3d wave equation interferometric migration of vsp free surface multiples interferometric migration of free surface multiples in vertical seismic profile vsp data has two significant advantages over standard vsp imaging 1 a significantly larger imaging area compared to migrating vsp primaries and 2 less sensitivity to velocity estimation and static errors than other methods for migration of multiples in this paper we present a 3d wave equation interferometric migration method that efficiently images vsp free surface multiples synthetic and field data results confirm that a reflectivity image volume comparable in size to a 3d surface seismic survey around the well can be computed economically the reflectivity image volume has less fold density and lower signal to noise ratio than that obtained by a conventional 3d surface seismic survey because of the relatively weak energy of multiples and the limited number of geophones in the well however the efficiency of this method for migrating vsp multiples suggests that it might sometimes be a useful tool for 4d seismic monitoring where reflectivity images can be computed quickly for each time lapse survey

least squares reverse time migration of multiples abstractthe theory of least squares reverse time migration of multiples rtmm is presented in this method least squares migration lsm is used to image free surface multiples where the recorded traces are used as the time histories of the virtual sources at the hydrophones and the surface related multiples are the observed data for a single source the entire free surface becomes an extended virtual source where the downgoing free surface multiples more fully illuminate the subsurface compared to the primaries since each recorded trace is treated as the time history of a virtual source knowledge of the source wavelet is not required and the ringy time series for each source is automatically deconvolved if the multiples can be perfectly separated from the primaries numerical tests on synthetic data for the sigsbee2b and marmousi2 models show that least squares reverse time migration of multiples lsrtmm can significantly improve the image quality compared to rtmm or standard reverse time migratio

interferometric daylight seismic imaging summary claerbout s daylight imaging concept is generalized to a theory of interferometric seismic imaging ii interferometric seismic imaging is defined to be any algorithm that inverts correlated seismic data for the reflectivity or source distribution as examples we show that ii can image reflectivity distributions by migrating ghost reflections in passive seismic data and generalizes the receiver function imaging method used by seismologists interferometric seismic imaging can also migrate free surface multiples in common depth point cdp data and image source distributions from passive seismic data both synthetic and field data examples are used to illustrate the different possibilities of ii the key advantage of ii is that it can image source locations or reflectivity distributions from passive seismic data where the source position or wavelet is unknown in some cases it can mitigate defocusing errors as a result of statics or an incorrect migration velocity the main drawback with ii is that severe migration artefacts can be created by partial focusing of virtual multiples

reverse time migration of multiples for subsalt imaging some hydrocarbon reservoirs are trapped beneath salt bodies where seismic imaging is greatly challenged due to poor illumination multiple reflections have different propagation wave paths from primary reflections and thus can be used to complement the illuminations where primary reflections from beneath the salt are not acquired consequently migration of multiples can sometimes provide better subsalt images compared to conventional migration which uses primary reflections only in this paper we propose to modify conventional reverse time migration so that multiples can be used as constructive reflection energy for subsalt imaging this new approach replaces the impulsive source wavelet with the recorded data containing both primaries and multiples and uses predicted multiples as the input data instead of primary reflections in the reverse time migration process multiples recorded on the surface are extrapolated backward in time to each depth level and the observed data with both primaries and mult

on the ulf magnetic ratio increase before the 2008 iwate miyagi nairiku earthquake by hirano and hattori 2011 abstract hirano and hattori 2011 hereafter cited as h h report the observation of anomalous increases in the ulf geomagnetic field spectral density ratio which the authors claim to be possible precursors of the 2008 iwate miyagi nairiku earthquake here the results of h h are reviewed taking into account the global geomagnetic activity level by means of k p index this paper cast serious doubts on the seismogenic origin of the magnetic signatures documented by h h showing that the anomalous signals are normal ulf magnetic variations induced by solar terrestrial interaction in summary h h s claims that magnetic field disturbances about a month before the mw 6 9 iwate miyagi nairiku earthquake on june 13 2008 are precursors to the earthquake are unlikely to be correct

integrated approach to subsalt depth imaging synthetic case study prestack depth migration has become almost mandatory in areas of complex geology for reasons that are abundantly clear in structurally complex media cmp traveltime curves differ significantly from hyperbolae thus when using standard time processing techniques we cannot produce a time section containing all information related to the medium that is in the prestack gathers

are there new findings in the search for ulf magnetic precursors to earthquakes moore 1964 in a letter published in nature reported disturbances in geomagnetic field data prior to the 27 march 1964 alaska earthquake after the publication of this report many papers have shown magnetic changes preceding earthquakes however a causal relationship between preearthquake magnetic changes and impending earthquakes has never been demonstrated as a consequence after 50 years magnetic disturbances in the geomagnetic field are still candidate precursory phenomena some researchers consider the investigation of ultra low frequency ulf 0 001 10 hz magnetic data the correct approach for identifying precursory signatures of earthquakes other researchers instead have recently reviewed many published ulf magnetic changes that preceded earthquakes and have shown that these are not actual precursors the recent studies by currie and waters 2014 and han et al 2014 aim to provide relevant new findings in the search for ulf magnetic precursory signals however in order to contribute to science alleged precursors must be shown to be valid and reproducible by objective testing here we will briefly discuss the state of the art in the search for ulf magnetic precursors paying special attention to the recent findings of currie and waters 2014 and han et al 2014 we do not see in these two reports significant evidence that may support the observation of precursory signatures of earthquakes in ulf magnetic records

