seismic tomography a window into deep earth abstract the goal of this paper is to provide an overview of the current state of the art in seismic tomography and trace its origins from pioneering work in the early 1970s to its present status as the pre eminent tool for imaging the earth s interior at a variety of scales due to length limitations we cannot hope to cover every aspect of this diverse topic or include mathematical derivations of the underlying principles rather we will provide a largely descriptive coverage of the methodology that is targeted at readers not intimately familiar with the topic the relative merits of local versus global parameterization ray tracing versus wavefront tracking backprojection versus gradient based inversion and synthetic testing versus model covariance are explored a variety of key application areas are also discussed including body wave traveltime tomography surface wave tomography attenuation tomography and ambient noise tomography established and emerging trends many of which are driven by the ongoing rapid increases in available computing power will also be examined including finite frequency tomography full waveform tomography and joint tomography using multiple datasets several practical applications of seismic tomography including body wave traveltime attenuation and surface waveform are presented in order to reinforce prior discussion of theory

tomographic determination of velocity and depth in laterally varying media estimation of reflector depth and seismic velocity from seismic reflection data can be formulated as a general inverse problem the method used to solve this problem is similar to tomographic techniques in medical diagnosis and we refer to it as seismic reflection tomography seismic tomography is formulated as an iterative gauss newton algorithm that produces a velocity depth model which minimizes the difference between traveltimes generated by tracing rays through the model and traveltimes measured from the data the input to the process consists of traveltimes measured from selected events on unstacked seismic data and a first guess velocity depth model usually this first guess model has velocities which are laterally constant and is usually based on nearby well information and or an analysis of the stacked section the final model generated by the tomographic method yields traveltimes from ray tracing which differ from the measured values in recorded data by approximately 5 ms root mean square the i

efficient 3 d migration in two steps the addition of a third dimension to seismic data acquisition offers significant improvement of our reflection images of geologic structures but potentially at a substantial increase in data processing cost here the authors study an approach to migration of three dimensional 3 d stacked data that can considerably reduce the burden of computation and data manipulation cascading two dimensional 2 d migrations in orthogonal lateral directions the authors obtain an efficient 3 d scheme that is exact for homogeneous media as applied to examples of synthetic data from inhomogeneous media the scheme introduces errors well below those attributable in practice to uncertainties in migration velocity application of this two step approach using both kirchhoff summation and finite difference algorithms to a 3 d seismic land survey over an area of known hydrocarbon traps additionally proves the importance of 3 d migration in stratigraphic imaging

structural architecture of the northern composite terrane the eastern ghats mobile belt india implications for gondwana tectonics abstract new data from structural mapping and tectonic evaluation in the northern parts of the eastern ghats mobile belt egmb north involving the interpretation of satellite images field traverses critical outcrop mapping and kinematic studies of macro as well as microstructures of the shear zone rocks together with the geometry and disposition of gondwana basins led to for the first time the elucidation of post grenvillian structural architecture of the terrane this helps in assessing the sequence of successive tectonothermal events that were responsible for the origin and progressive evolution of the permo carboniferous coal bearing sediments along the mahanadi rift that forms significant in the reconstruction models of east gondwana the composite terrane of high grade metamorphic rocks egmb north strikes e w in contrast to the regional ne sw trend of the egmb the structural architecture obtained from this study is controlled by the boundary shear zones and associated link shear zones the dextral kinematic displacements along the northern boundary shear zone nbsz as well as the mahanadi shear zone msz and koraput sonapur rairakhol shear zone ksrsz were derived from multi scale field based structural observations a n s structural cross section presents a crustal scale flower structure across the composite terrane exposing different domains displaying distinctive internal structures with widely varying different geological evolution history and strain partitioning separated by crustal scale shear zones deep seismic imaging and gravity signatures support flower structure model the pervasive first formed gneissic fabrics were continuously reworked and partitioned into a series of e w crustal scale shear zones the neoproterozoic regional dextral transpressional tectonics along the shear zones and their repeated reactivation could be responsible for initiation and successive evolution of gondwana basins and different episodes of sedimentation available geochronological data shows that the structural architecture presented here is post grenvillian which has been repeatedly reactivated through long lived transpressional tectonics the composite terrane is characterized by all the typical features of an oblique convergent orogen with transpressional kinematics in the middle to lower crust the kinematic changes from transpression to transtensional stresses were found to be associated with global geodynamics related to the transformation from rodinia to gondwana configuration

postmap migration of crosswell reflection seismic data vertical seismic profiling common depth point vsp cdp mapping is often preferred to crosswell migration when imaging crosswell seismic reflection data the principal advantage of vsp cdp mapping is that it can be configured as a one to one operation between data in the acquisition domain and data in the image domain and therefore does not smear coherent noise such as tube waves guided waves and converted waves as crosswell migration could however unlike crosswell migration vsp cdp mapping cannot collapse diffractions therefore the lateral resolution of reflection events suffers we present a migration algorithm that is applied to the crosswell data after they have been mapped by performing crosswell migration in two distinct steps mapping followed by diffraction stacking noise events can be identified and filtered in the mapped domain without smearing effects commonly associated with conventional crosswell migration operators tests on noise free synthetic crosswell data indicate that the two ste

joint inversion of pp and ps reflection data for vti media a north sea case study despite the significant advantages of combining pp and ps reflection data in anisotropic parameter estimation application of this approach has been hindered by the inherent complexity of ps wave moveout to overcome this problem v grechka and i tsvankin suggested a model independent procedure to construct the traveltimes of pure ss wave reflections from pp and ps data here we apply their method and anisotropic multicomponent stacking velocity tomography to a 2 d line acquired over the lower tertiary siri reservoir in the north sea the computed traveltimes of ss reflections from several horizons are used to estimate the effective ss wave normal moveout nmo velocities which are combined with the nmo velocities of pp waves in the tomographic velocity analysis comparison of the vertical and nmo velocities of the pp and ss waves provides clear evidence of anisotropy in the section above the reservoir the interval parameter estimation is performed under the assumption that the section is composed of

microstructural evolution of the serido belt ne brazil the effect of two tectonic events on development of c axis preferred orientation in quartz the polyphase evolution of the serido belt ne brazil includes d crust formation at 2 3 2 1 ga d 2 thrust tectonics at 1 9 ga and crustal reworking by d 3 strike slip shear zones at 600 ma microstructural investigations within mylonites associated with d 2 and d 3 events were used to constrain the tectono thermal evolution of the belt d 2 shear zones commenced at deeper crustal levels and high amphibolite facies conditions 600 650 degreesc through grain boundary migration subgrain rotation and operation of quartz q prism slip continued shearing and exhumation of the terrain forced the re equilibration of high t fabrics and the switching of slip systems from c prism to positive and negative a rhombs during d 3 enhancement of ductility by dissipation of heat that came from syn d 3 granites developed wide belts of amphibolite facies mylonites continued shearing uplift and cooling of the region induced d 3 shear zones to act in ductile brittle regimes marked by fracturing and development of thinner belts of greenschist facies mylonites during this event switching from a prism to a basal slip indicates a thermal path from 600 to 350 degreesc therefore microstructures and quartz c axis fabrics in polydeformed rocks from the serido belt preserve the record of two major events which includes contrasting deformation mechanisms and thermal paths c 2003 elsevier ltd all rights reserved

true amplitude imaging and dip moveout true amplitude seismic imaging produces a three dimensional 3 d migrated section in which the peak amplitude of each migrated event is proportional to the reflectivity for a constant velocity medium the standard imaging sequence consisting of spherical divergence correction normal moveout nmo dip moveout dmo and zero offset migration produces a true amplitude image if the dmo step is done correctly there are two equivalent ways to derive the correct amplitude preserving dmo the first is to improve upon hale s derivation of f k dmo by taking the reflection point smear properly into account this yields a new jacobian that simply replaces the jacobian in hale s method the second way is to calibrate the filter that appears in integral dmo so as to preserve the amplitude of an arbitrary 3 d dipping reflector this latter method is based upon the 3 d acoustic wave equation with constant velocity the resulting filter amounts to a simple modification of existing integral algorithms the new f k an

aspects of seismic reflection prospecting for oil and gas summary seismic reflection survey is a technology which has made and is making rapid advances by means of continuous marginal improvement over each of its subdivisions of data acquisition signal enhancement and geological interpretation it is wedded to the digital computer and as long as the real cost of digital computers and their peripherals continues to fall so long at least will reflection seismology continue to advance for there are many algorithms waiting only for more at the right price computer power before they are implemented the essence of the technique is simple echo sounding combined with large data redundancy and fairly complex signal enhancement and imaging procedures on land the source is normally a few kilograms of high explosive and at sea it is usually an array of airguns which is a device for releasing into the water a few litres of air at high pressure particle velocity detectors are used on land and pressure detectors at sea their output is digitally recorded on magnetic tape with a total dynamic range of some 180 db though resolution is limited to 14 bits arrays of sources and detectors are used and the first 10 12 stages in the signal processing chain are devoted to producing a record as close as possible to the hypothetical record which would have been obtained if the source and detector had been coincident on a horizontal datum plane and if there had been no noise and no multiply reflected echoes once the best such zero offset record has been obtained an imaging algorithm based on the acoustic not elastic wave equation is used in order to bring into focus as sharply as possible the seismic image of the subsurface this is normally done for vertical slices through the earth but increasingly attempts are being made to produce proper three dimensional images the models of the earth which underlie signal enhancement procedures are grossly simplified versions of reality a major development effort in iterative and interactive model fitting is just beginning with the aim of allowing more plausible models to be used interpretable echoes are commonly obtained from depths in sedimentary rocks of 5 km and more absorption limits penetration of the higher frequencies so that it is rare for echoes from the greater depths to have appreciable energy above 25 hz some information on the nature of the rocks and their depositional environment may be obtained from the reflections but essentially nothing may be deduced about whether their pores are filled with water oil or gas colour graphics work stations are just being introduced to aid in the geological interpretation of the computer enhanced signals but it will be some time before they can call up fast enough and display adequately the quantity of data involved in an average survey 1010 bits

