seismic imaging of complex structures by non linear traveltime inversion of dense wide angle data application to a thrust belt summary we have used a dense wide angle data set to test a two step procedure for the separate inversion of first arrival and reflection traveltimes data were collected in a complex thrust belt environment southern italy along a 14 km line with closely spaced sources 60 m and receivers 90 m we have applied a fully non linear tomographic technique specially designed to image complex structures to over 6400 first arrival traveltimes in order to determine a detailed velocity model a bi cubic spline velocity model parametrization is used the inversion strategy follows a multiscale approach and employs a non linear velocity optimization scheme the tomographic velocity model is adopted as the background reference medium for a subsequent interface inversion aimed at imaging a target upper crust reflector the interface inversion method is also based on a multiscale approach and uses a non linear technique for model parameters interface position nodes optimization we have applied the interface inversion method to over 1600 reflection traveltimes of a target event picked both in the near and in the wide angle offset range the retrieved interface is well resolved in the central part of the model where ray coverage mainly includes clear post critical reflections and the background velocity model is accurate in depth thanks to large offset deep turning rays the velocity and interface models thus determined are consistent with vertical seismic profiling data and correlate well with the geometry of known geological structures this study shows that the used inversion approach is efficient for target orientated investigations in complex geological environments

the vallo di diano fault system new evidence for an active range bounding fault in southern italy using shallow high resolution seismic profiling range bounding normal faults can presentsignificant challenges for seis mic exploration this is the case of the fault system bounding the vallo di diano the largest intermountain basin in the southern apennines seismic belt industry reflection profiles define the large scale structure of the basin but barely image the shallow fault system due to unfavorable topographic and near surface conditions along the foothills of the eastern range we present two high resolution hr wide aperture profiles recorded attheeastern marginof thebasin acrossunreported scarpsthat affect middle late pleistocene alluvial fans and slope debris the survey is aimed at identifying possible recent faulting across these challenging terrains and at understanding the re lationship between shallow structures and the master range bounding fault at depth common depth point processing of wide aperture reflection data and first arrival tra vel time tomography provide detailed images of the upper 200 300 m and sounding evidence of recent activity along previously unknown splays of the fault system these splays dissect the mesozoic limestone bedrock and alluvial fan sequences affecting theirdepositionalpattern veryhighresolutionvpandreflectivityimagesalsogivehints ofpossiblecoseismicsurfacefaultinginholocenecolluvia theseresultshaverelevant implicationsfortheevaluationoftheseismogenicpotentialoftherange boundingfault systemandforseismichazardassessmentofthedenselyurbanizedvallodidianobasin online material table of acquisition and data processing parameters and figures showing schematic stratigraphy of the vallo di diano basin restoration of fault dis placement along the hr profile and resolution of the p velocity tomographic images for the vhr profile

integrating high resolution refraction data into near surface seismic reflection data processing and interpretation shallow seismic reflection surveys commonly suffer from poor data quality in the upper 100 to 150 ms of the stacked seismic record because of shot associated noise surface waves and direct arrivals that obscure the reflected energy nevertheless insight into lateral changes in shallow structure and stratigraphy can still be obtained from these data by using first arrival picks in a refraction analysis to derive a near surface velocity model we have used turning ray tomography to model near surface velocities from seismic reflection profiles recorded in the hueco bolson of west texas and southern new mexico the results of this analysis are interval velocity models for the upper 150 to 300 m of the seismic profiles which delineate geologic features that were not interpretable from the stacked records alone in addition the interval velocity models lead to improved time to depth conversion when converted to stacking velocities they may provide a better estimate of stacking velocities at early travelt

multiscale deformable layer tomography traveltime tomography relies on broad ray angle coverage to constrain the spatial location of velocity anomalies when ray angle coverage is narrow cell or grid tomography can be plagued by smearing artifacts bearing imprints of the raypaths multiscale deformable layer tomography dlt which inverts for the geometry of velocity interfaces instead of velocity can be more effective than grid based tomography in mitigating such artifacts especially when velocity values are known for parts of the model the dlt model consists of geologically sensible layers represented by triangular prisms in areas of good ray coverage dlt can be used to invert simultaneously for layer velocities as well as interface geometry tests of synthetic models of crosswell refraction 2d tomostatics and 3d vertical seismic profiling vsp first arrival data sets show that dlt can produce solutions superior to those produced by cell or grid based tomography

