application of tephrochronology to the timing and correlation of palaeoceanographic events recorded in holocene and late glacial shelf sediments off north iceland abstract the north icelandic shelf is partly distinguished by the tjornes fracture zone featuring numerous active basins in a mud dominated shelf environment late glacial and holocene high resolution sedimentary records from this area have been studied with tephrochronology as the main tool for correlation and for exact timing of palaeoceanographic events in the area data from three new piston cores from the shelf demonstrate the importance of tephra markers for the first chronological evaluation and correlation of the cores the correlation is extended with lithological logs and with magnetic susceptibility records a detailed multidisciplinary study including biostratigraphy and tephrochronology of a late holocene record of predominantly muddy sediments at the same location demonstrates that marked variations in the distribution of water masses occurred repeatedly through the last 4500 cal yr of special interest is the exact timing of a marked drop in sea surface temperature in the area indicated by ice rafting debris concentration to about 50 years before the hekla 3 eruption which occurred at 2980 cal yr bp this appears to predate most records of a general cooling event in nw europe by a couple of centuries two different possible age models one based on 14c dates combined with tephra markers and one based on tephra markers alone are discussed in context with the problem of different marine reservoir ages of the water masses in the area

biostratigraphy interpretations of oppel s zones abstract zones like those of oppel and hedberg s oppel zone are commonly interpreted as rock units delimited temporally a more restricted view is that they are rock units empirically defined by bioevents that occur in the same order in all sections methods used by oppel and definitions proposed by hedberg are reviewed to assess their adequacy for definition of biostratigraphic units and their ability to support temporal inferences although they are usually interpreted as chronostratigraphic units oppel defined his zones in stratigraphic space without temporal reference in contrast hedberg required that bioevents for his oppel zone should be approximately isochronous across their distribution but provided no operational way to identify such bioevents neither author clearly indicated how boundaries should be defined recourse to a principle of biosynchroneity to support inferences that stratigraphically ordered bioevents are temporal markers conflicts with knowledge of the biogeographies of modern taxa evolutionary theory explains why some bioevents occur in the same stratigraphic order but does not support the inference that they are isochronous events since its inception biostratigraphy has focused on ordered classifications like those of oppel stratigraphic codes should allow for a complementary category of biofacies zones that reflect depositional environments and are not constrained to occur in a particular order

seismic versus sonic velocities a vertical seismic profiling study vertical seismic profiling vsp techniques provide a method to measure accurately the seismic velocity and lithologic structure near the borehole the analysis of a vsp survey can also provide insight into seismic wave propagation especially when related to sonic measurements but vsp and sonic log velocities or traveltimes are often found to disagree recent field evidence of these differences suggests that the vsp traveltimes are delayed with respect to the integrated sonic times especially in the deep section 3000 ft by about 2 0 ms 1000 ft on the average the vsp has numerous applications in exploration geophysics such as calibrating the sonic log it is thus important to understand why the two measurements differ differences in the geometries source frequencies and instrumental errors of the two surveys are reviewed more detailed analysis of seismic wave propagation in the vsp shows that short path multiples and velocity dispersion can have a significant delaying effect on the seismic tr

borehole guided avo analysis of p p and p s reflections quantifying uncertainty on density estimates seismic properties of isotropic elastic formations are characterized by the three parameters acoustic impedance poisson s ratio and density whilst the first two are usually well estimated by analysing the amplitude variation with angle ava of reflected p p waves density is known to be poorly resolved however density estimates would be useful in many situations encountered in oil and gas exploration in particular for minimizing risks in looking ahead while drilling we design a borehole seismic experiment to investigate the reliability of ava extracted density receivers are located downhole near the targeted reflectors and record reflected p p and converted p s waves a non linear wide angle based bayesian inversion is then used to access the a posteriori probability distributions associated with the estimation of the three isotropic elastic parameters the analysis of these distributions suggests that the angular variation of reflected p s amplitudes provides additional substantial information for estimating density thus reducing the estimate uncertainty variance by more than one order of magnitude compared to using only reflected p waves

