imaging granitic plutons along the iberseis profile abstract the parameters used for the acquisition of the iberseis deep seismic reflection profile in the sw iberian peninsula provide seismic images of the deep crust as well as a high resolution section of the shallow subsurface a very dense array of sources and receivers allowed high resolution tomographic studies in zones of special interest granitic plutons the three dimensional tomographic inversion produced velocity models along a 500 m wide and 1000 m deep strip along the iberseis transect in the areas of la bazana la dehesilla feria and villafranca in these high resolution velocity models sampled by 50 50 50 m cells high velocity anomalies indicate the geology and extension of the granitic plutons at depth this directly correlates with the surface outcrops moreover tomographic models provided valuable information for the geometry and characterization of fractured and fresh domains in a rock volume furthermore a piggy back seismic acquisition experiment using additional seismic instrumentation from the university of paris sud 40 channel dmt provided perpendicular offline recordings of the vibroseis sources this additional recording system was deployed perpendicularly to the main iberseis seismic reflection line and provided additional 3d control

magmatism in the iberian pyrite belt petrological constraints on a metallogenic model igneous formations associated with massive sulphide deposits in the iberian pyrite belt ipb are essentially composed of basic lavas and dolerites and dacitic to rhyolitic volcanites intermediate lavas are subordinate the basic rocks show variable geochemical characteristics lavas and dolerites comparable to recent within plate alkaline basalts seem restricted to the western and southern parts of the ipb whereas basic rocks comparable to continental tholeiites or arc related basalts occur across the whole belt the felsic rocks are classified as calc alkaline and belong to the low al2o3 and high yb type at given sio2 al2o3 and tio2 contents they show variable zr nb and hree contents heavy rare earth element fractionation decreases from the dacites to the rhyolites gd yb n 1 whereas the negative eu anomaly becomes more pronounced the characteristics of the rhyolites are typical of sulphide fertile volcanic packages trace element modelling suggests that the felsic rocks evolved from a dacitic parent magma through fractional crystallization of hornblende and plagioclase partial melting of an amphibolite protolith which appears as the most probable model for the origin of this dacitic magma requires a high t p gradient in the crust the occurrence of alkaline basalts and continental tholeiites is consistent with formation of the ipb in a tensional tectonic setting however the associated island arc tholeiites suggest a location in a domain of plate convergence emplacement in a fore arc basin over a recently accreted crustal segment is envisaged as a possible hypothesis to account for the geological and petrological constraints a high geothermal gradient and eruption in a submarine tensional basin could have been two key ingredients for the development of massive sulphide deposits within the ipb

elastic full waveform inversion of multi component obc seismic data elastic full waveform inversion of seismic reflection data represents a data driven form of analysis leading to quantification of sub surface parameters in depth in previous studies attention has been given to p wave data recorded in the marine environment using either acoustic or elastic inversion schemes in this paper we exploit both p waves and mode converted s waves in the marine environment in the inversion for both p and s wave velocities by using wide angle multi component ocean bottom cable seismic data an elastic waveform inversion scheme operating in the time domain was used allowing accurate modelling of the full wavefield including the elastic amplitude variation with offset response of reflected arrivals and mode converted events a series of one and two dimensional synthetic examples are presented demonstrating the ability to invert for and thereby to quantify both p and s wave velocities for different velocity models in particular for more realistic low velocity models including a typically soft seabed an effective strategy for inversion is proposed to exploit both p and mode converted ps waves whilst p wave events are exploited for inversion for p wave velocity examples show the contribution of both p and ps waves to the successful recovery of s wave velocity

reflection seismic investigations in the western part of the paleoproterozoic vhms bearing skellefte district northern sweden the skellefte district forms part of the svecofennian ca 1 90 to 1 80 ga supracrustal sequence and associated intrusive rocks in the northern part of sweden the western part of the skellefte district which is the most important metallogenic province in northern sweden today hosts major volcanic hosted massive sulfide vhms deposits e g the 23 million metric tons mt kristineberg cu zn pb ag au deposit in order to obtain a better understanding of the vhms ore potential at depth new seismic reflection data were acquired along two parallel and 25 km long profiles in the kristineberg area in 2003 the data were collected with the purpose of obtaining high resolution images of the top 10 km of the crust and are presented here for the first time although the structural setting is very complex the stacked sections reveal numerous reflections that can be correlated with surface geology visible on both profiles is a pronounced north dipping band of reflections marking a boundary between relatively transparent crust above and significantly more reflective crust beneath it we interpret this reflective crust to represent a structural basement to the ore bearing skellefte group possibly constituting bothnian basin metasedimentary rocks bordering the skellefte district to the south this new interpretation is important for the understanding of the tectonic evolution of the skellefte district and for defining exploration strategies in the area the seismic results suggest that the kristineberg and ravliden deposits occur on the northern limb of a kilometer scale local second order syncline within the hinge zone of a major antiform results from a profile located approximately 8 km to the west of the kristineberg mine indicate that the revsund granitoid has a thickness of about 3 to 3 5 km ultramafic rocks are also imaged clearly diffraction patterns and bright spot reflectivity is interpreted as originating from either mafic to ultramafic intrusions or a mineralization zone at 3 to 5 km depth these results help to identify new prospective areas and mineral potential both downplunge from known ores and on the same stratigraphic horizon on the southern limb of the ore bearing syncline the seismic reflection profiling has been effective in imaging the major structures around the kristineberg orebody demonstrating that this technique can be used for delineating complex structures significant for mineral exploration

atmospheric and depositional environments traced from unique chemical compositions of the snow over an inland high plateau antarctica surface snow samples from the inland high plateau east queen maud land antarctica show unique chemical characteristics they are characterized by high acidity accompanying the high electrical conductivity so that the hydrogen ion is the dominant ion coupling with chloride and nitrate ions the chemical composition differs sharply from sea salts these characteristics appear more clearly than previously reported in antarctica and are most strongly developed in snow samples from the high region the tritium content increases in the inland high area especially in the region higher than 3600 m asl above sea level which corresponds to the region where the effects of katabatic wind have vanished in the glaciological observations the highest tritium content occurs in the vertical profile at a snow pit 3761 m asl 77 00 s 35 00 e which corresponds to the snow deposition in 1966 the concentration is the highest ever reported in antarctica and is almost as high as the one observed in the precipitation in the inland region of north america which is the highest value ever reported in the world all the results suggest that most of the ions contained in snow samples from the inland high plateau especially higher than 3600 m asl are not brought directly through the troposphere from the sea around antarctica but from the higher atmosphere and that they are under the influence of the physicochemical reactions occurring there

imaging and interpreting the shallow crystalline crust abstract in the past many tests of high resolution seismic imaging methods utilizing off the shelf technology from hydrocarbon exploration have had only limited success some problems associated with the application of seismic profiling in the shallow crystalline crust differ from those encountered in sedimentary basins perhaps the most fundamental distinction stems from differences in statistical properties of the reflection coefficient time series analysis of power spectra derived from borehole data indicates that the reflection coefficients in the crystalline crust are depleted in the seismic bandwidth relative to sedimentary basins this fundamental difference must be considered in the design of shallow crustal seismic surveys in order to achieve success to a first approximation acoustic impedance variations in sedimentary basins are controlled by velocity changes whereas velocity and density can vary independently in the crystalline crust and changes in density may be relatively more important in controlling reflectivity in certain instances density fluctuations outweigh velocity fluctuations for example physical rock property data from mining districts in the canadian shield suggest that massive fe oxide or sulphide bodies will generate strong seismic reflections

a deep seismic sounding investigation of lithospheric heterogeneity and anisotropy beneath the iberian peninsula abstract the dimensions of the iberian peninsula the facility of firing large charges in the surrounding waters the well known and relatively uniform geology and prior knowledge of the crustal structure resulted in it being chosen as a study area for the investigation of the seismic structure of the lower lithosphere via refraction wide angle reflection seismic profiling the iberian lithosphere heterogeneity and anisotropy experiment iliha with a star shaped arrangement of six long range dss profiles was carried out in october 1989 the models derived from a first interpretation of the recorded data are presented three crustal profiles cover the same western and central part of the hercynian massif as the mantle profiles the resulting interpretations all include a middle as well as a lower crustal layer above the mantle the velocities of the layers in all three models are similar however the layer depths vary beneath the profiles interpretation of the mantle derived data suggests a layered lower lithosphere one reversed line and an intersecting unreversed line indicate the layering penetrates to at least 90 km depth the homogeneity of these layers contrasts strongly with the heterogeneous hercynian surface geology velocities derived from reflected data from the deep layers suggest the constituent materials are either anisotropic or that the layers suffer a slight regional dip