natural emissions of methane from geothermal and volcanic sources in europe it has recently been demonstrated that methane emission from lithosphere degassing is an important component of the natural greenhouse gas atmospheric budget globally the geological sources are mainly due to seepage from hydrocarbon prone sedimentary basins and subordinately from geothermal volcanic fluxes this work provides a first estimate of methane emission from the geothermal volcanic component at european level in europe 28 countries have geothermal systems and at least 10 countries host surface geothermal manifestations hot springs mofettes gas vents even if direct methane flux measurements are available only for a few small areas in italy a fair number of data on co2 ch4 and steam composition and flux from geothermal manifestations are today available for 6 countries czech republic germany greece iceland italy spain following the emission factor and area based approach the available data have been analyzed and have led to an early and conservative estimate of methane emission into the atmosphere around 10 000 ton yr 4000 16 000 ton yr basically from an area smaller than 4000 km2 with a speculative upper limit in the order of 105 ton yr only 4 18 of the conservative estimate about 720 ton yr is due to 12 european volcanoes where methane concentration in volcanic gases is generally in the order of a few tens of ppmv volcanoes are thus not a significant methane source while the largest emission is due to geothermal areas which may be situated next to volcanoes or independent here inorganic synthesis thermometamorphism and thermal breakdown of organic matter are substantial methane flux can reach hundreds of ton yr from small individual vents geothermal methane is mainly released in three countries located in the main high heat flow regions italy greece and iceland turkey is likely a fourth important contributor but the absolute lack of data prevents any emission estimate therefore the actual european geothermal volcanic methane emission could be easily projected to the 105 ton yr levels reaching the magnitude of some other natural sources such as forest fires or wild animals

image based near surface modeling for statics corrections the near surface is usually defined as the depth interval below the topography composed of a low velocity soil column and weathered rock layer in contrast with the near surface the subsurface is composed of relatively higher velocity rock layers this means that the interface between the near surface and the subsurface often gives rise to a strong shallow reflection the image based method described in this paper makes use of the strong reflection at the base of the near surface to estimate a model for the near surface for statics corrections the method is based on construction of constant velocity migration volume by prestack time migration of shot records the near surface rms velocities associated with the strong reflector at the base of the near surface are picked from the semblance spectra to create the near surface rms velocity field this laterally varying but vertically invariant velocity field actually is equivalent to the near surface interval velocity field which can be used to perform p

overthrust imaging with tomo datuming a case study through the use of iterative turning ray tomography followed by wave equation datuming or tomo datuming and prestack depth migration we generate accurate prestack images of seismic data in overthrust areas containing both highly variable near surface velocities and rough topography in tomo datuming we downward continue shot records from the topography to a horizontal datum using velocities estimated from tomography turning ray tomography often provides a more accurate near surface velocity model than that from refraction statics the main advantage of tomo datuming over tomo statics tomography plus static corrections or refraction statics is that instead of applying a vertical time shift to the data tomo datuming propagates the recorded wavefield to the new datum we find that tomo datuming better reconstructs diffractions and reflections subsequently providing better images after migration in the datuming process we use a recursive finite difference fd scheme to extrapolate wavefield withou

3d seismic traveltime tomography imaging of the shallow subsurface at the c o2 sink project site ketzin germany a 3d reflection seismic survey was performed in 2005 at the ketzin carbon dioxide c o2 pilot geological storage site the c o2 sink project near berlin germany to image the geological structure of the site to depths of about 1 km because of the acquisition geometry frequency limitations of the source and artefacts of the data processing detailed structures shallower than about 150 m were unclear to obtain structural images of the shallow subsurface we applied 3d traveltime tomography to data near the top of the ketzin anticline where faulting is present understanding the shallow subsurface structure is important for long term monitoring aspects of the project after c o2 has been injected into a saline aquifer at about 650 m depth we used a 3d traveltime tomography algorithm based on a combination ofsolving for 3d velocity structure and static corrections in the inversion process to account for artefacts in the velocity structure because of smearing effects from the unconsolidated cover th

a layer cell tomography method for near surface velocity model building using first arrivals variations in the topography and thickness of the near surface low velocity weathering zone negatively impact the quality of onshore seismic data and images such impact can be mitigated using accurate near surface velocity models we propose to estimate near surface velocities using a layer cell tomography method which sequentially combines the model parameterizations of multiscale deformable layer tomography dlt and cell tomography we first estimate the long wavelength velocity variations using multiscale dlt which maps the undulating geometry of the weathering base where the angular coverage of first arrival raypaths is limited then we convert the solution model of multiscale dlt into a cell model and further determine the short wavelength velocity variations using cell tomography we test the proposed layer cell tomography method using synthetic datasets and a field dataset the results indicate that the new method improves the resolution of the velocity model from the multiscale dlt and reduces the artifacts in the velocity solution compared to those observed in the cell tomography method reverse time migration of the synthetic data shows that using the velocity model from the layer cell tomography method gives more accurate images of reflectors for the field dataset test the layer cell tomography method yields a high resolution near surface velocity model that could be validated by the improvements in the tomostatic correction results