kirchhoff image propagation seismic imaging processes are in general formulated under the assumption of a correct macrovelocity model however seismic subsurface images are very sensitive to the accuracy of the macrovelocity model this paper investigates how the output of kirchhoff inversion migration changes for perturbations of a given 3 d laterally inhomogeneous macrovelocity model the displacement of a reflector image point from a perturbation of the given velocity model is determined in a first order approximation by the corresponding traveltime and slowness perturbations as well as the matrix of the beylkin determinant the required traveltime derivatives can be calculated with ray perturbation theory using this result a new computationally efficient kirchhoff inversion migration technique is developed to predict in parallel a series of subsurface images for perturbations of a given macrovelocity model during a single inversion migration process applied to the unmigrated seismic data these images are constructed by su

shallow structure characterization by 2d elastic full waveform inversion assessing the effectiveness of elastic full waveform inversion fwi algorithms when applied to shallow 2d structures in the presence of a complex topography is critically important by using fwi we overcome inherent limitations of conventional seismic methods used for near surface prospecting acoustic tomography and multichannel spectral analysis of surface waves the elastic forward problem formulated in the frequency domain is based on a mixed finite element p0p1 discontinuous galerkin method to ensure accurate modeling of complex topography effects at a reasonable computing cost the inversion problem uses an fwi algorithm to minimize the misfit between observed and calculated data based on results from a numerical experiment performed on a realistic landslide model inspired from the morphostructure of the super sauze earthflow we analyzed the effect of using a hierarchical preconditioning strategy based on a simultaneous multifrequency inversion of damped data to mitigate the strong nonlinearities coming from the surface waves this strategy is a key point in alleviating the strong near surface effects and avoiding convergence toward a local minimum using a limited memory quasi newton method improved the convergence level these findings are analogous to recent applications on large scale domains although limited sourcereceiver offset ranges low frequency content of the source and domination of surface waves on the signal led to some difficulties regarding the impact of data decimation on the inversion results we have learned that an inversion restricted to the vertical data component can be successful without significant loss in terms of parameter imagery resolution in our investigations of the effect of increased source spacing we found that a sampling of 4 m less than three times the theoretical maximum of one half wavelength led to severe aliasing

richardson mountains yukon northwest territories the northern portal of the postulated ice free corridor abstract the richardson mountains located to the east of the porcupine plateau and old crow basin constitute the first barrier that migrating organisms would have to traverse in order to follow any potential ice free corridor in western canada two gaps exist in the mountain range mcdougall pass in the north and peel river canyon in the south the existence of the ice free corridor in this region thus hinges upon the chronology and extent of laurentide glacial advances in these areas three glacial events have been recognized in the richardson mountains peel plateau region an initial event occurred during the middle quaternary depositing sediments subsequently reworked into the brown bear gravels of the central peel plateau a second glacial event occurred at some later time prior to the late wisconsinan this glaciation correlative to the buckland event of the yukon coastal plain reached mcdougall pass blocking the northern access through the richardson mountains the glaciation did not extend into the bonnet plume basin but the presence of glacial ice in the southern peel plateau impounded a large lake in the basin depositing lacustrine sediments and preventing use of the flooded peel river canyon route the most recent major glaciation of late wisconsinan age was less extensive than the second event in the central and northern peel plateau although glacial ice did not penetrate mcdougall pass at this time lacustrine impoundment in the pass and the bell basin precluded use of this route by any potential migrants during the late wisconsinan in the bonnet plume basin late wisconsinan ice advanced over the deformable previously deposited lacustrine sediment reaching its maximum position at hungry creek the southern passage through the richardson mountains was blocked by glacial ice during the late wisconsinan and thus no suitable gateways to the corridor route existed prior to the ultimate deglaciation of the region

aspects of the crustal structure of the western mojave desert california from seismic reflection and gravity data seismic and gravity data taken along line 1 of the 1982 consortium for continental reflection profiling cocorp mojave desert survey n s profile 30 km long have been used to characterize the upper crust north of the san andreas fault in the western mojave block of southern california consortium for continental reflection profiling seismic reflection data were reprocessed to emphasize the upper 5 seconds two way travel time the resultant common depth point cdp sections provided starting models for generating a refined geologic cross section using a combination of ray tracing forward modeling and gravity interpretation the forward modeling was used to validate the existence of faults and constrain their dips the gravity data were used to refine the overall model particularly in poor data areas on the cdp sections gravity data taken along three nearby profiles parallel to primary line of section were also used to determine the structural trend results from the first two seconds indicate the presence of a series of ene striking reverse faults beneath the late tertiary and quaternary sedimentary cover of the western mojave the faults dip northward and offset the sediment basement interface the largest such feature has an apparent throw of 1 8 km and exhibits a subtle scarp at the earth s surface suggesting holocene displacement the orientation of these faults although not an instantaneous representation of the present day stress field is consistent with nnw compression across the western mojave block and wnw striking san andreas fault as determined from nearby focal mechanisms and in situ stress measurements the faults also appear to be closing small sedimentary basins in the mojave block which may have formed during an earlier extensional phase similar to what is happening on a much larger scale in the los angeles basin to the south of the san andreas fault reflections between 2 and 5 s coupled with the local geology and gravity modeling are consistent with the presence of the pelona rand schist in the subsurface beneath the western mojave the upper surface of the schist i e vincent rand thrust equivalent rises southward toward the san andreas fault where it is displaced vertically up to the south at least 5 km along the e w trending hitchbrook fault such that the schist crops out between the hitchbrook and subparallel san andreas to the south the same structure may exist beneath the tehachapi mountains with the roles of the hitchbrook and san andreas faults played by the north and south branches of the garlock fault respectively the rising or arching of the basement toward the san andreas fault and toward the garlock is not only reflected in the geology and topography local to these faults in many places but is also generally observed on seismic reflection profiles in the vicinity of these faults in the western mojave furthermore the arching is also consistent with a strong component of fault normal compression

stability of source mechanisms inverted from p wave amplitude microseismic monitoring data acquired at the surface we study the stability of source mechanisms inverted from data acquired at surface and near surface monitoring arrays the study is focused on p wave data acquired on vertical components as this is the most common type of acquisition we apply ray modelling on three models a fully homogeneous isotropic model a laterally homogeneous isotropic model and a laterally homogeneous anisotropic model to simulate three commonly used models in inversion we use geometries of real arrays one consisting in surface receivers and one consisting in buried geophones at the nearsurface stability was tested for two of the frequently observed source mechanisms strike slip and dip slip and was evaluated by comparing the parameters of correct and inverted mechanisms we assume these double couple source mechanisms and use quantitatively the inversion allowing non double couple components to measure stability of the inversion to test the robustness we inverted synthetic amplitudes computed for a laterally homogeneous isotropic model and contaminated with noise using a fully homogeneous model in the inversion analogously amplitudes computed in a laterally homogeneous anisotropic model were inverted in all three models we show that a star like surface acquisition array provides very stable inversion up to a very high level of noise in data furthermore we reveal that strike slip inversion is more stable than dip slip inversion for the receiver geometries considered here we show that noise and an incorrect velocity model may result in narrow bands of source mechanisms in hudson s plots

post stack diffraction type migration of the signal component of the wavefield summary a new noise suppressing technique for post stack migration of cdp data is described the idea of the method in its simplest form is to slide in time a diffraction curve along a group of traces of a stacked section and for each point in time to check whether there is an interval along the diffraction curve within which this curve is tangent to some coherent energy if such an interval does exist its location is found a penalty function having maximal values within this interval and tending to zero outside it is constructed and the data samples along the diffraction curve are stacked with weights proportional to the values of the penalty function the method has been tested on variable fold seismic reflection data from a short seismic line at cajon pass in southern california results show substantial improvement in the ability to image a set of reflectors in the crystalline basement the uppermost reflectors were intersected by the cajon pass scientific drillhole and found to be related to fracture zones the source of a prominent reflector at 2 5 s about 6 2 km depth with an apparent dip of about 20 to the north is not known the northward dip of basement reflectors is common in seismic sections from the mojave block where it abuts the san andreas fault and is probably related to a strong component of compression across the fault

velocity analysis by iterative profile migration in conventional seismic processing velocity analysis is performed by using the normal moveout nmo equation which is based on the assumption of flat horizontal reflectors imaging by migration either before or after stack is done normally in a subsequent step using these velocities in this paper velocity analysis and imaging are combined in one step and migration itself is used as a velocity indicator because unlike nmo migration can be formulated for any velocity function migration based velocity analysis methods are capable of handling arbitrary structures i e those with lateral velocity variations in the proposed scheme each shot gather profile is migrated with an initial depth velocity model profile migration is implemented in the x domain but the actual implementation of profile migration is not critical as long as it is not done in a spatial wavenumber domain which would preclude handling of lateral velocity variations after migration with an initial velocity model the