estudio sismico de la corteza iberica norte 3 3 a seismic image of the variscan crust in the hinterland of the nw iberian massif an offshore vertical incidence reflection seismic study with simultaneous on land wide angle recording has been conducted as part of the estudio sismico de la corteza iberica norte escin project

palaeoclimatological and chronological implications of the vostok core dust record the2 083 m vostok ice core recovered by the soviet antarctic expeditions has provided much information of climatic and environmental interest for a period covering a full glacial inter glacial cycle1 6 here we present and discuss the dust record obtained down to 2 202 m the final depth to which this core was extended in 1986 first we document the fact that major changes in aeolian deposits as recorded in the vostok core appear to be of global significance and confirm the existence of a link between high latitude aeolian deposits and the earth s orbital parameters7 8 second we propose atmospheric dust as a strati graphic marker to compare timing with other records of palaeocli mate and use the magnetic susceptibility profile measured along the rc11 120 indian ocean core9 for this purpose assuming that major dust events correspond to common aeolian inputs this approach indicates that the vostok and marine records were roughly in phase at the previous glacial interglacial transition

seasonal evolution of water contributions to discharge from a greenland outlet glacier insight from a new isotope mixing model the greenland ice sheet gris subglacial hydrological system may undergo a seasonal evolution with significant geophysical and biogeochemical implications we present results from a new isotope mixing model to quantify the relative contributions of surface snow glacial ice and delayed flow to the bulk meltwater discharge from a small 5k m 2 land terminating gris outlet glacier during melt onset may and at peak melt july we use radioactive 222 rn and stable isotopes 18 o deuterium to differentiate the water source contributions atmospherically derived 7 be further constrains meltwater transit time from the glacier surface to the ice margin we show that 1 222 rn is a promising tracer for glacial waters stored at the bed and 2 a quantitative chemical mixing model can be constructed by combining 222 rn and the stable water isotopes applying this model to the bulk subglacial outflow from our study area we find a constant delayed flow stored component from melt onset through peak melt this component is diluted first by snowmelt and then by increasing glacial ice melt as the season progresses results from this pilot study are consistent with the hypothesis that subglacial drainage beneath land terminating sections of the gris undergoes a seasonal evolution from a distributed to a channelized system

applying waveform inversion to wide angle seismic surveys abstract multi scale frequency domain acoustic waveform inversion was applied to refraction and wide angle reflection seismic surveys of the upper crust across the plate bounding san andreas fault california and the chesapeake bay impact structure virginia this paper presents and compares the data the preconditioning and different inversion strategies the inversions start at 3 hz and they are stopped at 15 hz due to increasing non linear artifacts the resulting waveform models show a significant improvement in resolution compared to the traveltime starting models but they do not achieve the resolution demonstrated in synthetic studies possible reasons for this limitation are discussed especially the possibility of insufficient bandwidth and the effect of normalizing amplitudes the comparison of results from normalized amplitude and true amplitude waveform inversions indicates that amplitudes do not strongly constrain the velocity structure phase coherency weights are presented that mitigate the impact of noise maps of phase misfits provide a quantitative overview of the data residuals and some insight into the spatial distribution of errors they are helpful for monitoring the progress of the inversion and for determining if and when it must be stopped

seismic imaging of complex onshore structures by 2d elastic frequency domain full waveform inversion quantitative imaging of the elastic properties of the subsurface at depth is essential for civil engineering applications and oil and gas reservoir characterization a realistic synthetic example provides for an assessment of the potential and limits of 2d elastic full waveform inversionfwiof wide aperture seismic data for recovering high resolution p and s wave velocity models of complex onshore structures fwi of land data is challengingbecauseoftheincreasednonlinearityintroducedbyfreesurface effects such as the propagation of surface waves in the heterogeneous near surface moreover the short wavelengths of the shear wavefield require an accurate s wave velocity starting modeliflowfrequenciesareunavailableinthedata weevaluated different multiscale strategies with the aim of mitigating the nonlinearities massively parallel full waveform inversion was implemented in the frequency domain the numerical optimization relies on a limited memory quasi newton algorithm that outperforms the more classic preconditioned conjugate gradient algorithm the forward problem is based upon a discontinuous galerkin dg method on triangular mesh which allows accuratemodelingoffree surfaceeffects sequentialinversionsofincreasingfrequenciesdefinethemostnaturallevelofhierarchyin multiscale imaging in the case of land data involving surface waves the regularization introduced by hierarchical frequency inversions is not enough for adequate convergence of the inversion a second level of hierarchy implemented with complexvalued frequencies is necessary and provides convergence of the inversion toward acceptable p and s wave velocity models amongthepossiblestrategiesforsamplingfrequenciesintheinversion successive inversions of slightly overlapping frequency groups is the most reliable when compared to the more standard sequential inversion of single frequencies this suggests that simultaneous inversion of multiple frequencies is critical when consideringcomplexwavephenomena

interception of atmospheric fluxes by arctic sea ice evidence from cosmogenic 7be 1 the natural cosmogenic radionuclide 7be t1 2 53 4 d is supplied to the surface ocean from the atmosphere and in the arctic ocean can be used as a tracer of the efficiency with which sea ice intercepts the atmospheric fluxes of chemical species and of the importance of ice as a transport mechanism for particulate matter and chemical species analyses of 7be in samples of surface water surface sea ice water beneath the ice sea ice sediments and precipitation from the eurasian basin of the arctic ocean show that the fraction of sea ice coverage determines the amount of 7be in the surface water when sea ice coverage is 80 the water column inventory drops to 58 20 bq m 2 the 7be inventory in sea ice is 39 23 bq m 2 and mass balance calculations show that sea ice can intercept 30 18 of the atmospheric flux of 7be during the studied period we suggest that other atmospherically transported contaminants should be similarly intercepted 7be in the ice also can be used to estimate that the annual transport and release of sediment to the ablation area of the fram strait is 500 g m 2 a value comparable to previously measured fluxes in sediment traps deployed in the area

effects of near surface waveguides on shallow high resolution seismic refraction and reflection data waveguides formed between the earth s free surface and sharp subsurface velocity discontinuities such as the watertable may have a significant impact on high resolution seismic refraction and reflection data energy trapped in such waveguides may propagate over long distances with little loss due to geometrical spreading the dispersive nature of the guided waves results in pronounced shingling of the first arrivals this narrows the optimum reflection window between the first arrivals and the ground roll and may lead to mispicking of first breaks and difficulties in identifying and processing shallow reflections

recent deposition of 210pb on the greenland ice sheet variations in space and time detailed 210pb profiles were determined for four chernobyl dated snowpits sampled during a wide ranging survey of the greenland ice sheet during the 1988 season the profiles from widely separated pits show little or no coherence even for two pits only 40 km apart the profiles differ in detail there does not appear to have been any seasonality in the deposition of 210pb onto the ice sheet in the two years since the chernobyl accident the total d eposition of 210pb during this period 10 20 bq m 2 was about 20 times less than has been observed at mid latitude sites in the eastern united states the three pits west of th e ice sheet divide recorded very similar depositional f1uxe s while the one eastern pit had only two thirds the a verage of the others suggesting a west to east gradient in the d eposition of 210pb and perhaps other continentall y derived s ubmicron aerosols onto the greenland ice sheet

importance of lateral transport processes to 210pb budget in the eastern chukchi sea during summer 2003 chinese ipy research program national natural science foundation of china 41125020 fujian natural science foundation 2009j06026 state oceanic administration of china 201105022 4 ministry of education of china