advanced three dimensional seismic imaging of deep supercritical geothermal rocks in southern tuscany the seismic k horizon is the key to gaining understanding on the deep supercritical geothermal rocks in southern tuscany the k horizon is hosted in metamorphic rocks which cause strong seismic wavefield scattering resulting in a poor signal to noise ratio our study aims to reveal high resolution seismic images of the k horizon below a geothermal field in southern tuscany using an advanced threedimensional seismic depth imaging approach the key seismic pre processing steps in the time domain include muting a large amount of persistent noise based on the statistical analysis of the seismic amplitudes and tomostatics technique to correct for static effects we carried out seismic depth imaging using kirchhoff pre stack depth migration and fresnel volume migration techniques each migration technique was tested with constant and heterogeneous three dimensional velocity models due to the difficulties in determining emergent angles for this low signal to noise ratio data set the migration results with the heterogeneous three dimensional velocity model show less coherent reflections compared to the migration results using the constant velocity model both velocity models however lead to relatively the same structure and depth of the k horizon indicating the similarity of the average velocities along the wave propagation paths in both velocity models with both velocity models fresnel volume migration yields the k horizon with better reflection coherency and higher signal to noise ratio than standard kirchhoff pre stack depth migration nevertheless both migration techniques have been able to reveal the k horizon with relatively high resolution and provide a reliable basis for geothermal rock characterization as well as steering of the first geothermal well penetrating the k horizon

circulation and water mass transformation in a model of the chukchi sea 1 the circulation and water mass transformation in a regional ocean ice model of the chukchi sea are discussed the model has horizontal resolution of o 4 km is forced by fluxes derived from daily ncep reanalysis fields and has seasonally varying transport temperature and salinity imposed at bering strait many of the observed characteristics of the mean circulation and seasonal cycle in the chukchi sea are reproduced the discussion focuses on the branching of the inflow transport into pathways following herald canyon central channel and the alaskan coast the pattern of ice melt and the water mass transformation and formation of winter water and hypersaline water the ice melt pattern and timing is strongly influenced by advection through bering strait high frequency forcing results in a larger region of ice melt particularly over the shoals and in the northern chukchi sea compared to monthly mean forcing in the model the seasonal cycle of salinity in the southern and central chukchi sea is dominated by advection through bering strait while local atmospheric forcing and brine rejection are more important north of herald and hanna shoals and in barrow canyon however since the residence time in the chukchi sea is generally less than 1 year interannual variability in the bering strait salinity will be reflected in the salinity across the chukchi sea and at barrow canyon

recent applications of turning ray tomography recent applications of 2d and 3d turning ray tomography show that near surface velocities are important for structural imaging and reservoir characterization for structural imaging we used turning ray tomography to estimate the near surface velocities for static corrections followed by prestack time migration and the near surface velocities for prestack depth migration two dimensional acoustic finite difference modeling illustrates that wave equation prestack depth migration is very sensitive to the near surface velocities field data demonstrate that turning ray tomography followed by prestack time migration helps to produce superior images in complex geologic settings when the near surface velocity model is integrated into a background velocity model for prestack depth migration we find that wave propagation is very sensitive to the velocities immediately below the topography for shallow reservoir characterization we developed and applied azimuthal turning ray tomography to investigate observed apparent azimuthal traveltime variations using a wide azimuth land seismic survey from a heavy oil field at surmont canada we found that the apparent azimuthal velocity variations are not necessarily related to azimuthal anisotropy or horizontal transverse isotropy hti induced by the stress field or fractures near surface heterogeneity and the acquisition footprint also could result in apparent azimuthal variations

non uniqueness with refraction inversion a syncline model study non uniqueness occurs with the 1d parametrization of refraction traveltime graphs in the vertical dimension and with the 2d lateral resolution of individual layers in the horizontal dimension the most common source of non uniqueness is the inversion algorithm used to generate the starting model this study applies 1d 1 5d and 2d inversion algorithms to traveltime data for a syncline 2d model in order to generate starting models for wave path eikonal traveltime tomography the 1d tau p algorithm produced a tomogram with an anticline rather than a syncline and an artefact with a high seismic velocity the 2d generalized reciprocal method generated tomograms that accurately reproduced the syncline together with narrow regions at the thalweg with seismic velocities that are less than and greater than the true seismic velocities as well as the true values it is concluded that 2d inversion algorithms which explicitly identify forward and reverse traveltime data are required to generate useful starting models in the near surface where irregular refractors are common the most likely tomogram can be selected as either the simplest model or with a priori information such as head wave amplitudes the determination of vertical velocity functions within individual layers is also subject to non uniqueness depths computed with vertical velocity gradients which are the default with many tomography programs are generally 50 greater than those computed with constant velocities for the same traveltime data the average vertical velocity provides a more accurate measure of depth estimates where it can be derived non uniqueness is a fundamental reality with the inversion of all near surface seismic refraction data unless specific measures are taken to explicitly address non uniqueness then the production of a single refraction tomogram which fits the traveltime data to sufficient accuracy does not necessarily demonstrate that the result is either correct or the most probable