reinterpretation of seismic reflection data over the east pacific rise multichannel seismic reflection data over the axial region of the east pacific rise are depth migrated using detailed velocity information from laboratory measurements of ophiolite samples the data are from the lamont doherty geological observatory s line 17 shot across the rise at 9 sup 0 n the migrated data are interpreted to show the structure of the seafloor and what we believe is the magma chamber roof the polarity and apparent root mean squared velocity of the magma chamber roof are asymmetric with respect to the topographic axis of the ridge the asymmetry is probably real and not an artifact of data collection modeling of the magnetic data gathered along the line shows that spreading has not been proceeding normally a reasonable explanation for the asymmetry is the possibility that the line intersects a transform fault or an abandoned ridge segment near the ridge axis the shape of the roof reflection is convex upward with an approximate slope of 10 sup 0 and a width of 4 km if extrapolated symmetrically to the other side of the ridge axis the magma chamber roof event is consistent with the funnel shaped chamber proposed by pallister and hopson 1981 for the samail ophiolite if the chambermore roof steepens rapidly beyond the extent of the reflection it would be consistent with the mush filled model of sleep 1975 1978 and dewey and kidd 1977 less

analytical perils and progress in electron microprobe trace element analysis applied to geochronology background acquisition interferences and beam irradiation effects electron probe microanalysis epma of accessory minerals such as monazite xenotime and thorite for minor and trace element concentrations and geochronology requires consideration of beam irradiation effects increasing heat and charge as higher current densities and lengthy counting times are employed and requires careful detailed assessment of background intensities and interferences a carbon coat 250 a thickness is generally inadequate for prevention of absorbed current s uctuation and beam damage when using the high current densities applied for high precision e g 200 na focused beam beam irradiation effects include element mobility in monazite resulting in p loss relative to ree coating materials of higher electrical and thermal conductivity are indicated and use of gold 100 a is strongly suggested systematic compositional and therefore age variability can result simply from analytical effects requiring evaluation of all aspects of data acquisition the spectra relevant to measurement of y th pb and u are complex especially in ree bearing minerals acquisition of detailed spectral wavelength scans allows recognition of background and peak interferences as well as curvature background intensities can be extracted directly from scan data by regression x ray mapping allows delineation of domains guiding background acquisition and detailed quantitative analysis minor substitution or s uorescence of unexpected elements can compromise analyses one documented effect being the s uorescence of k in monazite adjacent to or hosted by k feldspar or micas this effect clearly evident within 10 micrometers from k feldspar can result in erroneous u concentrations leading to misinterpretation of rim ages as younger overgrowths absorption edges associated with th also become relevant to the measurement of u at high th concentrations because background intensity is sensitive to variation in average atomic number backgrounds must be acquired from each identi ed domain with particular attention being paid to th variation misapplication of background intensities can result in large age discrepancies for example application of backgrounds obtained from a high th domain in monazite to measurement of a low th domain 1 3 of the amount in the high th domain results in an overestimation of the um background intensity of 0 008 cps na and an overall increase in age of 70 m y

topographic lineament swarms clues to their origin from domain analysis of italy regional scale subparallel linear topographic features characterize almost all planetary surfaces this experiment concerns their tectonic meaning by focusing on map limits or domains through which individual swarms of these linear features are developed these domain limits are contrasted with map boundaries of better known structural features in a tectonically active region italy to seek clues to stress environments and times of origin of the lineaments the study uses regional raised relief maps to filter the lineament data to a very simple subset those strictly topographic features of a length and prominence capable of retaining detectability through generalizations required by relief map production a total of 5 372 lineaments were drawn using 4 differently lighted images of 1 1 000 000 scale relief maps of italy seven different tests were used for reliability and reproducibility of the data rose diagrams were prepared for 86 subareas by computer fitting of gaussians to azimuth frequency histograms individual azimuthal petals of these roses were then correlated to delimit the general area over which a given azimuthal swarm is developed the precise swarm boundaries were then located by computer contouring the population density of lines of each swarm on the original data set in this way 48 local swarms were mapped boundaries of these swarms correlate poorly with traditional litho tectonic provinces instead they seem to be associated with basin axes broad arches coastal flexures areas of normal fault swarms and projections of structural grain from adjacent sea floors the 48 domains may be grouped into 8 noncontiguous but azimuthally compatible super swarms covering much of italy the most prominent of the super swarms are greatly expanded versions of regional structural grains trend of the po basin axis of the upper adriatic basin zone of south alpine underthrusting and landward extensions of structural grain of the tyrrhenian sea from these relationships a model for lineament swarm origins is proposed involving very minor regional stretching of the thin brittle carapace of extremely large sometimes subtle structures deforming by ductile mechanisms at depth

trace interpolation by slant stack migration the slant stack migration formula based on the radon transform is studied with respect to the depth step z of wavefield extrapolation it can be viewed as a generalized trace interpolation procedure including wave extrapolation with an arbitrary step z for z 0 the formula yields the familiar plane wave decomposition while for z 0 it provides a robust tool for migration transformation of spatially undersampled wavefields using the stationary phase method it is shown that the slant stack migration formula degenerates into the rayleigh sommerfeld integral in the far field approximation consequently even a narrow slant stack gather applied before the diffraction stack can significantly improve the representation of noisy data in the wavefield extrapolation process the theory is applied to synthetic and field data to perform trace interpolation and dip reject filtration the data examples presented prove that the radon interpolator works well in the dip range including waves with mutual stepouts smaller than half the dominant period

the fractal shape of sutured quartz grain boundaries application as a geothermometer along grain boundaries of quartz from metamorphic and igneous rocks complex interfingering sutures may occur under the light microscope the lengths of the sutures range from approximately 10 1000 m the sutured grain boundaries are statistically self similar over one to two orders of magnitude they represent fractals their mathematical counterpart are koch curves which are developed after two to four iterations the fractal hausdorff besicovitch dimensions d of sutured quartz grain boundaries from different grades of metamorphism are between ca 1 05 and 1 30 the d value decreases with increasing temperature during formation of the sutures on a statistical basis d can be used as a measure of this temperature and therefore as a deformation related thermometer

prestack time migration velocity analysis using remigration trajectories abstractwe have developed a prestack time migration tool for local improvement of the seismic migration velocity model the method is based on remigration trajectories that describe the position of an image point in the image domain for different source receiver offsets as a function of the migration velocity it determines kinematic migration parameters using local slope information of migrated seismic reflection events these parameters in turn are used to locally correct the velocity model the main advantage of this technique is that it allows us to carry out a moveout correction not just at a fixed point in a zero offset poststack time migrated gather but varying through all offsets of a common image gather taking into account the reflection point displacement in the midpoint direction in other words it provides for time migration velocity analysis from prestack data we have tested the feasibility of the method on synthetic data from four simple models and the sigsbee2b data our tests deter

depth migration of imaged time sections none of the leading approaches to the migration of seismic sections the kirchhoff summation method the finite difference method or the frequency domain method readily migrates seismic reflections to their proper positions when overburden velocities vary laterally for inhomogeneous media the diffraction curve for a localized buried scatterer is no longer hyperbolic and its apex is displaced laterally from the position directly above the scatterer hubral observed that the kirchhoff summation method images seismic data at emergent image ray locations rather than at the desired positions vertically above scatterers in addition distortions in diffraction shapes lead to incorrect imaging i e incomplete diffraction collapse and hence to further displacement errors for dipping reflections the finite difference method has been believed to continue waves downward correctly through inhomogeneous media in conventional implementations however both the finite difference method and frequency domain a

finite difference calculation of travel times the travel times of the first arriving seismic waves through any velocity structure can be rapidly computed on a two or three dimensional numerical grid by finite difference extrapolation from point to point wavefronts rather than the traditional rays are tracked head waves are properly treated and shadow zones are filled by the appropriate diffractions differences of less than 0 03 per cent are found between the results of this technique and raytracing for a complex two dimensional model this scheme is useful for the windowing of finite difference calculations to increase computing speed and promises to aid in earthquake location tomographic inversion and kirchhoff migration in structures that have strong lateral velocity gradients

topography dependent eikonal traveltime tomography for upper crustal structure beneath an irregular surface seismic modeling of the crust with nonflat topography can be made by first arrival traveltime tomography which faces the challenge of an irregular free surface a feasible way to deal with this problem consists of expanding the physical space by overlapping a low velocity layer above the irregular surface in order to have a flat topography besides using the classical eikonal equation solver for traveltime computation however the undesirable consequences of this method include seismic ray deviations due to the transition from an irregular surface that is the free boundary to an inner discontinuity lying in the expanded computational space an alternative solution called irregular surface flattening which involves the transformation between curvilinear and cartesian coordinate systems has been recently proposed through the formulation of the topography dependent eikonal equation tdee and a new solver for forward modeling of traveltimes based on the solution of this equation we present topography dependent eikonal traveltime tomography hereafter tdett for seismic modeling of the upper crust first arrival traveltimes are calculated using the tdee solver and the raypaths with the minimum traveltime that can be found by following the steepest traveltime gradient from the receiver to the source by solving an algebraic equation system that connects the slowness perturbations with the already determined traveltimes these variables can be obtained making use of the back projection algorithm this working scheme is evaluated through three numerical examples with different topographic complexities that are conducted from synthetic data and a fourth example with somewhat more complicated topography and real data acquired in northeastern tibet the comparison of the results obtained by both methods i e physical space expansion above the irregular surface and irregular surface flattening fully validates the tomography scheme that is proposed to construct seismic velocity models with nonflat topography

dip moveout by fourier transform the conventional normal moveout nmo and common midpoint cmp stacking process enhances reflections having a particular moveout velocity while attenuating events such as multiple reflections having different moveout velocities unfortunately this process also acts as a dip filter applied to the cmp stack in other words nmo and stacking enhances reflections having a particular slope in the cmp stack while attenuating reflections having different slopes nmo and stacking like any dip filter degrades lateral resolution fortunately this dip filtering action can be suppressed by applying in addition to nmo a prestack process known variously as devilish prestack partial migration and dip moveout as the latter term implies this process is a dip dependent moveout correction that enables reflections from both horizontal and dipping reflectors to be stacked with the same nmo velocity stated another way nmo velocities estimated from dip moveout corrected seismograms are independent of the dips o