reverse time migration a prospect of seismic imaging methodology abstract reverse time migration rtm is a seismic imaging method to map the subsurface reflectivity using recorded seismic waveforms the practice in exploration seismology has long established a two fold approach of seismic imaging using velocity modeling building to establish the long wavelength reference velocity models and using seismic migration to map the short wavelength reflectivity structures among various seismic migration methods for different situations rtm is the only method that is capable to use all seismic wave types that can be computed numerically being initiated in early 1980 s rtm seeks an image of the subsurface reflectivity as the best match in an image space between the extrapolation of time reversed waveform data and the prediction based on estimated velocity model and source parameters judging the image quality in the same space of forming the images is more advantageous than the approaches of modeling and inversion which seek the solution in the model space but judge its fitness in data space considering that most seismic migration applications today still use primary reflection as the only signal the capability of rtm to use all computable wave types is unique and helpful reducing the imaging artifacts due to mistaking non primary waves as primary reflections hence we refer to those rtm algorithms using only primary reflections as the first generation rtm methods and the rtm algorithms making a full use of primary reflections multiple reflections and other non primary waveform data as the second generation rtm methods this paper reviews the development history of the rtm along with its major challenges current solutions and future perspectives

subsalt imaging using tti reverse time migration tti depth imaging technology is routinely applied to image structures that lie beneath dipping anisotropic overburden in the canadian foothills vestrum and vermeulen 2004 the north sea hawkins et al 2002 and offshore west africa ball 1995 until now ray based migration algorithms served as the only choice for tti imaging because upgrading ray based imaging algorithms for tti is straightforward and incurs minor additional computational cost unlike ray based algorithms tti wave based algorithms are difficult to formulate and their implementations are often unstable and computationally intensive unfortunately ray based algorithms perform poorly in comparison to wave based algorithms in imaging structures beneath a complex overburden such as the salt canopy in the gulf of mexico however the use of tti imaging in this area is impeded by the lack of appropriate check shots and offset vsps that can constrain anisotropic parameters

metallic ion binding by bacillus subtilis implications for the fossilization of microorganisms the silicification of bacterial cells has been followed in a laboratory simulation by electron microscopy and energy dispersive x ray spectroscopy structural degradation of the bacteria was not influenced by silica availability but the remains of degraded cells did promote the precipitation of silica in contrast cells reacted with ferric iron before aging remained intact and structures conforming to the original morphology of the bacteria were preserved by silicification the binding of metallic ions by microbial cells in particular the retention of iron is therefore considered to be an important contributing factor to the fossilization of microorganisms

geochemistry of ikaite formation at mono lake california implications for the origin of tufa mounds the mineral ikaite caco 3 6h 2 o not previously observed in lake environments precipitates seasonally along the shore of mono lake california where ca hco 3 spring water mixes with cold na co 3 lake water during the winter cold water temperatures and high concentrations of po 4 3 and organic carbon inhibit calcite precipitation allowing the metastable ikaite to form during the spring warming however ikaite decomposes to form calcium carbonate and water occasionally leaving pseudomorphs of the primary precipitate the identification of modern ikaite suggests that both pleistocene and holocene tufas in the mono basin originally precipitated as ikaite this mineral may also form in other lake environments but rapid recrystallization after warming destroys the physical chemical and isotopic evidence of formation and alters the geochemical record

possible association between anomalous geomagnetic variations and the molise earthquakes at central italy during 2002 abstract the molise region on 31 october and 1 november 2002 was struck by two earthquakes of moderate magnitudes mw 5 7 at depth between 12 and 24 km geomagnetic data from the circum pan pacific magnetometer network cpmn were analyzed in order to detect any anomalous geomagnetic variations in association with the molise earthquakes our results indicate the presence of long term anomalous variations in the geomagnetic components at l aquila laq station near the epicenter which started about four months before the occurrence of the molise earthquakes moreover the geomagnetic components recorded at laq are found to show no correlation with those at other conjugate stations in the period between june and october 2002 in addition anomalous ulf signal pc 3 10 45 s started few weeks before the molise earthquakes and lasted for about one week after the onset of the seismic activity the observed anomalous variations may be explained as a result of the crustal stress variations and the enhancement of the lithospheric conductivity in the molise region during the preparation period of the molise earthquakes

elastic reverse time migration elastic prestack reverse time finite difference migration of two component seismic surface data requires data extrapolation and application of an imaging condition data extrapolation involves synchronous driving of the vertical component and horizontal component finite difference meshes with the time reverse of the recorded vertical and horizontal traces respectively extrapolation uses the coupled elastic wave equation for variable velocity solved with a second order explicit finite difference scheme the imaging condition at any point in the grid is the one way traveltime from the source to that point elastic migrations of both synthetic test data and real two component common source gathers produce simpler images than acoustic migrations because of the coalescing of double reflections compressional waves and shear waves into single loci

geochemistry of great salt lake utah ii pleistocene holocene evolution sedimentologic and biostratigraphic evidence is used to develop a geochemical model for great salt lake utah extending back some 30 000 yrs b p hydrologie conditions as defined by the water budget equation are characterized by a lake initially at a low saline stage rising by about 17 000 yrs b p to fresh water basin full conditions bonneville level and then after about 15 000 yrs b p dropping rapidly to a saline stage again as exemplified by the present situation inflow composition has changed through time in response to the hydrologie history during fresh water periods high discharge inflow is dominated by calcium bicarbonate type river waters during saline stages low discharge nacl rich hydrothermal springs are significant solute sources this evolution in lake composition to nacl domination is illustrated by the massive mirabilite deposition free of halite following the rapid drawdown until about 8 000 years ago while historic droughts have yielded principally halite hydrologic history can be combined with inferred inflow composition to derive concentration curves with time for each major solute in the lake calcium concentrations before the drawdown were controlled by calcite solubility and afterwards by aragonite significant amounts of solutes are removed from the lake by diffusion into the sediments na cl and so42 are also involved in salt precipitation by including pore fluid data a surprisingly good fit has been obtained between solute input over the time period considered and the amounts actually found in lake brines pore fluids salt beds and sediments excess amounts are present for calcium carbonate and silica indicating detrital input

the lithobiontic ecological niche with special reference to microorganisms abstract revised terminology is proposed that describes the ecological niches of microorganisms within hard mineral substrates organisms attached to the external surfaces of the rock are termed epiliths while those in the interior of the rock are all termed endoliths the latter are called chasmoendoliths if they inhabit fissures in rocks cryptoendoliths if they dwell within structural cavities and euendoliths if they actively penetrate calcareous substrates