observed increases in bering strait oceanic fluxes from the pacific to the arctic from 2001 to 2011 and their impacts on the arctic ocean water column 1 mooring data indicate the bering strait throughflow increases 50 from 2001 0 7 sv to 2011 1 1 sv driving heat and freshwater flux increases increase in the pacific arctic pressure head explains two thirds of the change the rest being attributable to weaker local winds the 2011 heat flux 5 1020j approaches the previous record high 2007 due to transport increases and warmer lower layer ll temperatures despite surface temperature sst cooling in the last decade warmer ll waters arrive earlier 1 6 1 1 days yr though winds and sst are typical for recent decades maximum summer salinities likely set in the bering sea remain remarkably constant 33 1 psu over the decade elucidating the stable salinity of the western arctic cold halocline despite this freshwater flux variability strongly driven by transport exceeds variability in other arctic freshwater sources remote data winds sst prove insufficient for quantifying variability indicating interannual change can still only be assessed by in situ year round measurements

3 d tomographic static correction a 3 d tomographic inversion approach based on a surface consistent model for static corrections is presented in this paper direct reflected and refracted waves are used simultaneously to update the near surface model we analyze the characteristics of the first break traveltime in complicated low velocity layers to improve the accuracy for the velocity model the various first break times from direct reflected and refracted waves are considered for model inversion a fractal algorithm which overcomes the error caused by wavelet shape differences is applied to pick first breaks it also overcomes the leg jump of refractions the method can pick a large number of first breaks automatically the raypaths and traveltimes are calculated with a 3 d ray tracer that does not increase computation time for complicated geological models our method can determine the raypath associated with minimum traveltimes regardless of wave mode direct refracted or reflected we use a least squares approach in conjunction with a matrix decomposition to reconstruct a 3 d velocity model from the actual first break times obtained from 3 d data finally long and short wavelength static corrections are calculated concurrently based on the reconstructed velocity profile the method can be applied to wide line profiles crooked lines and 2 d and 3 d seismic survey geometries the results applied to a real 3 d data example indicate that the 3 d tomographic static corrections are effective for field data

shallow velocity depth model imaging by refraction tomography1 a tomographic imaging technique combined with coherence inversion is proposed for constructing a near surface model from refraction events a model obtained from coherence inversion serves as a good background model for the tomographic reconstruction a simultaneous iterative reconstruction technique sirt algorithm was used for this purpose this is a simple algorithm and can be easily adapted to irregular acquisition geometry and limited angular aperture using synthetic data it was shown that the proposed procedure can be used for determination of local velocity anomalies in a shallow subsurface the technique was also tested on a real data set

tomographic velocity model building of the near surface with velocity inversion interfaces a test using the yilmaz model first arrival traveltime tomography is a popular approach to building the near surface velocity models for oil and gas exploration mining geoengineering and environmental studies however the presence of velocity inversion interfaces viis across which the overlying velocity is higher than the underlying velocity might corrupt the tomographic solutions this is because most first arrival raypaths will not traverse along any vii such as the top of a low velocity zone we have examined the impact of viis on first arrival tomographic velocity model building of the near surface using a synthetic near surface velocity model this examination confirms the severe impact of viis on first arrival tomography when the source to receiver offset is greater than the lateral extent of the viis good near surface velocity models can still be established using a multiscale deformable layer tomography dlt which uses a layer based model parameterization and a multiscale scheme as regularization compared with the results from a commercial grid based tomography the dlt delivers much better near surface statics solutions and less error in the images of deep reflectors

traveltime tomographic inversion with simultaneous static corrections well worth the effort we have developed a first arrival traveltime inversion scheme that jointly solves for seismic velocities and source and receiver static time terms the static time terms are included to compensate for varying time delays introduced by the near surface low velocity layer that is too thin to be resolved by tomography results on a real data set consisting of picked first arrival times from a seismic reflection 2d 3d experiment in a crystalline environment show that the tomography static time terms are very similar in values and distribution to refraction static corrections computed using standard refraction statics software when applied to 3d seismic reflection data tomography static time terms produce similar or more coherent seismic reflection images compared to the images using corrections from standard refraction static software furthermore the method provides a much more detailed model of the near surface bedrock velocity than standard software when the static time terms are included in the inversion low velocity zones in this model correlate with other geologic and geophysical data suggesting that our method results in a reliable model in addition to generally being required in seismic reflection imaging static corrections are also necessary in traveltime tomography to obtain high fidelity velocity images of the subsurface