low thermal gradient hercynian metamorphism in the eastern pyrenees fluids with compositions in the system co2 h2o nacl were trapped in quartz veins enclosed in low grade metamorphic rocks chlorite zone on the southern flank of the canigo massif eastern pyrenees the veins which also contain arsenopyrite crystals were formed contemporaneously with the main hercynian foliation and metamorphism volumetric properties of the fluid and the results of arsenopyrite geothermometry suggest p t trapping conditions of 4 6 6 kbar and 450 530 c this implies that an episode of metamorphism with an average geothermal gradient of 25 c km 1 occurred during the main deformation event this episode preceded the low p high t metamorphism described around domes and to date considered as characteristic of the hercynian orogeny in the pyrenees

monte carlo analysis of seismic reflections from moho and the w reflector near normal incidence reflections have been used to image the moho and the w reflector structure in the lithosphere offshore northern scotland to determine the impedance variations at these reflectors we use a monte carlo technique which allows incorporation of geologically realistic a priori information as well as an extensive exploration of the model space after testing it on a synthetic data set the method is based on bayesian inversion theory the modeled moho consists of a series of layers with a total thickness of 2 4 0 3 km with an overall positive impedance contrast inversion of the w reflector results in a model of five seven layers with a total thickness of about 3 7 0 6 km and mostly nonpositive impedance contrasts the implied fine scale impedance structure of the moho is consistent with the broader velocity structure determined from previous wide angle reflection refraction profiles however the overall nonpositive impedance contrast at the w reflector requires a structure which is overlain or underlain by a broad increase in velocity in order to match amplitudes of reflected phases observed at large offsets interpreted previously to originate at a similar depth

anatomy of a strike slip fault controlled sedimentary basin permian of the southern pyrenees spain this study is mainly concerned with a 55 km long east west oriented strike section through a thick succession of dominantly clastic post variscan deposits in the spanish pyrenees south of andorra a subdivision into three formations can be made two formations of stephanian age composed of volcanic rocks and fluviatile sediments respectively and one of probable permian age which has been studied in detail and is entirely composed of terrestrial redbeds these three formations are characterised by large variations in thickness along strike thicknesses vary from 0 to 800 m in the volcanics from 0 to 300 m in the stephanian fluviatile sediments and from 0 to 1500 m in the permian redbeds the geometry of the sedimentary bodies in all three formations suggests deposition in an east west oriented graben like depression a conclusion which is confirmed by detailed sedimentological analysis of the permian rocks the vertical succession of permian sediments can be subdivided in six major basinfill sequences which unconformably overlie each other internally the basinfill sequences are built up of one or more coarsening upward fining upward cycles the genesis of these cycles is directly related to dip slip movement along the basin boundary faults whereas the unconformities between the basinfill sequences indicate that periods of basin subsidence and sedimentation were followed by periods of deformation and erosion these changes in tectonic regime as reflected in the sedimentary sequences are interpreted to indicate alternating phases of transtension and transpression in a strike slip zone other sedimentological observations substantiate this interpretation the basin depocentre shifted in space and time which resulted in the observed large variations in sediment thickness along strike together with eastward paleocurrent directions along the basin axis transtension was accompanied by outpouring of andesitic lavas and drape folding on a large scale during transpressional phases normal faults changed character to reverse faults while folding uplift and subsequent erosion resulted in the formation of intrabasinal unconformities alternation of transpressive and transtensive conditions should be the result of movement along non planar fault surfaces in a major strike slip zone a separate structural study of the variscan basement of the studied basin revealed the existence of a dextral simple shear deformation generation of probable stephano permian age simple shear in the basement on the one hand and brittle superficial strike slip faulting involving transtension and transpression on the other are considered to be expressions of the same regional stress field at different levels in the earth s crust