imaging of fracture zones in the finnsjon area central sweden using the seismic reflection method in 1987 the swedish nuclear fuel and waste management co skb funded the shooting of a 1 7 km long high resolution seismic profile over the finnsjon study site using a 60 channel acquisition system with a shotpoint and geophone spacing of 10 m the site is located about 140 km north of stockholm and the host rocks are mainly granodioritic the main objective of the profile was to image a known fracture zone with high hydraulic conductivity dipping gently to the west at depths of 100 to 400 m the initial processing of the data failed to image this fracture zone however a steeply dipping reflector was imaged indicating the field data were of adequate quality and that the problem lay in the processing these data have now been reprocessed and a clear image of the gently dipping zone has been obtained in addition several other reflectors were imaged in the reprocessed section both gently and steeply dipping ones correlations with borehole data indicate that the origin of these reflections are also fracture zones the improvement over the previous processing is caused mainly by 1 refraction statics 2 choice of frequency band 3 f k filtering and 4 velocity analyses in addition to reprocessing the data some further analyses were done including simulation of acquisition using only the near offset channels channels 1 30 and the far offset channels channels 31 60 and determining the damping factor q in the upper few hundred meters based upon the amplitude decay of the first arrivals the data acquisition simulation shows the far offset contribution to be significant even for shallow reflectors in this area contrary to what may be expected a q value of 10 determined from observed amplitude decay rates agrees well with theoretical ones assuming plane wave propagation in an attenuating medium

evidence for large scale rotations at spreading centers from the josephine ophiolite abstract repetition of the josephine ophiolite by large scale open folding and imbricate faulting provides an excellent opportunity to study the geometric relationships between sheeted dikes igneous layering and bedding measurements of over 600 dike margins from the sheeted dike complex and high level gabbro indicate that dikes are oriented at 20 40 to bedding in metasedimentary rocks overlying the ophiolite in addition igneous layering is discordant to gross lithologic layering and is oriented at a large angle to both sheeted dikes and bedding similar relationships may occur in several other ophiolites the inclined sheeted dikes and discordant igneous layering are best explained by a 45 rotation of new oceanic crust ophiolite immediately following formation at a spreading center rotation could have been accomplished by either subsidence or faulting further evidence for large scale rotations of oceanic crust include frequent anomalous magnetic inclinations in drill cores of layer 2 from the atlantic

variscan tectonics in the iberian pyrite belt south portuguese zone this paper aims to discuss the structural evolution of the iberian pyrite belt during the variscan orogeny it provides new structural data maps and cross sections from the eastern part of the iberian pyrite belt regional geology of the south portuguese zone and lithostratigraphy of the iberian pyrite belt are first briefly summarised three roughly homoaxial deformation phases are distinguished and are mainly characterised by south verging multi order folds axial planar cleavages and thrusts three structural units are distinguished the la puebla de guzman and valverde del camino antiforms are rooted units related to the propagation of southward directed thrust systems that may branch onto the lower decollement level of the south portuguese zone el cerro de andevalo is a structurally higher unit mainly composed of allochthonous d1 thrust nappes no evidence of sinistral transpression has been found in the transected cleavage and the strike of s3 with respect to s2 better evidence of transpression is the moderately to steeply westerly plunging folds that show s type asymmetry in down plunge view variscan deformation in the iberian pyrite belt is defined as the combination of a dominant southwards shear and a sinistral e shear caused by oblique continental collision between the south portuguese plate and the iberian massif

potential pitfalls of crooked line seismic reflection surveys during the last few years the geological survey of canada has pioneered the application of seismic reflection profiling to mineral exploration in close collaboration with canadian mining companies and with the lithoprobe project e g spencer et al 1993 milkereit et al 1994 because of the rugged terrain in crystalline rock environments dahle et al 1985 spencer et al 1993 vibroseis seismic surveys are frequently conducted along existing roads resulting in extremely crooked survey profiles crooked profiling geometry coupled with the complex nature of the geological targets pose special challenges for seismic data processing and interpretation many common midpoint seismic processing techniques are based on an implicit assumption of a straight line survey and are most effective with uniform fold and even offset distribution within common midpoint cmp gathers however with crooked line acquisition the cmp gathers are characterized by variable fold and uneven offset distribution based

seismic imaging of the enigmatic sudbury structure the sudbury structure on the southern margin of the superior craton was created by a catastrophic explosion in 1 85 ga and hosts one of the world s largest ni cu reserves as a unique terrestrial geological feature its genesis has been vigorously debated for more than a century in an effort to optimize the image from seismic reflection data acquired across the center of the sudbury structure we have developed a straight line common midpoint binning strategy and employed cross dip corrections these pseudo three dimensional seismic processing techniques coupled with standard processing methods have overcome limitations associated with conventional two dimensional seismic data processing and have substantially enhanced the seismic image warranting a more detailed and reliable structural interpretation the new image has revealed a major previously unrecognized zone of imbricated nw thrusts interpretation of these thrusts provides critical timing constraints relating the sudbury tectonic deformation to deposition of the sudbury basin sediments the onwatin argillites are penetrated by blind thrust faults whereas the overlying chelmsford turbidites are undeformed and most likely postdate the thrusting thus the uniform paleocurrent trends observed in the chelmsford reflect subsequent deformation history and may be dismissed as evidence against an impact origin based on our interpretation of the seismic image the original volume of the sudbury igneous complex likely an impact melt sheet is 1 104 km3 supporting the claim that the sudbury structure represents the eroded and tectonized remanent of one of the largest known impact structure 200 km on earth

seismic wide angle constraints on the crust of the southern urals a wide angle seismic reflection refraction data set was acquired during spring 1995 across the southern urals to characterize the lithosphere beneath this paleozoic orogen the wide angle reflectivity features a strong frequency dependence while the lower crustal reflectivity is in the range of 6 15 hz the pmp is characterized by frequencies below 6 hz after detailed frequency filtering the seismic phases constrain a new average p wave velocity crustal model that consists of an upper layer of 5 0 6 0 km s which correlates with the surface geology 5 7 km depths at which the velocities increase to 6 2 6 3 km s 10 30 km depths at which on average the crust is characterized by velocities of 6 6 km s and finally the lower crust from 30 35 km down to the moho which has velocities ranging from 6 8 to 7 4 km s two different s wave velocity models one for the n s and one for the e w were derived from the analysis of the horizontal component recordings crustal sections of poisson s ratio and anisotropy were calculated from the velocity models the poisson s ratio increases in the lower crust at both sides of the root zone a localized 2 3 anisotropy zone is imaged within the lower crust beneath the terranes east of the root this feature is supported by time differences in the sms phase and by the particle motion diagrams which reveal two polarized directions of motion velocities are higher in the central part of the orogen than for the siberian and eastern plates these seismic recordings support a 50 56 km crustal thickness beneath the central part of the orogen in contrast to moho depths of 45 km documented at the edges of the transect the lateral variation of the pmp phase in frequency content and in waveform can be taken as evidence of different genetic origins of the moho in the southern urals

tectonics and plate tectonics model for the variscan belt of europe abstract a plate tectonics model is presented to explain the tectonometamorphic characteristics of the european variscides after the closing of two oceanic domains by two sided subduction 500 420 ma and obduction 420 380 ma collision of the european and african continental plates occurred we propose that the subsequent complex intracontinental deformation 380 290 ma is the result of a double subduction of the continental lithosphere accompanied by crust mantle decollement this mechanism explains the progressive crustal thickening and migration of the deformation through time from the sutures toward the external parts of the variscan belt accounting for this model and for the relationships between the european variscides and the other paleozoic peri atlantic belts caledonides appalachian mauritanides and morocco we infer the relative positions of africa america and europe between the silurian and the permian

interpretation of the reflections in the siljan ring area based on results from the gravberg 1 borehole abstract during 1985 about 80 km of surface multichannel seismic reflection data were collected across the meteorite impact in the siljan ring area in central sweden the area consists mainly of granitic and gneissic rock ranging in age from 1400 to 2000 million years with remnants of palaeozoic sedimentary rocks preserved by downfaulting after a meteorite impact at 360 ma thereby forming a circular ring about 40 km in diameter dolerite intrusions ranging in age from 850 to 1700 million years are also present the seismic data revealed several high amplitude laterally continuous sub horizontal reflections in the northern part of the structure the high amplitude reflections and a possible intermediate low velocity zone were contributing factors in choosing the site for the gravberg 1 deep earth gas test well drilling and vertical seismic profiling vsp found that the reflectors were associated with dolerite sills which had intruded into the granite and which range in thickness from a few metres up to 60 m and with a pre impact area extent of at least 800 km 2 studies of amplitude and frequency versus offset afvo show the observations are compatible with a model of simple granite dolerite granite layering

