decaying from around 1690 but began to decrease more rapidly at around 1775 and even more rapidly after 1900 an intermittent synchronicity between the exponential field decay and the increase latitudinal speed of the south geomagnetic pole with similar decaying or growing timescales can be interpreted as evidence for a present persistent turbulence of the geomagnetic field from this work it emerges that the present situation is likely to persist further into the future probably for another century but longer predictions are not possible

geomagnetic secular variation we analyse over 175000 magnetic observations from an interval spanning 1695 1980 to produce a sequence of maps of the magnetic field at the core mantle boundary we find that even the earlier data enable us to determine reliable maps we produce these maps at approximately 60 year intervals through the eighteenth and nineteenth centuries and at 10 year intervals in the twentieth century this span of maps is long enough to render straightforward the distinction between static and drifting features in the field we observe that some features show no sign whatsoever of drift over the entire 285 year time interval although others drift westwards in particular we observe that the secular variation is very low beneath the pacific ocean but beneath southern africa and the south atlantic ocean we observe rapid secular variation we interpret the morphology of the static field in terms of a simple model of the dynamo and conjecture that interactions between the core and the mantle are an important element of the process as part of the static field we identify four main concentrations of flux two in each hemisphere at high latitudes these features largely account for the earth s axial dipole moment we find unequivocal evidence that magnetic flux has not remained frozen over the time span of our models much of the diffusive behaviour that we identify is associated with the formation of a pair of flux spots a core spot beneath southern africa early in this century nevertheless we are able to construct maps that satisfy a set of necessary conditions for frozen flux and use these maps to construct maps of the core surface fluid flow based on the steady flow hypothesis although we find no strong evidence against the steady flow hypothesis we do find some grounds on which to doubt the validity of the flow maps

longitudinal confinement of geomagnetic reversal paths as a possible sedimentary artefact the positions of the virtual geomagnetic pole vgp during a large number of reversals of the earth s magnetic field seem to show a remarkable confinement to longitudes over the americas or to antipodal longitudes1 2 although it has been argued that this confinement is statistically insufficiently constrained3 it has also been pointed out4 that the same bands of longitude appear in other geophysical observations such as the pattern of fluid motion in the outer core and regions of higher seismic velocities in the lower mantle suggesting a causal relationship here we show that longitudinal confinement of vgps can arise from the smoothing of non antipodal stable directions just before and after a geomagnetic reversal because of the filtering effect of the remanence acquisition process in sediments the origin of this non antipodality is still uncertain and must remain speculative until more reversal records become available that include sufficiently long pre and post transitional intervals