anisotropic velocity analysis for lithology discrimination a new velocity analysis technique is presented for analyzing moveout of signals on multichannel surface seismic or vsp data an approximate skewed hyperbolic moveout formula is derived for horizontally layered transversely isotropic media this formula involves three measurement parameters the average vertical velocity and horizontal and skew moveout velocities by extending dix type hyperbolic moveout analysis we obtain improved coherence over large source geophone offsets for more accurate moveout correction compared with the stacking velocity obtained by simple hyperbolic analysis methods the three velocity parameters estimated by this technique contain more physically meaningful geologic information regarding the anisotropy and or velocity heterogeneity of the subsurface synthetic p wave model experiments demonstrate that the skewed hyperbolic moveout formula yields an excellent fit to time distance curves over a wide range of ray angles consequently the measurement parameters are shown to re

late glacial and holocene palaeoceanography of the north icelandic shelf high resolution gravity cores and box cores from the north icelandic shelf have been studied for palaeoceanographic history based on lithological and biostratigraphical foramin iferal data results from two outer shelf cores covering the last 13 6 k 14 c yr bp are presented in this paper the sediments accumulated in north south trending basins on each side of the kolbeinsey ridge at water depths of ca 400 m sedimentation rates up to 1 5 m kyr 1 are observed during the late glacial and holocene the vedde and saksunarvatn tephras are present in the cores as well as the hekla 1104 a new tephra kol gs 2 has been identified and dated to 13 4 k 14 c yr bp and another tephra geochemically identical to the borrobol tephra has been found at the same level at present the oceanographic polar front is located on the north icelandic shelf which experiences sharp oceanographic surface boundaries between the cold east icelandic current and the warmer irminger current past changes in sedimentological and biological processes in the study area are assumed to be related to fluctuations of the polar front the area was deglaciated before ca 14 kyr bp but there is evidence of ice rafting up to the end of the gs 1 greenland stadial 1 younger dryas period increasing again towards the end of the holocene foraminiferal studies show a relatively strong gs 2 pre 13 kyr bp palaeo irminger current followed by severe cooling and then by unstable conditions during the remainder of the gi 1 greenland interstadial 1 bolling allerod and gs 1 younger dryas another cooling event occurred during the preboreal before the holocene current system was established at about 9 kyr bp after a climatic optimum between 9 and 6 kyr bp the climate began to deteriorate and fluctuate copyright 2000 john wiley sons ltd

hercynian low pressure high temperature regional metamorphism and subhorizontal foliation development in the canigou massif pyrenees france evidence for crustal extension a detailed study has been made of the structural and metamorphic history of part of the hercynian low pressure high temperature regional metamorphic terrane exposed in the canigou massif france microtextural relations between and compositional zoning within porphyroblast phases in pelitic rocks indicate that the pressure temperature time p t t trajectory followed by the massif during metamorphism involved decompression under prograde and retrograde conditions clockwise p t t trajectory the geometric relations between inclusion trails within these porphyroblasts and the structures in the adjacent matrix indicate that metamorphism was synchronous with the development of the regional subhorizontal foliation s3 the s3 foliation formed in response to noncoaxial bulk strain the absence of a marked hiatus in sedimentation in the pyrenean region during hercynian metamorphism and deformation and the widespread preservation of low grade hercynian metasedimentary rocks suggest that low p high t metamorphism occurred in an extensional tectonic environment and that extension manifested itself in asymmetric top to the northwest shear during d3

transdimensional change point modeling as a tool to investigate uncertainty in applied geophysical inference an example using borehole geophysical logs recently developed methods for inferring abrupt changes in data series enable such change points in time or space to be identified and also allow us to estimate noise levels of the observed data the inferred probability distributions of these parameters provide insights into the capacity of the observed data to constrain the geophysical analysis and hence the magnitudes and likely sources of uncertainty we carry out a change point analysis of sections of four borehole geophysical logs density neutron absorption sonic interval time and electrical resistivity using transdimensional bayesian markov chain monte carlo to sample a model parameter space the output is an ensemble of values which approximate the posterior distribution of model parameters we compare the modeled change points borehole log parameters and the variance of the noise distribution of each log with the observed lithology classes down the borehole to make an appraisal of the uncertainty characteristics inherent in the data our two examples one with well defined lithology changes and one with more subtle contrasts show quantitatively the nature of the lithology contrasts for which the geophysical borehole log data will produce a detectable response in terms of inferred change points we highlight the different components of variation in the observed data due to the geologic process dominant lithology changes that we hope to be able to infer geologic noise due to variability within each lithology and analytical noise due to the measurement process this inference process will be a practical addition to the analytical tool box for borehole and other geophysical data series it reveals the level of uncertainties in the relationships between the data and the observed lithologies and would be of great use in planning and interpreting the results of subsequent routine processing