seismic wave equation demigration migration reverse time migration is a well known method based on a single scattering approximation it is designed to obtain seismic images in the case of a complex subsurface it can however be a very time consuming task because the number of computations is directly proportional to the number of processed sources in the context of velocity model building iterative approaches require that one derives a series of migrated sections for different velocity models we propose to replace the summation over sources by a summation over depth offsets or time delays defined in the subsurface for that we have developed a new relationship between two migrated sections obtained for two different velocity models starting from one of the two images we obtain a second section correctly and efficiently for each time delay we compute a generalized source term by extending the concept of exploding reflector to nonzero offset we obtain the final migrated section by solving the same wave equation in the perturbed model with the modified source term our work included testing the methodology on 2d synthetic data sets particularly when the initial and perturbed velocity models differ greatly

format and philosophy for collecting compiling and reporting microprobe monazite ages microprobe monazite dating has been increasingly used to constrain the timing of deformation and metamorphism because of the potential to date very small monazite domains down to 5 m or less in structural and petrologic context this paper presents an analytical strategy presentation format and error considerations for microprobe monazite dating the strategy involves high resolution compositional mapping to delineate compositional domains within monazite crystals then for each compositional domain a series of th u and pb analyses are made and a single date and error are calculated the number of analyses in each domain is determined by the desired statistical precision of the date results from several monazite grains are typically combined and along with textural relationships are used to build an argument that the dates constrain the age of a deformation or metamorphic event the total error involves three components short term random error dominated by counting statistical uncertainty short term systematic error uncertainty in background correction conductive coating variation and calibration and long term systematic error uncertainty in standard composition mass absorption factors decay constants etc in homogeneous compositional domains short term random errors 2 of less than 10 m y can be obtained from five to ten analyses however short term systematic error mainly background estimation uncertainty would typically result in a doubling of the magnitude of random error microprobe dates are presented as a single gaussian probability distribution for each domain along with representative compositional maps it is recommended that a consistency standard be analyzed during each analytical session and the results be reported along with those from the unknown this proposed strategy and format are compatible with those of other geochronological techniques they incorporate analytical limitations associated with trace as opposed to major element microprobe analysis and will allow better comparisons to be made between labs and between different geochronological techniques

trace interpolation in seismic data processing trace interpolation of spatially aliased seismic data is used to improve the quality of multi trace processing especially that of migration we consider non rectangular sampling lattices after stating the real sampling requirements for two dimensional or 3 d digitized signals the paper explains geometrically the effect of migration and of the 2 d dip filtering upon the aliased events as well as our ability to interpolate traces from undersampled seismic data

u th pb monazite geochronometry of the eastern ghats belt india timing and spatial disposition of poly metamorphism abstract the eastern ghats belt of peninsular india exposes the deeply eroded high grade parts of a composite orogenic belt that including the rayner complex in antarctica once formed an important crustal component of proterozoic east gondwana recent work established four major crustal units separated by tectonic boundaries the late archaean jeypore and rengali provinces northwest and north the late palaeoproterozoic ongole domain southwest and the meso neoproterozoic eastern ghats province east to identify the timing and regional distribution of tectonothermal events within the composite belt monazites in 66 pelitic and felsic granulite samples from 26 localities were studied with the electron microprobe chemical dating technique in most cases the monazites are polygenetic with rock and grain scale chemical and chronological heterogeneities that document growth during episodes of partial melting and significant modifications through strain and fluid induced recrystallisation and replacement in the ongole domain monazite apparent age populations show that the fabric defining high to ultrahigh grade metamorphism occurred between 1650 and 1590 ma after the emplacement of basic and felsic plutonic complexes into the supracrustal granulites at ca 1 7 ga the monazites do not record pre intrusion tectonothermal events but give evidence of later imprints of ductile to brittle deformation accompanied by hydration between 1450 and 1350 ma that are correlated with mesoproterozoic crustal extension rifting and alkaline plutonism along the eastern cratonic margin the ongole domain was not involved in grenvillian crustal reworking and pan african tectonism 520 ma was focused in shear zones especially at tectonic boundaries in the eastern ghats province monazite apparent age populations in the time span between 1260 and 500 ma witness a prolonged and polyphase tectono metamorphic evolution during mesoproterozoic and neoproterozoic early phanerozoic times apparent ages of high y monazite grains armoured in garnet of khondalites and high mgal granulites bracket the timing of early ultra high temperature metamorphism between 1250 and 1100 ma dominant and comparable apparent age populations of matrix monazites in metapelitic and felsic granulites indicate that the fabric defining granulite facies metamorphism and anatexis of the entire terrane occurred between 980 and 940 ma synchronous with the intrusion of voluminous megacrystic granitoid plutons the easternmost chilka lake domain experienced renewed pervasive deformation and high grade metamorphism and melting at ca 800 ma coeval with emplacement of massif type anorthosite and further high grade reworking during neoproterozoic tectonism 680 650 ma apparent ages of 530 470 ma recorded in matrix monazites of strongly sheared mylonitic granulites indicate that the internal segmentation and structural configuration of the eastern ghats belt and its cratonic forelands occurred during late neoproterozoic early phanerozoic times through w to nw directed intracratonic deformation focussed on major shear zones and terrane boundaries when the grenvillian eastern ghats province rayner complex terrane collided with india

the isochron ray in seismic modeling and imaging the isochron the name given to a surface of equal two way time has a profound position in seismic imaging in this paper i introduce a framework for construction of isochrons for a given velocity model the basic idea is to let trajectories called isochron rays be associated with iso chrons in an way analogous to the association of conventional rays with wavefronts in the context of prestack depth migration an isochron ray based on conventional ray theory represents a simultaneous downward continuation from both source and receiver the isochron ray is a generalization of the normal ray for poststack map migration i have organized the process with systems of ordinary differential equations appearing on two levels the upper level is model independent and the lower level consists of conventional one way ray tracing an advantage of the new method is that interpolation in a ray domain using isochron rays is able to treat triplications multiarrivals accurately as opposed to interpolation in the dep

determining safod area microearthquake locations solely with the pilot hole seismic array data 1 in august 2002 an array of 32 three component geophones was installed in the san andreas fault observatory at depth safod pilot hole ph at parkfield ca as an independent test of surface observation based microearthquake locations we have located such events using only data recorded on the ph array we then compared these locations with locations from a combined set of ph and parkfield high resolution seismic network hrsn observations we determined the uncertainties in the locations as they relate to errors in the travel time picks and the velocity model by the bootstrap method based on the ph and combined locations we find that the c2 cluster to the northeast of the ph has the smallest location uncertainties events in this cluster also have the most similar waveforms and largest magnitudes this confirms earlier suggestions that the c2 cluster is a promising target for the safod main hole

the continent ocean boundary at the rifted margin off eastern canada new results from deep seismic reflection studies seismic reflection data were collected and processed to 20 s two way travel time along four lines which cross the rifted continent ocean boundary off the grand banks region of eastern canada specifically to examine the origin age and nature of this fundamental boundary this represents the first regional study of its kind the most important result is the presence of landward dipping reflectors near the foot of the continental slope these occur where oceanic crust appears to terminate against the continent we suggest that the dipping reflectors mark the continent ocean boundary and that they may represent magmatic material which has underplated or intruded the rifted and thinned lower continental crust adjacent to the boundary sedimentary basins lie just landward of the continent ocean boundary their subsidence history suggests significant heating and thinning of the lower lithosphere during rifting and this may be an important stage leading to continental breakup rift basins formed further landward on the grand banks do not exhibit this thinning other significant seismic results include the presence of strongly reflective zones in the lower continental crust near the continent ocean boundary also the oceanic crust exhibits a complexity of reflections some of which may be due to compositional zonation during magmatic crystallization finally our results may have important consequences for continental predrift reconstruction as oceanic crust appears to extend farther landward in the newfoundland basin than some recent studies have indicated

on the application of in situ monazite chemical geochronology to constraining p t t histories in high temperature 850 c polymetamorphic granulites from prydz bay east antarctica we present electron microprobe based in situ th u pb monazite chemical age data from granulite facies metapelites in prydz bay east antarctica the monazite age data define two distinct age populations late neoproterozoic to early palaeozoic c 570 520 ma and neoproterozoic c 950 820 ma that confirm the polytectonic nature of prydz bay our data suggest that similarity in lineation orientation along the prydz bay coast is not sufficient to necessarily indicate time equivalance the minimum duration of early palaeozoic tectonism spanning at least 60 ma is constrained from an mg al rich metapelite monazite hosted by coarse grained orthopyroxene defines the oldest early palaeozoic population whereas cordierite orthopyroxene symplectites define the youngest early palaeozoic population the spatial distribution of monazites and their ages is correlatable with the inferred mineral assemblage evolution we are able to characterize the evolution of early palaeozoic tectonism in quantitative p t t mineral assemblage space demonstrating that ultrahigh temperature tectono metamorphism in prydz bay is of early palaeozoic age the survival of neoproterozoic inheritance in fe al rich metapelites has implications for high pb retentivity in monazite when chemical and kinetic conditions are favourable the approach and logic applied herein are entirely and directly transferable to the interrogation of any other metamorphic terrane

