kirchhoff migration using eikonal equation traveltimes the use of ray shooting followed by interpolation of traveltimes onto a regular grid is a popular and robust method for computing diffraction curves for kirchhoff migration an alternative to this method is to compute the traveltimes by directly solving the eikonal equation on a regular grid without computing raypaths solving the eikonal equation on such a grid simplifies the problem of interpolating times onto the migration grid but this method is not well defined at points where two different branches of the traveltime field meet also computational and data storage issues that are relatively unimportant for performance in two dimensions limit the applicability of both schemes in three dimensions a new implementation of a gridded eikonal equation solver has been designed to address these problems a 2 d version of this algorithm is tested by using it to generate traveltimes to migrate the marmousi synthetic data set using the exact velocity model the results are compared with three other images an f x migration a standard for comparison a kirchhoff migration using ray tracing and a kirchhoff migration using traveltimes generated by a commonly used eikonal equation solver the f x migrated image shows the imaging objective more clearly than any of themore kirchhoff migrations and the authors advance a heuristic reason to explain this fact of the kirchhoff migrations the one using ray tracing produces the best image and the other two are of comparable quality less

unified imaging of multichannel seismic data combining traveltime inversion and prestack depth migration imaging 2d multichannel land seismic data can be accomplished effectively by a combination of traveltime inversion and prestack depth migration psdm referred to as unified imaging unified imaging begins by inverting the direct arrival times to estimate a velocity model that is used in static corrections and stacking velocity analysis the interval velocity model from stacking velocities is used for psdm the stacked data and the psdm image are interpreted for common horizons and the corresponding wide aperture reflections are identified in the shot gathers using the interval velocity model the stack interpretations are inverted as zero offset reflections to constrain the corresponding interfaces in depth the interval velocity model remains stationary we define a coefficient of congruence j that measures the discrepancy between horizons from the psdm image andtheir counterparts from the zero offset inversion a value of unity for j implies that the interpreted and inverted horizons are consisten

a practical approach to surface multiple attenuation multiply reflected events in marine seismic data can be divided into two classes surface multiples are events that have at least one downward reflection from the water surface and internal multiples on the other hand have all of their downward reflections at the water bottom or below surface multiples are usually stronger more broadband and more of a problem than internal multiples because the reflection coefficient at the water surface is much larger than the reflection coefficients found in the subsurface

paleoproterozoic basin development and sedimentation in the lake superior region north america abstract the peneplaned archean craton in the lake superior region was the platform upon which a continental margin assemblage was deposited extension resulted in localized rifts that received thicker accumulations of sediments and volcanic rocks than did adjacent parts of the platform seas transgressed onto the continent several times and an ocean basin opened south of the present day lake superior island arcs that formed during subduction collided with the craton margin as the ocean basin closed oceanic crust is poorly preserved as a dismembered ophiolite sequence the arc volcanics are preserved as the wisconsin magmatic terranes the collision resulted in a fold and thrust belt known as the penokean orogen to the north of the fold and thrust belt a northward migrating foreland basin the animikie basin developed thick turbidite successions were deposited along the basin axis and terrigenous clastics and lake superior type iron formation were deposited on the shelf along the northern margin of the basin the primary paleoclimatic indicators are 1 glaciogenic rocks at the base of the paleoproterozoic succession in michigan indicating ice house conditions 2 remnants of a paleosol on the glaciogenic rocks indicative of deep weathering probably under subtropical conditions and therefore of greenhouse conditions and 3 carbonate minerals after gypsum halite and anhydrite in stromatolitic dolomite indicative of aridity three second order depositional sequences are bounded by major unconformities and can be correlated throughout the lake superior region

vertical seismic profiling depth migration of a salt dome flank vertical seismic profiles vsps can supply information about both velocity and subsurface interface locations properly designed vsps can be used to map steeply dipping interfaces such as salt dome flanks mapping subsurface interfaces with vsp data requires careful survey design appropriate data processing interval velocity estimation and reflector mapping the first of these four ingredients is satisfied in most cases by preacquisition modeling the second is accomplished by careful data processing initial velocity estimates are provided by seismic tomography velocity model refinement is accomplished by a combination of iterative modeling and iterative least squares inversion finally the resultant interval velocities are used in depth migration of the processed vsp these four ingredients have been combined to map a salt dome flank