migration methods for imaging different order multiples multiples contain valuable information about the subsurface and if properly migrated can provide a wider illumination of the subsurface compared to imaging with vsp primary reflections in this paper we review three different methods for migrating multiples the first method is model based and it is more sensitive to velocity errors than primary migration the second method uses a semi natural green s function for migrating multiples where part of the traveltimes are computed from the velocity model and part of the traveltimes i e natural traveltimes are picked from the data to construct the imaging condition for multiples the third method uses cross correlation of traces the last two methods are preferred in the sense that they are significantly less sensitive to velocity errors and statics because they use natural data to construct part of the migration imaging conditions compared with the interferometric i e crosscorrelation imaging method the semi natural green s function method is more computationally efficient and is sometimes less prone to migration artifacts numerical tests with 2 d and 3 d vsp data show that a wider subsurface coverage higher fold and more balanced illumination of the subsurface can be achieved with multiple migration compared with migration of primary reflections only however there can be strong interference from multiples with different orders or primaries when multiples of high order are migrated one possible solution is to filter primaries and different orders of multiples before migration and another possible solution is least squares migration of all events a limitation of multiple migration is encountered for subsalt imaging here the multiples must pass through the salt body more than twice which amplifies the distortion of the image

imaging with deep water multiples acquisition of on bottom hydrophone data recording of a near surface source provides an opportunity to treat water column multiples as useful signal a ray equation based kirchhoff depth migration is used to image primary reflections and deep water multiples recorded on an ocean bottom hydrophone obh the image of the subbottom sediments is shown to be improved by inclusion of the deep water multiple in the imaging process field data jointly acquired by woods hole oceanographic institute and university of texas institute for geophysics at austin and consisting of an obh 2300 m depth recording a 10 800 cubic inch air gun array are used to illustrate the feasibility of the technique images are obtained from both the primary reflections and from energy that has undergone an additional passage through the water column comparison of these images reveals an excellent correlation of reflectors with the predicted polarity reversal observed in the multiple s image synthetic data are used to examine the d

mg silica precipitation in fossilization mechanisms of sand tufa endolithic microbial community mono lake california abstract the detection of mg silica associated with endolithic biofilms of cyanobacteria and bacterial communities living in lake shore sand tufa formations in mono lake california has been observed using scanning electron microscopy with backscattered electron imaging sem bse and energy dispersive x ray spectroscopy eds mg accumulated in cell walls of living cyanobacteria is interpreted to promote mg silicification with structural preservation while non cyanobacterial prokaryotes are not fossilized however extracellular polymeric substances around the living communities were prominent as nucleation sites for authigenic mineral precipitation a mg silicate served to stabilize the biofilm remains further diagenesis of these textures resulted in occlusion of the cytoplasm pore spaces by carbonate precipitates this precipitate damaged the microfossil structures but the mg silica fabric was still present the paleontological significance of mg silica precipitates as a possible biosignature of the former presence of endolithic biofilms is addressed

elastic p and s wave autofocus imaging with primaries and internal multiples conventional seismic imaging methods rely on the singlescattering born approximation requiring the removal of multiply scattered events from reflection data prior to imaging additionally many methods use an acoustic approximation representing the solid earth as an acoustic fluid medium we have developed imaging methods for solid elastic media that use primaries and internal multiples including their ps and sp conversions thus obviating the need for internal multiple removal and improving handling of internal conversions our methods rely on the elastic autofocusing method which estimates full multicomponent elastodynamic green s functions from virtual sources interior to the medium to receivers placed on the surface they require only surface seismic reflection data and estimates of the direct waves from virtual sources interior to the medium both of which are commonly available at the imaging step of seismic processing we tested our methods on a synthetic model with constant p and s velocities and vertical and horizontal density variations by producing for the first time to our knowledge pp and ss images from elastic autofocusing that are compared to reference seismic images based on conventional methods the effects of multiples are greatly attenuated in the images with fewer spurious reflectors than are observed when using born imaging

microstructure and mineral composition of holocene stromatolites from lagoa vermelha a hypersaline lagoon in brazil insights into laminae genesis

comment on possible association between anomalous geomagnetic variations and the molise earthquakes at central italy during 2002 by takla et al 2011 abstract takla et al 2011 documented the observation of seismogenic precursory signals in the geomagnetic field components of l aquila station laq which occurred before the 2002 molise earthquakes here these claims are reviewed taking into account the geomagnetic index kp time series and by means of data coming from the geomagnetic observatory of l aquila where the laq station is located this review shows that before the molise earthquakes the anomalous behaviour of laq geomagnetic field components was actually caused by a possible thermal drift of the instrumentation in conclusion there is no firm relation between the earthquakes occurrence and the observed magnetic anomalous signatures documented by takla et al 2011

modulation and daily banding of mg ca in orbulina universa tests by symbiont photosynthesis and respiration a complication for seawater thermometry the mg ca composition of calcium carbonate tests shells secreted by planktonic foraminifera is increasingly being employed to estimate past seawater temperatures and reconstruct paleocean and climate records spanning hundreds of thousands of years we show using two high resolution microanalysis techniques that the final chamber of the planktonic foraminifera orbulina universa typically comprises between three and six paired low and high mg growth bands the number and spacing of these bands is consistent with a diurnal origin modulated by changing ph within the foraminiferal microenvironment due to the day night photosynthesis respiration cycle of algal symbionts the amplitude of mg ca variation within individual tests and across many daily growth bands cannot be accounted for by seawater temperature in the shallow euphotic zone habitat of o universa our results indicate the mg ca composition of calcite precipitated by o universa in nature is strongly influenced by diurnal changes in the biological activity of algal symbionts and the host foraminifer this brings into question the fundamental premise often made in applying mg ca palaeoseawater thermometry that the mg ca composition of foraminiferal calcite is determined by seawater temperature and whether the mg ca composition of other planktonic species that are more widely used for palaeoseawater thermometry are subject to similar influences 2004 elsevier b v all rights reserved