geophysical evidence of a mantle derived intrusion in sw iberia 1 deep seismic reflection data acquired as part of the sw iberia europrobe project across the transpressional variscan orogen sample three tectonic terranes the south portuguese zone the ossa morena zone and the central iberian zone the seismic data reveal the existence of a mid crustal reflective body 140 km long and of variable thickness up to 5 km the iberian reflective body the conductivity image provided by coincident mt soundings the amplitude characteristics of the seismics mineralization studies related to magmatic ore deposits and the surface geology suggest that the irb is a mantle derived mafic intrusion the geophysical geological and petrological data suggest that the irb is most probably an early carboniferous approximately at 350 340 ma mantle derived intrusion possibly linked to plume activity that took place in europe in the carboniferous and permian

formation of uppermost oceanic crust the uppermost portion of oceanic crust extrusive basalts underlain and fed by diabasic dikes represents the chilled roof zone or lid to mid oceanic spreading center magma chambers this rigid lid forms and evolves through a steady state process of flexure subsidence and internal rotation all occurring within a couple of kilometers of the spreading axis the lid accommodates lateral spreading with dikes emplaced along a unique axis dikes tap underlying magma to feed overlying flows basalts accumulating along the axis of the lid load the lid which responds by flexing down and away from the intrusive axis this subsidence affects lid structure in the following ways emplaced dikes are tilted away from and basalt flows are tilted toward the intrusive axis lid subsidence and section rotation extend the base of the lid more rapidly at the axis than elsewhere consequently dikes preferentially intrude at the existing axis to maintain existing axis shape and relative position through continued spreading loading and flexure of the lid plus faster accretion at the base of the lid as a result of lid rotation cause the lid to be uplifted distal to the axis this creates flanking marginal highs and generates extension structures fissuring and faulting flanking and paralleling the loading extrusive axis the crustal structure and geometry produced by this process is compatible with structure and topography observed at present day mid oceanic spreading centers and with structure observed in ophiolites particularly the bay of islands ophiolite

potential coordinate mislocations in crosshole tomography results from the grimsel test site switzerland tomographic techniques based on borehole to borehole and tunnel to borehole traveltime data are now being employed in a wide range of studies associated with the exploration and exploitation of hydrocarbons and metallic minerals the disposal of chemical and radioactive waste diverse civil engineering projects and archaeology a fundamental assumption of currently employed tomographic inversion strategies is that the coordinates of the boreholes and tunnels containing the seismic sources and receivers are accurately known by inverting both synthetic and observed traveltime data we demonstrate that relatively minor coordinate errors 1 2 in the deeper parts of long boreholes 100 m may produce artifacts in the tomographic images that are comparable in extent and amplitude to true velocity anomalies to address this problem we introduce the coupled inverse method commonly used in earthquake studies as a means to determine simultaneously borehole coordinate adjustments and an estimate of the tomog

seismic reflection and borehole geophysical investigations in the matagami mining camp a high frequency vibroseis seismic survey was conducted by lithoprobe across a well known section of the matagami mining camp to assess the usefulness of seismic methods in lithological mapping the data have been reprocessed to enhance shallow reflections critical processing steps for the preservation of the shallow features include refraction static corrections cross dip corrections and first break mutes the seismic data were calibrated using geophysical logs from two boreholes adjacent to the profile physical rock property measurements suggest that strong reflectivity within the volcanic sequence occurs at rhyolite gabbro contacts the lower wabassee group a sequence of gabbros and basalts with interlayered rhyolite horizons has been identified as a seismic marker this reflective package overlays the horizon where most sulphide deposits are known to be located in the matagami mining camp seismic reflection methods can be used to map the deep volcanic stratigraphy and structures

the valhall field a geological overview abstract the valhall field is located within the central graben in the southern part of the norwegian north sea the discovery well 2 8 6 was drilled in 1975 and the field went on production in 1982 production is currently 55 000 barrels of oil per day the valhall field structure trends nw se and has an areal closure in excess of 60 000 acres the hydrocarbon column measures 200 m the structure is located on an elongate anticlinal feature called the lindesnes ridge which developed during late cenomanian to oligocene tectonic movements the geophysical evaluation of the field is complicated by the existence of a gas charged tertiary section which obscures the deeper seismic reflections vertical seismic profiles vsps have been utilized in an attempt to resolve the reservoir configuration reservoirs occur both in the tor formation maastrichtian and in the underlying hod formation turonian coniacian the tor formation is represented by a largely allochthonous chalk facies which thins regionally to the southwest a crestal graben is developed in which the tor formation is locally thick autochthonous chalks are characteristic of the hod formation primary porosity of the chalk varies between 36 to 50 on the crest of structure preservation of such high porosity is primarily due to formation overpressure inhibiting mechanical compaction of the chalk the field contains paraffinic oil of 36 api gravity it originated from the upper jurassic kimmeridge clay formation which is at peak oil generating capacity under the field at present day

high resolution quantitative seismic imaging of a strike slip fault with small vertical offset in clay rocks from underground galleries experimental platform of tournemire france imaging tectonic faults with small vertical offsets in argilittes clay rock using geophysical methods is challenging in the context of deep radioactive waste disposals the presence of such faults has to be assessed since they can modify the rock confining properties in the tournemire experimental platform tep france fault zones with small vertical offsets and complex shape have been identified from underground works however 3d high resolution surface seismic methods have shown limitations in this context that led us to consider the detection and characterization of the faults directly from underground works we investigate here the potential of seismic full waveform inversion fwi applied in a transmission configuration to image the clay rock medium in a horizontal plane between galleries and compare it with first arrival traveltime tomography fatt our objective is to characterize seismic velocities of a block of argilittes crossed by a subvertical fault zone with a small vertical offset the specific measurement configuration allows us to neglect the influence of the galleries on the wave propagation and to simplify the problem by considering a 2d isotropic horizontal imaging domain our fwi scheme relies on a robust adaptation of early arrival waveform tomography the results obtained with fatt and fwi are in accordance and both correlate with the geological observations from the gallery walls and boreholes we show that even though various simplifications are done in the inversion scheme and only a part of the data is used fwi allows to get higher resolution images than fatt and is especially less sensitive to the incomplete illumination as it uses also diffracted energy the results provided in this study highlight the complexity of the fault zone showing a complex interaction of the main fault system with a secondary system composed of decimetric fractures associated with the presence of water

seismic monitoring of diffraction images for detection of local heterogeneities diffracted waves contain valuable information regarding both the structure and composition of the media they are in in seismic data processing however these waves are usually regarded as noise in this paper we present an attempt to use scattered diffracted waves for the detection of local heterogeneities the method is based on the detection of diffracted waves by concentrating the signal amplitudes from diffracting points on the seismic section this is done using a correlation procedure that enhances the amplitude of the seismic signal at the location of the diffractors on the common diffraction point section d section the new local time correction for diffraction traveltime curve parameterization is based on the radius of curvature of the diffracted wavefront and near surface velocity we use the idea of seismic monitoring for detection and delineating local objects which may occur within the subsurface resulting from human activity or fast geological processes the method consists of continuou

ultrahigh resolution seismic reflection imaging of the alpine fault new zealand 1 high resolution seismic reflection surveys across active fault zones are capable of supplying key structural information required for assessments of seismic hazard and risk we have recorded a 360 m long ultrahigh resolution seismic reflection profile across the alpine fault in new zealand the alpine fault a continental transform that juxtaposes major tectonic plates is capable of generating large m 7 8 damaging earthquakes our seismic profile across a northern section of the fault targets fault zone structures in holocene to late pleistocene sediments and underlying triassic and paleozoic basement units from 3 5 to 150 m depth since ultrashallow seismic data are strongly influenced by near surface heterogeneity and source generated noise an innovative processing sequence and nonstandard processing parameters are required to produce detailed information on the complex alluvial glaciofluvial and glaciolacustrine sediments and shallow to steep dipping fault related features we present high quality images of structures and deformation within the fault zone that extend and complement interpretations based on shallow paleoseismic and ground penetrating radar studies our images demonstrate that the alpine fault dips 75 80 to the southeast through the quaternary sediments and there is evidence that it continues to dip steeply between the shallow basement units we interpret characteristic curved basement surfaces on either side of the alpine fault and deformation in the footwall as consequences of normal drag generated by the reverse slip components of displacement on the fault the fault dip and apparent 35 m vertical offset of the late pleistocene erosional basement surface across the alpine fault yield a provisional dip slip rate of 2 0 0 6 mm yr the more significant dextral slip rate cannot be determined from our seismic profile