seismic impedance inversion and interpretation of a gas carbonate reservoir in the alberta foothills western canada acoustic and simultaneous elastic impedance inversions of a 2d land seismic data set are performed to characterize a carbonate reservoir of mississippian age in the turner valley formation in the rocky mountain foothills of western canada the inversions produce p wave and s wave impedance sections ip and is respectively from which lame parameter density and sections are derived the ip data provide a separation between the clastics and carbonates the data provide an estimate of porosity distribution within the dolomitized limestone target deviations from baseline curves for water saturated carbonates of versus porosity versus porosity and is versus ip are interpreted as indicators of gas potential these indicators all provide similar spatial patterns of areas of high gas potential and are consistent with the gas occurence observed in a well

compositions and sources of metasediments in the upper dharwar supergroup south india texturally mature quartz rich sandstones texturally mature mafic rich sandstones and texturally immature graywackes are abundant in the late archean chitradurga group of the dharwar craton in south india when compared to graywackes of paleozoic age from northeastern north america the chitradurga graywackes have slightly higher average concentrations of na and ferromagnesian elements indicating the existence of a more tonalitic and mafic rich provenance during the late archean in karnataka the regional tonalitic gneiss peninsular gneiss is the predominant source of these sediments although mafic ultramafic volcanic debris from unidentified sources is also important neither intermediate nor felsic volcanics nor an extensive metasedimentary terrane were prominent in the provenance although the composition of the craton was dominantly sialic the presence of abundant ferromagnesian debris in some of the texturally mature sandstones that probably formed in shallow water intracratonic or platform

reverse time migration of offset vertical seismic profiling data using the excitation time imaging condition to apply reverse time migration to prestack finite offset data from variable velocity media the standard time zero imaging condition must be generalized because each point in the image space has a different image time or times this generalization is the excitation time imaging condition in which each point is imaged at the one way traveltime from the source to that point reverse time migration with the excitation time imaging condition consists of three elements 1 computation of the imaging condition 2 extrapolation of the recorder wave field and 3 application of the imaging condition computation of the imaging condition for each point in the image is done by ray tracing from the source point this is equivalent to extrapolation of the source wave field through the medium extrapolation of the recorded wave field is done by an acoustic finite difference algorithm imaging is performed at each step of the finite difference extrapolation by extracting from the propagating wave field the

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

3d acoustic prestack reverse time migration1 a prestack reverse time migration algorithm which operates on common source gathers recorded at the earth s surface from 3d structures is conceived implemented and tested reverse time extrapolation of the recorded wavefield a boundary value problem and computation of the excitation time imaging condition for each point in a 3d volume an initial value problem are both performed using a second order finite difference solution of the full 3d scalar wave equation the algorithm is illustrated by processing synthetic data for a point diffractor an oblique wedge and the french double dome and fault model