looking ahead with vertical seismic profiles several operations enhance our ability to predict the subsurface below the bottom total depth td of the well when applied to zero offset vertical seismic profiling vsp data other key issues regarding the use of vsp data in this fashion are resolution and look ahead distance an impedance log is the most useful form for presenting vsp data to look ahead of the drill bit the vsp composite trace must first tie reliably to the surface seismic section and to the well log synthetic seismogram the impedance log is obtained by inverting this vsp composite trace however before performing the inversion we need to 1 correct the composite trace for attenuation effects below td and 2 input velocities to provide low frequency information an exponential gain function applied to the vsp data below td adequately compensates for the loss of amplitude caused by attenuation a calibration of the seismically derived velocities with vsp velocities yields the necessary low frequency information these concepts ar

characteristics and mode of emplacement of gneiss domes and plutonic domes in central eastern pyrenees abstract gneiss domes and plutonic granitoid domes make up almost 50 of the pre hercynian terrains in the central and eastern pyrenees from a structural study of the shape and internal structure of the domes and of their relationships with the enclosing rocks it can be shown that both types of domes were emplaced diapirically during the major regional deformation phase and the peak of regional metamorphism the study also shows that the internal structure the overall shape and general behaviour relative to the host rocks are similar for plutonic domes and for gneiss domes this appears to be in good agreement with h ramberg s 1967 gravity deformation and the earth s crust academic press london 1970 model studies in relation to intrusion of plutonic bodies in mechanisms of igneous intrusion edited by newall g rast n geol j spec issue 2 261 286 model studies showing that dome or mushroom like structures similar to those observed develop when there is a small viscosity ratio between the rising body and its enclosing medium this implies a high crystal content for the granitoid magma this crystal content has been estimated by i calculating the viscosity and density in natural conditions from petrological data for the magma considered as a suspension using the model and program of j p carron et al 1978 bull soc geol fr 20 739 744 ii using the recent results of experimental deformation of partially melted granites of i van der molen m s paterson 1979 contr miner petrol 70 299 318 and ii comparing the preceding results with the data obtained by deformation experiments on rocks similar to those enclosing the domes the minimum crystal content for the development of a dome like structure has been thus estimated to about 70 i e a value very close to that estimated by van der molen paterson 1979 to be the critical value separating the granular framework flow from suspension like behaviour the effect of small variations in the viscosity of the rising body are then simulated by centrifuge experiments these small variations appear to exert a strong control on the shape and rate of rise of the domes they are thought to be sufficient to account for the variations in shape and structure and the level of emplacement of the different types of gneissic and plutonic domes finally more complex experiments with models built in order to simulate as closely as possible the natural structural evolution of the region as deduced from petrological and structural data are reported their implications for the regional interpretation of the relationships between gneissic and plutonic domes is then discussed