exploring some issues in acoustic full waveform inversion the least squares error measures the difference between observed and modelled seismic data because it suffers from local minima a good initial velocity model is required to avoid convergence to the wrong model when using a gradient based minimization method if a data set mainly contains reflection events it is difficult to update the velocity model with the least squares error because the minimization method easily ends up in the nearest local minimum without ever reaching the global minimum several authors observed that the model could be updated by diving waves requiring a wide angle or large offset data set this full waveform tomography is limited to a maximum depth here we use a linear velocity model to obtain estimates for the maximum depth in addition we investigate how frequencies should be selected if the seismic data are modelled in the frequency domain in the presence of noise the condition to avoid local minima requires more frequencies than needed for sufficient spectral coverage we also considered acoustic inversion of a synthetic marine data set created by an elastic time domain finite difference code this allowed us to validate the estimates made for the linear velocity model the acoustic approximation leads to a number of problems when using long offset data nevertheless we obtained reasonable results the use of a variable density in the acoustic inversion helped to match the data at the expense of accuracy in the inversion result for the density

toward a robust workflow for deep crustal imaging by fwi of obs data the eastern nankai trough revisited crustal scale imaging by the full waveform inversion fwi of long offset seismic data is inherently difficult because the large number of wavelengths propagating through the crust makes the inversion prone to cycle skipping therefore efficient crustal scale fwi requires an accurate starting model and a stable workflow minimizing the nonlinearity of the inversion here we attempt to reprocess a challenging 2d ocean bottom seismometer obs dataset from the eastern nankai trough the starting model is built by first arrival traveltime tomography fat which is fwi assisted for tracking cycle skipping we iteratively refine the picked traveltimes and then reiterate the fat until the traveltime residuals remain below the cycle skipping limit subsequently we apply laplace fourier fwi in which progressive relaxation of time damping is nested within frequency continuation to hierarchically inject more data into the inversion these two multiscale levels are complemented by a layer stripping approach implemented through offset continuation the reliability of the fwi velocity model is assessed by means of source wavelet estimation synthetic seismogram modeling ray tracing modeling dynamic warping and checkerboard tests although the visco acoustic approximation is used for wave modeling the synthetic seismograms reproduce most of the complexity of the data with a high traveltime accuracy the revised fwi scheme produces a high resolution velocity model of the entire crust that can be jointly interpreted with migrated images derived from multichannel seismic data this study opens a new perspective on the design of obs crustal scale experiments amenable to fwi however a further assessment of the optimal obs spacing is required for reliable fwi

application of waveform tomography to marine seismic reflection data from the queen charlotte basin of western canada to obtain a higher resolution quantitative p wave velocity model 2d waveform tomography was applied to seismic reflection data from the queen charlotte sedimentary basin off the west coast of canada the forward modeling and inversion were implemented in the frequency domain using the visco acoustic wave equation field data preconditioning consisted of f k filtering 2d amplitude scaling shot to shot amplitude balancing and time windowing the field data were inverted between 7 and 13 66 hz with attenuation introduced for frequencies 10 5 hz to improve the final velocity model two different approaches to sampling the frequencies were evaluated the limited maximum offset of the marine data 3770 m and the relatively high starting frequency 7 hz were the main challenges encountered during the inversion an inversion strategy that successively recovered shallow to deep structures was designed to mitigate these issues the inclusion of later arrivals in the waveform tomography resulted in a veloci

the arctic tropopause fold abstract analyses of research aircraft observation satellite total columnar ozone retrievals and synoptic upper air soundings are used to describe the structure of arctic jetstreams and their associated frontal zones and tropopause folds these analyses document the presence of major tropopause folding events within the arctic that occur at the flanks of large scale 2000 km polar vortices one example shows a polar vortex and its associated tropopause fold and arctic front that migrated from the high canadian arctic southward into midlatitudes over central north america the frigid cold air outbreak associated with this migration was an important component in the record setting daily minimum temperatures that were recorded from the great lakes to southern florida total columnar ozone measurements from the total ozone mapping spectrometer toms are shown to identify the location of polar vortices and the mesoscale 200 km ozone gradients at the flanks of these vortices which coincide with regions o

receiver coupling effects in seismic waveform inversions seismic waveform inversion offers opportunities for detailed characterization of the subsurface however its full potential can only be exploited when any systematic source and receiver effects are either carefully avoided or appropriately accounted for during the inversions repeated crosshole measurements in the mont terri switzerland underground laboratory have revealed that receiver coupling may significantly affect the seismic waveforms more seriously coupling conditions may vary during the course of a monitoring experiment to address this problem we have developed a novel scheme that estimates medium properties frequency dependent source functions and frequency dependent receiver coupling factors we demonstrate the efficacy of the new scheme via a synthetic 2d crosshole experiment in which realistic receiver coupling factors are incorporated because determination of medium parameters and estimation of source functions and receiver coupling factors are largely separated the method can be

application of diffraction tomography to fracture detection two diffraction tomography techniques are applied to crosshole field data to detect fractures in granitic rock the techniques used are the conventional back propagation method and a new quadratic programming method incorporating constraints in this formulation the born approximation is used for linearization of the inverse problem two dimensional 2 d pseudo spectral finite difference synthetic data are generated to demonstrate the inversion methods and justify use of the born approximation also using 2 d born synthetic data the velocity sensitivity of the inversion algorithm and reduction of fracture generated tube waves and s waves are investigated the inversion methods are applied to field data from the grimsel test site in switzerland the data are collected from a 10 21 5m rectangular area where fractures are known to exist data acquisition with 0 5 m spacing of three component receivers and a piezoelectric source is carried out so as to obtain a nearly complete coverage of the region cros

structure of the sheeted dike complex of the samail ophiolite near ibra oman the sheeted dike complex is a regionally mappable geologic unit with a consistent position in the samail ophiolite stratigraphy discontinuous exposures of the complex in the ibra area of the southeast oman mountains provide structural data that allow reconstruction of a paleo spreading ridge axis of 347 for the region and suggests a maximum spreading width of 275 km for the ophiolite contact relations within the well exposed al ahmadi section indicate that the complex was formed by multiple parallel intrusion over a narrow zone at the paleo ridge axis split dikes are abundant facing directions indicate a spreading ridge southwest of the present outcrop area in conflict with other spreading axis direction indicators

winagami reflection sequence seismic evidence for postcollisional magmatism in the proterozoic of western canada a recent lithoprobe seismic reflection transect in the peace river arch region of the western canada sedimentary basin traverses continental crust that was assembled in a series of magmatic and collisional events during the interval 2 0 1 9 ga the seismic data reveal panels of dipping reflectivity at tectonic domain boundaries but they are otherwise dominated by prominent subhorizontal reflections that occur between 3 5 and 18 5 km depth over an area of at least 120 000 km 2 these reflections here named the winagami reflection sequence overlap several tectonic domains and are comparable in geometry depth and areal distribution to sills and discordant intrusive sheets imaged by previous seismic profiling in the baltic shield and the basin and range province of the western united states as many as five distinct reflections are present some of which can be traced for more than 100 km contrasting reflection character in adjacent tectonic domains suggests an intrusive geometry controlled partly by the mechanical properties of the host rocks seismic modeling indicates that observed waveforms are consistent with reflections from tabular mafic bodies 70 m thick the winagami reflection sequence crosscuts the collisional fabric ca 1890 ma but the sequence is offset by a brittle fault with an estimated age of 1760 ma these timing constraints suggest that the winagami reflection sequence is a previously unknown component of a regional magmatic event emplaced during a period of brittle indentation of the western canadian shield by the slave province