acquisition and processing of high resolution reflection seismic data from a survey within the complex terrain of the bavarian folded molasse a high resolution reflection seismic survey was carried out in the southern part of the bavarian molasse basin in 1998 and 1999 the survey aimed to investigate the near surface structure of the complicated transition from the unfolded foreland molasse to the folded molasse and the folded molasse to the internally complicated thrust systems of the helveticum the ultrahelveticum and the rhenodanubian flysch the study is linked to the transalp seismic project and the results help to fill the gap between the surface and the upper 300 500 ms two way traveltime twt typical of deep reflection seismic experiments the environmental conditions encountered in the study area required that particular attention be paid to the acquisition parameters for the three seismic lines each about 4 km long the energy source was a small vibrator the geophone spread spacing and frequency range were adjusted to image reflectors which were expected to dip steeply southwards in general the unprocessed field records did not show signals that could be attributed to specific reflectors individual trace processing considerably improved the data quality taking into account the influence of the quaternary cover and also the strong lateral velocity variations of the shallow subsurface the effects of the various processing steps such as muting refraction statics residual statics and velocity analysis are discussed to assess the nmo velocities the qualitative analysis of the seismic energy in a common shotpoint gather offered advantages over an analysis in a common midpoint gather or in a stacked section and proved to be very effective as demonstrated along the miesbach 9801 line low velocity zones extend locally down to about 400 ms adjacent to zones of extremely high velocities close to the surface reflecting steeply dipping strata besides the quaternary cover on top the miesbach 9801 and miesbach 9802 lines exhibit many horizontal reflections in places down as far as 1400 ms twt indicating the sedimentary sequences of the unfolded foreland molasse the southern part of both lines is dominated by southward dipping reflection bands indicating units of the folded molasse the reflection pattern shown by the miesbach 9901 line suggests that there is almost no quaternary cover southward dipping elements reflect the internal structure of the folded molasse whereas a rather diffuse reflection signature may be attributed to rhenodanubian flysch units

paleoclimatology and provenance of the glaciogenic gowganda formation paleoproterozoic ontario canada a chemostratigraphic approach the gowganda formation dated as about 2 3 ga is widely but not universally interpreted as glaciogenic and our chemostratigraphic study supports such an interpretation the bulk compositions of diamictite matrix materials and associated argillites are considered to have resulted from the mixture of two detrital components one is glacial flour similar in composition to the unweathered archean upper crust whereas the other is chemically weathered detritus resembling average proterozoic shale in composition the first component has a chemical index of alteration cia value of 50 reflecting the abundance of feldspar and dearth of aluminous clay minerals the second component has a cia value of 70 and was produced primarily in weathering profiles by chemical weathering typical of temperate climatic regimes diamictite matrix materials 92 samples have an average cia value of 57 the argillites have higher values the average is 62 97 samples the lower cia values of the diamictites reflect a higher proportion of glacial flour suggesting a stronger influence of frigid climatic conditions than that which prevailed during deposition of the argillites systematic trends in cia values are developed across diamictite argillite boundaries and within a thick argillite between two major diamictite units these values reflect climatic variations with maximum amelioration during deposition of the middle section of the argillite rare earth element data and th sc and ti al ratios from the diamictite matrix materials all suggest a provenance that included a large supracrustal component 45 together with lesser amounts of tonalite trondhjemite granite and granite the proportion of supracrustal materials such as volcanic rocks and shales may have been exaggerated by selective comminution during glacial transport the matrix materials of the gowganda diamictites are severely depleted in ca and proportionally enriched in na relative to the source the anorthitic component of plagioclase has been selectively replaced by an albitic component during diagenesis and metamorphism a chemostratigraphic approach provides high resolution data for reconstruction of paleoclimatic conditions and valuable information concerning provenance and metasomatic processes

2 d traveltime and waveform inversion for improved seismic imaging naga thrust and fold belt india summary exploration along the naga thrust and fold belt in the assam province of northeast india encounters geological as well as logistic challenges drilling for hydrocarbons traditionally guided by surface manifestations of the naga thrust fault faces additional challenges in the northeast where the thrust fault gradually deepens leaving subtle surface expressions in such an area multichannel 2 d seismic data were collected along a line perpendicular to the trend of the thrust belt the data have a moderate signal to noise ratio and suffer from ground roll and other acquisition related noise in addition to data quality the complex geology of the thrust belt limits the ability of conventional seismic processing to yield a reliable velocity model which in turn leads to poor subsurface image in this paper we demonstrate the application of traveltime and waveform inversion as supplements to conventional seismic imaging and interpretation processes both traveltime and waveform inversion utilize the first arrivals that are typically discarded during conventional seismic processing as a first step a smooth velocity model with long wavelength characteristics of the subsurface is estimated through inversion of the first arrival traveltimes this velocity model is then used to obtain a kirchhoff pre stack depthmigrated image which in turn is used for the interpretation of the fault waveform inversion is applied to the central part of the seismic line to a depth of 1 km where the quality of the migrated image is poor waveform inversion is performed in the frequency domain over a series of iterations proceeding from low to high frequency 11 19 hz using the velocity model from traveltime inversion as the starting model in the end the pre stack depth migrated image and the waveform inversion model are jointly interpreted this study demonstrates that a combination of traveltime and waveform inversion with kirchhoff pre stack depth migration is a promising approach for the interpretation of geological structures in a thrust belt