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

the study of ionospheric anomalies in japan area during 1998 2010 by kon et al an inaccurate claim of earthquake related signatures abstract the problem of identifying precursory signals of earthquakes in the hope of mitigate the seismic hazard is a very important topic but inaccurate documentations of precursory signatures decrease the credibility of this field of research the statistical analysis by kon et al 2011 shows that there is tendency of positive total electron content tec anomalies to occur 1 5 days before 52 m 6 earthquakes which struck japan during 1998 2010 kon et al 2011 also report in detail three selected case studies claiming the occurrence of tec anomalies possibly related to large and destructive earthquakes this paper casts doubts on the possibility that in the three cases the tec disturbances were caused by seismic events suggesting that these tec changes could be induced by normal variations of the global geomagnetic activity as a consequence also the results of the superimposed epoch analysis performed by kon et al 2011 could be seriously influenced by global magnetospheric signals

the lamu basin deepwater fold and thrust belt an example of a margin scale gravity driven thrust belt along the continental passive margin of east africa in recent decades advances in seismic processing and acquisition of new data sets have revealed the presence of many deepwater fold and thrust belts dw ftbs often developing along continental passive margins these kinds of tectonic features have been intensively studied due to their substantial interest this work presents a regional scale study of the poorly explored lamu basin dw ftb a margin scale gravity driven system extending for more than 450 km along the continental passive margin of kenya and southern somalia east africa a 2 d seismic data set was analyzed consisting of both recently acquired high quality data and old reprocessed seismic profiles for the first detailed structural and stratigraphic interpretation of this dw ftb the system originated over an early to mid cretaceous shale detachment due to a mainly gravity spreading mechanism analysis of synkinematic strata indicates that the dw ftb was active from the late cretaceous to the early miocene but almost all of the deformation occurred before the late paleocene the fold and thrust system displays a marked n s variation in width the northern portion being more than 150 km wide and the southern portion only a few dozen kilometers wide this along strike variation is thought to be related to the complex tectonosedimentary evolution of the continental margin at the somalia kenya boundary also reflected in the present day bathymetry locally a series of volcanic edifices stopped the basinward propagation of the dw ftb a landward change in the dominant structural style from asymmetric imbricate thrust sheets to pseudo symmetric detachment folds is generally observed related to the landward thickening of the detached shales

going nano a new step toward understanding the processes governing freshwater ooid formation marine and freshwater ooids were historically thought to form by purely physicochemical processes in turbulent environments recently organomineralization has been identified as a key process for the initiation of freshwater ooid cortex formation but the exact biochemical mechanism s involved and subsequent contribution to the development of the growing cortex remain unknown here we show that photosynthetic microbes not only enhance early carbonate precipitation around the ooid nucleus but also control the formation of the entire cortex in freshwater ooids from lake geneva switzerland microbial extracellular polymeric substances are first permineralized as amorphous magnesium silicates am mg si before being calcified an 5 6 depletion of 13 c in ooid cortices compared to both bulk values and carbonate nuclei supports this photosynthetic microbial mechanism and argues against contributions from sulfate reducing bacteria or methanogens these data have significant implications for paleoenvironmental studies since photosynthetic microbes now provide an alternative to turbulent hydrodynamic conditions in the formation of freshwater ooids

prestack depth migration of primary and surface related multiple reflections part i imaging surface related multiples i e all seismic waves reflected at the free surface at least once often severely contaminate seismic recordings because conventional imaging techniques require input data that consist of primary reflections only significant processing effort is commonly dedicated to attenuating multiples prior to migration on the other hand surface related multiples provide additional illumination of the subsurface and therefore should not be considered as noise we present a prestack depth migration method that allows primary and multiple reflections to be imaged simultaneously depth imaging using primary and multiple reflections dipmr involves decomposing the datainto upgoing and downgoing wave constituents followed by downward extrapolation artifacts generated by interference of upgoing and downgoing events not associated with the same subsurface reflection points crosstalk are attenuated by using a 2d deconvolution imaging condition in contrast to existing methods dipmr doe

diagenetic pathways linked to labile mg clays in lacustrine carbonate reservoirs a model for the origin of secondary porosity in the cretaceous pre salt barra velha formation offshore brazil

formation of silica oncoids around geysers and hot springs at el tatio northern chile siliceous oncoids up to 4 cm in diameter are common on the laterally extensive sinter aprons that surround the spectacular geysers and hot springs at el tatio in northern chile many of these complex oncoids developed close to geyser and spring vents that discharge boiling water internally the oncoids which are composed of precipitated amorphous silica are formed of complex arrays of spicules and concentric laminae as well as detrital volcanic grains spicular growth is dominant in most examples the formation and growth of the spicules and concentric laminae were mediated by a microbial community which included filamentous microbes mucus and possibly bacteria the microbes and mucus were silicified by replacement and encrustation in some laminae the filamentous microbes lay parallel to the growth surface in other laminae most filaments forming the thin mats were suberect amorphous silica precipitated between the filaments occluded porosity and commonly disguised the microbial fabric the oncoids grew on the proximal sinter aprons around the geyser vents and hot spring pools most growth took place subaerially with the silica delivered to the precipitation sites by splashing water from the geysers and or periodic shallow flooding of the discharge aprons unlike silica oncoids at other geothermal sites vertical growth of oncoids that formed in some rimstone pools was not limited by water depth