appraisal of waveform repeatability for crosshole and hole to tunnel seismic monitoring of radioactive waste repositories countries worldwide are seeking solutions for the permanent removal of high level radioactive waste from the environment surrounding the waste with multiple engineered barriers and emplacement in deep geological repositories is widely accepted as a safe means of isolating it from the biosphere for the necessary 105 106 years as a precautionary measure society demands that repositories be monitored for 100 300 years after they are backfilled and sealed effective monitoring that does not compromise the engineered and natural barriers is challenging to address this issue we investigate the viability of crosshole and hole to tunnel seismic methods for remotely monitoring high level radioactive waste repositories measurements are made at two underground rock laboratories in switzerland one within granitic rock and one within clay rich sediments numerical simulations demonstrate that temporal changes of the monitored features i e bentonite plug excavation damage zone sand filled microtunnel sh

the abrupt termination of the younger dryas climate event previous studies on two deep greenland ice cores have shown that a long series of climate oscillations characterized the late weichselian glaciation in the north atlantic region1 and that the last glacial cold period the younger dryas ended abruptly 10 700 years ago2 here we further focus on this epoch defining event and present detailed heavy isotope and dust concentration profiles which suggest that in less than 20 years the climate in the north atlantic region turned into a milder and less stormy regime as a consequence of a rapid retreat of the sea ice cover a warming of 7 c in south greenland was completed in about 50 years

magnetotelluric survey of the electrical conductivity of the crust across the ossa morena zone and south portuguese zone suture abstract the transition between the south portuguese zone spz and the ossa morena zone omz is made up of a major geosuture which is indicated by the beja acebuches ophiolite several geological data suggest that this suture is the result of an oblique collision with a northward propagation between the spz and omz from the structural point of view the omz is much more complex than the adjacent central iberian zone and the spz the frequently complicated internal structure is the result of superposition of various structural elements developed during different tectonic events with the aim of providing new constraints to this complex deep structure a magnetotelluric mt survey was designed by the universities of lisbon and barcelona in the scope of the bilateral cooperation and the europrobe programs a first nne ssw profile across the spz and omz was carried out in september 1997 the profile is 40 km long and consists of nine deep mt soundings with periods ranging from 0 0039 to 4000 s the time series were processed using a robust algorithm after visual inspection the study of the dimensionality and directionality of the regional electrical structures is based on the analysis of the induction arrows and the groom bailey decomposition a strike of n125e was determined in accordance with regional geological features 2 d inversion was undertaken on both te and tm modes using the rri method the main feature of the model is a large conductive body

subhorizontal fractures in a granitic pluton their detection and implications for radioactive waste disposal several countries are actively investigating the feasibility of using crystalline rock bodies for the ultimate disposal of radioactive nuclear waste as part of the concept assessment phase in the canadian nuclear fuel waste management program a multidisciplinary research investigation is being conducted across the lac du bonnet batholith in southeastern manitoba no radioactive materials other than sealed sources or tracers are to be emplaced within this rock body the results of a high resolution seismic reflection survey together with information from a number of deep boreholes demonstrates that major subhorizontal fractures occur at depths of up to 800 m within the batholith an integrated interpretation of the seismic data with selected borehole logs shows that warm water is flowing up the major fractures from depth a natural corollary of these results is that a three dimensional seismic reflection survey combined with an appropriate exploratory drilling program has the potential for delineatin

seismic crustal structure in the southwest of the iberian peninsula and the gulf of cadiz abstract the crust under the southwestern iberian peninsula and the gulf of cadiz has been sampled by 1200 km of deep seismic refraction wide angle reflection profiles together with many seismic reflection lines and bore holes wide angle seismic data were collected during the last three decades commercial multichannel data provide a detailed image of the uppermost crust improving the confidence about the models of the deeper structures p wave velocities within the thick column of sediments in the gulf of cadiz range from 2 0 to 3 8 km s while the algarve and the sines areas have higher velocities of 4 3 to 4 8 km s the top of the palaeozoic basement rises to the northwest outcropping in the south portuguese zone of the iberian massif and is characterized by p wave velocities of 5 7 5 9 km s high velocities of 6 4 km s have been found at shallow depths of 7 to 10 km in the south portuguese zone that could be related to the mafic and ultramafic rocks in the beja acebuches zone lower crustal velocities are in the range of 6 7 6 9 km s the crustal thickness shows important lateral changes from 29 km beneath the guadalquivir basin iberian massif contact to 35 km in the southeastern part of the south portuguese zone from the interpretation of these seismic data a geodynamic model of the evolution of the crust in southwestern iberia and the gulf of cadiz is proposed the guadalquivir basin and its continuation at sea the gulf of cadiz is a flexure area of the crust that could be related to the overloading due to the overthrusting of the alboran domain over the iberian plate

effects of the shallow subsurface on upper crustal seismic reflection images abstract seismic imaging of the upper crust is strongly affected by the interactions of the seismic wavefield with the shallow subsurface in order to better understand these effects we have evaluated the seismic responses of a suite of canonical models of the upper crystalline crust and the near surface region our modelling is restricted to two dimensions but otherwise includes the full seismic wavefield notably the effects of attenuation and topographic variations along the free surface we find that s wave scattering is important in the upper crust and contributes significantly to the vertical component of the seismic reflection response the backscattered wavefield undergoes mode conversions when it interacts with the free surface even moderate topographic variations or velocity variations in the near surface region enhance these mode conversions and cause additional scattering much of the earth s surface is covered by thin layers of unconsolidated material or weathered bedrock with high attenuation and high velocity contrasts at the layer boundaries scattered and mode converted seismic energy gets trapped in these layers and thus interacts repeatedly with the free surface and its topography in analogy to large scale seismic resonance effects of sedimentary valley fills the level and duration of source generated noise depends heavily on the degree of attenuation in the shallow layers low attenuation causes high noise levels and vice versa in contrast the level of source generated noise is less sensitive to near surface layer thicknesses and velocity contrasts

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

validity of the acoustic approximation in full waveform seismic crosshole tomography abstractacoustic waveform inversions are an increasingly popular tool for extracting subsurface information from seismic data they are computationally much more efficient than elastic inversions naturally an inherent disadvantage is that any elastic effects present in the recorded data are ignored in acoustic inversions we investigate the extent to which elastic effects influence seismic crosshole data our numerical modeling studies reveal that in the presence of high contrast interfaces at which p to s conversions occur elastic effects can dominate the seismic sections even for experiments involving pressure sources and pressure receivers comparisons of waveform inversion results using a purely acoustic algorithm on synthetic data that is either acoustic or elastic show that subsurface models comprising small low to medium contrast 30 structures can be successfully resolved in the acoustic approximation however in the presence of extended high contrast anomalous bodies p to s conversions

electromagnetic imaging of variscan crustal structures in sw iberia the role of interconnected graphite abstract the western part of the iberian peninsula iberian massif is the best exposed fragment of the variscan orogen in europe its southern half was generated by an oblique collision between three continental terranes belonging to the margins of laurassia avalonia the south portuguese zone spz and gondwana the ossa morena zone omz and the central iberian zone ciz the boundaries between them are considered to be suture zones a 200 km long magnetotelluric profile across the three variscan terranes was done in a nne direction approximately perpendicular to the main tectonic features the results of two dimensional inversion of the mt dataset reveal high conductivity zones coinciding with the transitions spz omz and omz ciz these conductive bodies related to the sutures at depth were interpreted as graphite enrichments along shear planes formed due to the overall transpressive regime a high conductivity layer extending along the whole omz was found at a depth of 15 25 km the top of which spatially correlates with a broad reflector detected by a recently acquired deep seismic reflection profile the high conductivity was interpreted as caused by the precambrian serie negra graphite rich rocks carbon and oxygen x ray mapping with electron microprobe on polished sections of serie negra samples from omz revealed the presence of interconnected graphite which supports the hypothesis that graphite is determinant for the high conductivity two graphite types which help to record the geological evolution were identified graphite accumulations in the schistosity surfaces produced by folding and metamorphism and metallic films of graphite developed along late faults the conductive layer shows blobs of higher conductivity suggesting macro anisotropy additional mylonitisation and shearing produced by thrusting at depth can be the origin of these zones of enhanced conductivity given that the detachment level is located within the serie negra several high resistivity features were found in the upper crust related to devonian and carboniferous successions and probably to some unexposed plutons in the spz and the palaeozoic series of omz plus some granitic intrusions in the ciz a high resistivity zone extending to the whole crust is correlated with extensive late variscan granite intrusions