a new direction for shallow refraction seismology integrating amplitudes and traveltimes with the refraction convolution section the refraction convolution section rcs is a new method for imaging shallow seismic refraction data it is a simple and efficient approach to full trace processing which generates a time cross section similar to the familiar reflection cross section the rcs advances the interpretation of shallow seismic refraction data through the inclusion of time structure and amplitudes within a single presentation the rcs is generated by the convolution of forward and reverse shot records the convolution operation effectively adds the first arrival traveltimes of each pair of forward and reverse traces and produces a measure of the depth to the refracting interface in units of time which is equivalent to the time depth function of the generalized reciprocal method grm convolution also multiplies the amplitudes of first arrival signals to a good approximation this operation compensates for the large effects of geometrical spreading with the result that the convolved amplitude is essentially proportional to the square of the head coefficient the signal to noise s n ratios of the rcs show much less variation than those on the original shot records the head coefficient is approximately proportional to the ratio of the specific acoustic impedances in the upper layer and in the refractor the convolved amplitudes or the equivalent shot amplitude products can be useful in resolving ambiguities in the determination of wave speeds the rcs can also include a separation between each pair of forward and reverse traces in order to accommodate the offset distance in a manner similar to the xy spacing of the grm the use of finite xy values improves the resolution of lateral variations in both amplitudes and time depths the use of amplitudes with 3d data effectively improves the spatial resolution of wave speeds by almost an order of magnitude amplitudes provide a measure of refractor wave speeds at each detector whereas the analysis of traveltimes provides a measure over several detectors commonly a minimum of six the ratio of amplitudes obtained with different shot azimuths provides a detailed qualitative measure of azimuthal anisotropy and in turn of rock fabric the rcs facilitates the stacking of refraction data in a manner similar to the common midpoint methods of reflection seismology it can significantly improve s n ratios most of the data processing with the rcs as with the grm is carried out in the time domain rather than in the depth domain this is a significant advantage because the realities of undetected layers incomplete sampling of the detected layers and inappropriate sampling in the horizontal rather than the vertical direction result in traveltime data that are neither a complete an accurate nor a representative portrayal of the wave speed stratification the rcs facilitates the advancement of shallow refraction seismology through the application of current seismic reflection acquisition processing and interpretation technology

theory of supervirtual refraction interferometry summary inverting for the subsurface velocity distribution by refraction traveltime tomography is a well accepted imaging method by both the exploration and earthquake seismology communities a significant drawback however is that the recorded traces become noisier with increasing offset from the source position and so accurate picking of traveltimes in far offset traces is often prevented to enhance the signal to noise ratio snr of the far offset traces we present the theory of supervirtual refraction interferometry where the snr of far offset head wave arrivals can be theoretically increased by a factor proportional to here n is the number of receiver or source positions associated with the recording and generation of the head wave arrival there are two steps to this methodology correlation and summation of the data to generate traces with virtual head wave arrivals followed by the convolution of the data with the virtual traces to create traces with supervirtual head wave arrivals this method is valid for any medium that generates head wave arrivals recorded by the geophones results with both synthetic traces and field data demonstrate the feasibility of this method there are at least four significant benefits of supervirtual interferometry 1 an enhanced snr of far offset traces so the first arrival traveltimes of the noisy far offset traces can be more reliably picked to extend the useful aperture of the data 2 the snr of head waves in a trace that arrive later than the first arrival can be enhanced for accurate traveltime picking and subsequent inversion by later arrival traveltime tomography 3 common receiver pair gathers can be analysed to detect the presence of diving waves in the first arrivals which can be used to assess the nature of the refracting boundary and 4 the source statics term is eliminated in the correlation operations so that the timing of the virtual traces is independent of the source excitation time this suggests the possibility of applying this method to earthquake data recorded by receivers that are inline with the refraction paths and source locations