microbialite formation in seawater of increased alkalinity satonda crater lake indonesia abstract the crater lake of the small volcanic island satonda indonesia is unique for its red algal microbial reefs thriving in marine derived water of increased alkalinity the lake is a potential analogue for ancient oceans sustaining microbialites under open marine conditions current reef surfaces are dominated by living red algae covered by non calcified biofilms with scattered cyanobacteria and diatoms minor caco3 precipitates are restricted to the seasonally flooded reef tops which develop biofilms up to 500 m thick dominated by the cyanobacteria pleurocapsa calothrix phormidium and hyella microcrystalline aragonite patches form within the biofilm mucilage and fibrous aragonite cements grow in exopolymer poor spaces such as the inside of dead lysed green algal cells and reef framework voids cementation of lysed hadromerid sponge resting bodies results in the formation of wetheredella like structures hydrochemistry data and model calculations indicate that co2 degassing after seasonal mixis can shift the carbonate equilibrium to cause caco3 precipitation increased concentrations of dissolved inorganic carbon limit the ability of autotrophic biofilm microorganisms to shift the carbonate equilibrium therefore photosynthesis induced cyanobacterial calcification does not occur instead passive diffusion controlled eps mediated permineralization of biofilm mucus at contact with the considerably supersaturated open lake water takes place in contrast to extreme soda lakes the release of ca2 from aerobic degradation of extracellular polymeric substances does not support caco3 precipitation in satonda because the simultaneously released co2 is insufficiently buffered subfossil reef parts comprise green algal tufts encrusted by microstromatolites with layers of fibrous aragonite and an amorphous unidentified mg si phase the microstromatolites probably formed when lake satonda evolved from seawater to ca2 depleted raised alkalinity conditions because of sulfate reduction in bottom sediments and pronounced seasonality with deep mixing events and strong co2 degassing the latter effect caused rapid growth of fibrous aragonite while mg si layers replaced the initially mg calcite impregnated biofilms this could be explained by dissolution of siliceous diatoms and sponge spicules at high ph followed by mg calcite dissolution and mg silica precipitation at low ph due to heterotrophic activity within the entombed biofilms

fluid inclusion analysis of twinned selenite gypsum beds from the miocene of the madrid basin spain implication on dolomite bioformation this research work is centred on continental lacustrine gypsum deposits of miocene age cropping out in the easternmost part of the madrid basin these gypsum deposits accumulated in a continental saline lake are characterized by a spectacular distinctive christmas tree morphology and a peculiar dolomite replacement a combination of microscopic petrography and scanning electron microscopy and analytical techniques fluid inclusion microthermometry x ray energy dispersive spectroscopy and x ray diffractometry was used in order to study the crystallographic distribution and the composition of the fluid inclusions within the gypsum the objectives were to characterize the continental brine from which the mineral precipitated and to detect mineral and element traces that could indicate early diagenetic processes altering the gypsum deposits data from primary fluid inclusions indicated that gypsum precipitated from an aqueous fluid lake water of low to moderate total salinity between 20 and 90 g l nacl secondary fluid inclusions represent interstitial lake brine in contact with gypsum slightly enriched in total salt content as crystal formation proceeded textural ultrastructural and microanalytical analysis indicate that the presence of dolomite precipitates inside the gypsum layers is related to the microbial colonization of the gypsum deposits and the biomineralization of the cell walls and extracellular polymeric substances around the cells our investigation emphasizes necessity of a multidisciplinary approach to assess geobiological processes

increasing illumination and sensitivity of reverse time migration with internal multiples reverse time migration is a two way time domain finite frequency technique that accurately handles the propagation of complex scattered waves and produces a bandlimited representation of the subsurface structure that is conventionally assumed to be linear in the contrasts in model parameters because of this underlying linear singlescattering assumption most implementations of this method do not satisfy the energy conservation principle and do not optimally use illumination and model sensitivity of multiply scattered waves migrating multiply scattered waves requires preserving the non linear relation between the image and perturbation of model parameters i modify the extrapolation of source and receiver wavefields to more accurately handle multiply scattered waves i extend the concept of the imaging condition in order to map into the subsurface structurally coherent seismic events that correspond to the interaction of both singly and multiply scattered waves this results in an imaging process referred to here as non linear reverse time migration it includes a strategy that analyses separated contributions of singly and multiply scattered waves to a final non linear image the goal is to provide a tool suitable for seismic interpretation and potentially migration velocity analysis that benefits from increased illumination and sensitivity from multiply scattered seismic waves it is noteworthy that this method can migrate internal multiples a clear advantage for imaging challenging complex subsurface features e g in salt and basalt environments the results of synthetic seismic imaging experiments including a subsalt imaging example illustrate the technique

diversity and origin of quartz cements in continental carbonates example from the lower cretaceous rift deposits of the south atlantic margin abstract silica precipitation in continental carbonates is a common process occurring during sedimentation and diagenesis the lower cretaceous rift deposits of the south atlantic equatorial margin which are intensively explored by petroleum companies provide good examples of such silicifications in carbonates exhibiting a wide diversity of petrographic habit of early to late quartz cements in order to understand the palaeoenvironmental and diagenetic conditions leading to this diversity we integrated detailed petrography of diagenetic sequences and quartz habit with 18oquartz measurements by sims of individual cements observed in samples from the offshore and onshore basins of the west african margin the petrographic description highlights the omnipresence of early fibrous microquartz cements exhibiting either length fast or length slow habit in addition to laminated microquartz and micro or mega quartz forms amongst the isotopic analysis the 18oquartz data show that length slow cements are generally strongly enriched in 18o 18oquartz ranging from 31 to 37 smow whereas length fast forms show less elevated values 25 c and

can we image beneath salt a decade has passed since petroleum geologists first recognized that jurassic salt in the extensional gulf of mexico often appeared to be thrust between layers of younger sediments since then many teams of geoscientists have been faced with the problems of either 1 improving subsalt quality through better data collection or 2 making subsalt interpretations from data of very poor quality these problems aren t new in fact they have faced the seismic industry from the beginning in the gulf of mexico however the cause of the problem salt is obvious in particular high velocity structurally contoured salt bodies act as sonic lenses that disperse or concentrate seismic signals in haphazard ways many advances in seismic imaging technology have been in response to these salt related problems of velocity and structure