seismic waveform inversion in the frequency domain part 2 fault delineation in sediments using crosshole data a crosshole experiment was carried out in a layered sedimentary environment in which a normal fault is known to cut through the section initial traveltime inversions produced stable but low resolution images from which the fault could be only vaguely inferred to image the fault wavefield inversion was used to produce a velocity model consistent with the detailed phase and amplitude of the data at a number of frequencies our wavefield inversion scheme uses a classical descent type algorithm for decreasing the data misfit by iteratively computing the gradient of this misfit by repeated forward and backward propagations our propagator is a full wave equation frequency domain acoustic finite difference method the use of the frequency space domain yields computational advantages for multisource data and allows an easy incorporation of viscous effects by running wavefield inversion on the field data a quantitative velocity image was produced that yielded a significantly improved image of the fault when compared with the original traveltime inversions because the original field data were noisy and contained a high degree of multiple scattering from the layering of the sediments the transmitted arrivals were selectively windowed to enhance the image the sediments at the site were strongly attenuating we therefore used a viscoacoustic model during the modeling and the inversion that correctly simulated the observed decrease in amplitude with increasing frequency and source receiver offset furthermore since the traveltime inversion indicated a high degree of anisotropy at the site a fixed homogeneous level of anisotropy was used during the inversion tests at varying levels of anisotropy confirmed the improvement in image quality and in data fit when anisotropy was incorporated the final image was verified by examining the distribution of the residuals in the frequency domain by comparing time domain modeled wavefields with the observed data and by direct comparison with borehole logs

beryllium 7 and lead 210 in the atmosphere and surface snow over the greenland ice sheet in the summer of 1989 the concentrations of 7be and 210pb were measured in surface air and fresh and aging snow samples from summit 72 20 n 38 45 w and dye 3 65 10 n 44 45 w greenland during june and july 1989 the aerosol concentrations of these radionuclides showed rapid variations at both sites but were nearly twice as high on average at summit concentrations in the 16 fresh snowfall events that were sampled also showed wide variability but the averages were the same at the two sites the apparent difference in air snow fractionation and the lack of coherence in the concentration in air time series between the two sites indicate previously unsuspected complexity in atmospheric dynamics over the ice sheet improved understanding of atmospheric processes and how the results of those processes are recorded in snow and ice is crucial for full interpretation of the information about past atmospheric chemistry and climate contained in the snow and ice of glaciers around the world

210pb as a tracer of shelf basin transport and sediment focusing in the chukchi sea abstract activities of dissolved particulate and sedimentary 210pb were measured in the shelf slope region of the chukchi sea samples were collected as part of the shelf basin interactions sbi phase ii process study 6 may 15 june 2002 along three shelf basin transects identified as west hanna shoal east hanna shoal and barrow canyon distributions of 210pb and suspended particulate matter indicate efficient removal of 210pb over the shelf by particle scavenging low 210pb activities measured throughout the halocline of the canada basin are attributed to shelf scavenging and subsequent advective transport into the interior basin additionally 210pb inventories were used to construct a water column sediment budget of 210pb and determine regions of particle export and deposition on the continental shelf and slope sediment focusing calculated with this 210pb budget was observed throughout the shelf slope region particularly in shallow 100 m shelf waters at barrow canyon despite elevated concentrations of suspended particulate matter in barrow canyon the 210pb budget does not indicate that sediment transport occurred from the west and east hanna shoals into barrow canyon

towards an improved seismic imaging technique for crustal structures the lithoprobe sudbury experiment under the canadian lithoprobe program conventional low 10 55 hz and high frequency 30 140 hz vibroseis reflection surveys were conducted across the sudbury structure the main objective of the study was to evaluate the performance of seismic exploration techniques in a complex mainly crystalline geological setting the sudbury experiment demonstrated that high input frequencies can be preserved through appropriate acquisition and processing procedures consequently the high frequency seismic data proved capable of imaging complex shallow crustal structures not seen by the conventional low frequency seismic survey in crystalline environments improved high frequency seismic exploration technology will provide a better link between seismic images and surface geology

seismic reflection imaging over the south portuguese zone fold and thrust belt sw iberia 1 we reprocessed an 35 km long part of the ibesreis seismic reflection profile which runs over the iberian pyrite belt section of the south portuguese zone sw iberia with the goal to image the upper crust 15 km depth the applied processing sequence enhanced numerous prominent reflections and diffraction patterns within the uppermost 5 s travel time relative to high amplitude source generated noise a complex subsurface characterized by conflicting dips and a survey following winding roads require a crooked line prestack migration scheme for coherent imaging to interpret sources of diffracted energy we additionally employed a diffraction imaging scheme which enhances diffractions at the expense of reflections the final seismic images show south vergent imbricate fold and thrust tectonics documenting the contractive deformation that the south portuguese zone experienced during the variscan orogeny based on surface geological information we correlate a low reflective unit with the shallow upper carboniferous flysch group a highly reflective unit ranging in depth from 2 to 4 km with the middle carboniferous volcano sedimentary complex group which hosts massive sulfide deposits and a moderately reflective unit with the upper devonian phyllite quartzite group below these units another low reflective facies is present which may represent older paleozoic metasediments in addition the seismic and diffraction images reveal bands of high reflectivity and distinct diffraction patterns that were interpreted as extensive layered mafic intrusions these proposed mafic bodies may be related to the same event that triggered a huge hydrothermal activity assumed in early carbonifereous times

shallow 3d seismic reflection imaging of fracture zones in crystalline rock a limited 3d seismic reflection data set was used to map fracture zones in crystalline rock for a nuclear waste disposal site study seismic reflection data simultaneously recorded along two roughly perpendicular profiles 1850 and 1060 m long and with a 400 x 400 m receiver array centered at the intersection of the lines sampled a 680 x 960 m 2 area in three dimensions high levels of source generated noise required a processing sequence involving surface consistent deconvolution which effectively increased the strength of reflected signals and a linear tau p filtering scheme to suppress any remaining direct sv wave energy a flexible binning scheme significantly balanced and increased the cmp fold but the offset and azimuth distributions remain irregular a wide azimuth range and offsets 800 m are only found at the edges of the site three dimensional dip moveout and 3d poststack migration were necessary to image events with conflicting dips up to about 40 degrees despite the irregular acquisition geometry and the high level of source generated noise we obtained images rich in structural detail seven continuous to semicontinuous reflection events were traced through the final data volume to a maximum depth of around 750 m previous 2d seismic reflection studies and borehole data indicate that fracture zones are the most likely cause of the reflections

the effects of out of plane seismic energy on reflections in crustal scale 2d seismic sections abstract crustal scale seismic surveys mostly collect data along single profiles and the data processing has an underlying assumption that the data have imaged two dimension 2d structure striking at right angles to the seismic profile however even small amounts of out of plane topography on a reflector can result in reflections that do not map the reflector shape accurately out of plane energy will migrate within the plane of the section to an apparent depth represented as two way time twt that is greater than the depth of the reflection point out of the plane of the section it will fall within the plane of the section at depths less than equal to or greater than the intersection of the reflector with the plane of the section depending on both the amount of out of plane topographic relief on the reflector and the offset of the topographic relief from the plane of the section reflectors that are a single surface can therefore be manifested in the seismic section as a band of several reflections rather than a single reflection more complex reflectors such as shear zones that have a finite thickness because they are made up of several to many anastomosing layers of altered and anisotropic rock embedded in protolith will appear as laterally short reflections within a laterally continuous reflection band other examples of such reflectors would be the moho in some places and rock with compositional layering with increasing out of plane topographic relief on the reflector the top of the reflection band for both single and multi layer reflectors will be a poor indicator of the top of the reflector in the earth the bottom of the reflection band will always be a poor indicator of the bottom of the reflector because out of plane energy can arrive at twts that are different from those of the reflector in the plane of the section out of plane energy has the potential to interfere constructively or destructively with the in plane energy in synthetic data calculated for a simple model assuming one layer and topographic relief of 250 m over wavelengths of 4 5 km similar to that imaged in a real sub horizontal detachment amplitudes ranged up to 2 6 times the expected amplitude for the layer a model with anastomosing layers built to resemble a thick shear zone rather than a discrete fault surface allowed tuning between layers the effects of out of plane energy when combined with the effects of tuning caused amplitudes up to 3 1 times those expected larger amplitudes could be achieved if a suitable model was contrived the results indicate that care must be taken when calculating impedance contrasts using real data the highest amplitude reflections are likely to yield overestimates of the true impedance contrast