strain partitioning along the mahanadi shear zone implications for paleo tectonics of the eastern ghats province india abstract during indo antarctic collision at c 1 0 ga eastern ghats province egp granulites amalgamated with the archean indian craton the northern boundary of the egp was subsequently reworked undergoing dextral strike slip shearing at 0 5 ga this study documents a phase of dextral shearing within the egp along wnw ese trending shear planes in c 0 5 ga mylonites of the mahanadi shear zone regional structural trends in the egp show a swing from ne sw to the south of the shear zone to wnw ese to its north the mylonitic shear zone foliation has a sub horizontal lineation associated with a prominent dextral shear sense in near horizontal sections electron back scatter diffraction ebsd studies on quartz confirm that mylonitisation was associated with dextral strike slip movement in the greenschist facies north of the mahanadi shear zone strain was partitioned into narrow dextral strike slip shear zones along which the older granulite fabrics were transposed parallel to later wnw ese trending shear planes at lower grades of metamorphism this regional scale shearing at 500 ma possibly resulted in a significant dextral shift of the northern egp with respect to the south the shear zone was reactivated in the permian time during deposition of gondwana sediments in the mahanadi basin

seismic migration problems and solutions historically seismic migration has been the practice science technology and craft of collapsing diffraction events on unmigrated records to points thereby moving migrating reflection events to their proper locations creating a true image of structures within the earth over the years the scope of migration has broadened what began as a structural imaging tool is evolving into a tool for velocity estimation and attribute analysis making detailed use of the amplitude and phase information in the migrated image with its expanded scope migration has moved from the final step of the seismic acquisition and processing flow to a more central one with links to both the processes preceding and following it in this paper we describe the mechanics of migration the algorithms as well as some of the problems related to it such as algorithmic accuracy and efficiency and velocity estimation we also describe its relationship with other processes such as seismic modeling our approach is tutorial we avoid presenting the finest details of either the migration algorithms themselves or the problems to which migration is applied rather we focus on presenting the problems themselves in the hope that most geophysicists will be able to gain an appreciation of where this imaging method fits in the larger problem of searching for hydrocarbons

2 5d sh wave propagation in heterogeneous crustal wave guides using the phase screen method abstract one way approximations for regional phases slice the half space crustal wave guide into a number of slabs perpendicular to the propagation direction the moho discontinuity can be easily treated as a perturbation from the crustal background the advantage of one way propagation methods is their great saving of computing time and memory compared to full waveform numerical methods especially for 3d simulations in this article we present general 2 5d sh formulas for the thin slab method and a generalized screen propagator gsp for simulating sh wave propagation in the heterogeneous half space crustal wave guide comparisons of numerical results with a finite difference fd algorithm for a standard flat crustal model show excellent agreement demonstrating the validity and accuracy of the 2 5dgsp for the half space problem we then compare the curves of energy attenuation calculated by the 2d and 2 5d formulas the comparison shows that the energy attenuation of the 2 5d case is about 1 5 3 times greater than that of the 2d case when the trapped mode is formed the sh screen propagator is also used to simulate two laterally unvarying in the y direction crustal models showing the path effects on regional wave propagation

glaciotectonic and till or tillite genesis examples from pleistocene glacial drift in central sweden abstract a few different wasy of formation of diamictons that to some extent could be considered till are discussed shearing brecciation and flow as a result of glacier action deformed and mixed waterlaid sediments to become diamictons the structures observed illustrate the ways of formation similar structures found in e g precambrian sediments could be an indication of glacial deformation

joint 3d processing of active and passive seismic data passive seismic provides additional illumination sources in producing reservoirs improving the earth s imaging obtained by standard 3d seismic surveys the joint tomographic inversion of surface and borehole data both active and passive even allows the delineation of thin reservoirs that cannot be resolved by reflection tomography as an application example we present a feasibility study for a real case of co 2 geological storage showing that this operation may benefit both environment and reservoir monitoring the origin time of micro earthquakes due to production operations is critical for merging active and passive data we show here that the wadati s method is not accurate for borehole data in a layered earth model when the ratio between p and s velocities is not constant as occurs in most hydrocarbon reservoirs this drawback can be solved by deploying a few receivers at the surface close to the well

distribution of magma beneath the east pacific rise between the clipperton transform and the 9 17 n deval from forward modeling of common depth point data we have reprocessed seven cross axis common depth point cdp profiles between the clipperton transform and the 9 17 n deval deviation in axial linearity on the east pacific rise epr to understand the relationship between axial magma chamber amc width and seafloor morphology forward modeling of cross axis cdp profiles suggests a segmented amc in which significant variations in width occur across minor rise axis discontinuities e g devals the modeled rise segment widths bounded by the 9 53 n 9 35 n devals the 9 35 n 9 17 n devals and south of the 9 17 n deval were 0 7 km 1 0 1 2 km and 4 15 km respectively transition in amc width across these discontinuities is unclear due to the sparseness of cross axis line spacing however a simple association of devals with decreased magma supply is doubtful the minimum 250 m and maximum 4150 m amc widths are found near the 9 35 n and 9 17 n devals respectively the reprocessing of cdp profiles included repicking stacking velocities to ensure a proper stack of the amc reflector and its associated diffractions imaging postcritical reflections from the base of layer 2a finite difference time migration ray theoretical depth migration and travel time modeling of amc diffraction patterns constraints on amc width were derived from forward modeling techniques based on analytic raytracing velocity models were constructed from seabeam bathymetry and modified expanding spread profile esp velocity depth functions esp velocity models were altered to compensate for off axis thickening of layer 2a as imaged in the cdp reflection data continuous two dimensional velocity models constructed from modified esp velocity depth functions and seabeam bathymetry should account for ray bending at the seafloor basalt interface and any lateral velocity gradients induced by a thickening layer 2a stacked data were time migrated using a finite difference algorithm and extrapolated to depth using ray theoretical depth migration reflector positions were input into our forward modeling scheme to produce a zero offset travel time response of our migrated solution the travel time response was then overlain on the stacked section to ensure an adequate match especially to diffractions generated at the amc edge forward modeling of amc diffraction patterns reveals that original amc width estimates were too large the under migration of the amc reflector resulted from the conversion of stacking to interval velocities without accounting for topographic effects on individual cmp gathers thus resulting in improperly collapsed diffraction hyperbolae ship wandering relative to the amc edge can account for variations in amc reflector amplitude and dropout on the along axis cdp line the continuity of the amc appears unbroken across several ridge axis discontinuities between the clipperton transform and the 9 17 n deval which suggests an amc whose along axis dimension exceeds 75 km reflectivity modeling of cmp gathers suggests that the available data are consistent with a magma chamber comprising only a thin layer of melt overlying a zone of partially solidified crystal mush such a thin layer of melt might inhibit along axis mixing of magmas and thereby account for variations in magma composition along the rise crest this melt lens model for the amc would also produce strong diffraction patterns as imaged in the cdp data

from the hagedoorn imaging technique to kirchhoff migration and inversion the seminal 1954 paper by j g hagedoorn introduced a heuristic for seismic reflector imaging that heuristic was a construction technique a string construction or ruler and compass method for finding reflectors as an envelope of equal traveltime curves defined by events on a seismic trace later kirchhoff migration was developed this method is based on an integral representation of the solution of the wave equation for decades kirchhoff migration has been one of the most popular methods for imaging seismic data parallel with the development of kirchhoff wave equation migration has been that of kirchhoff inversion which has as its objectives both structural imaging and the recovery of angle dependent reflection coefficients the relationship between kirchhoff migration inversion and hagedoorn s constructive technique has only recently been explored this paper addresses this relationship presenting the mathematical structure that the kirchhoff approach adds to hagedoorn s constructive method and showing the relationship between the two

nonlinear 3d tomographic least squares inversion of residual moveout in kirchhoff prestack depth migration common image gathers velocity model estimation with seismic reflection tomographyisanonlinearinverseproblem wepresentanewmethodforsolvingthenonlineartomographicinverseproblemusing 3d prestack depth migrated reflections as the input data i e our method requires that prestack depth migration psdmbeperformedbeforetomography themethodisapplicable to any type of seismic data acquisition that permits seismic imaging with kirchhoff psdm afundamental conceptofthemethodisthatwedissociatethepossiblyincorrect initialmigrationvelocitymodelfromthetomographicvelocity model we take the initial migration velocity model and the residual moveout in the associated psdm common image gathers as the reference data this allows us to consider themigrateddepthoftheinitialpsdmastheinvariantobservation for the tomographic inverse problem we can therefore formulate the inverse problem within the general framework of inverse theory as a nonlinear least squares data fitting between observed and modeled migrated depth the modeledmigrateddepthiscalculatedbyraytracinginthetomographic model followed by a finite offset map migration in the initial migration model the inverse problem is solved iteratively with a gauss newton algorithm we applied the method to a north sea data set to build an anisotropic layer velocitymodel

tomographic determination of three dimensional seismic velocity structure using well logs vertical seismic profiles and surface seismic data a tomographic technique traveltime inversion has been developed to obtain a two or three dimensional velocity structure of the subsurface from well logs vertical seismic profiles vsp and surface seismic measurements the earth was modeled by continuous curved interfaces polynomial or sinusoidal series separating regions of constant velocity or transversely isotropic velocity ray tracing for each seismic source receiver pair was performed by solving a system of nonlinear equations which satisfy the generalized snell s law surface to borehole and surface to surface rays were included a damped least squares formulation provided the updating of the earth model by minimizing the difference between the traveltimes picked from the real data and calculated traveltimes synthetic results indicated the following conclusions for noise free cases the inversion converged closely from the initial guess to the true model for either surface or vsp data adding random noise to the observations and performin

time migration velocity analysis by image wave propagation of common image gathers image wave propagation or velocity continuation describes the variation of the migrated position of a seismic event as a function of migration velocity image wave propagation in the common image gather cig domain can be combined with residual moveout analysis for iterative migration velocity analysis mva velocity continuation of cigs leads to a detection of those velocities in which events flatten although image wave continuation is based on the assumption of a constant migration velocity the procedure can be applied in inhomogeneous media for this purpose cigs obtained by migration with an inhomogeneous macrovelocity model are continued starting from a constant reference velocity the interpretation of continued cigs as if they were obtained from residual migrations leads to a correction formula that translates residual flattening velocities into absolute time migration velocities in this way the migration velocity model can be improved iteratively until a satisfactory result is reached wi