generalized staining algorithm for seismic modeling and migration abstractthe staining algorithm is introduced to improve the signal to noise ratio s n of poorly illuminated subsurface structures in seismic imaging however the amplitudes of the original and the stained wavefield i e the real and the imaginary wavefields differ by several orders of magnitude and the waveform of the stained wavefield may be greatly distorted we have developed a generalized staining algorithm gsa to achieve amplitude preservation in the stained wavefield the real wavefield and the stained wavefield propagate in the same velocity model a source wavelet is used as the source of the real wavefield however the real wavefield is extracted from the stained area as the source of the stained wavefield the gsa maintains some properties of the original staining algorithm the stained wavefield is synchronized with the real wavefield and it contains only information relevant to the target region by imaging with the stained wavefield we obtain higher s ns in images of target struct

carbonate and silicate biomineralization in a hypersaline microbial mat mesaieed sabkha qatar roles of bacteria extracellular polymeric substances and viruses in a modern peritidal microbial mat from qatar both biomediated carbonates and mg rich clay minerals palygorskite were identified the mat ca 5 cm thick shows a clear lamination reflecting different microbial communities the initial precipitates within the top millimetres of the mat are composed of ca mg si al s amorphous nanoparticles few tens of nanometres that replace the ultrastructure of extracellular polymeric substances the extracellular polymeric substances are enriched in the same cations and act as a substrate for mineral nucleation successively crystallites of palygorskite fibres associated with carbonate nanocrystals develop commonly surrounding bacterial bodies micron sized crystals of low mg calcite are the most common precipitates together with subordinate aragonite very high mg calcite dolomite and ankerite pyrite nanocrystals and framboids are present in the deeper layers of the mat calcite crystallites form conical structures circular to triangular hexagonal in cross section evolving to crystals with rhombohedral terminations some crystallite bundles develop into dumb bell and stellate forms spheroidal organo mineral structures are also common within the mat nanospheres a few tens of nanometres in diameter occur attached to coccoid bacteria and within their cells these are interpreted as permineralized viruses and could be significant as nuclei for crystallite crystal precipitation microspheres 1 to 10 m in diameter result from intracellular permineralization within bacteria or the mineralization of the bacteria themselves carbonates and clay minerals are commonly aggregated to form peloids tens of microns in size surrounded by residual organic matter magnesium silicate and carbonate precipitation are likely to have been driven by ph saturation index redox changes within the mat related to microenvironmental chemical changes induced by the microbes extracellular polymeric substances viruses and their degradation

reverse time migration using water bottom related multiples abstract reverse time migration of multiples can be used to construct subsurface structures where primaries cannot illuminate well however the images generated using multiples suffer from severe artefacts due to the cross talks created by interference among unrelated multiples we developed a migration approach using water bottom related multiples to reduce these cross talk artefacts this approach first isolates primaries from the original data and predicts water column primaries the nth order water column multiples can be obtained by auto convolving the water column primaries n times followed by convolving the nth order water column multiples with the primaries to extract the n 1 th order water bottom related multiples the approach takes the nth order water column multiples as the secondary source and regards the n 1 th order water bottom related multiples as the receiver wavefield followed by a cross correlation imaging condition numerical examples from synthetic and field data sets reveal that our approach can provide images with substantially fewer cross talk artefacts than conventional reverse time migration using multiples as well as greatly improving shallow imaging compared with reverse time migration of primaries

metal fixation by bacterial cell walls all biomass contains a significant quantity of metallic constituents and mineralization in living and dead biodebris may contribute to element transport from the hydrosphere into sediments the anionic cell walls of bacteria are remarkable in their ability to fix metals and provide sites for nucleation and growth of minerals results presented show the types of cell wall polymers that are responsible for metal binding in walls of gram positive and gram negative bacteria

reverse time migration of internal multiples for subsalt imaging abstractinternal multiple reflections have different propagation paths than primary reflections and surface related multiples thus they can complement illumination where other waves are unavailable consequently migration of internal multiples may provide vital images in the shadow zone of primary reflections we have developed an approach to image structures with internal multiples with this method internal multiples can be separated and used as data for reverse time migration instead of applying the imaging condition directly to the source and receiver wavefields we decomposed the extrapolated source and receiver waves into up and downgoing waves then we applied the imaging condition to upgoing source and receiver waves to form an up up uu image and also to downgoing source and receiver waves to form a down down dd image the image of the true reflector and the artifacts behave differently in the dd and uu images a similarity analysis is conducted to the uu and dd images to separate true i

in situ silicification of an icelandic hot spring microbial mat implications for microfossil formation transmission electron microscopy and energy dispersive x ray analysis revealed that filamentous phototrophic bacteria resembling chlorojlexus aurantiacus underwent rapid silicification in an icelandic hot spring microbial mat the mineralization associated with the cells occurred both extracellularly within and on the external sheaths of the bacteria and intracellularly within the cytoplasm the exceptional preservation of the bacterial sheaths is due to the presence of distinct mineral nucleation sites this results in the production of silica casts of the bacteria which bear a striking resemblance to microbial remains in ancient microfossil assemblages rcsumc les analyses par microscopie clectronique i transmission et par 1 6nergie dispersive des rayons x ont rcvcl6 que les bactkries filamenteuses phototrophes ressemblant b chlorojlexus aurantiacus ont subi une silicification rapide au sein d un tapis microbien de source therrnale en islande la mineralisation associee aux cellules se presente sous forme extracellulaire b l interieur de et sur la membrane externe des bacteries et sous forrne intracellulaire i l interieur du cytoplasme la preservation exceptionnelle de la membrane des bacteries est due b la presence de sites de nucleation minerale disperses le resultat est la production des moulages siliceux des bactkries affichant une resemblance remarquable avec les vestiges microbiens d anciens assemblages microfossiles traduit par la raaction fossil evidence of the earth s earliest biosphere shows that complex communities of microorganisms were abundant and well distributed throughout the precambrian era among the richest beds of microbial fossils are those found in the gunflint iron formation north of lake superior in canada barghoorn and tyler 1965 the early archaean apex chert of australia schopf and packer 1987 schopf 1993 and the riphean section of the anabar uplift in russia sergeev 1993 all ranging in age from 0 8 to 3 5 ga the distribution and types of microorganisms preserved as cellularly intact microfossils are remarkably similar to those in modern microbial mats in which organic rich layers formed by cyanobacteria and or other types of filamentous bacteria alternate with layers that are comprised of detrital inorganic sediment and authigenic mineral precipitates morse and mackenzie 1990 these benthic microbial communities are considered to be analogous and possibly homologous with those that were preserved billions of years ago as stromato