tracer studies of pathways and rates of meltwater transport through arctic summer sea ice 1 at the surface heat budget of the arctic ocean sheba program s field site in the northern chukchi sea snow and ice meltwater flow was found to have a strong impact on the heat and mass balance of sea ice during the summer of 1998 pathways and rates of meltwater transport were derived from tracer studies h218o 7be and release of fluorescent dyes complemented by in situ sea ice permeability measurements it was shown that the balance between meltwater supply at the surface averaging between 3 5 and 10 5 mm d 1 and ice permeability between 10 9 m2 determines the retention and pooling of meltwater which in turn controls ice albedo we found that the seasonal evolution of first year and multiyear ice permeability and surface morphology determine four distinct stages of melt at the start of the ablation season stage 1 ponding is widespread and lateral melt flow dominates several tens of cubic meters of meltwater per day were found to drain hundreds to thousands of square meters of ice through flaws and permeable zones significant formation of underwater ice composed between 50 of meteoric water formed at these drainage sites complete removal of snow cover increase in ice permeability and reductions in hydraulic gradients driving fluid flow mark stage 2 concurrent with a reduction in pond coverage and albedo during stage 3 maximum permeabilities were measured with surface meltwater penetrating to 1 m depth in the ice and convective overturning and desalination found to dominate the lower layers of first year and thin multiyear ice enhanced fluid flow into flaws and permeable zones was observed to promote ice floe breakup and disintegration concurrent with increases in pond salinities and 7be advective heat flows of several tens of watts per square meter were derived promoting widening of ponds and increases in pond coverage stage 4 corresponds to freeze up roughly 40 of the total surface melt was retained by the ice cover within the ice matrix as well as in surface and under ice ponds with a total net retention of 15 based on this work areas of improvement for fully prognostic simulations of ice albedo are identified calling for parameterizations of sea ice permeability and the integration of ice topography and refined ablation schemes into atmosphere ice ocean models

electromagnetic imaging of a transpressional tectonics in sw iberia forty one magnetotelluric soundings were carried out along a 200 km long profile approximately nne ssw across the three major geotectonic units in sw iberia a model obtained from two dimensional inversion of the magnetotelluric data set reveals high conductivity zones in the middle lower crust 10 30 km two of these zones correspond to the transition between the main geotectonic units one between the south portuguese zone and the ossa morena zone interpreted as having been caused by metasediments and the other between the ossa morena zone and the central iberia zone associated with a shear zone and metasediments another high conductivity anomaly related to black shales with major graphite impregnation was detected within the ossa morena zone the resistive features located preferentially in the upper crust 1 10 km coincide with gabbroic and granitic complexes

gently dipping fracture zones in paleoproterozoic metagranite sweden evidence from reflection seismic and cored borehole data and implications for the disposal of nuclear waste 1 the svecokarelian orogen in the coastal area north of stockholm central sweden is characterized by a complex deformation belt in this belt bedrock domains strike wnw nw dip steeply and are affected by high ductile strain these domains anastomose around tectonic lenses that are up to a few tens of kilometers in length and are a few kilometers wide the bedrock within the lenses is folded and in general affected by lower ductile strain site investigations for the disposal of highly radioactive nuclear waste are presently in progress within one of these lenses approximately 16 km of high resolution 10 m source and receiver spacing 100 channels seismic data were acquired in 2002 along five separate profiles 2 to 5 km in length within this lens in the southeast reflection orientations are well determined where profiles cross one another here there is a nearly one to one correlation of distinct reflections on the seismic sections with hydraulically conductive fracture zones in cored boreholes indicating that these fracture zones control the reflectivity however smaller lenses of amphibolite within the fracture zones may enhance the reflectivity the reflections dip gently to the sse and se and the zones in the boreholes are dominated by fractures that show the same dip direction or are subhorizontal a working hypothesis for the gently dipping fracture zones suggests that these structures formed as minor thrust faults during the later stages of the svecokarelian orogeny mineral fillings and coatings along fractures in these zones indicate later reactivation the current stress field a high intensity of fractures in the zones at a high angle to 3 high in situ stresses in the bedrock and rapid glacial unloading with the development of high differential stresses may explain the large volume of groundwater in these zones the reflection seismic data in combination with the verifying work presented in the cored borehole data have steered the subsequent focus in geoscientific studies to the northwestern part of the candidate area

tectonic rotations in extensional regimes and their paleomagnetic consequences for oceanic basalts paleomagnetic samples of many deep sea drilling project dsdp basalts have shallower inclinations than would be inferred from the paleolatitude at which the basalts were extruded at mid ocean ridges rather than arising from peculiarities of the geomagnetic field the anomalous magnetic directions may result from tectonic rotations along the listric normal faults which often characterize extensional tectonic regimes several lines of evidence support this conclusion observations of the basin and range province an extensional tectonic regime occurring in continental crust observations of the troodos complex cyprus an oceanic extensional tectonic regime now exposed on land and analysis of possible fault patterns associated with the magma chambers postulated to occur at some mid oceanic ridges the tectonic rotations resulting from movement along the listric normal faults occur about a horizontal axis generally perpendicular to the extension direction and typically amount to 30 to 50 although rotations as large as 70 to 90 are not uncommon the paleomagnetic consequences of these rotations can be calculated for dsdp basalts if the original strike and latitude of a ridge are known at four dsdp sites for which adequate data are available namely sites 319 332 410 and 417 rotations ranging from 50 to 70 are required to explain the paleomagnetic results if the original strike and location of the ridge are not known as is the case for most of the older parts of the pacific ocean the paleomagnetic data and the inferred rotation determine a relationship between possible ridge strikes and locations this information combined with a similar but independent relationship derived from the marine magnetic anomalies puts severe constraints on the original strike and location of the ridge

crosshole seismic waveform tomography i strategy for real data application summary the frequency domain version of waveform tomography enables the use of distinct frequency components to adequately reconstruct the subsurface velocity field and thereby dramatically reduces the input data quantity required for the inversion process it makes waveform tomography a computationally tractable problem for production uses but its applicability to real seismic data particularly in the petroleum exploration and development scale needs to be examined as real data are often band limited with missing low frequencies a good starting model is necessary for waveform tomography to fill in the gap of low frequencies before the inversion of available frequencies in the inversion stage a group of frequencies should be used simultaneously at each iteration to suppress the effect of data noise in the frequency domain meanwhile a smoothness constraint on the model must be used in the inversion to cope the effect of data noise the effect of non linearity of the problem and the effect of strong sensitivities of short wavelength model variations in this paper we use frequency domain waveform tomography to provide quantitative velocity images of a crosshole target between boreholes 300 m apart due to the complexity of the local geology the velocity variations were extreme between 3000 and 5500 m s 1 making the inversion problem highly non linear nevertheless the waveform tomography results correlate well with borehole logs and provide realistic geological information that can be tracked between the boreholes with confidence

preserved collisional crustal structure of the southern urals revealed by vibroseis profiling the vibroseis reflection profiling component of the urseis 95 experiment provides a high resolution crustal scale image of the unextended southern uralide orogen a marked lateral and vertical variation of reflectivity throughout the entire crust differentiates the former margin of the east european craton to the west from accreted terranes to the east between these regions is a less reflective zone corresponding to the magnitogorsk volcanic arc and the crustal root zone continuous reflections and reflective domains that correlate at the surface with major tectonic features define a bivergent orogen in which the paleozoic collisional structure has been largely preserved

reflection seismic investigations of the siljan impact structure sweden reflection seismic measurements were carried out during 1985 in the area of the siljan impact structure of sweden a total profile length of 80 km was shot to get a clearer picture of the subsurface geometry of impact structures and the processes that formed them than could be obtained from surface observations or theoretical modeling studies standard stacked sections have been interpreted in terms of current impact models the transient crater diameter is found to be approximately 22 km considerably smaller than current estimates based on interpretation of surface geology the structural uplift is estimated to be 3 km at the 5 to 8 km level and the material flow necessary to support the uplift is mainly horizontal rather than vertical