environmental changes off north iceland during the deglaciation and the holocene foraminifera diatoms and stable isotopes abstract a combined study of foraminifera diatoms and stable isotopes in marine sediments off north iceland records major changes in sea surface conditions since about 15 800 cal years yr bp results are presented from two gravity cores obtained at about 400 m water depth from two separate sedimentary basins on each side of the submarine kolbeinsey ridge the chronology of the sedimentary record is based partly on ams 14c dates partly on the vedde and the saksunarvatn tephra markers as well as the historical hekla ad 1104 tephra during the regional deglaciation the planktonic foraminiferal assemblages are characterised by consistently high percentages of sinistrally coiled neogloboquadrina pachyderma however major environmental variability is reflected by changes in stable isotope values and diatom assemblages low 18o values indicate a strong freshwater peak as well as possible brine formation by sea ice freezing during a pre bolling interval greenland stadial 2 corresponding to the heinrich 1 event the foraminifera suggest a strong concurrent influence of relatively warm and saline atlantic water and both the foraminifera and the diatoms suggest mixing of cold and warm water masses similar but weaker environmental signals are observed during the younger dryas greenland stadial 1 around the level of the vedde ash each freshwater peak is succeeded by an interval of severe cooling both at the beginning of the bolling allerod interstadial complex greenland interstadial 1 and during the preboreal presumably associated with the onset of intense deep water formatiom in the nordic seas the holocene thermal optimum between 10 200 and about 7000 cal years yr bp is interrupted by a marked cooling of the surface waters around 8200 cal yr bp this cold event is clearly expressed by a pronounced increase in the percentages of sinistrally coiled n pachyderma corresponding to a temperature decrease of about 3 c a general cooling in the area is indicated after 7000 6000 cal yr bp both by the diatom data and by the planktonic foraminiferal data after a severe cooling around 6000 cal yr bp the planktonic foraminiferal assemblages suggest a warmer interval between 5500 and 4500 cal yr bp minor temperature fluctuations are reflected both in the foraminiferal and in the diatom data in the upper part of the record but the time resolution of the present data is not high enough to pick up details in environmental changes through the late holocene

constraints on sst estimates for the northern north atlantic nordic seas during the lgm abstract a map of estimated calcification temperatures of the planktic foraminifer neogloboquadrina pachyderma sinistral tnps for the nordic seas and the northern north atlantic for the last glacial maximum was produced from oxygen isotopes with support of mg ca ratios to arrive at the reconstruction several constraints concerning the plausible salinity and 18 o fields were employed the reconstruction indicates inflow of temperate waters in a wedge along the eastern border of the nordic seas and at least seasonally ice free waters the reconstruction from oxygen isotopes shows similarities with mg ca based paleotemperatures in the southern and southeastern sector while unrealistically high mg ca values in the central nordic seas prevent the application of the method in this area the oxygen isotope based reconstruction shows some agreement with temperature reconstructions based on the modern analogue technique but with somewhat lower temperatures and a stronger internal gradient inside the nordic seas all told our results suggest a much more ice free and dynamic high latitude ocean than the climap reconstruction

monte carlo bayesian look ahead inversion of walkaway vertical seismic profiles we describe a method to invert a walkaway vertical seismic profile vsp and predict elastic properties p wave velocity s wave velocity and density in a layered model looking ahead of the deepest receiver starting from bayes s rule we define a posterior distribution of layered models that combines prior information on the overall variability of and correlations among the elastic properties observed in well logs with information provided by the vsp data this posterior distribution of layered models is sampled by a monte carlo method the sampled layered models agree with prior information and fit the vsp data and their overall variability defines the uncertainty in the predicted elastic properties we apply this technique first to a zero offset vsp data set and show that uncertainty in the long wavelength p wave velocity structure results in a sizable uncertainty in the predicted elastic properties we then use walkaway vsp data which contain information on the long wavelength p wave velocity in the reflection moveout and on s wave velocity and density contrasts in the change of reflectivity with offset the uncertainty of the look ahead prediction is considerably decreased compared with the zero offset vsp and the predicted elastic properties are in good agreement with well log measurements

bayesian estimation in seismic inversion part i principles1 this paper gives a review of bayesian parameter estimation the bayesian approach is fundamental and applicable to all kinds of inverse problems its basic formulation is probabilistic information from data is combined with a priori information on model parameters the result is called the a posteriori probability density function and it is the solution to the inverse problem in practice an estimate of the parameters is obtained by taking its maximum well known estimation procedures like least squares inversion or l 1 norm inversion result depending on the type of noise and a priori information given due to the a priori information the maximum will be unique and the estimation procedures will be stable except in theory for the most pathological problems which are very unlikely to occur in practice the approach of tarantola and valette can be derived within classical probability theory the bayesian approach allows a full resolution and uncertainty analysis which is discussed in part ii of the paper