migration of common shot gathers three depth migration methods which operate on common shot data are presented the first migration method maps digitized horizons from the x t domain to the x z domain because this method is based on ray tracing its computation time is short it is suggested for iterative velocity analysis the other two migration methods map the entire common shot gather into a depth section the common shot migration requires calculation of the arrival time of the direct wave from the source to all the depth points and is done through a direct solution of the eikonal equation all three methods are suitable for areas with both lateral and vertical velocity variation

subsalt velocity estimation by target oriented wave equation migration velocity analysis a 3d field data example abstractwe apply target oriented wave equation migration velocity analysis to a 3d field data set acquired from the gulf of mexico instead of using the original surface recorded data set we use a new data set synthesized specifically for velocity analysis to update subsalt velocities the new data set is generated based on an initial unfocused target image and by a novel application of 3d generalized born wavefield modeling which correctly preserves velocity kinematics by modeling zero and nonzero subsurface offset domain images the target oriented inversion strategy drastically reduces the data size and the computation domain for 3d wave equation migration velocity analysis greatly improving its efficiency and flexibility we apply differential semblance optimization dso using the synthesized new data set to optimize subsalt velocities the updated velocity model significantly improves the continuity of subsalt reflectors and yields flattened angle domain common image gathers

what is a seismic reflector like the spatial region that is in the vicinity of an interface and actually affects the interface response and hence the reflected wavefield is of particular interest for the characterization of reflectors from a seismic viewpoint this region is represented by a volume of integration of medium properties above and below the interface whose maximum lateral extent corresponds to the lateral extent of the interface fresnel zone and whose maximum vertical extent is equal to a thickness we evaluate approximately for subcritical incidence angles for a plane interface as well as for curved interfaces of anticline and syncline type the maximum vertical extent might be larger than the seismic wavelengths for subcritical incidence angles close to the critical angle and for a strong impedance contrast at the interface although the part of the reflector volume lying below the interface and affecting traveltime measurements actually is smaller than described in previous studies the whole part of the reflector volume that affects the amplitude of the reflected wavefield is larger than estimates in previous studies which considered only the spatial region below the interface for a syncline respectively an anticline it is larger respectively smaller than described for a plane interface in addition to providing more physical insights into the wave reflection process this study might have significant implications for seismic interpretation using amplitude variation with angle methodologies

event oriented velocity estimation based on prestack data in time or depth domain1 a 2d reflection tomographic method is described for the purpose of estimating an improved macrovelocity field for prestack depth migration an event oriented local approach of the layer stripping type has been developed where each input event is defined by its traveltime and a traveltime derivative taken with respect to one of four coordinates in the source receiver and midpoint half offset systems recent work has shown that the results of reflection tomography may be improved by performing event picking in a prestack depth domain the authors adopt this approach and allow events to be picked either before or after prestack depth migration hence if events have been picked in a depth domain such as the common shot depth domain or the common offset depth domain then a depth time transformation is required before velocity estimation the event transformation may for example be done by conventional kinematic ray tracing and with respect to the original depth migration velocity field by this means they expect the input events for velocity updating to become less sensitive to migration velocity errors an interactive updating approach is used based on calculation of derivatives of event reflection points with respect to velocity the event reflection points are obtained by ray theoretical depth conversion andmore reflection point derivatives are calculated accurately and efficiently from information pertaining to single rays less

migration moveout analysis and depth focusing prestack depth migration still suffers from the problems associated with building appropriate velocity models the two main after migration before stack velocity analysis techniques currently used depth focusing and residual moveout correction have found good use in many applications but have also shown their limitations in the case of very complex structures to address this issue we have extended the residual moveout analysis technique to the general case of heterogeneous velocity fields and steep dips while keeping the algorithm robust enough to be of practical use on real data our method is not based on analytic expressions for the moveouts and requires no a priori knowledge of the model but instead uses geometrical ray tracing in heterogeneous media layer stripping migration and local wavefront analysis to compute residual velocity corrections these corrections are back projected into the velocity model along raypaths in a way that is similar to tomographic reconstruction while this approa

linesmanship and the practice of linear geo art the theory of linesmanship and the practice of linear geo art are explained as critiques of methods observed in commonly published classic to recent papers devoted to fracture analysis and related forms of self delusion

glacial or non glacial origin for the bigganjargga tillite finnmark northern norway bigganjargga is the classic locality for the varangian ice age in northern scandinavia the presence of two sets of striations on the underlying quartzite basement has been taken as evidence for a glacial origin of the bigganjargga diamictite however soft sediment surface structures on the underlying sandstone indicate that it was not consolidated when the diamictite was emplaced together with structures in the diamictite this shows that the deposit formed as a debris flow moving northwestwards over an unconsolidated substrate there is no evidence for glacial origin or contribution for the diamictite material

influence of models on seismic event localization source localization is a fundamental problem in seismology current localization techniques often rely on homogeneous isotropic models even if a survey region is known to be geologically complex or anisotropic we investigated a model s influence on localization using data from a hydraulic injection experiment at the continental deep drilling kontinentale tiefbohrung or ktb site in germany we performed the localization with a grid search algorithm and two additional methods for verification from previous work a homogeneous isotropic model based on a borehole check shot and a heterogeneous 3d isotropic model were available vertical seismic profiling vsp and borehole data as well as laboratory data from rock samples provided homogeneous anisotropic models although localization with the isotropic and anisotropic models led to the same magnitude of residuals and therefore to a comparable quality of fit the distribution of the event cloud differed significantly for isotropic and anisotropic media

a robust approach to time to depth conversion and interval velocity estimation from time migration in the presence of lateral velocity variations the problem of conversion from time migration velocity to an interval velocity in depth in the presence of lateral velocity variations can be reduced to solving a system of partial differential equations in this paper we formulate the problem as a non linear least squares optimization for seismic interval velocity and seek its solution iteratively the input for the inversion is the dix velocity which also serves as an initial guess the inversion gradually updates the interval velocity in order to account for lateral velocity variations that are neglected in the dix inversion the algorithm has a moderate cost thanks to regularization that speeds up convergence while ensuring a smooth output the proposed method should be numerically robust compared to the previous approaches which amount to extrapolation in depth monotonically for a successful time to depth conversion image ray caustics should be either nonexistent or excluded from the computational domain the resulting velocity can be used in subsequent depth imaging model building both synthetic and field data examples demonstrate the applicability of the proposed approach

chime dating of monazite xenotime zircon and polycrase protocol pitfalls and chemical criterion of possibly discordant age data abstract this paper outlines the chime chemical th u total pb isochron method dating method which is based on precise electron microprobe analyses of th u and pb in th and u bearing accessory minerals such as monazite xenotime zircon and polycrase the age mapping technique that is applicable to young monazite and zircon is also described chime dating consists of analyzing multiple spots within homogeneous age domains that show sufficient compositional variation and then these data are used to construct a pseudo isochron from which an age can be obtained via regression this method when coupled with discrimination of possibly concordant age data by chemical criteria such as the ca si th u pb s ratio for monazite and ca and s contents for zircon has the potential advantage of significant precision and the ability to work with minerals that have a significant initial common pb component this technique can identify two or more homogeneous domains that are separated by age gaps smaller than the error on individual spot age analysis many features that are insignificant in major element analysis can have major impact in the acquisition of trace element data critical factors include the roles of collimator slit detector gas background estimation accelerating voltage probe current x ray interferences and count rate in affecting the accuracy and a way to apply the th and u interference correction without pure th and u oxides or synthesized pure thsio4 the age mapping procedure for young monazite and zircon includes acquiring pbm or pbm intensity of individual pixels with multiple spectrometers correcting background with background maps computed from a measured background intensity by the intensity relationships determined in advance of the measurement calibrating of intensity with standards and calculating of ages from the th u and pb concentrations this technique provides age maps that show differences in age domains on the order of 20 ma with in monazite as young as 100 ma the effect of sample damage by irradiation of intense and prolonged probe measurement is also described

migration by extrapolation of time dependent boundary values migration of an observed zero offset wavefield can be performed as the solution of a boundary value problem in which the data are extrapolated backward in time this concept is implemented through a finite difference solution of the two dimensional acoustic wave equation all depths are imaged simultaneously at time 0 the imaging condition and all dips right up to vertical are correctly migrated numerical examples illustrate this technique in both constant and variable velocity media

systematics of time migration errors even if the correct velocity is used time migration mispositions events whenever the velocity changes laterally these errors increase with lateral velocity variation depth of burial and dip angle our analyses of two model types one with an implicit gradient and one with an explicit gradient yield simple rules of thumb for these errors to first order in the lateral gradient the x error is a 1 3tan2 and the z error is 2atan3 where the quantity a a x z contains the information about depth of burial and magnitude of lateral gradient these rules can be used to determine when depth migration is needed further analysis also shows that the image ray correction to time migration is accurate only at small dip for dipping events the image ray correction must be supplemented by a shift in x of the form 3atan2 and a shift in z given by 2atan3 these time migration corrections take the same form for both the models we have studied suggesting a general scheme for correcting time migration w