identification of cross valley faults in the maynardville limestone oak ridge reservation tennessee using seismic refraction tomography first arrival times from p wave refraction and reflection seismic surveys along bear creek valley on the oak ridge reservation tennessee were inverted to produce refraction tomographic velocity images showing seismic velocity variations within thinly mantled karstic bedrock to a depth of approximately 20 m inverted velocities are consistent with two distinct bedrock groups the nolichucky shale 2 730 5 150 m s and maynardville limestone 3 940 7 575 m s low velocity zones 2 700 4 000 m s in the tomographic images correspond to previously inferred cross valley strike slip faults in places these faults create permeability barriers that offset or block groundwater flowing along bear creek valley these faults may also force groundwater contaminants such as dense non aqueous phase liquids to migrate laterally or downward spreading contamination throughout the groundwater system other previously unmapped cross valley faults may also be visible in the tomographic images borehole logs suggest the low velocity values are caused by low rigidity fractured and vuggy rock water zones cavities and collapse features surface streams including bear creek tend to lie directly above these low velocity zones suggesting fault and fracture control of surface drainage in addition to the subsurface flow system in some cases fault zones are also associated with bedrock depressions and thicker accumulations of unconsolidated sediment

evolution and dynamics of the flow through herald canyon in the western chukchi sea abstract the flow of summer and winter pacific water masses through herald canyon is investigated using data from a high resolution hydrographic velocity survey conducted in summer 2004 the survey was part of the russian american long term census of the arctic rusalca program and consisted of four cross canyon transects occupied over a 2 day period at the time of the survey dense winter water was entering the western side of the canyon from the chukchi sea flowing alongside a poleward jet of summer water on the canyon s eastern flank as the dense water progressed northward it switched sides of the canyon and underwent a sudden increase in layer thickness this coincided with vertical mixing near the interface of the winter and summer water producing a new water mass mode exiting the canyon all of these features are consistent with the notion of hydraulic activity occurring in the canyon a three layer hydraulic theory is applied to the flow which suggests that it is supercritical and that hydraulic control is likely a lock exchange formulation accurately predicts the northward transport of the winter water the origin of the winter water and the manner in which it drains into the canyon is investigated using satellite ice concentration data atmospheric re analysis fields historical in situ data and a simple circulation model finally the fate of the pacific water exiting the canyon and its connection to the chukchi shelfbreak current is discussed

pathways of pacific water across the chukchi sea a numerical model study 1 pathways of pacific water flowing from the north pacific ocean through bering strait and across the chukchi sea are investigated using a two dimensional barotropic model in the no wind case the flow is driven only by a prescribed steady northward flow of 0 8 sv through bering strait the resulting steady state circulation consists of a broad northeasterly flow basically following the topography with a few areas of intensified currents about half of the inflow travels northwest through hope valley while the other half turns somewhat toward the northeast along the alaskan coast the flow through hope valley is intensified as it passes through herald canyon but much of this flow escapes the canyon to move eastward joining the flow in the broad valley between herald and hanna shoals another area of slightly intensified currents there is a confluence of nearly all of the flow along the alaskan coast west of pt barrow to create a very strong and narrow coastal jet that follows the shelf topography eastward onto the beaufort shelf thus in this no wind case nearly all of the pacific water entering the chukchi sea eventually ends up flowing eastward along the narrow beaufort shelf with no discernable flow across the shelf edge toward the interior canada basin travel times for water parcels to move from bering strait to pt barrow vary tremendously according to the path taken e g less than 6 months along the alaskan coast but about 30 months along the westernmost path through herald canyon this flow field is relatively insensitive to idealized wind forcing when the winds are from the south west or north in which cases the shelf transports tend to be intensified however strong northeasterly to easterly winds are able to completely reverse the flows along the beaufort shelf and the alaskan coast and force most of the throughflow in a more northerly direction across the chukchi sea shelf edge potentially supplying the surface waters of the interior canada basin with pacific water the entire shelf circulation reacts promptly to changing wind conditions with a response time of 2 3 days the intense coastal jet between icy cape and pt barrow implies that dense water formed here from winter coastal polynyas may be quickly swept away along the coast in contrast there is a relatively quiet nearshore region to the west between cape lisburne and icy cape where dense water may accumulate much longer and continue to become denser before it is carried across the shelf

tomostatics turning ray tomography static corrections applications of tomographic velocities to static corrections have drawn much attention recently in computing field statics with the help of seismic tomography geophysical prospecting 1987 w n de amorin et al showed that the waves refracted at the base of a low velocity layer can be used successfully for computing and refining field static corrections in tomographic inversion for velocity plus statics seg expanded abstracts 1991 livia squires et al pointed out that velocity variations and static shifts caused by near surface effects and positional uncertainties can be solved simultaneously using constraint tomography