composition and origin of volcanic ash zones in late quaternary sediments from the reykjanes ridge evidence for ash fallout and ice rafting abstract seven quaternary volcanic ash zones including the well known ash zones az i and az ii have been investigated in five well dated sediment cores from the reykjanes ridge between 59 and 60 n the zones were defined on their geochemical composition morphological variation of the particles and stratigraphic position the ash zones az i and az ii show a mixture of colorless bubble wall shards and brown blocky glasses whereas ash zones vz 1 to 5 are composed of dark to light brown volcanic glasses ranging in shape and texture from blocky nonvesicular shards to highly vesicular pumiceous types the dark brown glass shards of ash zones vz 1 to 5 represent subalkalic and low k subalkalic basaltic series geochemical correlation of the ash zones with possible source areas indicates that they all derived from iceland the colorless shards in az i and az ii are of rhyolitic composition the eastern volcanic zone on iceland is the most important source with abundant volcaniclastics being produced by subaerial subglacial eruptions and secondary erosion of volcanic rocks explosive volcanism on iceland is manifested by marine ash layers az i and az ii most of the volcaniclastic zones vz 1 to 5 coincide chronologically with peak abundances of coarse ice rafted material ird indicating that most of this volcanic glass was incorporated into the icelandic ice sheet and released to the ocean by iceberg calving and subsequent melting however two layers in vz 1 and vz 5 have no correlatives in the ird record suggesting that they were probably transported by local sediment gravity flows from this we must infer that most of the ice rafting episodes bearing volcanic particles as observed in sediments from reykjanes ridge monitor glacier fluctuations on iceland

the variscan geology of the pyrenees abstract the main outlines of the geology of the variscan part of the pyrenees are discussed rocks involved in this cycle are high grade basement gneisses palaeozoic sediments and their metamorphic equivalents late intrusive granodiorites and early pre variscan granites the main features of the stratigraphy of the palaeozoic are given structures fall into two domains a low grade suprastructure essentially with steep folds and cleavages and a high grade infrastructure with dominantly low dipping foliations an important phase of early pre cleavage folding occurs in low grade rocks mainly along the southern border of the axial zone in high grade rocks most structures and the metamorphism postdate the main cleavage phase in low grade rocks the influence of the alpine orogeny on the variscan structures consists mainly of faults steep reverse faults in the northern and south directed thrusts in the southern part of the pyrenees metamorphism took place under high geothermal gradients and low pressures as indicated by the abundant occurrence of andalusite and cordierite

solution of the vsp one dimensional inverse problem the acoustical impedance distribution of the substratum or equivalently the reflection coefficient sequence is determined from vsp data this nonlinear inverse problem is solved by a least squares method as the wavelet is unknown the impedance distribution and the neumann boundary condition which characterizes the excitation of the medium are simultaneously identified the inversion method is applied to synthetic and field vsp s the result is satisfactory even when strong noise corrupts the data provided that a suitable constraint on the impedance distribution is introduced in order to ensure the stability of the inverse problem the reliability of the inversion result in the case of field vsp is confirmed some ways in which this result may be used are illustrated calibration of the seismic surface data multiple identification prediction ahead of the bit

le devonien et le carbonifere inferieur du sud ouest de la france pyrenees massif de mouthoumet montagne noire sedimentation dans un bassin flexural en bordure sud de la chaine de collision varisque une fois annules les effets des deplacements pyreneens et tardi varisques on peut reconstituer l evolution du domaine de sedimentation d ge devonien carbonifere inferieur du sw de la france pyrenees mouthoumet montagne noire episode de distension au passage silurien devonien sedimentation dans un bassin borde au nord par une plate forme peu profonde au devonien inferieur nouvelle distension au devonien moyen approfondissement relatif au devonien superieur carbonifere inferieur precedant une subsidence active avec accumulation de depots terrigenes a partir du viseen superieur cette succession d evenements peut traduire la naissance et l evolution d un bassin flexural a soubassement continental situe en bordure sud de la chaine de collision varisque