seismic demigration migration in the curvelet domain curvelets can represent local plane waves they efficiently decompose seismic images and possibly imaging operators we study how curvelets are distorted after demigration followed by migration in a different velocity model we show that for small local velocity perturbations the demigration migration is reduced to a simple morphing of the initial curvelet the derivation of the expected curvature of the curvelets shows that it is easier to sparsify the demigration migration operator than the migration operator an application on a 2d synthetic data set generated in a smooth heterogeneous velocity model and with a complex reflectivity demonstrates the usefulness of curvelets to predict what a migrated image would become in a locally different velocity model without the need for remigrating the full input data set curvelets are thus well suited to study the sensitivity of a prestack depth migrated image with respect to the heterogeneous velocity model used for migration

evolution of stretching lineations in granulite hosted ductile shear zones eastern ghats province india role of temperature strain rate and pre existing stretching lineations abstract shear zone kinematics are interpreted from sections x z parallel to the stretching lineation x direction and perpendicular to the mylonitic foliation x y the stretching lineation is a product of finite strain defined by minerals or mineral aggregates that deform plastically in quartzofeldspathic rocks deformed below 600 c rheological contrasts between quartz and feldspar are amplified and the mesoscopic stretching lineation in the shears may be indistinct in outcrop as quartz ribbons anastomose around relatively competent feldspar clasts in high strain rate zones rheological contrasts between these minerals are reduced and a stretching lineation suitable for kinematic interpretation develops in late amphibolite and greenschist facies shear zones transecting pre lineated quartzofeldspathic granulites of the eastern ghats province india the stretching lineation represents the resultant of pre existing stretch and superposed increments of later shearing until the stretch accumulated in the x direction of the later deformation exceeds the earlier finite stretch the pre existing stretching lineation progressively rotates towards the later x direction with decreasing aspect ratio these intermediate lineation orientations have no kinematic relevance to either earlier or later deformations this example highlights the importance of temperature strain rate and orientation of pre existing stretching lineations in the appearance and kinematic interpretation of stretching lineations in ductile shear zones

delineation of lithostructural domains in northern orissa india from landsat thematic mapper imagery abstract image based reconnaissance geological mapping at 1 100 000 scale using landsat tm data has delineated a college of precambrian lithostructural domains within a 50 000 km 2 region which encompasses the northern portion of the archaean to proterozoic granulite grade eastern ghats tectonic province and the adjacent archaean mesoproterozoic singhbhum craton the domains identified in the present study display distinctive internal structures on satellite imagery most are bounded by clearly recognisable major shear zones and faults on imagery displacement directions may frequently be ascertained through local reorientation of planar structures adjacent to large strike slip shear zones and through juxtaposition of domains with different structuring the macroscopic megascopic structural overview provided by the landsat interpretation supported by preliminary field investigation suggests that the n s shortening e w fold thrust packages with associated ne and nw strike slip faulting is the dominant structural style in the northernmost part of the eastern ghats tectonic province followed by regional dextral transpression expressed in the form of major strike slip faults the largest of these structures kerajang fault which may be traced for over 250 km has demonstrable paleozoic mesozoic dextral motion associated with coal basin formation a precursor kerajang shear zone with dextral movement in excess of 100 km of indeterminate age may also have been instrumental in juxtaposing the singhbhum nucleus into its present position to the north of the eastern ghats tectonic province

dip moveout processing for depth variable velocity in dip moveout dmo processing the seismic velocity is often assumed to be constant while several approximate techniques for handling vertical velocity variation have recently become available here we propose a method for computing the kinematically exact dmo correction when velocity is an arbitrary function of the depth while not known in advance the raypath from source to receiver and the corresponding zero offset raypath satisfy several relationships which are used to form a system of nonlinear equations by simultaneously solving the equations via newton raphson iteration we determine the mapping that transforms nonzero offset data to zero offset unlike previous schemes that approximately handle vertical velocity variation this method makes no assumptions about the offset dip velocity function or hyperbolic moveout tests using both synthetic and recorded seismic data demonstrate the effectiveness of this variablevelocity dmo these tests show this method accurately handles vertical veloc

structure of the irian jaya mobile belt irian jaya indonesia image interpretation and field investigation over the central ranges of irian jaya has provided a structural overview of the inaccessible and largely unmapped irian jaya mobile belt ijmb in the region bounded by longitudes 136 30 e and 141 00 e throughout much of the southern para autochthonous part of the belt structural facing directions are visible on imagery through selective illumination of hogbacks and cuestas formed on thick bedded mesozoic siliciclastics and extensive late cretaceous neogene platform carbonate sequences north of the regional derewo fault zone the derewo metamorphics are regarded in part as correlatives of the metamorphosed om beds in western papua new guinea and display a regional structural grain parallel to the orogen the limited extent of spectrally anomalous features associated with ultramafic rocks indicate that ophiolites are confined to the southern part of the irian jaya ophiolite belt which may be largely composed of metamorphic rocks interpreted macroscopic and megascopic structures suggest the presence of seven discrete structural domains in the ijmb from north to south these are 1 north coast basin region consisting of paleogene neogene volcanics and sediments overlain by a pliocene pleistocene successor basin sequence 2 an allochthonous terrane composed of ophiolites and high grade metamorphic rocks 3 the derewo metamorphic assemblage displaying polyphase deformation 4 a complex marginal zone within mesozoic paleogene miogeoclinal sediments which includes steep duplex structures and remnant klippen 5 a 40 to 50 km wide partly inverted synclinorium composed of miogeoclinal sediments 6 a regional south vergent overturned anticlinorium formed by incompetent paleozoic sediments and 7 a foreland thrust domain involving both mesozoic cenozoic miogeoclinal cover and a deformed neogene foreland molasse basin sequence the regional disposition of interpreted structures in the irian jaya mobile belt and reference to published mapping in western irian jaya and western papua new guinea strongly indicates polyphase deformation late oligocene miocene docking of metamorphics island arc assemblages and ophiolites produced tectonically stacked e w trending structures thrust on to the north margin of the australian continent with its miogeoclinal cover late miocene pliocene collision with the melanesian arc brought the accreted australian margin into contact with the westward moving pacific plate and instituted a regime of oblique transpressional convergence resulting structures include e w sinistral wrenching and nw thrusts along lateral e w ramps

estimation of multivalued arrivals in 3d models using wavefront construction part i1 a new 3d wavefield modelling approach based on dynamic ray tracing is presented this approach is called wavefront construction and it can be used in 3d models with constant or smoothly varying material properties s and p velocity and density separated by smooth interfaces wavefronts consisting of rays arranged in a triangular network are propagated stepwise through the model at each time step the differences in a number of parameters are checked between each pair of rays on the wavefront new rays are interpolated whenever this difference between pairs of rays exceeds some predefined maximum value a controlled sampling of the wavefront at all time steps is thus obtained receivers are given multiple event values by interpolation when the wavefronts pass them the strength of the wavefront construction method is that it is robust and efficient

restoration of missing offsets by parabolic radon transform1 restoration of missing offsets and trace interpolation is an interesting and important problem in seismic data processing based on the parabolic radon transform a method is presented for missing offset restoration resampling and regularization of prestack individual cmp gathers the method is also valid for resampling spatially aliased seismic data the method is based on the parabolic assumption of the seismic events which is generally verified after a partial nmo correction in the cmp organization of the data the essence of the method consists of a band limited forward parabolic radon transform of the data containing zero traces at the missing offset locations the curvature range is chosen to map properly the coherent energy while the zero traces map beyond that range after inverse transform the originally zero traces are partly filled with information several iterations of forward and inverse transform every time replacing the zero traces in the original gather with the partially reconstructed ones almost fully restore the zero traces efficient and fast algorithms can be built up to process data having a uniform geometry examples on synthetic as well as on field data demonstrate clearly the robustness of the method

phanerozoic history of the mahanadi region india the mahanadi basin is a nw se stretching gondwana basin perpendicular to the east coast of india the metamorphic basement to the basin mainly comprises rocks of the eastern ghats mobile belt egmb which experienced a complex thermotectonic evolution involving several episodes of deformation and magmatism it can be inferred from 40ar 39ar ages on hornblende 845 ma and biotite 622 and 593 ma from basement rocks that the last high grade metamorphism in the western part of the egmb i e the western khondalite zone occurred during the late grenvillian with any pan african overprint only reaching anchimetamorphic conditions only pseudotachylite intrusions and their related thermal imprint with ages around 515 ma display a direct pan african signature fission track analyses on nine zircon 419 313 ma and 49 apatite 443 119 ma samples indicate a rift related formation of the mahanadi basin as an asymmetrical half graben in two stages during the permo carboniferous and the middle triassic moreover the gondwana breakup in eastern india is signified by dyke intrusions and the thermal overprint of thrusts and faults in the mahanadi region and the onset of ocean floor production in the mahanadi offshore basin at around 120 ma

migrating around in circles part ii this paper proposes 1 simulating a zero offset zo section from common offset co sections of different offsets and 2 a possibility to find the best velocity to do this simulation more precisely the paper discusses the kinematic aspects of a common reflection point crp stack for an unknown velocity model

orientation of three component geophones in the san andreas fault observatory at depth pilot hole parkfield california to identify and constrain the target zone for the planned safod main hole through the san andreas fault saf near parkfield california a 32 level three component 3c geophone string was installed in the pilot hole ph to monitor and improve the locations of nearby earthquakes the orientation of the 3c geophones is essential for this purpose because ray directions from sources may be determined directly from the 3d particle motion for both p and s waves due to the complex local velocity structure rays traced from explosions and earthquakes to the ph show strong ray bending observed azimuths are obtained from p wave polarization analysis and ray tracing provides theoretical estimates of the incoming wave field the differences between the theoretical and the observed angles define the calibration azimuths to investigate the process of orientation with respect to the assumed velocity model we compare calibration azimuths derived from both a homogeneous and 3d velocity model uncertainties in the relative orientation between the geophone levels were also estimated for a cluster of 36 earthquakes that was not used in the orientation process the comparison between the homogeneous and the 3d velocity model shows that there are only minor changes in these relative orientations in contrast the absolute orientations with respect to global north were significantly improved by application of the 3d model the average data residual decreased from 13 to 7 supporting the importance of an accurate velocity model we explain the remaining residuals by methodological uncertainties and noise and with errors in the velocity model