imaging internal multiples from subsalt vsp data examples of target oriented interferometry seismic interferometry has become a technology of growing interest for imaging borehole seismic data we demonstrate that interferometry of internal multiples can be used to image targets above a borehole receiver array by internal multiples we refer to all types of waves that scatter multiple times inside the model these include for instance interbed intrasalt and water bottom multiples as well as conversions among them we use an interferometry technique that is based on representation theorems for perturbed media and targets the reconstruction of specific primary reflections from multiply reflected waves in this interferometry approach we rely on shot domain wavenumber separation to select the directions of waves arriving at a given receiver using a numerical walkaway waw vsp experiment recorded by a subsalt borehole receiver array in the sigsbee salt model we use the interference of internal multiples to image the salt structure from below in this numerical example the interferometric i

dolomite formation within microbial mats in the coastal sabkha of abu dhabi united arab emirates microbial mediation is the only demonstrated mechanism to precipitate dolomite under earth surface conditions a link between microbial activity and dolomite formation in the sabkha of abu dhabi has until now not been evaluated even though this environment is cited frequently as the type analogue for many ancient evaporitic sequences such an evaluation is the purpose of this study which is based on a geochemical and petrographic investigation of three sites located on the coastal sabkha of abu dhabi along a transect from the intertidal to the supratidal zone this investigation revealed a close association between microbial mats and dolomite suggesting that microbes are involved in the mineralization process observations using scanning electron microscopy equipped with a cryotransfer system indicate that authigenic dolomite precipitates within the exopolymeric substances constituting the microbial mats in current models microbial dolomite precipitation is linked to an active microbial activity that sustains high ph and alkalinity and decreased sulphate concentrations in pore waters such models can be applied to the sabkha environment to explain dolomite formation within microbial mats present at the surface of the intertidal zone by contrast these models cannot be applied to the supratidal zone where abundant dolomite is present within buried mats that no longer show signs of intensive microbial activity as no abiotic mechanism is known to form dolomite at earth surface conditions two different hypotheses can reconcile this result in a first scenario all of the dolomite present in the supratidal zone formed in the past when the mats were active at the surface in a second scenario dolomite formation continues within the buried and inactive mats in order to explain dolomite formation in the absence of active microbial metabolisms a revised microbial model is proposed in which the mineral template properties of exopolymeric substances play a crucial role

dolomite silica stromatolites in miocene lacustrine deposits from the duero basin spain the role of organotemplates in the precipitation of dolomite this research provides an ancient analogue for biologically mediated dolomite precipitation in microbial mats and biofilms and describes the involvement of highly structured extracellular polymeric secretion eps templates in dolomite nucleation the structure of eps is shown to match the hexagonal trigonal lattice geometry of dolomite which favoured the epitaxial crystallization of dolomite on the organic substrate this structure of eps also matches the arrangement of silica nanospheres in opal which further accounts for the organically templated formation of opal enabling the non replacive co existence of dolomite and silica the study is focused on a 50 m thick dolomite succession that is exposed in central areas of the tertiary duero basin and was deposited in a mudflat saline lake sedimentary complex during the middle to late miocene 9 to 15 ma in the intermediate intervals of the succession poorly indurated dolomite beds pass gradually into silica beds on the basis of sedimentological compositional geochemical and petrographic data silica and dolomite beds have been interpreted as mineralized microbial mats the silica beds formed in marginal areas of the lake in response to intense evaporative concentrations this resulted in the rapid and early precipitation of opal silicification accounted for the exceptional preservation of the microbial mat structure including biofilms filamentous and coccoid microbes and eps extracellular polymeric secretions have a layered structure each layer being composed of fibres which are arranged in accordance with a reticular pattern with frequent intersection angles at 120 and 60 therefore the structure of eps matches the lattice geometry of dolomite and the arrangement of silica nanospheres in opal additionally eps binds different elements with preference to si and mg the concurrence of suitable composition and surface lattice morphologies in the eps favoured the crystallization of dolomite on the substrate in some cases dolomite nucleation took place epicellularly on coccoid micro organisms which gave way to spheroid crystals organic surfaces enable the inorganic mineral precipitation by lowering the free energy barrier to nucleation most of the microbial mats probably developed on the lake floor under sub aqueous conditions where the decomposition of organic matter took place the subsequent formation of openly packed dolomite crystals with inter related si enriched fibrils throughout is evidence for the pre existence of fibrillar structures in the mats miocene dolomite crystals are poorly ordered and non stoichiometric with a slight ca excess up to 5 which is indicative of the low diagenetic potential the microbial dolomite has towards a more ordered and stoichiometric structure this confirms that microbial imprints can be preserved in the geological record and validates their use as biosignatures

least squares data to data migration an approach for migrating free surface related multiples free surface related multiples can provide extra illumination of the subsurface and thus can be usefully included in migration procedures however most multiple migration approaches require separation of primaries and free surface related multiples or at least prediction of multiples in advance which is time consuming and prone to errors the data to data migration ddm method migrates free surface related multiples by forward and backward propagating the recorded full data containing primaries and free surface related multiples for ddm there is no need to predict or separate multiples but the migration results suffer from the crosstalk generated by crosscorrelations of undesired seismic events e g primaries and second order free surfacerelated multiples we have developed least squares ddm lsddm for marine data to eliminate the crosstalk generated by ddm in each iteration the forward propagated primaries and free surface related multiples are crosscorrelated with the backward propagated primary and free surface related multiple residuals to form the reflectivity gradient we use a three layer model and the marmousi model for numerical tests the results validate that lsddm can provide a migrated image with higher signal to noise ratio and more balanced amplitudes than ddm the lsddm approach might be valuable for general subsurface imaging for marine seismic data when the migration velocity is accurate and the acquired data have sufficient recording time