relationships between compressional wave and shear wave velocities in clastic silicate rocks new velocity data in addition to literature data derived from sonic log seismic and laboratory measurements are analyzed for clastic silicate rocks these data demonstrate simple systematic relationships between compressional and shear wave velocities for water saturated clastic silicate rocks shear wave veloci ty is approximately linearly related to compressional wave velocity and the compressional to shear velocity ratio decreases with increasing compressional velocity laboratory data for dry sandstones indicate a nearly constant compressional to shear velocity ratio with rigidity approximately equal to bulk modulus ideal models for regular packings of spheres and cracked solids exhibit behavior similar to the observed water saturated and dry trends for dry rigidity equal to dry bulk modulus gassmann s equations predict velocities in close agreement with data from the water saturated

joint avo inversion wavelet estimation and noise level estimation using a spatially coupled hierarchical bayesian model the main objective of the avo inversion is to obtain posterior distributions for p wave velocity s wave velocity and density from specified prior distributions seismic data and well log data the inversion problem also involves estimation of a seismic wavelet and the seismic noise level the noise model is represented by a zero mean gaussian distribution specified by a covariance matrix a method for joint avo inversion wavelet estimation and estimation of the noise level is developed in a bayesian framework the stochastic model includes uncertainty of both the elastic parameters the wavelet and the seismic and well log data the posterior distribution is explored by markov chain monte carlo simulation using the gibbs sampler algorithm the inversion algorithm has been tested on a seismic line from the heidrun field with two wells located on the line the use of a coloured seismic noise model resulted in about 10 lower uncertainties for the p wave velocity s wave velocity and density compared with a white noise model the uncertainty of the estimated wavelet is low in the heidrun example the effect of including uncertainty of the wavelet and the noise level was marginal with respect to the avo inversion results

anisotropic velocity analysis1 results from walkaway vsp and shale laboratory experiments show that shale anisotropy can be significantly anelliptic heterogeneity and anellipticity both lead to non hyperbolic moveout curves and the resulting ambiguity in velocity analysis is investigated for the case of a factorizable anisotropic medium with a linear dependence of velocity on depth more information can be obtained if there are several reflectors the method of dellinger et al for anisotropic velocity analysis in layered transversely isotropic media is examined and is shown to be restricted to media having relatively small anellipticity a new scheme based on an expansion of the inverse squared group velocity in spherical harmonics is presented this scheme can be used for larger anellipticity and is applicable for horizontal layers having monoclinic symmetry with the symmetry plane parallel to the layers the method is applied to invert the results of anisotropic ray tracing on a model sand shale sequence for transversely isotropic media with small anisotropy the scheme reduces to the method of byun et al and byun and corrigan the expansion in spherical harmonics allows the p phase slowness surface of each layer to be determined in analytic form from the layer parameters obtained by inversion without the need to assume that the anisotropy is weak

orbital tuning as an inverse problem chronology of the early aptian oceanic anoxic event 1a selli level in the cismon apticore 1 orbital tuning the process of fitting sedimentary cycles to orbital periodicities can estimate with high resolution the timing and duration of key events in the geological record we formulate here orbital tuning as the inverse problem of finding the variation in sedimentation rate that matches sediment cycles with orbital periodicities instead of obtaining a single best estimate we apply a bayesian formulation and define a probability distribution of sedimentation rate variations that result in powerful orbital periodicities by sampling this distribution with a monte carlo method we quantify the uncertainty in the inferred sedimentation rates due to uncertainties in the tuning periods and to components of the sedimentary signal that are unrelated to orbital cycles the method is applied to the chronology of a 30 m interval in the cismon apticore borehole southern alps italy that includes the early aptian oceanic anoxic event 1a selli level estimated to last 1 11 0 11 ma 95 interval the 13c record shows a sudden negative shift of about 1 at the base of the selli level 22 47 ka followed by a recovery to preshift values in 240 ka consistent with a scenario where light carbon is quickly added and then flushed out of the ocean atmosphere system