the chocolay group lake superior region u s a sedimentologic evidence for deposition in basinal and platform settings on an early proterozoic craton the early proterozoic chocolay group in the lake superior region is a cratonal sequence consisting of a basal unit overlain successively by quartzite dolomite and slate formations two regional depositional environments are described 1 present day structural troughs marquette menominee and felch michigan show evidence of having sedimentary basin precursors and 2 other areas gogebic range michigan show evidence of sedimentation in platform settings the basal unit in the marquette and menominee troughs is as much as 160 m thick and was probably deposited by alluvial processes in the gogebic range the basal unit is less than 10 m thick and resembles lag deposits at the base of transgressive sequences the overlying quartzite contains cross beds that show two regional sedimentary vector paleocurrent distributions 1 unimodal low variance distributions with sedimentary vectors oriented subparallel to local structural trough axes marquette menominee and felch troughs and 2 polymodal distributions with sedimentary vectors showing greater dispersion about the mean gogebic range as indicated by many varieties of sedimentary structures silicified evaporites cryptalgal structures intraclast breccias and so on the carbonates and interstratified siliciclastics of the chocolay group were deposited in subtidal to supratidal conditions taylor noted in 1972 that certain members of the chocolay group carbonates thicken toward the center of the marquette trough with concomitant facies changes the uppermost pelite unit of the chocolay group was probably deposited below normal wave base it is concluded that the basins and platforms were formed on a craton undergoing rift type tectonism

prestack depth migration of an alberta foothills data set the husky experience we produce depth images for an alberta foothills line by iteratively using a number of migration and velocity analysis techniques in imaging steeply dipping layers of a foothills data set it is apparent that thrust belt geology can violate the conventional assumptions of elevation datum corrections and common midpoint cmp stacking to circumvent these problems we use migration from topography in which we perform prestack depth migration on the data using correct source and receiver elevations migration from topography produces enhanced images of steep shallow reflectors when compared to conventional processing in addition to migration from topography we couple prestack depth migration with the continuous adjustment of velocity depth models a number of criteria are used in doing this these criteria require that our velocity estimates produce a focused image and that migrated depths in common image gathers be independent of source receiver offset velocity models are estimated by a series of iterative and interpretive steps involving prestack migration velocity analysis and structural interpretation overlays of velocity models on depth migrations should generally show consistency between velocity boundaries and reflection depths our preferred seismic depth section has been produced by using prestack reverse time depth migration coupled with careful geological interpretation

seismic exploration in complex terrains a processing experience in the southern apennines we discuss a data processing sequence adopted to reprocess a seismic line that crosses the italian southern apennines from the tyrrhenian sea to the adriatic margin and investigate both the overthrust and foreland areas we first determine the main causes of the very low s n ratio in the field data and then propose a processing sequence aimed at exploiting the signal content also making use of a priori geological knowledge of this area our work indicates a combination of causes for the very low quality of the seismic data these include length of the spread about 20 km that is unfavorable because of the rapid variation in the near surface geology tectonic complexity crooked line acquisition and the rough topography associated with outcropping rocks characterized by highly variable velocities based on the outcome of this data analysis we present a processing sequence driven by knowledge of the regional tectonic setting and by knowledge of the shallow subsurface geology the main effort is in remov

applications of seismic travel time tomography summary this paper describes the application of tomography to seismic travel time inversion there are various implementations of travel time tomography in reflection tomography sources and receivers are on the surface of the earth and the principal seismic events are reflections from subsurface velocity discontinuities in transmission tomography sources and or receivers may be buried beneath the surface and the events correspond to direct or unreflected arrivals this is the analogue of medical tomography there are also cases in which both direct as well as reflected arrivals are important such as in vertical seismic profiling the latter is a direct application of the first two but is not discussed in any detail here it is also shown how the iterative use of travel time tomography and depth migration can produce much enhanced subsurface images examples of both transmission tomography and reflection tomography combined with depth migration illustrate the methods