microbial silica interactions in icelandic hot spring sinter possible analogues for some precambrian siliceous stromatolites silicified deposits such as sinters occur in several modern geothermal environments but the mechanisms of silicification and crucially the role of microorganisms in their construction are still largely unresolved detailed examination of siliceous sinter in particular sections of microstromatolites growing at the krisuvik hot spring iceland reveals that biomineralization contributes a major component to the overall structure with approximately half the sinter thickness attributed to silicified microorganisms almost all microorganisms observed under the scanning electron microscope sem are mineralized with epicellular silica ranging in thickness from 5 lm coatings on individual cells to regions where entire colonies are cemented together in an amorphous silica matrix tens of micrometres thick within the overall profile there appears to be two very distinct types of laminae that alternate repeatedly throughout the microstromatolite microbial layers are predominantly consisting of filamentous intact vertically aligned biomineralized cyanobacteria identified as calothrix and fischerella sp and weakly laminated silica layers which appear to be devoid of any microbial component the microbial layers commonly have a sharply defined base overlying the weakly laminated silica and a gradational upper surface merging into the weakly laminated silica these cyclic laminations are probably explained by variations in microbial activity active growth during spring summer allows the microorganisms to keep pace with silicification with the cell surfaces facilitating silicification while during their natural slow growth phase in the dark autumn winter months silicification exceeds the bacteria s ability to compensate i e grow upwards at this stage the microbial colony is probably not essential to microstromatolite formation with silicification presumably occurring abiogenically when conditions once again become favourable for growth recolonization of the solid silica surface by free living bacteria occurs cell motility is not responsible for the laminations we have also observed that microbial populations within the microstromatolite some several mm in depth appear viable i e they still have their pigmentation the trichomes are not collapsed cell walls are unbroken cytoplasm is still present and they proved culturable this suggests that the bulk of silicification occurred rapidly probably while the cells were still alive surprisingly however measurements of light transmittance through sections of the microstromatolite revealed that photosynthetically active light pal only transmitted through the uppermost 2 mm therefore the deeper microbial

stevensite in the modern thrombolites of lake clifton western australia a missing link in microbialite mineralization microbialites form the earliest macroscopic evidence of life and have always been important in particular aquatic ecosystems they demonstrate the remarkable ability of microorganisms to provide the foundation for structures that can rival coral reefs in size microbialites are generally assumed to form by microbial trapping and binding of detrital grains by carbonate organomineralization of microbial biofi lms or by inorganic mineralization around microbial templates here we present a signifi cant discovery that modern thrombolitic microbialites in lake clifton western australia gain their initial structural rigidity from biofi lm mineralization by the trioctahedral smectite mineral stevensite this nucleates in and around microbial fi lament walls when biological processes suppress carbon and ca activities leaving mg to bind with silica and form a microporous framework that replaces and infi lls the fi lament web after microbial materials are entombed local carbon and ca activities rise suffi ciently for aragonite microcrystals to grow within the stevensite matrix and perhaps replace it entirely with eradication of biogenic textural features this may explain why many ancient microbialite carbonates lack clear evidence for biogenicity stevensite may provide the missing link between microbial organomineralization and subsequent abiotic calcifi cation

staining algorithm for seismic modeling and migration abstractin seismic migration some structures such as those in subsalt shadow zones are not imaged well the signal in these areas may be even weaker than the artifacts elsewhere we evaluated a method to significantly improve the signal to noise ratio s n in poorly illuminated areas of the model we constructed a phantom wavefield an extension of the wavefield to the complex domain the imaginary wavefield was synchronized with the real wavefield but it contained only the events relevant to a target region of the model which was specified using a staining algorithm the real wavefield interacted with the entire model however all structures except for the target were transparent to the imaginary wavefield which is excited only when the real wavefront arrives at the target structure the real and the imaginary source wavefields were crosscorrelated with the regular receiver wavefield the results were revealed in two images the conventional reverse time migration image and an image of the target

magnesite formation by microbial activity evidence from a miocene hypersaline lake abstract this paper provides an ancient analogue for biologically mediated magnesite in lacustrine hypersaline environments thin beds of massive to crudely laminated magnesitic marls occur interbedded with mudstone and evaporite facies deposited in a saline lake mudflat sedimentary system during the lower miocene in the madrid basin central spain exposure of this succession in a recently excavated tunnel and in collected cores offered a good opportunity to study magnesite precipitates that have been preserved in unaltered conditions by primary evaporitic minerals specifically magnesite arranged as host matrix between and enclosed by displacive halite and glauberite crystals occurs closely associated with abundant microfossils embedded in sheets of organic matter interpreted as the remains of extracellular polymeric substances eps even many magnesite clots are seen to have resulted from the agglutination of fossilized bacterial bodies forming a biofilm the close and pervasive association of microorganisms and magnesite is used as evidence that microbes played a fundamental role in the precipitation of this mineral additional traces of microorganisms and microbial activity observed in magnesite beds include the isotopic ratios in the carbonates with the 13c pdb value averaging 6 2 the presence of carbonaceous film like structures and the association with patchly distributed pyrite and celestite minerals on the basis of the combined sedimentological mineralogical chemical and morphological signatures of the magnesitic deposits it is postulated that carbonate crystals precipitated in microbial mats magnesite crystals together with celestite local barite and or pyrite have been also observed to replace silicate and sulphate minerals it is suggested that magnesite precipitation was biochemically coupled with the early dissolution of the associated sedimentary minerals microbes may have used the associated minerals as source of energy and or of essential elements