volcaniclastic sediments from mid oceanic kolbeinsey ridge north of iceland evidence for submarine volcanic fragmentation processes volcaniclastic sediments of the active kolbeinsey ridge north of iceland reflect changing fragmentation mechanisms due to increasing sea level during the last deglaciation glass shards deposited on the western flank of the mid oceanic ridge before 13 4 ka are vesicular 20 60 whereas shards deposited since 13 4 ka are mostly vesicle free and of hyaloclastic origin the vesicle rich shards display morphology thought to be atypical for submarine eruptions at this depth 400 500 m the environmental conditions during the deposition of these layers regional ice cover more than 200 km from the nearest subaerial source preclude a subaerial origin for these shards we propose an eruption mechanism in which hot vesiculating bombs from a low energy eruption are transported upward through the water column by convection these particles reach their volcanic fragmentation depth vfd and undergo secondary fragmentation creating the vesicle rich shards recovered near the ridge rising sea level since the end of the last glaciation was sufficient to prevent the erupted particles from reaching the vfd thus ending the formation of vesicle rich shards this model explains the presence of vesicular shards in sediments of mid oceanic ridges where no subaerial source can be inferred

hercynian tectonometamorphic evolution of the bosost dome french spanish central pyrenees metamorphic and structural studies show that the bosost dome central pyrenees is a metamorphic as well as a structural dome developed in cambro ordovician to devonian metasediments around a migmatitic core during the hercynian orogeny the structural metamorphic dome occurs in epizonal terranes and is twice as large as originally described the structural dome shows a concentric zoning of d2 and d3 local deformations developed around the migmatites and superimposed on the d1 regional hercynian deformation the structural data suggest that migmatites were emplaced diapirically into palaeozoic metasediments during d1 and are responsible for the local d2 and d3 deformations an early medium pressure high temperature metamorphism occurred during d1 leading to anatexis under water rich conditions then evolved progressively to a high temperature low pressure metamorphism during the d2 and d3 deformations the corresponding p t time path is a clockwise loop representing the evolution of the metamorphism from migmatiza tion to the diapiric emplacement of migmatites in a compressional setting pressure values obtained from geobarometry are three times as high as expected from sedimentological data this suggests that the hercynian metamorphism developed in a thickened crust during the n s shortening responsible for the large scale e w folding observed throughout the pyrenean axial zone

late quaternary terrestrial tephrochronology of iceland frequency of explosive eruptions type and volume of tephra deposits close to 100 silicic tephra layers have been identified in icelandic terrestrial soils of holocene age the majority of these tephras were erupted at the hekla torfajokull oraefajokull askja snaefellsjokull eyjafjallajokull and katla central volcanoes by far the most active is hekla with close to 50 identified silicic tephra layers in 8000 years showing an inverse relationship between eruption frequency and volume of erupted tephra volumes of uncompacted silicic tephra layers range from 10 km3 with nearly 50 of known tephra volumes lying between 0 1 and 0 5 km3 seven closely spaced hekla eruptions in the period 950 550 bc with identical major element composition illustrate the need of stratigraphic control in short and long distance correlations a 2600 year old tephra geochemically indistinguishable from the borrobol tephra is a reminder that no tephra is geochemically unique the major producers of basaltic tephra are the volcanic systems that are partly covered by ice or partly lying within areas of high groundwater or extending offshore i e the grimsvotn katla veidivotn bardarbunga reykjanes kverkfjoll and vestmannaeyjar volcanic systems the best known eruption record is that of the katla system with over 170 identified basaltic tephra layers and more than 300 estimated in 8400 years the grimsvotn system is currently the most active volcanic system with over 70 eruptions during the last 1100 years volumes of uncompacted basaltic tephra layers range from 20 km3 the majority of known tephra volumes lying between 0 1 and 1 km3 the most voluminous basaltic tephra deposit the 10 200 yr old saksunarvatn tephra may however represent more than one eruption on the grimsvotn system deposition of approximately 800 basaltic tephra layers during the last 9000 14c years is estimated but many of those erupted from volcanoes within ice caps have not been preserved copyright 2007 john wiley sons ltd