increasing the number and signal to noise ratio of obs traces with supervirtual refraction interferometry and free surface multiples summary the theory of supervirtual interferometry is modified so that free surface related multiple refractions can be used to enhance the signal to noise ratio snr of primary refraction events by a factor proportional to ns where ns is the number of post critical sources for a specified refraction multiple we also show that refraction multiples can be transformed into primary refraction events recorded at virtual hydrophones located between the actual hydrophones thus data recorded by a coarse sampling of ocean bottom seismic obs stations can be transformed in principle into a virtual survey with p times more obs stations where p is the order of the visible free surface related multiple refractions the key assumption is that the refraction arrivals are those of head waves not pure diving waves the effectiveness of this method is validated with both synthetic obs data and an obs data set recorded offshore from taiwan results show the successful reconstruction of far offset traces out to a source receiver offset of 120km the primary supervirtual traces increase the number of pickable first arrivals from approximately 1600 to more than 3100 for a subset of the obs data set where the source is only on one side of the recording stations in addition the head waves associated with the first order free surface refraction multiples allow for the creation of six new common receiver gathers recorded at virtual obs station located about half way between the actual obs stations this doubles the number of obs stations compared to the original survey and increases the total number of pickable traces from approximately 1600 to more than 6200 in summary our results with the obs data demonstrate that refraction interferometry can sometimes more than quadruple the number of usable traces increase the source receiver offsets fill in the receiver line with a denser distribution of obs stations and provide more reliable picking of first arrivals apotentialliabilityofthismethodisthatlong offsetrefractionarrivalsextractedbyinterferometry might not necessarily be head waves from deeper refraction interfaces the extracted arrivals might be from a shallower interface and so only supply redundant information about that portion of the subsurface nevertheless our tomography example shows the value of these arrivals in reducing artefacts and increasing resolution in the tomogram

application of advanced imaging technologies to carbonate reservoirs in southern china seismic imaging of geologic structures and the definition of reservoir properties is generally difficult in the marine carbonate depositional provinces of southern china this is partly because the near surface exhibits extreme topography and strong lateral velocity variations and also because the subsurface layers are highly deformed as a result of a series of vertical and horizontal movements at different geologic times

refraction traveltime and amplitude corrections for very near surface inhomogeneities the performance of refraction inversion methods that employ the principle of refraction migration whereby traveltimes are laterally migrated by the offset distance which is the horizontal separation between the point of refraction and the point of detection on the surface can be adversely affected by very near surface inhomogeneities even inhomogeneities at single receivers can limit the lateral resolution of detailed seismic velocities in the refractor the generalized reciprocal method statics smoothing method grm ssm is a smoothing rather than a deterministic method for correcting very near surface inhomogeneities of limited lateral extent it is based on the observation that there are only relatively minor differences in the time depths to the target refractor computed for a range of xy distances which is the separation between the reverse and forward traveltimes used to compute the time depth however any traveltime anomalies which originate in the near surface migrate laterally with increasing xy distance therefore an average of the time depths over a range of xy values preserves the architecture of the refractor but significantly minimizes the traveltime anomalies originating in the near surface the grm statics smoothing corrections are obtained by subtracting the average time depth values from those computed with a zero xy value in turn the corrections are subtracted from the traveltimes and the grm algorithms are then re applied to the corrected data although a single application is generally adequate for most sets of field data model studies have indicated that several applications of the grm ssm can be required with severe topographic features such as escarpments in addition very near surface inhomogeneities produce anomalous head wave amplitudes an analogous process using geometric means can largely correct amplitude anomalies furthermore the coincidence of traveltime and amplitude anomalies indicates that variations in the near surface geology rather than variations in the coupling of the receivers are a more likely source of the anomalies the application of the grm ssm together with the averaging of the refractor velocity analysis function over a range of xy values significantly minimizes the generation of artefacts and facilitates the computation of detailed seismic velocities in the refractor at each receiver these detailed seismic velocities together with the grm ssm corrected amplitude products can facilitate the computation of the ratio of the density in the bedrock to that in the weathered layer the accuracy of the computed density ratio improves where lateral variations in the seismic velocities in the weathered layer are known

geophysical characterisation of a rockslide in an alpine region a combination of geophysical methods has been used for the characterisation of a rockslide area located in the graie alps nw of italy where the construction of a roadway tunnel has been planned electrical resistivity tomography ert seismic refraction tomography srt and spectral analysis of surface waves swm have been adopted to determine the overburden thickness and the geometry of the sliding surfaces the geomechanical characterisation of rock mass was performed by means of horizontal seismic profiles hsp and sonic log in a horizontal borehole the results have been calibrated by comparing the geophysical responses with the geological evidence derived from boreholes and with the movements recorded using inclinometric probe moreover borehole geophysics results were validated by the results of laboratory measurements performed on core samples the integration of geophysical and geotechnical results allowed the definition of a subsoil model for the investigated area and provided the necessary data for the design of the roadway tunnel the study shows that surface and borehole geophysical techniques could be an efficient and cost effective tool in rockslide investigations