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

multiple removal results from delft university at the workshop comparison of seismic multiple attenuation techniques held in conjunction with seg s 1997 annual meeting three data sets were distributed to a number of participants a gulf of mexico subsalt data set an offshore western australia data set and seg s 2 d synthetic salt model data set

carbon isotope record for the onset of the lomagundi carbon isotope excursion in the great lakes area north america carbonate units of the chocolay group in the lake superior area usa and the gordon lake formation of the correlative upper huronian supergroup ontario canada were deposited after the last paleoproterozoic glacial event and an episode of intense chemical weathering the huronian supergroup contains at the base 2 45 ga volcanics and is intruded by the 2 22 ga nipissing diabase dykes and sills while the chocolay group is bracketed in age between 2 29 and 2 20 ga the lomagundi 2 22 2 1 ga carbon isotope excursion started after the paleoproterozoic glacial epoch and before a plume breakout event at 2 22 ga therefore the chocolay and upper huronian carbonates were deposited either before or during the onset of the lomagundi event notably thin carbonates of the basal gordon lake formation and thick carbonate succession of the kona dolomite in the northeastern exposures of the chocolay group record 13 c values as high as 9 5 versus v pdb similar to other successions deposited during the lomagundi event both units contain pseudomorphs and molds after sulfates this observation suggests that seawater sulfate contents rose dramatically in association with the onset of the lomagundi event and the rise of atmospheric oxygen carbonates in the western and southern exposures of the chocolay group randville and bad river dolomites and saunders formation previously assumed to be equivalent to the gordon lake formation and kona dolomite have carbon isotope values close to 0 based on basin analysis we infer that these carbonate units were deposited during a negative carbon isotope excursion after the lomagundi event started and are slightly younger than the kona dolomite and gordon lake formation this interpretation implies that the carbonate platform in the great lakes area transgressed to the west over shallow marine and fluvial deposits the negative carbon isotope excursion in the lake superior area might correspond to similar 13 c values of the mooidraai dolomite in the griqualand west basin south africa supporting correlation between paleoproterozoic successions of north america and south africa and the notion of three global glaciations in the paleoproterozoic era carbonates of the mille lacs group trout lake glen township and denham formations in minnesota have 13 c values ranging from 1 2 to 2 5 combined with geochronologic constraints these data suggest that these units were deposited after the lomagundi excursion and are related to the rifting event that led to development of the so far unrecognized 2 0 ga passive margin in the lake superior area 2006 elsevier b v all rights reserved

topographic migration of georadar data implications for acquisition and processing application of conventional elevation static corrections and migration to wavefield data recorded on irregular surfaces may result in poor reconstructions of complex subsurface features particulary poor images may be obtained at locations where the depths to target structures are comparable to undulations in the surface topography for example topographic relief of only 1 2 m may be important for the processing of georadar data we describe an algorithm that allows georadar data to be migrated directly from gently to highly irregular acquisition surfaces when applied to a variety of complicated synthetic data sets topographically migrated images are observed to be markedly superior to those produced by two standard processing schemes extensive tests demonstrate that topographic migration should be considered in regions characterized by surface gradients 10 i e dips 6 for effective topographic migration lateral and vertical coordinates of the georadar antennas should be determined to better

book review iron formation facts and problems developments in precambrian geology 6 edited by a f trendall and r c morris elsevier sciences publishers b v amsterdam oxford new york tokyo 1983 xiv 588 pp price us 106 00 u s a and canada dfl 275 00 rest of world isbn 0 444 42144 0 hardback