fault distribution in the precambrian basement of south norway abstract mapping of the structural pattern by remote sensing methods landsat spot aerial photography potential field data and field study of selected structural elements shows that the cratonic basement of south norway is strongly affected by a regular lineament pattern that encompasses fault swarms of different orientation age style attitude and frequency albeit counting numerous fault and fracture populations the faults are not evenly distributed and n s to nne ssw nnw sse and ne se ene wsw systems are spatially dominant n s to nnw sse structures can be traced underneath the caledonian nappes to the western gneiss region in western and central south norway emphasizing their ancient roots dyke swarms of different ages are found within most of these zones also the ostfold oslo trondheim and the mandal molde lineament zones coincide with trends of sveconorwegian post collision granites we conclude that the n s trend includes the most ancient structural elements and that the trend can be traced back to the proterozoic svecofennian and sveconorwegian orogenic events some of the faults may have been active in neoproterozoic times as marginal faults of rift basins at the western margin of baltica remnants of such fault activity have survived in the cores of many of the faults belonging to this system the ancient systems of lineaments were passively overridden by the caledonian fold and thrust system and remained mostly but note entirely inactive throughout the sub cambrian peneplanation and the caledonian orogenic collapse in the silurian devonian the system was reactivated in extension from carboniferous times particularly in the permian with the formation of the oslo rift and parts of it remain active to the present albeit by decreasing extension and fault activity

a successive three point perturbation method for fast ray tracing in complex 2d and 3d geological models abstract this paper presents new 2d and 3d ray tracing methods that can be applied to traveltime and ray path computations for transmitted reflected and turning seismic waves in complex geologic models the new ray tracing scheme combines segmentally iterative ray tracing sirt and pseudo bending methods to address both stratified and arbitrarily shaped block models the new method robustly extends our previous constant block models and constant gradient block models to generally heterogeneous block models and incorporates cubic splines or triangulated interfaces to boundaries of complex geological bodies the method is thus more widely applicable to practical problems a successive three point perturbation scheme is formulated that iteratively updates the midpoints of a segment based on an initial ray path the midpoints are corrected by applying first order analytic formulae to locations of the midpoint inside the block or on the boundaries of the blocks which are then updated with the pseudo bending method and sirt algorithm instead of the traditional iterative methods empirical applications including an example addressing the bohemian massif demonstrate that this successive three point perturbation scheme successfully performs kinematic ray tracing in heterogeneous complex 2d and 3d media

smiles and frowns in migration velocity analysis reliable seismic depth migrations require an accurate input velocity model inaccurate velocity estimates will distort point diffractors into smiles or frowns on a depth section for both poststack and prestack migrated sections high velocities cause deep smiles while low velocities cause shallow frowns on migrated gathers however for prestack images in the offset domain high velocities cause deep frowns while low velocities cause shallow smiles if the velocity is correct there will be no variation in the depth migration as a function of offset and no smiles or frowns in the offset domain we explain migration responses both mathematically and graphically and thereby provide the basis for depth migration velocity analysis

late tectonic reorientation of lineaments and fabrics in the northern eastern ghats province india evaluating the role of the mahanadi shear zone abstract south of the msz that cuts across the eastern ghats province egp of india lineament trends and early granulite facies fabrics rotate from nne ssw in the south towards the wnw ese striking northerly dipping mahanadi shear zone msz in the north using field microstructural and ebsd techniques we demonstrate that d1 and d2 planar fabrics are reoriented by an e w trending northerly dipping d3 extensional shear zone that has a subordinate sinistral strike slip component d3 microstructures are pervasive with quartz and feldspar microstructures characteristic of amphibolite facies metamorphism monazite chemical ages from the d3 shear zone yield ages in the range 730 680 ma the msz representing d4 deformation in the area is a predominantly dextral strike slip deformation during d4 microstructures evolve exclusively through plastic deformation of quartz and culminates in s4 mylonitic fabric formation in high d4 strains zones under greenschist facies conditions these microstructures are restricted to the msz and localized high d4 strain zones unlike the d3 microstructures we conclude that the earlier post granulite extensional deformation d3 and not the msz was responsible for the swing in lineament and fabric trends in the northern egp

determination of full moment tensors of microseismic events in a very heterogeneous mining environment abstract the determination of source parameters and full moment tensors in mines is a difficult task especially when the velocity model of the mining environment is complex and strongly heterogeneous the heterogeneities in the velocity model are usually caused by the presence of ore bodies host rocks tunnel systems and large excavations due to mining activity the mined out cavities introduce strong velocity contrasts in the model cause multiple scattering of waves and result in a complex wave field with long coda waves we have analysed five blasts and five induced microseismic events recorded at the pyhasalmi ore mine finland and suggest a strategy of successfully inverting for the seismic moment tensors we compute accurate locations using an eikonal solver and perform the time domain moment tensor inversion from full waveforms using a generalized linear inversion green s functions are computed using a 3 d finite difference visco elastic code capable of reproducing complex interactions of waves and structures to suppress the sensitivity of the inversion to inaccuracies of locations and the velocity model we analyse the data in the frequency range from 30 to 80 hz the analysis of blasts and microseismic events proves that the moment tensor inversion is successful the moment tensors of blasts display a high percentage of positive isotropic components however the presence of minor shear faulting triggered during blasting cannot be excluded on the other hand the moment tensors of microseismic events display significant negative isotropic and compensated linear vector dipole components this indicates that the predominant mechanism of the events is probably related to the collapse of rock due to mining activity

a platform for kirchhoff data mapping in scalar models of data acquisition kirchhoff data mapping kdm is a procedure for transforming data from a given input source receiver configuration and background earth model to data corresponding to a different output source receiver configuration and background model the generalization of nmo dmo datuming and offset continuation are three examples of kdm applications this paper describes a platform for kdm for scalar wavefields the word platform indicates that no calculations are carried out in this paper that would adapt the derived formula to any one of a list of kdms that are presented in the text platform formulae are presented in 3d and in 2 5d for the latter the validity of the platform equation is verified within the constraints of high frequency asymptotics by applying it to a kirchhoff approximate representation of the upward scattered data from a single reflector and for an arbitrary source receiver configuration the kdm formalism is shown to map this kirchhoff model data in the input source receiver configuration to kirchhoff data in the output source receiver configuration with one exception the method does not map the reflection coefficient thus we verify that asymptotically the ray theoretical geometrical spreading effects due to propagation and reflection including reflector curvature are mapped by this formalism consistent with the input and output modelling parameters while the input reflection coefficient is preserved in this sense this is a true amplitude formalism as with earlier kirchhoff inversion a slight modification of the kernel of kdm provides alternative integral operators for estimating the specular reflection angle both in the input configuration and in the output configuration thereby providing a basis for amplitude versus angle analysis of the data

superposed deformation and inherited structures in an ancient dilational step over zone post mortem of the rengali province india abstract in the eastern indian shield a dextral strike slip system juxtaposed the archaean singhbhum province against the proterozoic eastern ghats belt at 490 470 ma two wnw ese trending strands of the strike slip system enclose a multiply deformed d1 to d3 intervening domain called the rengali province with d3 representing dextral shearing in a granulite lens within the province an early fabric sgr was deformed by an amphibolite facies d1 d2 deformation continuum in the late archaean time forming cylindrical folds in the surrounding quartzofeldspathic gneisses quartzites and mica schists of the province superimposition of syn d3 shortening on d1 d2 folds generated complex non cylindrical geometries the granulites escaped d3 strain microstructures in the province bounding shear zones confirm that d3 deformation was associated with mylonitization dynamic recrystallization and greenschist facies metamorphism in the quartzites syn d3 folds can be correlated with rotation of d1 d2 structures through the shortening zone of bounding dextral shears since the province bounding shears form a step over zone the structural complexity within the rengali province arises from superposition of syn d3 shortening structures on initially asympathetically oriented inherited cylindrical d1 d2 folds hydrous fluid channeling causing greenschist facies metamorphism and quartz vein emplacement accompanied d3 as the step over zone was dilational in nature

a new correlation of sapphirine granulites in the indo antarctic metamorphic terrain late proterozoic dates from the eastern ghats province of india abstract uranium lead and thorium lead ages were obtained from perrierite and zircons in two samples from the eastern ghats province of south india a sapphirine bearing granulite from anakapalle and a charnockite from visakhapatnam the isotopic data are concordant or close to concordant at 1 ga which is interpreted to be the age of granulite facies metamorphism and charnockite plutonism in the vicinity of anakapalle and visakhapatnam and provides the first report of a late proterozoic granulite facies event in the eastern ghats province in reassemblies of gondwanaland based on dutoit s original proposal the eastern ghats province is juxtaposed against a high grade terrain in east antarctica affected by a granulite facies event and charnockitic plutonic activity 800 1100 ma ago the ages reported here suggest that a portion of the eastern ghats province represents an extension of the late proterozoic terrain from antarctica into india the radiometric results on the anakapalle rocks are the first evidence in south india for sapphirine formation during the late proterozoic other south indian sapphirine localities appear to be archaean in age a review of sapphirine localities in the indo antarctic metamorphic terrain indicates that sapphirine developed under a range of pressure temperature conditions during the archaean and proterozoic evidence for a systematic change in peak metamorphic conditions with time is obscured by these regional variations characterization of the entire pressure temperature path in addition to estimates for the peak conditions is needed to distinguish the archaean and proterozoic metamorphic regimes