constrained deformable layer tomostatics although first arrival tomography provides an effective waytoestimatenear surfacevelocitiesandstaticcorrections the undulation of velocity interfaces such as the base of the weatheredzonemaynotbeeasilydeterminedbythismethod the main reason is that first arrivals are insensitive to small geometric changes in velocity interfaces because their raypaths tend to traverse along those interfaces to improve the solution of interface geometry we developed a deformable layer tomostatics method that approximates the near surface velocity field as several layers of constant velocity and variablethicknessthatcanbeinvertedforthegeometryofthevelocityinterfaces weuseamultiscalemodelparameterization in the inversion for interface geometry synthetic and field datatestsshowedthatthemethodcandeterminetheinterface geometry constraining the depth range of the basal boundary of the weathered zone increases the convergence rate of the iterative inversion process tests on field data showed greater reflection coherency in a stacked section based on constrainedstaticcorrectionsthaninonefromunconstrained static corrections the method yielded a better match with statics computed from sand dune curves than does a match obtained by using two commercial grid tomography packages

seismic tomography studies of cover thickness and near surface bedrock velocities reflection seismic imaging of the uppermost kilometer of crystalline bedrock is an important component in site surveys for locating potential storage sites for nuclear waste in sweden to obtain high quality images refraction statics are calculated using first break traveltimes these first break picks may also be used to produce tomographic velocity images of the uppermost bedrock in an earlier study we presented a method applicable to data sets where the vast majority of shots are located in the bedrock below the glacial deposits or cover typical for northern latitudes a by product of this method was an estimate of the cover thickness from the receiver static that was introduced to sharpen the image we now present a modified version of this method that is applicable for sources located in or on the cover the general situation for nuclear waste site surveys this modified methodalso solves for 3d velocity structure and static correctionssimultaneously in the inversion process the static correcti

early arrival waveform tomography on near surface refraction data we develop a waveform tomography method for estimating the velocity distribution that minimizes the waveform misfit between the predicted and observed early arrivals in space time seismograms by fitting the waveforms of early arrivals early arrival waveform tomographyewt naturally takes into account more general wave propagation effects compared to the high frequency method of traveltime tomography meaning that ewt can estimate a wider range of slowness wavenumbers another benefit of ewt is more reliable convergence compared to full waveform tomography because an early arrival misfit function contains fewer local minima synthetic test results verify that the waveform tomogram is much more accurate than the traveltime tomogram and that this algorithm has good convergence properties for marine data from the gulf of mexico the statics problem caused by shallow gassy muds was attacked by using ewt to obtain a more accurate velocity model using the waveform tomogram to correct for statics the stacked section was significantly improved compared to using the normal moveout nmo velocity and moderately improved compared to using the traveltime tomogram inverting high resolution land data from mapleton utah showed an ewt velocity tomogram that was more consistent with the ground truth trench log than the traveltime tomogram our results suggest that ewt can provide supplemental shorter wavelength information compared to the traveltime tomogram for both shallow and moderately deep seismic data

tomographic joint inversion of first arrivals in a real case fromsaudi arabia a joint inversion of both first and refracted arrivals is applied on a seismic line acquired onshore in order to obtain a well resolved velocity field for the computation of static corrections the use of different arrivals in the inversion involves exploiting the information derived from the different raypaths associated with each wave type thus enhancing the reliability of the inversion the data was gathered by saudi aramco in an area of the arabian peninsula characterized by strong lateral variations both in topography and shallow velocity and where therefore a well defined near surface velocity field is important in addition to velocity the depth distribution of the quality factor q is computed from the tomographic inversion of the seismic signal frequency shift thus the q factor field is used to perform an inverse q data filtering and improve the resolution of the final stacked section

scattering effect on shallow gas obscured zone imaging in bohai pl19 3 area seismic imaging is challenging in the bohai bay pl19 3 area offshore china bohai bay field is seismically obscured making well penetrations the only reliable source of data for subsurface interpretation with the help of turning ray tomography we are able to obtain a reliable near surface velocity model approximately down to 700 m below sea level using the first arrivals picked from streamer data depth migration using velocities estimated from turning ray tomography has improved shallow structures and fault definition however reservoir level structures from 800 to 1500 m are still poorly imaged a viscoelastic modeling study with assigned variable q and shallow velocity profiles with and without shallow gas induced scatterers demonstrates that scattering is the primary controlling phenomenon causing imaging difficulty within the obscured zone due to scattering imaging tests at the target level were unsuccessful even with precise knowledge of velocity