the lower proterozoic fern creek formation northern michigan mineral and bulk geochemical evidence for its glaciogenic origin bulk chemical data indicate the fern creek formation early proterozoic marquette range supergroup lower chocolay group originated as a glacial till resolving an old argument about the correct interpretation for diamictite containing units within the formation there are poor correlations among sio2 al2o3 and k2o and the chemical index of alteration averages 55 these are qualities characteristic of tills and other sediments accumulated in the absence of chemical weathering and transport induced sorting and inconsistent with an origin as a fluvial deposit the principal alternate hypothesis the archean carney lake gneiss might be the source for at least the lower portions of the fern creek formation compositions are similar and rare earth element trends in the fern creek formation are consistent with derivation from an archean high grade terrane pegmatites in the gneiss could be the source for monazite huttonite and an unnamed fluor hydroxy rare earth mineral that occurs in the midsection of

topographic corrections to slowness measurements in parsimonious migration of land data parsimonious migration requires that incident and emergent angles be measured via apparent slownesses from the seismic data being migrated when slownesses are measured from land data it is the apparent slowness along the topography not the horizontal slowness that is being measured thus errors are introduced into the incident and emergent angle estimates which are defined via horizontal slownesses these errors can be corrected using the local topographic dips a 2d field data example shows that after correction a migrated image has significantly improved coherency

least squares migration of incomplete reflection data a least squares migration algorithm is presented that reduces the migration artifacts i e recording footprint noise arising from incomplete data instead of migrating data with the adjoint of the forward modeling operator the normal equations are inverted by using a preconditioned linear conjugate gradient scheme that employs regularization the modeling operator is constructed from an asymptotic acoustic integral equation and its adjoint is the kirchhoff migration operator we tested the performance of the least squares migration on synthetic and field data in the cases of limited recording aperture coarse sampling and acquisition gaps in the data numerical results show that the least squares migrated sections are typically more focused than are the corresponding kirchhoff migrated sections and their reflectivity frequency distributions are closer to those of the true model frequency distribution regularization helps attenuate migration artifacts and provides a sharper better frequency distribution of estimated reflectivity the least squares migrated sections can be used to predict the missing data traces and interpolate and extrapolate them according to the governing modeling equations several field data examples are presented a ground penetrating radar data example demonstrates the suppression of the recording footprint noise due to a limited aperture a large gap and an undersampled receiver line in addition better fault resolution was achieved after applying least squares migration to a poststack marine data set and a reverse vertical seismic profiling example shows that the recording footprint noise due to a coarse receiver interval can be suppressed by least squares migration

the chemical discrimination of clastic sedimentary components abstract the detrital composition of siliciclastic rocks from six localities in the taconic and acadian sections of new york and quebec can be described as mixtures of two or more differentiable sedimentary components a sedimentary component is defined as a mineral or group of minerals that sort independently of other sedimentary constituents because of grain morphology phyllosilicates usually dominate the finer grained component and quartz or mixtures of quartz and feldspar dominate the coarser grained components the chemical composition of a sedimentary component is distinctive and the differentiation of sedimentary components usually results in the concomitant separation of associated elements typical separations include the enrichment of k2o and al2o3 in the finer grained phyllosilicate rich fraction and the enrichment of sio2 and na2o in the coarser grained tectosilicate rich fraction fe2o3 and mgo are also enriched in the fine fraction if they are abundant constituents in the phyllosilicates the chemical composition of the graywackes from north america was compared to the chemical composition of unlithified turbidite sediments from the black sea the chemical variation trends found in the deeply buried sedimentary rocks are similar to the trends found in the suite of slightly buried and unlithified deep sea sediments this indicates that burial alteration does not necessarily result in large scale redistribution of the constituent elements it is possible therefore to use the chemical variations observed in a suite of deeply buried and mineralogically altered sedimentary rock to infer the original clastic assemblage and to help clarify the processes that produced the sediment